# Portable all discrete all bipolar class A amp



## NelsonVandal

I finally finished this stripboard amp. It could be called lil' classe A, since it's basically a Hiraga le Classe A adapted for 9 V battery use and a vbe multiplier to set the bias current. Active ground channel of course, and TLE2426 as VG circuit. There's a LM317 as a charger, and like in Hiragas amps, the batteries are powering the amp even when wallwart is plugged in. It's more transportable than portable, boxed in a Hammond 1455J1201 with 8 AAA's. Regarding the number of components and how tightly they're packed, I'm surprised it's working without the need of error searching. The noise is just audible using IEM's, but higher than I expected after making a messy one channel prototype. Using normal cans it's dead quiet. No obvious oscillation, but I have no oscilloscope and I doubt it would look perfect in one. There's some DC-offset drift, maybe 10 mV or so. I don't know if it's normal, or if it's a sign of instability. There's no caps in the signal path, just electrolytics for decoupling. There should probably be some caps in there like in the feedback loop and from collector to base in the VAS. Maybe the open loop gain should be limited as well.

 Since it was many hours of work, I think I'm biased when giving listening impressions, and it's too early to tell if there are irritating colorations. I think it sounds very good, very neutral tonality, great imaging, detailed, energetic without being harsh. I don't listen to "inner details" "depth and height" etc. I have the output biased to 20 mA, which I think is plenty enough for low Z phones such as HD650. I don't know what the voltage swing is, but it's enough for me.

 Since I don't know electronics or ampmaking, comments are welcome. I did this looking at other amps and by trial and error. I hope more people will build all discrete portables. Most of the chipamps are just not good enough IMO, and a discrete amp looks cooler. A fellow DIY'er has layed out a board, common 2 ch topology, not 3 as in this one, to fit the same box. I'm going to order some boards later. I'm thinking of laying out an amp like this for SMD parts to see it can fit a Hammond C801 or 1201 or something in between. I've built a 3 ch JISBOS in a C1201 with through hole components and 10 AAA's, so maybe it's not impossible.

 Most of the components are on sockets, not the output transistors and vbe multiplier, so it's easy to change the values. Like I said please help me improve it. There are some free sockets to the bases of the output transistors so I can retrofit open loop reducing resistors or a capacitor like in most of amb's amps. Maybe I should have lower values for the feedback resistors.

 One funny thing is I tried the "high grade" and more expensive Toshiba transistors 2SA970/2SC224 instead of the BC's. The Toshiba's sound artificial and hard in comparison
	

	
	
		
		

		
			





. I can see why amb and a lot of other "real" audio designers use the common BC's.


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## NelsonVandal

Schematic. And pictures of the actually working and stable prototype
	

	
	
		
		

		
		
	


	




.

 The ground channel looks the same except for R11 = 2.2k


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## majkel

Job well done! How much current does it draw?


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## NelsonVandal

Quote:


  Originally Posted by *majkel* /img/forum/go_quote.gif 
_Job well done! How much current does it draw?_

 

I never measured it after the final adjustments, but I will to find out there are no supraaural mad oscillations. I would guess 80 mA.


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## bearmann

Hi NelsonVandal,
 your 3ch version looks absolutely stunning! Great (soldering) job! 
	

	
	
		
		

		
		
	


	



 The 2N5486 seems to be a quite popular JFET for audio circuits, doesn't it? The only problem is, that it's really pricey and hard to get here at Germany/Europe. Do you know substitutes?

 best regards and keep up that good work!
 bearmann


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## FallenAngel

Quote:


  Originally Posted by *bearmann* /img/forum/go_quote.gif 
_Hi NelsonVandal,
 your 3ch version looks absolutely stunning! Great (soldering) job! 
	

	
	
		
		

		
		
	


	



 The 2N5486 seems to be a quite popular JFET for audio circuits, doesn't it? The only problem is, that it's really pricey and hard to get here at Germany/Europe. Do you know substitutes?

 best regards and keep up that good work!
 bearmann_

 

J310 as per Lisa III?

 By the way, NelsonVandal, gorgeous job on making it on perfboard! I hate working with the stuff usually but it does make prototyping much eaiser and less expensive. I'm definitely building one of these soon, right after the Lisa III on perfboard


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## steinchen

Quote:


  Originally Posted by *bearmann* /img/forum/go_quote.gif 
_The 2N5486 seems to be a quite popular JFET for audio circuits, doesn't it? The only problem is, that it's really pricey and hard to get here at Germany/Europe. Do you know substitutes?_

 

BF245*C*, but note the reversed pinout


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## bearmann

Thanks a lot both of you, steinchen and FallenAngel.
 0.18€ per BF245C ... perfect! 
	

	
	
		
		

		
		
	


	




 I think I'll give this little amp a try...

 best regards and a nice weekend.
 bearmann


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## NelsonVandal

The 2N5486 is there for isolating the small signal rails from the return currents, like in LISAIII (and in B22 I think). I've actually listened to the rails, and they're almost dead quiet using this topology. If not using active ground, but still the output class A biased, the rail noise is nasty and loud. I don't know if they do any good. There's no dramatic changes in sound, but it's cool to have them there.

 I changed R7/R8 to 1k. This gives a current of 0.95 mA. Hiraga suggested 0.8 - 1.0 mA in the original amp. This seems to make it more stable since there's less offset drift now. The sound is still the same.

 I also changed R9/10 to 120 Ohm, and now the current through them are 3.5 Ohm. I've no idea of what's adequate here. Hiraga says nothing about it. Please help me with it.

 The currents through R3/R4 and R5/R6 are 0.5 mA. Maybe this should be raised some, please help me on this one too.

 The final mod is lowering the feedback resistors in L/R. R11 is now 2.2k and R12 is 560 Ohm.

 After these changes the noise is lower, and like I said it seems more stable with less offset drift. The noise level is lower or on par with the high performance Mini3.

 Like I thought I would, I've found out that this amp isn't totally neutral. There's a slight edginess, a bit like AD8397 (Mini3 etc). On the other hand it has only played for a couple of hours, but I'm not sure I believe in burn in. Still this is a good listen with smashing soundstage, probably thanks to the 3 ch topology. It's very energetic and powerful. It does more to low Z phones I'm afraid since I made for my 650's. I give more impressions later after some burn in and maybe some more tuning.

 I'm sorry I don't have a 3 ch JISBOS to compare it to, but I remembered it being a bit softer/darker. The 2 ch JISBOS plays well with 12V supply. I'm quite sure it could be made for 9V supply. There's less parts in JISBOS, making it easier to fit in a small box. Since it's 2 ch, the soundstage is not as impressive as in the amp above.


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## NelsonVandal

I've found out that the voltage swing is worse than the JISBOS amp, but that's powered by 10 AAA's. The noise level is higher in JISBOS.

 If you're going to build a discrete portable amp, I think JISBOS is a safer bet since it's known stable and the simulation by amb is extraordinary. There are also less parts, and it could be made smaller. There's also the edginess in this amp. I don't know if it's distortion or any other measurable problem since I don't have a 'scope, or if it's just the sound signature of this design. I hope either burn in or fine tuning make it smoother. It's more lively though, and the imaging is very very good.


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## NelsonVandal

I've found the major error - the ground channel. Too high output impedance? The edginess is gone after having the ground channel in unity gain. 

 I've altered some resistor values, and now it plays louder without distortion. Loud enough for me. I can turn the volume all the way up when using Iriver IHP120 as source (max volume on this too). I'm sure these aren't the perfect values. I guess they should be calculated with regards to Hfe, Vcc and gain, but since I don't know what I'm doing, I have to work by trial and error.

 The amp seems very stable now. After warming up there's almost no DC offset drift, just within a mV or so. This kind of amp is very thermally unstable I think. If I blow gently on the transistors, the offset starts to drift.

 Before this important changes I added capacitors between bases and collectors on the Q3/Q4. I think it smoothened it a bit, but I'm not sure. The values are to high, 50 pF, but that's what I had. I'm going to remove them later to see if they make any difference at all and maybe add 10 pF instead.

 I have to give it some time to be sure it sounds ok now. It takes some listening to hear colorations. I'm also going to try the Toshibas again. I wonder if it's wiser to use different transistors in the different stages. If a transistor has a certain sound. Spreading the coloration with three different components might lessen colorations
	

	
	
		
		

		
		
	


	




.


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## NelsonVandal

I'm very happy with the sound of this amp now using the values above. It's very close to neutral. Good details, good soundstage, no sibilance, alert. It's slightly brighter than JISBOS with more of a focus on the mid-high mids while JISBOS has focus on mid-low mids. Treble is very clean. Bass is snappy but full. JISBOS sound more woodwind while this sound like brass
	

	
	
		
		

		
		
	


	




. It's hard to explain the very slight colorations. This is airier when JISBOS is weightier and more down to earth. Bipolars vs JFETs
	

	
	
		
		

		
		
	


	




.

 Everything seem very stable. It plays very loud without clipping and sounds clean and powerful while playing loud. Noise is totally absent with full size cans and barely audible with IEMs. No turn on thump at all, even with IEMs, and just a slight click when switching off (only heard with IEMs). No measurable voltage peaks while turning on or off either, it rises to maximum 10 mV when turned on and falls to -30 mV when turned off. There's absolutely no hum and no picking up radio frequencies or other interference. Offset drift stays within 0.1 mV while fully warmed up.

 I ordered the board layed out by Nico Ras. It's 2 channels, and no VBE multiplier for finetuning of the bias.


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## TzeYang

hmmm, you should probably try some JFets for the input stage instead?


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## NelsonVandal

It's funny.. I've finally started to use LTSpice. I had to use other transistors. The list of parts (except for Linear chips) is very limited. I'm not sure I understand distortion analysis yet, just frequency, phase, DC and AC. The values I've got by listening and using a simple DMM is also what simulated best. Trial and error is not such a bad thing
	

	
	
		
		

		
		
	


	




. I hope I can learn some more Spice and enter the correct parts. As I expected the simulated voltage swing is +/- 3V, not much but enough for me. Phase and frequency response are flat. Idle currents are about the same as in real life. Distortion seems bad, but as I said I don't really understand it.

 This amp still performs very well and I prefer it to my 2 ch JISBOS or any other amp I have.


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## 00940

Quote:


  Originally Posted by *NelsonVandal* /img/forum/go_quote.gif 
_It's funny.. I've finally started to use LTSpice. I had to use other transistors. The list of parts (except for Linear chips) is very limited. I'm not sure I understand distortion analysis yet, just frequency, phase, DC and AC. The values I've got by listening and using a simple DMM is also what simulated best. Trial and error is not such a bad thing
	

	
	
		
		

		
		
	


	




. I hope I can learn some more Spice and enter the correct parts. As I expected the simulated voltage swing is +/- 3V, not much but enough for me. Phase and frequency response are flat. Idle currents are about the same as in real life. Distortion seems bad, but as I said I don't really understand it.

 This amp still performs very well and I prefer it to my 2 ch JISBOS or any other amp I have._

 

To get meaningful distortion results in ltspice, you have to turn compression off in the control panel. Use those parameters: 

 V3 N011 0 SINE(0 5 9k765625) AC 1 (voltage source at the input)
 .tran 0 5.12m 1.024m 0.015625u 
 .fourier 9k765625 V(out) (replace V(out) by your output voltage node)

 If you use FFT: 

 262144 points
 Hann window
 5 points binomial smoothing


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## NelsonVandal

Thank you, I'll try it tonight. I could never have figured this out by myself. Looks very complicated, by engineers for engineers?


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## 00940

Me neither 
	

	
	
		
		

		
			





 Those values were given here : diyAudio Forums - Diamond buffers w. CFP outputs


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## NelsonVandal

So I've been simulating and tweaking a bit.

 1. The caps C7/8 are needed in real life for stability. As I've reported before I've used 50 pF with absolute stability and now I've tried 10 pF which seems borderline stable, so I guess 33 pF would be ideal.

 2. When simulating the circuit in unity gain (like in the ground channel) and I've found peaking at 33 MHz. This peak disappears with a higher resistance R1, at least 3k is needed.

 3. To increase the voltage swing, I've dropped the filtering JFETs/capacitance multipliers or what they're called. The voltage swing is now +/- 4V with 8 cells = 9.6V. I can have both the amp and iRiver at max volume without distortion, and that's very very loud.

 4. While simulating I've found that when the voltage drops, the bias currents drop dramatically. The solution is to have a constant current source to the input stage. I get the best results from this standard two BJT CCS, much better than BJT/LED or BJT/diode. Now the currents are almost maintained down to 4 V supply (but the voltage swing goes down of course).

 I still haven't got my boards, very slow delivery for sure, or mybe I've been screwed. I'm tempted to lay out a SMD class AB version small enough for a Hammond C801.

 I've simulated a pocket Gilmore lite. It seems to work fine at 9.6 V supply. The voltage swing is a bit lower +/- 3.5V, but I think it's enough for most of us. I think it'll be the next project just for the hell of it. Posters in the amp section has been kicking dirt on us portable users and keep whining about the impossibility of a portable amp with the sound of a G-lite.










 Pocket Gilmore:


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## FallenAngel

Buddy, lay out this thing in SMC and fit it into a C801 or even C1201 case and I'll be jumping up and down for joy! Too long have there not been enough portable amps with discrete output stages, even wrapped in an opamp feedback loop (all I can think of is the Sijosae Class-AB and modified 3-channel versions which sound pretty damn good too, and of course the Lisa III). A fully discrete portable would be very nice, lets get our hands on those boards 
	

	
	
		
		

		
		
	


	




 If you have ordered a few extras, I would love to get my hands on one (or two).


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## Nebby

I'd be in for one or two boards myself


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## TzeYang

lol, not to be picky but why did you use BC337/BC327 for the BJT ccs instead of the BC550/BC560?


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## wquiles

Quote:


  Originally Posted by *Nebby* /img/forum/go_quote.gif 
_I'd be in for one or two boards myself 
	

	
	
		
		

		
		
	


	


_

 

Same here. I would love to build one of these as well - and surface mount is no problem 
	

	
	
		
		

		
		
	


	




 Will


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## TzeYang

Oh i forgot to add, i'd grab 3-5 boards just for the sake of me and my local buddies.


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## NelsonVandal

Why BC327/37? Because I have a bunch of them from the time when I struggled to find matching pairs for Sijosae buffers. I guess any BJT will do.

 I don't know anything about laying out a PCB either, so I guess I have to learn how to handle Eagle and read about ground planes, star ground, supply planes, parasitics etc. There are some interesting transistors from Philips - matched pairs PNP/PNP NPN/NPN and PNP/NPN, BC847BS, 857BS and 847BPN. They are _very_ small, SOT363-6, 1 x 2 mm, maybe too small for manual soldering. Otherwise BC850/60 (and BCX53/56, output) could be used, but it would be tedious to match SMD transistors. Some kind of socket would be needed.

 I don't have an oscilloscope, so I don't know how the amp behaves in real life other than from listening, measuring offset, offset drift and current draw. This is a risk project.


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## MASantos

Quote:


  Originally Posted by *NelsonVandal* /img/forum/go_quote.gif 
_Why BC327/37? Because I have a bunch of them from the time when I struggled to find matching pairs for Sijosae buffers. I guess any BJT will do.

 I don't know anything about laying out a PCB either, so I guess I have to learn how to handle Eagle and read about ground planes, star ground, supply planes, parasitics etc. There are some interesting transistors from Philips - matched pairs PNP/PNP NPN/NPN and PNP/NPN, BC847BS, 857BS and 847BPN. They are very small, SOT363-6, 1 x 2 mm, maybe too small for manual soldering. Otherwise BC850/60 (and BCX53/56, output) could be used, but it would be tedious to match SMD transistors. Some kind of socket would be needed.

 I don't have an oscilloscope, so I don't know how the amp behaves in real life other than from listening, measuring offset, offset drift and current draw. This is a risk project._

 

Maybe you could kindly ask AMB's help on this. Layout help, simulating and even parts selection. He has the tools and equipment and certainly has the knowledge. Time might be a problem!


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## TzeYang

I believe the best route to go is to somehow use only matched transistors on the input stage.

 The rest can go unmatched i'm sure.

 output transistors should use the regular MJE243/MJE253/BD139/BD140s and the input transistors can just go with 2SK170/2sJ toshiba fets.

 Rest could very well be SMDs without much trouble. Matching isnt needed.


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## MASantos

Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_I believe the best route to go is to somehow use only matched transistors on the input stage.

 The rest can go unmatched i'm sure.

 output transistors should use the regular MJE243/MJE253/BD139/BD140s and the input transistors can just go with 2SK170/2sJ toshiba fets.

 Rest could very well be SMDs without much trouble. Matching isnt needed._

 

Even though not needed, matching would reduce DC offset problems and reduce THD right?


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## NelsonVandal

Quote:


  Originally Posted by *MASantos* /img/forum/go_quote.gif 
_Maybe you could kindly ask AMB's help on this. Layout help, simulating and even parts selection. He has the tools and equipment and certainly has the knowledge. Time might be a problem!_

 

I think he's tired of my stupid questions at Headwize.com, is working on his speaker amp, and I think he prefers FET transistors.

  Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_I believe the best route to go is to somehow use only matched transistors on the input stage._

 

That's why I was thinking of the above mentioned transistor pairs. They should be very well matched.





  Quote:


 output transistors should use the regular MJE243/MJE253/BD139/BD140s and the input transistors can just go with 2SK170/2sJ toshiba fets.

 Rest could very well be SMDs without much trouble. Matching isnt needed. 
 

I was thinking about SMD in the output stage as well.

 2SK170/SJ74 are JFETs, and I want an all bipolar amp. With those transistors it would be a JISBOS. JISBOS isn't really suitable for portable use since the output swing is far lower with the need of 12V supply, and the current draw of the driver stage is very high. I'm going to try these transistors in a pocket Gilmore lite instead. I have some of those transistors intended for JISBOS, but I think a Gilmore is more interesting.


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## NelsonVandal

Damn! I've found that my ground channel still has too high output impedance. 0.05 Ohm (both measured and simulated) is too much! I got a better sound using OPA690 in that position, but since this is supposed to be an all discrete amp, I've made a simple discrete opamp. In unity gain it has an output impedance of 0.004 (simulated, not measured). If I remove the emitter resistors in the differential pair, the output impedance is 0.002 Ohm. The difference in sound is a fuller bass/lower mid and less hollow center. I can't really say this discrete opamp sounds better than OPA690, but now it's there. The current sources are JFETs in real life. Not shown in the schematic is a ferrite bead on the output. It's needed for stability. R5/R13 is a potentiometer. 

 I could lower the output impedance to "unmeasurably low" levels by using a darlington pair, but then I'd lose some voltage swing - down to +/- 3.5V. I guess that would be OK too.






 So now there's another project waiting to be fulfilled - a discrete opamp-amp. The voltage swing is about +/- 4 V at 10 V supply, so it could be used with a single 9V, and be played loud.


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## amb

Just curious, what method are you using to measure the output impedance?

 In your discrete opamp above, having that current mirror _and_ removing the degeneration resistors will result in a large amount of gain in the first stage. Your VAS stage also has very high gain. This of course results in a lot of loop feedback and reduces output impedance. But a huge open loop gain and lots of feedback is what most modern discrete designers strive to avoid. Also keep in mind that transistor degeneration gives you local feedback and linearizes the devices.


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## NelsonVandal

I use a 1 kHz sine wave and measure the voltage drop by adding a 10 Ohm parallel resistor. I've set up one of the L/R channels in the same manner as the old ground channel. I don't know how to measure the ground channel directly. That's why I haven't measured the new ground channel.

 Zo= Rl(V-Vo) / Vo.






 I know that I've added a lot of negative feedback, and it's probably bad in a way, but an active ground channel has to have a very low output impedance to sound good. At least that's what I've found by listening. Maybe I should use a CFB output stage or a Darlington instead. CFB seems a bit scary and Darlington reduces the output swing. Everything is a compromise. I think I'm going to be inspired by this link A new happy AKSA user , and I'll probably remove the current mirror, add a Darlington output stage and lessen the open loop feedback. AKSA seems to be the most regarded DIY amp, in close competion to JLH, so why not be inspired. I haven't seen the AKSA circuit, but it's said to be very similar to Project 3a. I've tried to tweak the JLH for 10V supply, but with no success.

 I'm very glad for suggestions to improve this amp, or ideas for an even better portable amp. This amp sounds very good though.


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## NelsonVandal

BTW, constant current sources and current mirrors are hard to replace in portable amps because of the voltage drop when the battery drains.


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## NelsonVandal

I did some tweaking again and reduced the gain by dropping the current mirrors for resistors. Like I thought, some of the fullness - the center information is lost, and the amp sounds leaner and a bit harsh. This could be placebo, but I think it's essential that the center amp has a very low output impedance.

 I wish the sound was the same, or even improved, because the amp is more stable at this lower gain. There's some offset drift when using current mirrors. I wonder if there's a way to improve stability. I've changed the VAS transistor (BC337) to BC550 and use a larger cap (100pF), but the slight instabiltiy is still there. The sound is the same I think.

 I also added a VBE multiplier with a pot instead of diodes. It doesn't change stability, but I want to find out how the amp sounds class AB.

 Finally, I've tried to use JFETs as differential pair, 2SJ74. I like the sound of bipolars better. The slight instability was the same.

 This is the setup without current mirrors:


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## NelsonVandal

Amb's right of course, he always is. I should've worked more on the original circuit with low global feedback. Even though the soundstage is much better with the opamp as ground channel, there's now more of a "transistor sound".

 Sigh! So I rebuilt the ground channel to the original circuit and added a diamond buffer. I haven't measured the output impedance, but it simulates very low. The buffer is hanging on cabels without adequate decoupling etc, and it's unstable without a zobel network on the output. The buffer is one of my old class AB buffers with JFETs as current source. I think it's biased to something like 8 mA quiescent. There's no hole in the center of the soundstage, it doesn't sound thin, and I think it's less transistor sounding, but I haven't done any extensive listening since it doesn't fit the box right now. It will be a bit problematic to squeeze it in. This could be the solution
	

	
	
		
		

		
		
	


	




. I certainly hope so. I'm starting to get tired of doors leading to other doors.

 The values for R16/24 is in real life 10 Ohm and R10/15 4.7 Ohm.


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## TzeYang

nelson, can you share your spice models with me? I really do not have the proper working BC550/560 and BD139/BD140 models. Been using TINA TI but it's sorta buggy in a way. i very much prefer LTSpice.

 thanks in advance 
	

	
	
		
		

		
		
	


	




 EDIT: Whoops, saw your diamond buffer output stage.Is it really necessary to add another different output stage for it?


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## amb

Adding a diamond buffer (which is two cascaded complementary emitter followers connected upside down) to an existing complementary emitter follower is truly redundant. Too many stages = more poles from junction capacitances, and leads to complex high frequency amplitude and phase deviations, hence feedback loop stability issues.


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## NelsonVandal

Quote:


  Originally Posted by *amb* /img/forum/go_quote.gif 
_Adding a diamond buffer (which is two cascaded complementary emitter followers connected upside down) to an existing complementary emitter follower is truly redundant. Too many stages = more poles from junction capacitances, and leads to complex high frequency amplitude and phase deviations, hence feedback loop stability issues._

 

Amb, can you help me on this one. What's the best solution/compromise for low output impedance, low global fb, high voltage swing? Do you think it would be better to use a CFB on the output? I've tried this "classic current feedback topology", the picture below, with no success.








 And TzeYang, I can post the spice models tonight. You haven't got a spice model for AD797 ready to run in LTSpice?


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## amb

While low output Z is a very good thing, I wouldn't obsess about it when it's lower than some milliohms range. The headphone jack contact will add more resistance to that alone, not to mention the headphone cable.

 You schematic in the above post still has FIVE stages in it, compared to the β22's three (and the latter is a far more complex amp)...

 Just a comment: Your goal of an all-discrete portable amp is noble, but I think you're seeing now that given all the constraints (size, power consumption, performance, output current and voltage swing, complexity, ease-of-build, etc), it is difficult to do without sacrificing _something_. This is why I went with high output current rail-to-rail opamps in the Mini³. I found no better way to do a small portable amp that would really delivers on all those fronts. I am not trying to discourage you, but I had traveled down that road before (and you know I like all-discrete circuits)...


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## NelsonVandal

Quote:


  Originally Posted by *amb* /img/forum/go_quote.gif 
_Just a comment: Your goal of an all-discrete portable amp is noble, but I think you're seeing now that given all the constraints (size, power consumption, performance, output current and voltage swing, complexity, ease-of-build, etc), it is difficult to do without sacrificing something. This is why I went with high output current rail-to-rail opamps in the Mini³. I found no better way to do a small portable amp that would really delivers on all those fronts. I am not trying to discourage you, but I had traveled down that road before (and you know I like all-discrete circuits)..._

 

Yes, I've wondered why you settled for monolitic opamps in Mini3. Now I know, but I haven't quite given up yet. I like the sound of this amp better than my chipamps.

 I still want to try a battery powered Gilmore lite. What about adding a complementary differential VAS to it? Wouldn't it be something like a simplified B22 without the cascoding? Did you ever consider a simplified B22 when you did Mini3?

 About adding a diamond buffer after a complementary emitter follower, isn't this how it's done in a lot of amps like PIMETA, PPA, LISAIII etc. Isn't the final stage in a lot of opamps a complementary emitter follower.


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## amb

Quote:


  Originally Posted by *NelsonVandal* /img/forum/go_quote.gif 
_I still want to try a battery powered Gilmore lite. What about adding a complementary differential VAS to it? Wouldn't it be something like a simplified B22 without the cascoding?_

 

Yes, it would.

  Quote:


 Did you ever consider a simplified B22 when you did Mini3? 
 

Even vastly simplified and with all-SMD parts, it won't fit the target Hammond 1455C801 case with all board-mounted jacks/pot/battery. It would also make the amp much harder to build and set-up, and the build cost would be much higher too. The circuit topology is not optimized for such low supply voltages, and it'd won't be rail-to-rail like the AD8397/OPA690 or LMH6643/LMH6642 are.

  Quote:


 About adding a diamond buffer after a complementary emitter follower, isn't this how it's done in a lot of amps like PIMETA, PPA, LISAIII etc. Isn't the final stage in a lot of opamps a complementary emitter follower. 
 

Yes, most of the time the opamp internal output stage is emitter follower. Not always complementary, though. Sometimes it's quasi-complementary (i.e., two NPNs or two PNPs). 

 Avoiding too many stages is one of the reasons why I didn't go the diamond buffer route on the M³, opting for a simpler complementary MOSFET source follower instead. During M³ development I actually built and tested a prototype with a diamond-buffered MOSFET output stage, and it didn't perform as well. Due to the simpler output stage, and avoiding multiloop feedback, M³ has proven to be rock-solid and stable.


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## TzeYang

Quote:


  Originally Posted by *NelsonVandal* /img/forum/go_quote.gif 
_Amb, can you help me on this one. What's the best solution/compromise for low output impedance, low global fb, high voltage swing? Do you think it would be better to use a CFB on the output? I've tried this "classic current feedback topology", the picture below, with no success.








 And TzeYang, I can post the spice models tonight. You haven't got a spice model for AD797 ready to run in LTSpice?_

 


 something to add to this circuit. The output stage is running at Class B. You forgot to add in emitter resistors for your input driver transistors.


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## NelsonVandal

Thanks for your clearifying replies amb.

 TzeYang, the image is just to show the topolygy. When I ran the real circuit, I used those emitter resistors and JFETs as current source/sink. The problem was that the bias to the output transistors just died when the input of the output buffer connected to the collector from the input stage. It simulated perfectly, but even after extensive error searching, I couldn't get it to work properly, so I dropped it.


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## NelsonVandal

I tried a CFP output, and like I thought everything went crazy. I guess you have to have a nice PCB for stability. Now I'm not sure what to do. I think the diamond buffer variant gives the best sound, but it's hard to squeeze it in.


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## NelsonVandal

Hi it's me again. I think I finally got the ground channel like I want it, and this is the "optimal" ground circuit in this amp. I added the diamond buffer against better knowledge and the sound was very "right", but it was a bit unstable, just like amb predicted. So I tried to do the "classic current feedback amplifier" circuit again. This time it works! I don't know what went wrong the first time. Now the diamond buffer isn't added to another emitter follower.

 There are no diamond buffers in the left/right channels, just the complementary emitter follower. I don't think a highish output impedance is an issue there, but it certainly is in the ground channel.

 Now I just have to add some current sources instead of R17/18, replace some junk carbon resistors and maybe resolder some ugly joints (messy after a lot of tweaking), and I think I'm satisfied. I've got a couple of boards for the 2 ch amp with some kind of discrete rail splitter made by Nico Ras. I'm looking forward to compare it to this 3 ch amp.


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