Mic Preamp Improvements? (Soundcraft 800B)

[morinix]

Originally Posted by RJM1

C6 should be a non-polarized capacitor, but fitting one in there might be a problem.

C1 and C2 should be non polar too if possible.

[ds23man]

There is one important thing to consider if you are going to swap opamps: power consumption! If you swap all your opamps for a type with a much higher consumption, the power supply may give in!

[Charles Darwin]

Good point!

Again I find myself agreeing with Jim Williams' advice: Get a larger Soundcraft psu than the one that came with the console regardless of upgrades.
They come up on ebay regularly and sometimes quite cheap.

[ronned2tm]

I have found non-polarized capacitors at RS at the right values, in sized that could be mounted on the board.

At first I will try to do the upgrades on a few channels, so I can compare them to the stock channel strips.

I am well aware about the problems of changing opamps and the power consumption. I am fully capable of making a more powerful powersupply, at least for the +/- 17 Volt audio supply myself, out of components and materials from my work. Many of these standard components (Low-ESR caps, resistors and opamps) I can either get from my work, or order relatively easy. So cost is less of an issue.

Would there be any point in maybe having a couple of transformer coupled channels? Could that have any sonical benefits?

I also found this post at Gearslutz from Jim Williams about the Soundcraft 1600 (which is basically the same as 800B but with a patch bay):

1600's and 2400's used a standard instrumentation amp design for the mic preamps. It's a pair of noisy 2N4403 switching transistors in front of a 072 opamp.
Modern Japanese transistors will lower the noise, but THD is high with this design at higher gains. It's the same design as used in the TS-24, early 600's and others of that era.

They can be rewired into trans-amps by cutting the opamp feedback loop and tying that feedback resistor to the inverting transistor emmiter. That will lower preamp noise below that of a mic and will lower THD levels considerably. You can get -129.6 EIN with that preamp if reworked.

Jim Williams
Audio Upgrades

And thank you for all your great advice. I am very happy about this desk, and since it is entirely built upon standard components, it will serve me well for many years. I fortunately have a couple of extra channel strips and a group strip lying around.

[AuroraB]

Off the top of my head, that preamp looks very much like the 400B I used for quite some years. Nevertheless, I don't think you will gain much in terms of noise by changing the opamps, as it is the two input transistors that sets the noise level. 5532s should be OK in terms of power consumption. I'm not really sure you'd get much lower noise by changing the transistors either, as 2n4403 is concidered being quite good in that respect, and getting any real improvement with other transistors may need a total redesign. Maybe shopping for some used external mic preamps is a better choice.

[morinix]

True, reducing noise for the micpre amp will be nearly impossible. But lower distortion and better bass - generally a more open sound can be obtained. Upgrade the power supply, then opamps and caps AND ADD A CURRENT SOURCE TO BIAS THE OPAMP FURTHER INTO CLASS A OPERATION. At least do this to the opamp associated with the mic pre circuit. A lm334 set to even ONE MILLIAMP (62ohm set resistor if memory serves me) will help with low level detail linearity. I learned this from Jim Williams.

[Charles Darwin]

Originally Posted by ronned2tm

I have found non-polarized capacitors at RS at the right values, in sized that could be mounted on the board.

At first I will try to do the upgrades on a few channels, so I can compare them to the stock channel strips.

I am well aware about the problems of changing opamps and the power consumption. I am fully capable of making a more powerful powersupply, at least for the +/- 17 Volt audio supply myself, out of components and materials from my work. Many of these standard components (Low-ESR caps, resistors and opamps) I can either get from my work, or order relatively easy. So cost is less of an issue.

Would there be any point in maybe having a couple of transformer coupled channels? Could that have any sonical benefits?

I also found this post at Gearslutz from Jim Williams about the Soundcraft 1600 (which is basically the same as 800B but with a patch bay):



And thank you for all your great advice. I am very happy about this desk, and since it is entirely built upon standard components, it will serve me well for many years. I fortunately have a couple of extra channel strips and a group strip lying around.

Glad you still found a suitable Jim Williams post at gearslutz since he has been deleting a lot of his posts there as I previously mentioned.
I wasn't sure if its cool to mention other fora by name here, some moderators at some fora can get quite touchy about that kind of thing.
-129.6 EIN is not to be sniffed at if you can get that or close to it.

Output transformers should give you a different flavour sound which is/can be a good thing. Output transformers can have more influence on the sound than valves do. In my experience solid state mics with output transformers sound more like vintage valve mics than modern transformerless valve mics.

Enjoy your console and the fiddling with it.
May all your recordings be hits!

[ds23man]

And something very important: The SOUND of a console is mostly determined by its EQ section.

[Charles Darwin]

Then please explain the existence of analogue summing mixers which mostly do not have an eq section yet sound different from each other or digital summing.
Lots of people buy analogue consoles for their sound yet never use the eq section.

[imix500]

It's true, put a Midas XL-88 next to a Rane MLM mixer and there is no EQ, but a huge difference in sound. (They measure differently too!)