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Old 21st March 2012, 08:43 PM   #3901
tvrgeek is offline tvrgeek  United States
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Playing with the DH-120 in simulation, the ziener stack across the gates is causing about 10dB of the output distortion. Two questions. I am not really sure what protection this provides as all three nodes will under normal conditions never exceed a couple volts difference, so why 20V clamp? Second, why is the center tied to the output? Is this a case of a little bit of protection is a bad thing? Lastly, its it just the capacitance that is causing the issue? The outputs already have the same network internally. So, redundant as well.
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Old 21st March 2012, 11:46 PM   #3902
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Originally Posted by gk7 View Post
If you donīt like the term "DF" feel free to invent your own. But DF _is_ a well defined number and the impact on bass response can be calculated,
simulated, measured and is audible. What else do you want ?
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Old 22nd March 2012, 12:18 AM   #3903
tvrgeek is offline tvrgeek  United States
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I thought I found another "euraka" in the base stoppers to the drivers, but what I really found was a lot more about how LTSpice FFT works. Kind of funny actually as I have seen posts that I know know clearly are not correct.

Tiny adjustments to the gate stoppers makes pretty big differences in balancing the standing bias current and in even order distortion. 420 and 330 seem to be the magic values in the sim. Higher "nominal" values greatly increase distortion. So do lower. Notice the ration is not what the OEM suggests. Until I test n the bench, all conjecture.

Removing the 27 Ohm resistor in the emitter of the VAS cascode as it is no longer part of a pole-zero helped by a couple of dB.
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Old 22nd March 2012, 12:30 AM   #3904
jcx is online now jcx  United States
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Cherry recommends the VAS degen R for improved stability in simailar BJT output amps

it is also necessary for the "correct" cascode bias AC return which bcarso has been advocating - hanging the cascode bias LED from the degenerated VAS emitter will likely give bigger distortion improvement at high frequencies - the Hawksford article shows the variations, measures distortion improvement - even if Hawksford didn't invent the circuits
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Old 22nd March 2012, 04:26 AM   #3905
bcarso is offline bcarso  United States
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Quote:
Originally Posted by tvrgeek View Post
I thought I found another "euraka" in the base stoppers to the drivers, but what I really found was a lot more about how LTSpice FFT works. Kind of funny actually as I have seen posts that I know know clearly are not correct.

Tiny adjustments to the gate stoppers makes pretty big differences in balancing the standing bias current and in even order distortion. 420 and 330 seem to be the magic values in the sim. Higher "nominal" values greatly increase distortion. So do lower. Notice the ration is not what the OEM suggests. Until I test n the bench, all conjecture.

Removing the 27 Ohm resistor in the emitter of the VAS cascode as it is no longer part of a pole-zero helped by a couple of dB.
Gate stoppers shouldn't change the bias current at all. In fact I doubt that most simulators will give any useful information about gate stoppers, unless they include stray and package inductances that result in high frequency (~100's of MHz) resonances.

The return of the upper two bias LEDs to the 27 ohm, as mentioned by jcx, may have some benefit, but the effectiveness of this in reducing C at the output of the second stage cascode is mitigated somewhat by the ratio of the upper Q emitter impedance to the 27 ohms. Since we're running a little under 3mA, that emitter impedance is around 9 ohms, so the signal returned from the lower collector is divided between the 27 ohms and that 9 ohms. That output node will have voltage-dependent capacitances from the Ccb of the emitter followers, the reflected output Z of the output FETs, and the voltage-dependent C of the current source. So there will be some potential improvement from an "Aldridge" (aka Hawksford) connection, but it won't be tremendous.

Beware as well of zener models. My sim program doesn't model zeners well at all --- they are made to look like voltage sources regardless of bias! <searches for Smilie for dumbs#it>
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Old 22nd March 2012, 05:27 AM   #3906
bcarso is offline bcarso  United States
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BTW, given that the second stage's maximum positive swing is limited to of order three volts below the positive rail, there's little reason to have the lower current source swing any closer to the negative rail. And with that in mind, although the Jaeger JFET/bipolar cascode source used for the input diff pair takes up too much voltage burden, a hybrid using your existing "ring of two" source with a driven cascode (one LED bias sufficient) will work quite nicely to raise the current source impedance. Again, how much the following loads will make this less important I don't know offhand, and if one is hell-bent on improvement of everything, probably adding an emitter follower to the second stage to make it a better integrator is a better place to spend a transistor.

The nice thing about the cascoded ring-of-two, though, is that the ring of two's local feedback makes the impedance at the emitter-210 ohm R junction really low . So virtually all of the cascode device's base current change gets pumped back, as opposed to the situation mentioned with the second stage amp, where such currents are split between the emitter Z and the 27 ohms. Aside from the temperature coefficient of the output current being substantial (which in some situations may even be an advantage), this is a three-transistor circuit with remarkably high performance.

Brad
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Old 22nd March 2012, 05:46 AM   #3907
bcarso is offline bcarso  United States
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Default Ring of two Aldridge cascode

A picture to clarify.
Attached Files
File Type: pdf ring of two cascode schematic 03-21-12.pdf (7.9 KB, 38 views)
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Old 22nd March 2012, 08:24 AM   #3908
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Brad, doesn't this ring of two have poor neg supply PSRR?

jan
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Old 22nd March 2012, 08:31 AM   #3909
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by ThorstenL View Post
I will probably fit an additional circuit to synthetically increase the output impedance to 3 Ohm, which is the target source impedance for all of my personal speakers.....
I quite like the idea of simply adding a 3 Ohm power resistor in series with the output, since it also provides very effective protection for the output devices. Worst case, with a short across the speaker terminals, the output stage just has to drive the resistor, which should be no problem.

Overall efficiency is reduced a bit and maximum output is about 3dB less, but I don't consider that too much of a problem. If high efficiency and power savings were a top priority nobody would be using valves or class A.

YMMV, Different strokes for different folks etc.

Quote:
Originally Posted by bcarso View Post
A picture to clarify.
Thanks! That helps no end.
The circuit looks like it will have very high output impedance, but lousy PSRR as shown, since any ripple current through R2 is fed straight through to the output. One could always add some filtering or regulation for R2's voltage though.

edit: Oops, crosspost with janneman
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File Type: gif isrc2.GIF (4.1 KB, 98 views)
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Old 22nd March 2012, 09:22 AM   #3910
tvrgeek is offline tvrgeek  United States
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Much to take in here. In the last few days, I have learned as much about the sim as the circuit. I am both impressed, and disappointed.
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