DIP8 Discrete Op Amp advice

[jdurango]

I just bought a Sound Workshop Series 40 console and I'd like to swap the LF351's out for some other flavors of op amp in a couple channels, just to try a little variety. There are plenty of IC options, but can anyone recommend a good DIP8 package DOA that would work well with this circuit? It is 36 channels and has two power supplies, I also plan on adding larger caps to each PSU.

The circuit is nearly identical to a 1600 and it has the black box Transamp pres. Schematics for input module, EQ and master section are attached. Thanks fellas!

 

[jdurango]

EQ schematics

 

[jdurango]

Master section schematics.....would be really great to "upgrade" this first! Already planning on bigger caps in most places (obviously avoid caps used in timing circuits, talkback, etc)

 

[jensenmann]

I would not change anything if I were in your place. from a quick look at the channel schematics I´d say that there are no bandwidth limiting caps anywhere around. If you put in some fancy new highspeed opamps then you will end up with an AM transmitter and/or blow the new chips. If you want new chips then you need to redesign the circuits.

In general there´s no "flavor" in chips. It´s the circuit which dominates the performance. If you just throw something in without knowing what you do you will most likely get a worse performance than before. Second guessing a professional designer´s thoughts can be done, but only with serious knowledge, not by luck. And then you have to know which parameters you want to improve and got for that, not just flavor. Don´t buy this internet-BS-blahblah about the "sound" of chips. There are too many halfeducated people around or even worse, few educated ones who won´t tell you the entire truth what to do.

If you want to learn what is necessary to improve the performance of a circuit then get a good book, read, do the maths(!) and ignore the internet´s opinion.

 

[gyraf]

Thanks Jens!

 

[ruffrecords]

Thanks Jens!

PLUS 10

Cheers

Ian

 

[radardoug]

PLUS 20!!!

 

[jdurango]

Thanks very much fellas, especially Jens! I will take your advice and hold off on the op amps.

I have been advised by several techs in the SW Yahoo group that upping the value on PSU caps by as much as a factor of 10 is wise, and elsewhere in the audio electronics by as much as double. Apparently SW skimped on the value of caps throughout the board to save $$$, especially the PSU. What do you think? On one hand I don't want to second guess the engineers who designed these circuits. On the other hand, it sounds very much like the ideal circuit may have been compromised non-trivially by costs concerns.

I can't think of any reason NOT to up the value in the PSU to reduce ripple current, but I know the audio circuitry gets a bit more complicated. Supposedly no caps are used actively in the EQ circuitry, nor for any timing related circuits. What do you think?  Perhaps only up the value on caps going directly to ground? I'm going to have to recap the board anyway, so might as well make improvements where I can. Thanks!

 

[radardoug]

If you have been doing sound for any period of time, you would know that the high pass filter is your friend. The effect of lower value coupling capacitors is to raise the low frequency cutoff corner. So having smaller caps is not that much of an issue for most signals. Of course, if you are recording the low frequency vibrations from wind machines or the subsonics of jet engines, it might be usefull to eke out a bit more lf response. But quite frankly, live with the designers call. By all means do the calcs.

 

[jensenmann]

Depending on the age of the desk it might have been designed with tape and/or lousy PAs in mind. That means anything below 50Hz was not considered to be important, more the opposite as stated by Doug. Nowadays tape doesn´t mess up the lowend anymore and we have speakers which are able to reproduce a solid lowend down to 25Hz. In that scenario a vintage desk with small coupling caps could be a limiting factor in lowend reproduction. When I restore desks I always tell this to their owners and usually they decide to open up the lowend at least one octave. That would mean to double every coupling cap in the signal path while recapping.

There is a reason not to got too crazy with the big caps in the PSU. The larger they are the shorter is the time available for the rectifier to charge them. At a given current drew from the desk this means that a certain amount of energy has to be pushed through the rectifier in a shorter time. In worst case this could create problems like local overheating, etc. All that depends on many factors, e.g. if the rectifier is overengineered or working close to it´s limits, how good cooling is done, thermal grease renewed,...