Thread: Valve preamp.
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Old 15th Jun 2019, 6:45 am   #28
Diabolical Artificer
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Location: Sleaford, Lincs. UK.
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Default Re: Valve preamp.

I've built one phono circuit to test so far, IE the Valve Wizard ECC83 based jobbie. I built this using Tektronix ceramic wafers, I can get the whole circuit on two 11 "pole" wafers. It seems to work ok however I have 50hz modulating the OP. With the circuit knocked together on an old chassis 50hz was at about 60mV P-P, I reduced this to 20mV P-P by lifting the ground and reduced it a bit more by moving the circuit away from the mains tfmr/PSU section and putting it in a shielded steel box, it's still too much though.

Another issue I've hit is testing said phono stage using an inverse RIAA filter. I built this one - http://sound.whsites.net/project80.htm the signal in is really attenuated, but after some thought I suspect this to be normal. I thought at first I could IP a 2mV sine, a typical cart OP but there's nowt comes out, obviously in retrospect.

I'm a little uncertain about R3's value, there are a few inverse RIAA filters I've found, all use the same R's and C's more or less, EG 833k, 75k, 3n6 and 1n, but use different value's for R3, anywhere from 600r to 3k3. More testing of my circuit is needed but suspect R3's value needs tweaking to suit one's sig gen OP Z.

I've also finished building the wood box/chassis, a nightmare to do but doesn't look too bad.

Andy.
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