How low should PSU ripple voltage be in a preamp?
I'm in the process of building a single stage valve preamp using triode connected EL84's with plate loading chokes and NiMH bias.
Although high voltage electros are awkward to get hold of where I am, 80H 560 ohm dcr chokes that'll handle upwards of 70ma can be obtained with relative ease.
I'm using 2 as my plate loads but I thought, Hey, why not use one in the PSU as well?
Plugged a few values into Ben Duncan's "psu designer 2" and with 200u, 80H series, 200u, 200r series, 200u, 200r series, 200u and a draw of 50mA I'm getting the terrifying total of 470nV (that's right! NANOVOLTS!) of ripple on my 270V line.
The plan was to have the main psu tranny and CLC in an offboard enclosure. The RCRC was to split to both channels as soon as the PSU feed entered the preamp housing. I accept that on a 270v valve preamp rail this is overkill but the choke's easy to get hold of and of known quality. Should I just run with it?
As low ripple as possible - In Pre Amps
PreAmps should of course have as low PSU ripple as possible.
To give a nuber on how many mV is acceptable,
Different amplifiers are diffeently sensitive for ripple.
They have different PSRR. Power Supply Rejection Ratio.
But I would say it is good if you have only 10-50mV ripple.
/halo - ripples a lot
Re: How low should PSU ripple voltage be in a preamp?
To spite the sim results, with any standard preamp sized transfrmr, your hum will probably be worse than a similar supply with only 100uF of capacitance.
15H-->100uF-->500 ohms-->100uF is all the filtering you would need to have.
I used that for a mic preamp - and I get no hum in headphones.;)
ripple and noise
the CAD models (I use multisim) will show you results without the real world crud which comes about when circuits heat, etc.
You have to distinguish between "ripple" and "noise" while the CAD model puts the ripple at nV, there are probably tens of millivolts of noise modulating the ripple -- this is why a good low noise scope is so helpful.
BTW, the TEK 5XXX scope are dirt cheap and very good for this purpose. They have plug-ins which are capable of differential measurements of 5uV/cm . This series of scopes were used in biomedical applications in the 1970's and they are a bit bulky but you can find them for sale on EBay (with time base and plugins) for less than $50. They have a much bigger screen than the TEK 7XXX series.
<b>80H 560 ohm dcr chokes that'll handle upwards of 70ma can be obtained with relative ease.</b>
Do you have a source for those chokes that you'd care to share (by email if you'd like)? I'm in NSW, and might even have some electros available.
I also agree with Joel and Jack, that for a line stage 100uF is enough. I never bother with more, except in poweramps.
Brett, I was going to source my chokes through Lancroft. They have been very favourably recommended by a freind who does LOTS of valve work in the ACT.
I was going to use the CHSFO4 at $110 ea for my plate loads.
On second glance however, the chokes are current limited until I go up to FS05 for my PSU purposes as they'll be taking the draw from both channels.
Hope this helps.
The 100uf 400v caps i can get from Jaycar if necc.
BTW, was planning on diode rectifying not tube.
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