F5 measurement/tuning

[doolav]

Sorry was busy at work, so could not get the pics earlier. The good news is the amp is still alive and sound good (at least to my unspoiled taste). With some good speakers was hard to believe that there is no subwoofer involved.
The pictures were taken by web camera hence the quality...
Here is the overview and the source resistor proximity to the FET (which is buried under the thick aluminium bracket)

[doolav]

Originally Posted by h_a

Well, commonly signal generators assume a 50 Ohms load and for that load they're calibrated.

So, unless your signal generator is a different type, you need a 50 Ohm resistor (higher wattage type for serious signal) in parallel to the amp input; also I recommend koaxial 50 Ohms wiring from the generator to the input.

Hannes

- yes, mine says 50 Ohm at its output too, but I don't have such cable, only 75 Ohm. Would 50/75 Ohm impedance matching adaptor help?

[DJNUBZ]

You cannot test an amp using a speaker as a load. I thought I was having problems as well until someone told me that

[doolav]

Originally Posted by DJNUBZ

You cannot test an amp using a speaker as a load. I thought I was having problems as well until someone told me that

OK, that explains some of my results then. I don't have 22Ohm resistors as Andrew T. advised, so have to wait to make 'proper' tests.
I think I also need a 'proper' (50 Ohm) coaxial cable to connect signal generator to the amp.

Nonetheless, here are some of my current measurements - taken at 10k, 50k, 100k and 200k for both channels. Whatever distortion is contributed by wrong cables/load/etc there is still quite a discrepancy between the channels.

It obviously also degrades badly at higher frequency.

I am going to take it apart to check how this asymmetry may depend on transformers location.

The first row (first 5 images) is for the left channel. Note on the 5th image (probs_swap...) the signal was affected if I did swap the probe's polarity (i.e. connecting probe gnd to the amp hot rather than to the amp gnd). I tried to push/rotate the rca connector - no change. When I swap probe (gnd to the amp gnd gnd, hot to the amp hot), it looks better (pic. 1).
This prob swapping effect is only seen on the left channel.

Thanks,
Alex.

[Nelson Pass]

Interesting.

Probe swap means 0, probably probe not compensated

some ringing between 100 Khz and 200 Khz on 1st ch

weird high frequency loss on 2nd ch

where's House when you need him?

[doolav]

Originally Posted by AndrewT

Add a standard 22r across the output. The power will be ~ 0.14mW

Hi Andrew,

0.14mW or 0.14W (or 14mW)?
How is it calculated?

Thanks,
Alex.

[doolav]

Here are some results with 3x22R [parallel] resistors, as Andrew advised.
The signal in is square 100mVpp (not sure is it same as 100mVac?) at 100kHz.

Looks a bit cleaner at this amplitude, but the general picture is the same - overshoot on the left and h. freq loss on the right.

Any idea what may cause this? (should not be layout, as I ripped of the heatsinks with PCBs, moved them away from the amp and tested each channel individually - same results. The only thing I missed to try - swap the PSUs, and my right ch. trans has some buzz - is it possible it affects the signal in the way described?)

[6L6]

Are you sure you have genuine input transistors in your amp?

[doolav]

Hi 6L6,

having got it from tech-diy, I had assumed so, never tested them.

Does it look like they cause the difference?

Thanks,
Alex.

[6L6]

They are genuine.