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Old 29th April 2004, 11:39 AM   #1
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Cool op-amps for phono stage

Hi,

I am experimenting with different opamps in a kit phono stage (which I am comparing with a commerical Groove phono stage). The original configuration involved three gain stages based around LT1028 / OPA627 / OPA627. In general this phono sounded significantly better than the Groove especially in terms of rythym, dynamics, the frequency extremes and seperation. However it lagged behind considerably in terms of precision, speed vitality and etching in the midband - especially noticeable on electric guitar and female vocal which form a disproportionate amount of my record collection.

Swapping the LT1028 front end for an OPA637 improved female vocal and guitars but made the sound warmer and a bit softer. In addition it raised the noise floor a lot and the dc offset became a real problem when using the stepped attenuator of the volume control unit. Swapping the OPA627 for AD825 made a much bigger improvment and removed a lot of the softness and warmth. However, as they AD825 wore in and became less edgy there was still some warmth and softness apparent which I attribute to the OPA637 now in the first gain stage. As I assume the OPA637 is just a decompensated OPA627 and the AD825 beat the OPA627 hands down it seems likely that finding a replacement for the OPA637 will also bring further improvements.

Does anyone have any suggestions? Anyone compared these opamps with AD8610, AD8099, AD8065, AD811, LM6171? Has anyone experiemtned with high speed voltage regulators? According to the designer the Lithos regulators in the Groove are the major contributory factor to its superiority in this area .

Thanks for your time!

Nick.
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Old 29th April 2004, 03:49 PM   #2
Werner is offline Werner  Europe
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The Michell Iso HR had the LT1028 as front-end opamp.

The Groove consists of two Iso PCBs wired for dual mono (each with
one channel unstuffed), and fed from one positive and one negative Lithos reg. One wonders about the opamp used in the Groove, but given noise restrictions it can almost only be the 1028 or AD797.

BTW, a Groove/Iso is a two-stage phono preamp, not three, with all-in-one-go passive RIAA between the two, and DC-servo (plus something proprietary of, IMHO, very little practical value) around the output.
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Old 29th April 2004, 04:41 PM   #3
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thanks do you have any more detailed info about the groove and lithos? I had wondered whether faster regs or battery operation might help my diy phono. do u know what the other opamps in the iso were?

Yes I know its two stage I opened mine up Couldnt see what the op-amps were though, not for lack of trying. Tom did tell me something but I cant remember.

The alternative kit phono I have is three stage. The RIAA equalistation is handled in two active stages.

Any advice welcome!

Thanks,

Nick.
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Old 29th April 2004, 04:43 PM   #4
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Nick

I greatly prefer the 8610 to a 637 as input stage. Replaces a lot of the warmth with speed and clarity. Not to mention much lower offset.

cheers
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Old 11th June 2004, 06:22 PM   #5
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Hi,
I'd like to second nick's question concerning the lithos regulators. Is it anything in the near around the heavily discussed superreg? Shunt? ...

And Nick, do you have any findings which op-amps suit you better?
Rüdiger
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