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Pre Amp help

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briantrey6

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Hi,

I got this preamp, and I use it for my microphone. It does a good job on pre amping it and it has no staic. But it sounds like its going through a low pass filter. there are vary little mids and no highs.

How would I mod this so it and sound clear? I know its not ment for a mic but it close to it. I can't spend $100 on a pre amp. This thing cost only 20 bucks. and it has a simple desine from what I can tell. I'm not shore if you can find the specs on this or not, but any help will be good!

PA-111 says on the board. Thats All I know about it.


Thanks.
 

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By your description I reckon you have a phono pre-amplifier. The unusual frequency response would be explained by the RIAA compensation system it uses for magnetic cartridges. You need to use a mic preamplifier, a simple one can be built using an op-amp.
 
Yes, it's a phono preamplifier, it's easily changed for a microphone, and most kit versions give you the option of making either?. It's simply a question of removing the RIAA feedback components, and replacing them with a single resistor.

However, we would need the circuit to advise you, but the feedback components look to be R13, R14 and the two green capacitors below them. The other channel are the ones opposite.
 
Yes, thats an option. I imagine the circuit is something like this:

**broken link removed**

After you remove the filter components, you may notice more noise or "static". You can try replacing the 4558 for a NE5532 if this is a problem, it should work as a direct replacement (although it will need bypass caps if none are currently fitted).
 
.......

Thanks everyone.

After you remove the filter components, you may notice more noise or "static". You can try replacing the 4558 for a NE5532 if this is a problem, it should work as a direct replacement (although it will need bypass caps if none are currently fitted).

I don't know what bypass caps are...

A bit of noise is OK as long as its not to loud. So, your saying all I got to do is take off R13, R14 and the two green capacitors below them? and where would the resister go? just saying if thats (R13, R14 and the two green capacitors below them) is accurate. And what Oms resisters should I use. Potentiometer maybe?

Thanks,
Brian
 
Re: .......

briantrey6 said:
Thanks everyone.

After you remove the filter components, you may notice more noise or "static". You can try replacing the 4558 for a NE5532 if this is a problem, it should work as a direct replacement (although it will need bypass caps if none are currently fitted).

I don't know what bypass caps are...

A bit of noise is OK as long as its not to loud. So, your saying all I got to do is take off R13, R14 and the two green capacitors below them? and where would the resister go? just saying if thats (R13, R14 and the two green capacitors below them) is accurate. And what Oms resisters should I use. Potentiometer maybe?

The new resistor is the feedback resistor, from the ouput of the opamp to it's inverting input. We can't say exactly where it goes, or what value it needs to be, without the circuit.
 
....

Ok, I spent some time and made the circuit into a schematic drawing. I'm not to good at this, so please bare with me. I just put my multimeter on the resistors to be accurate as I can. it was hard to see them color bands :)

I only did the one channel and I did not illustrate the power ether.

but, now I hope you can show me what I need to do to get this working for a mic per-amp.

Thanks
Brian
 

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I think you need to check your colour codes?, the values don't make much sense!.

Anyway, remove the two resistors and capacitors we mentioned earlier (between pins 6 and 7), and replace then with a single resistor from pin 6 to pin 7 - as your values aren't correct, I suggest you try 100K for a start, and see what gain that gives you. Make it larger for more gain, and smaller for less gain.
 
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