Add-on for MM-RIAA to EQ 78 RPM records?

[DeonC]

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

 

[peranders]

http://www.vadlyd.dk/English/RIAA_and_78_RPM_preamp.html
http://sound.westhost.com/project91.htm
http://www.rfwilmut.clara.net/repro78/repro.html

 

[DeonC]

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

 

[Bill Fitzpatrick]

Why don't you build the Project 91 and plug it into one of the high level inputs on the pre-amp?

 

[DeonC]

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

 

[Bill Fitzpatrick]

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.

 

[DeonC]

Hi Bill

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

 

[Cobra2]

Or....get schematics for the PreAmp-phono-section, and modify it, make a switch to enable the modified curve...

Arne K

 

[Bill Fitzpatrick]

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.

 

[DeonC]

Hi Bill

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

 

[sreten]

Or....get schematics for the PreAmp-phono-section, and modify it, make a switch to enable the modified curve...

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?

Seems to me a switch for the bass time constant and a set of
switches for the treble time constant are the way to do it.

sreten.

 

[EC8010]

Seems to me a switch for the bass time constant and a set of
switches for the treble time constant are the way to do it.

Agreed. A while ago, I looked into this problem for a friend with rather a lot of LPs. Just for early LP, I found that you needed a 9 position switch for the bass time constants, and another 9 position switch for the treble time constants. I hate to think what would be needed to cover all the different 78 "standards". Realistically, there's no way it can be implemented as an add-on to an existing disc stage without completely compromising the noise performance - let alone response flatness or headroom.

 

[sreten]

Agreed. A while ago, I looked into this problem for a friend with rather a lot of LPs. Just for early LP, I found that you needed a 9 position switch for the bass time constants, and another 9 position switch for the treble time constants. I hate to think what would be needed to cover all the different 78 "standards". Realistically, there's no way it can be implemented as an add-on to an existing disc stage without completely compromising the noise performance - let alone response flatness or headroom.

So the disc stage needs modifying to support two pots,
one bass eq 500 to 200 (or lower ?) and another treble
eq 2.2 to 6k (or higher ?)

Hopefully the preamp has two stage EQ.

Point is to accurately set the RIAA values at the extreme
of each dual pot (stereo), accurate values for switches
for 78's don't seem to be an issue.

I did read somewhere that you could get a MM to play 78's
by slugging it with innapropriate loading, this in combination
with a passive eq box could work, at the expense of needing
more gain.

Or was it a crystal cartridge playing 78's into a MM input
works in a fashion due to innapropriate loading ?

sreten.

 

[EC8010]

So the disc stage needs modifying to support two pots,
one bass eq 500 to 200 (or lower ?) and another treble
eq 2.2 to 6k (or higher ?)

Unfortunately, it's a bit worse than that. Bass eq is inevitably comprised of two time constants (as is treble eq, but it's rarely explicitly stated). And that means that you can't solve the problem with a single (calibrated) linear variable resistor.

I never realised it was so awkward until I tried to solve the problem.

 

[sreten]

Unfortunately, it's a bit worse than that. Bass eq is inevitably comprised of two time constants (as is treble eq, but it's rarely explicitly stated). And that means that you can't solve the problem with a single (calibrated) linear variable resistor.

I never realised it was so awkward until I tried to solve the problem.

???? how come ? doesn't make any sense at all !?

sreten.

 

[EC8010]

Start by considering that you are at 1kHz. You now need bass boost, so you set a time constant to determine where that starts. But the replay equalisation (assuming a magnetic cartridge) can't continue boosting at 6dB/oct for ever, so you have to specify a lower frequency bass cut-off, at which point, the recording company has a complementary turning point.

Conversely, the recording company sets the treble boost frequency, but they can't keep boosting treble by 6dB/oct because cutter heads are driven by power amplifiers (perhaps 500W) and expensive, so they add a plateau frequency.

In practical terms, the extra LF time constant only became significant with LP - but it varied considerably until the pairing was standardised on 3180us and 318us. The treble pairing is often not appreciated, but Decca used 10.2us as the upper pairing to 110us. RIAA is 75us with an (unstated) upper time constant (dependent on cutters) of 3.5us or 3.18us.

 

[ticktock]

deon,


if you need scematics email me.

 

[dvautier]

The entire business of trying to get decent or even representative sound from 78s has so far eluded me and driven me to one conclusion—digitization. I run my 78s from an old (big) ceramic stylus directly into my computer sound card as raw wav file. From there I avail myself of the multitude of equalizations and filters at my disposal to develop a reasonable sound rendition. I do have a few pages on this but I’m by no means a purist so forgive my ruthless ways.

http://dvautier.home.comcast.net/records/records.htm