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Old 4th January 2009, 10:02 AM   #1
jitter is offline jitter  Netherlands
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Default unhappy marriage (oscillations)

About ten years ago I built myself a headphone amp that has been working flawlessly for all this time.

I recently purchased a Micromega DAC1 which sounds great. But if I hook it up to the headphone amp directly, I get some funny noises. It sounds like oscillations to me. They're not steady, keep coming and going, are dependent on the volume setting, can be stopped just by touching the frame. They can be alo be evoked just by turning the volume knob as if you were tuning into a radio station.

The DAC1 seems OK, if I hook it up to a regular Marantz amp: no problem. The headphone amp hooked up to anything but the DAC1 is also fine. I'm assuming it has something to do with the output stage of the DAC1 reacting to the input of the headphone amp.

The problem seems to be be dependent on the interlink used. Cheapo or long HQ interlinks are OK. Short HQ interlinks and the funny noises are back.

Has this got something to with the capacitance of the interlink used? I ask this because there seems to be no kind of HF bypassing on the inputs of the headphone amp whereas my Marantz amp has ceramic capacitors on every input.

The attachment is of the L channel input of my headphone amp. P1 is the volume control.
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Old 4th January 2009, 10:35 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
you could add a 47pF to 100pF in parallel to the 22k Zin resistor.
100pF may reduce the treble more than you like, particularly at high volume settings (worst at -6dB).
It might help cure your problem. I suspect the Micromega is sending some/much non audio rubbish along with the wanted output.
Consider also adding 100pF directly across the input RCA, input pin to earthing tag.

BTW,
have you ever experimented with that 22uF DC blocking capacitor? A 4.7uF metallised polypropylene film would be adequate.
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Old 4th January 2009, 10:46 AM   #3
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Since there is no problem with the Micromega in another setup, I doubt it is that unit. The giveaway is the difference with different interlinks. I looks to me that with the HQ interlinks you are getting some radiated RF on the cable. Some expensive interlinks are made low-cap by skimping on the screening. Is this a 'directional' interlink?

Andrews' tip will probably work. Or you can re-label your interlinks

Jan Didden
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Old 4th January 2009, 12:25 PM   #4
jitter is offline jitter  Netherlands
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Thanx for the input!

Quote:
Originally posted by AndrewT
[B]Hi,
you could add a 47pF to 100pF in parallel to the 22k Zin resistor.
100pF may reduce the treble more than you like, particularly at high volume settings (worst at -6dB).
This was certainly something I had in mind. Tomorrow I will get some different value ceramics. (BTW, the Marantz seems to use 151 (=15p?) between input pin and earth-pin and another 103 (=1n?) between input pin and ground-plane, but I still have to take a closer look at that to be sure.

Quote:
It might help cure your problem. I suspect the Micromega is sending some/much non audio rubbish along with the wanted output.
The opamps used in the (unbalanced) output are actually video opamps (LT1227) that seem to be DC-coupled to the output (there are multiturn potmeters close by, presumably for offset-nulling). Unfortunately I don't have schematics of the DAC1.

Quote:
Consider also adding 100pF directly across the input RCA, input pin to earthing tag.
I will, thanx for the tip.

Quote:
BTW,
have you ever experimented with that 22uF DC blocking capacitor? A 4.7uF metallised polypropylene film would be adequate.
Not yet, I just put Roedersteins EB-GPF in when I built the thing. But now that you mention it, I might experiment a little with MKT.

Quote:
Originally posted by janneman
[B]Since there is no problem with the Micromega in another setup, I doubt it is that unit. The giveaway is the difference with different interlinks. I looks to me that with the HQ interlinks you are getting some radiated RF on the cable. Some expensive interlinks are made low-cap by skimping on the screening. Is this a 'directional' interlink?
It doesn't seem to be a screening/shielding issue. And in another setup the headphone amp is also fine. There seems to be some unwanted interaction between the two.

I tried the following interlinks:
- Oehlbach 'NTV-C30vvs' (approx. 60 cm between connectors)-> problematic.
- van den Hul 'The Source' (approx. 56 cm) -> problematic.
- cheapo 1 (approx 90 cm) -> no problem.
- cheapo 2 (approx 90 cm) -> no problem.
- cheapo 3 (approx 160 cm) -> no problem.
- cheapo 4 (approx 300 cm) -> no problem.
- MITerminator '5' (approx. 95 cm) -> no problem.
- Transparent 'MusicLink Plus' (approx. 95 cm): no problem.
The latter two are directional.

The two problematic interlinks are actually shielded very well, especially the Oehlbach. It has two (isolated) conductors for the signal as well as mesh AND foil shielding around those two.

I'm assuming the the longer interlinks have a higher capacitance than the first two preventing oscillations.

I'll report back soon if some ceramics will solve the problem.
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Old 4th January 2009, 01:20 PM   #5
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So the shorter ones are problematic. Yes, probaby you and Andrew are right and it is just a matter of 'missing' capacitance.
Intersting problem, though.

Jan Didden
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Old 4th January 2009, 02:00 PM   #6
jitter is offline jitter  Netherlands
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I just did some measurements of which the results seem to point in this direction.

I remembered I had a Velleman PC-DSO stashed away somewhere that I built ages ago. The software needed to run this 'scope also has a spectrum analyzer on board. Nice.
First I hooked up the probes to the input, but that was no good: they prevented the oscillations from occurring, so I moved them to the output (headphones connected).

I looked at some FFTs and they comfirm what I hear. Clearly above the noise floor I can see several frequency components moving to and fro in (opposite directions, sometimes crossing one another). With careful turning of the volume control knob I "steadied" one of those in the audioband. For reference I played a 5 kHz/-20 dB testsignal (test-CD). See attachment.

I also did some measurements with the rudimentary C-meter in my Extech EX830 on some of the interlinks.

Oehlbach: 0.06 nF
vdH: 0.03 nF
Cheapo 1: 0.12 nF
MIT: 0.30 nF
Tr: 1.24 nF
All values are corrected for the approx. 0.1 nF measure-lead capacitance. (BTW, the probe's added capacitance is only about 0.04 nF, so probably a 47 pF across the input is where I'll start.
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Old 4th January 2009, 02:17 PM   #7
jitter is offline jitter  Netherlands
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And another experiment. Having no capacitors handy I used RCA Y-splitters on the input of the headphone amp (HA).
I checked with only the Oehlabachs between DAC1 and HA -> problem. When I add another interlink to the Y-splitter (other end just hanging loose, of course) -> problem solved! That's gotta be the extra few pF of capacitance at work...

This must be one of those "worst case" scenarios that just happened to happen with my setup.

Edit: spectrum with one Oehlbach interlink.
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Old 4th January 2009, 02:21 PM   #8
AndrewT is offline AndrewT  Scotland
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which seems to confirm that the headphone amp is susceptible to the HF rubbish that the Micromega is sending out with the audio signal.
i.e. there are two problems, not just the amplifier.
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Old 4th January 2009, 02:31 PM   #9
jitter is offline jitter  Netherlands
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I think so, I just measured the spectrum up to 4 MHz with one interlink and again with an extra parallel on the input. There's a lot of rubbish around 700 kHz (see post #7).

Same measurement but with extra capacitance:
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Old 4th January 2009, 02:37 PM   #10
jitter is offline jitter  Netherlands
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One last image of the noise of the DSO itself (input switched to GND) to rule its shortcomings out:
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