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Steady thumping noise in preamp

Unit: MFA Luminescence preamp. Separate power supply, tube regulation, octal tubes, decoupling electrolytic on each tube. Just replaced large coupling caps with Dynamicaps 4 mf

Problem: Not motorboating in the classic sense, but a steady thump, not heavy, short, like a soft rim shot in a snare drum. If I were playing LPs, I'd say it's a steady once-per-second or so sound of a stuck record. But I'm playing CDs. The CD plays, the music is there, but so is the thump. It's much louder on one side than on the other. I'm assuming the cause is on the loud side and is being picked up on the other.

I'm thinking maybe it's a defective Dynamicap (or solder joint, but I'm not there to check it) or a decoupling cap. I'm assuming it's not the power supply because of the disparity between the two channels. Thoughts?
 
What were the capacitors you replaced?

4 microfarad coupling caps.

Does the preamp have negative feedback?

Yes.

It sounds like motorboating i.e. LF instability.

It does, but always associated motorboating with faster sounds, not this slow. But from what I've gathered, I may be wrong. I plan on putting the old coupling cap back in and if that doesn't get rid of the noise, I'll try replacing the decoupling caps. They have some mileage on them.
 
What were the capacitors you replaced?

4 microfarad coupling caps.

Does the preamp have negative feedback?

Yes.

It sounds like motorboating i.e. LF instability.

It does, but always associated motorboating with faster sounds, not this slow. But from what I've gathered, I may be wrong. I plan on putting the old coupling cap back in and if that doesn't get rid of the noise, I'll try replacing the decoupling caps. They have some mileage on them.

Of course there is always the possibility of what I call 'the dripping faucet syndrome' where an issue exits but for whatever reason has not been noticed and after it is, it's nearly impossible to ignore. Are you absolutely certain it wasn't doing it before you changed the caps or are you noticing it now because you're listening for changes (improvements)?

 
It could be that the original preamp was only just stable, so any minor increase in loop gain or phase shift would tip it over the edge. A big peak in gain at a few Hz would probably not be noticed by either the designer or users, provided everything was back to normal by about 20-30Hz. Do you have a circuit diagram?
 

kevinkr

Administrator
Paid Member
It could be that the original preamp was only just stable, so any minor increase in loop gain or phase shift would tip it over the edge. A big peak in gain at a few Hz would probably not be noticed by either the designer or users, provided everything was back to normal by about 20-30Hz. Do you have a circuit diagram?

These caps are actually not within the feedback loops of their respective gain blocks.

I've worked on this particular unit before, (as well as a number of other Lumis) and the OP just needs to get out his scope and track down where this thumping is coming from. It is quite possible that it is originating in one of the phono channels and is talking through the PSU which uses RC decoupling from the regulator which becomes totally ineffective below a couple of Hz.

First identify which gain block is oscillating by removing the appropriate 6DN7/6EM7 from the 2nd stage of each gain block. Once the individual channel gain block is identified (L/R phono, or L/R line stage) it should be possible to identify what has gone wrong.. Bias supply ok?

This unit runs very hot, and many of the electrolytics are probably in need of replacement.
 
Thumping Lumi

Hello Trombone

I have a Lumi, an early A-1 unit, and it started motorboating on me a few months back. Turned out the zener diode chain in the high voltage regulator circuit went caboom. I would start by checking the voltages at tubes. Since your problem is different for one channel, I would suspect the decoupling caps. 4uF is not a lot for that unit, I've used as high as 8 with no problem. Which version do you have? I have a different problem with mine now, I will post it.

PS Ahhh, I wish I could find another Lumi, ar MC Ref...
 
Last edited:

seanfir

Member
2020-08-02 9:59 pm
I've had a similar problem in a VERY dissimilar circuit. I realize there are tons of reasons this could happen, but I'm restoring an old CRUMAR ORCHESTRATOR synth... and some of the transistors were bad, so I ordered some new ones, though the BC style transistors weren't available and the ones in the thing were different than the ones in the schematic. I'll show you the particular part of the schematic I'm dealing with. I'm wondering if the decoupling and or coupling caps in this would even be suspect, or am I just dealing with a controlled feedback circuit that I need to balance with high or lower gain transistors? Also I'm struggling a little with the schematic and the polarities of the transistors in the circuit...it took me DAYS to get this far and I could kick myself, as I had the thing working last night, but it wasn't quite the way I remembered it sounding, so I moved a few around... lol... I will never do THAT again! I've been inside this thing every day for a week now... fixed everything in there, but this "brass" setting, is a separate synth circuit that is basically discrete from the other functions and sounds. And it's the coolest, of course!


Pics of the circuit here.
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Pics of the circuit here.
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