Cartridge Loading

[vpharris]

I have a Sumiko Blue Point cartridge. This is a high output (2.5mv) Moving coil cartridge. Manufacturer gives 47k as proper loading for this, similar to standard MM cartridges.

I built a Pass Pearl preamp, and had some problems with an oscillation at the preamp input with the cartridge connected, and I tried different loads.

I ended up with 4.7k. I've noticed that the output level increases with a greater load, and seems to peak in the 4.7k to 10K range. The signal output from the preamp is definately louder when loaded like this than with simply the 47K.

1) Why might this be?
2) Is there any possible compromise to using a greater load than that specified by the manufacturer?

Thanks!

 

[dice45]

Vince,

cannot help much. Do not know your preamp, is it MM sensitive or MC sensitive? In any case, adding an input load should not cause the thing to oscillate at all. Is it HF oscillation? maybe shunting the input resistor with 100 or 200 pF helps. Should be a 1st quality capacitor however. Mica or potted styrene or NP0 ceramic.

What surprises me is that reducing the damping at the input by reducing the input resistor value reduces/kills oscillation. But in in any case this preamp seems to operate at the margin of stabioliy even with 4k7. Please hunt the mistake withing the preamp and ask the SS gurus what could be wrong with your rig.

 

[vpharris]

Bernhard:

Thanks for your reply. I was hoping I'd hear from you, as you seem to be the highly experienced analogue guy here.

The preamp is basically intended for MM cartridges, as its gain is 60dB. My cartridge is a high-output MC, intended for use without a MC head amp. (2.5mv nominal)

To clarify, with the 'stock' 47K shunt resistive load on the input and the cartridge connected, it oscillates at about 250khz. There's some other component there too, as the waveform is nothing like a clean sine. When I play a record like this, it gets modulated and causes an audible squeal in the 1khz range (if I recall correctly) with the music audible and sounding ok.

Shunting with lower and lower resistances at the input, I still have some oscillation (but no squeal) at 10K, and I no longer see an oscillation when I shunt with 4.7K.

I've yet to complete all that the Pass folks suggested and hope to do so today. But I still wonder... generally if a given load increases the signal level transferred from the cartridge to the preamp, is there any reason not to use it, or could it influence the frequency response characteristic?

If I wanted to use my generator to do some tests on the preamp, how do I set that up? I'm guessing that I build an attenuator that looks like 47K to the preamp to reduce the output of the generator to a low level.

I'll post again after I finish trying the suggestions. Thanks!