In an attempt to do a discrete vs monolithic folded cascode opamp shootout, I've breadboarded an amp for my favourite opamps - AD797 as left/right and AD829 comp pin out as active ground.
I simulated AD797 and AD829 in LTSpice and came up with the schematic below. While a 22 pF cap for the distortion cancelling circuit simulated the best, a 47 pF one as suggested in the data sheet sounds better. I put this cap from the buffers output to pin 8 as proposed by Walt Jung. This cap is absolutely essential to get a good sound from AD797. Without it it sounds worse than NE5534.
Much have been said about AD797, and how cranky it is, but I've had a hard time finding schematics showing a good implementation. So, please tell me how to get the best of this great opamp!
I simulated AD797 and AD829 in LTSpice and came up with the schematic below. While a 22 pF cap for the distortion cancelling circuit simulated the best, a 47 pF one as suggested in the data sheet sounds better. I put this cap from the buffers output to pin 8 as proposed by Walt Jung. This cap is absolutely essential to get a good sound from AD797. Without it it sounds worse than NE5534.
Much have been said about AD797, and how cranky it is, but I've had a hard time finding schematics showing a good implementation. So, please tell me how to get the best of this great opamp!
Attachments
Hi nelson,
Sorry, but I don't get it: you have chosen high impedance node from AD829 for your active ground, as well as LED CCS (again high impedance) for AD797 supply? Could you elaborate please?
Edit: Ah there is another sch😀
Sorry, but I don't get it: you have chosen high impedance node from AD829 for your active ground, as well as LED CCS (again high impedance) for AD797 supply? Could you elaborate please?
Edit: Ah there is another sch😀
I'm not so clever. I used LED CCS since it was what I had at hand, and it could be squeezed inside the small box I use. I prefer JFETs because of simplicity but i don't have any with that idss, 30 mA something. What CCS would be better? Do you think just using resistors, say 100R, would be better than this CCS?aparatusonitus said:
Hi Nelson !
Did you build the circuit with AD797 or your conclusions come from simulations?
Sorry, but it's not clear from your post - you wrote about simulations and you comment on sound in the same time. It would be nice to know about real life circuit because I used AD797 only as a unity gain buffer.
Did you build the circuit with AD797 or your conclusions come from simulations?
Sorry, but it's not clear from your post - you wrote about simulations and you comment on sound in the same time. It would be nice to know about real life circuit because I used AD797 only as a unity gain buffer.
You are using LED based CCS to supplay power to AD797 (pins opa+/-), but intristic property of every CCS is that they represents high impedance output, you need as low as possible impedance from PSU, regulated or not.
The clues are all in the datasheet; read it closely - it's excellent.
One essential item is the input damping resistors, min. 100ohms , at the 797s signal inputs. Everyone leaves them out, then wonders why the 797 is 'cranky'! It isn't - it's simply a very high performance part, and one cannot expect simply dropping it into conventional 'jellybean opamp' circuits to get the best out of it.
So I shouldn't use CCS to the opamps at all? The amp is powered by two 6F22 NiMH batteries in series. I can try to bypass the CCS and see what it does to the sound, but that will be tomorrow since it's getting late now.aparatusonitus said:
I'll try to increase these resistors tomorrow as well.martin clark said:
Thank you for your suggestions.
I'll try to increase these resistors tomorrow as well.martin clark said:
Thank you for your suggestions.
nelsonvandal said:
You could use this LED based CCS to supply current to some type of shunt voltage regulator (aka super-shunt voltage regulator), but be avare that you need to shunt at least the value od AD797 quiescent current, so your LED CCS need to supply at least 2 x AD797 quiescent current.
nelsonvandal said:
I don't know about 6F22 batteries output impedance, but as I allready noted, your AD797 needs low impedance power supply (be it a linear or shunt type voltage regulator, not current regulator)
I did measure the batteries (DC) resistanse a while ago, and as far I can remember it's 4 Ohm each = 8 Ohm total.aparatusonitus said:
Many of the better headphone amps like PPA and LISAIII use this kind of capacitance multiplier (but with JFETs) to the opamps to separate them from the power section. Doesn't the capacitors provide low impedance? I'm not that clever, so I have to ask, and I'm willing to learn. Could I use a TL431 like in figure 13 in this data sheet? http://www.fairchildsemi.com/ds/TL/TL431A.pdf
Will the low power section of the M3 be a better solution. http://www.amb.org/audio/mmm/
nelsonvandal said:
You can use figure 10 or 12 and if you use CCS (like in figure 13) instead of plane resistor (the first one from the left in both figures), you will have a very good super-shunt regulator.
nelsonvandal said:
Well, I'm not sure, I'm done with series regulators for now, so mine answer is that I would go for super-shunt regulator😉
Nelsonvandal,
Yes, caps C2 and C5 in this diagram (and parallel caps) provide low impedance:
http://www.diyaudio.com/forums/attachment.php?s=&postid=1633205&stamp=1224101759
(Assuming I read the power supply diagram correctly. It's a bit unorthodox in the layout, IMHO.)
Yes, caps C2 and C5 in this diagram (and parallel caps) provide low impedance:
http://www.diyaudio.com/forums/attachment.php?s=&postid=1633205&stamp=1224101759
(Assuming I read the power supply diagram correctly. It's a bit unorthodox in the layout, IMHO.)
nelsonvandal said:
Now I'm confused.
Are you hooking up a constant current supply to the operational amp?
You can't force-feed the OpA with current. It needs a set voltage, and it needs the ability to draw currents as needed -hence the need for a low impedance source. ("Impede" is the same as "obstruct", or "hindra" in Swedish.)
Perhaps I misunderstood the circuit?
Regards
a voltage regulator strives to have low output impedance.
A current regulator strives to have a high output impedance.
If you feed the opamp with a CCS (current regulator) then two conditions must follow.
1.) the CCS current MUST EXCEED the maximum transient current draw of the opamp.
2.) there must be another circuit alonside the opamp to regulate the supply voltage. Most would use a shunt voltage regulator to lower the supply impedance.
A current regulator strives to have a high output impedance.
If you feed the opamp with a CCS (current regulator) then two conditions must follow.
1.) the CCS current MUST EXCEED the maximum transient current draw of the opamp.
2.) there must be another circuit alonside the opamp to regulate the supply voltage. Most would use a shunt voltage regulator to lower the supply impedance.
1. It doesAndrewT said:
2. I'm convinced. I'll see what I squeeze in. The space is quite limited.
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