Soundcraft Spirit Live (1990) mic input gain reduction mod

[mihaelbele]

Hi,
It seems that mic preamps have +10db gain when the gain pot is in it's zero position. Is there a mod to reduce gain to unity at pot zero position? Or even implement as trim/gain function? I think that the mod would affect the line input gain too, so I would probably leave few pairs of inputs as they were.
I'm mostly using electrets, and can only resort to resistive pads if mic capsule has a built in FET, but even the AKD D112 gets clipped when used in bass drum.
I've found a channel schematic this thread: link
Thanks!

 

[JohnRoberts]

Try reducing the values of R17 and R18. To drop the gain 10 dB you want to reduce the 12k to 1/3 or roughly 4k.

Maybe only modify one channel first to see how it works, before doing more.

JR

 

[ccaudle]

Try reducing the values of R17 and R18

Couldn't you also increase the value of R16? That looks like the classic instrumentation amp front end where decreasing resistance increases gain, and R16 sets the minimum resistance when the pot is all the way at one end. Changing R16 has the benefit that you don't have to worry about matching the replacement resistors (although for that class of device they may not be matched super closely to start with).

 

[JohnRoberts]

Couldn't you also increase the value of R16?

No.. R16 establishes the maximum gain not minimum gain (OP's complaint). You would need to replace the 5k gain pot with a larger value which also involves an unusual non-linear taper making them even harder to source.

That looks like the classic instrumentation amp front end where decreasing resistance increases gain,

It's commonly known as a "Cohen" topology.

and R16 sets the minimum resistance when the pot is all the way at one end.

minimum resistance commands maximum gain...

Changing R16 has the benefit that you don't have to worry about matching the replacement resistors (although for that class of device they may not be matched super closely to start with).

I would recommend using 1% resistors for R17 and R18, I didn't suggest a specific value because I don't remember them all. Should not be hard to find.

JR

 

[ccaudle]

No.. R16 establishes the maximum gain not minimum gain

Thanks, dangers of posting before the coffee brews.

 

[mihaelbele]

Thanks guys!!!!
I'll try it over the weekend and let you know

 

[mihaelbele]

Thanks guys!!!!
I'll try it over the weekend and let you know

It works! Tested with linkwitz modded WM55 into Zapnspark's Schoeps generic electret front:
12k || 5k6 (piggyback) = 3k8 results in ~6db reduction
12k || 5k6 || 5k6 = 2k3 results in ~9db reduction
12k || 1k8 = 1k6 results in ~12db reduction
12k || 1k8 || 1k8 = 840R results in ~14db reduction and 2db headroom on soundcard input (clipped at capsule or mic preamp)

WM61 handles higher SPL, but with more than 20db of input gain, mic preamp/capsule noise starts to be noticable on peak meters, so I'll try to match that with -20db reduction (more than that would probably degrade S/N ratio). If these resistors are bridged, will preamp operate normaly (or, how low can I go)?

PS. I got +10db number (in my first post) by comparing Spirit with my SSM2019 based mic pre at unity gain. If that comparison was correct, that would mean that when R17/18 are lower than 2k3, preamp actually trims the signal?

 

[mihaelbele]

I need a bit more help, please.

This mod introduced some sort of oscilation that looks like shifting resonant peak. When the desk is turned on, the resonance slowly moves up/down the whole audible frequency range (on modded channels only). As time passes, it seems that the shifting speed slows down, and that frequency span becomes more narrow, eventualy stabilising across very narrow band after a few hours. Peak amplitude is about the same on all the affected channels, but the peak frequencies are not the same/correlated. I used 1% resistors. This is with no microphone connected. When mic is connected, more resonant sweeps appear (some with wider q), and the sweeps are faster.

If it's too complicated to reduce preamp gain, would a 10k pot between R34 and gnd, with wiper connected to send/tip, work ok for variable reduction before the soundcard input?

Thanks

 

[JohnRoberts]

That old schematic link is not working now... The Cohen topology should be pretty stable. If you reduce the feedback resistors around the op amps too low that increases the open loop gain working with the Rs connected to the device collectors and op amp - input. Try reducing the value of those collector resistors a commensurate amount to the reduction in the op amp feedback resistors.

I have never seen this instability before, nominally in the Cohen topology the collector load resistance is similar to the feedback resistance for a unity feedback factor. Making the feedback resistors smaller than the collector load effectively introduces voltage gain into the feedback path.

Of course there may be other remedies but that is where I would look first.

JR

 

[k brown]

Why can't you just put an inline pad between the mics and the mixer?

https://www.shure.com/en-US/products/accessories/a15as?variant=A15AS
https://www.ebay.com/itm/2753692796...puhInn34r+d3l9wjGo7WdcNFM=|tkp:Bk9SR4zIwKmqZQ

 

[JohnRoberts]

Reducing the gain of the preamp should be lower noise than padding down the signal before boosting it up again, higher than desired, but with hot signals the preamp ein may not dominate the noise floor.

JR

 

[dai h.]

 

[JohnRoberts]

Regarding the above schematic (that must not be reproduced or disclosed), the 12k feedback resistors and 5.1k collector resistors nominally attenuate the open loop gain more than 6 dB helping stability. BUT the 22 pF capacitors across the 12k feedback resistors are actually adding some HF boost to that open loop gain. This may actually be working against stability (for stability we want the open loop gain to drop below unity gain by the time that the phase shift caused by internal time delay reaches 180' making negative feedback positive).

See if removing the 22pF caps improves the stability.

JR