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common mode input voltage range

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Jony130

Active Member
Hi, today I find a low noise preamplifier circuit diagram based on ssm2210 and NE5532.
**broken link removed**

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As you can see the long tail pair current is 1mA. And this means that collector current will be equal to 0.5mA. This current will create 1.5V drop across R5 and R7. So the voltage at the op amp input will be equal to 12V - 1.5V = 10.5V. But as we can read in NE5532 data sheet the common mode input voltage should be less then Vcc - 2V = 10V. Also from what I know NE5532 don't show any phase inversion due to exceeding Vcm.
https://www.ti.com/lit/ds/symlink/ne5532.pdf
So what do you think about this? Will it work ?
 

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MrAl

Well-Known Member
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Hi,

If you want to build a low noise preamp i would think you would want an all FET front end, not a bipolar. FET front ends are usually quieter than bipolars due to the noise the bipolars can generate. We could look at this more if you like. Maybe we can find something online.

On the data sheet i see the input might have to be as low as Vcc-3v, so with a 12v power supply that means 9v max at the input. And likewise for the negative rail. If the power supply is -12v then the min input is -9v.

Not hard to change the input circuit though to meet these requirements.
 
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Jony130

Active Member
Hi,
If you want to build a low noise preamp i would think you would want an all FET front end, not a bipolar. FET front ends are usually quieter than bipolars due to the noise the bipolars can generate.
No, I don't want to build one. FET's are better for low noise? FET's has smaller voltage noise than BJT even if a signal source impedance is small?

On the data sheet i see the input might have to be as low as Vcc-3v, so with a 12v power supply that means 9v max at the input. And likewise for the negative rail. If the power supply is -12v then the min input is -9v.

Not hard to change the input circuit though to meet these requirements.
Yes, we can fix this issue quite easy, but I'm curious how can they sold this circuit if it doesn't work?
 

crutschow

Well-Known Member
Most Helpful Member
The lowest voltage noise op amps are bipolars for low impedance applications but JFETS can have lower noise from a high impedance source. For low impedance sources you are unlikely to find a significantly lower noise preamp than using an OP-27 by itself.
 

audioguru

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Most Helpful Member
The datasheet for the NE5532 says that SOME ICs will need their inputs 3V away from the supply voltage. Then the long tail current will need to be doubled.
 

alec_t

Well-Known Member
Most Helpful Member
but I'm curious how can they sold this circuit if it doesn't work?
It will work, but not as well as it could if modified slightly. Besides, there's all sorts of stuff sold that doesn't live up to its promise. There are plenty of gullible customers out there :).
 

MrAl

Well-Known Member
Most Helpful Member
No, I don't want to build one. FET's are better for low noise? FET's has smaller voltage noise than BJT even if a signal source impedance is small?


Yes, we can fix this issue quite easy, but I'm curious how can they sold this circuit if it doesn't work?


Hi,


Oh so you just want to buy one, or are just looking at the spec's of one that is already built. I understand that.

They might be banking on the fact that some op amps will work without the 3v drop from the positive rail to the input. However, it appears to me that the 1ma current is not the only thing to consider here. Consider that one transistor draws more current than the other, that means one input is closer to the positive rail than the other. I dont like that at all.

Either we are missing something here or else they are not showing something in the schematic, or else it is just not designed right in the first place and this would not be the first time this has happened. It could be that the circuit was originally designed for rail to rail op amp and then someone came along and stuck a different op amp in there without reviewing the complete spec's for that new IC. It could also be that they actually use a different IC and just wrote down the wrong part number. Too many possibilities about what could have happened when something isnt right and we cant contact the original author. Contacting the original author is the only way to know what went wrong, but definitely if you build one or buy one you should be ready to modify it to get it to work properly.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Oh great cant wait to hear what happens.
 

audioguru

Well-Known Member
Most Helpful Member
I was going to agree that somebody changed the opamp. Then I wondered why does this circuit use a very modern and new preamp with a very old opamp?

I betcha that the company making an audio product with the very old NE5532 opamp got a new engineer straight out of school. The new engineer was told that the old circuit had too much noise. Fix it.
So the new engineer added the new extremely low noise SSM2210 preamp without looking at the problem caused with the input common mode range of the NE5532.
 
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