Folded Cascode Sound?

[wrenchone]

Ive been doing a little PSpice simulation work with various amplifier and preamplifier types, looking at the differences in harmonic distribution between various topologies and using this information to decide which circuit to prototype. I realize that the simulator won't tell the whole picture about a design, but I found that with folded cascode topologies , the harmonic spectrum was rather strange, with the odd harmonics predominating 10-20dB over the even. This difference is rather persistent, and holds true both with simple and much more complesxdesigns.

I also remember reading about the development of the Pass A75. It seems a folded cascode topology was tried for a while with the A75, but abandoned for sonic reasons. (Mr. Pass, can you comment?) Has anybody here had the chance to measure the harmonic spectrum of a folded cascode amplifier? I'm curious to see if the simulations correlate in any way with real-life results. I am definitetly going to try it some time, but first, I have to learn to master the Audio Precision analyzer at my work place...

[DarkOne]

As I know, mr. Pass uses current folded cascode in X amps. I think that it is the best amnplification stage yet. But you have to fine tune this circuit, differential stage have to be biased quite high, and on the output side of circuit you need as low impedance as possible to get good results.
I use current folded cascode in my amplifier and I'm biasing differental stage at 12mA, while using only 2mA for signal and impedance at the output is 10kohm, I also use current mirror at the output.

If you can use very high bias at differential stage with at least 20mA for signal, you can drive output mosfets directly (if you will use mosfets) with very low dostortion.

[Nelson Pass]

I thought the A75 sounded fine with folded cascode - it was
just one of several options we presented.

Properly done, folded cascode behaves just like a regular
cascode and sounds the same. The real difference is that
you can use a folded cascode simultaneously as a level shifter
to get a direct coupled output with a single-stage (or more
complex, if you like) amplifier.

The emphasis is on properly done; The CCS which supplies
both the input gain device (the one being cascoded) and the
cascode device has to be high quality, and there has to be
adequate voltage margins provided for it - which is not always
done.

Folded cascode has been most popular in very fast op amps,
typically video stuff, and is often found in conjunction with
current feedback (also known as low impedance feedback, where
the feedback network attaches to the Source/Emitter pins of the
input devices instead of the Gate/Base).

[sreten]

I'm certainly no expert on circuit topologies but NP's comments
confirmed my dimmed with time observation (which I wasn't
now sure about) that a folded cascode is effectively a re-
arrangement of an LTP pair.

And in a circuit simulator this with give nearly perfect cancellation
of all even order harmonics, leaving odd order dominating.

As I understand it an LTP arrangement reduces even order distortion,
but not to the extent typically suggested by a simulator.

sreten.

[GRollins]

Cascodes (straight, folded, or kinky) don't necessarily have to be part of differentials. The
Differentials, however, do tend to suppress even harmonics.

Grey

[wrenchone]

I did a simulation of a symmetrical complementary common source design using current source loading and symmetrical folded cascode output. The harmonic signaure from the simulation was pretty disturbing, with prominent odd order components out to the 7th harmonic, and the evens suppressed by 10X. Maybe the symmetry here is also cancelling the even component. However, if I substitute another common source stage for the output instead of the folded cascode transistors, so that the design now looks like a symmetrical compound pair, the distortion signature shows a smooth tapering off of harmonics as a function of frequency. If anyone is interested, I can post the schematics and an output file for each, so that you can see the difference.
Obviously, the next step is to build up both circuits to see if the difference manifests itself in real life. I also have a folded cascode power amplifier that I also want to evaluate when it comes out of surgery ( higher bus voltage, higher voltage drivers, bigger output FETS). I'm really posing this question to see if anyone has noticed a different sonic character for the folded cascode topology.

[wrenchone]

I took a closer look at the two circuits I mentioned in the previous post, and gave the folded cascode stage a little more voltage drop, so that the bases of the cascode transistors were about 5V away from the rails. This gave the current source load transistors a little more voltage to work with. The nasty harmonic character went away, and the folded cascode stage is now about 0.05% distortion at 10kHz, or about ten times that of the symmetrical compound pair at the same frequency and drive level. This is to be expected, as the folded cascode circuit won't have as much open loop gain. I still will be doing both circuits for test, to see if PSpice really can give a reasonable prediction of circuit behavior. I also want to see if I can tell any diffrence between the two in listening tests. Thanks all, for giving me a tip to straignten things out. When I get some results, I'll post in a separate thread.

[Peter Daniel]

FYI, here's an older thread, dealing with the sound of cascode:
The sound of cascodes

[Nelson Pass]

Originally posted by wrenchone
I did a simulation of a symmetrical complementary common source design using current source loading and symmetrical folded cascode output. The harmonic signaure from the simulation was pretty disturbing, with prominent odd order components out to the 7th harmonic, and the evens suppressed by 10X. Maybe the symmetry here is also cancelling the even component. However, if I substitute another common source stage for the output instead of the folded cascode transistors, so that the design now looks like a symmetrical compound pair, the distortion signature shows a smooth tapering off of harmonics as a function of frequency.

This would be expected, as matched pairs operated out of phase
with each other (as with a diff pair) suppresses even order
harmonics. If you load them with a common source stage, the
output of that stage is likely to contain a high percentage of
even harmonics. A lot of this is dependent on the specifics of
the circuit and parts themselves.

The cascode transistor imposes very little of its own character
on the signal, but the common source device does by
comparison.

[wrenchone]

Just for reference, the schematics below show the circuits I was simulating. The one on the left is a symmetric compound pair, while the circuit on the right is a symmetric folded cascode using current source loading. This is all part of an examination of simple current feedback gain blocks for use in an RIAA preamp. The first is a simple version of a circuit used long ago by Erno Boberly as a MC preamp. The second has probably been done by someone else too, though I haven't heard about it. The simple FET cascodes used for the circuit on the left are to get a little voltage margin on the second stageFETs because of the 25V breakdown voltage of the 2SK74. If someone could suggest some complementary Japanese devices that would serve better for cascode FETs than the J175 and PN4393, I'd be happy to hear about it. The source follower in the circuit on the left also needs FETs with higher breakdown voltage than the J110s. I'll probably end up using PN4393s, as I have scads of them.
In practice, I'll be selecting source resistors based on the matching results for the 2SK170BL and 2SJ74BL I have in my lab stock. This will all happen when I find out where I packed my FET matching jig (just moved).
I'll let folks know the outcome when I'm finished playing...