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Old 8th October 2012, 08:42 PM   #41
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Hi again Christian

There are a couple of obvious board-mods, no major changes or op-amp stuff though.
Until you've cleaned everything however, you won't know if they work or not.

...here's that lovely 60 diy mic-pre.

There has been loads said about it on the net; so see what people say.

Better still; just order a pair NOW, with your plastic.
Couple of hours soldering; gets you a valve based mic-pre with op-amp front-end; that you would be hard-pushed, not to know didn't cost $1000 ( serious ).

Probably wouldn't impress people much on gearslutz though.
Oh well; they won't be renting your studio; they'll be too busy waxing on about Neve & Manley; whilst having anxiety attacks about how much the latest video-bandwidth op-amp is going to set them back & how many pico-volts of noise reduction they MAY get ( if they fit it correctly, that is ! ).

Anyway; nuff ranting...
...here's the best upgrade since meths, pipe-cleaners & Lexicons ( whoops, I mentioned some gear; what a slut ! ).

Attached Images
File Type: gif www.paia.com 9704 Valve Mic Preamp - Schematic.gif (11.8 KB, 170 views)
File Type: gif Buy A Pair NOW !!!.gif (10.4 KB, 163 views)
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Old 8th October 2012, 08:52 PM   #42
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One question... Where do I ordere that DIY Mic Pre? Is there a PCB for it? As I can see, I already have most of the components in stock at my workshop, or they have it at my work.
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Old 8th October 2012, 09:09 PM   #43
morinix is offline morinix  United States
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Go to Gyraf Audio and click "DIY-projects" for a tube micpre and more.
Lounge Audio
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Old 8th October 2012, 09:17 PM   #44
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Hi Christian

Smart move on the self-built cables; shop stuff rubbish !
( clean the pins ! )

Personaly I would not track through the board further than a direct-out from the strips ( pre-EQ ).
That's just me though.
Record flat; EQ the mix.

Having reduced the massive potential noise from likely dirty contacts; you could try something on the board.
Not for noise reduction as such ( might help though ).

I would look at replacing components c1 c2 c7 c8 on the schematic; all of which are before the first op-amp.
I think after the big contact clean-up; you listen ( hopefully be TOTALY amazed ! ), and then do the strip-mod on 1 channel & listen to that.
See if it is noticable.

Components c1 & c2 block the 48 Volt phantom-power from getting to the first-stage ( leakey, noisey, electrolytics, not good ).
Any DC leaked through from phantom-power obviously makes the first transistor pair VERY UNHAPPY ( sob...sob...).

My thought is that the component value here ( I think it says 47uF on the schematic ) does not have to be this large a value.
10uF should be fine.
The schematic states c7 & c8 to be 10uF also.
I would replace all 4 of them with 10uF 63 Volt Polyester capacitors.

The orange-blob shape ones are great.
They come to this 10uF, in a very modest sized package.
Can be mounted slightly off-board on short insulated leads etc. if needed.

In the UK I would order CPC part # CA05390 about 1 euro each.
These are actualy 100 Volt, I am sure RS have similar in 63 Volt maybe.
I'm sure you know this component as lecey-guy, very easy to get & compact.


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Old 8th October 2012, 09:33 PM   #45
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Go to PAiA Corporation - Analog Synths, Theremins, Preamps, and More ( a USA based company ).

They have everything for about 60 euro's.
Inc skts, valve etc. etc.
Comes with PCB.
They do not sell PCB's alone.

I would throw some 'trick parts' at it myself ( it is worth it )...
...but the unit works great, as is, simples...
Also some component matching as well..

You could buy a pair to test ( great for precision mid-side etc.)...
...then copy the board youself.

As said the Gyraf G9 'all-tube' mic-pre is well worth checking out.
Strangely enough....I'm building one at the moment !!!!!!

There is however no kit available.
There is a PCB though.
But I hear it has 'issues'.
Personaly I am P2P on the Gyraf G9.
The design is WAY more complex than the Paia.
And WAY more expensive; by a VERY large factor.

I am hopeing for good things from the Gyraf G9...
...but honestly...in a blind AB test...
...I do not think one would sound BETTER than the other; just DIFFERENT.

Nice Danish chap the Gyraf designer...
...bung some Lundahls in...BINGO
Back to ye olde days.

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Old 8th October 2012, 10:36 PM   #46
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Of course the other obvious thing to do is...
...Just get rid of the phantom-power blocking capacitors c1 & c2 altogether...
...and replace them with an input transformer.

Benifits :-
No input capacitors before your first gain stage.
RF effectivley blocked from saturating the first sensitive gain transistors.
FREE GAIN !!! ( can't be a bad thing ).

Could try a modest 1:2 or 1:3
Thus backing off the mic-gain control & getting much less noise.

Don't forget also...
...all your inputs don't have to be the same, like a factory board.
Transformer inputs for your best ( probably less output ) mics.
Cheap transformer inputs for your next best, etc etc.
Can fit them one at a time and check brands & results as you see fit.
Still replace c7 & c8 as well.

When the transistor came along, input transformers were dropped from most ( affordable ) desks.
This MAY have not been such an issue way back when...
...but these days, particularly with the cell-phone density the way it is...
...need I say more.


I have used cheap OEP transformers from RS etc.
And expensive Lundahl's from the even more expensive Canford Audio.

You do get what you pay for though.
The main point being, the lower the resistance of the windings; the less noise.
More wire, bigger wire, bigger transformer, more expensive transformer.

Best spec. to look for ???
Pretty much just buy 'em by weight !!!
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Old 8th October 2012, 10:40 PM   #47
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Check out the input transformer market on eBay.

Sometimes there are some great 'bulk' deals to be had.

People who want just 1 or 2 input transformers don't bid on the bulk lots & sometimes you can get a WAY BETTER deal if you bid on a whole load at once.

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Old 10th October 2012, 09:30 PM   #48
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My assistant and I were in the studio this evening cleaning all connectors out. We need a few of the loose microphone cables, but all connectors on the Soundcraft are cleaned thorougly. Next step is recapping the console, which I will start doing on monday when I receive the caps in the mail.
I am looking at eBay for input transformers, waiting for a good deal, then I will modify some of the channels to be transformer coupled, and maybe use some transformers to build some valve mic pre's. I have a bunch of old Philips SQ-40 high-voltage (100V) amplifiers, where the cabinets/cases/whatever they are called, will be excellent for making some "vintage" looking tube preamps. They also have some components I can reuse for the project.
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Old 15th October 2012, 05:42 AM   #49
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Old 21st March 2014, 11:09 PM   #50
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Hey, just wondering if there were any updates on your 600. Especially what mods you did (and how they sound) and if you ended up putting transformers on some channels.

I just picked up a 600 and have begun the restoration process and am very curious how yours is going.
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