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Old 25th February 2004, 08:50 PM   #1
DeonC is offline DeonC  South Africa
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Default Add-on for MM-RIAA to EQ 78 RPM records?

Hi guys

Does anyone know of a circuit that can be put in front of an existing RIAA MM stage that can do pre-EQ to work with the differing curves of 78 RPM records?

I am looking for a circuit which will take into account the EQ that the RIAA stage will do, and then allow EQ to be adjusted for the various curves for 78 RPM records. There are a few projects on the net for stand-alone 78 RPM EQ pre-amps, but they will not work in this case. Can anyone help, or is anyone adventurous enough to try and design this?

Thanks,
Deon
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Old 25th February 2004, 09:01 PM   #2
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http://www.vadlyd.dk/English/RIAA_an...PM_preamp.html
http://sound.westhost.com/project91.htm
http://www.rfwilmut.clara.net/repro78/repro.html
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Old 25th February 2004, 09:33 PM   #3
DeonC is offline DeonC  South Africa
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Hi Per

Thanks for the links, but the problem is that these are all stand-alone pre-amps. Let me explain my situation- I am in the process of buying a Linn pre-power set-up (the LK1 and LK280). The Linn LK1 has two phono inputs: MM and MC, albeit that both go through the same phono-stage (the MC just has more boost). I want to use the MC input for playback of normal LPs, but I want a pre-amp that I can plug into the MM input, but one that will EQ to compensate for 78 RPM records. Now if I plug the signal directly into the MM stage, the said stage will EQ the signal to RIAA standards, i.e. a turnover at 500 Hz down to 50 Hz will be turning the bass up with 6 dB/octave (first order filter) up till +17 dB at 50 Hz, and at a turnover of 2120 Hz, will be turning the treble and middle down with 6 dB/octave until -13,7 dB at 10 kHz. This is fine for RIAA records, but not for most 78 RPM records, SO, I need a circuit that will re-EQ the signal to compensate for the RIAA stage, and add the correct turn-over frequencies.

Here is an example: with some 78RPM records the bass turn-over frequency is at 200Hz, so this stage will have to a 6 dB/octave (first order filter) CUT between 500Hz and 200Hz to oppose what the RIAA stage is doing (remember, it boosts from 500Hz down to 50Hz), and then let it go (as it must be boosted below that). Same for the high frequencies where the turn-over frequencies can vary between 3.4kHz and 6.6kHz, and some have none. So therefore a 6 dB/octave (first order filter) boost will need to be added between 2.12kHz (the place where the RIAA curve starts cutting) and the relevant turn-over frequency of the label being played to oppose the RIAA cut (various lables had their own curves), or until a preset upper cut-off frequency (for those not having a upper frequency turn-over). Both the bass and treble turn-over frequencies also need to be independantly adjustable to be able to correctly EQ a specific 78 RPM record. Can you see where my dilemma lies? I do not know of any such circuit or pre being available anywhere. Any ideas?

Enjoy,
Deon
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Old 25th February 2004, 09:37 PM   #4
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Why don't you build the Project 91 and plug it into one of the high level inputs on the pre-amp?
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Old 25th February 2004, 09:45 PM   #5
DeonC is offline DeonC  South Africa
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Hi Bill

All taken up I'm afraid. It has the MM and MC input, two tape-loops, a tuner input and the Aux input. The MM and MC I want to utilize as speced above. The one tape-loop will be used by a reel-to-reel deck, the other for my PC. The tuner input will be used for just that, and the Aux input for the CD-player. Now if the LK1 had about 10 inputs I might not have had this problem

Enjoy,
Deon
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Old 25th February 2004, 10:19 PM   #6
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OK.

I have 2 ideas.

One:

Build Project 91 and add 2 RCA jacks for the tuner. Use a DPDT switch to select which goes to the output.

Two:

The turntable goes into Project 91, a volume control, a passive inverse RIAA, a buffer and finally to the MM input of the pre.
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Old 25th February 2004, 10:28 PM   #7
DeonC is offline DeonC  South Africa
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Hi Bill

Quote:
Two:

The turntable goes into Project 91, a volume control, a passive inverse RIAA, a buffer and finally to the MM input of the pre.
That was one option I was thinking about, but I do think that is a bit of a schlep (adding all that extra stuff in the signal path). I really think my idea is a lot cleaner and easier, but that's just IMHO.

Then again, I'm sure I am not the only one that would appreciate something like this. I am sure there are other people that would like to convert a phono unit to be able to accept 78 RPM records. I mean, if you have a very good phono-stage why discard it just to be able to play 78s. With a pre like the one I'm looking for you can use the existing phono pre and just add a bypass in the the circuit for when you want to listen to normal LPs (if you use the same stage for normal LPs that is). Am I really alone in this quest? I wonder...

Deon
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Old 25th February 2004, 10:41 PM   #8
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Or....get schematics for the PreAmp-phono-section, and modify it, make a switch to enable the modified curve...

Arne K
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Old 25th February 2004, 10:42 PM   #9
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Well, you could reduce the schlepiness factor by replacing the volume control with two fixed resistors once you determine the amount of cut you need.

Let's face it, the quality from a 78 isn't likely to be degraded by this little bit of circuitry.
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Old 26th February 2004, 02:55 AM   #10
DeonC is offline DeonC  South Africa
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Hi Bill

Quote:
Well, you could reduce the schlepiness factor by replacing the volume control with two fixed resistors once you determine the amount of cut you need.
That is true, but it still seems to me to be a very, for the lack of a better word, clumsy way to do it. I am still hoping that one of the circuit geniuses frequenting these fora will take up the challenge and develop this little circuit. Like I said, I doubt if I am the only one that will want this type of circuit. While it might not be the ultimate solution (something like Project 91 into line-level input will be MUCH better), it is IMHO way better than the other proposed solution. I for one will remain being positive and keep on hoping someone will take up this challenge

Enjoy,
Deon
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