Electronics > Beginners
super-low frequency PSU oscillations + AC coupling with electrolytics
pepelevamp:
Hi guys. Riddle me this one.
My buddy and I (noob #1 and noob #2) are trying to figure out how to fix this awful noise floor of this audio tube amplifier: There is low frequency oscillation of the signal all by itself when nothing's plugged in. We are seeing these wiggles when simply recording silence on the PC (we know its not the audio interface, that goes past -120dB).
We're seeing sub and super-harmonics of 50hz, and at around 4hz its @ -84dBFS. When we turn the gain up & down on the amplifier, the wiggles/oscillations remain at constant amplitude.
Thus we suspect power supply in the amp and we suspect it is _after_ signal-amplification somewhere. We suspect something like the mains rectification diodes.
Now we don't have much gear. We just got ourselves a wee pocket oscilloscope. We want to probe around & find the oscillations in the circuit. But here is the problem:
The power supply inside the amp is 400 volts by the time it touches anything. -84dB wiggles of 400V are 40mV. So we're like ok cool - bring on AC coupling. No wakkas.
BUT:
We are uncertain if the AC-coupling in our wee DSO-112A pocket oscilloscope is cutting off all the low-frequencies we wanna see. And with a 10x probe to bring the DC into the scope's 50v tolerance, we fear we've attenuated the wiggles to nothing. We're not even certain if -84dB is the true value since its so close to DC that we fear its being attenuated by the AC-coupling of both our audio interface, the AC-coupling of the amplifier, and the DSO-112A pocket scope. My mate is the one doing the legwork here & can't make heads or tails of what the scope shows.
So I came up with an idea. Lets make our own AC-coupling which reaches unity gain at 1hz and above. Put this on the nose of our oscilloscope & go probing. But to make this AC-coupling / low cut front-end I need large capacitor values, namely whats provided by an Electrolytic capacitor.
My questions to the audience are:
* Is it safe to do AC coupling like this with an Electrolytic capacitor (even if we put several in series to achieve 400V tolerance)?
* Is Star Trek Picard going to be good?
* Is there any credibility to my claims of attenuated dB from the low-cut?
* Does anyone have a better approach?
The amp is the Sebatron vmp 4000e.
Yes we have changed the tubes & it hasn't resolved the problem.
Attached is the signals we can see (along with a different amp for comparison. Everything is at unity gain)
Thank you guys I know this is a bit of a corky one because its not actually me doing the arm/leg work but my mate is with us via 'tired-after-work' online comms. We are looking at building audio gear ourselves so this is a great exercise handed to us for our learning.
Thanks again everyone.
TimFox:
During the vacuum-tube era, many power supplies used a relatively high-inductance filter choke between two filter capacitors (usually less than 100 uF). I have seen resonant frequencies of such circuits below 10 Hz, with Q > 1, that can magnify power-line fluctuations or interact with the amplifier. What values are found in your amp’s power supply?
mike_doom:
Hi Tim,
I'm the guy with the Sebatron VMP Mic preamp. *See pics below*. I don't believe there are any chokes in this psu that i can see. I'm wondering if I just start replacing parts in the psu like the rectifying diodes and so forth and so on.
exe:
Afaik, an RC-snubber reported to be able to cause this. If so, it's fine. The article was on Rod Elliot's website. Can't find it atm.
T3sl4co1l:
Motorboating is due to a low frequency feedback path in the amplifier circuit. It has little to do with power supply, though the feedback can be introduced through accidentally poor value RC or LC filters between stages (much as a phase shift oscillator works).
Just because an amp cost something, or has some weight in its name, means absolutely nothing. The highest end amps are just as often, just as trash as the lowest. This is not my experience directly, but a close friend does tube amp repair (among other things) regularly, and this is his experience.
He often fixes design faults in these things, as well as incidental repairs (burned parts, etc.). (Most of which are caused by design faults anyway, i.e., that could've been reduced or eliminated through proper design. Electrolytics placed beside stinking hot vacuum tubes, for example.)
(The low level 100Hz ripple also suggests poor design, either in filtering, balance or shielding.)
Tim
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