Anybody try this JLH phono preamp ?

gaetan8888

Member
2007-04-08 5:11 pm

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Bonsai

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2003-07-25 10:44 pm
Europe
www.hifisonix.com
Using good modern opamps it will certainly be possible to remove the electrolytic caps from the nfb decoupling. Those 10nF in the MC load look interesting if a little controversial. Certainly a worthwhile topology to explore.

Probably RFI suppression. I’m working on an MC stage now and have done exactly the same thing. The cart impedance and resistance is so low that any pole thus formed will be well above the audio band but quite ok for filtering out RFI.

For example, with a 50 ohm coil resistance, the pole is about 3 MHz. Of course this excludes wiring and pickup coil inductance, but it will still be well out of the audio BA d.
 
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dreamth

Member
2010-11-18 9:12 am
Hello


I hesitate between a opamp phono circuit and a discrete phono circuit.

There is the audiolab 8000a phono stage circuit who have a very good reputation.

Gaetan
I personally think you should start with the op-amp based circuit to have a quick reference for anything you might do.JLH shunt preamp is a good place to start! You can buy an Audiolab 8000A anytime at a good price ...no need to build it.
 
Using good modern opamps it will certainly be possible to remove the electrolytic caps from the nfb decoupling. Those 10nF in the MC load look interesting if a little controversial. Certainly a worthwhile topology to explore.


Why would 10nF be anything to worry about at 100 ohms impedance? - filters out RFI very effectively. Its the very high 220pF for MM that worries me, that's an order of magnitude lower cutoff frequency.

You can't use any old opamp for MM and MC, it tightly constrains you to low voltage noise and low current noise, there are only a few devices that can manage both reasonably well. And if you want to have low offset voltage as well that's another way to make things harder. Beware bias-current cancellation schemes when picking that low current noise low offset opamp too...

Basically that's the main problem with the circuit, its trying to do two very different jobs with the same stage. A separate MC stage with low voltage noise allows the MM stage to be optimized for low current noise.