I want to build a tube mic preamp. Any recommendations?

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Terryb

Active member
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May 24, 2013
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32
I'm thinking either an old RCA circuit (50s era) or maybe NYD's Mila.

Basically, I want something low noise for quiet vocals, but still a large, 3-d image that's thick and compelling.  Not really looking for a distortion circuit.  I want big and clean.

Also, I'm hoping to go with a circuit design that's as simple as can be.  Not looking for a kit.  I'd like to build the whole thing from scratch. 

Any recommendations?
 
Thanks guys.  I have looked at the altec  1566 schematic.  It doesn't look too difficult a build, but i've also read it's more of a color piece.  I'd like to build one anyway.
 
I just build Altec 1566 and also build RCA OP-6 clone
They are both high gain around 100db and sounded very good

I recommend RCA OP-6
Here is good clone (hybrid)  Chris from Preservation made
http://www.preservationsound.com/?p=2605
one things is loud tube mic pres are very easy to make so much noise
use DC to fire your tube

And no distortion, it is sparkling clean (at least mine is,,)
I use for vocals all the time
http://ameblo.jp/shokomiya/image-11920258037-13056963104.html

 
A few months ago I designed a PCB for what I called the 'Classic' mic pre to be one of the boards available for the the ongoing EZTube Lunch box project. It is based on my very early mic pre designs which follow the RCA topology of a gain stage followed by an attenuator followed by another gain stage. The original designs used mu followers for each stage for low distortion but the 'Classic' can have the stages configured as an SRPP stage instead if desired which gives better drive capability at the expense of distortion and gain. On the board are separate twin triode gain stages and  footprints for one or two in put transformers (Jensen, Cinemag or Sowter). You can use a variety of tubes  which allows the board to be configured as one or two mic pres.

For example, if you use a couple of 6922 tubes configured as SRPPs , each stage will provide about 30dB of gain. This combined with 20dB gain from the input transformer allows you to build a wo channel mic pre each with 50dB of gain.

My original 'Classic' design used the 6CG7 tube which is a 6SN7 in a B9A envelope. Configured as a mu follower this gave 26dB of gain. I put a pot between stage a la RCA and with a 20dB gain from the input transformer, this two stage mic pre will provide more than 70dB of gain. My very first tube mixer used this exact topology and is still in daily use in a studio in Switzerland.

Of course, there are plenty over other tube tubes you can try like the classic 12AY7 and and 12AV7 and so on.

Anyway, I have never had time to build the EZTube 'Classic' PCB so I am happy to send you one free of charge if you want to try it out.

PM me if interested.

Cheers

Ian
 
I was actually looking for that circuit Ian.  Do you have it posted anywhere?  I have a pile of 6SN7's that need to get used, and a really cool chassie to put it in.  (Sorry for the hijack)
 
usekgb said:
I was actually looking for that circuit Ian.  Do you have it posted anywhere?  I have a pile of 6SN7's that need to get used, and a really cool chassie to put it in.  (Sorry for the hijack)

No problem. Here is the schematic of my original mic pre circuit. I built two of them into a 19 inch rack as a demo unit, hence the schematic title 'twin channel'. I sent this to a guy in the USA who was very keen on it and promised to demo it to loads of people and get me orders for  mixers. He says he lent it to a C&W band who never gave it back. He never paid for it.

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/TwinChannel/micprecct.jpg

Cheers

Ian

P.S I just found this document about it:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/TwinChannel/twinchannelassy.pdf
 
That looks pretty straightforward to build point to point.  I should be able to use a 6SN7 in place of the 6CG7, correct?  Thanks for sharing Ian!
 
usekgb said:
So, with a 4:1 output transformer, the overall gain would be right around 58dB, and it could drive a 600 ohm load, right?

Point to point is quite straightforward. My early prototypes were built this way.  A 6SN7 mu follower to this design should be able to output about +28dBu into a 10K load so via a 10K:600 output transformer you should be able to get up to +16dBu into 600 ohms.

Cheers

Ian
 
this is really nice Ian - as usual!!

hey i've got just 1 question - what does providing the dummy load on the phantom power circuit accomplish? I can't say i have ever noticed that before…


thanks for the smarts Ian.
Timothy




ruffrecords said:
usekgb said:
I was actually looking for that circuit Ian.  Do you have it posted anywhere?  I have a pile of 6SN7's that need to get used, and a really cool chassie to put it in.  (Sorry for the hijack)

No problem. Here is the schematic of my original mic pre circuit. I built two of them into a 19 inch rack as a demo unit, hence the schematic title 'twin channel'. I sent this to a guy in the USA who was very keen on it and promised to demo it to loads of people and get me orders for  mixers. He says he lent it to a C&W band who never gave it back. He never paid for it.

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/TwinChannel/micprecct.jpg

Cheers

Ian

P.S I just found this document about it:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/TwinChannel/twinchannelassy.pdf
 
mutterd said:
this is really nice Ian - as usual!!

hey i've got just 1 question - what does providing the dummy load on the phantom power circuit accomplish? I can't say i have ever noticed that before…


thanks for the smarts Ian.
Timothy

I am not 100% sure. That design must be at least 5 years old. I think it was my second tube mic pre design. I suspect I might have been trying to minimise clicks due to switching the phantom power on or off. I do  not use that circuit any more,  I now have just a switch in series with the two 6K8 resistors.

Cheers

Ian

 
ruffrecords said:
I am not 100% sure. That design must be at least 5 years old. I think it was my second tube mic pre design. I suspect I might have been trying to minimise clicks due to switching the phantom power on or off. I do  not use that circuit any more,  I now have just a switch in series with the two 6K8 resistors.

Cheers

Ian

Thanks for that Ian…

hmmm, maybe if the switch was above that pair of 6k81's? so it switched between either two 6k8's to ground or two 6k81's to pin 2/3?

that way at least the overall load seen by the PSU would be constant?

thats how the bus switching in my console design is - so the total bus impedance is always the same no matter how many channels are connected to it…

T.

 
mutterd said:
ruffrecords said:
I am not 100% sure. That design must be at least 5 years old. I think it was my second tube mic pre design. I suspect I might have been trying to minimise clicks due to switching the phantom power on or off. I do  not use that circuit any more,  I now have just a switch in series with the two 6K8 resistors.

Cheers

Ian

Thanks for that Ian…

hmmm, maybe if the switch was above that pair of 6k81's? so it switched between either two 6k8's to ground or two 6k81's to pin 2/3?

that way at least the overall load seen by the PSU would be constant?

thats how the bus switching in my console design is - so the total bus impedance is always the same no matter how many channels are connected to it…

T.

I would nt recommend that as the two 6k8 resistors to ground load is the same as if a mic input was shorted and will draw about 14mA from the phantom supply. Most phantom powered devices take about 4 or 5mA so the load would not be constant and if all channels were switched off the load would be excessive.

Cheers

Ian
 
ok - so your thinking originally was to switch in the 2nd two 6k8's to set the current without the microphone load to 5.6mA - or approximately equal to what the microphones current draw would be?

and did it not really work, or did it just prove unnecessary?

sorry, i know you said it was a while ago - but I'm working on a phantom power circuit right now so i'm just trying to see how other people are doing it…. for me your thought process is just as important as the execution...
 
mutterd said:
ok - so your thinking originally was to switch in the 2nd two 6k8's to set the current without the microphone load to 5.6mA - or approximately equal to what the microphones current draw would be?

I think so, yes.
and did it not really work, or did it just prove unnecessary?

sorry, i know you said it was a while ago - but I'm working on a phantom power circuit right now so i'm just trying to see how other people are doing it…. for me your thought process is just as important as the execution...

I built a twin channel rack mounting unit to this design and sent it to a 'friend' in the USA who was going to show it to a lot of interested people and get me some orders. After about 6 months it was lent to someone who apparently forgot to return it. Have not heard from my 'friend' since. So I never got any feedback about if it was any better.

I just realised, the other thing about that switch arrangement is that the two phantom resistors are completely disconnected from the mic input circuit when the phantom power is switched OFF. I suspect I was aiming to ensure there was no loading of the input by the phantom power network. I am not sure why. I used a slightly different form of this in my first 6 into 2 mixer. There the phantom resistors remain across the mic input but the phantom power is switched to a dummy load when off (see attached schematic). That mixer is now in use in a studio in Switzerland.

Cheers

Ian

 

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ruffrecords said:
I just realised, the other thing about that switch arrangement is that the two phantom resistors are completely disconnected from the mic input circuit when the phantom power is switched OFF. I suspect I was aiming to ensure there was no loading of the input by the phantom power network. I am not sure why. I used a slightly different form of this in my first 6 into 2 mixer. There the phantom resistors remain across the mic input but the phantom power is switched to a dummy load when off (see attached schematic). That mixer is now in use in a studio in Switzerland.

Cheers

Ian

oh thats really great - thanks so much Ian - that circuit is pretty similar to what i was talking about - do you know offhand what that resistor value is?  9k6 gives us 5mA if we follow the same thinking of maintaining a more constant current flow…

would this work over 16 channels tho? would it be too much? that would mean there would be at least an 80mA draw off the phantom power tap…

I really have no concept of what  i need to keep in the bank for this stuff…

i'd love to hear about that variable HPF - is that helios inspired?

thanks again Ian.
 
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