How to measure JLH Class A with a Scope

[chris ma]

Hi Karl,

The version of the high power is here:
http://www.diyaudio.com/forums/showt...08#post1517808

By the time I try it tonight after work it will be two in the morning in your part of the world I think.

I was hoping the moderators would have merged that thread with this one the other day but I guess my english is not good enough to express that point clear enough to be understood precisely.

I will be checking that feedback resistor 2.7k (R8) might need a cap across it as the others were saying. But I only have a 100pf smallest at hand. Do you think that can help?

Best Regards,

[Mooly]

Hi Chris,
See your on line, is it the drop across R15/16 that is not equal.

Edit, Well you were !

[Mooly]

Had chance to read your other post again. What DC voltage have you at the emmiter of Q4, just interested on that one
I suspect the outputs are not sharing the current equally for a couple of possible reasons.
1. The characteristics of the transistors vary enough that the very low 0.1 ohm resistors are unable to allow the currents to equalise properly. Although the efficiency would be slightly worse perhaps try 0.33 ohms or 0.47 ohms and see if that helps.
2. Are the outputs thermally coupled i.e. on the same heatsink ? The characteristics change wildly with changes in temperature.
Adding a cap across R8 won't change the DC conditions unless the amp was unstable and oscillating to begin with. A small current flows through R8 to provide Q4 with its correct operating conditions and this will very slightly ( not what you are getting ) imbalance the current in the output halfs (Q1/Q1A and Q2/Q2A combined).

[chris ma]

Hi Karl,

The top and bottom half are not on the same heat sink but they are identical heat sinks though.

Emmiter voltage of Q4 will be measured across the emmiter and ground? Same as the base of Q3 and ground? I hope it is not too stupid a question.

Best Regards,

[Mooly]

Hi Chris,
Emmiter of Q4, goes to junction of R6/8 and collector of Q6.
Am I looking at the right circuit ?

[chris ma]

Sorry Karl,

That's my mistake I thought emitter is the one that with the arrow

So it is to measure voltage across the the collector of Q6 junction of Q4 emitter and R6/8 and ground then? Thank you for your effort. Will post result later on tonight..

Cheers,

[Mooly]

Hi Chris,
Have a look at this, just been posted today-- if I knew (could get to work more like ) I would put a link in.
Look in the " everything else " forum, the thread you want is "Oscilloscope in audio tutorial" post # 57. Could be just what you want.
As I type this it's next one to this thread

[chris ma]

Thank you Karl,

I will take a look tonight can't do that from work

[chris ma]

Originally posted by Mooly
Had chance to read your other post again. What DC voltage have you at the emmiter of Q4, just interested on that one
.

Hi Karl,

Measured across the emitter resistors (100m) after 20 minutes warm up.

Bottom half
For Q1 R14 ---> 49 mV
For Q1a R16 ---> 85 mV

Top half
For Q2 R13 ---> 37 mV
For Q2a R15 ---> 96 mV

The DC voltage of emitter of Q4 is 605 mV

DC offset stable around -6 mV at speaker outputs
The emitter of Q2a and speaker Vout is 100 mV

DC Supply rails +26.7 V and -26.7 V at C6 and C7

Best Regards,

[Mooly]

Hi Chris,
That's fine, what I was getting at was to make sure C4 was not reverse biased at all -- it's not - just !
Going back to the current inequality- in percentage terms it's a lot. Do try higher value emmiter resistors, that might well do the trick, and even at 2 amps per transistor the loss is small. A 0.47 ohm 3 watt resistor (non inductive) would be fine.


No ripple voltage yet ! Get it measured. If you post the result here at least we know you lived to tell the tale.