# wait! I meant lin-log converter

Magnus Danielson magnus at analogue.org
Sat May 16 22:44:41 CEST 1998

```>>>>> "AR" == Aaron Ross <dryo at best.com> writes:

AR> Oops, sorry, what I really meant was I need a box to convert PAiA's LINEAR
AR> output voltages to 1v/oct LOG voltages, which is what my Roland and SCI
AR> synths want to see. I have found log-lin converter schematic in the Penfold
AR> book, but it is not what I need.

AR> Chalk it up to "rookie mistake"

Ah well. This lin/log stuff is sometimes confusing.

If you go and look at for instance go and look the ASM-1 homepage you
will find the exponential converter based around a matched transistor
pair (MAT-02 or something similar). This exponential core can be used
to do a log function by the addition of an resistor and a op-amp.

There is a logarithmic converter as found in PMI's MAT-02 datasheet is
very similar to the exponential converter that is used in the ASM-1
VCO to convert CV to linear current. That linear current is a simple
thing to convert to linear voltage, and op-amp and a resistor in the
feedback to the negative input and hook the current in there. The
positive is put at ground.

I think that you can find a lin/log converter over at Anders Sponton's
(who BTW was over at my place last night) webpage:
http://www.tellus.vallentuna.se/gymninfo/personal/anders/andersus.html

One way to get the log function out of an exponential converter is to
use 1 opamp. You apply the output of the op-amp to the input of the
exponential converter. You apply the output voltage of the exponential
converter to the negative input on the op-amp. You apply you signal to
the positive input of the op-amp and now will the output of this
op-amp be the logarithm of the value you apply to the input.
This is a very unlinear controlloop but it works great!

Either way, have fun!

Cheers,
Magnus

```