UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Diabolical Artificer · 26th Jun 2019, 6:33 am

Thanks again Peter, I'll rebuild it with your tweaks after I've done with this Shure M65. I can configure the PSU to have a negaitve rail as the HT secondary has a CT. Have slowed down a bit on this project but did build the Shure M65 circuit over the weekend, but found it had too much gain on the top end, swapped C9 470p for a 1n which gave better results. Overall though response isn't flat and this circuit is the less accurate of the ones I've tested.

Which brings me neatly to ask in response to this and other posts above - " How accurate does an RIAA stage need to be?" On some schematics and builds caps like 99.3n are required which means hours of endless fun measureing caps to 3 decimal places and paralleling em or fitting big air adjustable caps and spending more hours of endless fun doing response graph's.

Which leads me to another question that I've not found an adequate answer to - FB or no FB? I think David did try and answer this but I didn't understand his response. So far my tests show no FB design's to be less troublesome and more accurate.

Andy.

26th Jun 2019, 6:33 am	#69
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Valve preamp. Thanks again Peter, I'll rebuild it with your tweaks after I've done with this Shure M65. I can configure the PSU to have a negaitve rail as the HT secondary has a CT. Have slowed down a bit on this project but did build the Shure M65 circuit over the weekend, but found it had too much gain on the top end, swapped C9 470p for a 1n which gave better results. Overall though response isn't flat and this circuit is the less accurate of the ones I've tested. Which brings me neatly to ask in response to this and other posts above - " How accurate does an RIAA stage need to be?" On some schematics and builds caps like 99.3n are required which means hours of endless fun measureing caps to 3 decimal places and paralleling em or fitting big air adjustable caps and spending more hours of endless fun doing response graph's. Which leads me to another question that I've not found an adequate answer to - FB or no FB? I think David did try and answer this but I didn't understand his response. So far my tests show no FB design's to be less troublesome and more accurate. Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.