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Author Topic: Mic Preamp with a 5534  (Read 2183 times)
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Lefty,

I'm building that circuit, figure it might be the easiest way to go about this... But I'm not sure what the Cb is on the 100nF cap between the +12V and the -12V terminals.

What does Cb mean?  I've never seen that designation. Is it just a ceramic disc?

Please help!

Thanks!!
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Bypass Capacitor (Capacitore bipasse  smiley-twist )
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Lefty,

I'm building that circuit, figure it might be the easiest way to go about this... But I'm not sure what the Cb is on the 100nF cap between the +12V and the -12V terminals.

What does Cb mean?  I've never seen that designation. Is it just a ceramic disc?

Please help!

Thanks!!

Yes, just a bypass wired across the two power voltage rails placed as close to the op-amp as practical. Any small ceramic cap in any package style is fine. And yes Cb is not a common symbol, maybe it's just an Aussie thing?

Lefty


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Must be!

It took me a while, as I kept screwing up, but I finally got that circuit working!  There's not a whole lot of gain on it, so I increased the feedback resistor, using a 100kOhm pot, and then the output of that goes into a TL074 where it gets amped again.  However, there seems to be more noise than there should be.  It's really pretty noisy.

Where would I start to look to lower the noise?

Other than that, it works pretty well!

I'm going to try to build the 5534 again, thinking that might be less noise, but unbalanced.

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The 5534 is very good for an opamp, 3.5nV/root-Hz, but for cutting edge noise performance you want more like 1nV/root-Hz, and there are only a handful of op-amps that can manage that - individual low-noise transistor differential input stage would be cheaper.  Make the input stage have lots of gain so that the next (noisey) stage can't add more effective noise to the signal (30x voltage gain perhaps).

And no resistors in the input signal path, they just add noise (all resistors generate noise).  Feedback network needs to be very low impedance to reduce noise (perhaps 1k / 33ohm resistor divider) - the noise in the 33 ohm resistor will add to the signal but 33 is low enough (noise voltage proportional to square root of R, in fact about 0.13 sqrt(R) in nV/root-Hz, thus 33 ohms is 0.75nV, 1k is 4.1nV)

To get best low-noise performance from a transistor you have to set the collector current at the sweet spot for the device in question, often around 0.1mA.

Google for suitable circuits and look for noise performance near the 1nV/root-Hz range.

Incidentally 1nV/root-Hz is about 0.14 uV RMS across the audio band.  4nV/root-Hz is 0.56uV RMS...  For 60dB S/N ratio the Vrms of the signal needs to be 1000 times the noise Vrms.

If you do want op-amp design I can recommend the AD8656 dual opamp - 2.7nV/root-Hz (better than NE5534 and low-voltage rail-to-rail so can use from single 5V supply).
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