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ruffrecords 31st October 2008 02:54 PM

Low Frequency Mains Noise
 
On several tube mic preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +-20 to 30mV.

Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it?

Cheers

Ian

coloradosound 31st October 2008 03:40 PM

Your description sounds more like popcorn noise.
I've read it being more common in older solid state OP amps
but haven't ever heard anybody say anything about
popcorn noise from tubes. Could also be microphonic tubes?

kevinkr 31st October 2008 03:54 PM

I have seen this same phenomena in several of my designs using unregulated supplies and it definitely appears to be caused by small long term (low frequency) variations in the mains voltage. I have also done experiments on the bench where I looked at a rectified and well filtered unregulated dc supply loaded into a resistor bank and I can see it there as well.

Some of my tube based regulators have enough loop gain to remove most of this variation, and the maida solid state regulators definitely do.

Practically speaking there isn't much you can do about this short of using voltage regulation on your supplies or designing power supplies with huge time constants (probably not a good idea).

This becomes a problem in phono stages and microphone amplifiers where there is lots of gain and where minute LF shifts in the first stage plate voltage get amplified by successive stages.

I've also noted that in these applications minute variations in filament voltage can also amplify the effect so regulating filament supplies may help here..

Choosing rc time constants carefully can help a lot wrt to this issue, but won't eliminate it totally, and carried to an extreme you can also loose useful bass response..

One of the problems this can create in tape recording applications is that it might, depending on recording amplifier design, result in saturation of the recording head or tape in extreme cases. Pretty unlikely but not impossible.

kevinkr 31st October 2008 03:59 PM

Quote:

Originally posted by coloradosound
Your description sounds more like popcorn noise.
I've read it being more common in older solid state OP amps
but haven't ever heard anybody say anything about
popcorn noise from tubes. Could also be microphonic tubes?

Popcorn noise by definition sounds a lot like popcorn, it is random bursts of noise generated typically by early operational amplifiers - it was generally caused by a fabrication process defect, and is quite uncommon in modern op-amps.

The problem described which I have also encountered is generally inaudible but may result in woofer "breathing" in systems with electrical response down to near dc. Usually microphone pre-amplifiers and phono stages or tape head amplifiers are most susceptible to this problem because of the large amounts of loop gain available.

Jeb-D. 31st October 2008 06:02 PM

I've seen this too on the mains if it's non-sinusoidal and fairly random. Especially when the gear is sharing a power strip with a temperature controlled soldering iron(because it toggles on and off). Or when the washer/dryer, dish washing machine, ect.. is running.



If it's a continuous sub-sonic sine wave that presents itself at the output. It's possible your getting positive feedback between stages on the B+. This occurs in some circuits when the coupling networks f3 is close-to or lower-than the decoupling networks. Just for re-assurance, I generally design so that subsonic B+ feedback is always negative if it were to occur. That's for un-regulated, in regulated supplies it's not a problem.

ruffrecords 31st October 2008 07:26 PM

Quote:

Originally posted by coloradosound
Your description sounds more like popcorn noise.
I've read it being more common in older solid state OP amps
but haven't ever heard anybody say anything about
popcorn noise from tubes. Could also be microphonic tubes?

That was my first thought, that or some weird form of 1/f noise. However, I get exactly the same without any tubes connected, just the PSU on its own with a dummy resistor load so it is definitely NOT the tubs this time!

Cheers

Ian

ruffrecords 31st October 2008 07:26 PM

Quote:

Originally posted by kevinkr
[B]I have seen this same phenomena in several of my designs using unregulated supplies and it definitely appears to be caused by small long term (low frequency) variations in the mains voltage. I have also done experiments on the bench where I looked at a rectified and well filtered unregulated dc supply loaded into a resistor bank and I can see it there as well.
That was my most recent test - just the PSU with a dummy resistive load - no tubes - same result. Good to hear someone else has come across the same thing.
Quote:

Some of my tube based regulators have enough loop gain to remove most of this variation, and the maida solid state regulators definitely do.

Practically speaking there isn't much you can do about this short of using voltage regulation on your supplies or designing power supplies with huge time constants (probably not a good idea).
A regulated supply was my first thought - good to hear it can do the trick. I'll check out the maida regulators.
Quote:

This becomes a problem in phono stages and microphone amplifiers where there is lots of gain and where minute LF shifts in the first stage plate voltage get amplified by successive stages.

I've also noted that in these applications minute variations in filament voltage can also amplify the effect so regulating filament supplies may help here..
Interesting - at present the effect seems to be the same on a dummy load as with tubes with unregulated ac filaments. I'll keep an eye out for that.

Cheers

Ian


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