Author Topic: Heavy Overtones/Intermodulation

March 28, 2021, 06:38:49 pm
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log8tom

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Hi there,
first of all: Thanks to JPK and the team for the great quality of your modules and the build guides! Unfortunately, some problems came up while building the TS500, and I don‘t know how to solve it. I measured high distortion in the output signal, even with unit switched off! Input signal was sine, 1kHz, 1Vrms. I checked the audio interface and the cables, there was no problem. Sending in a mixed signal (2 sines or more complex material like e.g. an 808 drumloop) causes a lot of sidebands and interference.
I already did all the basic checks (proper components and polarization), resoldered everything, even tested all opamps (one was faulty, I‘m afraid I killed it while testing, but temporarily replaced it by a 072), but the problem persists.
Any hints?
Thanks J.

March 29, 2021, 09:33:47 am
Reply #1

JPK

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If you get distortion with the unit switched off, then the issue is probably in the output stage, U12a/Q1/U13.
Check there is no DC voltage on TP7.
JPK

March 30, 2021, 09:57:20 pm
Reply #2

log8tom

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I probed TP7 with a scope, there was no DC.

I checked my audio interface (RME FF400) again, +4dBu output, pad on line in, to make sure that there is no distortion caused by the interface (maybe that was the case last time). Then, I levelled a 1kHz signal to 1Vrms (about -1,5V...1,5V) at TP7 in "Off" mode. Finally, with TS500's output to max, I set the "drive" to get the same level at TP7 in "On" mode (1Vrms). Tape speed: 15, saturation: middle pos, bias: right pos (flat).

I'll attach some spectrum screenshots: 1= 1kHz, TS500 off; 2= 1kHz, TS500 on; 3= 1kHz+1.5kHz, TS500 off; 4= 1kHz+1.5kHz, TS500 on. The distortion seems to be low or moderate when switched off, but is very present when on, even with low input signals.

I also measured U12a: (2), (3) are around 0,5Vrms input with no DC offset, but (1) is 2Vrms with DC offset of 1V. U12b seems to be unused (?), there are no signals at all.

Finally, I measured U13 (below the output trafo). I don't understand it's purpose, but was wondering if it's ok: (1): -15.5V, (2): -14V, (3): 1kHz, 1Vrms

I hope that this info helps somehow :-)


March 30, 2021, 09:58:18 pm
Reply #3

log8tom

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April 05, 2021, 07:09:22 pm
Reply #4

log8tom

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Any idea, how to go on here?
Thanks ;-)

April 07, 2021, 11:21:35 am
Reply #5

JPK

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This is what the distortion looks like on a bypassed working unit, with a 1kHz sine, 1Vrms signal on input.
It is normal that some distortion remains in bypass because the signal still goes through the in and out transformers and the output amp stage.
JPK

April 07, 2021, 07:29:20 pm
Reply #6

log8tom

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Ok, that looks quite similar to my device in bypass, about 60dB difference between 1st and 3rd harmonic. But what happens when switched on? Could you tell me some hints/testpoints to measure the signal path, so that I can find the issue here? 

April 13, 2021, 09:10:05 pm
Reply #7

log8tom

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Hello,
I still don‘t have a clou, how to get my problem solved. Could you help me? As I told, the problem comes up with the unit switched on.
Thanks and greetings
J.

April 16, 2021, 11:00:43 am
Reply #8

JPK

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You could try to "listen" to the sound you get from TP1, TP2, TP3, TP4, TP5, TP6, TP7 by connecting these points to an input of your monitoring.
Hearing where the distortion starts will give us an indication of where to look for.
JPK

April 17, 2021, 05:57:10 pm
Reply #9

log8tom

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Hi,
thanks for the advide! I measured the spectrum on all testpoints, and there seem to happen something bad between TP2 and TP3. I'll attach some screenshots.
Could you tell me what components I should have a closer look at?
Thanks
J.

April 17, 2021, 06:13:05 pm
Reply #10

log8tom

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Hm, dropping images here doesn't work today :-/

To describe what happens: On TP3, the level of overtones jump up, especially the uneven ones (3kHz, 5kHz, 7...).

April 19, 2021, 04:36:57 pm
Reply #11

JPK

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TP3 is just after the tape saturation emulator circuit.
What is the level displayed on the meter in your test? It should stay green for minimum distortion.
Can you check the aspect of the sine on TP2. It should be clean, in particular with the TAPE SAT switch set to minus. 
Look for wrong component value or bad solder joints on U2, U3, U4, U5, R11...R25, C6...C22.
JPK