While I agree that schematics ofr symmetric amps are artistically attractive, there should be technical justification for them, given that they effectively double the parts cost and halve the reliability of a given design. I hate to say it, but a good single ended design seems to work just as well at half the cost. They may be quite boring to look at but meet my design objectives better.
The justification in this case is that it takes a balanced/bridged circuit in order to have an X (aka SUSY) circuit. If you assume that the circuit will have a complementary output--as opposed to an Aleph-style output--then you've already got the P devices in the output and there's very little extra needed in order to make an X front end carrying complementary differentials. Given that it's rarely the front end of an amp that breaks, there's not much of a reliability problem, and you get an extra distortion-reducing mechanism for free. And one that doesn't involve negative feedback, at that.
Grey
Grey
good point
"here's very little extra needed in order to make an X front end carrying complementary differentials." - GR
Perhaps you can draw a schematic on how to do this. I am sure that there would be a great interest in a two jfet pair front end Aleph X and a home-brew version of the jfet input X amps( the second voltage stage should be pretty easy). It would give you opportunity to come up with another X-name and share more of Mr. Pass's design principles with the DIY community. Is anybody else interested in seeing something like this?
"here's very little extra needed in order to make an X front end carrying complementary differentials." - GR
Perhaps you can draw a schematic on how to do this. I am sure that there would be a great interest in a two jfet pair front end Aleph X and a home-brew version of the jfet input X amps( the second voltage stage should be pretty easy). It would give you opportunity to come up with another X-name and share more of Mr. Pass's design principles with the DIY community. Is anybody else interested in seeing something like this?
For me it is a big surprise. I always tought that Mr. John Curl and Mr. Erno Borbely are in the field of complementary differential, while Mr. Pass is on the single differential. Just my imagination. It turns out every master can use everything exist, and makes good amp with any circuit.
Just imagining. Maybe the X150-350 looks like A75 mirrored in horizontal direction, using both legs of differential for left and right part of amp (no, no, how the folded cascode goes with this?)
Only X150-350? What about X1000? Still single differential?
I do.
Is this mean for every half (left or right half), X150-350 have 3 stages (Differential(shared together)+push-pull VAS+Curent amplifier), and X1000 only 2 stages (Folded Cascode(shared together)+Current amplifier)?
Although I haven't seen schematics, I would not be surprised if they were also two stage amps.
Grey
Grey
GRollins said:
Technically, the later circuits would be considered 3 stage, as
the initial stage has a diff pairs driving common source level
shifters. The composite gain has the same open loop as the
original folded cascoded diff pairs, so we let the literature be.
The initial Xamp and the current Xamp has changed the topology, now without Folded Cascode?
It seems that it is a 3 stages power amp, complementary differential(shared together), driving pushpull VAS, and Current amp in the final.
It seems that it is a 3 stages power amp, complementary differential(shared together), driving pushpull VAS, and Current amp in the final.
= VAS?
The X1000 and X600 retain the original topology with the
folded cascode, all other Class AB X amps use the circuit
described above, and the XA amps use a circuit very similar
to Grey's as published in the forum, and have no cascoding.
folded cascode, all other Class AB X amps use the circuit
described above, and the XA amps use a circuit very similar
to Grey's as published in the forum, and have no cascoding.
In this forum I read that some expert entering a highend audio show, finds that the best soundsystem is made by X250 (If I dont forget).
Mr. GRollins, would you mind to start a new thread on this amp backengineered? Mr. Pass have give a clue, that it is different than X100 backengineered (it is not using folded cascode).
Just experimenting with cascoding yesterday. The results are "THEY ARE GOOD". But only doing it in Differential and VAS (small signal transistors). The sound improves alot.
But have doubt to do this in output stage (Mr. Pass patent place cascode in every stage of power amp), after reading some people's comment. Is it different result if we are doing it in the output stage (with power transistors)? Why?
as this discussion's gone off-topic....
I read it few times in this forum. It's not true. I was told that small signal is below Ut (thermokinetic potential?!), but VAS swings from one rail to the other.
lumanauw, I don't say it to be annoying, but to be sure that I understand it all clearly. 🙂
I read it few times in this forum. It's not true. I was told that small signal is below Ut (thermokinetic potential?!), but VAS swings from one rail to the other.
lumanauw, I don't say it to be annoying, but to be sure that I understand it all clearly. 🙂
"Common Source level shifters = VAS?"
Lots of things could comprise a VAS, including the folded cascode.
As I said before, topologically you could see the voltage gain
stage of the X150 through 350 as consisting of two stages,
however we set the open loop gain to give the equivalent
of the folded cascode. This saves us the heavy voltage loss
and complexity of the folded cascode and allows a single set
of rail voltages for the front end (passively decoupled) and the
output stage.
Lots of things could comprise a VAS, including the folded cascode.
As I said before, topologically you could see the voltage gain
stage of the X150 through 350 as consisting of two stages,
however we set the open loop gain to give the equivalent
of the folded cascode. This saves us the heavy voltage loss
and complexity of the folded cascode and allows a single set
of rail voltages for the front end (passively decoupled) and the
output stage.
Maybe this is out of theory. But I often see the output stage (current amplifier) as a totem pole (with bias). A totem pole have 2 transistors, NPN in the top and PNP in the bottom. The base tied together, the emitors also tied together. This is usually found in mosfet gate driver (for smps usually). The emitors will follow whatever voltage in the base. The output stage of power amp have similiar topology, but with bias.
So, if the output only follows what is in the base, consequently the base driver must have already full swing. This is done by VAS, where the VAS is done by taking output from the collector (this can give full swing, doesn't depend on the VAS transistor's base voltage, like emitor follower, where Vbe is always 0.6V). So, it is right, that in VAS all the voltage swing has happened, just don't have enough current to drive speaker. This is done by final stage (amplifying the current).
xocoatl,
What I mean by small signal is the size of the transistor, not the function of transistor. 🙂
*Mr. Pass, you haven't read the emails at nelson@passlab.com?*
So, if the output only follows what is in the base, consequently the base driver must have already full swing. This is done by VAS, where the VAS is done by taking output from the collector (this can give full swing, doesn't depend on the VAS transistor's base voltage, like emitor follower, where Vbe is always 0.6V). So, it is right, that in VAS all the voltage swing has happened, just don't have enough current to drive speaker. This is done by final stage (amplifying the current).
xocoatl,
What I mean by small signal is the size of the transistor, not the function of transistor. 🙂
*Mr. Pass, you haven't read the emails at nelson@passlab.com?*
I’m very interested on how these two very different input topologies sound, when compared to each other. I’m also interested as to why a different topology was used for the lower wattage / less costly versions. My guesses are:
a) people wanted singal ended inputs
b) the transformer needs less taps or you need less transformers
a) people wanted singal ended inputs
b) the transformer needs less taps or you need less transformers
lumanauw said:
(It's nelson@passlabs.com ), you left off the "s"
Your analysis is correct, the X series has complementary followers
as you describe, and the full voltage swing comes off a Drain of
a transistor (which you can call a VAS), but this Drain can be from
a device operated either as Common Source or Common Gate.
In the X600 and X1000 it is Common Gate, in the other X amps
(non XA, which do not have followers) it is Common Source.
LBHajdu said:
True on both a) and b)
As to the difference in sound, we have tested these changes
(there is yet a 3rd, 4th and 5th front end you haven't heard
about and which are not in production) on unknowing subjects,
and nobody noticed a difference. The later circuits we deem
more appropriate for amplifiers where we have chosen to
include the output stage in the loop, although it's perfectly
suitable for the case where it is not.
The design of the newer front end also does not lend itself to the
higher voltages of the X600 and X1000, and since they were
already so well loved and reviewed, we opted not to alter them
at all.
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