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MIDIMOGUL 18th March 2014 08:16 AM

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.

tubesaurus 7th March 2019 09:43 AM

this must be the elektor MD phono preamp from 11/1990

HerrFlick 7th March 2019 11:55 AM

RIAA preamp 2 x OP27
2 Attachment(s)
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.

MIDIMOGUL 7th March 2019 10:03 PM

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.

tubesaurus 11th March 2019 08:50 AM

2 Attachment(s)
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".

carlmart 29th September 2019 11:31 AM

What about swapping the order of the RIAA eq stages, putting the bass lift stage first and the low pass stage second?

This would follow what Erno Borbely did with this discrete FET RIAA preamp in The Audio Amateur many years ago.

Unfortunately Borbely's all FET designs became impossible to do nowadays, because FETs have vanished from the market. So chips are the only thing to work with in DIY.

ticknpop 29th September 2019 12:56 PM

Borbely did various phono preamps but mostly HF filter after the first stage and bass boost in the second. There are also all passive RIAA options for a couple of them, but never bass boost in the first stage or HF filter in the second stage.

MIDIMOGUL 29th September 2019 11:52 PM

There is a very good reason to to use a low-pass stage at the input of sn RIAA phono preamp : clicks, which are caused by dirt or fine scratches. These clicks have fast transients and a surprisingly high amplitude and can and do overdrive input stages, which not only distorts the unwanted transients but also widens the resulting audio dropout. Using a low-pass filter with low or no voltage gain will lessen the clicks and even reduce the amplitude, so they become much less pronounced. As these transients do not contain much low-frequencies the high pass filter of the bass lift will not amplify the clicks. This way slight contamination of a vinyl records could be rendered inaudible.

stocktrader200 3rd October 2019 10:48 PM

virtually the same as the 2*op27 1990 design. the 2nd one has about 1/2 the output voltage. I tried op27 back in the day as a cassette head amp, I found it to be rather harsh on the high end and prone to instability. moving to 2 stage flat transistor and LF353 NAB worked orders better.

MIDIMOGUL 3rd October 2019 11:59 PM

I am not surprised, if the RIAA preamp does not sound right if used as a cassette head preamp. The equalisation curves are very different between vinyl records and cassette tape. Also there are different equalization curves for different types of cassette tapes, and none is compatible with the RIAA curves. However it is possible to play back ferrro tape running at 3 i.p.s. with the "Chrome" position of a cassette head preamp. The two curves are sufficiently similar and result in a very slight lift at the high end, which is useful to compensate for the high end loss when copying the tape. Indeed I used a modified cassette deck running ferro tape at 3 a very portable live recording machine. The master tapes would be copied to open reel for editing and ended up as normal cassette copies.
However the 2-stage equalisation has no real advantage in a taoe head preamp, because tapes do not produce steep and high amplitude transients caused by dust or scratches, the very reason for tge 2-stage equalisation design. It effectively prevents the preamp from being overdriven (and hard limiting the amplitude with associated severe distortion).

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