-=> Mike Ross said to Greg Mayman
 -=> about "BAG OF CHIPS" on 02-05-04  07:53.....

 MR> I think your above network would tend to make a ceramic cartidge sound
 MR> a little extra "strident" but I could be wrong. Well,
they all sound a
 MR> little strident already... ;-)

Unfortunately I got the two resistors swapped over in the circuit
I gave. See my message to Roy.

The network will lift the level of stuff above about 2KHz by
about 6dB. This could make the sound "brighter", "shriller",
"more strident" or "harsher" depending on the quality you started
with.

 MR> But I don't think that fixing the riaa wiggle is a real priority in a
 MR> type of transducer which has so many sonic anomalies. An article I

Agreed.

 MR> The only problem seems to be due to loading the capacitive impedance
 MR> nature of the pickup that tends to lose bass response and as such a
 MR> bit of bass boost is usually added. (+12dB starting at 250Hz?) The
 MR> load resistance is often in the megohm range but I've seen Philips
 MR> using a 100K load but with added correction.

The pickup looks like a voltage source in series with a
capacitor, 0.002 uF is a typical figure.

At that value, a 3db LF rolloff at 50Hz would need the load
resistor to be 1.59 megohms.

If the RL was 100k, the response would be roughly 24dB down at
50Hz, and 10.5 dB down at 250Hz. So your figure of 12dB boost at
250Hz looks about right.

Interestingly, adding a capacitor across the low load resistor
can extend the bass response. For example in the above case, a
0.033uF cap connected across the 100k resistor will lower the
output voltage of the pickup to 1/17.5 of its previous level due
to voltage divider action, but the effective impedance will be
equivalent to the .002uF and .033uF in parallel!

This will bring the 3dB rolloff point down to about 48 Hz. So it
provides the necessary bass boost, admittedly at the cost of
having to add extra gain in the system.

But a passive boost network elsewhere in the amp would need extra
gain anyway...

 MR> Also, since the output sensitivity level is in the 100's of millivolts
 MR> and more, often only a unity gain buffer is used between ceramic
 MR> pickup and the amplifier.

Some high quality ceramic pickups were as low as about 10mV
output. The ones in cheap record players had a MUCH higher
output. I heard of one that gave nearly a volt.

You could always add a simple gain stage after the LM311, if
necessary.

   ,-./\
  /     \ From Greg Mayman, in beautiful Adelaide, South Australia
  \_,-*_/    "Queen City of The South"    34:55 S  138:36 E
       v

... Reading the small print is education; not reading it is experience.
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