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catman

If you have the best dual op amps, why not use them?

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G'day all, I'm talking about LM4562 dual op amps which are by all accounts about the best of the present crop for audio use, at least judging by the data sheets!  So after doing a few wiring repairs on my existing DIY ESP P06 phono stage, I whipped out the existing dual op amps and put in two LM4562's from my existing stocks. 

 

Yeah it sounds pretty good and the hiss is noticeably less so I guess that I'll leave them in there.  I mean with 140 db open loop voltage gain, high slew rate (20 v/us) and ultra low noise and distortion (.00003% THD), they've got to be better, right?  Regards, Felix.      

Edited by catman

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Note that the through-hole version of the LM4562 has been obsoleted.  So going forward only surface mount and metal can (expensive!) versions will be available.  

 

All in the brave new world of Texas Instruments' acquired businesses (National Semiconductor and Analog Devices).  Like the now-obsolete lowest noise pnp dual transistors available - the SSM2220 and MAT03 http://www.analog.com/media/en/PCN/ADI_PDN_16_0034_Rev_-_Form.pdf .  The MAT12 is still available at UKP18 *each* in metal can only.

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G'day mate, yes they are and they seem to be readily available from the usual online suppliers for the moment anyway.  Interestingly enough Element 14 (Farnell's) show the same device available at two different prices.  I'm not sure what that means. :unsure:  They're not too expensive in any case!  Regards, Felix. 

Edited by catman

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Yes - they are compatible.  The OPA2134 has more voltage noise than an LM4562 (8nV/rootHz for OPA and 2.7nV for LM).  

 

The lowest noise opamp is the AD797 at 0.9nV/rootHz, but it is only a single opamp in a package - and is good enough for higher resistance moving coil cartridges all by itself (like the DL103).  For lower noise than that you have to go for a discrete transistor front end.

 

The originals for the ESP06 were NE5532, which has 5nV/rootHz, so broadly the LM4562 will be about 5dB quieter.  The other key thing is that the 1/f noise is lower with the LM, so there will be less low frequency noise.  Definitely a worthwhile upgrade for the ESP06.

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G'day mate, thanks for all that.  Whilst I really like the OPA2134 for a number of reasons, it is audibly quite 'hissy' in the P06.  I have actually discussed this with Rod Elliott who confirms this, but downplays its importance in actual playback, something that I don't really agree with.  As you say the use of LM4562's is a definite upgrade for the DIY ESP P06.  Regards, Felix.       

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For sure the LM4562 is a whole lot quieter than the OPA2134.  It is difficult to say precisely how much because of the rather complicated impedance character of a moving magnet cartridge, and then RIAA EQ.  

 

But, a useful figure of merit is the "noise factor" which compares the total noise from the cartridge connected to the amp to the thermal noise from the cartridge resistance alone.  For a typical moving magnet cartridge with 500 ohms resistance the noise factor for the OPA is 9.3dB - in other words the opamp+cartridge is 9.3dB noisier than the noise from the cartridge resistance alone.  For the LM it is 2.7dB (which is rather good).  So the upshot is that confirms that the OPA is 6.6dB noisier than the LM.  Which is very clearly audible - it is a major league difference.

 

What does this mean with a 5mV cartidge?  Well you multiply the total noise from amp+cart in nv/rootHz by the root of the frequency range, 20kHz.  For the OPA that is 8.5 * root(20,000) = 1.2uV, giving an SNR of 72dB.  A high quality 200g pressing with have an SNR approaching 80dB, so there is more noise from an OPA than is coming from the vinyl.  Same calculation for the LM gives SNR of 79dB, which is quiet enough not to degrade significantly the best pressings you can buy.

Edited by CraigS

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G'day mate, the cartridge that I mainly use is a Shure M97xE with a coil resistance of 1550 ohms.  That makes it a lot worse with the OPA2134 I think.  Regards, Felix. 

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I've just realised that there is another factor that can dominate - the input noise current.  That is tiny (3fA/rootHz) in the OPA because it is FET input, and irrelevant.  It is much higher for the LM because it is bipolar (1.6pA/rootHz), and contributes a bit to the noise.

 

For your fairly high resistance cartridge that makes the noise factors a closer run thing:

 

OPA2134 - 5.5dB

LM4562 - 1.9dB

 

So the LM is still 3.6dB quieter with your cartridge - still worthwhile

 

Since your high resistance cartridge is 1550 ohms, the noise from that is 5nV/rootHz and your cartridge is 4mV output - so the ultimate signal to noise ratio, if the amplifier was noiseless and all the noise came from a 1550 ohm resistor, is 75dB.  The LM degrades that to 75-1.9 = 73.1dB, and the OPA gives 75-5.5 = 69.5dB

 

The LM part is still worth that 3.6dB in my opinion, and a 1.9dB noise figure for your cartridge is about as good as you will get.

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