A138b - DC offset?

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jasonjasonson
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A138b - DC offset?

Post by jasonjasonson » Tue Feb 04, 2020 10:08 am

So, I recently noticed my signal distorted somewhere and started looking for clues.

Multed the sine-output of my Doepfer A-110 VCO to input 1 of my oTool (green wave on attached image) and input 1 of my 138b mixer. Signal was displayed, as expected as a full, perfectly centered +/- 5V sine wave.

I then multed the output of my 138b mixer to input 2 of my oTool (red wave on attached image) and to my speakers. The sine wave now sounds clipped and I noticed in the oTool that input 2 was offset by about -3V.
All the other inputs of the A138b mixer give the same result and if I swap input 1 and 2 in the oTool, the DC offset also changes (so the oTool is fine).

I also have the A138a linear version and I know you can generate DC offsets with that if you don't plug anything into the first socket (that works as expected), but if I turn my first input all the way to zero, I have no DC offset at all (as expected).

I can't get the same result with the A138b, every input shifts downwards by -3V, so my guess is that the 138b generates an (unwanted) DC offset and is therefor broken, am I correct?
IMG_3236.jpg
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Navs
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Re: A138b - DC offset?

Post by Navs » Tue Feb 04, 2020 10:20 am

There could be two things at play.

To test the mixer alone, just patch the output to the O'Tool and check for DC.

As for the distortion, is it possibly just the level on the mixer, i.e. is it cleaner if you set the input pot to 30%?

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cptnal
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Re: A138b - DC offset?

Post by cptnal » Tue Feb 04, 2020 10:29 am

Spotted this in the manual. Perhaps check the jumper?

"From about middle of 2004 the module is equipped with an additional offset function for input 1. An internal jumper is used to decide if control input 1 works as a positive or negative DC offset generator provided that no plug is inserted into input 1."

jasonjasonson
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Re: A138b - DC offset?

Post by jasonjasonson » Tue Feb 04, 2020 10:51 am

Navs wrote:
Tue Feb 04, 2020 10:20 am
There could be two things at play.

To test the mixer alone, just patch the output to the O'Tool and check for DC.

As for the distortion, is it possibly just the level on the mixer, i.e. is it cleaner if you set the input pot to 30%?
Ok, checked it and did some further testing, even more confused.

0) Mixer output straight into interface/speakers: distorted sound. Of course I could turn the input to 10% (to not get a clipped sine wave), but that renders the mixer virtually useless.
1) Mixer output straight into oTool: no DC offset visible
2) Mixer output with (any) tiptop stackable (passive multi), one output to oTool, other output to speakers: -3V DC offset and distorted/clipped sine.
3) Mixer output to buffered multi, one output to oTool, other output to speakers: no DC offset (and no distorted sound, so the buffered multi "fixes" my offset?)

:hmm:

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Re: A138b - DC offset?

Post by jasonjasonson » Tue Feb 04, 2020 11:43 am

cptnal wrote:
Tue Feb 04, 2020 10:29 am
Spotted this in the manual. Perhaps check the jumper?

"From about middle of 2004 the module is equipped with an additional offset function for input 1. An internal jumper is used to decide if control input 1 works as a positive or negative DC offset generator provided that no plug is inserted into input 1."
Yeah, I read that aswel (and the more detailed description):

"The pin header labelled JP4 (located behind input 1 on the pc board) is used for this option. With no jumper on JP4 the offset option is disabled. If a jumper is put to JP4 in the right position (near the edge of the pc board) a positive offset voltage (~ 0...+5V) is generated by control 1 provided that no plug is inserted into socket 1. If a jumper is put to JP4 in the left position (direction to the front panel) a negative offset voltage (~ 0...-5V) is generated)."

Now I checked the back of both my modules (A138A and A138B), they both look exactly the same. It's a bit unclear to me what they mean with "jumper", I see a connection labeled "JP4" (encircled it on the image), but there's no jumper, left or right position.
In any case, my 138a (linear) mixer does have the DC offset feature (if I turn input 1 with nothing connected, DC shifts, as expected, handy feature), my 138b seems (to me) to (unintentionally) offset DC, but turning or plugin in something in input 1 doesn't make any difference.

Since both look exactly the same, I don't think the issue is jumper-related.
IMG_7BC74411A8DD-1.jpeg
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Navs
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Re: A138b - DC offset?

Post by Navs » Tue Feb 04, 2020 3:37 pm

jasonjasonson wrote:
Tue Feb 04, 2020 10:51 am
0) Mixer output straight into interface/speakers: distorted sound. Of course I could turn the input to 10% (to not get a clipped sine wave), but that renders the mixer virtually useless. ....

tiptop stackable ...
...output to speakers
A few more things ;)

Does setting the mixer input to 10% solve the distortion? If so, then I would suspect the LM324. Search for 'Pimp my Doepfer' (or similar) here. There are cleaner quad opamps, like the TL074. Is the module 2nd hand or is Doepfer still using these? Anyway, chip aside, that would mean it's simply a question of headroom.

Point two: what is your signal chain? Are you going directly from the modular to your speakers somehow? Then, it's no wonder you're getting a distorted signal - you need to attenuate before hitting, e.g., an audio interface.

Do you have any normal cables you could try instead of the Stackcables?

The jumper is the plastic bridge above the marking JP3 ... unfortunate if the description says JP4 :doh:

If you have the O'Tool, you can locate the source of your offset ... if that's what it really is. Just be methodical :tu:

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ersatzplanet
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Re: A138b - DC offset?

Post by ersatzplanet » Tue Feb 04, 2020 4:07 pm

jasonjasonson wrote:
Tue Feb 04, 2020 11:43 am
cptnal wrote:
Tue Feb 04, 2020 10:29 am
Spotted this in the manual. Perhaps check the jumper?

"From about middle of 2004 the module is equipped with an additional offset function for input 1. An internal jumper is used to decide if control input 1 works as a positive or negative DC offset generator provided that no plug is inserted into input 1."
Yeah, I read that aswel (and the more detailed description):

"The pin header labelled JP4 (located behind input 1 on the pc board) is used for this option. With no jumper on JP4 the offset option is disabled. If a jumper is put to JP4 in the right position (near the edge of the pc board) a positive offset voltage (~ 0...+5V) is generated by control 1 provided that no plug is inserted into socket 1. If a jumper is put to JP4 in the left position (direction to the front panel) a negative offset voltage (~ 0...-5V) is generated)."

Now I checked the back of both my modules (A138A and A138B), they both look exactly the same. It's a bit unclear to me what they mean with "jumper", I see a connection labeled "JP4" (encircled it on the image), but there's no jumper, left or right position.
In any case, my 138a (linear) mixer does have the DC offset feature (if I turn input 1 with nothing connected, DC shifts, as expected, handy feature), my 138b seems (to me) to (unintentionally) offset DC, but turning or plugin in something in input 1 doesn't make any difference.

Since both look exactly the same, I don't think the issue is jumper-related.

IMG_7BC74411A8DD-1.jpeg
JP3 not JP4. The manual must be wrong.
-James

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cptnal
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Re: A138b - DC offset?

Post by cptnal » Tue Feb 04, 2020 4:16 pm

There's also this... http://www.doepfer.de/a100_man/A138_Jumper.pdf

(Sorry if we've moved on from jumpers... I've been AFK. :oops: )

jasonjasonson
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Re: A138b - DC offset?

Post by jasonjasonson » Wed Feb 12, 2020 5:12 am

Thanks for your thoughts & input!
It's been a while, but it's the school's modular, so I didn't have it with me all the time to do further testing.

I did yesterday however and found out that the 138b is fine. Turns out the culprit is the patchbay.

When I output the A138b straight into an audio interface, a mixer, headphones etc... all is fine (soundwise as on the scope)
However, when I output the A138b into the Samson S Patch, the sound distorts and on the scope the wave shifts some -3V downwards.

I have no clue why, however. Thought it maybe had something to do with plugging an unbalanced output into a balanced patchbay, but then again, the interface inputs are also balanced and when I plug it straight into the interface (bypassing the patchbay), it doesn't happen.
So maybe the entire patchbay is DC offset somehow? But then again, why does it only happen if I take the A138b mixer output? If I take the output of the A110 oscillator and plug it straight into the patchbay's interface input, it sounds and looks normal. If I take the output of the A110, send it to the A138b mixer and plug it into the same patchbay input, it shifts -3V and sounds distorted.

Anyone? Or should I call Hercule Poirot? 8-)

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