Hello,
I have a sudden issue with microphonics in my tube preamp, and I'm wondering if C0G ceramics are the cause. I added a 0.1uF TDK multilayer radial-leaded C0G across the heater pins at the socket of all the signal tubes in the pre, and it did seem to result in a bit blacker background, so I was happy with them, but I've noticed that if I tap on any part of the preamp chassis, I get a fairly loud thump coming through my speakers. It doesn't seem to be related to any particular tube - they all thump if tapped, as does the chassis - and it was definitely not present before the addition of the caps.
I have read that C0G ceramics are much less prone to microphonics than the other ceramic types, so I'm surprised by this. I plan to take them out or perhaps replace them with film caps, but I have two questions:
1) Has anyone else experienced this problem with C0G caps across heater pins?
2) Might I be better off with two caps per tube (from each side of the heater to ground) than with one cap across the heater pins?
Any help/experience sharing is greatly appreciated.
Here is a link to the exact caps used:
Digi-Key - 445-4756-ND (Manufacturer - FK20C0G1H104J)
I have a sudden issue with microphonics in my tube preamp, and I'm wondering if C0G ceramics are the cause. I added a 0.1uF TDK multilayer radial-leaded C0G across the heater pins at the socket of all the signal tubes in the pre, and it did seem to result in a bit blacker background, so I was happy with them, but I've noticed that if I tap on any part of the preamp chassis, I get a fairly loud thump coming through my speakers. It doesn't seem to be related to any particular tube - they all thump if tapped, as does the chassis - and it was definitely not present before the addition of the caps.
I have read that C0G ceramics are much less prone to microphonics than the other ceramic types, so I'm surprised by this. I plan to take them out or perhaps replace them with film caps, but I have two questions:
1) Has anyone else experienced this problem with C0G caps across heater pins?
2) Might I be better off with two caps per tube (from each side of the heater to ground) than with one cap across the heater pins?
Any help/experience sharing is greatly appreciated.
Here is a link to the exact caps used:
Digi-Key - 445-4756-ND (Manufacturer - FK20C0G1H104J)
I will certainly try removing them, I was just looking to see if this had happened to others with these caps. The reason I don't think it's a tube issue is that ALL of the tubes make the thump when they are tapped, and tapping the chassis anywhere also produces the sound. I didn't notice it until a couple of weeks after I installed the caps, but that's probably because I didn't happen to tap on the chassis during that time.
C0G/NP0 ceramic caps are a world better than regular (say, X7R) ceramic caps, and are much less prone to microphonics, but you never know.
Also, because you're using them on the heater, I wouldn't expect microphonics to be an issue there, but you never know (at least I never know.).
..Todd
Also, because you're using them on the heater, I wouldn't expect microphonics to be an issue there, but you never know (at least I never know.).
..Todd
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I would guess a sensitive (and high potential) wire may be in a different location, possilby closer to the chassis gnd than before
or (seems unlikely) the cantilever mass of the added caps at the sockets are mechanically coupling the thumps into the tubes better
electrically the caps seem hard to explain as a source of microphonics
or (seems unlikely) the cantilever mass of the added caps at the sockets are mechanically coupling the thumps into the tubes better
electrically the caps seem hard to explain as a source of microphonics
Some ceramic capacitor dielectrics are piezoelectric. But I doubt that's the case for C0G or NP0 types of dielectric. In any event, the force you're applying to the caps while thumping the chassis is minuscule and if any voltage was induced on the cap through microphonics it would have to couple through the heater to the active elements in the tube. I just don't see that as a plausible explanation.
I'd look for cold solder joints and the like.
~Tom
I'd look for cold solder joints and the like.
~Tom
Thanks for the replies. So it seems that the caps are a longshot as the problem source. I'll need to open it up to remove/change the caps anyway, so I'll do a thorough sweep through and see what I can find. Maybe I'll swap in some new tubes before that just to see if I have a badly microphonic one as well.
Any advice on whether it is preferable to put one cap across the heater pins or two caps, one from each pin to ground?
Any advice on whether it is preferable to put one cap across the heater pins or two caps, one from each pin to ground?
Could it be the way you mounted it? My guess is that piezoelectric effects would be different for different vibration modes. An SM component has better mechanical coupling than an old-fashioned leaded component, as the soft leads provide some isolation. Here you want poor mechanical coupling!
They are mounted with one lead to each of the heater pins on the sockets - the leads are just long enough to reach from pin to pin when bent outward at 90 deg. I used these because I was under the impression that C0Gs are excellent caps for bypassing HF noise, and are low in microphonics, so the microphonics were a surprise, especially in the heater circuit. The preamp is all point-to-point wired, and I had to move some of the wires around to get access to the pins for installation, so maybe there is a wire where it shouldn't be (I thought I moved them all back to where they were, but maybe not).
Sy, I was wondering if maybe the multilayer design of these caps contributes to more microphonics versus a simple disk design. I am going to try some film caps in this position versus the C0Gs to check that out - I have some .01uF Multicap PPMFX on order.
Here is a pic of these caps in a different application of mine - in an OPA827 opamp-based EQ unit. I'm using them both as PS decouplers and across the +/- rails, with no microphonics that I can detect - completely different application, of course, in the DC power supply. They are nice and compact for their 0.1uF capacitance.
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I'm not disagreeing with you - I make more than my share of errors, that's for sure.
In any case, I'll wait for the new caps (.1uF PPMFX, not .01 as I stated above) to come in and then open her up and poke around - no sense in going in there twice if I don't need to - the pre still sounds very nice, just perplexed by the new microphonics.
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OK, you guys are going to think I'm nuts, but guess what - the microphonics are gone! They seemed to just go away on their own...
No matter what I did tonight, I could not make any sound come from the speakers by tapping the chassis or the tubes; yet for the previous three days, even gentle tapping produced quite loud thumps from the speakers.
Do these symptoms ring any bells? Ghosts in the Machine?
Guess this rules out the caps once and for all.
No matter what I did tonight, I could not make any sound come from the speakers by tapping the chassis or the tubes; yet for the previous three days, even gentle tapping produced quite loud thumps from the speakers.
Do these symptoms ring any bells? Ghosts in the Machine?
Guess this rules out the caps once and for all.
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