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Old 25th August 2005, 05:33 PM   #1
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Default RIAA Preamp

As I plan to « design » an RIAA preamp to upgrade the EMT 938 one (lots of 741 op-amps, but the sound isn't so bad at all !!!!) and after reading some tips particularly from John Curl (Vendetta / Blowtorch) and Jonathan Carr (Lyra-Connoisseur) as well as in various web sites, it seems that a good topology would be : first stage gain + passive RIAA equalization for the higher part of the RIAA curve then a 2nd gain stage with feedback around the lower part of the RIAA correction.

My first question is why a second stage with feedback or more precisely which are the benefits of a feedback stage ?
- Simpler design ? (simpler could be best)
I mean in the case of a passive equalisation for this 2nd stage we need a third stage (fet/bjt follower) after the lower part of the passive RIAA network.

Or probably this stage with feedback just around the 20Hz-500Hz part of the equalization is less sensitive to the classic drawbacks associated with feedback.

Which are the benefit of such a configuration?

Some hints and explanations about the various topologies are welcome!

Thanks.
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Old 25th August 2005, 07:24 PM   #2
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You should read these:

dimitri has written this paper on RIAA topologies and an active/passive RIAA preamp.

National Semiconductor has AN-346 which describes an active/passive RIAA preamp.

Analog Devices AN-124 describes 3 designs.

You will probably find no consensus on topology choice, or clear description of why one is better than another. The topology you describe is similar to EAR 834 and dimitri's preamp.
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Old 25th August 2005, 07:53 PM   #3
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Walt Jung has also written a good peice about RIAA amps. Check his book which you can download.

Thick book by Walt Kester and Walt Jung
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Old 1st September 2005, 07:35 PM   #4
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I am doing some test with a folded cascode fet input design (2 stages) and 2 passive RIAA networks equalization, not bad at all! My Cartridge now is an MC (EMT TSD15).
I will post the schematic as soon as will be completely satisfied with the design ( o.k. for my ears, and my wife!).

What I would like to know is, if somebody here, has tested various capacitors for the RIAA network and which is their first choice (as they are quite expensive...) Actually I listen to it with the Mica ones.

Mica / Ploypropilene / Teflon (for the last ones, like Rel-Cap or V-Cap) ???
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Old 2nd September 2005, 01:52 AM   #5
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Teflon first, followed by polypropylene, then mica, if you choose by dielectric absorption. Polystyrene has slightly beter da than polypropylene. Beyond this you are in the subjective realm.
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