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Old 18th March 2014, 08:16 AM   #1
MIDIMOGUL is offline MIDIMOGUL  United States
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Default Unusual phono preamp

A long time ago (probably in the mid 1990s) Elektor published a rather innovative RIAA phono preamp based on two cascaded opamps. The first opamp would act as the low pass section of the RIAA equalizer, the second opamp was used for the necessary bass lift, as far as I can remember. This was published as part of a larger collection of schematics in one of the July/August double issues or in a Christmas issue, but it could as well have been in one of the later 30x Circuits books. The clever thing with this design is, that it is far less susceptible to input overload due to the very low gain of the first stage and its attenuation of the transients caused by record damage, and thus it actually reduces the effect of surface crackles caused by debris or scratches in the groove of a vinyl record. I never got to buoild this circuit mainly due to lack of time. Can anyone remember this circuit and possibly post a scan of the schematic ? I would like to test this circuit, as it may be very advantageous to use for digitizing old records. Your help would be deeply appreciated.
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Old 7th March 2019, 09:43 AM   #2
tubesaurus is offline tubesaurus  Germany
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this must be the elektor MD phono preamp from 11/1990
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Old 7th March 2019, 11:55 AM   #3
HerrFlick is offline HerrFlick  Netherlands
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Default RIAA preamp 2 x OP27

Or is it the RIAA preamp published in Juli/August 1990 with 2xOP27? This preamp was build by me and it sounds very good. I mounted it inside the Philips 877 pickup together with a +15V and -15V power supply. 3 weeks ago I sold it and was still working very well. I do have the hand made PCB layout, probably some help for you if you want to build this preamp.
Attached Images
File Type: jpg Elektor RIAA-1.jpg (621.5 KB, 102 views)
File Type: jpg Elektor RIAA-2.jpg (384.5 KB, 102 views)

Last edited by HerrFlick; 7th March 2019 at 12:18 PM. Reason: pictures uploaded
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Old 7th March 2019, 10:03 PM   #4
MIDIMOGUL is offline MIDIMOGUL  United States
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Smile Thanks !

Thanks, and I am quite sure, that you have indeed found the right preamp. I am not surprised, that it worked well after many years, because it uses no components susceptible to accelerated aging. Metal film resistors are very stable and reliable under the negligible load in this circuit, and so are all solid dielectric capacitors.
The only components likely to fail after many years are the aluminum electrolyic reservoir capacitors in the power supply. If you use the 78L15 and 79L15 regulators (ideally with steering diodes), the total load for each power rail will be just under 20mA for a stereo preamp.
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Old 11th March 2019, 08:50 AM   #5
tubesaurus is offline tubesaurus  Germany
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Hi,
I also made a PCB, with a dual mono design, for each channel a separate power supply. It is a modified MD Elektor circuit.
When you use other OP`s, you must be aware that the current draw can be different, which can be important for the current sources setup . More info under "power supplies".
Attached Images
File Type: jpg MD_bestueckung1.jpg (162.0 KB, 53 views)
File Type: jpg MD_phono_TL431.regler.JPG (180.6 KB, 56 views)
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