[Agent 86]

I'm slapping together a preamp from 360bm modules. It's in a box and it
sounds pretty cool. I'd like smaller gain steps than the 10dB steps based
on the info in the catalog. Bel strongly recommended that I NOT use a pot
in the feedback loop, but his explanation was a bit over my head. I'm
wondering what formula I might use to calculate FB resistor values.

Thanks.

[Geoff]

Agent 86 wrote:
I'm slapping together a preamp from 360bm modules. It's in a box and
it sounds pretty cool. I'd like smaller gain steps than the 10dB
steps based on the info in the catalog. Bel strongly recommended that
I NOT use a pot in the feedback loop, but his explanation was a bit
over my head. I'm wondering what formula I might use to calculate FB
resistor values.


Google opamp basics.

Pots are not good in feedback loops as they can cause extyreme crackles when
moved, or cause full gain when they go o/c.

geoff

[Scott Dorsey]

Agent 86 wrote:
I'm slapping together a preamp from 360bm modules. It's in a box and it
sounds pretty cool. I'd like smaller gain steps than the 10dB steps based
on the info in the catalog. Bel strongly recommended that I NOT use a pot
in the feedback loop, but his explanation was a bit over my head. I'm
wondering what formula I might use to calculate FB resistor values.

It's on the data sheet on the Opamp Labs website. Because there is so much
parasitic leakage in the thing, and the open-loop gain isn't anything even
approaching infinite, the actual numbers fall very short of what you'd
see on an ideal op-amp.

Here is the datasheet: http://www.opamplabs.com/360bm.htm. If you want
values other than the ones listed, plot out the curve on some graph paper
and pick whatever points you want on it. Note that it's asymptotic... you
cannot get more than 60 dB of gain no matter how hard you try.

I'll say, though, that I have a console around here that uses the 360B
modules and has trim pots, and they work just fine. However, the pots
are reverse log taper so they give log gain when used in a feedback network,
and finding reverse log pots today is pretty much impossible.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[David Grant]

However, the pots
are reverse log taper so they give log gain when used in a feedback
network,
and finding reverse log pots today is pretty much impossible.
--scott

One of the panasonic pots available from Digikey can be converted into a
reverse log. I think it's one of the 6-pin pots and you have to jumper some
of the pins to make it rev-log.

Dave



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