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Old 25th September 2005, 09:47 AM   #1
mp741 is offline mp741  Australia
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Default RIAA opamp phono stage advice

Greetings All,
I wish to build a RIAA phono stage using opamps to be used with MC cartridges. I have read an earlier thread The best phono preamp on op-amps?
with comments by "rjm" where he suggests a two stage passive RIAA approach is a good first step. I am leaning towards this schematic (I'm more of a constructor than a designer!) offered by Linear Tech (see attachment) using the old chestnut - LT1115.
I am seeking advice on several matters:
Does this schematic have any major pitfalls?
Secondly, I would like to alter the over gain of the preamp by -20dB in one instance and alternatively boost the gain by about +10dB in another. Can this be achieved successfully by simply altering the 2 resistor gain network around the second opamp using a dil switch for selection?
Many commercial (e.g. Trichord Dino, Project Phono-Box) phono stages offer “compatibility” with both MM and MC cartridges. Is this attempted by offering a selection of input impedances, loads and gain to suit each type of cartridge but essentially staying with the original topology?
Many thanks for advice, MP.
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File Type: jpg lt1115mcoilschematic.jpg (35.3 KB, 2181 views)
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Old 25th September 2005, 10:42 AM   #2
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Default Re: RIAA opamp phono stage advice

Quote:
Originally posted by mp741
Does this schematic have any major pitfalls?
Walt Jung has made a deeper study about this design. See the book from Analog Devices which you can download free of charge.

http://www.analog.com/Analog_Root/st...minar2_02.html
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Old 25th September 2005, 11:07 AM   #3
rjm is offline rjm  Japan
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For the record what I originally said was:

Quote:
The most ironclad DIY option for those who don't know how to design op-amp circuits is the two stage, passive RIAA, approach [...] Insert your flavor-of-the-month IC and chances are it will have low noise and sound pretty decent.
However, that advice was for rather for moving magnet rather than moving coil gains. Ask much more than 50 dB from this circuit and you start to see the stretch marks. A three stage passive (Hag. Tech Bugle?), or two stage active RIAA design (Phonoclone, etc) starts to make increasingly more sense as gain increases.

To answer your questions:

No it has no major pitfalls.

No you cannot just fiddle around with the gain without consequences.

Most commercial MM/MC stages switch in another gain stage when the MC input is selected.

/R
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Old 26th September 2005, 08:41 PM   #4
mlloyd1 is online now mlloyd1  United States
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i'd think very seriously about using the AD797 if i was building for moving coil and using op amps (versus discrete devices)

mlloyd1
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Old 27th September 2005, 05:45 AM   #5
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For reasons i don't fully understand (free opamp samples?) the INA103 seems these days completely forgotten. And yet it makes for a very nice input stage compatible with very low output MCs.

I last used the INA103 about 10 years ago with a 100uV cart. It sounded at least as good as any opamp i had at hand; noise was extremely low and it could be configured for balanced input.
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Old 27th September 2005, 03:32 PM   #6
mp741 is offline mp741  Australia
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Thank you gentlemen for your advice and interest. The phono amp (MC) I wish to build needs to be a little generic to work with a range of cartridges. For example:
Benz-Micro L2 @ internal impedance 13R / 0.3mV and
Ortofon Rondo Blue @ internal impedance 6R / 0.5mV
Considering the earlier comments, I’m leaning towards a two stage device. First opamp (INA103 suggested, looks good – maybe with a DC servo) with about 30dB gain (no RIAA) which feeds into a second opamp stage (AD845?) – single amp with active RIAA and 34dB gain - overall gain of about 64dB.
I’m not against discrete components but I don’t wish to spend long evenings measuring Vgs’s or Hfe’s…besides, you need a technical mind to work those DMM’s… Uncomplicated discrete front end schematics (which can precede the second opamp RIAA stage and replace the first opamp) most welcome here.
This preamp is to feed an M-Audio PCI card resident in a DAW, i.e. archiving LP’s to CD. The front ends range from Linn LP12 with Origin Live tweaks, Rega 3 to MicroSeiki.
I was hoping to include a MM stage as well. Maybe the best solution here is to build a separate MM stage and house it in the same chassis as the MC amp and have the outboard supply plugged into one or the other.
Cheers, MP.
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Old 27th September 2005, 03:49 PM   #7
mlloyd1 is online now mlloyd1  United States
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analog_sa:

good point. the INA103 is a nice part, too.
i'm thinking the AD797 a better choice in some aspects, but not having built a RIAA moving coil preamp with either, i guess i really can't say.

your feedback sounds consistent with users who i know have used the INA103 in such a way.

mlloyd1

Quote:
Originally posted by analog_sa
... the INA103 seems ... forgotten ... It sounded at least as good as any opamp i had at hand ...
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Old 27th September 2005, 04:23 PM   #8
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Are you sure that you want to use a '797????

BJT inputs...........connected to an antenna. WJ switched from using that part in his reg, if I am correct, to eliminate RFI/EMI issues.

I guess we could ask our resident "fact collector". He has gleaned most everything possible from WJ already. [joke]

A buddy said that he used an '825, and it was great. I have never heard it, so we only have his word. I believe he paid for the op-amp, so he may be biased.

Jocko
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Old 27th September 2005, 06:50 PM   #9
tubenut is offline tubenut  South Africa
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I tried the AD825 in a single op amp TI application note RIAA (MM) stage once and it was very good sounding. Better then OPA176, 604, 134, 712, 847 and 275. (I know there are duals and singlles in this list, i used adaptors.)

I used a 3 op amp electronics and wireless world design for many years that sounded very good.
Passive RIAA, shunt feedback I think. Still have spare boards somewhere.....
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Old 27th September 2005, 07:28 PM   #10
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Quote:
Originally posted by Jocko Homo
Are you sure that you want to use a '797????

BJT inputs...........connected to an antenna. WJ switched from using that part in his reg, if I am correct, to eliminate RFI/EMI issues.
... mmm, that was true but that is comparing cherries with strawberries.

Walt's application was a gain 1 application together with a power stage AND with a certain pcb, a pretty difficult task I must say. This is a high gain application with no extra circuits involved at the output, a completely different task. If the amp is designed properly with options for RFI filters an AD797 will work pretty good.

Personally I have tested, LT1115, LT1028, AD797, NE5534(!), uA741 (as a test only, not so good... ), various JRC, OPA134, TL071, maybe more. For MM cartridge, LT1115, LT1028, Ad797 and OPA134 were the best. I'm sure many AD models will be alright also.
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