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'Optimised' for a particular op amp?


catman

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G'day all, I did an interesting thing this afternoon.  I modified one of my Bruce Heran design phono stage boards with a high quality 8 pin op amp socket and tried a bit of op amp 'rolling'. 

 

With this particular design I've always used an OPA2134 which sounds superb and was used in the original design, but I substituted a brand new LM4562 thinking that 'this should be quieter'.  Well believe it or not, it wasn't. 

 

It was actually (just) noticeably noisier (at full gain) than the OPA2134!  Not the expected result!  I presume then that the RIAA network was/is indeed optimised for the (FET input) OPA2134.  Interesting!  Ah the mysteries of analog circuit design!  Regards, Felix. 

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If there is distortion as gain is increased with the LM4562 and the opamp is warm or hot to touch then the opamp is oscillating and not performing within Ti datasheet specs and this isn't just simply matching voltage reference within specified range there are other factors as well. 

 

The dual OPA2134 is hard to beat in most cases as it offer's a nice balanced sound signature. 

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G'day mate, no all is stable with both op amps when in use.  I have been advised elsewhere that the 'impedance' of the RIAA network can affect the noise performance of the op amp.  Certainly that seems to be the case here.  Regards, Felix.   

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I would expect the LM4562 to be noisier as it is optimized for low source impedance around 2k ohms for lowest noise.

 

I would expect it to be about 2dB worse wrt noise than the OPA2134.

 

The old NE5532A would be about 0.9dB quieter than the OPA2134, while the NE5534A would be a further 2dB quieter, as it is optimized for about an 8k ohm source resistance, but it is a single opamp not dual.

 

Usually in the interest of lowest noise the RIAA network would be kept to a low impedance.

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One further question, how is 'low 'impedance' defined? Regards, Felix. 

The lower the better, even in the 1st app note for the LM4562 the resistance of the first stage -ve input resistor to ground is 150 ohms.

 

The optimal source impedance for lowest noise is 'en'/'in' ohms. for the LM4562 those figures are typically en=2.7nV & in=1.6pA giving an omptimal source impedance of around 1687 ohms.

 

For the NE5534A, en=3.5 , in=0.4 . so this equates to Rs Optimal for noise as 8.75K ohms, which is much closer to the impedance of an MM cartrigdge at higher frequiencies. ~12K Ohms

 

For the negative input, the lower limit is what the output can drive.

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G'day mate, sadly mathematics is not my strongest point but I can follow your reasoning.  Many thanks and yes the NE5532 is the quietist dual op amp of the lot in this phono stage and it actually sounds quite good with a rather cool/laidback sound. 

 

To be honest I'm finding this stuff absolutely fascinating and I'm actually starting to think that op amp 'sound' is at least in part determined by the circuit and component values around the op amp.  Regards, Felix. 

Edited by catman
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To be honest I'm finding this stuff absolutely fascinating and I'm actually starting to think that op amp 'sound' is at least in part determined by the circuit and component values around the op amp. Regards, Felix.

You hit the nail in the head. If a circuit is optimised for a particular Opamp then there is a reason not roll it out as it may behave drastically different. Just like tube rolling unfortunately I've seen to many idiots on the web tube roll there amps leading to problems and fast tube wear because it has the same pin count.

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Yep.   The fact that changing tubes and opamps results in different sound  .... says something about the circuits (the circuit is no longer the same) ..... but people often like to impart these qualities onto the device they substituted.    Isn't usually the correct analysis.

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Yep.   The fact that changing tubes and opamps results in different sound  .... says something about the circuits (the circuit is no longer the same) ..... but people often like to impart these qualities onto the device they substituted.    Isn't usually the correct analysis.

Wtf? i can change tubes from the same manufacturer some new others 200hrs and they sound different no circuit changes........

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Wtf? i can change tubes from the same manufacturer some new others 200hrs and they sound different no circuit changes........

 

Yes of course.

 

 

What I mean is that if you had an amp.... and you used an EL34 in it  ......  then a KT120 in it .... and you didn't change anything (except like bias).     We could say that they tubes have a "different sound" .... but this wouldn't be sensible without considering if the circuit needed to be changed to get the equivalent performance from the tubes.

 

Same goes for opamps.     It is very like that you should change the rest of the circuit (yes, sometimes only subtly) when you have replaced an opamp with a different type.

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Someone probably has a warehouse full of old opamps somewhere I guess ;)

Don't get started on that Dave, there are guys that swear can packaged variant of the same opamp sounds better than dip8.

All an elaborate distraction to avoid using smt parts though like the rest of the world does for most decent circuit designs now :P

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Don't get started on that Dave, there are guys that swear can packaged variant of the same opamp sounds better than dip8.

 

As myself being one of these people I find the HA variants better sounding from Ti's offerings.

 

As with NOS opamp's. With digital circuits, the newer stuff are much better in almost all aspects. But of course older basic general amplifier opamp's that have been around such as the JRC4556 and NE5532 and OPA2134 stuff are still pretty much used by alot of manufacturers because they are cheap, decent parameter circuit specs and easy to implement. 

 

Unlike tubes, 99% of the older stuff are better because they were made with quality in mind as they were used almost everywhere since the early 1900's due to the absence of semi-conductors and advanced silicon. Also to mention they were used in all sorts of applications from radio's to tv's to military applications and etc.

Edited by DefQon
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