Capacitor Values

[abechap024]

Hello,
I am rebuilding a Qauntum QM-168 console, I'm replacing the lot of capacitors it uses 22uf 25v for the line in and the coupling between opamps.

http://bolstad.net/quantum/Quantum%20QM-168%20Manual.pdf 
page 23

is the input card schematic.


My question is what is the benefits of using larger input capacitors? And is there an adverse effect when you use very large capacitors? Like if I replaced all the capacitors with 100uf, would it be worse audio wise than sticking with the 22?? .I've searched all across the board but can't find a straight explanation.  I know I can just easily replace all the caps with the same value parts, but would there be an advantage to using 47uf or maybe even 100uf? am I correct to think it will help with low frequency response? I'm still learning and am a little splotchy on the information of coupling capacitors.
Thanks in advance for any help
AC

 

[JohnRoberts]

Advantages include pushing down the LF -3dB pt, and improving LF CMR since the lower the pole frequency the less error caused by parts tolerance up at mains frequency.

Disadvantages include, a lot more energy to dissipate into clamps or circuitry if an input cap charged to phantom voltage gets momentarily shorted to ground, driving the other end to -48V.

I have seen most people using 22-47uF. I would advise caution about using 100uF with a studio patch bay in line. Normal input jacks and cables should not throw inadvertent shorts onto input pins, but TT or TRS patch bays can be circuit killers, depending on which end of the cable you plug up first. 

JR

 

[abechap024]

Thank you,
that makes sense, so I'll probably go with 47uf. I appreciate the input! Can't wait to get this old board fired up, supposedly the board uses richenbach(sp?) transformers and they made jensen transformers before changing into cinemag(at least thats the word on the internet).
Regards
AC

 

[pstamler]

The Quantum QM-168 -- I have a lot of memories of that board. Made some good recordings using one. Yes, the input transformers are very clean. If you have the version with LM318 opamps, they're clean too.

There are a couple of gotchas you need to know about. The ICs used in the EQs are a quad opamp with pretty poor performance -- poor slew rate and high distortion, plus a weird asymmetrical slewing performance. Unfortunately, it's an unusual pinout not compatible with modern quad opamps unless you use a conversion carrier, which is a pain and not too reliable. When we had that board I replaced the EQ opamps with TL075s, which had the same weird pinout. Unfortunately, they're not so clean either, and in any case no longer available. Once you have the board recapped, consider finding some way to replace these quad opamps, or maybe just don't use the EQ (the quad opamp is hard-wire bypassed when the EQ is off). You may want to find something cleaner to replace the LM301s in the output stages; they're used in feedforward mode, which is their best, but still.

Also, the version of this board we had was Pin 3 hot on its inputs and outputs, which can jump up and bite you if you're not careful. The outputs are XLR, but unbalanced -- not even impedance-balanced. You probably ought to wire the outputs for Pin 2 hot, assuming the rest of your gear has that hookup (most post-1980 gear does). While you're at it, you should put in the resistors for impedance-balancing it; you'll only spend a buck or so, and it's worth it.

I occasionally had problems with the input sensitivity: on U 87 mics or others with similar outputs, sometimes the level with the pad on wasn't quite enough to drive the tape recorder to 0 VU, but with the pad off the input clipped. Some changes in gain-setting resistors may be in order. Of course, lower-level recordings on a 24-bit digital system aren't quite as critical as they were on the Ampex we used ours with...

Peace,
Paul

 

[pstamler]

Oh, and the 1.8kHz EQ on the Quantum turns out to be very useful. Pull it back about 1-1.5dB, and you take the squawk right out of a KM 84 or KM 85.

Peace,
Paul

 

[abechap024]

Ha! Awesome Paul, I'm eagerly scratching up any useful information about this console I can, your post is very helpful. Thank you. I was not aware about the pin 3 that will be most helpful when I get back to wiring the thing back together. Right now I took all the cards out and am buying some molex connectors so I don't have to solder and unsolder the cards inputs and out puts ever time I want to modify something. I was aware of the weird 4136 opamps in the eq and I think I'm going to solder little wires onto some replacement quad op amps and then solder the wires into the correct pin outs - time consuming yes but hopefully it will provide an cleaner eq sound. now when you talk about impedance matching the outs I assume your talking about the Buss outs? what would I do just wire a 2.5 k resistor in series? or 10k? Thanks again for the Info!
Regards,
AC

 

[conleycd]

Hi there,

Check out the top of page 3 from Jensen Transformers on how to convert an unbalanced signal to impedance balanced using caps and resistors.

http://www.jensen-transformers.com/an/an003.pdf

CC

 

[abechap024]

Sweet,
I printed out and this is going in the Electronics file cabinet for future reference ;D
Thanks!