# Cascode Current Source



## ppl

I have been an advocate of using Constant current Sources to bias an Opamp driving a Buffer into Class A for over a decade. The Advantages is to remove the Possibility of Crossover Distortion From the Class b or Class ab Opamp output stage. Some Opamps like The AD-845 Have a Class A output stage and as sutch will not be improved by this technique. Some Opamps notably all the Most used Burr Brown Types used by Head-Fi'ers show a dramatic improvement with this method. My most often used method of suppling this Constant current is via one Of the Current regulator Diodes. or using a Jfet with the Gate and Source connected together and this connecting Between the Vee (Neg. Rail) and the Opamp's output. This works alot better than the simple resistor most folks use to do this as the CRD will not load the Opamp's output stage with a low resistence from a Resistor and thus removing the Dynamic Benifits of using a Buffer. This method while Great is still not the Best way. The Nonlinear capacitence of the CRD or j-FET will have a negitiive impact upon the sound quality of the Opamp. I have got around this in the Past by Placing a resistor in series with the CRD and the Opamps output to Buffer this capacitence. Since most of my recent amps have been battery powered The Possibility of the CRD droping out of regulation on large signal swings is a possibility. I have thus been testing Current sources made from cascoding Two J-FET's. A cascode circuit has a constant capacitence and Leakage current with voltage. This may at first seem a minor Point unless like Me you Think god is in the details. and if your search for the Holy grail in sound quality from an Opamp based Headphone amp I cay say without reservation The Cascode Current source is so mutch of an improvement over a Current regulator diode in sound quality that I was Most impressed. The most notable improvement is in the Midrange upward. My first Impresions using an OPA-637 Driving a stack of EL-2002's were staggering almost like another Otave was added to the Reproduction and this changed what is normaly a slightly dark sounding Opamp with a Normal CRD into a High and Mid frequency sonic Wonder. All BB OPA's without any Resistor or CRD sound Dark and vailed to me. using a 5K resistor strips all the dynamics away, while still only providing trivial improvements. replacing the resistor with a resistor Bufferd CRD brings the dynamics back while almost Compleatly removing the vail over the Sound image. Replacing This with a cascoded low capacitence jfet pair made this dark sounding opamp into a nice and detailed Performer. This same improvement happens with AD Opamps like the AD-825 & AD-823 also. Infact the sibelense sometimes present on the AD-825 is dramiticly reduced. 

 In conclusion If your going to get the best sound out of your Opamp Buffer Combo cascode a Pair of jfets like the MPF-103 or a Lower capacitence type and enjoy a sonic improvement You will find hard to resist.


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## MacDEF

I think this belongs in DIY, no? I'm gonna move it


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## aos

ppl is at it again 
	

	
	
		
		

		
		
	


	




. Very nice article. I'll try it when I start experimenting with DAC/headphone amps again.


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## Possum

I'm sold! I'll get a few of those jfets with my next orders.


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## Apheared

trust in the ppl, the ppl is good... hasn't steered me wrong once. that's just me, though... heh

 oh incidently, where was this MacDEF, that you had to move it?


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## Daniel Pumphrey

Great idea PPL. I am laying out an amp right now and will include this. I have examined some headphone circuits lately and all have extra transistor pairs like this. 
 I have two old Denon portable CD players. I repaired one with a digital out last week and disassembled another one and both had this type of circuit. The two designs wer very different although from the same era. These were very expensive players and considered the best portables ever made. 
 The second one which I have had for years is full of Elna Electrolytic caps. These things eat batteries but have the best headphone amps I've heard on any portable. No shock stability but they sound great.
 Dan


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## thomas

Dan

 I can see a commercial headphone amp having a push-pull transistor pair as an output stage, but i seriously doubt any consumer product would use this type of design to bias opamps. These transistors are not used in the signal path, they only bias the output stage transistors of the opamp into class-A.

 I'm sure this design sounds amazing, but i wouldn't use it in a portable unless you have a monster 16AA battery pack. 

 PPL: Can this same technique be used in buffers like BUF 634 or EL2001? Seems like it should work, but i've never seen it done before...


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## ppl

Oh Darn! I forgot to Change Fourms before i posted this I was reading the Music Reviews and forgot to Change to the DIY fourm, Sorry. Gess that is what Moderators are for. Anyway moving along Regarding Thomas Idea of Biasing Buffers into Class A. Elantec Recomended placing a 100 ohm Resistor between V- and The Output of the EL-2008 for a Video Application in order to get the Best Possible Diff. Phase and gain. On an AC powered Headphone Amp Using the EL-2009 I at First used this Resistor and as expected The dynamics were somewhat Compressed so i changed That to a Current source using a Bipolar transistor with an LED for a Reference. This made a slight Improvement and was most noticeable using the Amp as a Line Level Preamp. Havent used a Jfet in that Application because of the amount of Current Needed. Using Elantec's recomended 100 ohms at 12 volts results in 120 Ma. I dont Know any Jfet with an Idss that High. As far as usiing My Cascode Current source Idea on the Opamp in a battery Powered amp. The Current source only Needs to be like 1-2 Ma. The LS-840 Dual J-Fet i used in my Present amp Design as discribed, supply 0.68 Ma in cascode Mode and 1.75 Ma per J-fet half if used in a Normal non cascoded Current source. However this was maintained from 1.2 -15 Volts. where as an 1N5298 CRD went from 0.7 to 1.1 Ma Under the same Voltage range and if a Buffering resistor is added then the Voltage could only Go down to 2.2 Volts while sourcing 0.7 Ma. This may seem unimportent untill you consider that most Opamps can swing to wihin 2 Volts of the rail and Some Like the OPA-132 series and The AD-823 Opamps can swing to within less than 1 volt of the rails. Now you may be Thinking that no one is going to Listen to There Headphones at that level and this would be Good Thinking but The Change in current within the Normal Operating ranges can Impact the Opamps Performance and most surly affects the sound quality as this is just one reason The Cascode Current source made sutch an Improvement. I am evean starting to Like The of an OPA-637. With an AD-825 and AD-823 OH Ya.


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## aos

ppl, is it necessary to use dual JFET? I was thinking of using two separate ones. I do have some duals but it's really a sacriledge (not to mention a major $$$ issue) using low noise dual JFETs for biasing, instead of differential input stage...


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## ppl

It is Not required to use Dual jfets for the cascode Current source. I Did because i was replacing a CRD and this is the size of a 1/8 watt resistor. My portable amp board is quite tight so space was an Issue. It had to fit in the same space as the CRD and Conect into the same Hole's. Also the LS-840 has a low Pinchoff Voltage so this also was a factor since the lower this voltage the Lower the Voltage will be that the Current source Drops out of regulation. Capacitence is also a issue since i wanted a low capacitence current source. It is true that cascoding reduces the Capacitence vs just the Normal non cascoded Current source but having lees to start with will result in less in the End. I temperly Tried an MPF-102 and these also improved the sound. but i think a lower capacitence type like on semiconductor's type 2N5484 & 2N5486 would be evean better.


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## topher

ppl, 

 Did the guy at Linear Systems ever give you a 
 price on the LS-480? Or anything else?

 I asked him twice months before I gave you there 
 address, and I still haven't received a response.

 He said he would on the phone, but it hasn't 
 happened.


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## ppl

Hmm... So you decided to go with the spendy Dual's. Heck thay are one hell of a great fet. I am going to use some of these as the Diff Amp In my Winter Project An AC powered All Cascoded Discreet component Opti-Fet Dual mono ultra reference Headphone amp. So far i got the Power Supply Done and I done some Tricks to Totaly Eliminate DC Drift with tempature on the Regulators. Evean Kevins and Jungs supper regulator drifts alot with Temp. Heck i Blow air across the Circuitry and get a change of a volt or two. On my latest design No change rock solid +/- 15.0 Volts. I am glad the dramatic amount of work i put into that paid off. Any way sorry i got side tracked. LSI has a sales rep that is sometimes hard to deal with But i Think he sed somthing like $5.50 Ea for the LS-840. in Small Qunt. Ill will check and let you Know. I tell ya this company is one of the Better Nitch Analog fab Plants around. there 60 Volt Dual BJT's actualy spec out on a curve Tracer at 95-115 Volts. try that with a fairchild or Siliconex Device.


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## aos

I'd also be interested in getting those LS-480's. I want to try them in my portable DAC (which is progressing nicely btw, just about ready to start laying out the PCB). If you guys figure out a way to get them, could you let me know?


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## topher

I want to play with their BJT's as much as the 
 ls-840's. You don't see other 600MHz pnp's with 
 good voltage, let alone good current gain.

 I sure wish the heterostructure parts would go 
 commercial...

 Anyway, a price list would sure be nice.


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## ppl

I Requested a price list so when i get it ill email you a copy.I also like some of there BJT's but i use the dual PNP/NPN Ls-312 & LS-352. These are only 200 MHz. But real low noise and High gain like over 300@ 1Ma Tested. When I am done with my Opti-Fet headphone amp that uses these as well as the LS-840 Ill email you a copy. So far the PS is done and sounds Killer on the Gilmore amp i am using to tweek the Regulator circuit. Boy after listening to this I may never go back to IC reg';s again Night and day difference. IC regulators are still in the dark ages. I think evean a simple Series pass transistor and zener reference sounds better than any IC reg evean the Linear tech types. and i doo tweeks to designs using these most people dont Bother with, these tweeks improve the sound of the IC's alot But still way short of a discreet component design, but i gess there is only so mutch you can do with a sow's ear.


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## aos

ppl, you should look at the latest voltage regulators. The classic 3-pin 1970's designs are way inferior to everything being out now. I spend tons of time over the last months looking at them for my DAC. There are references out there which have all the key parameters - load, line, temperature, dropout and noise - **SEVERAL** orders of magnitude better than those old regulators. For example, just couple of hours ago I placed an order for Analog's ADP3303-xxx voltage regulators. Don't knock out them just yet. Besides, what are you going to use in a discrete regulator to give you reference voltage? A LT1027/LT1036 or another one of many of voltage references made by LT, AD, NS, TI and others. 1ppm temp coefficients, less than 1uV p-p noise, virtually independent of line/load, ultra high initial precision etc...

 However, I would like to look at your discrete regulator. Not for the portable DAC, but for the stationary it's always good to have the option. The one I used before (shunt reg) has only a resistor in the rail path and MOSFET as parallel regulating element, and opamp is used to compare the voltages. I suppose opamp can be replaced with a Darlington transistor or something, but you say you have your own design which has low temperature drift, so I'd love to see that too.


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## ppl

My discreet Regulator is your basic Series pass transistor witn a Gain transistor and Current sourcing Everything. The Great tempature performance comes from Current sourcing but more importently using an LED as the Voltage reference. LED temp Coe. is just the Oppisit of silicon so positive changes in one are canciled by negitive changes in the Other. LED's as a voltage reference is not New it is used in BJT current sources so as to maintain A constantant Current with Tempature as well as Lower noise and lower dynamic impedance over a rather wide bandwidth. I haven't seen an LED used as a Voltage reference in Voltage regulators yet. So with me always trying new unproven Ideas i used a radio Shack Blue LED as the Primarry voltage reference and Red led's to Bias the Current Sources whitch Resulted in Greatly improved Tempature stability. LED's also have lower noise than a Zener as well as Substantualy wider bandwidth that the Buried Zener devices like the BB (Ref-01) Current sourcing any voltage reference is required to obtain The best possible Performance out of any Ref. Most people still use a resistor rather than a current source and the Performance of the Reference Suffers as a result. Constant Current Sourcing eather in the form of BJT or FET will improve the rejection ratio about 30dB over just a resistor. The shunt Regulators like the TL-435? are alot better than the Typical Three termanal devices, However most people use a resistor to drop the voltage. Again about 30dB of additional PSRR can be obtained and 60dB is possible if the Current source is a Cascode type. this is because of the Higher output impedance the cascoding supplies. Ill chek out the AD device you mentioned.


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## aos

Some TI and AD references have PSRR of 60dB still at 1MHz and about 40 at 10MHz. AD regulator I mentioned provides up to 100mA and is stable with any output capacitor (so they call them AnyCAP (TM)). This is quite non-typical for classic references which loose any semblance of regulation when they reach about 100kHz.

 But hey, current sourcing the reference, that's a cool idea. Although the ones I'm looking at have such extremely low dependency on load and line - just a couple of parts per million! - that it's not really necessary. Well, partially true, line regulation is really almost perfect but load regulation would still probably get improved by using the cascoded current source... Mmmm... Gotta try it.


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## ppl

These devices may have that kind of performance under static conditions, However i would like to see how thay change with large transients causing them to heat up and thus Change. Now i understand that the Feedback used will correct for this somewhat but unless the open-loop bandwidth is verry large then we can get a similar effect to TIM in amplifires where the Amp looses feedback for a moment and is thus momentarly overloaded. This showes up as ringing on Fast transientts and High frequency Sq waves. Now regarding the temp stability issue evean if these References did not drift at all and are used as a reference for another silicon based device the Change of the other device will still Change regardless of wether or not the Reference is stable and this is where the LED comes in.as it also changes but in the Other direction and can cange quickly well into the MHz. range. Oh ya Try the current sourcing unsteed of a resistor Regardless of what ref you are using dramiticlc improvement so if your chosen reference has 60 dB of PSRR to start with then Current sourcing will add another 30dB or so to that. I have Seen References combined with cascode J-fet/BjT current sources aproach 120db of PSRR from DC-200MHZ providing the ref is that fast whitch LED's are.


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## aos

All the references come with transient response graphs. Some are better than the others. I was looking specifically for high perfomance at high frequencies and found AD and TI. TI even advertises its thing as "fast transient, RF", for use in digital assistants and cell phones. I'm using these for digital supplies.

 I like the idea, anyway. Now even the solid-state equipment, like my Kevin Gilmore amp, are glowing. Who needs tubes 
	

	
	
		
		

		
			





.


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## ppl

What? are you refering to in the Coment about Kevins Amp. that you used Current sources or the References you mentioned?


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## ppl

Walt jung discribes the advantage of Current sourcing in the EDN artical about his supper regulator. http://archives.e-insite.net/archive...97/01di_03.htm


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## aos

Kevin's amp uses LEDs as voltage references for current sources. So there is a glow coming from inside the amp when it's working 
	

	
	
		
		

		
		
	


	




.


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## ppl

Ok i understand. It is true that LED's have been used for voltage references in Current sources for a long time. Thay were used for the Reasons i cited in my above Post. Walt jung also dose the same and for the same Reason. I just wanted to point out about the Bennifit of using LED's as the Voltage reference in Voltage regulator circuits as well as point out the advantages of Current sourcing these Voltages References. Take for example the Classic BJT Current source using a LED as the Voltage reference. Note that the LED is still receiving it's current Via a resistor. This Resistor can be easly changed to a CRD Diode or jfet Current source. But I gess that's just me I like to cover All possible Bases in Circuit Design and leave nothing to Chance.


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## ppl

Attached is a copy of the schematic to Walts supper regulator. This is also available from the Link i provided above


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## transducer

I have a bunch of 2SK389's and 2SJ109 dual JFET's sitting around. Can I use these for your cascaded current source? Also, which leg does the little dot on the FET represent?

 How do I calculate the current supplied to the opamp?

 Can this be used on an output opamp, that is one that is not buffered by another opamp as you show in your schematic? I was thinking of doing this to my OPA627 line stage which also doubles as a headphone amp, it's just one opamp feeding either the amp or the headphones.

 Thanks,
 RonS


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## transducer

I have a bunch of 2SK389's and 2SJ109 dual JFET's sitting around. Can I use these for your cascaded current source? Also, which leg does the little dot on the FET represent?

 How do I calculate the current supplied to the opamp?

 Can this be used on an output opamp, that is one that is not buffered by another opamp as you show in your schematic? I was thinking of doing this to my OPA627 line stage which also doubles as a headphone amp, it's just one opamp feeding either the amp or the headphones.

 Thanks,
 RonS


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## ppl

transducer: If you are refering to the Dot on the Schematic i posted as an attachment, Then that dot represents the source of The FET. The gate and source on an N Ch Fet go twards the Negitive rail. The amount of Current supplied by a FET connected as a Current source is determand by the Idss specification of the FET in normal situations However since the Gate to source Voltage on the Cascoded fet is nor 0 Volts the actual amount of Current supplied by the cascode Current source will be less than the Devices nominal Idss Spec. What you Need to do is Connect the Dual N Ch fet up as a cascoded Curent as depicted in the Schematic with the Dot on the Schematic being Represented by the Source Lead. Then conect the Compleated Cascoded Current source up to a Battery with the Source twards the Negitive Battery termanal. then connect a Meter to the remaining drain lead and the battery Positive termanal. This set the Meter to measure on the 20 Ma range. The Meter will then read the actual amount of Current being supplied by the Current source. I then recheck this with a 1.2-1.5 volt Battery to see if the Curent remains the same. This is importent for battery operated amps, But an AC powered amp operating on + & - 15 to 18 Volts then just using the 9 Volt battery is Enough. The 2SK389's are an N channel device and thus are referenced to the Neg. Supply. the 2SJ109 are P Ch Devices and suould be referenced to the Positive rail. I don't have a data sheet on the 2SK389 but i Reviewed the 2SJ109 and as far as DC parameters go Idss of 2.6-20 Ma should be about right for a 1-2 Ma Cascoded Current source. The Capacitence of this device is quite High 95 Pf so if it were me i would use another Fet. A Pair of MPF-102's ar Better yet a BF-256 or the American equiv 2n5484 with a capacitence of 1.0 Pf would be alot better. As far as what the results in sound quality would be doing this to an opamp Drictly driving Headphones I think that since any reasonable Volume level is going to Require more current than the Current source is suppling and thus no longer Keep the Opamp into Class A it may be a Moot point. As far as an opamp driving another Active device like a Power Amp then the Bennifits will be realised because it would still be driving a High Impedance Load. Most power Amp's have a input impedance of 50,000 Ohms or more thus allowing the OPamp from the Line stage driving it to remain in Class A. On my other Posts i stressed the fact that i dont like to require an Opamp to drive low impedance loads like Headphones and Thus A Buffer will Dramiticaly improve the sound of the Opamp under these Conditions Regardless of wether or not the Opamp is into Class A. Hope this Helps.


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## aos

Ha! I knew those 2N5484 would come in handy one day!


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## transducer

Hi PPL,

 Thanks for the reply. The 2SK389 has a capacitance of 25pF, so still to high I guess. I'll get some of the FET's you recommend.

 I'm wondering what you think of this schematic which was posted at the Elliott Sound Products page, it shows an OPA2134 feeding a low impedance input on a THS6012 CFB opamp. I had thought of trying this circuit with a Class A biased input opamp. The article is also intereting if you haven't read it before.

 Cheers,
 RonS (transducer)


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## ppl

You could try the 2SK389 as i discribed above using a battery and see what kind of current you get. The Isolation resistor showen in my attached schematic will isolate that kind of Capacitence and Cascoding will also reduce it alot futher but it is always best to start with a low capacitence device to begin with. As far as the circuit you posted in the gif attachment see my post on walt jungs Multiloop I think it will be alot better as the link will inlighten as to why.


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## transducer

Thanks ppl. I looked at Walt's article, actually saw it years ago, but it is well worth revisiting. I will see about implementing his suggestions into the gif schematic that I posted. Really want to try the THS6012, it's spec's are amazing. Will use the OPA627BP biased into Class A. Might also try a trio of Buf634's.

 Cheers,
 transducer


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## Possum

How do you troubleshoot something like this? I ended up using MPF 102's with a 61.9 Ohm resistor between the outputs of an AD823 and the -9V rail. I get sound, but it's very distorted and fuzzy. I've double-checked the MPF 102 datasheets to make sure I used the correct drain, source, and gate pins.


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## aos

Measure the current it's drawing. That's the voltage accross your 61.9 Ohm resistor divided by 61.9. It should be a couple of mA, something like 2 or 3.


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## Possum

Ah, thanks, it's working now. Current draw is ~3.5 mA. Problem was caused by some parts of the transistors lightly touching parts of my circuitboard. I had to put the transistors and resistor setup on the underside of my PCB since the layout was planned without these extra parts in mind, leaving no room on top.


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## aos

So, did you notice improvement in sound?


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## Possum

Err, I guess that's what separates the "audiophiles" from the circuit makers/tweakers. 
	

	
	
		
		

		
		
	


	




 I honestly wish I could A/B the sound, but as long as the theory behind the aspects of a circuit or a tweak sound reasonable, then I'm satisfied if whatever I hear coming out is enjoyable. I guess I actually would A/B it if I wasn't in the middle of cramming for an important exam I have tomorrow. I took a break from studying because I just had to try this out (my Digikey parts came 1 day early - tracking number was expecting delivery tomorrow).


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## aos

2N5484 gives only about 1.7mA. I'd prefer about double of that. MPF102 looks like a more fitting choice here.


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## ppl

aos: 1.7 Ma. is about right for an opamp driving a high impedance load like a Buffer. I have 1.2Ma using a selected LS-840 with 0.75 Ma. Typical of the LS-840 in cascode. Years ago when i first investigated Biasing Opamps into Class A I found most of The App Notes where this was used (See Data sheets for LT-1115,EL-2008, EL-2009 For starters) were Biasing at 1/10th the Nominal rated Outtput Curent of the Device. After lots of Phone conversations with app Eng's. from several Semiconductor Manufactures Thay all seemed to sugest that this 1/10 was the Max Current and would not Recomend Going Higher. My thinking on using less is the fact that if the Opamp is driving a load of several Meg's as is the case with an Opamp Buffer Combo. The Opamp is not going to see anywhere close to a Ma. of output Current. So why Over-Bias it. 

 My AB comparrisons on this were easy for me to do because i put the resistor that goses from the Current source to the Opamp in Those machiened IC socket Pins so as i can just pull out the resistor to be able to to see what it is like without it. and i can do this while the Circuit is still Powered. Although Doing AB testing was not the reason for wanting to disable the Current source it did help alot in AB evaluations. The reason i wanted to disable the Current source is to be able to use the AD-744 in it's best sounding mode when Driving a Buffer and that is to use Pin#5 rather than Pin#6 as the Opamps Output. This bypasses the AD-744's internal Output stage and allows one to Connect the VAS stage of the Opamp Direct to the Buffers input and thus avoiding the Added Distortions and time delays any output stage adds. The AD-744 operated in this way dose not require a Current source to Operate into class A since the VAS stage of the Opamp is allready in Class A. The sound Change from operating the 744 in this mode vs using the Opamps internal output stage Biased with a current source or not is quite Impressive. I now never use the AD-744 in a conventional way when it's driving a Buffer. WARNING do not do this without a buffer between the AD-744 and the load since only about 2 ma of current is available and you want to Keep capacitence low so as not to lose Bandwidth. Remember Pin 5 is at a High Impedance Node. Using the AD-744 in this way and driving a buffer results in oune of the Best overall warm and musical Configuations I have Heared. While other opamps may provide more detail or added Brightness. Maby more Bass Slam with some, I think that for Overall Musicality the AD-744 Driving a buffer operated with Pin#5 as the Output of the AD-744 is a hard one to beat. and it seems that no matter what Opamp i try that after awhile i always go back to Silky Smooth Euphoric sound of the AD-744 using pin#5 as stated above. Opamps like the AD-825 and others provide better Detail and slightly better sound stage but also force you to Hear the Bad Recording Job done on most recordings and this is starting to Irritate me. I may make this into another thred as it makes for an easy search in the future.


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## Tomo

Hello,

 I am raising this discussion back into our focus.

 TOMO


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## Joobu

Finally got around to reading some more app notes. The Linear LT1575 is an ultrafast low dropout regulator. Designed to drive microprocessors with large transient currents on the order of nano seconds.

 Basically it's a linear regulator with a huge amount of bandwidth. Due to this, designs can do away with the huge bulk decoupling usually employed. Of course you still need some local ceramic decoupling. Nonetheless, definitely seems like a better approach than putting down hundreds of thousand of farads of capacitance


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## ppl

Joobu; LTC has been saying the same thing for years starting with the old LT-1085 & 1086. However look at any Mother-Board next time your at a PC shop and you will see Big Electro's Bypassing whatever Regulator is used. Years ago I used to think as you did Thanks to mark brandsfield of the Then MSB technology but after lots of listening over the Years i have Found Big caps Sound Better will smooth any Glitch from the reg's. All Regs regardless of Specs have Ringing on fast High Current pulse. Granted the magnatude is small but the Big cap reducesit Even futher. Sound Quality is also improved. This is only My View and other views with equal technical Support are available from other sources and some on these fourms. So Ya all save your Flames


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## Joobu

You'll always have bulk decoupling. However, I wonder how much of that bulk decoupling is actually powering the processor. Power circuitry for processors is an involved design. Step-down switchers (I'm pretty sure that's what all those toroids are for) and of course linear regulators. 
 I'm never tried the regulator so I don't know how valid its merits are but it's definitely an interesting implementation.


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## ppl

Joobu: Power supply design for High Current High data rate Digital Circuits are in a different class than that of Low Noise Analog Circuits. In Analog design switchers are often not Recomended, However thay still are used in some situations. aos and tomo might be the ones to eloberate upon Fast regulators.


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## aos

I've read tons of datasheets for regulators, but I looked only at linear regulators. Some very fast linear regulators will regulate well in the MHz region and are usable for digital circuits. The ones I use are meant to be used for Bluetooth and handheld devices. The voltages there are usually so low that I guess they don't need switching to up the voltage.

 I'd love to see a nice article where someone professional, with profi equipment naturally, has done comparison of vanilla LM317/337/7815 etc. with these new regulators. But I would encourage people to look beyond those old 3-pin regulators.


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## ppl

Thanks aos, now i gess we can keep this thred on topic, However Tomo may have,as he most often dose! have some insights into Fast regulators.


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## ppl

Some Members of this fourm have expressed concern of the amount of Current supplyed by the cascode Current source changing slightly after applying Power to the Current source.

 This is normal since all JFETS will have greator leakage current as thay warm up. this is also true of Opamp's that use JFET inputs. The reason that the JFET make sutch a nice and easy to implement Current source is by virtue of one of the drawbacks of JFET's Known as Leakage Current. This is sometimes undesireable, sutch as on the Input of an Opamp. However when using the FET as a method to supply an almost Constant current to a Load this leakage now becomes an attrabute. since the leakage current will change with Tempature and the fact that the Current being supplied will cause a small increce in the Die Tempature of the FET the actual amount of Current supplied to the Load will also go up slightly. with the larger TO-92 and TO-18 Packages of most JFET Transistors this rise in die Tempature is small and as sutch the rise in Current is also small, it is like less than 5% of the Nominal Current. 

 All Active Devices within any circuit require some time to stabilize and the amount of time is dependent upon anoung other things as the thermal envioment around the Circuit, however 2-5 min. is generaly enough time to allow the circuit to stabilize. by this time the Current source has also stabilized and will remain constant unless the Device is futher heated or Cooled. Tempature changes resulting in parramectric changes to semiconductors is widly Known and Evean the Bipolar transistor will have gain and other paramitors change with temp like the base to emitter voltage (Vbe) and this has also been exploited in Electronic Tempature sensors.


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