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Old 8th Jul 2020, 9:37 pm   #1
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Order to place circuit elements in a Valve preamp.

I'm getting back into a few projects at long last, It's been a somewhat rough personal life last few months, least said the better there.
Anyway I have been sifting through the "roundtuit" pile and after getting the old Ravensbrook fettled, I fancy a go at a job that's been on the back burner for years.
I have the case and front panel from a Leak "Point One Stereo" preamplifier, along with the original potentiometers. It dates from the times when these things had no value, you could pick one up for a tenner or less. Not the case now. So no beating me up over its present state.
So I have a nice cabinet to build a pretty little control amplifier in that will be suitable to use with any of my power amplifiers.
I've got the PSU sorted which will be outboard and connected by an umbilical.
So I have the tone control circuit sorted which will be the early triode connected EF86 baxandall control, that bit is simple enough, using the original and still useful pots. Theres no point reinventing the wheel here the leak circuit works and I have the right potentiometers.
So I need a buffer amp in front of the tone controls and also something like a Cathode follower downstream of it to give a nice low output impedance as it has to work into at least a 50k input resistance.
So do I put a gain stage, probably using an ECC82 type at the front of the tone network, bearing in mind that I need to be wary of overloading the EF86 valves, they'll be biased up to about 1.2V so that will dictate how much I can have at the input.
Given that I will need say 6dB to cover the insertion loss from the balance control that's easily achievable from the circuit I have roughed out, I'm not after squeezing the maximum out of the valve.
So do I put a simple cathode follower unity gain buffer at the very front of the circuit and thus not overloading the tone network, then put balance and volume controls after the output of tone controls which is fine in terms of impedance matching as leak simply bunged a balance and volume pot at the end and fed the result from the Volume control wiper to the input of their power amps. Followed by the ECC82 gain stage and feed off the anode which I think will be about 7k but I'll have to calculate it. That's not too extreme for a metre or less of connecting cable into the 1meg input Z of the valve amps and TBH I don't see me connecting it to the quad 405 but you never know.
I need that gain as the tone network is unity gain. Connecting as a pentode would give the tone control a bit of gain but For some bizarre reason I want to go triode.
OK a lot of info, and if anyone can guide me here I will be very grateful.
I should maybe add that I intend to fit a IR remote volume control using a known to work eBay module with motorised alps pot which I have used on another lash up and I am satisfied its working fine.
Many Thanks in advance.

Andy.
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