# Dirt Cheap Stax Amp DIY - new schematic updated!



## wdiabc

This is the final circuit I have, see page 5 for details:
   

   
   
   
   
   
   
  Update 2: Somehow I managed to kill two MPSA42 transistors in the circuit. One is for soft start one is burnt when running at 300V and bearing 450mw for a to92 seems to be too much. Now I replaced all four transistors with TIP500, which can stand 400V, and they dissipates about 1w without heat sink, so I will be good for a while. 
   
   
   
  Updated again with 5 cell lipo supply, on man, what a difference. My old lambda is singing with 600v peak to peak voltage swing! 
   

   
   
   
  Showing how this little happily drives the ECR500 now. Scope is showing 50v/div, so it looks like 250v peak to peak playing music in my normal listening level, will distort at full output. However its the mighty inefficient ecr500, for stax normal bias version lambda, it singg without any distortion.

   
   
  PSU

   
   
  Schematic, note Vcc can be changed from 150v up to 300v, but current is always 6ma per channel, so loading resistor needs to be adjusted accordingly. 
  Input can be balanced or not, does not matter. At the output, it will always be balanced. 
  One concern I have is, some people like to ground all their devices together, cause ground loop noise or whatever people may call it. In this circuit, the input is floating. 
  needs 2v peak to peak to reach full output swing. 
  Re=330 ohm. 

   
   
  Testing with one channel: 

   
  Hi all,
   
  [updated with edits: I raised the power supply to +300V by doing a voltage multiplier from switch power supply and feed it with 20 volts, output swing is now 600V peak to peak, with idle power consumption near 4 watts, man what a difference! ]
   
  Recently I have been trying to work on a very cheap stax amp that aims to drive electrostatic headphones. Thanks to the help of the headfiers here, especially to Frank, who offered to send me the 6pin female connectors for free.
   
  So far, this thing costs nothing compared to other fancy builds:
  all the parts needed for this amp, excluding enclosure is around $20. So cheap that I feel many people won't even bother do look at the measurements. However the results are really good and I really like the sound of this cheap amp. 
   
  Technical Details:
  Power supply: battery or 12V supply, so it can be powered by 3 cell lipo or 12v wall adaptor. ( now I am good with 19V )
  Output: 600v peak to peak.
  Idle Power: 300V *12ma=3.6 watts, that translates to 6 ma per channel, and 3ma per leg of each differential pair.( surprisingly to92 package with 600mw heat dissipation is more than enough, so I am not using any head sinks at all). 
  Input impedance: 2k 
  FR: DC to 15Khz db, rolls off to -3db at 20Khz
   
  Please offer some thoughts and comments. I need some help building the enclosure, will really appreciate if anyone can provide some resources. this thing can be made quite small so mental project enclosure is currently my top choice, just want to know how you guys drill holds for these enclosures? what tools do I need?


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## wdiabc

quick discussion:
   
  Power Supply: I want to design the amp so imperfect power supplies can be used. We have been trying to deal with imperfect power supplies for a long time, and ideal power supplies do not exist. Some class A amps uses very large cap banks( 20,000 uf, which are very costly and honestly, may be an overkill), so I designed the circuit to be class A with differential pairs, so the power supply will only be supplying a constant DC current, filtered by a ccs. 
   
  Output: Personally I did some measurements, I feel an output of 300 V peak to peak seems to drive the old STAX lambda well. However, for those new 580v bias staxs, 300v pp may not be enough. But this circuit can be modified to take 400V power supply and has the potential to swing 800v pp, which to me is more than enough to drive these staxs. 
   
  Input power is only 3-4 watts, since my current setup is 150V DC loaded at 12 ma for two channels. ( 3ma for each leg of the differential pair). That is only 1.8W total power consumption, and if we use batteries, only 150ma is required, so a 2100mah li-po battery can go for more than 10 hours. I also play with a lot of RC car stuff, so the charger and battery wont be an issue for me, but those who don't have lipo chargers and batteries, these can cost $100( $20 for the battery, $80 for a decent charger) But 12v wall adaptor can be obtained on ebay for only $5 ( these HK sellers) 
   
  A quick note here: eXStatA only uses 3ma each leg, if you look at the circuit. So I feel 3ma is more than enough to drive the headphone. 
   
  Input Impedance: currently around 1K or 2K I guess, didnt really measure it, but now most DAC and preamps can do very well with 600 ohm load. I try to make the input impedance small so the interconnect cables' inductance and capacitance won't impact the sound that much. ( Trying to avoid those insanely expensive cables) 
   
  FR is a little tricky to measure, I recently got a tek 222 scope so it will be very helpful to take some measurements.


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## kevin gilmore

definitely interested in seeing the schematics.
   
  I have seen decent quality lipo chargers for far less I have the
  link at home somewhere. The RC batteries are definitely the
  way to go, really good power density.
   
  Assuming you are using a high frequency switcher to make
  the high voltages, even smaller caps rated for voltage are
  more than sufficient. Small series inductors help.
   
  Low input impedance is not an issue these days.


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## wdiabc

A quick discussion about the noises and so called 'dynamic ranges' and THD:
   
  Here is a measurement of fiio E10, which is a cheap DAC.
   

 Fiio E10: Sine Wave Perfect 20-20K, rolls of a little beyond 18K   Square Small Ringing, but well controlled   Clip Clips clean, but overshoot with inductive load   Noise 1mv(5mv little spikes) noise floor at lineout, at 200mv max output level, lineout is very clean 
  In comparison, I have measured a popular Class D amp, which I use to drive the energizer to power up ECR500 and stax lambda: 
   

 Class D Amp: Sine Wave Class D, made of small steps when loaded with 8 ohm, 20-18K flat 2V PP, 18K-20K 2.5V pp output   Square at 1Khz, at almost full output, looks OK, but low output has 100mv HF noise and Rings badly, at 100HZ, distorts like an OTL amp   Clip Clips clean, but overshoot with inductive load   Noise: 50mv noise floor, constant ****ty, at 800K-1000K, somewhat sine wave
   
  So a quick conclusion:
  A fiio E10 is considered to have clean sound: it has about 200 mv peak to peak line out, with a noise floor of 1mv. so output is around 200 times to the noise floor. I am bad with math so I will just use this new ratio I come up with: ( noise floor / output peak to peak)  and E10 is at 1/200
   
  For the Class D amp, noise floor is at 100mv peak to peak, vs its output of 24V peak to peak. so the ratio here is 1/240, very interestingly, in this ratio, this nasty distorted class D amp wins over fiio E10.
   
  For this little I designed, I measured the noise floor at around 200mv peak to peak, with output of 300 peak to peak, then this ratio is 1/1500, it is a winner, hands down. 
   
   
   
  Appendix
  Class D amp output wave form: made of small steps, becuase it is basically a high frequency switch, like an imperfect function generator. I don't have the square output shot for 100hz square wave, but it looks nasty: 
   

   
   
  Class D noise floor at + - 50mv. 

   
   
   
   
  DIY AMP: Output, 1Khz, square wave, notice how this DIY amp perfectly captured the imperfect ringing coming from fiio e10, and these little ringings are at 30khz so this amp is very well extended into high frequencies. 
   

   
   
  Input from fiio e10, please notice the little ringing at 1khz:


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## wdiabc

Thanks for the reply. I try to keep the input impedance low because super high impedance like 1M will cause the cable to have a significant impact on the sound.


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## nikongod

Cool to see new projects! 
   
  Low input impedance is usually used to avoid noise. Considering that so many newer sources are able to drive it, its a shame more DIYers don't take advantage of this. 
   
  Cables are usually very low DCR and fairly low capacitance. Even cheap ones from the dollar store are very low DCR. 
   
  When measuring the switching/class-D amplifier did you have a load attached?


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## kevin gilmore

Where did you acquire the high voltage power supply board?
  I want to acquire some. Then I will modify to make +/-300.
   
  With mpsa92 you can go to 300v.
   
  Some people have a problem with lifting the dc voltage of the stators.
  A significant shock to ground from one of the output voltages can be
  present. not a good thing. You can always add output capacitors.
   
  Lifting the voltage this way requires an even higher bias voltage.
  So for 150 vdc, you need 730 volts for the bias.
   
  So a voltage trippler of the top of the secondary should work.


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## wdiabc

nikongod said:


> Cool to see new projects!
> 
> Low input impedance is usually used to avoid noise. Considering that so many newer sources are able to drive it, its a shame more DIYers don't take advantage of this.
> 
> ...



Yes it's loaded with te ecr500 transformer box, which is 8ohm. But it is somewhat resistive plus inductive load.


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## wdiabc

kevin gilmore said:


> Where did you acquire the high voltage power supply board?
> I want to acquire some. Then I will modify to make +/-300.
> 
> With mpsa92 you can go to 300v.
> ...



I got it on eBay. The 12v to 110v inverter, this cheap one spits RF out badly. And with the voltage multiplier its 270v no load and with 6 ma load it went down to 240v. I wish I could go to 400v with a few tip50 at hand. However I am also trying to get some cheapo solution for the HV supply. I need 400v at 12ma output, merely 5w though, however such things are hard to find nowadays. 

PM me if you need the link to the eBay item.


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## wdiabc

kevin gilmore said:


> Where did you acquire the high voltage power supply board?
> I want to acquire some. Then I will modify to make +/-300.
> 
> With mpsa92 you can go to 300v.
> ...




Bias is not a problem. I have the 230v bias version. And it can be properly driven even if I only feed it with - 120v bias( by connecting the bias directly to the ground) 

Or I can simply use another voltage multiplier with a 5M resistor to restrict the current, like the bias circuit in stax srd transformer boxes.


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## kevin gilmore

That's a 50 or 60 hz switcher. Not what I am looking for.
Besides which it makes nasty square waves.

Total efficiency going to be under 50%

Still you have some interesting ideas


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## wdiabc

kevin gilmore said:


> That's a 50 or 60 hz switcher. Not what I am looking for.
> Besides which it makes nasty square waves.
> 
> Total efficiency going to be under 50%
> ...



Surprisingly, this is a cheap inverter from HK and its a 50khz fast switching circuit. If you take a look at this circuit, 50hz switcher will need a huge transformer. The efficiency of this circuit maybe around 80-90%. The to220 transistor does not get hot at all.


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## wakibaki

Cool.
   
  I always like to see 'dirt cheap' in a thread title.
   
  w


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## jezz

Nice design; it reminds me of the Son of Zen. If you haven't already, you could improve distortion a bit by matching the output transistors for hFE.


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## wdiabc

Quote: 





jezz said:


> Nice design; it reminds me of the Son of Zen. If you haven't already, you could improve distortion a bit by matching the output transistors for hFE.


 
  Thank you. Yes its more of a fully balanced Zen. No matching is required if you look close of the design of this amp. I just need to carefully adjust the main load resistor so the voltage drop is always half of Vcc, which will ensure maximum output level


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## jezz

"Son of Zen" is a different amp than Zen (article here). Pass calls it "balanced single-ended."


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## wdiabc

Quote: 





jezz said:


> "Son of Zen" is a different amp than Zen (article here). Pass calls it "balanced single-ended."


 
  Thanks for the article. I guess they are almost identical. Then this is a BJT version for Son of Zen.


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## wdiabc

Updates: 
   
  So far I start to realize that this amp rolls off at the high end, near 15-20Khz, because the stax headphone impedance drop to around 50K to 100K near that frequency and given the huge output impedance of this circuit ( near 50K), certainly the 20Khz end will roll off, which is OK to me and that really made Stax Lambda sound like my ortho, haha. 
   
  I will try to play with idle current but does any one know what idle current stax is using for some of their high end amps? I am referring to T1 T2 323 etc. I have looked at some other amp designs, and many of the tube versions simply used a 50K load resistor for the output stage, which is identical to my design, so I feel the output impedance is really the same, I dont know how these amps deal with the high frequency roll off. 
   
  Please help.


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## kevin gilmore

sr-007 and sr-009 are j66k at 20khz
   
  srm727-2 is 8.5 ma per output driver, and is 3db down at 100khz with a standard 120pf load
  all the other solid state amps are similar. Remember that there is up to 20db of feedback to
  keep the frequency response flat.
   
  the tube designs are 50k plate resistors, and 3db down at about 50khz. Also 20db of feedback.
   
  The T2 is 20ma of bias per output tube and is 3db down at about 230khz.
  (lots of other tricks going on here)


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## wdiabc

kevin gilmore said:


> sr-007 and sr-009 are j66k at 20khz
> 
> srm727-2 is 8.5 ma per output driver, and is 3db down at 100khz with a standard 120pf load
> all the other solid state amps are similar. Remember that there is up to 20db of feedback to
> ...




Thank you so much for the info. Now I am really having a hard time if i want to boost up the idle current or not. I will try another layout to handle this roll off issue


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## wdiabc

updates, using this new layout can fix the high freq roll off, sorry guys I used negative feedback, but it helps to make the FR flat: 
   
  It is a little trick that discovered, this transistor is 'tube like', because its now biasing itself, and also the changes in the 'plate voltage' really cause the transistor to change its Ic, which means now this transistor is like more like a resistor. So it does two things, 1) achieved negative feedback for a better frequency response and 2) lowered its dynamic resistance, and increased output.
   
  I always try to avoid capacitor when applying feedback, my oscilloscope told me these inductors and capacitors really makes the oscillation happen. 
   
  btw, the two small Re sets the gain of the stage, and the variable resistor sets the idle current. Some math here: 
  V of LED: 1.8v - 0.6V( Vbe) = Idle Current * Re+2*Idle Current *Rvar
   
  For example:
  1.8v-0.6v = 3ma * 200ohm + 6ma * 100ohm, so we need the var resistor to be 100 ohm for 3ma idle current. of course this number is definitely off, but not by much, since we have to consider the Ibe


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## wdiabc

now I am happy, time to ditch these testing resistors, I am going to replace them with carbon Comp resistors, my all time favorite!


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## wdiabc

Quote: 





wdiabc said:


> Surprisingly, this is a cheap inverter from HK and its a 50khz fast switching circuit. If you take a look at this circuit, 50hz switcher will need a huge transformer. The efficiency of this circuit maybe around 80-90%. The to220 transistor does not get hot at all.


 
  with extensive tests, this little power unit is capable to handle 19volts DC input, with an output of 320V continuously at 12ma. However, it will need a soft start circuit, in order to allow this thing to start oscillation, otherwise with the heavy load it won't start to oscillate and the power supply voltage will not go up. I simply used a transistor for that circuit, I broke up another mpsa42 during experiment and it died because of this imperfect 'soft start circuit', so i am not posting a misleading schematic for it.


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## wdiabc

Update:
   
  I killed one channel of the fiio e10 during testing....Now I need to fix it. But I can still use the lineout, I need to be more careful next time. 
  I will soon post a new circuit which allows this amp to be used by unbalanced inputs.


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## jcx

Quote: 





kevin gilmore said:


> ...The T2 is 20ma of bias per output tube and is 3db down at about 230khz.
> (lots of other tricks going on here)


 
  has this changed? - schematic claimed 10 mA ccs


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## kevin gilmore

original T2 was 20ma. Which is why it ran at stupidly high temperatures.
   
  Diy T2 runs at less current. And still gets very hot.


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## spritzer

"Stax SRM-T2, fry some eggs while you listen"...  
	

	
	
		
		

		
			




   
  I've been working on possible ultra simple Stax amps for a few months now but but I'm so frustrated by the disappearance of high voltage transistors that I'm going with all tubes for this one.  That has problems as well but I'm fairly sure ECC83's will remain in production for decades to come.


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## wdiabc

Ecc83 can be another great option. Maybe you can consider a cascaded hybrid circuit. Simply cascade two tubes on top of the two transistors then you will have enough gain. 





spritzer said:


> "Stax SRM-T2, fry some eggs while you listen"...
> 
> I've been working on possible ultra simple Stax amps for a few months now but but I'm so frustrated by the disappearance of high voltage transistors that I'm going with all tubes for this one.  That has problems as well but I'm fairly sure ECC83's will remain in production for decades to come.


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## spritzer

That is possible but one has to thing about the filament-cathode voltage limit in that regard.  Gain is not really the problem with these amps though as most are run without feedback.  Add feedback and it becomes an issue.


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## wdiabc

Quote: 





spritzer said:


> That is possible but one has to thing about the filament-cathode voltage limit in that regard.  Gain is not really the problem with these amps though as most are run without feedback.  Add feedback and it becomes an issue.


 
  The design with tubes ususally comes with no gloabl feedback, and with a 50K plate reisistor, I was wonderng how does the circuit deal with high frequency roll offs? ( plate resistor is big and can signiciantly cause a roll off in output when working at 20Khz, since the stax headphone has about 50K resistence at 20Khz)


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## kevin gilmore

All of the stax tube amps do have global feedback. So the high frequency roll off
  is compensated for.
   
  Better to use constant current sources for the plate loads. Then you end up
  with ixys parts and then you decide you need bigger tubes, and then you
  decide to reference the outputs to ground, and sooner or later it looks like
  a T2.


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## spritzer

Yes, even the very old Stax amps had feedback.  Only the super simple "A" design they released for DIY use in 1968 was without feedback.  I'm listening to that design now modified with a differential front end and it's ok for something so simple.  Works off a single filament supply too which makes sourcing a transformer that much easier. 
   
  The plate resistors are compromise but what else can we do on a budget?  We could use tubes as current sources but then we enter filament hell.
   
  One thing I'm going to try soon is run small encapsulated transformers backwards off 24VAC to generate high voltages on the cheap.  18V/230V would give us plenty of voltage to play with and regulating it would be easy.


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## jcx

are we going to keep hearing this FUD about properly selected, properly applied MOSFET right up to the moment you guys finally "discover" how to use them right?
   
  just read the datasheet - you do have the one with the full plots?  http://ixapps.ixys.com/DataSheet/DS98809C(IXTP-U-Y01N100D).pdf
   
  if you can spare 25 V then 12 pF Coss, 2 pF Crss aren't crippling with ~100 pF headphone load
   
  if you draw up the full bridge, headphone load and device parasitics then only ~ 15 % of the current is wasted in the MOSFET parasitic C, 1/2 that if the IXTP01N100 MOSFET are only used for CCS


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## kevin gilmore

Already using those parts in the kgsshv.
   
  problem is that they have a dramatic drift with temperature
  so you need a good servo to deal with that.
   
  10m90s much better, but will not go as low in current.
   
  Its not the output capacitance that is the issue with some
  of the mosfets, but rather the input capacitance which
  can be as high as 700pf. Makes them hard to drive at
  20khz.


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## jcx

servos are cheap - just add to parts count
   
  but you can also cascode another current regulating circuit with the depletion mode devices - looks like there is >2 V @10mA to work with with the IXTP01N100
   
  plenty for LM334, even with the diode TC comp hack
   
  or 2 BJT feedback CCS could reduce I bias drift from Vgs TC ~50x, but still would have BJT Vbe TC
   
  could use some 3-terminal regulator chips, even precision refs
   
   
   
  the specific part under discussion IXTP01N100, should handle enough current for ES headphones - has small die: Cgs ~ 100 pF not that much more than BJT, only has to be modulated ~ 0.5 V
   
  acutally less current required than EL34 triode connected with mu <20 where you need >50 Vgk across 15 pF


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## wdiabc

spritzer said:


> Yes, even the very old Stax amps had feedback.  Only the super simple "A" design they released for DIY use in 1968 was without feedback.  I'm listening to that design now modified with a differential front end and it's ok for something so simple.  Works off a single filament supply too which makes sourcing a transformer that much easier.
> 
> The plate resistors are compromise but what else can we do on a budget?  We could use tubes as current sources but then we enter filament hell.
> 
> One thing I'm going to try soon is run small encapsulated transformers backwards off 24VAC to generate high voltages on the cheap.  18V/230V would give us plenty of voltage to play with and regulating it would be easy.




Running little transformers backwards may lead to low efficiency. Since the primary winding has very high DC resistance.


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## wdiabc

jcx said:


> are we going to keep hearing this FUD about properly selected, properly applied MOSFET right up to the moment you guys finally "discover" how to use them right?
> 
> just read the datasheet - you do have the one with the full plots?  http://ixapps.ixys.com/DataSheet/DS98809C(IXTP-U-Y01N100D).pdf
> 
> ...



This MOSFET looks like a HV jfet. Which can be used as SRPP. Are they expensive?


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## wdiabc

I am trying to work on a transistor version SRPP, which is cheap( only using 8 BJT in total) and output impedance is really low for ES phones. 

Initial tests shows its hard to bias the upper transistor and its prone to temp/voltage drifts. If any of you worked on tube SRPP in the past, how do you adjust the output to be 1/2 of the power supply? Does tubes not drift at all? I only worked in SE tube amps and my knowledge about tubes are limited.


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## kevin gilmore

And this is the major problem.
   
  One solution is 2sa1968. which is impossible to get these days.
   
  Another solution is ixys10m90s which is designed to have sharp
  cutoff and thus stable current with temperature. But needs to
  run at 5ma minimum.
   
  latest kgsshv has a servo because the 01n100d as a current
  source in the 3rd stage has temperature drift. Not a cheap
  design. Lots of parts.
   
  With a ground referenced design and a servo, you can definitely
  control the output voltage to 1/2 of B+


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## spritzer

The thought of SRPP and 'stats just brings nasty flashbacks for me.  Kevin and I came up with the B-10 design which is a fixed version of the RSA A-10 but one could argue that it isn't really SRPP since the load isn't fixed.  I ran the tubes very hard and it's pretty stable but sounds utterly terrible. 
   
  Quote: 





wdiabc said:


> Running little transformers backwards may lead to low efficiency. Since the primary winding has very high DC resistance.


 
   
  Yeah, that is the main drawback but I'm going to the local store which has a huge selection of 230V units and pick the best ones.  Hell, I've used some of the same transformers for audio use (tiny Stax adapters) so this has to perform better...


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## jcx

the IXTP01N100D are currently ~$4 ea, need another few bucks of heatsink, hardware
   
  not "dirt cheap" if you need 8 for 2 channels, dual stator drive - but could still be less than cost of tubes, sockets, isolated heater supply windings
   
   
  I have protoed one stator driver with the IXYS, and "SRPP derived" modulated current source - but used op amp gain inside the global feedback loop so the DC servo was built in so to speak
   
   
  too lazy to handwire point-to-point any more than the "proof of concept" - need to brush off the Eagle and layout some test boards
   
  especially since I plan to totem pole/cascade 2x each Q for a ESP-950 +/-600 Vsupply amp which doubles the parts count, and all the cool op amps today are smt anyway


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## wdiabc

Quote: 





jcx said:


> the IXTP01N100D are currently ~$4 ea, need another few bucks of heatsink, hardware
> 
> not "dirt cheap" if you need 8 for 2 channels, dual stator drive - but could still be less than cost of tubes, sockets, isolated heater supply windings
> 
> ...


 
  Can you share any schematics based on this idea? It sounds very interesting.


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## wdiabc

what do you think of this ? That looks like a MOSFET SRPP with a global feedback, still can be done cheap with OP amps and MOSFETs, I feel IRF730/740 can be acquired pretty cheap. Just don't know how stable this circuit is.


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## jcx

Gilmore does have a point if you choose MOSFET that big with too large parasitic C - the IXYS part seems to be very specially suited to ES headphone amps - can be bought at DigiKey
   
  you really don't want multi-Amp rated parts for the ~ 10mA you may actually use
   
  the BSP125 in that schematic are more reasonably sized for the job - if a bit low V, low case pwr rating and smt so harder to proto with


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## kevin gilmore

the schematic posted above is that of the sennheisser hev60.
  Most people who have listened to that amp don't like it.
   
  Its only SRPP if the load impedance matches the cathode resistor.
  In this case the source of the top fet. Therefore its not SRPP but
  rather constant current.
   
  In fact, due to C20 its more like a white cathode follower done
  with solid state.


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## jcx

the Tube Cad articles on SRPP do mention padding the cathode with additional R, there is a continuum from ccs to "optimum push-pull" to "mu follower" and the articles describe SS versions as "SRPP derived"  - clearly there's a strong similarity in operating principles - but it does seem like "SRPP" label is pushing the wrong buttons for some of you
   
http://www.tubecad.com/2009/09/blog0171.htm
   
  with depletion MOSFET having high gm R padding is needed to get the "optimum" "push-pull" relation at a specified (low) current - looking at the circuit as a modulating current source is useful - "impedance multiplier in the blog
   
  as I pointed out above a possible use of the depletion mode MOSFET is to cascode another current regulator circuit - Walt Jung audioXpress ccs articles shows some options - should give circuit designers some ideas
   
  tapping the current sense resistor to get the push-pull current modulation "impedance multiplication" is applicable to several of the ccs circuits, the stability and TC of the set current, accuracy of push-pull matching can be improved, made nearly independent of the depletion cascode Vth
   
   
  while I haven't measured my test circuit that carefully - sims with a square law gm model extracted from real parts measurements show the modulating current source simply looks like the load Z has been reduced by 1/2 to beyond 1 MHz
   
   
  and I don't see the justification for such blunt dismissals of a output stage circuit on grounds of its "sound" - seems too simplistic
   
  if a circuit can drive the load reasonably linearly over the range of expected signal then the rest of the amplifier circuit can control it to varying degrees of accuracy via global and/or local feedback, "distortion compensation" potentially give a range of "sound" - or preferably just be "transparent"


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## wdiabc

special thanks to Frank. He sent me an STAX transformer amp ( SRD-7), still in excellent condition and I really want to thank him for his generosity. I will do some tests with it( loaded and unloaded and see how it performs, obviously I only have one TA2020 amp to drive it, not great power amp, but at least I can use the oscilloscope to compare the input vs. output.) I will post pictures before I tear it apart to build the dirt cheap amp in it.


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## wdiabc

Quote: 





kevin gilmore said:


> the schematic posted above is that of the sennheisser hev60.
> Most people who have listened to that amp don't like it.
> 
> Its only SRPP if the load impedance matches the cathode resistor.
> ...


 
  with R43 and R44, C20 and C20, if the output point is at the negative end of C20, then we have a CCS loaded circuit, not SRPP. But since the output is taken between R44 and R43, I still think this is a SRPP, but with R43 and R44, I guess the amplification is more linear? I dont really quite understand how the two resistors work together.


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## wdiabc

Quote: 





jcx said:


> the Tube Cad articles on SRPP do mention padding the cathode with additional R, there is a continuum from ccs to "optimum push-pull" to "mu follower" and the articles describe SS versions as "SRPP derived"  - clearly there's a strong similarity in operating principles - but it does seem like "SRPP" label is pushing the wrong buttons for some of you
> 
> http://www.tubecad.com/2009/09/blog0171.htm
> 
> ...


 
  I was thinking about this yesterday. and please help me out ( correct me if I am wrong)
   
  - Resistor load: most stable and sounds nice and linear, but with ES phones, high freq roll off is really some issue if no feedback is in place. 
   
  - CCS load: I feel this one is very tricky to get right without DC feedback. a very small change in Ic the lower transistor( or tube in the schematic below) will cause a significant change in the DC output. I dont know if tubes are very stable or not since I never played with CCS for one of my tube amp I built years ago, but many people are using CCS load for tubes and sees to get very linear frequency response, however if the lower tube is changed to a transistor, the output looks very different, in stead of being linear, the output can get clipped very easily( with voltage swing from 0v to V supply), because of the difference in the V-I graph between tube and transistor. So its very tricky to use CCS load without global feedback. and similar things may also apply to SRPP. I feel its very hard to get a flat response with SRPP I build with NPN transistors, even though it has very low output impedance, but the gain factor changes so much when input frequency changes ( say under the say 0.1V input, I get +-50V output at 100hz to 1000hz, but the output rolls off to +-10V at 15KHZ, under a proto NPN SRPP circuit I built)


----------



## wakibaki

What are the drive requirements here? I'm unlikely to ever own a pair of Stax phones, but so what...


----------



## kevin gilmore

at least 1600 volts peak to peak stator to stator.
   
  2000 volts is better.


----------



## jcx

stators are pretty much always driven balanced with 2 driver stages per channel
   
  the load seems to range from 70-120 pF by published ES headphone specs - as seen by each individual stator driver the load is effectively double if you calculate based on just that driver's output V since the other side is being driven in opposition
   
  cable is significant part of the Cload, and extensions are sold - could add to the max load design target
   
  the relation for current vs load is different than usual audio amps/transducers - at low frequency next to no current is needed so "big bass" only depends on Vswing
   
  relatively high current is needed to charge/discharge the Cload at higher audio frequency - while we never listen to 20 kHz sine at full amplitude that could be one goal for setting max current output
   
  piling together all worst case assumptions you may want 20 mA or so peak output current, practically anything over 5 mA should be quite listenable - most music has <5 kHz power bandwidth - even exotic moving coil phono cartridges won't track that high
   
  for digital source, close miced, it is possible to record faster signals but rare slew rate limiting distortion is even less audible than rare clipping - which really isn't as easy to detect as most would think - it really is crossover/low level distortion that is clearly audible - and all direct drive ES headphone amps are at least "deep A" Class AB biased and avoid many low level problems
   
  most project amps are Class A - simpler if power hungry - sliding bias or AB would be seen by most as unescessary compromise and circuit complication - but again the E9 energizer sold with the Koss ESP-950 is listenable, the electrostat quality shows thru despite the things the E9 amp "does wrong" by audiophile standards


----------



## wakibaki

Thanks jcx, my next question was going to be what kind of a load do the phones present...


----------



## jcx

driving the feedback R can be an annoyingly large fraction of the output current budget - but again doesn't impact low level linearity which is most obviously audible
   
  with ~ 100 dB SPL @ 100 Vrms sensitivity spec few hundred kOhm feedback R are drawing less than 1 mA @100 dB - but the feedback R could be soaking up 1/2 of the current at higher 500 Vpk (one side) and low bias 5 mA SE Class A


----------



## wdiabc

jcx said:


> driving the feedback R can be an annoyingly large fraction of the output current budget - but again doesn't impact low level linearity which is most obviously audible
> 
> with ~ 100 dB SPL @ 100 Vrms sensitivity spec few hundred kOhm feedback R are drawing less than 1 mA @100 dB - but the feedback R could be soaking up 1/2 of the current at higher 500 Vpk (one side) and low bias 5 mA SE Class A




So what about this: my schematic in the first stage and a ccs loaded emitter follower in the second stage, I can do 1ma at first stage and 5ma in second stage. That comes to 24ma in total, with + - 300v supply we are looking at 12w in total that will give 1200v pp output with a very low output impedance and no feedback resistor. Damn now I need a heatsink.


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## tyre




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## wdiabc

tyre said:


>



This one does not have feedback. With load resistor as high as 60k, a significant roll off will happen beyond 15khz


----------



## tyre

Quote: 





wdiabc said:


> This one does not have feedback. With load resistor as high as 60k, a significant roll off will happen beyond 15khz


 
   
  It's just a derivation of an amp from tubecad. The load resistor value could be adjusted.
   
  http://www.tubecad.com/november99/page9.html


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## spritzer

The SRM-T1/T1S/006T all use 66k for plate resistors with the 6CG7.  SRM-600 uses 60K and the 007t uses 47K.  The latter has been the "golden standard" by Stax since the 60's though.


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## wdiabc

Today I come up with this circuit, not really SRPP, not really a cathode follower. its a hybrid, dont know what to call it, but it is now my newer version of the dirt cheap amp. 
  please see the attached circuit.


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## wdiabc

Test done under 260V power supply. 

basically the 100K load resistor has about 1.3ma idle current in it. 
the lower transistor is taking about 4-5ma current( didnt bother to measure it, but its biased with one LED and 240 Re, so about 4-5ma).

Then I adjust the variable resistor of the upper transistor to make the output at 1/2 of Vcc. so the upper transistor is seeing about 2.7ma of current and yet it is a combination of CCS and emitter follower. 

I did some tests, absolutely flat FR from 20 - 20K with or without load

loaded with ECR500 ( with is about 100-120pf and i measured my pair of ECR500 is at 50Kohm at 18-20Khz), and with this circuit, the output is clean and did not roll off at 20k! Yet it's done without global feedback. 

Now I have a circuit that is dirt cheap, simple to build, gloabl-feedback free and yet a very good performance. I will soon put all the stuff together in a box.


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## wdiabc

test: 
  triangle wave, 1khz, +-100V output:
   

   
   
   
  6KHZ square wave, because i only have fiio e10 as the source and the wav is 44.1Khz, so only 7 points per cycle, very poor resolution there, and this output is actually very close to the input:

   
   
   
  saw wave at 1Khz, +-100V output:

   
   
   
  1khz square wave, exactly the same as the input:

   
  These are loaded output wave forms, so I feel very happy about the result I have so far, this little circuit is really something!


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## wdiabc

I did a quick draw out of the circuit I have, if any of you feel this can be a interesting project, you can do a quick PCB prototyping. I don't mind if any of you want to produce it or not, I already post the schematic so I am sharing it and giving it away for free. But if anyone designed a PCB, please send me a few pieces, thanks!
   
  Q1 Q2 Q3 Q4 I used TIP50, which is about $0.25 per piece.. so cheap
  and Q5 can be any small bjt, as long as it can work around 20V. 
   
  R3 R4 and R7 shall be trimpots, 500ohm
   
  R3 and R4 sets the output voltage, R7 sets the idle current.
   
  Output is taken from the upper side of R3 and R4


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## wakibaki

Hi tyre
   
  Had to change it a bit to get it to work...
   
   

   

   
  ...doesn't look too bad in simulation...
   
  This is 580V supply, 250V at the anode.
   
  Input is 1V pk-pk, thru a 5k-5k divider into 100k. Loaded with 1M || 120pF. Opamps are NE5532
   

   I'll have a look at the transistor one in a minute.

   
  w


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## tyre

Thanks for looking it over. Most of the part values were chosen somewhat arbitrarily. Any improvements that you think are worthwhile, feel free to add.
   
  On a side note, this schematic of the SRM-001 by kevin gilmore might be of interest.
   
  http://gilmore.chem.northwestern.edu/stxp.gif


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## kevin gilmore

12at7 cannot handle 600 volts. In fact it can handle less voltage
  than 6cg7. The only small tube that works is a 6s4.


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## wdiabc

tyre said:


> Thanks for looking it over. Most of the part values were chosen somewhat arbitrarily. Any improvements that you think are worthwhile, feel free to add.
> 
> On a side note, this schematic of the SRM-001 by kevin gilmore might be of interest.
> 
> http://gilmore.chem.northwestern.edu/stxp.gif




I am looking at the circuit as well, it's a great little circuit but it has op amps in it. My experience with op amps is that it requires another supply(12v) and also introduce high freq noises. I used to own sr001 and I really enjoyed its smooth sound, I guess the 130k resistor really helped to roll off the high end a little bit so I can't hear any harshness and its quite forging towards sources


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## wakibaki

Quote: 





kevin gilmore said:


> 12at7 cannot handle 600 volts. In fact it can handle less voltage
> than 6cg7. The only small tube that works is a 6s4.


 
   
  This is obviously a consideration and one which had not escaped me, but the quiescent plate voltage is 250V, the swing is 150V, and a 6SN7, which is what is shown in the tubecad circuit, is good for 450V. and is shown run at a B+ of 700V with its plate at 300V. Who am I to argue with Broskie, although it could be a typo.
   
  Anyway I didn't seek to optimize the circuit, only show that it had some potential, and shouldn't be dismissed out of hand as lacking in HF response.
   
  @wdiabc
   
  Can you please check your schematic, I don't see how it is going to work as drawn.
   
  w


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## kevin gilmore

A 12AT7 has a rating of a maximum of 300 volts on the plate. Try it and you will see.
  A 6SN7 has a rating of a maximum of 450 volts on the plate, with peak excursions exceeding 1000v.
   
  6CA7 and 6SN7 are the best cheap tubes for this job.


----------



## wakibaki

Quote: 





kevin gilmore said:


> A 12AT7 has a rating of a maximum of 300 volts on the plate. Try it and you will see.


 
   
  Why would that be necessary? As I pointed out, _not a consideration which had escaped me_. Please read the posts before presuming to condescend.
   
  w


----------



## wdiabc

wakibaki said:


> This is obviously a consideration and one which had not escaped me, but the quiescent plate voltage is 250V, the swing is 150V, and a 6SN7, which is what is shown in the tubecad circuit, is good for 450V. and is shown run at a B+ of 700V with its plate at 300V. Who am I to argue with Broskie, although it could be a typo.
> 
> Anyway I didn't seek to optimize the circuit, only show that it had some potential, and shouldn't be dismissed out of hand as lacking in HF response.
> 
> ...




Sorry the bias is not right. The capacitor connects to the upper side of 20k resisitor and provide bias via two 20k resistors to q3 and q4


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## kevin gilmore

Quote: 





wakibaki said:


> Why would that be necessary? As I pointed out, _not a consideration which had escaped me_. Please read the posts before presuming to condescend.
> 
> w


 
   
  not condescending.  But you should look at the real waveforms generated when you push
  tubes like that past their limit.  Same thing with 6cg7. Top of the waveform gets very nasty
  when you push the tube more than 300vpp. In addition to the high voltages, the Rp of the
  tube very much effects the top part of the waveform. Serious compression occurs.
  This is why current sources are better than the resistors.


----------



## spritzer

That is indeed why the Stax amps perform better with other tubes.  It's also not the 6SN7 that can do 450V, just the 6SN7GTA/B's.  The normal ones are identical to the 6CG7's in this regard. 
   
  My super symmetry amp runs GTB's at +600V and they are fine but it was mostly chosen because of the transformer I used.


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## wakibaki

OK, point taken.
   
  I can't get the Broskie circuit to work, the opamps are stuck on the rails.
   
  wdiabc, I still can't get your circuit to work in the simulator.


----------



## wdiabc

wakibaki said:


> OK, point taken.
> 
> I can't get the Broskie circuit to work, the opamps are stuck on the rails.
> 
> wdiabc, I still can't get your circuit to work in the simulator.




I don't know why but simulators never likes my design. Haha. But I tested the circuit as shown and it works perfectly fine. R7 R3 and R4 are not accurate and needs to be trimpots. otherwise the bias won't be 1/2 of the power supply.


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## wakibaki

Voltage sources are all 1.5V.
   
  w


----------



## wdiabc

Quote: 





wakibaki said:


> Voltage sources are all 1.5V.
> 
> w


 
  Thank you very much for your help running the simulation. looks great to me. I dont know how THD is measured but 1.6% is a lot. I am looking to optimize it by altering the resistor values. I set up this circuit last night for a quick listen and some tests shows resistor value can greatly impact the output. Do you have any thoughts and comments about the circuit? Thanks!


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## kevin gilmore

an open loop circuit like this with that much voltage gain...
  1.6% thd is very reasonable. Adding a differential cascade
  at the bottom is going to help a bunch.


----------



## wakibaki

The swings you are seeing there, ~150V pk-pk, are using 2N3904 transistor model throughout. The simulator doesn't care that 2N3904 blows up, it doesn't tell me that, just carries on regardless.
   
  When I put in TIP50 + 2N2222, things get much worse. Swing falls to +/-90V, THD goes way up. I increased the drive to 2.5V pk-pk and moved the quiescent voltage to ~180V (you can see the increasing assymmetry, with compression of the sinewave on the negative half of the cycle), to get 211V pk-pk out at ~5.5% THD.
   
  Things are slightly better when output is taken from the bottom of R3 & R4, drive must be increased again (to ~2.7V pk-pk) to get 211V pk-pk out, but THD is better at ~4.5%.
   
  I tried to attach the schematic so you can run the simulation yourself, both as a .txt file and as a .zip, but the forum is not cooperating.
   
  I am not entirely happy with the circuit, I don't feel I understand what is going on in it. I don't like having two current sources feeding a current sink. This means that the circuit is very tightly balanced with just the base currents of the LTP making up any difference in current, and the quiescent point setting is very critical. I can't help but feel that it will not be very stable with temperature.
   
  I was a programmer and then an FPGA designer, I came to discrete audio design via software radio, RF design and only latterly IC audio design. Since ill-health forced my retirement this has been a good way to stay out of trouble, but you will get better advice from some of these guys who have been doing it longer than I.
   
  w


----------



## wdiabc

Quote: 





wakibaki said:


> The swings you are seeing there, ~150V pk-pk, are using 2N3904 transistor model throughout. The simulator doesn't care that 2N3904 blows up, it doesn't tell me that, just carries on regardless.
> 
> When I put in TIP50 + 2N2222, things get much worse. Swing falls to +/-90V, THD goes way up. I increased the drive to 2.5V pk-pk and moved the quiescent voltage to ~180V (you can see the increasing assymmetry, with compression of the sinewave on the negative half of the cycle), to get 211V pk-pk out at ~5.5% THD.
> 
> ...


 
  thank you for your comments. I built the circuit using actual transistors and resistors as shown in the circuit, and based on my experience, once adjusted, the bias is pretty stable and will not drift that much ( Once adjusted, within in 60 seconds the DC output will move up and down a little before transistors warms up but later become pretty stable at 1/2 of V supply) This circuit is basically a transistor version of SRPP and actual listening shows I really like the sound, much smoother, better clarity and better base definition,  compared to the sound from TA2020 amp + the stock SRD7.


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## wakibaki

Tried a lot of variations on this. I found the spice model for IXCP10M90S (strictly speaking a regulator) is just a MOSFET.
   
  These are far from elegant, require a second PSU, but interesting nevertheless.
   
  This is nice and symmetrical with an ideal CCS...
   

   
  ...but even with a cascode it's deteriorated a bit.
   

   
  What can we try to improve things a bit? Here's what we can get with the IXCP10M90 with CCS loads...
   

   
  At 80Vpk-pk, 0.25% THD
   

   
  40Vpk-pk, 0.117%
   

   
  Now with the DN2540, 170Vpk-pk, 0.07% THD
   
   
   
  These are _all just simulations_, I've got no reason to build any of them. Kept me out of trouble for a couple of hours tho'...
   
  w


----------



## wdiabc

Quote: 





wakibaki said:


> Tried a lot of variations on this. I found the spice model for IXCP10M90S (strictly speaking a regulator) is just a MOSFET.
> 
> These are far from elegant, require a second PSU, but interesting nevertheless.
> 
> ...


 
  Hi W,
  Can help comment on this circuit? I come up with this last night and did a quick build, it performs very nicely with almost no distortion, yet no feedback is used.


----------



## wdiabc

Q2 and Q3 are bjts that will take inputs, + and - or can be unbalanced 
  Output are from the emitters of Q8 and Q9. 
   
  This is a SRPP + emitter follower. almost flat down to 30Khz, according to my scope measurement


----------



## wakibaki

I made R10, R11, 20k, R5 300, D1 1.5V but emitter of Q8, Q9 is ~1.7V.
   
  Can you give me some spot voltages?
   
  w


----------



## wdiabc

wakibaki said:


> I made R10, R11, 20k, R5 300, D1 1.5V but emitter of Q8, Q9 is ~1.7V.
> 
> Can you give me some spot voltages?
> 
> w



Sorry. Power supply is 400v. Bias set at 200v. First stage at 1.3ma and the second stage at 3.9ma. 

Donny


----------



## wakibaki

I have 1.3mA in R9 and 3.9mA in R13, but emitter of Q8 is still only ~2V. Can you see what the problem is?
   
  w


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## jcx

even if working as intended it has a basic problem - the load is a nearly pure C, the current demand is 90 degrees phase shifted, orthogonal to the resistor/mirror multiplied current
   
  the R current adds nothing at the time of C load I demand peaks
   
  I thought the Broskie tube cad articles were clear on the "SRPP derived"  impedance multiplier concept
   
  nothing like that is going on in this cirucit - nothing is in a place to "sense" the load current - you are just adding a fixed impedance/phase current modulation


----------



## wakibaki

Quote: 





jcx said:


> I thought the Broskie tube cad articles were clear on the "SRPP derived"  impedance multiplier concept


 
   
  Got a link, please, jcx?
   
  w
   
  OK, found it...


----------



## wakibaki

@jcx OK, I understand what you are saying, if this circuit is truly SRPP then one would expect to see some feedback from the collector of Q5 to the base of Q2. I've been taking the view that I'd allow the poster to to correct the circuit rather than trying to second-guess what his intentions are, or with the presumption that his idea of what constitutes SRPP is conventional.
   
  @wdiabc You need to look at the circuit you have posted, check it for errors, and provide some more details, particularly of spot voltages at various points and show exactly how you intend the inputs to be applied and the outputs taken. I also need to know if the values I have chosen for R10 and R11 are in the range you intended. I am simply slavishly copying your circuit into the simulator without thinking too much about how it is intended to function.
   
  w


----------



## wdiabc

Hi W,
  the base of Q8 and Q9 shall connect to the upper side of each 150K resistor. 
   
  Sorry about the confusion but my schematic was pretty clear about where the base of the Q8 and Q9 shall be connected at. 
   
  Let me explain a little about this circuit, this circuit is like a class A "full bridge" driver and the two upper output transistors are driven by the 150K resistor, acting like an emitter follower, the two lower output transistors are connected to the first stage using current mirror, so this circuit becomes a interesting class A push-pull slash emitter follower slash SRPP circuit. 
   
  Sorry I was traveling for the past two days was not able to get back to you sooner. 
  -D
  Quote: 





wakibaki said:


> I have 1.3mA in R9 and 3.9mA in R13, but emitter of Q8 is still only ~2V. Can you see what the problem is?
> 
> w


----------



## jcx

while according to the Tube Cad articles the name SRPP is a recent invention - with ambiguous interpertations - I don't think it can be stretched to cover the present circuit
   
  you have "feedforward" push-pull - the bias/pull down current is modulated by the amplified signal V, scaled by a R and current mirror
   
  there is no connection to the load current demand - it could be helpful if it "lined up" with the load demand - but with ES headphones being a fair capacitor it doesn't happen here


----------



## wdiabc

Quote: 





jcx said:


> while according to the Tube Cad articles the name SRPP is a recent invention - with ambiguous interpertations - I don't think it can be stretched to cover the present circuit
> 
> you have "feedforward" push-pull - the bias/pull down current is modulated by the amplified signal V, scaled by a R and current mirror
> 
> there is no connection to the load current demand - it could be helpful if it "lined up" with the load demand - but with ES headphones being a fair capacitor it doesn't happen here


 
  I agree, while I was testing the output wave form, I found out even though this circuit has overall flatter FR, but I also tried loading the upper two transistors with resistors or CCS load. and resistor load clips very clean and smooth, sounds like a triode, CCS is a little harsh when clipping, while this SRPP thing clips very harsh. I will try to play with higher idle current once I received the heat sinks I ordered a while ago. 
   
  I think a CCS loaded emitter follower can do what you were describing here. emitter follower makes sure the voltage is right and CCS is making sure the current is there when demanded by the C load, especially when output voltage is at max, when current is 0 or when voltage is at 0 and output current is at max.


----------



## GL1TCH3D

Sorry if this is gravedigging...
I'm probably going to end up building my own stax amp and don't even know where to start xD
If someone could help me, that'd be awesome


----------



## wink

Go to the other site.........


----------



## Lil' Knight

Just start with the T2. Live hard or die trying.


----------



## spritzer

Yeah, there site would be good and also what you'd want to build (tube, SS etc.).  Since I just posted it in the Stax thread, this is quite cheap:
   

   
  Not really done for high performance and more for being odd.


----------



## ervstil

Sorry, stupid impulse post. When i stopped and think abut it i realized it is a dangerous, stupid idea.
  
 Hi
  
 I needd a Stax amp quick for testing purposes and this seems to fit the bill pefectly! Just one question, I live in Sweden we have 230v. Thats about 330v retified. Some cleaning and a simple regulator could take it down to 300. Same goes for the bias supply. Do i need a transformer at all?
  
 Best Regards Christian


----------



## chinsettawong

For safety sake, yes you need a transformer.


----------



## davidsh

This is quite interesting.. Suitable for a first 'real' project?


----------



## T N Args

I am interested in having a go at the updated circuit in post #1. I have a couple of questions please.
  
 1. Is the OP still watching?
  
 2. The ‘final circuit’ in post #1 shows 2N3904’s, but text below refers to MPSA42’s — which it then says are replaced by TIP500’s (what’s that?).  So, OK, what are the final-choice transistors that I should use?
  
 3. I cannot see a PSU schematic in the thread. What should it look like? Including the 580V bias feed to the stator?


----------



## T N Args

t n args said:


> I am interested in having a go at the updated circuit in post #1. I have a couple of questions please.
> 
> 1. Is the OP still watching?
> 
> ...


 

 Bumping my own post after 6 days, hope that's okay. Maybe others have the answers? Tho OP looks to have left.


----------



## Robert Neil

hi chinasettAWONG, do you think it might be possible to build stators like the srl700 or sr009?? I might try and buid them, or my friend whohas a cnc is going to be doing a set for me to retrofit in a set of 507s I see the plates are like nb lambda ones in that they have smaller holes closer together we will be milling them from brass plate and it shall be exiting to hear the outcome .. might take about 3;4 weeks il keep you posted .p.s...have you made any esoteric stator designs?? I personally think the more rigid the stator, the better the sound will be .. cheers.


----------



## Robert Neil

spritzer where can I find the schematic of the small pcb you have pictured with 8 small valves?


----------



## TypodCrowd

Robert Neil said:


> spritzer where can I find the schematic of the small pcb you have pictured with 8 small valves?



Hey, did you ever find this?


----------

