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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old 12th April 2012, 04:15 PM   #2361
mikelm is offline mikelm  England
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Quote:
Originally Posted by Bonsai View Post
JLH . . . a genius who understood the value of simplicity!
I agree about the genius but he also understood the value of complexity is his highly respected later designs
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Old 12th April 2012, 04:32 PM   #2362
PMA is offline PMA  Europe
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Quote:
Originally Posted by AndrewT View Post
I am trying to adjust the tempco of the input transistor and it's CCS to minimise drift in the output offset.
I have been at it for over 4 hours today and a couple of hours yesterday and I'm not in the ballpark yet. I think I am getting closer.
Andrew, I want to keep the original capacitively coupled version. One of the reasons is the possible output DC offset instability, and frankly speaking I do not like the 'improved' circuit designs that appeared after the original version. I bypassed the output capacitor with the 3u3 high quality foil type, and added 2 resistors - 1st to discharge input capacitor to ground (1M) and 2nd to discharge output capacitor when load is disconnected (10k). I also had to add 220pF capacitor + 1k resistor as an input RC, for the reason of faster modern devices used, it was necessary. Output devices are MJL21194 and they do a great job.
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Old 12th April 2012, 08:49 PM   #2363
Jonathan Bright is offline Jonathan Bright  Australia
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Hi kp93300. Noticed your post #2349 re; bias adjustment. I am not sure if you particularly want to use the original 1969 circuit but JLH put out an amended design the following year with a better system for the bias. Are you aware of Geoff Moss's site that is about 95% JLH Class A? Mountain of info' on this amp. Just Google "Class-A amp site" and it should come up. Cheers, Jonathan
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Last edited by Jonathan Bright; 12th April 2012 at 08:51 PM.
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Old 12th April 2012, 09:54 PM   #2364
kenpeter is offline kenpeter  United States
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The bootstrap resistors should be equal, for least parasitic load on the output.
I have no idea why JLH's resistors are unequal. 355 + 355 would give the same
series resistance and current, but roughly half the parasitic load of 150 + 560.
The corner frequency is lower for the same value bootstrap cap too.

Last edited by kenpeter; 12th April 2012 at 10:03 PM.
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Old 12th April 2012, 10:29 PM   #2365
AKSA is offline AKSA  Australia
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Try LTSpice with different bootstrap resistor ratios, Ken. I did this work with my amps a few years ago, all use bootstraps. You find that the harmonic profile is altered with different ratios. Very interesting.

Hugh
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Old 13th April 2012, 09:39 AM   #2366
homemodder is offline homemodder  Portugal
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Quote:
Originally Posted by AndrewT View Post
I am trying to adjust the tempco of the input transistor and it's CCS to minimise drift in the output offset.
I have been at it for over 4 hours today and a couple of hours yesterday and I'm not in the ballpark yet. I think I am getting closer.
Andrew dont forget to temp compensate the current source as well, only way youll get it 100 percent stable, if youre using the 2 transistor ccs, you could just use a low voltage zener in series with the controlling transistor, zeners have closer tempco with transistors.
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Old 13th April 2012, 09:52 AM   #2367
AndrewT is offline AndrewT  Scotland
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Yes I said input transistor and it's CCS.

Last night I gave up with the PCB as is and started to thermally couple Q4 to Q5, but the leg lengths I left were too short (by about 1mm each).
I am not ready to build up the second PCB so Today I will replace those two transistors and thermally couple them to see if that makes a difference.

It is certainly very clear that Q4 moves the offset in one direction and Q5 moves the offset in the opposite direction. And that Q3, Q5, Q6, Q7 and Q8 have a minor effect in comparison to the strong effect of 4&5. I have added a sink to Q8 since it dissipates ~ twice as much heat as Q3.

I am away looking for the link to the schematic.
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Old 13th April 2012, 10:15 AM   #2368
homemodder is offline homemodder  Portugal
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Do you have problem with dc servos, easiest solution.
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Old 13th April 2012, 10:28 AM   #2369
AndrewT is offline AndrewT  Scotland
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I had to report my own unfinished post and this is what I sent.
Quote:
your 30minute edit window expired before I could find and attach the link to the schematic. Why?
Because your system went into daytime maintenance and I had to wait and wait and wait before the Forum would talk to me again.

Please stop ******* us non USA Members about.

Substitute this wording for my unfinished post.
Yes I said input transistor and it's CCS.

Last night I gave up with the PCB as is and started to thermally couple Q4 to Q5, but the leg lengths I left were too short (by about 1mm each).
I am not ready to build up the second PCB so Today I will replace those two transistors and thermally couple them to see if that makes a difference.

It is certainly very clear that Q4 moves the offset in one direction and Q5 moves the offset in the opposite direction. And that Q3, Q5, Q6, Q7 and Q8 have a minor effect in comparison to the strong effect of 4&5. I have added a sink to Q8 since it dissipates ~ twice as much heat as Q3.

JLH 10 Watt class A amplifier
link to the schematic. I am using the MJ15003, BD139/140 & BC560c semiconductor options.
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Old 13th April 2012, 10:34 AM   #2370
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by homemodder View Post
Do you have problem with dc servos, easiest solution.
I have no problem with DC servos.
I contributed to one of the most detailed discussions on DC servos ever held on this Forum.

But the JLH inherently suits AC coupling in it's original form.
The dual polarity version cries out for some form of build that is inherently offset stable.
That is what I am hoping to get closer to.

This same misbehaviour is seen in other circuits and if I can get a handle on this one, I may be able to transfer some of the lessons learned to other cases. I am somewhat in awe at Roender's tempco additions, my mind struggles with that even though he has shown me the answers.

It is likely I will never use this circuit to power a full range nor a bass only speaker. In this case I am likely to revert to the AC coupled single polarity version to drive treble and mid units which need the output capacitor anyway.


I think some forget, that I have two reasons for being here. To learn and to share.
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