ssl 9k mic preamp

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mich

Well-known member
Joined
Oct 31, 2004
Messages
449
Location
Israel
hi all.
just reporting an ssl 9k mic preamp build. (keith andreus schematic)
so i have built this thing ,using a tl72 for the tl51/52(impossible to get)
and powering it with the +/-18v rails.(i also left out the cx capacitors)
the gain of this thing is really high...so noise is high too...(i guess..)
ive tested it with the unbalanced output,into a crapy soundcard,so i dont know how much of this noise should really be there...i also have a noisy
computer next to the mike so...
can someone point out how to perform a noise measurment,using a windows scope?or some tips on this build?any entry will be apreciated!
 
TL072 is not good for the servo. You will need the low-offset TL052 type for that.

For noise measurement, short the input of the preamp with a resistor like your microphone's impedance (e.g. 600 Ohms) and measure output noise level (in e.g. dBm)

Then send a signal through the micamp, and measure it's gain.

Your equivalent input noise (for the selected input resistance) is then your gain (dB) plus the measured noise (dB). Should be on the good end of 110dB, or there's something wrong.

Jakob E.
 
thanks for the info guys,ive orderd some samples of the tl052 from ti....
and im about to go right into measuring the noise of the preamp,so lets see if i understand correctly:
1. put a 600r resistor from pins 2+3 to 1 on the input xlr
2. measure db level(e.g recording silence into sound forge and check levels?)
3. measure the gain of the pre(any suggenstions for a simple proceder?)
4. sum the two
:roll:thanks
 
> the gain of this thing is really high...so noise is high too...(i guess..)

Put the thing in a proper box, away from the computer.

Connect a microphone. The signal should be more room acoustic noise than amplifier noise (unless your room is VERY quiet).

It may be normal to have high noise when there is nothing connected to the input.

There should be a gain control: how is the noise when you are at low gain?

The 072 has higher DC offset voltage than the 052, but that won't make any difference to the noise.
 
the schematic calls for a dual reverse audio pot of 2.5k.
i could only find a normal dual log pot of 4.7k.
i paraleled two 4.7k resistors (on each section of the pot)
and conected it backwards.
the preamp is in a proper box,and with low gain i have no significant noise.
 
I believe I did use TL052 (don't have it in front of me now). I tested mine on the bench sitting on a piece of paper...no box or shielding and really had very little noise. I wasn't using a pot though rather a switch for gain (I think I measured ~92dB). Maybe the pot(s) you're using are funky??
 
Do any of you guy have a parts list? I am about to build this project and would like help on a parts list. Also tommytones if you can illustrate how you wired a gain swith I would be interested in that.
 

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