Although only practical with MM/MI cartridges, this method gives several other advantages: potentially lower noise from matching to the lower load resistance (typically several K Ohms), and practical removal of the cartridge inductance + loading and stray capacitance LC resonance (typically in the upper octave of the audio range).
IOW, this gives MM/MI cartridges the frequency response of MC cartridges, for better or for worse.
All good fortune,
Chris
Whether the noise gets better or worse or stays the same depends on how you make that low input resistance. When you bluntly shunt the cartridge with a low-valued resistor, it only gets worse.
You can see that by using the Norton equivalent of a noisy resistor: an ideal resistor in parallel with a noise current source with power spectral density 4 k T/R. The lower R, the more noise current it injects into the signal path.
If you prefer more complicated calculations, you can use the Thevenin equivalent (ideal resistor in series with a noise voltage source with power spectral density 4 k T R) and calculate the impact of lowering R on both the noise and the transfer of the signal. You'll then find that lowering R lowers the signal more than it lowers the noise, so the signal to noise ratio gets worse. Add some extra gain to get back to the target output signal level and the output noise will be larger than it used to be.
You can see that by using the Norton equivalent of a noisy resistor: an ideal resistor in parallel with a noise current source with power spectral density 4 k T/R. The lower R, the more noise current it injects into the signal path.
If you prefer more complicated calculations, you can use the Thevenin equivalent (ideal resistor in series with a noise voltage source with power spectral density 4 k T R) and calculate the impact of lowering R on both the noise and the transfer of the signal. You'll then find that lowering R lowers the signal more than it lowers the noise, so the signal to noise ratio gets worse. Add some extra gain to get back to the target output signal level and the output noise will be larger than it used to be.
SY has a calculator on his website: SYclotron Audio | RIAA Noise Calculator
Simplified models can get us into trouble with a MM/MI's impedance and RIAA weighting included.
All good fortune,
Chris
Simplified models can get us into trouble with a MM/MI's impedance and RIAA weighting included.
All good fortune,
Chris
There is no need to neglect anything, just use the Norton equivalents of both the cartridge and the shunt resistor and take into account that the transfer from the cartridge's short-circuit current to the amplifier output has to be kept constant.
See sections 10 and 11 of http://www.cordellaudio.com/preamplifiers/vinyltrak.shtml for the order of magnitude of the noise increase. Mind you, the differences are much smaller when you also take record surface noise into account.
See sections 10 and 11 of http://www.cordellaudio.com/preamplifiers/vinyltrak.shtml for the order of magnitude of the noise increase. Mind you, the differences are much smaller when you also take record surface noise into account.
Last edited:
Just pulled up my well-worn LA Vol 4.
The motor can only put out so much current -- and loading it too heavily (6.7k vs 47k) makes additional gain necessary in the first stage.
Thank you for pointing this out. Cordell now gives a noise penalty, using a Shure V15 V as example, of 2.5 dB with his damped loading when compared to conventional (47K) loading. It also looks like the correction was due to your efforts, so thanks for that too!
All good fortune,
Chris
I frown at the accentuated l.f. part just carrying on and on after the lowest useful l.f., doing nothing but harm. With a correctly fashioned active feedback circuit, one could also have a useful 12- 14dB/octave l.f. cut below say 30 Hz or whatever one's lowest frequency is, without any extra components.
I have a line amp on the drawing board which includes a 3rd order infrasonic filter and I intend these to be used together. I could however move the f3 up a tad but would lose a little bit of 30Hz that way. I decided to put the infrasonic in the line amp because I have some CDs with strong infrasonics (1812 Overture) that tax the subwoofer. I am curious as to how you would choose to implement that however.
Last edited:
MC requires three major things - correct loading, (much) more gain and (very) good shielding from RF interference.
MM is in some ways much easier: the loading is pretty much fixed, requires less gain than MC (which is also pretty much fixed), and (usually) less attention to shielding.
However MM requires much more care to be taken with regard to input capacitance - this includes valve/tube miller capacitance, valve/tube socket capacitance, connector capacitance and all cable capacitances.
When it comes to noise floor, MC should beat MM pretty much all the time. I only ever attempted noise floor measurements on MM carts (that spreadsheet from Sy is very useful). However due to all the finicky implementation, I prefer MM over MC these days. Just make sure input miller capacitance is low enough and be careful with the connections, etc. for them. They tend to be much cheaper than MC too.
I sold pretty much all the MC carts I had a couple of years ago and have not looked back.
MM is in some ways much easier: the loading is pretty much fixed, requires less gain than MC (which is also pretty much fixed), and (usually) less attention to shielding.
However MM requires much more care to be taken with regard to input capacitance - this includes valve/tube miller capacitance, valve/tube socket capacitance, connector capacitance and all cable capacitances.
When it comes to noise floor, MC should beat MM pretty much all the time. I only ever attempted noise floor measurements on MM carts (that spreadsheet from Sy is very useful). However due to all the finicky implementation, I prefer MM over MC these days. Just make sure input miller capacitance is low enough and be careful with the connections, etc. for them. They tend to be much cheaper than MC too.
I sold pretty much all the MC carts I had a couple of years ago and have not looked back.
Last edited:
Modern MM catridges have obviously improved performance, such as frequency response comparable to MC at much lower price, thanks for advancements of magnet technologies.
But MC catridges can still offer better dynamics and THD, if the signal can be processed in DC mode amplifier. That means that the phono amp will be configured to work in DC mode, with MC catridge directly coupled to the phono amp and the phono amp processing the signal as a DC amp, eliminating any and all AC coupling elements on the signal path, such as step-up transformer, coupling capacitor or output transformer.
Then it will sound, in some cases, like a DSD player. Don't know it's a good or bad omen.
But MC catridges can still offer better dynamics and THD, if the signal can be processed in DC mode amplifier. That means that the phono amp will be configured to work in DC mode, with MC catridge directly coupled to the phono amp and the phono amp processing the signal as a DC amp, eliminating any and all AC coupling elements on the signal path, such as step-up transformer, coupling capacitor or output transformer.
Then it will sound, in some cases, like a DSD player. Don't know it's a good or bad omen.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.