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Old 13th November 2009, 03:07 PM   #1
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Default About a phono preamp

Hello

Here is a phono preamp using the LM318 but at pin 1 and 5 it use a super-matched LM394 transistors pair.

The LM318 are a fast op-amp with no rise of distortions in high frequency or at very low level, its an old but good op-amp.

There is more modern op-amps but most of them do have a strong rise of distortions in high frequency or at very low level.

This circuit use an external LTP input to keep the speed but not the noise.

Any opinions, ideas ?

Bye

Gaetan
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Old 13th November 2009, 04:45 PM   #2
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It's better to use a modern opamp. This technique was a sign of missing good low noise opamps. You may check for opamps with noise figures of 1 nV/Hz or so. AD797, LT1028, AD4898 etc.
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Old 13th November 2009, 04:58 PM   #3
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Originally Posted by peranders View Post
It's better to use a modern opamp. This technique was a sign of missing good low noise opamps. You may check for opamps with noise figures of 1 nV/Hz or so. AD797, LT1028, AD4898 etc.
Hello peranders

Yes I know that there is op-amps with much lower noise, but looking in data sheets, most of them do have a strong rise of distortions in high frequency and at very low level, and that are not very good since it's an indication of an input common mode distortions and output crossover distortions.

Thank

Bye

Gaetan

Last edited by gaetan8888; 13th November 2009 at 05:10 PM.
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Old 13th November 2009, 05:09 PM   #4
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What do you mean by strong rise, from what to what? Any examples?
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Old 13th November 2009, 05:35 PM   #5
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Originally Posted by peranders View Post
What do you mean by strong rise, from what to what? Any examples?
Hello peranders

Here is an examples of strong rise of distortions in high frequency.

Bye

Gaetan
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Old 13th November 2009, 05:56 PM   #6
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0.04% at 20 kHz at a rather heavy load AND 60 dB is not much and if you think so you have use two opamps.
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Old 13th November 2009, 08:08 PM   #7
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I must add that the gain is not constant in the table you have shown. The gain in only 10 at 20 kHz so the LT1028 for example will have pretty low distortion. The AD4898 has 0.0003% distortion at 100 kHz.
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Last edited by peranders; 13th November 2009 at 08:11 PM.
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Old 14th November 2009, 06:14 AM   #8
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Hi,

distortion values must rise with increasing frequency, since the openloop gain is only constant to a couple of Hz (150dB@ 0-5Hz for the LT1028). With rising frequency the OL-gain sinks (<120db@100Hz, <80dB@10kHz), which means less feedback for a constant closed loop gain, hence distorton values must rise.

jauu
Calvin
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Old 14th November 2009, 04:18 PM   #9
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Hello guy's

An op-amp with a high open loop gain and high slew-rate would be better but a bit more noisyer and more prone to oscillate. There is the THS4031 who are very good.

Maby I should go back to the olf faithfull OPA637.

Thank

Bye

Gaetan
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Old 14th November 2009, 04:48 PM   #10
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you can take advantage of the fact that the transfer linearity in most op´s is much better
then the common mode case. that requires to use the 0p with phase inverting shunt feedback. you can see that the LT1028 distorts much less if you go for an inverter stage (see distortion at -1000)
there is another circuit that has no common mode distortion and high input impedance if the input impedance of the shunt stage is a problem.
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