How to measure JLH Class A with a Scope

[Mooly]

Hello Chris,
What you are seeing is called overshoot, the reason why -- well thats another thing altogether. Have you used your own wiring layout for constructing it all ? I can't remember now without re reading it all. Is it on PCB's ?
In all honesty this is where it gets a bit difficult now. There are so many posible reasons why. It could be the way the returns to the zero volt line are made. Every piece of wire has to be treated as a potential "resistance" that could cause an unwanted voltage (signal) to appear across it. The returns for the C4/C2 & R2 are the most important in the amp. They should all go to the star earth not to just any "convenient" earth. I am not saying you have done this -- it's just something to bear in mind.
C2 the 330 pf -- thats fitted is it.
I don't really know what else to advise Chris, at the end of a keyboard there are so many posible pitfalls.
If you are really keen you could try rewiring one channel at a time to a star grounding scheme.

[chris ma]

Ok,

then in grossly/ losely term when amp showing overshot behaviour what ill effect it would have to the sound quality of reproducing?

What do you mean C2 330pf ?
This amp is point to point hard wired, no pcb , the butcher style


Sorry C2 yes I have C2 intalled in amp

[chris ma]

Hey Karl,

Problem identified. It was due to the wiring of the volume pot I just move the aligator clip and the wiring a bit and it changes the ringing to no overshot.

It was due to this mess of wires

[chris ma]

So I should have a better setup for the pot and wiring instead of loose wires soldered to the pot and to 4 aligator clips. And they are not shielded. Now the trace look exactly the same as the CD player connected direct to the scope except the vertical divisions is more with the amp output control by the pot.

This is the amp trace at 1kHz



PS. now I twisted the wires together and the overshot can be reduced now. It seems the amp is reproducing the squarewave without adding further extra freebies.

[Mooly]

That looks great !!. It's all in the detail isn't it. Earlier on you asked about the quiescent current and I mentioned 1.6 Amp for the original. The updated JLH (not yours exactly -- the one I sent you) mentions I think 2 amps.
What you may find interesting to do is to connect your 'scope across the speaker when it's all playing and see just what voltage you actually put out when it's turned up loud. Try and calculate then what the peak and RMS power is into 8 ohm. You may be suprised how little power you need for normal listening. For ex 6 volts peak (not peak to peak) would be 4.5 watt's peak into 8 ohms. If it were a sine wave swinging 6 volts above and below ground that would then be 12 volts peak to peak or 12/2 which is 6 divided by root 2 ( 12/2=6. 6/1.414=4.24 watts RMS)
So whats next ???

[chris ma]

I will check and re-bias the other 3 JLH amps before I build more.
I am using regulated PSU Mj15003 for mid, CLC PSU Mj21194 for high and Chipamp for subs doing active 3 ways per channel.

These JLH amps are so picky about the matching of output transistors may be I should build a matching tool with regulated power supply? Any suggestions?

[chris ma]

Originally posted by Mooly
What you may find interesting to do is to connect your 'scope across the speaker when it's all playing and see just what voltage you actually put out when it's turned up loud. Try and calculate then what the peak and RMS power is into 8 ohm. You may be suprised how little power you need for normal listening. For ex 6 volts peak (not peak to peak) would be 4.5 watt's peak into 8 ohms. If it were a sine wave swinging 6 volts above and below ground that would then be 12 volts peak to peak or 12/2 which is 6 divided by root 2 ( 12/2=6. 6/1.414=4.24 watts RMS)
So whats next ???

Do I set the scope at DC or AC ?
I guess it would have to be AC.

[Mooly]

Think about it The output is at zero volts DC. You only need set it to AC coupling if the signal you want to look at is sat on top of a DC voltage. Yours isn't so use DC coupling. You will find if you switch back and forth there is no difference.

[torroid88]

Chris,

You might want to read these freebies from TEK:

http://www.tek.com/Measurement/App_N...60W_6053_9.pdf

http://www.tek.com/Measurement/App_N...03W_8605_2.pdf

They outline how to make measurements with your scope ...

You could also find the User's Manual for your scope at the TEK site ...

[chris ma]

Thank you very much