Tone controls

From: Little Monster (root_at_localhost.localdomain)
Date: 08/09/04 

Date: Mon, 09 Aug 2004 23:16:47 +0100

Hello all,

Not done any electronics for a while so rusty!

I am building an external (Baxandall) tone stack, with switched inputs,
to go with a Leak Stereo 20 valve amp I restored some time ago. The
inputs will be for radio, tape etc. At the moment the radio (the only
thing I have to connect to it) is fed across a 20k dual log pot, with the
wipers connected to the amp's inputs, which makes a not-quite-satisfactory
volume control (I guessed the radio's output would be 20k). Neither the
amp nor the radio have their own vol etc controls. There will also be an
output to connect to a tape deck record input, or to the 'puter's line in.

Mainly I need to know about impedances. I realise the amp has 1M inputs,
but what I would like to know, is what is the standard output impedance
for transistor hi-fi separates (the case in question being a 70's model of
Realistic turner). I assume it is some kind of standard.

Next question, there will have to be some kind of impedance matching
circuit inside the tone box, with a bit of gain to compensate for the
insertion loss. What is the best way to implement this?

Also, are there any 3 knob (ie bass, mid, treble) Baxandall circuits out
there? I haven't seen any on the 'net.

Final question (probably) I've designed a small logic circuit to provide
soft switching between the 4 inputs. I think the best (or at least
simplest) way is to couple its outputs (buffered) to the emitters of some
npn transistors connected in the path of the inputs, so they can allow the
signal through when that particular output is low. Apart from hard
switching, is there a better way to do this? If I use this arrangement,
is it also my best option for Z matching?

Thanks in advance,
Monster 

-- 
Of course I can!  I'm British.
www.the-monstruum.co.uk

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