folded-cascode FET/opamp MC phono

[ThorstenL]

Hi,

Originally Posted by Onvinyl

I don't get where the other end of the 1Meg resistor goes. One of the supplies?

Okay, simple.

Feedback Loop = 316K//10n & 36K, 750R to the collector of Q1 from negative input of U1.

CCS from Q1 Collector to ground or negative supply.

Resistor 1M from the collector of Q1 to the positive input of U1.

Capacitor 1uF or larger (high quality) to ground from positive input of U1.

This does not eliminate the offset of U1 of course, but offset should be minimised and it will track any thermal shift in the input folded cascode. The Offset of U1 can then be easily trimmed off or you can use a 2nd order Servo with a very high value resistor into the inverting input of U1 minimising the sonic impact.

Ciao T

[Onvinyl]

Thanks Thorsten.

Fiddling with it in a simulator indicates it might not be that easy to make it working. I will try on the bench.

However, your first answer indeed backs up my feeling that the input stage deserves something (more) decent as a following stage than 1 opamp.

Rüdiger

[ThorstenL]

Hi,

Originally Posted by Onvinyl

Fiddling with it in a simulator indicates it might not be that easy to make it working. I will try on the bench.

A Fet Input Op-Amp may help...

Originally Posted by Onvinyl

However, your first answer indeed backs up my feeling that the input stage deserves something (more) decent as a following stage than 1 opamp.

Well, I have a "near other commercial stuff" comparison Phono a design with an INA103 set up as Howland current pump (so it effectively behaves as transconductance cell (so it behaves a lot like a folded cascode).

The second stage set up as inverting Op-Amp with suitable input impedance and feedback network to affect the HF timeconstant with a cap to ground and the input resistor, plus the LF timeconstants in the feedback loop.

OPA is of course the OPA637.

No Servos.

No Coupling Caps. Hand trimmed offset using Vishay trimmers.

All Rhopoint wirewound copper alloy resistors for the critical stuff and mil spec east german made polystyrene caps (1985 vintage) for the EQ.

Massively overbuild unregulated supply, using four 50VA transformers with full wave schottky rectifiers and 10 pcs 10,000uF Caps in RCRC...RC filters per voltage.
Seperate 10R resistors and 80uF/35V Film Caps for each Op-Amp with some additional SMD Film Caps for decoupling.

This phono stage was in part inspired by the Emitter and the Vendetta plus some twists of my own.

Two section chassis, non-magnetic & copperclad in the audio section.

It takes some very SERIOUS phono-pre to better this little "illustration of a good commercial Phono".

So far not much commercial, solid state or hollow state did much better (except some extreme tubes stuff).

So, that "better than one op-amp" may take some doing.

The Phonostage that was designed with this one (and many others) as reference is not entierly inexpensive and was the one I referenced above.

Ciao T

[Onvinyl]

Thorsten, for sure I did not want to sound pretentious. I you get great results with that opamp-stage this is good news to me, and I will try to get there.

Rüdiger