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Old 10th February 2008, 09:08 AM   #1
gmikol is offline gmikol  United States
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Default Opinions on Mic Pre-Amp circuit

Hi everyone--

I'm trying to design a balance mic preamp circuit for myself, and I need some feedback on a couple of different options for the last stage which is going to drive the line-in of my sound card.

In the first one, the drive stage has fixed gain, which I moderate with an attenuator. The second one uses a pot in the feedback to adjust gain.

Another question I have is regarding noise. Is it better to have more gain early?

Right now I have the circuit drawn with OPA2134. I've been considering OPA2228 or perhaps LM4562. Can anyone recommend any other good, low-noise opamps for a design like this?

Also, I haven't started the PCB layout yet, and I don't really have any experience with high-gain circuits like this...any tips or tricks on the layout side?

Thanks for everyone's help--

Greg
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Old 10th February 2008, 09:09 AM   #2
gmikol is offline gmikol  United States
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Here's the other design option with a pot in the feedback to set gain.

--Greg
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Old 10th February 2008, 09:32 AM   #3
Gordy is offline Gordy  United Kingdom
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I have never designed one, but these guys have...

http://www.prodigy-pro.com/forum/viewtopic.php?t=16909
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Old 10th February 2008, 10:09 AM   #4
AndrewT is offline AndrewT  Scotland
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I would be tempted to replace IC1a & b with the 2228 option.
The input resistors, r10 to r13 inclusive, should be matched to better than 0.1% and preferably <=0.01%.

Use a very low noise regulator for the power supply.
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Old 10th February 2008, 06:24 PM   #5
gmikol is offline gmikol  United States
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Quote:
Originally posted by Gordy
I have never designed one, but these guys have...

http://www.prodigy-pro.com/forum/viewtopic.php?t=16909

Holy **** that's a lot of info!!! How did I not find that one while I was searching? It's gonna take me weeks to go through it all.

While I'm reading...anybody out there care to comment on which of my 2 options is a better choice, and if the gain structure is OK? I've forgotten what little I learned about noise analysis.

This thing is going to be battery powered with 9.6V NiMH batteries, 4 in total. 2 for phantom power, and 2 for the opamp supplies.

--Greg
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Old 10th February 2008, 06:51 PM   #6
Minion is offline Minion  Canada
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You know you can get pretty much that whole curcuit in an IC (Ballanced Differential input and unballanced output) useing a THAT1510 or a INA217 and it will have lower noise and the Gain can be set with either a fixed/switched resistor or a Gain pot...It would save money as these Chips are less that $5 each and it will save in PCB size...I have built Basic Mic preamps with these Chips that were less than 2in Square and totally quiet and have a Max gain of 66db (72db if you add a ballanced line driver or Output transformer)......


Cheers
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Old 10th February 2008, 10:37 PM   #7
gmikol is offline gmikol  United States
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Quote:
Originally posted by Minion
You know you can get pretty much that whole curcuit in an IC (Ballanced Differential input and unballanced output) useing a THAT1510 or a INA217....
Yeah, I know about those chips. I've got a design using the INA127 on the drawing board as well.

This is as much about learning for me as it is anything else. So I need to go down the "wrong way" path and see where it takes me.

Wrong Way #1 (unpublished): 100K/20K feedback resistors and a TL072 with 47dB of gain makes for a poor mic preamp.

--Greg
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Old 11th February 2008, 08:16 AM   #8
tINY is offline tINY  United States
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I bet that the TL072 pre amp could be made a lot better with an impedance matching transformer, but that's a $100 option for a good one....

Is there some reason you want 18v phantom power? - seems to me you could stack the batteries and at least get 36v without trouble.




-tINY

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Old 11th February 2008, 04:30 PM   #9
gmikol is offline gmikol  United States
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Quote:
Originally posted by tINY
Is there some reason you want 18v phantom power? - seems to me you could stack the batteries and at least get 36v without trouble.
Since I'm using 9V batteries, that's 2 for phantom power and 2 for the opamp supplies. Something in the back of my head is telling me I can't hang the mic power across the +9 and -9 rails without bad things happening. Am I wrong?

Aside from a lower Max SPL capability, what's the downside of a lower phantom supply? From doing some reading, I realize that the 6k8 resistors should be reduced to something like 1k2.

If I *can* hang the phantom supply across the +9 and -9, then I can use only 2 batteries total instead of 4.

--Greg
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Old 12th February 2008, 01:48 PM   #10
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Quote:
Originally posted by gmikol



Aside from a lower Max SPL capability, what's the downside of a lower phantom supply? From doing some reading, I realize that the 6k8 resistors should be reduced to something like 1k2.


--Greg
Depends on the mic. Not all mic preamps are happy with funny voltages, some may simply move outside there normal bias points and be grossly distorted - for example, if it is a valve mic, bad stuff may happen unless it gets the expected 48V.

I cannot see any harm in stacking the phantom voltage on top of your +9V rail, which would get you up to 27ish volts.
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