I'd like to ask for guidance on the type of phono pre-amp for a low output MM cart.
For example: Technics EPC-100c
output 1.3mv
DC resistance: 30 ohms
inductance: 33mH
impedance: 210 ohm, 1khz
This is not a typical MM cart so I wonder if it needs an unusual phono stage? Or just any good MM phono with enough gain? How much gain?
thanks!
For example: Technics EPC-100c
output 1.3mv
DC resistance: 30 ohms
inductance: 33mH
impedance: 210 ohm, 1khz
This is not a typical MM cart so I wonder if it needs an unusual phono stage? Or just any good MM phono with enough gain? How much gain?
thanks!
About four times the gain of a regular MM phono preamplifier (so about 400, 52 dB, at 1 kHz) and you can allow about fifteen times the noise current of a good regular MM phono preamplifier.
As long as white noise dominates, minimizing the RIAA- and A-weighted noise is equivalent to minimizing the spot noise at 3852 Hz. The impedance of your cartridge at 3852 Hz is about 800 ohm, so the noise optimum would be that combination of equivalent input noise voltage and current that gives the smallest total at 800 ohm. An AD797-based amplifier would be close to noise optimal. (An amplifier that is not so close to the noise optimum can still have a noise level well below the record surface noise, though.)
As long as white noise dominates, minimizing the RIAA- and A-weighted noise is equivalent to minimizing the spot noise at 3852 Hz. The impedance of your cartridge at 3852 Hz is about 800 ohm, so the noise optimum would be that combination of equivalent input noise voltage and current that gives the smallest total at 800 ohm. An AD797-based amplifier would be close to noise optimal. (An amplifier that is not so close to the noise optimum can still have a noise level well below the record surface noise, though.)
Last edited:
Elaborating on that last point: if you don't care about the noise level in between records, have records with a surface noise of -60 dB RIAA- and A-weighted with respect to 5 cm/s, 1 kHz and don't want to lose more than 1 dB of signal to noise ratio, an amplifier with 11.6 nV/sqrt(Hz) equivalent input voltage noise and negligible equivalent input current noise would just be acceptable, as would an amplifier with 14.5 pA/sqrt(Hz) equivalent input current noise and negligible equivalent input voltage noise. Practically that means that almost anything goes noise-wise, except dirt cheap TL07x-based phono preamplifiers.
MarcelvdG,
That went over my head but I will study your words. So a phono pre with AD797 in the 1st gain stage is ok?
Now I thought of different topologies that I know of:
1.Step up transformer (seems to be useful with MC carts only)
2.discrete type phono pre.
3.tubes
How can I narrow down my search, in other words which should I rule out?
That went over my head but I will study your words. So a phono pre with AD797 in the 1st gain stage is ok?
Now I thought of different topologies that I know of:
1.Step up transformer (seems to be useful with MC carts only)
2.discrete type phono pre.
3.tubes
How can I narrow down my search, in other words which should I rule out?
I agree with rayma that transformers meant for MC cartridges and high-gain MC stages probably won't work well, while a lower gain MC stage with a relatively high input impedance should do fine.
Tubes could still be acceptable if you only care about the noise level with a record playing, not about how much softer the noise gets when the record stops. An optimally biased ECC83 can do much better than the 11.6 nV/sqrt(Hz) I calculated in post #3.
Tubes could still be acceptable if you only care about the noise level with a record playing, not about how much softer the noise gets when the record stops. An optimally biased ECC83 can do much better than the 11.6 nV/sqrt(Hz) I calculated in post #3.
The output of the cartridge is too low for a transformer, there are things like reluctance and hysteresis to think of.
Making it is also difficult, and a ferrite core might or might not work.
Think of a two stage low noise chip amp, with lots of ground planes. metal film resistors, super quality capacitors, and a linear power supply rated at about 3/4 of the amp max. volts.
The output of the chip is proportional to the input in general, so a higher voltage supply to the same chip will give you more output, hopefully less noise as well.
I have seen designs that use both pre-amps in a chip in series, 5532, TL072 and similar have two amps with a common power supply in the same package.
Same chip used as a two stage amp.
Normal phono cartridges give out 20 to 50 mV, 1.3 is quite low.
Stray noise pickup will also cause trouble.
Making it is also difficult, and a ferrite core might or might not work.
Think of a two stage low noise chip amp, with lots of ground planes. metal film resistors, super quality capacitors, and a linear power supply rated at about 3/4 of the amp max. volts.
The output of the chip is proportional to the input in general, so a higher voltage supply to the same chip will give you more output, hopefully less noise as well.
I have seen designs that use both pre-amps in a chip in series, 5532, TL072 and similar have two amps with a common power supply in the same package.
Same chip used as a two stage amp.
Normal phono cartridges give out 20 to 50 mV, 1.3 is quite low.
Stray noise pickup will also cause trouble.
The attached may be of interest. This is an impedance measurement of the EPC205CIIL which is a lesser cousin of the 100 but similar low output and I have no reason to believe it is hugely different in coil construction. This was something Hans Polak did on a thread where we were looking at different ways of electrically loading cartridges. The interesting part is the equivalent electrical model which may not be perfect, but is better than anything measured before.
The bottom graph shows how loading affects the cartridge, which shows is relatively insensitive to load. Sadly it's not a great candidate for current mode operation which is an interest/fetish of mine.
None of which gives the answer you want, but might inform the discussion.
The bottom graph shows how loading affects the cartridge, which shows is relatively insensitive to load. Sadly it's not a great candidate for current mode operation which is an interest/fetish of mine.
None of which gives the answer you want, but might inform the discussion.
Attachments
Apparently it has a JFET input, which means its equivalent input noise current can't be excessively large, so that's good.
Its equivalent input noise voltage is low enough for moving coil, so it is definitively low enough for your cartridge.
I don't know about the input impedance and gain.
Do you have any information about whether the input impedance can be set to something in the 10 kohm to 47 kohm range? What gain does it have?
Salas Simplistic Folded works with MM and has adjustable gain.
Simplistic NJFET RIAA
The input impedance is 47K and can be set to any value you put on it.
Schematic is also in the post linked above.
I guess this unit will work good?
Simplistic NJFET RIAA
The input impedance is 47K and can be set to any value you put on it.
Schematic is also in the post linked above.
I guess this unit will work good?
That's an interesting one - MM ... but low output (as distinct to being a HO MC - which typically has a similar output!).
As rayma said - any phono stage that offers 52dB of gain (so 4 times the gain of a standard MM phono stage - which expects 5mV input) will cope ... provided it has the ability to offer a high loading to the cart.
Andy
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.