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Diabolical Artificer · 18th Jun 2019, 7:01 am

Thanks lads and thanks again Peter for going out your way to help. I see you paralleled the caps in your RIAA filter, I was wondering how to get 8n2 and 560p, though thinking about it am sure I have some 560p silver mica caps.

I built Fred Nachbaur's phono amp in the interest of science and all that, attached schematic below again, but found it to lively. Just tweaking it to get the DC conditions right took a lot of doing and I still couldn't get it bang on. Also the OP was "boingy", a bit like a wein bridge oscillator with a lamp as the gain control. So I won't be building it.

The one thing that has taken a lot of time on this project is finding caps, I have draws full of the ******* but still had to root through the sheds and parallel caps to get odd values in schematics.

This order of circuits has been suggested - RIAA ( with it's own buffer ) , input selection, attenuator ( volume control), line amp + tone controls, output buffer. Another good idea is to put pads or trimpots on IP's, 47k is the usual line level IP Z isn't it? I have some 100k and 50k pots that should do the job there.

Andy.

18th Jun 2019, 7:01 am	#44
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: Valve preamp. Thanks lads and thanks again Peter for going out your way to help. I see you paralleled the caps in your RIAA filter, I was wondering how to get 8n2 and 560p, though thinking about it am sure I have some 560p silver mica caps. I built Fred Nachbaur's phono amp in the interest of science and all that, attached schematic below again, but found it to lively. Just tweaking it to get the DC conditions right took a lot of doing and I still couldn't get it bang on. Also the OP was "boingy", a bit like a wein bridge oscillator with a lamp as the gain control. So I won't be building it. The one thing that has taken a lot of time on this project is finding caps, I have draws full of the ******* but still had to root through the sheds and parallel caps to get odd values in schematics. This order of circuits has been suggested - RIAA ( with it's own buffer ) , input selection, attenuator ( volume control), line amp + tone controls, output buffer. Another good idea is to put pads or trimpots on IP's, 47k is the usual line level IP Z isn't it? I have some 100k and 50k pots that should do the job there. Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.