# stax srm717/srm727-11



## duncan1

Hello! I am relatively new to this forum .Although I have loved stax earspeakers since buying a stax lamba in 1982.and have had several the latest being the 007mk1= my old equipment was a srd7-sb.I spent some months modding the srm 717 which might interest some and I also have a new srm727-11. This I have started modding not quite the same as the "Spritzer mod" and I intend to carry out other mods like the ones on the 717.I am retired from many years working on anologue electrionics including valve and solid-state although I am "solid" behind solid-state.If any body is interested  I will continue.


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## chinsettawong

Quote: 





duncan1 said:


> Hello! I am relatively new to this forum .Although I have loved stax earspeakers since buying a stax lamba in 1982.and have had several the latest being the 007mk1= my old equipment was a srd7-sb.I spent some months modding the srm 717 which might interest some and I also have a new srm727-11. This I have started modding not quite the same as the "Spritzer mod" and I intend to carry out other mods like the ones on the 717.I am retired from many years working on anologue electrionics including valve and solid-state although I am "solid" behind solid-state.If any body is interested  I will continue.


 

 Please do continue.


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## padam

Yes we are are looking forward to your findings 
  I think the stock 717 is rather excellent and better than the 727 but the latter might be slightly better still with some proper mods.


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## duncan1

Thanks for the encouragement I dont like "reading on other peoples toes.When I bought the 727-11 in the UK the dealer was surprised I was buying it"we dont get asked for many of those"=In the UK if its not valve[tube] is not of any good.But I knew I could modify it as I had done to the 717.Yes the 717 "sounds better" than the 727-`11 as bought in the shops.But on opening it up I wasnt pleased by the active components=big publicity is given to the "dual fet" input in laymans terms okay but not in technical terms. two fets pressed together by a plasic "shrinkdown is no my idea of a dual fet. Which is two fets on the one slice of silicon. The same applies to the "dual bjt[bipolar junction transistor] same cap .The 2sa1156 were of the cheapest quality[improved in the 727] . The low-level[+/- 15volts] regulators were only one pair. Not good for power supply p overhearing >Power supply runs to the daughter boards werent decoupled at the connectors[if it is over a few w inches run distortion can be picked up by induction/transmission. The power supply ripple could be lowered by increased capacitance or a solid state regulated high voltage power supply if there was room.The high current output devices on each board dont have heatsinks although they get hot[any active device when heated gives off additional noise] The input fuse  on the 717 tends to blow this can be uprated to around 1 amp without any worries [I shorted each leg in turn of the full wave rectifiers and the fuse blew every time so its okay no other damage was done.There are other minor things but going to the 727. I tried Spritzers mod  and yes it works but if you look at the 717 negative feed back it is taken from the two 100000K series resistors[voltage limits] The 150000K resistor in Spritzers is attached to the same two resistors but at the wrong end [feedback from the output-unless it is nested feedback ] is taken to the input active device to acheive better overall fidelity .so I bought 8 0.6 watt miniture metal resistors with minute noise levels soldered two in series and soldered one end to where Spritzer has shown and the other end to the TOP of the two original resistors ie-going directly to the input fets >This reduces the level of silibance down to a very low level.I do need to point out that as Proven technically your solid-state equipment must be on for 3days to get the benefit of higher fidelity ALL solid state must be on 24/7 or you are wasting your time trying to get the highest fidelity. I intend to carry on with the mods in the 727 later.But this is enough for themoment till i spend more listening time on it.Everybody has heard of the person weith the $1000000 system .Well I want to make those that cant affoard it have the same high quality at a cheaper price-excuse the mistakes I am a qiuck clicker of keys=Duncan


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## spritzer

Comparing the 727 and 717 is a bit fruitless since the circuits are very different and designed by different people.  The 717 was never a Stax design and built under a license so when that license ran out they released the 727 which is a souped up SRM-Xh/SRM-313. 
   
  Placing two J74 transistors next to one another is the only thing possible today.  There are no monolithic P-channels available in the world as the LSJ109 has been delayed for years now.  It is also possible to get far better matching by hand than Toshiba ever did with the K389/J109. 
   
  Also, please use the return key a few times...


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## duncan1

The two circuits different?-Only at the out-put remove the comp.capacitors and they are relatively the same. I have both boards in my hands and see they were wise enough to add heatsinks to the 727 which were left out of the 717.Unable to obtain Dualmatched monolithic j-fets+ bjts-wrong I can still obtain them  p  -Read -Horowitz and Hill: The Art of Electronics [Paul Horowitz Harvard University] After detailed technical analysis he even lists the ""best" j-fets/bjt in the US for use in audio equipment -Yes they are Very expensive but if you want the best you pay for it. And I intend to help those that dont have deep pockets.Its too bad you dont like my writing layout but you cant please every body in life and I dont intend to change .So if you cant live with it???


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## padam

So what's the difference between a (modded) 727, Cavalli LL and High-amp.de Transistoramp (V4.0)?
   
  They are supposedly based on the same basic design yet quite different.


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## duncan1

Sorry I havent tried or seen the other two.Most of the equipment I worked on in the UK was Stax or related also Mullard design tube amps and various solid -state. and other anologue test equipment-scopes-tube+ solid state etc.If as you say the circuits are similar then the four PNP bjts are an improvement  r on the NPN bjts which are the same as the ones on the main large heat-sink[717] the bandwidth is up from 5.5mhz to 6.omhz there is negative feedback from there as the output was disconnected but now with the overall negative feedback reinstated I should probably  g diconnect that but at the moment it isnt causing a problem I aim to acheive a level of fidelity equal to equipment costing many times more. This isnt magic because those solid gold unit only sound better because of very expensive parts and better laid out PCB .Any person with some technical knowledge can do this and the cost will be low in comparison to the $$$$$ of extreme equipment.This was even accepted in the UKs Electronic World/Wireless World. I am a disciple of the late great JLH- John Lindslay Hood a UK Audio amp and others he designed an audio design engineer.He helped introduce star earthing in the UK and the acceptance of the different qualities of capacitors .At the time he was ridiculked for that view by fellow design engineers but was proved right in the end when an engineer proved by science he was right.I do not use an earth as he was right that the normal mains earth inputs loads of noise and distortion. .If needed run a wire to the garden [if you have one and hammer a spike of copper alloy three feet into the ground it will be better than any mains earth. Be aware that with the high voltages in energizers you do so entirely at your own risk.I will post a list of the best dual j-fets and bjt when I look through the book mentioned in the last post by me.-You will need a good soldering iron /as said in other posts a digital multi meter isnt ideal for testing all parts of an energizer because of the very high impedance required although it will be okay if you stick to the daughter boards [testing elsewhere operates the cutt-out.excuse the mistakes in spelling-duncan


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## duncan1

First chance to get back.I have fitted 470UF capacitors plus polyprop bypass to the underside of the sockets of the channel boards for the +/_ 15 volts contacts.I also have fitted 100UF capacitors in the place of the 10UF ones on each board. I will be fitting 2 additional voltage regulators to the two already there to split the supply from providing both boards to each pair providing one board-this cuts down over-hearing between the channels due to the common power supply.This takes time due to waiting 3 days for the energiser to reach thermal equilibrium before judging the audio improvement As it stands just now the reproduction has changed from detailed but with a harsh sibilance to a lot more smoother and purely expressed vocals .and clearer low level detail.As the signal amplifiers are so simple they require a very high quality power supply -to stop power supply noise injection.and the ones installed arent that.This comes down to cost which is why there can be various modding "upgrades to this 717.I will get back later.


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## duncan1

Further to the modification of the 727.I have now added 2-additional 15V+- regulaters fitted these are TSitems no additional components are needed but I still added them .Both channels now have a pair of their own so no PS o/hearing from that source.May I add at this time that due to the 717/727 being DC connected any feedback applied by modification will change the DC voltage output at the ear-speakers.So I hope anybody who reconnects the output will -1=have to balance the two voltages-2=the working voltage will need to be reajusted so  you will need to use your digital MM at the red/white-green/yellow terminations at the main board.The transformation of the quality of reproduction is now changed completely from prominate sibilance and a slight harshness to a smooth valve like reproduction but with even deeper detection of very low level content.due to noise from the power supply having been reduced.Vocals are now "pure" and are a joy to listen to.I am not finished yet as I will be replacing the so called "dual" fets/bjt for real ones with a much highewrv -higher spec as detailed in every serious semi-conductor data books in the world.But as it sounds so good just now I will give it more listening time.My whole idea here is to give those without the money  to raise the level of the 727 up to a standard costing $10000 or more reproduction. It isnt magic its just spending a small amount of money. manufactureres build to a cost. Any amp can be uprated by those capable of working on audio electronic circuits.If I was younger I would do an upgrade service.


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## duncan1

After a while listening I have now changed all thr  the electrolytic capacitors in the power supply .The large ones to double their value and the small ones triple their value.The feedback resiastors resistors that I added to the TOP of the two in series I have increasd the value to 750000Kohms as the 200000K ohms resisters were giving out too much deatr   detail.I have not yet changed the semiconductors .Remember to adjust the balance voltage every time you move or change the value of them.Do not connect them to the bootom or middle of the two series feedback resistors as that will make it sound as if it is smooth but on prolonged listening is actually dull.Connect ONLY to the top of the two feedback resistors.I will get back.


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## duncan1

I have now spent some time in listening tests and the reproduced sound is now at a very high level=sibilance very low=purity of vocals excellent=plenty of detail and above all musical. I have not changed the active devices as when tested some do not conform to their identity as printed on them=eg=2sc1815 BL=standard audio bjt but not the ones fitted which are current mirrors one has beta[gain] of 20 the other the gain went off the scale of my digital tester =more than 1500. these are not standard[to the book]values and is not a darlington but are specialised .But what I have now is of a much higher standard than the original 727. I also bypassed all the electrolytics with 0.1 uf polyprop=400V working.So thats about it . I hope its of some use to those of a practical nature.


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## Number9

dup


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## Number9

Duncan1, you seem to have said a lot here, but its nearly impossible to follow. With no spaces, line or paragraph breaks... It makes for very difficult reading!
   
  Can you please summarize the changes you've made, and maybe your insight in modding the 727 will be better understood by everyone. If you don't want to change your writing habits to make yourself be better understood, I guess so be it. Your choice. But it does turn off people because it is considered poor form.
   
  Commenting as encouragement rather than criticism. I'm very interested in what you have done here with this mod.
   
  .


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## duncan1

Thank you for your email NUMBER9 I dint put English phrasing high up on my list of priorities. I notice the mis-speller doesn't work right here[misses some] but that is technology for you.What I do care about is more political and not right for here. As a follower  of JLH-UK audio-design engineer[sadly no longer with us] I use his 100% tried and tested  methods in  addition to some of my own[based on his]. He was a Minimalist and DC connected where possible/Proved "capacitor sound way before it was recognised and was criticised for it[proved right in the end] .Star-Earthing etc.Every thing I posted was to "open up"the circuit while some find it a lot easier to "dumb down" a circuit introducing filters etc-wrong way to go! . It is harder opening up as it shows up even smaller amounts of noise/distortion.But what you get is a deeper revealer of detail not found in many "dumbed down" to make it smooth circuits. But I like a challenge.ALL electronic circuites from the Government down are built to a price.But in Audio why should you accept the status quo when you can spend as much time as you want AND outlay a very small amount of money to achieve a level of Fidelity that would cost  10s of 1000s of $$$$$. in the shops . The only people unhappy about that would be manufacturers and those with shares in the Audio industry.And dont think I am a lone voice.I have the copies of the magazine -Electronics World/ Wireless World  where some design engineers agree! I must be able to put my point first as then people will understand where i  "come from"-I like helping people but some see this as interference but you cant win everybody! I wanted you to read this first-Duncan1


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## Number9

So beefing up the power supply makes sense to me. But I'm a little confused here as to your variation-twist to the Spritzer mod. Come again as to why the additional resistors are needed? Can you post a photo of what this looks like?


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## drp

duncan1,
   
  Thanks for posting your modifications and thoughts on the 727ii. Those of us with the 727ii, and others looking for a mid-cost electrostatic solution, really appreciate the time and effort you put into typing out your findings, however they may be formatted.
   
  I made my way through the entire thread with even greater interest because I performed Kevin and Spritzer's negative feedback loop mod a few months ago; have been curious as to what level of power supply mods could be performed. RE: The experts in this community (Kevin and Spritzer) pointed out the PSU as another weak link in an otherwise good+ stat amp (once modded).
   
  In your first post, you wrote "If anybody is interested  I will continue." The fact that the typed English language is obviously quite low on your list of priorities, and you've only gotten a few very gentle nudges to use the Enter key, should speak volumes as to the very high level of interest we have in your thoughts and modifications (other topics would have been less accepted for the given writing style). Please continue sharing, even in your own way.
   
  You also wrote (I think) that you want others to be able to perform your mods and get our 2K amp to a much much higher level of performance. With regard to actually making it possible for the layman that's handy with a soldering iron and volt meter (like me) to actually perform these modifications, an easier to follow procedure would be needed.
   
  If you would be interested in writing up your full modifications, by section and with part values, etc., and then PMing them to me, I would be happy to work with you to translate this information into a step-by-step procedure. The most important aspect would be defining the relative level of knowledge/experience needed, as we are talking about a device with very high voltages. 
   
  Thanks for sharing, and welcome to the community.


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## duncan1

NUMBER9= Spritzer connected a 150KOHM resistor to the beginning of the chain of two resisters in series coming from the new output on the channel cards back.two were used because of the low voltage of each PCD resistors to make up the voltage .I have found out you only need =one=500KOHM-upwards[to suite yourself] on listening ] if you use 250V working ones.There are several things that must be taken into account=1=Connecting Two feed-back resistors to each other is no t right =it means BOTH go down the same path.You will think=smoother. but on listening the detail is recessed.not right.so it must be fitted at the TOP end of those two feed-back resisrtors. This gives a very clear and detailed output=BUT=doing thid   this before doing the other mods will just make it sound maybe even worse as you are opening it out without doing anything else=2=ANY change to the feed-back produces a proportional change to the VOLTAGE output =did Spritzer not tell you to check the balance of the two sides on each card??=and because of being DC connected there is INTERACTION between the two adjustment presets on each card meaning you will have to adjust the voltage as well[ usually upward as feedback lowers output.Wiy   without doing that you will find the voltages very unbalanced.You can do it but dont expect miracles without doing ant  any other mod.


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## duncan1

DRP =Thanks for your nice post .Your right I am what you would call in the UK a bit ex centric=BUT this in no way imp ares my intelligence or " down to earth approach.You are right about the high voltages while my health has never been good . I am Lucky enough to be born with a high body resistance to voltages due to thick skin otherwise I would be dead long ago and yes I have had several shocks from the HV-power supply -very sore others with thin /moist skin would be wise to make sure that the voltages in the capacitors is drained before starting.But if you take a chance -use ONE hand only put the other behind your back [so the HV doesnt run through your heart] to earth. The level of knowledge required is Medium-ie used to working with the basics of electronic circuits AND can recognise resistors/capacitors Buy a good so;soldering iron -better still a quality Soldering station=it will last years longer than any single soldering iron. Buy ex-industrial for good quality and at a lower price than new.As regards the digital multi-meter Spritzer is right ONLY use it to test the output voltages at the main board as it runs up to the socket for the head-phones any other place  the 727-11 will cut out-due to the low impedance of the meter--10MEG OHMS is not good enough. I have a HP industrial bench multi-Meyer with 100MEG input .I must make this point -the upgrade parts -do NOT go to Radio -Shack and ask for some components-you MUST buy High quality parts especially the HV capacitors. can you give me more info on emailing you for you to make the above more acceptable to Head-Fi posters??


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## padam

I just found some pictures of a very recent 727II and what I noticed that it looks a little bit different to the earlier ones
   

   
  Earlier one:
   

   
   
   
   
   
  I guess the circuit is identical so the two sound should identical
   
  So my question is:
  Are the parts supposed to be equal grade or Stax decided to go just a bit cheaper?


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## Number9

Quote: 





duncan1 said:


> ...so it must be fitted at the TOP end of those two feedback resistors.


 
   
  Duncan1, this is where you lose me. What do you mean here by "TOP" end?
  Thanks.
   
  P.S. You can send a private message to DRP by moving your mouse cursor over his name, you should see a drop-down appear saying: Private PM. You can communicate directly this way without relying on email.


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## drp

9 - It reads like he is replacing the two 150k resistors that are in series with a combination of resistors with different values. 
   
  Visualize two resistors in series; oriented vertically. With the current mod, you replace just the lower 150k surface mount res with an axial lead of equal value, soldering from the alternate edge connector to the _bottom_ of the res that's left in place. If you replace both resistors, you'd be soldering to other end of the res that was previously in place, hence, the "top." You'd actually be removing both and soldering to the top-most surface mount pad. I'm practicing my interpretation skilz here..... A completed sequence would include a picture, but search the mod for current pics.  I dropped Duncan a PM yesterday afternoon.
   
Padam - thanks for uploading the 727 pron. I took pics when I opened mine (recent version); will post pic of amp and individual amp board this eve.


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## duncan1

Padam  Looked at the 2 pictures I have the latest one the earlier one looks as though it has kept the feed-back/stability capacitors of the 727-1 mine has them removed.This means the tonal qualities wont be identical. The most sensitive part of an amp of any sort is the feed-back even the type of resistors and capacitors change the sound. And you have "hit the nail on the head"-Cheaper active and passive components[to save money . that is why it is possible to "upgrade" the fidelity so easily. But the new heatsinked outputs on each card is an improvement from the bare BJT[bipolar junction transistors] on the 727-1 .The 727-11 is more open but shows up the defects more than the previous model.When All the mods are done you will not need to "upgrade" to a stax -oo9 I have the omega-1 headphones and they sound fantastic now miles more der detail -depth sound-stage etc and very musical . Your feet wont stop tapping in harmony with the music


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## duncan1

DRP-Sorry if I am confusing you I realise its the way I write.The two smd resistors on the pcb are left in place they come from the output on the CHANNEL card [the heat-sinked bjt] Leave them there changing them will cause instability [I tried it] so the 500KOHM resistor[250v working-metal film or better] is soldered to the TOP-looking from the bottom of the card upwards-of the 2 smd resistors which are in series-you can upgrade them if wanted but the soldered 150k goes at the top connecting it below gives what you think is a smoother sound but it is really recessing the detail. You cant put 2 feed-backs through the same resistors each feed-back must go to the input.You will notice the difference but I warn you -unless you upgrade the rest  it wont be as good as you think as now you will hear noise and distortion products causing roughness to the sibilance. There are 2 types of feed-back overall -output direct to input and "nested"-meaning local feed-back  the norm is direct feed-back but some amp designs have nested feed-back-you take your pick and decide yourself.


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## padam

Quote: 





duncan1 said:


> Padam  Looked at the 2 pictures I have the latest one the earlier one looks as though it has kept the feed-back/stability capacitors of the 727-1 mine has them removed.This means the tonal qualities wont be identical. The most sensitive part of an amp of any sort is the feed-back even the type of resistors and capacitors change the sound. And you have "hit the nail on the head"-Cheaper active and passive components[to save money . that is why it is possible to "upgrade" the fidelity so easily. But the new heatsinked outputs on each card is an improvement from the bare BJT[bipolar junction transistors] on the 727-1 .The 727-11 is more open but shows up the defects more than the previous model.When All the mods are done you will not need to "upgrade" to a stax -oo9 I have the omega-1 headphones and they sound fantastic now miles more der detail -depth sound-stage etc and very musical . Your feet wont stop tapping in harmony with the music


 
  So if I get it right:
  That means the earlier 727II is easier to feedback mod than the latest 727II where those capacitors need to be superseded so it is a bit more complicated?
   
  I have the Omega1 and 717 and I loaned the 727 from my friend. Without the feedback mod on the 727 the 717 easily sounds better but a few aspects are better in the 727.


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## duncan1

Your right Padam with the feed-back capacitors fitted on the earlier 727-11 the addition of the feed-back resistor will have a different effect.But they found removing them opens the sound up a bit but because of the cheap components that makes it easier to hear small amounts of noise/distortion and so overall doesnt sound so good as the 717 and anyway the 717 has overall feed-back from the main output which the 727-11 hasnt thats why Spritzer complained about It  and added the 150K resistor to the bottom of the chain to smooth it out.But that meant 2 feed-backs were going through the same resistors not done in commercial practise .So I have put a 500K resistor directly attached to the input semi-conductors bypassing the two smd resistors on the pcb This give an "upfront sound" but unless you upgrade the rest it wont sound as good as I am listening to .And remember ANY changes to the feed-back means you MUST re adjust the output voltages     both the balance and the voltage up/down as they interact because of being DC connected [no capacitors in signal path] a good thing! The front 4 wires feeding the socket when looking from the front the pr on the RHS are for the RHS channel card and the pr on the left for the left channel. Its okay to use your digital MM to test there BUT not anywhere else[except the power supply at the fuses] doing so will operate the cut-out because of system in balance.duncan1


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## drp

Yes, Duncan, it would appear I totally missed the mark. I'm sure we'll get it straightened out once we start collaborating off line. BTW - I am looking at the schematic as I try to follow your logic and changes; feel free to write the reference designators and pin numbers to which you are referring.
   
  One thing that would really be helpful is pictures of your mods. Can you snap a pic of your amp board the next time you're in your amp?
   
  For additional comparisons to Padam's pics, here are a few camera phone pics of my 727II, (quickly) taken before I started my mod.
   
  Irrespective of the ways of modding the 727, I've been listening to mine every day with the SR-007(SZ3), driven by an RA Opus 21, and I think it is a great sounding system; most enjoyable I've owned to date.
   
  ETA: Found a partial shot of back side of amp board; shows the two referenced resistor pairs


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## Number9

This is getting interesting.


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## duncan1

Yes DRP your second photo shows the two vertical resistors in series on the right -hand side of the photo .They go directly back to the input fets providing neg-feedback and 2 contacts along show the un-connected neg-feedback from the main output     ] to the headphones sock] strike that]  back towards the same input. As you can see thats where Spritzer connected his 150k resistor to the BOTTOM of the vertical pr. I connected a 500k resistor to the TOP of that pr.You are actually ahead of me as try as I might I cannot obtain a circuit diagram as Stax wont sell me one nor does the Stax dealer have any as repairs are sent back to Japan on 3 months boat journeys I f you could possible send me a copy of the 727-11 circuit I am sure I will be able to add more modifications . While I have good visualization senses and can look over a circuit board and get the gist of a circuit it still is not as good as having the circuit in front of me.This side of the Atlantic they are very secretive in their allowing access to circuits that they want to keep commercially to themselves. for financial reasons. Even my usual circuit company web-site hasn't v got one.This applies especially to the active components as if I can see exactly what they do then maybe be able to change some to better quality as they certainly arent the best quality that you can buy.eg- the 2 2sc1815 bjt which look like a CC source/ current mirror . They are not low noise although they are audio bjt there can be many transistors with the same lettering  but ending in different codes [letters/numbers] that change their active quality.-duncan1


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## drp

K, Duncan; thanks for the additional information via PM and email. Reads like your mod is best represented by the following very simple MS Paint edit of the solder side of a 727II amp board. Your stated resistor specs: 500k, .1%, .25 watt, 250v, 5 - 10 PPM. And of course, adjust balance and DC offset for both channels after the amp reaches operating temperature.


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## duncan1

Yes Dave. exactly right.also for the adjustment when hot .Solid -state audio equipment i should be left on 24/7 and give it 3 days at least to judge it . If it is kept being switched off/on then you will never hear the best fidelity.Valves[tube] isnt the same doesnt need as long to judge it .valve equipment heats up a lot quicker and the actual amplification is usually all valve so other parts dont need to heat up. I am talking a pure valve set up not a hybrid valve/so;id-state.Once f valves are "run in"for the first time of use the fidelity doesnt change much on switch-on only thing to watch is the bias adjustment/unequal gain which will increase distortion and every piece of valve amps are heavily dependent on the quality of the out-put transformer although in energisers they wouldnt normally be used.Reproduction should be a lot clearer and open [smaller detail heard] but that depends on not only the other mods but the quality of your other equipment.My own-build buffer is 2 individual amps with their own ott PS and an very low distortion audio chip in each /plugged into gold-plated contact sockets so chips can be changed to suite. JLH proved to his own satisfaction that at low signal voltages it was near impossible to tell the difference between them and a discrete circuit.Star earthing polyprop caps used as bypass for electrolytic caps-105 deg high quality double voltage regulators[2 stage drop in voltage] this gives me a neutral output although if some like it warm then just change the chips to suite.I should add if you are not changing the caps in the PS then at least change them [as I have done in the channel cards to 100uf-10uf as fitted is not good enough and change bypass caps to 0.1uf polyprop /soldered to the back to give room to fit the larger capacitors[side-ways] The other 2 small /very low value caps I changed to polystyrene.Duncan


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## kevin gilmore

that 500k resistor had better be rated for at least 400 volts.
  otherwise you are going to be burning up front end fets.


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## duncan1

Kevin- I have been running the 727-11 for over a month  with no problems! The difference to the "bog standard" 727-11 is "night and day" .It is "well seeing" that Stax has cut-corners with the quality of components and the amount used.
  It is "cheapskate" and not top hi-Fi just to use 1 pr of 15V regulators for BOTH channels . I am sure you know about channel overhearing via the power supply . In this case the low voltage PS to the critical input

 This lowers the perceived stereo  effect and openness.And that's just 1 comment /many others are there.
  I listen on Stax 007-MK1s that people say can be recessed -not on my system.I go for openness and minimalism and my own build mono-buffers are on near as straight through as you can get .
  If it makes people feel better then okay buy 500V -500K resistors/ or  2-250k-250V working I dont have problem with that.After ALL the mods it produces upmarket reproduction which is a mile better than the standard item.
  Even the dealer in the UK whom I bought it from couldn't figure out why I was buying it as -his words-nobody buys this amp here! They all buy tube amps. But I knew what I was getting and in the future will be making some more mods.  The ONE advantage   that the channel circuits have is that they are so simple with little active devices in the signal path that they reproduce a clearer sound without being "processed" by many stages.


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## duncan1

After extensive listening tests  I have found that  -in my ears- That I am hearing too much detail-NOT because of uneven FR as NEG-feedback flattens the FR but because of increased CLARITY-drop in noise/distortion.
  I could hear analogue drum skin bounce and every vocal articulation from the singer somewhat like the OM2-009s. on my 007s-MK1 AS  I like a fully musical presentation I therefore increased the feed-back resistors value to 1MEG-OHM this cured it.
  I also realise this amount of detail is what a lot of people want so its "horses for  courses"


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## Number9

In effect, increasing the value to 1 Meg Ohm does what? Increase negative feedback?


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## duncan1

NO reduces the Neg-feedback.The reproduction was too "crystal clear"-too much feed-back for me as the sibilance  was also reduced to a minute sharp point. which isnt really  correct musically but it gave highly detailed reproduction  .Even with the increased value of the resistors there still is a lot of detail but the sibilance now sounds "normal" and more natural.
  The 727-11 circuit is not the same completely as the 727-1 or 717 although more than half is the same .The presentation in the standard version 727-11 is slightly more open than the others. I have made it [if you do all the mods] a lot more open but with much higher fidelity.   achievable  because of the lowering of noise/distortion. 
  If you just change the resistors you would hear more clearly the noise which affects the sibilance making it rougher.Some people dont like sibilance at all but in that case what you would be listening to wasnt  Natural musical reproduction Just listen to a live performance in a hall or football stadium.and you will hear the difference.But there are people who cant live with it so its up to the individual what makes him/her happy.
  the PS is very basic and I doubled the capacitance . But really a stablised PS is the ultimate answer. But changing the 2- 10 UF caps on each board to 100UF-400V helps[fitted sideways.Change the bypass low values to high quality  poly-prop and the 2 smaller value comp.  caps to polystyrene -fit both types on the rear of the PCB [to allow the 100UF caps to fit flat.


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## duncan1

I have now spent time constantly listening  and it is now exactly as I want it to be. Natural musical reproduction smooth but not too smooth so that all fine detail can be heard completely natural.Even 50s rock-n-roll not remastered  to digital now sounds alive and you get a fine sense of that era.
  I have a wide taste in music only not keen on jazz. and classical music played has a sense that you are part of the audience.
  There is no similarity between the standard 727-11 and what I am listening to now. I have an old but upmarket record -deck and it is not too dissimilar to that- considering I use a CD player that's saying something. Reproduction is on a par with a $10000 unit.

 All this can be done by anybody who     knows electronic  components . All you need is a digital MM to adjust the DC V+DC balance .
  People dont realise that you can with some work improve the fidelity of Audio equipment beyond the makers set level. It isnt magic just a bit of work.


----------



## Number9

Hi Duncan, this is very encouraging news. With DRPs help, I think its very easy to understand your variation to the NFB or "Spritzer mod is all about.
   
  The power supply changes are much less clear to me. It is obvious that there is need for improvement in the PS (as you've stated several times). But what specifically should be done? 
   
  You have said:
   
But changing the 2- 10 UF caps on each board to 100UF-400V helps [fitted sideways].
Change the bypass low values to high quality poly-prop and the 2 smaller value comp. caps to polystyrene-fit both types on the rear of the PCB [to allow the 100UF caps to fit flat. 
   
Can you please elaborate in such a way that this can be easily replicated by someone much less knowledgeable (like myself)?
   
THanks.


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## duncan1

HelloNumber9-When a audio circuit is designed  many things need to be taken into mind including the amount of neg.feed-back.The same applies if you change or amend an existing circuit design

 The parameters of the  original circuit are different from the modified circuit. The reason I do not supply photos is simple .Unusually I have never got round to buying a digital camera.
  On the front of the PCB are 2- 10UF caps-unsolder them-also remove the two small by-pass caps and the 2 even smaller comp.caps.
  The 100UF caps are big so must be fitted side-ways-either use stiff solid core wire to solder them in[to support them] or super glue them and use flexible wire.
  On the reverse of the  PCB -solder in 2 poly-prop-400V -0.01UF bypass caps-the 2 smaller comp.caps- solder in 2- polystyrene caps.same value as the old ones.= labelled-221J=220PF
  This allows the large 100UF caps to be flat on the PCB.
  Get back if more info required.


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## duncan1

Just a word to let you know Number 9 still sounding great!
        tried out over the days in all types of music very detailed with a realistic reproduction[sounds natural]
        You get "the feel" of the atmosphere of the music it isnt just a "smooth sound" which can mean you arent hearing deep detail.and therefore cant feel the emotion in the music.


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## Number9

Hi Duncan, thanks forms sharing your mod. I think I will indeed try it at some point.


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## duncan1

Number 9- I realise my problem is no photos . And this can cause problems in people understanding what I post.
                      At the first opportunity I will buy a Digital camera.


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## padam

I recently compared a modified 727 (original version of the mod) against my own 717
   
  It is a bit fiddly as I constantly need to pull out and connect everything and the gain is also different so difficult to match precisely. The volume pot is disabled in both amps.
   
  Even so, overall I would say they are quite close in sound.
   
  The 727 is a bit more neutral and a bit more precise in the bass so maybe has a little bit better clarity.
  Instrument positioning is a little different maybe the 727 has a slight edge again. Maybe.
   
  But with the SR007 the modded 727 does sound a little more anemic and I somehow prefer the 717s presentation a bit more it has a slight bloom to the midrange that matches it well.
  With SR Omega SR Sigma Pro and SR Lambda I prefer the 727 but I wouldn't be unhappy with the 717 either.
   
   
  Considering the 717 costs less to buy and needs no fix at all to sound good (the stock 727 is quite different) it is still great value in my opinion.
   
  Both are excellent solid state amps it would be interesting to see what else duncan1 did to his 727 and what those mods can lead to in terms of sq.


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## Argybargy

Duncan, fascinating mods.

Would you expect to see similar gains in sound quality by making analogous capacitor upgrades to the SRM 007t(II)? Or is the tube circuit an entirely different beast?


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## spritzer

Put the largest cap you like in there but it won't change how badly overdriven the tubes are.


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## Argybargy

Quote: 





spritzer said:


> Put the largest cap you like in there but it won't change how badly overdriven the tubes are.


 

 I guess I'll take a look at the 6S4A mod.
   
  What's the best way to remove the top layer of the PCB to expose the bare copper trace?  I noticed in Justin's high res photos of the mod, that he did this several times?
   
  No need to change any resistor values?


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## duncan1

Thanks for the positive comment Argybargy I have been put off adding to this because of the general negative comment.
                         That I wasn't presenting this by the "way it is done in Head-Fi"
                            And yet looking at some posts they go on for greater lengths to mine --in the same lay-out.
                              My whole approach and attitude is bound up in the way that JLH [john linsay  hood ] .
                                  approached it but very condensed- I love music but reproduction of it isn't always perfect
                                   Like John I believe in complete openness so that any fault in the electronic components is shown up    NOT to be "dummed down" to make it smooth by adding many frequency changing and stability components like small value capacitors . To me that is cheating.
                                       A certain ADE wrote in EW/WW that while on a breadboard his new design performed beautifully BUT transferred to a lovely to look at PCB he had to add comp capacitors.They affect the reproduction -yes to stop HF oscillation but also the perceived sound reproduction. 
                           Johns designs were always open -and latterly -high input impedance/high sensitivity any ADE will tell you that that is harder to achieve  low distortion than a low sensitivity/low input impedance.
                             John also used Mos-Fets at that time he was criticised for using them under the heading-"they are not as LINEAR than BJT"-quite right -BUT a lot more neg feedback can be applied as they have a higher bandwidth and as some know here have a more clear presentation.
                MY whole aim here is to achieve  a lot more open and natural sound-as you would hear live at a concert hall 
           Not smoothness for smoothness sake with rounded ends which after a while you feel inside yourself that you are missing something in the music but a feeling you are in the stalls listening to it live. I have no intention of changing my values learned from JLH. I know ADE have blazing  rows with each other on audio issues but I wont be changing mine.


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## spritzer

Quote: 





argybargy said:


> I guess I'll take a look at the 6S4A mod.
> 
> What's the best way to remove the top layer of the PCB to expose the bare copper trace?  I noticed in Justin's high res photos of the mod, that he did this several times?
> 
> No need to change any resistor values?


 
   
  Probably just scraped it off.  I cut off a part of the trace to make sure there was no chance of arcing.


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## Argybargy

Thanks. I'm a little hesitant to take a razor blade to an amp I paid $900 for, but, hell for better sund quality, why not?


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## Argybargy

Duncan, keep up the good work.

It's the ideas that matter, not the form of the message nor the messenger for that matter.

I don't currently have a 727 or 717 but will probably get one in the future, so I copied and saved your posts into a word file for future reference.


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## spritzer

Quote: 





argybargy said:


> Thanks. I'm a little hesitant to take a razor blade to an amp I paid $900 for, but, hell for better sund quality, why not?


 
   
  The first amp I modified was a brand new 007II...


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## duncan1

The 727-11 produces quite a bit of heat-it does run in class-A. But heat means noise and this too will have be removed from the amp by  NEG. feedback.[more work for it]
          It is supposed to work by convection but it only takes a quick look at the size of the holes on the base plate to realise that the efficiency will be near nil.
                 I understand that companies are  under legal restraints by governments  as tiny fingers could reach underneath and touch a live part. causing a massive legal claim.
        For those who dont have that problem- I got my old 717 base plate   and was going to drill  large holes in it bit as I wont be selling it I just removed the whole area of holes.Try using that base in front of your PC extraction fan and watch the CPU temp rise.
             That isnt enough as the bottom clearance to allow a good stream of cold air isnt enough.
                Using the old base plate I added new feet to it these I got from an old 70s  Japanese record player being used as suspension these raised up the base high enough to allow more air to get in.
                        I realise this isnt for everybody but just as a test  IF you have re instated the feedback as I suggested
                             run the amp without the top cover for 2 days to see if it makes an improvement.
                                I am talking about a very small   improvement as it sounds very good as it stands. 
                                     And remember IMO- SS equipment should never be switched off.
                                          The company I bought all my input equipment says the same -CYRUS Hi-Fi-UK


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## duncan1

I said in a previous post that to work on the Stax 717/727 1/2 energizers that you would need  a higher impedance than your normal MM of 10M ohm input  because of the very high  impedance of the circuits and that 10M would cause the safety relay to cut out the power to the units inside.  I said I had a HP bench MM industrial version of very high input impedance.
       But I bought an Avo MK 4 VCM and noticed on the sites on that web page that Valve/Tube Voltmeters  were for sale . While these are old technology they still beat  most SS ones as far as input impedance is concerned and unlike SS measure a high voltage with the meter set at a low one only the needle would be damaged as the valve are nearly "bullet proof". At the moment they sell cheaply on EBay and others as nobody wants them but those unable to work on energizers because they don't have the right MM this is the answer. REMEMBER --there are MANY circuits for them ONLY buy ones that have an input impedance of say around 100M ohm . Many old Heath-kit  only have a 11 M ohm  input -not high enough.
       The only hard part is you might have to adjust a couple of controls  to set them up.And you have usually bought[depending on the circuit] a TT or two.


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## duncan1

Further to my above post I contacted a ex-government equipment seller in the UK as I saw  a Bradley CT471 very high impedance meter for sale[ 100M  ohms above 12V ]  No price was on it as it was on sale . Phoning them  I found it was selling very cheaply as it did not have a probe and no front cover [don't need the front cover and unless you are testing RF equipment   don't need the probe]. It was a total of including shipping of £37  probably $60 or so I ordered one. I have dealt with this company before all their equipment is guaranteed to work. That is a bargain! as one sold on eBay in ropey condition for £150.
                  I don't want to mention the company as some business people might want to buy them as they have more and resell at a profit.
                       But if any Head -Fiers are interested I could  post it.


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## duncan1

Further to the above post . My Bradley Electronic MM type CT471 arrived today cost £25.[approx $38]. Works just fine . Was able to test high impedance  in the 717/727 [  12V and upwards at 110 MEG input impedance]  As nobody seems bothered then the company is Stewart of Reading.co.uk  ex government electronic equipment seller for past 30yrs all equipment guaranteed working.


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## duncan1

I realize that the feedback resistor I fitted isn't a complete answer. So I made changes to each channel PCB. It wasn't logical that both feedback's should work together on the same connections So I  decided to "go the whole hog" and rearranged the feedback to something I am sure will meet with universal approval. I cut away the original feedback connection to the inner BJTs and  connected the ends of the 2 -150K+ 150K resistors directly to spade terminal 5[from the end of each side count the spaces as well] So that leaves only them and not the inner circuit connected directly to the output on the mother board.Doing this removes any conflict with BOTH sets of feedback still being attached so now there is only  TWO feedback's from each end of each card-5 in either side. Which is like the original 717 design  --as far as feedback goes- NOT IN OTHER RESPECTS. There is also a circuit diagram on this website supposedly showing a 727 diagram- This is NOT an ORIGINAL STAX 727 diagram - There are various changes to component  numbers  even if you eliminate  the added switch section . So beware if you are using it the input side also has some differences. Tested on my scope with a 1KHz square wave it looks pretty good as there now is no longer any conflict with 2 feedback's an "improvement" should be heard . YOU decide just how much once you do it as I have been criticized for sounding SUBJECTIVE- but maybe that's because I am not a robot who hears zeros and ones . Most people on this website talk subjectively try talking another way and still describe the sound quality . I mean describe by  human ears - not my or others THD meters.I would like to add that there are 2  small value caps connected to both plus and negative INPUTS. These are bandwidth limiting caps  or called "low pass" filters They are usually there to help stability BUT they do impact on  SUBJECTIVE sound response checking out under my scope  the amps will work without them and the square wave now has sharper edges.--meaning extended response and quicker delivery.- but its up to you as to the amount of minute detail you want.so you you could try it if it doesnt suite then its easy  to replace them.--- I should also have added that the verticals can hardly be seen when caps removed signifying a much faster  "rise time" My scope is a  Tex -150 Meg with 1 nanosec rise time so it is not limited in its speed of display.Those with "virtual" scopes on PCs might have a different rise time but I dont think it equals that speed although will anybody specify the type and make of the virtual program so I can check it out.


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## duncan1

Well that's about 2 weeks of listening trials since I reverted the 2 -150K resistors back to where they originally were in the -717-BUT  remember the rest of the feedback between both the 717/727 is different. Actually there are now a lot less local feedback caps on the 727 than on the 717[check it out-small brown caps] and that's the way I like it as it proves it is more STABLE than the 717[although I not saying the 717 isn't stable] its just that the 717 NEEDS those caps when the 727 doesn't. Which is a good thing for openness and clarity. Every small comp caps implies a COMPROMISE in the design . If you are thinking of buying an amp check out how many-LOCAL feedback  there are  to see how good the design is. The best have very little signifying good design in circuit layout and parts used. There is now more full negative feedback [from output to input] and it shows in even more clarity without sounding--edgy/sharp/etc as more TOTAL feedback doesn't just lower the THD but also the noise floor.The music flows beautifully and realistically and does full justice to my Stax 007-MK1s . While the 009s can sound good on tube technology the same -IMOP-does not apply to solid-state.unless it has been "tuned" to sound like tubes.by that I mean some "dumbing down" of the signal. I am even happier now with the results. .I might add that the +/- 15V supplies that I separated and used 2 regulators  in-EACH  channel with smoothing caps both at the regulators and attached to the channel cards socket pins means that even on my lowest setting of my scope  I can hardly see any AC ripple and remember that's PEAK/PEAK.[RMS value much lower.] ---In case some have not read my previous posts on this . You MUST CUT-OFF[use a sharp knife] to cut the printed circuit copper that leads to the original feedback BEFORE connecting the feedback resistors to the original position of the feedback  as on the -717.


----------



## duncan1

I contacted the company I bought the 727 from to tell him that I realize that I have lost the guarantee  as I had modified it and spoke to the owner. He told me that an Italian had bought a 007 MK1 from him as they are very popular and that the Italian had modified his 727 and was very pleased with it. So I am not the only person to make some major changes to the design. Its funny when you start doing something the urge to carry on and do more is strong. So in the future I intend to  convert the power supply to a stabilized version. This isn't as hard as you think its actually pretty simple as I wont be removing any parts and will just add it in series with the high voltage fuses on a small PCB . The only amount of bulk would be the heat sink and as the current drawn is only approx-60 MA it wont have to be large.


----------



## duncan1

One of the faults of all amps [power] is  correcting the offset . Many things have to be taken into account= the identical spec. of the input pair/ the amount   of current through them/the stability of the CCs/ variations of the power supply and so on . Careful design in the choice of components and actual circuit design itself. There are ways of combating that by a DC servo and other means  but in this day and age as things get smaller and smaller.this means that all components must be of a high caliber. If you take the actual adjusters of a standard offset adjuster the small preset resistor. physics cant be overcome by this means of adjustment. Small track means small selective area of adjustment  That doesn't help to keep the set adjustment steady[within a small range] .I cured the same type of thing in my old cheaper of the two tube tester-Taylor   45C when testing large pentodes with large gain -needle couldn't be adjusted by "piggy backing a small value variable resistor to the main one --it worked.So in old/low tech. you use either a small value resistor to help steady the adjustment OR you remove the small variable resistor and fit one 3 times as big --much longer amount of adjustment due to the longer TRACK OR you fit a HIGH quality -10 turn pot. These will all give you finer adjustment and less chance of the offset straying  into many volts Old fashioned yes/ old technology yes/ too simple yes but not for me.Because it works for a small amount of soldering.


----------



## duncan1

This might be a big money saver for some technically proficient Head-Fiers . Having had a close and extended look at the two circuit boards on each model of the 717 and the 727-11 I have to say that's its perfectly possible for a 717 board to be converted to a 727-11 board with someone who has done many changes  to electronic equipment over the years  and has various test equipment Both motherboards have differences but they are in positioning of the volume control and the cut-out relay . Why do you think it can easily be converted back  from the 727 negative feedback to the 717 version?. Many companies save money by keeping most of the layout the same on PCBs but make smaller changes to them look at the two motherboards . The essential ,components to make it operate are the same it hasn't changed . Think of saving about $2000 by DIY -BUT ONLY to be undertaken by those with years of training or knowledge of electronics and have the test equipment.----Also you will need the original circuit diagrams for both Remember ORIGINAL. Unless you like me can "read" a circuit  just by looking at it.----I will also add that only 20-25% of the layout needs to be changed . The biggest is the removal  of the last two-C5466s and replacing them with the new type . You notice heat-sinks are now fitted to them in an earlier post I fitted heat-sinks to the output BJTs as Stax didn't fit them-[717]. The more heat produced the more low level noise  you might not hear it as noise but you will hear the sibilance  rougher. And you wont need any comp.caps[small brown objects] which is a good thing because it implies a more stable circuit and comp caps effect the sound quality . That's why when they are used in the neg.feedback they must be top film cap types and small value local feedback caps should be polystyrene. They seem to have shot up in price used to be cheap not now.------I had a look at the costs it would be no more than $50 for parts and you could get better quality  heat-sinks with wings than the right angled aluminum plates fitted[better dispersion of heat]  The only real cost is time taken and precision build no cutting corners or trying to do it fast. Take your time slower is better here--less  mistakes.------Its occurred to me that there must be many enterprising people in the US or Australia and that if you charged say $450 per conversion which would still be a bargain for someone upgrading to a 727-11 as compared to buying a new one.I am sure somebody will see the financial benefits and if they buy in bulk even more of a saving in initial outlay.  ---Or they could include the mods I have mentioned above  as a "stage 2" with an additional cost of say $200.


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## Furlan

Hello! 
  
 Some time ago I bought an SRM-727A amplifier and headphone SR-007 MKII. 
  
 I already bought the resistors to perform Spritzer Mod and i want to make the service soon. 
  
 So, I have a doubt: 
  
 How do I adjust the balance DC? 
 Where do the adjustments? In which components? 
 How do the measurement? 
 Could someone explain me step by pass, please? 
  
 thank you


----------



## jgazal

furlan said:


> Hello!
> 
> Some time ago I bought an SRM-727A amplifier and headphone SR-007 MKII.
> 
> ...


 
  
http://www.head-fi.org/a/adjusting-bias-on-stax-tube-amplifiers
http://gilmore.chem.northwestern.edu/dsc_1583mod.jpg
http://img829.imageshack.us/img829/2255/p1000579o.jpg


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## Furlan

Dear jgazal, 
  
 It was just what I needed. 
  
 Thank you very much for your help.
  
 Best regards


----------



## duncan1

Just a word to say I have added a stabilized  power supply using a mos-fet power transistor It is not an original design by me as there is a limit in the number of variations in solid-state design unless you make some specialized overload cut out. I used a constant current design which was pretty simple. Mos-fets are much more reliable in solid-state power supplies when used as pass transistors many normal transistors(BJT) need a lot of protection to stop them failing mos-fets dont and are used in millions of switch-mode power supplies World-wide. The result is a much reduced noise floor and power supply injected distortion products (cleaner -clearer sound ) as opposed to  simple power supply provided in a 717/727. I see many people upgrading the power supply capacitors while this is a positive thing it entails a good bit of time and work. While not a first glance looking any easier actually fitting a stabilized power supply is a good bit quicker and if you can read a circuit diagram no harder. It is easy enough to cut the pcb copper that runs to the two output fuses and solder flying leads to the power supply . It can be even simpler just fitting a pass mosfet and little more. I cut a section of the casing away next to the fuses  that allowed me to fit a small heatsink and mount the components directly onto the two mosfets. This kept the distances between components very short improving stability with no oscillation but you could make pcb to suite or even a strip board although that could introduce small amounts of capacitance. Active components must be at least 500 V working. The perceived fidelity when compared with just changing the caps is large . This is a mod you will hear right away making a large difference and will reward those with Omega 2 earspeakers with a large increase in detail due to decreased noise/distortion on the supply lines. Its no use having an amp with 0.001 thd when your power supply is outing large amounts of noise unless your amp has power supply rejection of the same amount.


----------



## Interben

Hi guys,
 I am entering the world of Electrostatics and am looking at the 717 for my solution.....
 I have always loved playing with electronics and am confident to solder, replace resistors, caps etc but I really have no idea regarding how to test and measure circuits.
 My question is can the 717 be easily converted from 110 to 240V?
 Would it require a new transformer?
 I note the caps are 400V so does that mean the circuitry would be fine with the change?
 I would also like to thank you all for this thread...
 As I said before I love to tinker and could see myself trying to perform the mods as described in this thread in the future .......
 Thanks


----------



## jgazal

interben said:


> My question is can the 717 be easily converted from 110 to 240V?
> 
> Easily is a relative term. You can try the following posts:
> http://www.head-fi.org/t/223263/the-stax-thread-new/23625#post_9627206
> ...


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## Interben

Awesome... That was exactly the info I need....  Also thanks for the Heads up with the capacitors .  When I was young I had a friends dad who would charge them up and throw them at us to catch... We obviously weren't that smart because we kept catching them 
 Thanks again jgazal....


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## duncan1

I had forgotten to add that the feedback resistors total 300K OHMS in the unmodded version . If you check the circuit diagram you will see that it is a     version of a widely modded standard circuit. All the other versions show the series resistance as 200 K OHMS from the output to the input I had already reduced the series resistance to 200K OHMS using a parallel resistor across one of the 150KOHMS feedback resistors in the original circuit that means that 2 resistors are paralleled in each board. With the extra feedback it does make a positive difference I still stick by the fact that if you want a major positive difference to the 727 /11  then make the power supply stabilized or at least use a pass transistor or mosfet.Believe it or not its not hard to do a even a simple design  makes a world of a difference blocking a lot of  mains harmonics and noise and distortion. Many easy to follow designs are on the web.


----------



## duncan1

I noticed some people have trouble adjusting the balance and near zero voltages  I do not think that the ones fitted are up to the job considering the price of the total unit. To make life easier and up grade them I fitted Multi-turn presets .These allow a much finer adjustment of the voltages . They are not the large volume control types as found in high end test equipment but much smaller and narrower 1 inch and a quarter long - 3 eighths height-3 sixteenth broad made out of high impact plastic by =Paintonbourns England they are called Trimpot.I havent checked to see if the firm is still in business but I am sure there must be many US manufacturers who can supply them or equivalent ones .These are industrial types -do NOT buy ones made for the general public. There is a small adjusting screw at the end of each.


----------



## duncan1

There are many who like extreme detail both with the amp and the headphones but that does not apply to some who like a smooth musical presentation. It might not appear to some that the 717/727 boards are not really much different ,the main thing being the twin output devices on the newer board as compared to the original but the circuit of the newer board still has its original circuits still in place (except for the output circuit) . The openness and highly detailed output from the mods above might not suite some especially if they are using the latest top end Stax(too much detail) .A way round this is to re install the feedback to the input circuit please note this is not the mods as above  but also re-instating the small value capacitors which existed in the 717 which were fed back from the board output devices to the circuit feeding the input .These were series-ed using two 250 Volt caps (total 500 V working of 50 PF value) If you want the best reproduction then they must be replaced by 500 Volt Polystyrene caps with a value of 25 PF of VERY high quality , no they arent cheap but if you want the best you pay for it . If you cannot get them then look for polypropylene caps of the same high quality ,if you cannot get them the Silver Mica ,again of very high quality would do to give you an idea of the new reproduction. The result is a slight lowering of detail at the high end and a slightly more powerful bass and remember feedback tends to even out the frequency response                 some call it nested feedback. This gives a smoother quality of reproduction which can be  reproduced at a very high level without any perceived distortion in other words a very Musical reproduction  that will be welcome by some who arent convinced extreme detail =musicality. There is no problem with your foot tapping up and down with the music. I added this to the other mods  but it could be tried as stand alone ,its only 4 caps to be soldered in so it is not brain surgery but again I emphasis they MUST be 500 Volts working and of very high quality as they are stopping 375 or so volts from being fed back. Dont try and test this unless you have a digital/analogue test meter of of 100 MEG OHMS INPUT IMPEDANCE or more your 10 meg digital meter will trip the relay.


----------



## fuzzzy

Inappropriate comment removed.


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## duncan1

Hello fuzzzy if you have had many years of electronic service then you will know about primary feedback and secondary feedback-aka nested feedback . Everything I have stated I carried out myself on the 717 and 727. Electronics isnt some black magic anybody can do it if they read up on it and that is my whole point the inside of a power amp/pre-amp shouldnt be beyond the capabilities of the ordinary person .If they read books go to college/night school etc they soon learn that it is really easy to do.I dont patronise people I credit them with intelligence .It takes a lot of courage for somebody who paid $1000s for their  equipment to mod it themselves but if they do and dont electrocute themselves they can raise the fidelity of that product much cheaper than paying for it . As the opening warning on Head-Fi says this site isnt for semi or commercial profit but for earphone users to experience better Hi-Fi/Musicality by doing it themselves not paying another to do it.I would never patronise somebody I judge them equally as that is how life has taught me.


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## fuzzzy

You are right as all you say is good. No intend to patronize you at all, just a bad day at work.
 Apology for my comment on your thread, I shall remove it.
  
 (deleted a bunch of technical crap, never mind).

 Just one more general comment to this post:
  
  
 Not far above you conclude:
  
 "The openness and highly detailed output from the mods above might not suite some especially if they are using the latest top end Stax(too much detail) ."
  
 Really?  There is such a thing as too much detail?
 But else where you discuss these mods are to improve detail and clarity. This is contradictory.
  
  
 "A way round this is to re install the feedback to the input circuit please note this is not the mods as above  but also re-instating the small value capacitors which existed in the 717 ..."
  
 Now continuing you suggest that for good sound to partly restore the amp back to its original configuration add backward mods towards the 717 and for "best reproduction" mod the amp again and buy expensive high quality parts, etc.

 This does not make sense.


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## duncan1

I realise most people like the latest Stax but not everybody ,it was aimed at them. But ,yes you can have too much detail as it gets in the way of the music ,as you are listening to the detail not the music .I speak from a UK perspective as most of the "golden ears " over here come down pretty strong on it and its probable that US users like a different balance accenting the higher frequency range. Each part of a high -fi system adds or removes highs or lows to a small degree depending on the philosophy of the designer  and his choice of components. I myself have a very open system so my Stax MK1 -007,s are the correct choice to make sure the frequency response appears flat to my ears. I work on the teachings of  John Lindsay Hood who was an open system  advocator with his insistence of the different sound qualities of capacitors long before  it was applied to hi-fi.So anything I build is DC connected with top quality components and all feedback using polystyrene caps with as little value is practically possible without causing insipid oscillation. Without the latest feedback as above the 727 is very open especially if you use a stablised power supply using a  pass mosfet. which lowers power supply noise/ripple/distortion which I fitted in mine ,and its a bigger improvement than changing the caps  and for that matter easier to do . What does happen is that it is so open minute aberrations not heard because of PS noise can then be heard and you have to go to a deeper level of investigation. Adding those caps flattens out even more the frequency response as well as sounding more integrated and musical . But I know in life what one persons idea of music isnt necessarily another's that why its an option.


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## duncan1

Its possible that I have not put a concise rely to Fuzzzy . When I say the latest feed-back I am referring to my own modifications to as detailed in previous posts above .The Stax corps had a "brainwave " to remove most if not all feedback from its 727 to make it more open in practice this wasnt the total success that it hoped . It was instigated when the UK hi-industry went through a -"no feed-back is good" policy  I have re-instated with mods the original 727 layout .This applies to the 2 channel cards that have DOUBLE -ie- 4 output BJT,s on heatsinks not the 717 with the single pair with no heat sinks (I of coarse added heatsinks to the original 717 as they produced some heat  in bjt,s (bipolar junction transistors ) any bjt when heated produces noise so adding heat sinks lowered the noise floor allowing more detail to be heard . So although I was re-instating the caps as per the original feedback in the 717 it was applied to my mods in the 727 so I was not going back to the original sound only modifying the boards I had already modified. This was a try and see if you like it mod which could easily be removed by unsoldering the four caps without disturbing the rest of the board . I would also add for those still confused that the original 717 had 2 feed-backs 1 overall and 2 a secondary feed-back . The difference in the 717 board and the 727 board is that the 727 has TWIN DOUBLE output bjt,s unlike the 717 with only one pair otherwise the PCB copper tracking is the same. If you read my previous posts you will find I changed the caps added a stablised mosfet power supply to isolate power supply noise as well as the feedback mods. If you can do nothing else to your 717/727 just do the stablised power supply ,it really does make a big difference and its easier than pulling apart the chassis to replace the smoothing caps just fit it BEFORE the fuses not after .


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## okw3188

Hi Ducan,
  
 I would like to confirm with you on your final value of the feedback resistor. For soldering of the resistor onto the existing PCB, do I need to de-solder the top smd chip resistor?
  
  
 Regards
 Tachi.


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## duncan1

Hello okw3188 Which Perth do you live in ?  Its been a long time since I  posted much on this but are we talking here of the 2 vertical smd  resistors in series  in the feedback in the Stax 727 ? if so I left them as they are but I have now re-instated overall feedback from where the 717  originally had it rather than some local feedback as advertised as -"low feedback                   " to give a "better " sound . .


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## okw3188

Hi Duncan, I'm in Perth, Western Australia and yes, I am referring to the 2 vertical smd resistor in series.


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## duncan1

Hello okw3188 the reason I asked about Perth is that I live 50 miles from Perth (Scotland ) I think many Scots settled in your area as ( I am told ) it is like Scotland in certain aspects or maybe just because they liked the area. Anyway I removed my 727 left-hand board to look at it again as it is a while since I worked on it and your right I did change the value of it .Instead of the 150k ohm resistor I replaced it with a  75 k resistor ,now if you have not got one then parallel the top resistor with another lower value one till it equals =75K .This is done to BOTH vertical series feedbacks which you will see spaced apart by four smd resistors . Each resistor must be 250v WORKING as that is the reason for the two smd resistors in series  which total 500v WORKING . As you might think you can replace BOTH smd resistors with even higher quality metal -500v working resistor across both soldering tags on the pcb  the value then being =225k ohm TOTAL .Hope this helps Duncan


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## Audioholic123

duncan1 said:


> Perth is that I live 50 miles from Perth (Scotland )


 
 A fellow scot on Head Fi
	

	
	
		
		

		
			





 what are the odd's of finding another scot on here! I'm from Lanarkshire... Just incase anyone who doesn't know where Scotland is, is wondering...It's scorching  sunshine all year round and never rains


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## duncan1

Hello Audioholic123 -Yes I was born in Glasgow went to live in Clydebank (as a baby )  came back to Glasgow and now live in a wee fishing village in the East-Neuk of Fife at the moment there is fog and sea mist rolling into the village just like James Herbert,s =The Fog. .Scorching sunshine all year round ? wow !


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## okw3188

Hi Duncan,
  
 thanks for the advise.


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## okw3188

Hi Duncan, one of the reason I like Perth (WA) is that we have winter in June and summer in Christmas time


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## duncan1

That would take me a bit of getting used to okw3188 but I like Australia for the quality of life there.


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## okw3188

duncan1 said:


> Hello okw3188 the reason I asked about Perth is that I live 50 miles from Perth (Scotland ) I think many Scots settled in your area as ( I am told ) it is like Scotland in certain aspects or maybe just because they liked the area. Anyway I removed my 727 left-hand board to look at it again as it is a while since I worked on it and your right I did change the value of it .Instead of the 150k ohm resistor I replaced it with a  75 k resistor ,now if you have not got one then parallel the top resistor with another lower value one till it equals =75K .This is done to BOTH vertical series feedbacks which you will see spaced apart by four smd resistors . Each resistor must be 250v WORKING as that is the reason for the two smd resistors in series  which total 500v WORKING . As you might think you can replace BOTH smd resistors with even higher quality metal -500v working resistor across both soldering tags on the pcb  the value then being =225k ohm TOTAL .Hope this helps Duncan


 
 Hi Ducan,
  
 After reading your advise, could I confirm with you if I want to replace the 2 x SMD resistor with better quality ones:
  
 Desolder the 2 x SMD resistor and connect a total of 225K ohm of resistor (in series and parallel) from the top solder pad of the 2 SMD series resistor to the connect card (gold finger) directly.
  
  
 Cheers.


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## duncan1

You,ve got it  okw3188 -third gold finger/spade in from each side ,same applies to the 717 . If you want the highest quality  remove the two SMD resistors and replace with a single 225K or high quality combination that totals it BUT they must be 500 V working this is very important if you can get a higher voltage - good . The only reason you have two SMD  resistors in series is that they are 250 V working  to a manufacturer production methods save money and fitting all the same types makes live easier as well as "looking good " I am more concerned with audio reproduction . There is no more vital part of any high quality audio amp than the negative feedback the sound quality of the components fitted can make or brake even a high grade amp this applies to any capacitors as well connected in any way to the negative feedback dont skimp .


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## okw3188

Thank you,Duncan


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## okw3188

Modification done, didn't know SR507 can sound so different, for the better


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## duncan1

Glad you are happy okw3188 I spent a year or more making modifications to see if they made a positive improvement to the sound quality rather than just a change of tonal quality. I believe the policy of Stax in removing the negative feedback in the 727  was in relation to a fad in the UK of the less negative feedback the more "realistic"  the sound was . While this can be applied to single-ended class A audio output circuits it didnt always work in other cases.Re-instating overall feedback  ie- from input to output in the 727 was for me the correct thing to do.. My mosfet pass transistor power supply I fitted after the fuses is still working fine no blowout from transients switch on peaks  etc I really didnt even need the finned heat sink I fitted as the mosfet doesnt get hot even though I picked a high speed one used in SMPS,s . By fitting it it removed the need to keep changing the supply capacitors as years go by as their influence is greatly reduced and the output has a much reduced mains/ earth noise interference floor level when tested on my scope . A good Stax move in the 727 was fitting heatsinks to the power transistors there were none on the 717 I had to fit my own as they got hot and heat is what you dont want in a solid- state active component when used in an audio circuit as it increases noise..


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## okw3188

Hi Duncan, 
  
 It would be greatly appreciated if you could email me a rough sketch circuit diagram of the MOSFET power supply modification, I will PM you my email address. Thanks a million


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## duncan1

Hello again  okw3188 hope you got what I sent as there are 1000,s of pass transistor diagrams designing a truly individual one would involve many components and go against the principles of keeping it easy to build every person has their own level of electronic building expertise so this is so simple most competent posters on Head-Fi could build it ,if you do build it  remember to test it first before fitting it . If you dont have the test equipment to try it out first then connect it but leave the output side of the fuses disconnected till you make sure the voltage is okay mines measures -350 V approx so you should have + 350V and -350 V  it doesnt matter if the voltage is a bit higher but not over 380 V . You also have to take into consideration your mains voltage . Another change I made was splitting the low voltage regulators as I found only 2 were feeding Both sides ,this induces overhearing via the power supply I split the PCB copper runs to the sockets of the channel boards and connected separate voltage regulators but this is not straight forward  and requires electronic knowledge of active components like the 7815 and 7915 voltage regulators .


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## okw3188

Thanks Duncan for the email, I will study it and communicate with you via email, if I have further query.


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