folded-cascode FET/opamp MC phono

Werner · 18th June 2004, 11:34 AM

It's too quiet in here, therefore I present this conceptual diagram of a phonostage I am working on. Mind that is it not complete and that R and C values can differ slightly from what is optimal.

A rough prototype with DC servo, LM317/337 regs for the opamps and zener-follower for the front-end succeeds in sounding remarkably dynamic and very transparent in the lower registers, this with a 0.4mV cartridge.

Noise is almost absent, and measured distortion is -70dB, H2 mostly, with only a tiny little bit of H3 coming in above 2kHz.

Elso Kwak · 18th June 2004, 11:41 AM

Quote:

Originally posted by Werner
It's too quiet in here, therefore I present this conceptual diagram of a phonostage I am working on. Mind that is it not complete and that R and C values can differ slightly from what is optimal.

A rough prototype with DC servo, LM317/337 regs for the opamps and zener-follower for the front-end succeeds in sounding remarkably dynamic and very transparent in the lower registers, this with a 0.4mV cartridge.

Noise is almost absent, and measured distortion is -70dB, H2 mostly, with only a tiny little bit of H3 coming in above 2kHz.

Hi Werner,
That's pretty crude compared to this one:
Jfet/bipolar PCB and Schematic (finaly)

Werner · 18th June 2004, 11:56 AM

Elso, if I wanted to design a discrete folded-cascode opamp I would have done just that, don't worry. But the overall aim was simplicity.

And in case you wonder why the cascode current is set with just a resistor, I can tell you that I was following a quite illustrious example there (and I hope no-one will take revenge on me for that).

Elso Kwak · 20th June 2004, 02:45 PM

Quote:

Originally posted by Werner
Elso, if I wanted to design a discrete folded-cascode opamp I would have done just that, don't worry. But the overall aim was simplicity.

And in case you wonder why the cascode current is set with just a resistor, I can tell you that I was following a quite illustrious example there (and I hope no-one will take revenge on me for that).

Hi Werner,
Your frontend is discrete.

Marinos · 5th July 2004, 08:55 AM

Hi Werner,

Could it be adapted for MM gain also ?

Upupa Epops · 5th July 2004, 09:16 AM

For optimal noise parametres by MC is better to use BJT devices - fets are optimal for MM.

Elso Kwak · 5th July 2004, 09:22 AM

Quote:

Originally posted by Upupa Epops
For optimal noise parametres by MC is better to use BJT devices - fets are optimal for MM.

Hi Upopa Epops,
FETs give better sound quality. John Curl was able to achieve fenomenal signal to noise ratios in the Vendetta MC-phono amp with FETs.
I do not complain about noise either with off the shelve 2SK389BL's

Upupa Epops · 5th July 2004, 09:36 AM

Yes, I agree, in my experiences fet's sound is better. But for lower noise with MC you must to use 4 - 8 pieces in parallel ( look at MC preamp designed by Erno Borbelly - it is in my opinon the best preamp for this using )

.

Werner · 5th July 2004, 09:41 AM

"better to use BJT devices"

Wouldn't it be wonderful if people just read what is written? When I say that noise is almost absent, then I mean that noise is almost absent.

And I mean so even compared to the sub-1nV/sqrt(Hz) BJT-stages I have lying around.

Anyway, try to build the same/similar design with BJTs. Hawksford did, and could not do without a number of elcaps in the front-end signal path.

Elso, from you last reply I gather that you found the hidden reference in the third posting from the top?

"Could it be adapted for MM gain also ?"

Sort of. You could try with a lower gain from the opamp, running the front-end still at 30dB or so. This works, but drives the input FET at much higher input and output levels, and distortion may rise.

You could lower the FET's gain to 10 or even less, simply by scaling down R1. But in that case you'd have to scale up C1 to maintain proper RIAA equalization.

Fine if you can get decent 330nF caps, precision-matched and of known value.

Even so I'd still expect more distortion.

As an aside, given the non-linearities that moving coils exhibit in their frequency response I am slowly being driven to conclude that trying to get a phono preamp to within .1 dB of the RIAA standard is utterly useless, and may actually be counter-productive.

Upupa Epops · 5th July 2004, 10:05 AM

Discussion about noise of phono preamps is quite academic and we can talk about this only in time, when tonearm " is excited "

- when fall down, all discussions must be ended, 'cos vinyl's noise is much more bigger

.

5th July 2004, 08:55 AM	#5
Marinos diyAudio Member Join Date: Mar 2003 Location: Athens	Hi Werner, Could it be adapted for MM gain also ? __________________ Marinos

5th July 2004, 09:36 AM	#8
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	Sound quality Yes, I agree, in my experiences fet's sound is better. But for lower noise with MC you must to use 4 - 8 pieces in parallel ( look at MC preamp designed by Erno Borbelly - it is in my opinon the best preamp for this using ) .

5th July 2004, 10:05 AM	#10
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	Noise Discussion about noise of phono preamps is quite academic and we can talk about this only in time, when tonearm " is excited " - when fall down, all discussions must be ended, 'cos vinyl's noise is much more bigger .

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18th June 2004, 11:56 AM	#3
Werner diyAudio Member Join Date: Jan 2002 Location: Belgium	Elso, if I wanted to design a discrete folded-cascode opamp I would have done just that, don't worry. But the overall aim was simplicity. And in case you wonder why the cascode current is set with just a resistor, I can tell you that I was following a quite illustrious example there (and I hope no-one will take revenge on me for that).

5th July 2004, 09:16 AM	#6
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	For optimal noise parametres by MC is better to use BJT devices - fets are optimal for MM.

5th July 2004, 09:41 AM	#9
Werner diyAudio Member Join Date: Jan 2002 Location: Belgium	"better to use BJT devices" Wouldn't it be wonderful if people just read what is written? When I say that noise is almost absent, then I mean that noise is almost absent. And I mean so even compared to the sub-1nV/sqrt(Hz) BJT-stages I have lying around. Anyway, try to build the same/similar design with BJTs. Hawksford did, and could not do without a number of elcaps in the front-end signal path. Elso, from you last reply I gather that you found the hidden reference in the third posting from the top? "Could it be adapted for MM gain also ?" Sort of. You could try with a lower gain from the opamp, running the front-end still at 30dB or so. This works, but drives the input FET at much higher input and output levels, and distortion may rise. You could lower the FET's gain to 10 or even less, simply by scaling down R1. But in that case you'd have to scale up C1 to maintain proper RIAA equalization. Fine if you can get decent 330nF caps, precision-matched and of known value. Even so I'd still expect more distortion. As an aside, given the non-linearities that moving coils exhibit in their frequency response I am slowly being driven to conclude that trying to get a phono preamp to within .1 dB of the RIAA standard is utterly useless, and may actually be counter-productive.

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