Mic Preamp Improvements? (Soundcraft 800B) - diyAudio
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Old 23rd September 2012, 08:22 PM   #1
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Default Mic Preamp Improvements? (Soundcraft 800B)

I am not sure if this is the right place to ask for this kind of advice.

I am currently running and managing a recording studio, which I have built myself. The studio is based around a Soundcraft 800B mixing desk which is the live counterpart to a Soundcraft 1600. The difference between the two mixers is only the configuration of the modules and the fact that the 1600 has an optional patchbay. Other than that, the input stages and channel strips are the same.

I do like the sound of the desk, and I am in no doubt that it's a high quality piece of equipment, but I do feel there is room for improvement. I do feel that the preamps on the console could use a bit tweaking. They do sound OK, but I suspect that there are some improvements that can be done to give them more life and maybe less noise. I know this is not a clean sounding console, neither is the purpose of the studio, since I mostly record the typical blues, rock and occasional metal bands. I have attached the schematich for the front end of the input modules. As you can see, the very front end is discrete, followed by a couple of opamp stages. The transistors are 2N4403 and the opamps are TL072.

I am willing to do major modifications, but I would like to ask you NOT to advice me to either scrap the console or buy outboard preamps. Outboard pre's are not that practical, both because of the price of a nice pre will be above the 1000$ mark, and 32 channels of nice pre's are not really anything I can afford... Ever...

Give me your suggestions for improvements. Maybe I could benefit from changing the resistors from carbon to metal film, other transistors, constant current source in the emitters of the first stage. Gimme all you've got!

The complete schematics are here: http://www.soundcraft.com/downloads/...guides&id=1282
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Old 23rd September 2012, 08:41 PM   #2
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Excerpt from the Schematics
Attached Images
File Type: png Soundcraft preamp.png (59.1 KB, 329 views)
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Old 23rd September 2012, 10:59 PM   #3
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The easiest thing would be to contact Jim Williams of Audioupgrades as he specialises in just this kind of thing.
Audio Upgrades

Personally I would get rid of the TL072s as a first step.
I replaced the ones which were in my active xovers with OPA2134s and got a significant improvement. Other and better opamps are available but would possibly need extra bypass caps to prevent them oscillating.
Talking about capacitors it might be about time to have your Soundcraft recapped.

That's all I've got, I'm crap at electronics.
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Old 23rd September 2012, 11:28 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

2N4403's are low noise, the TL072's not so much, I'd replace them
with 5532's as a matter of course, but try one or two channels first.

Quite probably changing IC1 only will make the biggest difference.

Years ago for a friend I installed quality transformers into half of his mic
channels of his desk. Not cheap but it worked noise wise, very well.

I only changed half (8 of 16) because they were mic / line level switchable.

Seem to recall though they they had no pre analogue front end to
the IC's, which could explain why the transformers worked so well.

rgds, sreten.

Changing IC1 to 5532 is the cheap obvious no brainer,
and must be done first before anything else, IMHO.
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Last edited by sreten; 23rd September 2012 at 11:54 PM.
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Old 24th September 2012, 04:46 PM   #5
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PS: I would strongly advise you to hang on to that console.
They are very much overlooked gems and very tweakable, built like the proverbial tank and fully modular. A replacement in todays money would cost many thousands if you can actually find that build quality nowadays. To get an idea of the cost I think the closest you'd get would be an Audient console which is only semi-modular.
A Jim Williams tweaked Soundcraft would be better/cleaner than an SSL (that man really does know his business).
Personally I'd tweak some channels myself with OPA2134s or 5532 as sreten suggested, some by Jim and possibly leave some alone. That way you'd have a choice of flavours to cover nearly all eventualities. Jim's mods would get noise down to about -132dB or so and I'd seriously consider having the master module done by him. At $275 it is a bargain.
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Old 25th September 2012, 07:33 AM   #6
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Thank you for the suggestions. I will be ordering some OPA2134's. I do already have a couple of hundred 5532's and I do have other different dual-opamps and I have some tubes of IC-sockets too, so there is room for experimenting.
Well I guess there is no reason to change the transistors in the front end then, if they are already a low noise type. Could there be any design changes done there? What about adding a transformer on a couple of channels?

Yes I could send a module to Jim Williams, but if I could do the work myself, I could save some money.
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Old 25th September 2012, 01:43 PM   #7
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On another forum Jim was very generous with giving out information on what he does to the modules but for some reason he has started to delete all his posts over the last two weeks. May be it cost him business, I don't know.

What I do remember is that he used Linear Technology opamps and favours Panasonic caps. His mods make the modules as transparent as is technologically possible which is why I suggested may be having the master module done by him.
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Old 25th September 2012, 03:10 PM   #8
morinix is offline morinix  United States
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Jim uses very high speed opamps. One of my favorites he turned me on to is the LT1358/9. He also puts current sources on the outputs for class A operation. Biasing Op-Amps into Class A
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Old 25th September 2012, 03:22 PM   #9
morinix is offline morinix  United States
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Get the best caps you can afford that will fit for c1, c2, c6, c7, c8. Raise the capacitance value a factor of 10.
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Last edited by morinix; 25th September 2012 at 03:27 PM.
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Old 25th September 2012, 06:14 PM   #10
RJM1 is offline RJM1  United States
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C6 should be a non-polarized capacitor, but fitting one in there might be a problem.
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