Favorite op-amps for RIAA preamp?

[jackinnj]

Speaking of oscillations -- I have noted that with the AD797 as you null the offset the opamp will start to sing -- in the case of a design I was working on yesterday it was modulating a 1kHz sine wave at 6.6 MHz --- so I tamed it with 100R in series with 56pF parallel to Rf. (Remember to read the datasheet to the end.)

[MikeBettinger]

Does anyone any experience with the OPA27 used in a MC amp? Seems like a good choice.

Regards, Mike

[Conrad Hoffman]

Mike, no experience with the OPA27, but have used lots of the old OP27 and OP37. These seem to be modernized souped up versions of the old parts. The old ones were really optimized for DC performance, with decent BW on the 37, but I don't know how good they were for audio. My guess is the new ones will be better all around, but the only way to know for sure is get a couple and test 'em!

Even though my parts aren't here yet, I couldn't keep my hands off the preamp and did some more comparisons. I don't think the LT1028 is the best choice for the first stage. Yes, it has spectacular noise performance, but a mm phono amp isn't that challenging noise-wise. OTOH, because of the eq on the incoming signal, I think the AD845 is a better choice- it's strong suite is quick settling, and it seems to sound cleaner. I did have to select them for low offset. The OPA627 is the winner so far in the RIAA half of the thing, but I have really high hopes for the new National parts.

Right now, I think the preamp is ever so slightly harsh. I still have a couple silver micas in the eq section, and DA on silver micas isn't great. OTOH, I can't really account for any of this on the test bench. Differential comparisons between the channels, using different opamps, shows some differences in recovery time after an edge. These are in a frequency range that might matter, not up in the tens of kHz, but they're subtle, as are the sonic differences, if they exist at all. The builder is probably the worst person to evaluate the circuit!

[jackinnj]

Originally posted by Conrad Hoffman

Right now, I think the preamp is ever so slightly harsh. I still have a couple silver micas in the eq section, and DA on silver micas isn't great.

Try these guys for polystyrene caps -- http://www.surplussales.com/Capacitors/Poly-Unelco.html

[MikeBettinger]

Originally posted by Conrad Hoffman
[B]Mike, no experience with the OPA27, but have used lots of the old OP27 and OP37.My guess is the new ones will be better all around, but the only way to know for sure is get a couple and test 'em!

I have really high hopes for the new National parts.
/B]

I have a 1/2 dozen of them so I decided to build a board. The specs looked impressive as far as mating with my MC cartridge. Actually I was also debating trying some of the new National parts as well. I'm using the passive/active design from their application note for the LM4562. Decided to try it as a clean slate option to my age old discrete design, which hasn't fared well as my amp and preamp have evolved. I'm assuming the LME49710 would be a drop in swap in the same circuit. Keep us posted as to what they sound like.

I hesitated jumping into the National chip because of it's high-end pretentions... Normally this is a code phrase for "choke the life out of the sound with ultra low distortion" , thought I'd try the OPA27s first since they were already at hand.

It might make sense to use the OPA27's in the first stage and the Nationals in the second. The spec sheet on them indicate they are best, noisewise, with a low impedance source. Any thoughts?

Regards, Mike.

[Juergen Knoop]

thank you for pointing at the source impedance.
The datasheet doesn't mention that specially, except for the noise issue of course.
I remember to do my first evaluations with source impedances below 50 Ohm and I didn't recognised this annoying oscillations then.
regards

[jcx]

sorry but the LT1028 won't be the noise winner with mm cart, the ~5K coil resistance turns the heavily biased bipolar Q input noise current into much higher noise than you would have with a decent fet input op amp

http://www.diyaudio.com/forums/showt...t=#post1193893

[Conrad Hoffman]

jcx, I thought about the same thing, but thought mm source Z was lower. What I don't understand is how to evaluate it in terms of the noise formula. My Ortofon is only about 700 ohms DC resistance, but that's not (I think) the output impedance for an AC signal, and I'm sure it varies over the audio range. I could measure it, but fear damaging the cartridge. How much voltage do you thing those itty bitty wires could stand? If I could put a couple tenths of a volt across it, I could sweep the audio range and plot a curve. Or is the Z looking in the same as looking out? Though they don't say, I assume the LME49710 is a well biased bipolar, like the LT1028, based on the input current numbers, but aimed at a little higher impedance than the 1028. I think the OPA627 is down in the low pA, and little will beat it for higher Z applications. National is pretty quiet about the process used for the 49710 series, but this weekend will tell the tale soundwise. I also want to try the passive eq shown in their data sheet. BTW, nobody talks about the AD845, but it has a good combination of specs- what do you think of it?

[SY]

Conrad:

http://www.national.com/an/AN/AN-104.pdf

[Conrad Hoffman]

Sy, thanks- I'm familar with that app note, but had forgotten that it had the phono cartridge example. I wonder if most mm cartridges have similar values?