Hi Roy,

On 05-Feb-04, Roy J. Tellason wrote to Greg Mayman:

 GM>> They were unity gain, remember.

 RJT>> I don't think that's likely to be a problem.

 GM>> It would be if it was a magnetic pickup. The output of those
 GM>> usually needs quite a bit of gain.

 RJT>> It's a ceramic pickup,  basically the same idea.  The only
 RJT>> thing I need to put in there is something to give me the
 RJT>> proper equalization...

 GM>> Usually no equalization was used.

 RJT> No?

No! Something to do with the output characteristics of a ceramic
cartridge being such that equalisation isn't necessary.

 GM>> I don't know whether the pickups contained some inbuilt correction
 GM>> for the "wiggle" in the RIAA curve, but I rather suspect it was
 GM>> ignored.

It was ignored, the basic characteristics were such that no reasonable
network was possible to match their output properly to the RIAA specs,
and they approximated to it reasonably well. (That means they were
closer to the RIAA curve than the amplifier/speaker system they were
used with was to a flat response)

 GM>> If you want, you can use the following network either between the
 GM>> pickup and the preamp or between the preamp and the following
 GM>> stage.

 GM>> in ------+---- C1 ------+------- out
 GM>> .        |              |
 GM>> .         ---- R1 ------+----- R2 ---- gnd

 GM>> Typical values:

 GM>> (a) between p/u and preamp: R1 = 510k, R2 = 2.2M, C1 = 150 pF
 GM>> R2 is chosen to give a suitable load to the pickup.

 GM>> (b) following preamp: R1 = 2200, R2 = 10k, C1 = .033uF
 GM>>     R2 may be a 10k volume control.

But only for a magnetic cartridge...

 RJT> Sounds reasonable to me.  What really bugs me is that I *know*
 RJT> I've seen schematics for this sort of thing,  bunches of times
 RJT> over the years, and can't remember where the heck they've
 RJT> gotten to.  I've done a little bit of digging, but having a
 RJT> 4-drawer file cabinet of stuff all mixed together I didn't get
 RJT> too terribly far into it.  One of these days I'll probably get
 RJT> back to that, when I can get to that file cabinet again

 GM>> Values have been rounded to nearest readily available values.
 GM>> Values may be scaled as long as the time constants are preserved;
 GM>> C1R1 = 75uS, C1R2 = 318uS

 RJT> Noted.

 RJT> Worse yet,  I kicked this around quite extensively a while
 RJT> back,  and lost a bunch of the archived echomail when a drive
 RJT> failed,  so I lost all that discussion.  Oh well

Happens :-(.

 RJT>> I have plenty of small caps and ferrite beads and such stuff
 RJT>> around to deal with those issues,  and no particular strong
 RJT>> sources of radio nearby,  so I'm not terribly worried about
 RJT>> it.

 GM>> The only other factor is the possible noise in the amplifiers.

 RJT> Yes.  But the output of a ceramic cartridge is pretty hefty,
 RJT> so I suspect that won't be much of a problem either,
 RJT> particularly if I end up using a unity-gain buffer and some
 RJT> equalization as a "preamp".  The load is going to be the line
 RJT> input of my sound card..

As said above, no equalisation is needed, just impedance buffering.
A ceramic cartridge needs a high impedance input (at the very least
100k) and most inputs are designed round the standard 10k.

George

--- Terminate 5.00/Pro