SHURE FP-11 is it possible to improve the noise ratio

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didordure

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Joined
Sep 8, 2022
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8
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pyrénées
I have bought for 20$ a shure fp11, expecting a noiseless preamp but it appears to be the opposite. There is a lot of headroom but a lot of hiss too. It's a quasi constant hiss, whatever the knob settings are so I was thinking about changing all the capacitors (they are really old) but would it change something?
Would it be a good Idea to change the opa's? and with what?
I surprised by all this noise for a battery powered transformer preamplifier.

https://www.electronica-pt.com/esqu...k,58de2f683b65d91a592dee964d2ff9b5/no_html,1/

Any idea is welcome :)
thank you.
 
thanks I know, this more about a challenge. I'll change the capacitors anyways. Post the results. If someone has an idea. I want to repair not to buy.
 
The capacitors won't usually make anything noisy

There's always a noise tradeoff in mic preamps - if you're seeing huge headroom, you're using it wrong: whatever input you're sending it to should have low enough sensitivity so that the mic preamp can clip (just) before the input. If you don't set it like this, you don't extract everything possible from the pre.

That said (and I'm usually no believer in opamp rolling)

The RC4156 quad opamp in there - https://www.farnell.com/datasheets/28336.pdf - is 5uV input noise @ 20Hz-20KHz bandwidth
Perhaps a MC33079 could make it a bit quieter - https://www.onsemi.com/pdf/datasheet/mc33078-d.pdf - at 4.5 nV/√Hz and 20KHz bandwidth you should expect some 0.65 uV input noise if everything else is optimized, which it ain't.. ..like the 33K always in series with the optical cell..

/Jakob E.
 
For the moment I have found this :

Shure portable preamp mod

and this:

Shure stepped attenuator: why these values?

If I swap the capacitors which are 30 years old at least and this fp11 has suffered a lot during his life, won't this help a little?
What about the 33k? I can't remove it without disturbing the signal path no?
Yesterday, during tests, the noise timbre changed a little while I was moving it around, like a faulty shielding. This was subtle though.
 
Is ti possible that IC1 being used in the first stage of the preamp and the opto-compressor part in the circuit would be adding so much noise?
 
That's the same amp as in Urei LA-4's. So the mods people do to those might be good for clues. JLM used to have an adapter board that goes in place of one of these and lets you populate it with dual amps like 5532s. I expect the FP11s might lose quite a bit of their (to me cool) dirty tone if the 4136s come out. But yeah, losing the noise would be sweet. I have two of these that if slightly cleaner would see quite a lot of use as limiters.
 
According to the schemo, the input xfmr is a 1:10.7, which is optimum for the RC4156. Replacing it with 5532's (with an adapter) would better the noise level by about 6dB, which is not bad, but I doubt it would make it enormously better.
The comment "It's a quasi constant hiss, whatever the knob settings are" made me look at the rest of the schemo.
The output stage is an 'integrated circuit, power amplifier", so I assume it's similar to an LM380 (which it is not, for the pin-out is different).
This type of circuit is known to be quite noisy.
I would suggest you replace it with a 5534, which would require some kind of adapter or flying leads.
That would be also an opportunity to review the gain of the output stage, which is probably too high for many applications. As is the maximum overall gain is more than 85dB.
I would reduce it by at least 20-25dB for most common purposes. I seldom use more than 45dB of gain for music recording. Now for recording dipteria farts, it's another story.
 
Oh yes! thanks!
The output stage IC is an LM386 (JRC386) with no amplification (pin 1 and 8 are unconnected) which makes 20dB according to the datasheets. Even though you are right, the amplification should be reduced.
I made a mistake, the hiss is getting louder at the end of the amp steps, the 2 or 3 last steps. I am assuming this is because of the logarithmic scale. I don't know.
I have watched with an oscilloscope the output of the first and second (and last) stage of the preamp ans there is noise even at the output of the first opamp. This is between 20mV and 40mV.
I can't realize if this is a lot or an expected amount, but with no input or even with the ground connected to the inputs it seems a lot to me.
I am not sure the reading I had for the output are accurate, there must be something about the impedance, I had a great lot of mucho signal (no input just noise) at the output, over 50V.
I am going to desolder the bridge between the two stages (name 13 and 14) and see what it shows on the oscilloscope for both the opamps.
thanks.
 
didordure said:
Oh yes! thanks!
The output stage IC is an LM386 (JRC386) with no amplification (pin 1 and 8 are unconnected) which makes 20dB according to the datasheets. Even though you are right, the amplification should be reduced.
I made a mistake, the hiss is getting louder at the end of the amp steps, the 2 or 3 last steps. I am assuming this is because of the logarithmic scale. I don't know.
Click to expand...
This is not unexpected. There comes a point where the input amp's noise becomes high enough to emerge within the output noise.
didordure said:
I have watched with an oscilloscope the output of the first and second (and last) stage of the preamp ans there is noise even at the output of the first opamp. This is between 20mV and 40mV.
Click to expand...
Did you load the input? It is essential for measuring noise of most equipment.
Now an oscilloscope is definitely not adequate for evaluating noise, for several reasons:
  • The oscilloscope measures noise over its bandwidth,which is much larger than teh audio spectrum (20 or 22kHz). Noise depends on BW's square root, so noise measured with a 20MHz BW is 31.6 times (+30dB) that measured over a 20kHz BW.
  • The oscilloscope displays the peak-to-peak value, which is necessarily higher than 2 times the rms value (in theory, it could be infinitely higher, although it is seldom higher than 12dB in practical observation
You need to make these measurements with a level indicator. Typically, a soundcard with a measurement software such as REW is perfectly adequate.
 
Last edited:
didordure said:
Oh yes! thanks!
The output stage IC is an LM386 (JRC386) with no amplification (pin 1 and 8 are unconnected) which makes 20dB according to the datasheets.
Click to expand...
LM386 is very noisy. If I were hacking this, I would study that output, redesign it and make an oshpark pcb to adapt to the 386 footprint. Find a good single, low min supply voltage, low quiescent current amp with good drive. There are lots of new op amps to select from.
 

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