[sdiy] Summing resistors in transistor circuit

[Roman Sowa]

Since it's the follower and not virtual ground input, it's not the best 
spot to add summing there, because summing will be affected by what is 
connected to every summed input. If the sources are of comparable 
impedances, amplitude of each one will be altered by a few dB when you 
connect next one, or leave it empty. If one of them is high impedance, 
it will be attenuated a lot.
But if it's going to be patched once and left forever, with every source 
more or less the same impedance, then I'd suggest using 10k resistor for 
every input. 47k is also fine, it all depends on the source you want to 
connect.

But this is all not really the way to do it.
You should rather make new followers for new inputs exactly as they are 
built now, and do the summing AFTER the transistor follower. This way 
you make it independent of the source and if there is something 
connected or not.

Roman

W dniu 2013-12-02 05:58, David Ingebretsen pisze:
> Is there a rule of thumb for picking a summing resistor for mixing inputs in
> a transistor based amplifier? I've been experimenting with different values
> and am trying to understand the advantages/disadvantages of picking higher
> values (47K) as opposed to lower (4.7K) or whatever values make sense.
>
> The circuit I'm looking at is a "vintage" design with an emitter follower
> for input conditioning. There is a 2.5uF cap in series with the input to the
> base, a 100k resistor on the "input" side of the cap to ground, and then two
> 100k biasing resistors on the "output" side of the cap connected to the
> base, one goes to +12 and one to ground. The emitter basically connects to
> ground through a 2.2k resistor. There is a 220 ohm resistor in series with
> the collector to +12 volts.
>
> I'd like to be able to sum two or three inputs to this and so would
> appreciate any thoughts on how to pick the summing resistor values.
>
> David
>
>
>
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