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Diabolical Artificer · 16th Nov 2019, 9:38 am

Made some progress on this, finally decided to make a separate line preamp and RIAA stage. The line pre will power the RIAA on a separate chassis via an umbilical cord.

The circuit is an odd one, namely a CCDA - constant current draw amplifier, the idea being from what I gather is that if an amp stage's current draw is constant, the PSU specs need be less critical, but the main reason I went with it was it's simple, has the right gain - about 10 and works.

I used two VR150's to clamp the HT in order to give the 820u 400v cap an easy ride and they look pretty too as well as filling out an otherwise empty chassis.

Siting and drilling holes for the 12 100k presets was a nightmare, I just don't have the tools or skill to work to high tolerances of 1mm or so. I got it wrong quite a few times so there's a few holes to fill in.

Wiring in the pics is temporary as is the pot, just wanted to get everything working and take some voltage readings.

Andy.

16th Nov 2019, 9:38 am	#79
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Valve preamp. Made some progress on this, finally decided to make a separate line preamp and RIAA stage. The line pre will power the RIAA on a separate chassis via an umbilical cord. The circuit is an odd one, namely a CCDA - constant current draw amplifier, the idea being from what I gather is that if an amp stage's current draw is constant, the PSU specs need be less critical, but the main reason I went with it was it's simple, has the right gain - about 10 and works. I used two VR150's to clamp the HT in order to give the 820u 400v cap an easy ride and they look pretty too as well as filling out an otherwise empty chassis. Siting and drilling holes for the 12 100k presets was a nightmare, I just don't have the tools or skill to work to high tolerances of 1mm or so. I got it wrong quite a few times so there's a few holes to fill in. Wiring in the pics is temporary as is the pot, just wanted to get everything working and take some voltage readings. Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.