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Old 15th December 2012, 09:51 PM   #11
doolav is offline doolav  United Kingdom
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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)
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File Type: jpg overwiev.jpg (564.7 KB, 365 views)
File Type: jpg source_resistor.jpg (30.8 KB, 345 views)
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Old 15th December 2012, 10:16 PM   #12
doolav is offline doolav  United Kingdom
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Quote:
Originally Posted by h_a View Post
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?
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Old 15th December 2012, 10:43 PM   #13
JZatopa is offline JZatopa  United States
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You cannot test an amp using a speaker as a load. I thought I was having problems as well until someone told me that
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Old 15th December 2012, 11:51 PM   #14
doolav is offline doolav  United Kingdom
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Quote:
Originally Posted by DJNUBZ View Post
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.
Attached Images
File Type: jpg L_10khz.jpg (574.4 KB, 323 views)
File Type: jpg L_50khz.jpg (558.8 KB, 289 views)
File Type: jpg L_100khz.jpg (594.7 KB, 266 views)
File Type: jpg L_200khz.jpg (515.1 KB, 45 views)
File Type: jpg probs_swap_only_seen_on_the_left.jpg (642.5 KB, 43 views)
File Type: jpg R_10.jpg (556.3 KB, 43 views)
File Type: jpg R_50.jpg (586.0 KB, 42 views)
File Type: jpg R_100.jpg (568.3 KB, 33 views)
File Type: jpg R_200.jpg (583.5 KB, 32 views)

Last edited by doolav; 16th December 2012 at 12:01 AM.
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Old 16th December 2012, 02:29 AM   #15
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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?

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Old 23rd December 2012, 02:03 PM   #16
doolav is offline doolav  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
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.
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Old 23rd December 2012, 02:49 PM   #17
doolav is offline doolav  United Kingdom
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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?)
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File Type: jpg 100mVpp_100kHz_left.jpg (554.0 KB, 22 views)
File Type: jpg 100mVpp_100kHz_right.jpg (539.7 KB, 20 views)
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Old 23rd December 2012, 03:11 PM   #18
6L6 is offline 6L6  United States
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Are you sure you have genuine input transistors in your amp?
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Old 23rd December 2012, 09:36 PM   #19
doolav is offline doolav  United Kingdom
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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.

Last edited by doolav; 23rd December 2012 at 09:39 PM.
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Old 24th December 2012, 01:57 AM   #20
6L6 is offline 6L6  United States
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They are genuine.
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