opamp labs 350p racked

[mrphotodude]

Had a few Opamp Labs 350p's so i racked a pair into a dual mic pre. Sounds good on preliminary  test, haven't tried recording with it into my rig yet. Wanted to post what i did, see if any of the more experienced people can give me suggestions to make it better or things i did wrong. I have only copied other designs so far and still have alot to learn.
Here is the link to Opamp Labs page on the modules i used: http://www.opamplabs.com/products/logic-module-plug-in/microphone-pre-amps/350p.html

Here is a pic of the build:

here is a basic schematic of what i did:

I wasn't sure about if the pot on the output was a good idea, how does that affect my output impedance?
I have considered putting a 1:1 transformer instead of the blocking caps. Isn't the point of those caps to block the dc from the phantom power from going into the pre circuit? If i used a transformer, does it need any protection or is it good to go since dc does not induce through? Also, do i need a load resistor or cap or "zobel network" if i used a transformer?
Any thoughts and schooling appreciated

 

[Winetree]

These have been sitting in a drawer for awhile.
Been wondering what to do with them.

 

[okgb]

you could use a transformer on the output to balance it , I believe you don't need a zobel there
are there app notes on the opamp lab site ? there maybe another way to control gain without
the output pot .

 

[mjrippe]

Those specs say 20Hz-20kHz *** +/- 20dB ***  That is a HUGE range and probably the reason they mention things like public address systems, government installations, etc.  They might be an interesting effect preamp but don't expect them to be too hi-fi.  The 360BM is a much better choice.

 

[bruno2000]

Although "rated" at +/- 24 VDC, you will be running these pretty close to the edge for no good reason.  This has been my experience with all Opamp Labs products.  Run them at +/- 18 V instead (my $0.02).
If you "really" need that much output, use a 1:2 transformer.
Best,
Bruno2000

 

[emrr]

Those specs say 20Hz-20kHz *** +/- 20dB ***  That is a HUGE range and probably the reason they mention things like public address systems, government installations, etc.  They might be an interesting effect preamp but don't expect them to be too hi-fi.  The 360BM is a much better choice.

360BM has the same spec, I assume a typo.

 

[mjrippe]

You're right of course, that did seem to be too extreme to be true    Probably supposed to be 2.0dB but my 1997 paper catalog also looks like 20dB!  The 360BM just says 20-20k, no variation range.

 

[rafafredd]

Use a 1k pot on the output instead, so that you can drive modern 10k line inputs on all settings without major frequency response and distortion problems.

Also, a good thing would be a 0-40dB at the input, like JLM audio posted once in a Langevin AM-16 thread. Can't find it now though, but would be simple to calculate.

By the way, how does it sounds?

 

[PRR]

> I wasn't sure about if the pot on the output was a good idea, how does that affect my output impedance?

Well obviously the signal has to go through "some part of 25K" to get out.

And "studio impedance" is more like 10K, 600r, or <100r. So "some part of 25K" seems like a lot.

In fact, if the 350p's output is zero, the output at the pot wiper can be zero to 6.25K. Kinda high for studio loads.

> the link to Opamp Labs page on the module

If you study that, OpAmpLabs uses a 1K in every example. Logically that gives 1/25 the Zout of a 25K pot, or like 250 ohms. Which is "low" compared to most studio loads.