[sdiy] HF Trim in Expos

[Magnus Danielson]

From: "Rob Keeble" <rob at emulatorarchive.com>
Subject: [sdiy] HF Trim in Expos
Date: Sun, 27 Feb 2005 18:33:18 -0000
Message-ID: <JJEIIOABBJMPAPBIBEHNMEOIIOAA.rob at emulatorarchive.com>

Rob,

> I am building an E-mu modular VCO from 1973 and I need some advice on the
> expo generator circuit.
> 
> The HF trim uses an Op Amp and needs a resistor of unknown value to be
> selected. I'd like to use a trimmer to make it much easier to set up the
> VCO. Can anyone advise on a suitable circuit? The link below has a picture
> of both the original circuit, and a possible solution from the E-mu Audity,
> which uses the same LM394. Any reason why I shouldn't use the Audity design
> or do I need something a bit different?
> 
> Any views or ideas most welcome! I was also thinking of using the SSM2210
> rather than LM394.

Go with the E-MU Audity variant. The point is to compensate for the bulk
resistance rBE in the transistors. You can avoid/reduce this my using low rBE
transistor pairs.

Cheers,
Magnus