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-   -   Mic Preamp Improvements? (Soundcraft 800B) (https://www.diyaudio.com/forums/analog-line-level/220251-mic-preamp-improvements-soundcraft-800b.html)

ds23man 28th September 2012 04:46 AM

Of course consoles can sound differently with eq's switched off. But the main use of a mixing console is to "mix" and there is where you use the eq's and are the most contributing part of the sound. That is why you can buy famous eq's in a external box.

I have seen consoles which sound nice with the eq's flat and sound terrible when the eq's are used and vica versa.

Charles Darwin 28th September 2012 10:29 AM

True that but if the Soundcrafts I have used (Ghost and my own Delta DLX) are anything to go by the eqs are actually pretty good.

ronned2tm 4th October 2012 10:54 AM

Update!
I ordered a couple of so-called high performance opamps. I did the master-section and one channel strip. In the same round, I also did recap the master section and the channel. I had finally given in to all the audiophile hype around changing opamps to get "darker background, wider image, more detailed but less hyped highs, violins comming clearer through" and all the other subjective terms to describe differences in sounds, and decided to try to "upgrade" the opamps. I have never believed in all the hype, and I am sorry to say to all of you "audiophiles" that I still think it's a load of cr**. I was very dissapointed when I plugged the "upgraded" channel strip in, and connected a couple of microphones (Neumann U87 and a Shure SM57) and a DI-box to both the channels with a Y-splitter cable. I made sure that both channels were gained identically, checking the level in my DAW so they were matching within 0.4 dB. I recorded some voiceovers, snare hits, bass guitar notes and other stuff,. I A/B'ed the recordings (not a blind test), and other than the improved low-end of the recapped channel, I couldn't hear a difference. There was no darker background (whatever that means) no relaxed mid, no non-hyped highs, just an identical channel with improved low end because of the recapping. The monitor system I use is a set of Yamaha NS-1000M and a Yamaha MX-1000 amp, so I would expect to be able to hear a difference if there were any. Even the noise floor was just about the same for the two channels, any small difference could just as well be differnce in component tolerances. All that hype about super fast opamps is, in my opinion, completely nonsense. The good old 5532 is still one of the best suited opamps for these tasks.
After that, I did some measurements. I did use my DAW and sound interface. Some of you might say that an audio interface and a VST plugin from a DAW is not the best tools for doing measurements, but I would actually think that it will be the most appropriate, since that's where the signals eventually will end. I put a test tone in to the two channels, and monitored the frequency graphs of the two channels (layered on top of each other so that I could compare the two graphs). I could see that there were slightly better distortion characteristics of the reworked channel, but that could just as well be because of the new and better Low-ESR caps I used to recap the channel.
I also measured intermodulation distortion, inputting two different frequencies into the channels at a quite high level, far beyond where the peak lamps where lighting up (though the desk still has some headroom above the peak-light), and I couldn't measure any considerable differences between the two channels and their distortion characteristics.
So based on these tests, with both measurements and listening, I don't believe that changing op amps is the way to improve this console. This will also save me the hassle of acquiring a larger power supply
A total recap of the console is of higher priority. I could measure a 2 dB drop at around 50 Hz, when comparing the recapped and the non-recapped channel. That is a considerable amount when we're talking bass guitar or kick drum.
I tried the mod described by Jim Williams, cutting the feedback loop of the first opamp and connecting that to the emitter of the inverting transistor in the discrete first stage of the preamp, but that didn't work. Can any one of you tell how exactly to do that mod? I couldn't get it to work.

ds23man 4th October 2012 03:49 PM

I am very happy you did discover this yourself....

I have worked many years with Soundcraft consoles in live sound and studio, from the earliest ( series one and two) 400, 400b, 800b, 200b, 1600,2400, 500b, 600b, 8000, Delta and so on. Except for the oldest ones ( transistor and single 24volt rail), they sound all the same. But that is no suprise if you look at the schematics, it is all recycling of the same basic concept. All manufactures do that, every brand has its own sound. But then came along the Crest Century, completely dc coupled inside, balanced mix busses, state of the art ssm input and output chips ( including mic stage). That was a big difference!!!!!

I had a Tac Matchless in my studio, did some work on it ( better opamps, nice ground bus and so on). The only difference it made was the total noise level became a bit better. The "sound" was unaltered!

morinix 4th October 2012 04:19 PM

What op-amps did ya' try?

ronned2tm 4th October 2012 06:24 PM

I tried OPA2134, OPA2604, LM6172, OPA2277 and others. The extensive testing I did, was with the OPA2134 which I often see highly appraised for it's capabilities in the audio field. As I said, I was not able to hear any difference, except improved low end on the reworked channel which probably has more to do with the recapping.

I would still like a more thorough description of the mod I quoted on page two by Jim Williams of Audio Upgrades:
Quote:

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.
I tried that but it didn't work.

morinix 4th October 2012 07:28 PM

Did you actually try listening to real audio with the LM6172?

ronned2tm 4th October 2012 08:09 PM

Yes, I tried real audio through the LM6172 (though the very first stage of my channel modules only use a single op amp, since it does not have a line input at in the schematic, so I used a OPA134 in this position in all the tests). I also activated the EQ (flat though) and both the Phase and Low cut switch, inserting at least five op amp stages in the signal chain. I let the signal exit from the DAW, allowing further op amp stages in the chain.

I would expect that a complete recap of the console will have much more impact on the sound, than changing the op amps. What I have heard during my own tests didn't convince me that it is worth the high cost. These types of op amps are expensive even at bulk prices. Lucky for me, I can get Low-ESR 105 degrees caps pretty cheap throug my work.

Just out of curiosity, I took a look at the schematics of a couple of high-end pieces of studio equipment, among others, the world famous SSL 4000G desk and some other stuff, and most of it were stuffed with 5532's, so I would expect that it is the circuit topology and many of the surrounding components around an opamp that defines the sound, rather than the opamp itself.

My next plan would be to maybe try to find other ways to upgrade, or at least make a couple of channel strips different than the others, to have some options/different colors of sound.

AuroraB 4th October 2012 09:19 PM

Really not trying to be a smartiepants, but a few of us told you.... :rolleyes:
All this op-amp changing is largely based on belief rather than reason. At least it requires a chip-specific design in order to optimise the circuit. As I said earlier, in that circuit noise and several other features will mainly be determined by first transistors in that compound.

If you really want to change the 'sound' I think you should try external mic preamps, tube or SS, buy or build... there's quite a lot of info out there....

ronned2tm 5th October 2012 06:20 AM

I had never believed in all the audiophile hype, not the stuff about opamps either, but while I built the studio, I spent more time hanging around forums where audiophiles and musicians hang around and that apparently got me to give in a little to this hype, but I was confirmed in my original belief, and I am very happy that I didn't give in to all that crap.

The only thing I see can be done here to this desk, is the Jim Williams mod to reduce the noise and to recap the whole thing with high quality caps. Other than that I could spend my time making music and renting out the studio so I can get money to buy more cool gear. I am not at all dissatisfied with the sound of the console, it's not clinical and clean, but as I mostly will be recording rock and the occasional heavy metal bands, it suits my needs fine. I just thought that if there were any easy improvements, I would try them.

Are there any of you who can explain the mod to the first stage of the preamp as I described above?


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