[ginger]

The RIAA curve is applied to the phono signal to exactly oppose the frequency shaping applied to the cutting head when the master disc is made.

There is a 75us time constant (2122 Hz low pass) which is applied to oppose the boost in the cutting hed amp. This 6dB/octave roll off goes on forever. Obviously the cutting head amp was'nt boosted for ever. That means we MAY be missing out on a bit of high frequency information which MAY effect the spatial image.

The 75us time constant is made up of R6 parallel R7 and C3

Change R6 and R7 to 2K7 parallel 3K4
Lift the end of C3 which joins to R6/R7 and add 68 Ohm resistor between that end of C3 and the old R6/R7 point.

What we are doing is adding 3.18us (50kHz) high pass to limit the 75us roll off above 20kHz.

Report back to this thread with:
A/ Yep thats better
B/ Nope - waste of time
Cheers,
Ginger

[andyr]

Hi Ginger,

Sorry that no one's taken U up on your suggested mod!  

What U say sounds very reasonable.  Do U know whether other commercial RIAA equalisation stages use this trick?

A GEA friend of mine made the following comment when he came round to listen to my system a few weeks ago (we were comparing my GK-1 MC phono stage/AKSA "Paris" head-amp combo against a complete external MC phono stage which I have):

"... the way Hugh has performed the 75us equalisation.  In my experience it is impossible to get a good result except by using passive EQ for this time constant (ie. not feedback!).  I also believe that the 75us rolloff must be blocked at 50KHz (or somewhere between 30KHz and 100KHz) by inserting a resistor in series with this cap.  I discovered this way back when I was working with Trevor (Lees), and read recently that Allen Wright (Vacuum State Logic) is a also a keen advocate."

Is this the same as the mod you are suggesting?

Regards,

Andy

[ginger]

Andy,
YES - exactly the same.
Cheers,
Ginger

[AKSA]

Hi Ian,

I will examine this in detail, Ian.  I think you are right, I've been thinking on this for some time and it's not optimum.

Thanks for your valued input,

Cheers,

Hugh

[andyr]

Andy,
YES - exactly the same.
Cheers,
Ginger

Thanks, Ginger,

You said in your original post: "we MAY be missing out on a bit of high frequency information which MAY effect the spatial image".

Yes, OK, so we insert that res and stop the 6dB/octave roll-off of HFs above a certain frequency.

However, "folklore" has it that MCs have a rising top end - or at least a peak at some high frequency (like 50Khz) - which some people find objectionable (though I have no idea whether my Benz does this or not!).  So the 6dB roll-off which goes on for ever might actually be preventing this rising top end from getting past the phono stage.

IE. is this mod more appropriate to MM cartridges (which I understand don't suffer from this "artefact") than MC???

Regards,

Andy

[ginger]

Andy,

If as you suggest, there is a resonance or a rising impedance due to inductance of the coils (which would be likely) in MC Carts at 50 to 60 kHz then this mod may well agravate that and NOT sound great. I would say however that if its a resonance, then it is likely to be over a very narrow frequency range and so this mod would still give some benefits. Its likely to make about 8 to 10dB difference at 60 kHz. In other words the response will be down about -22dB with respect to the 2122 Hz corner frequency (75 us timeconstant) compared to about -30dB for the unmodified circuit. No Flames please - figures are back of the envelope scratchings and not "proper" engineering calculations.

Once the mod is implemented you can play around with it to your hearts content.

The Calcs are:
New 3.18 us time constant (50kHz high pass) consistes of the existing 47nF cap and the added resistor (68R) - actually gives 3.196 us

To keep the 75us time constant accurate R6 parallel R7 need to be reduced by the 68R you've added.

For 75us with 47nF you want TOTAL resistance of 1596 Ohms. Subtract the 68 Ohms and say 10 Ohms for the OP Amp Output Impedance and you get
1518 Ohms. The suggested resistors 2K7 || 3K4 give 1505 Ohms. Its a bit pointless trying for closer when your using 1% resistors and the cap is 1% tolerance (if that good) as well.

Actually the phono amp gurus suggest its important to get the various timeconstants (corner frequencies as accurate as possible) BUT more important to get both channels the same. So if you have access to capacitance and resistance measuring gear then match the components you use in the 2 channels. All thats getting a "bit over the top" for me BUT if you are a serious Vinyl Freak then you can be as anal as you like.

If you want to shift the roll off limit higher (to keep response down a bit at 50 to 60 kHz) try 56R with R6||R7 = 1530 Ohms (2k7 parallel 3K6 is close)

Cheers,
Ginger

[andyr]

Thanks, Ginger, for your explanation.

Regards,

Andy