Konnichiwa,
Actually, on the principle "where does current flow" this folded cascode is very neat.
In a nomral cascode the signal output is effectively referenced to to positive supply rail but the following stage commonly amplifies the differences between some usually ill-defined "ground" and the input terminal. Under ideal conditions the supply rail and ground are the same in AC terms, in reality they rarely are.
Now the folded cascode as Werner shows:
1) The BJT's base is referenced (AC) to the positive rail, as is it's emitter and it's input signal, namely the current in the J-Fet.
2) The collector current of the BJT is controlled by the J-Fet current and pretty much little else, it, just like the J-Fet operats as variable current source.
3) A resistor (be it a direct collector resistor as Werners original shows or a virtual resistor as in my Vendetta inspired idea) then converts this AC current into voltage which is referenced to the so-called ground and which is fairly well independent from the supply rail noise and modulation (with ideal BJT's, J-Fet's and capacitors it would completely unperturbed by anything on the supply rail).
Now I call that very smart and tricky....
It might be fun to do a folded cascode "Pacific"!
Sayonara
Pjotr said:
Actually, on the principle "where does current flow" this folded cascode is very neat.
In a nomral cascode the signal output is effectively referenced to to positive supply rail but the following stage commonly amplifies the differences between some usually ill-defined "ground" and the input terminal. Under ideal conditions the supply rail and ground are the same in AC terms, in reality they rarely are.
Now the folded cascode as Werner shows:
1) The BJT's base is referenced (AC) to the positive rail, as is it's emitter and it's input signal, namely the current in the J-Fet.
2) The collector current of the BJT is controlled by the J-Fet current and pretty much little else, it, just like the J-Fet operats as variable current source.
3) A resistor (be it a direct collector resistor as Werners original shows or a virtual resistor as in my Vendetta inspired idea) then converts this AC current into voltage which is referenced to the so-called ground and which is fairly well independent from the supply rail noise and modulation (with ideal BJT's, J-Fet's and capacitors it would completely unperturbed by anything on the supply rail).
Now I call that very smart and tricky....
It might be fun to do a folded cascode "Pacific"!
Sayonara
Kuei Yang Wang said:
Hi Sayonara,
Ahhh… I see and R8 does not inject a bunch of PSU related noise into the emitter? Actually combined with R1 there is a gain of app. 2 concerning PSU related noise. Therefore C2 is needed to relax that, but it works only at higher frequencies.
Maybe replacing R8 by a current source will do better, but that needs to be very low moise then.
Cheers
Konnichiwa,
No, as the controlling voltage appears between base and emitter and thus C2 clamps this...
Well, the turnover for this RC combo is 1.6Hz, so at 16Hz noise is clamped down to 20db less and so on. Increeasing the capacitor to 1,000uF is trivial and increases the supply rejection for any given frequency by 20db and increasing nthe resistor values in the divider for the base by a factor of 10 gives another 20db, so 60db supply line noise suppression at 16Hz above your case of -6db should be non too difficult to attain....
Sayonara
Pjotr said:
No, as the controlling voltage appears between base and emitter and thus C2 clamps this...
Pjotr said:
Well, the turnover for this RC combo is 1.6Hz, so at 16Hz noise is clamped down to 20db less and so on. Increeasing the capacitor to 1,000uF is trivial and increases the supply rejection for any given frequency by 20db and increasing nthe resistor values in the divider for the base by a factor of 10 gives another 20db, so 60db supply line noise suppression at 16Hz above your case of -6db should be non too difficult to attain....
Sayonara
Kuei Yang Wang said:
Domo arigato. But as said, the credit goes to JC and that Kirchoff guy.
Too bad that the present prototype is housed together with its
multi-transformer PSU in one box. Baaad situation. I must rectify that. Which is kinda hard with my present corporate-life and daddy-life combined.
Pjotr said:
In fact the other part of the trick is that the original folded cascode allows the use of a simple emitter-follower regulator as supply (*), one per stage. This is of very low noise, and unperturbed by the actual signal as the folded cascode's current consumption is purely DC.
So the clamping capacitor is not even strictly required. Just nice to have.
Hi!
While waiting for parts for another project to arrive,
I finished a protoboard of this litte phono pre I had in mind since it was posted.
I found it has some remarkable features, both sonically and practically.
At first, I build it as shown in the first posting, and then with a servo. This made a substantial sonic improvement, allowing me to omit C3. It can reveal a lot of 'microresolution' on my vinyl now. I clearly like what I hear. Schematic as shown.
1) Technical discussion.
What might not be obvious at a first glance: this circuit falls nicely together on the protoboard, a nice layout is easily made.
It is very quiet and totally relaxed when it comes to hum. I placed it directly on top of my line stage, above a not so small transformer. Even well made boards fail this test and hum badly. Not with this circuit!
It is a headscratcher, though, where Q1 gets his current from, with the servo working, because the non-inverting input of U1 is at 0V now. The sim indicates, that U2 serves as current source for Q1. I freely admit that I didn't really get it. (It was a mistake, I forgot to tie Q1 to the negative supply) There is probably room for improvement.
Since I will probably rewire my tonearm for balanced signalrouting, I have a very simple mod for the input stage on the drawing board which allows balanced input with just one additional FET which simulates great as well.
I have the vague feeling, though, that Werner's simple input stage deserves something better (simpler?) than the currently following opamp stage.
2) Sound
It sounds good.
Lots of microdetail, maybe more than the two other pre's I currently have in the listening room. It's the typical FET-sound of relaxedness and resolution.
However, it lacks the final firmness in the bass region, which the other pre's have, more or less. This relative weakness was already present with C3 in place. It is not unpossible, though, that another servo arrangement would improve things here. A bigger C5 might be all whats needed.
Thanks for listening,
Rüdiger
While waiting for parts for another project to arrive,
I finished a protoboard of this litte phono pre I had in mind since it was posted.
I found it has some remarkable features, both sonically and practically.
At first, I build it as shown in the first posting, and then with a servo. This made a substantial sonic improvement, allowing me to omit C3. It can reveal a lot of 'microresolution' on my vinyl now. I clearly like what I hear. Schematic as shown.
1) Technical discussion.
What might not be obvious at a first glance: this circuit falls nicely together on the protoboard, a nice layout is easily made.
It is very quiet and totally relaxed when it comes to hum. I placed it directly on top of my line stage, above a not so small transformer. Even well made boards fail this test and hum badly. Not with this circuit!
It is a headscratcher, though, where Q1 gets his current from, with the servo working, because the non-inverting input of U1 is at 0V now. The sim indicates, that U2 serves as current source for Q1. I freely admit that I didn't really get it. (It was a mistake, I forgot to tie Q1 to the negative supply) There is probably room for improvement.
Since I will probably rewire my tonearm for balanced signalrouting, I have a very simple mod for the input stage on the drawing board which allows balanced input with just one additional FET which simulates great as well.
I have the vague feeling, though, that Werner's simple input stage deserves something better (simpler?) than the currently following opamp stage.
2) Sound
It sounds good.
Lots of microdetail, maybe more than the two other pre's I currently have in the listening room. It's the typical FET-sound of relaxedness and resolution.
However, it lacks the final firmness in the bass region, which the other pre's have, more or less. This relative weakness was already present with C3 in place. It is not unpossible, though, that another servo arrangement would improve things here. A bigger C5 might be all whats needed.
Thanks for listening,
Rüdiger
Attachments
Forget the servo arrangement. With this, the whole immuneness against hum is gone. This is no big surprise, because the current loop of the input stage is not small anymore (through R5 and back to the input), but goes right through the negative supply and back through the entire ground bus to the input stage.
But with the intermediate coupling cap, the sound isn't as transparent as it could be.
Back to complimentary design?
Rüdiger
But with the intermediate coupling cap, the sound isn't as transparent as it could be.
Back to complimentary design?
Rüdiger
Last edited:
Hi,
Why not use U1 inverting, with a feedback network of ten times the impedance and then use a 1M Resistor plus a 1uF (Wima?) cap to the non-inverting input, to cancel DC.
I used a similar circuit in a phono stage with a similar frontend and a valve section following and it works very well there.
Plus in your phono Stage it retains correctly the polarity, unlike now where the phono inverts.
Ciao T
Why not use U1 inverting, with a feedback network of ten times the impedance and then use a 1M Resistor plus a 1uF (Wima?) cap to the non-inverting input, to cancel DC.
I used a similar circuit in a phono stage with a similar frontend and a valve section following and it works very well there.
Plus in your phono Stage it retains correctly the polarity, unlike now where the phono inverts.
Ciao T
Hi Thorsten,
yes, this is sensible. Here is Werners last incarnation.
I will try it.
I don't get where the other end of the 1Meg resistor goes. One of the supplies?
Rüdiger
Hi,
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
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
Hi,
A Fet Input Op-Amp may help...
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
A Fet Input Op-Amp may help...
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
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