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Common Emitter OPS?
Common Emitter OPS?
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Old 22nd November 2016, 10:28 AM   #21
soundbloke is offline soundbloke  United Kingdom
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Quote:
I am not convinced about the wisdom of a floated supply in amp optimized for really low distortion
I disagree entirely.

Disregarding Cherry's compound BJT attempts, the use of MOSFETs with grounded sources and a floating supply delivers exemplary results - with reliability as a bonus. I have also implemented this with an AD797 but the difference between that an a run-of-the-mill 5534 is negligible in terms of distortion due to the added open loop gain supplied by the output stage. Noise was improved slightly as expected with the AD797 but stability needs attention in both cases.
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Old 22nd November 2016, 10:30 AM   #22
soundbloke is offline soundbloke  United Kingdom
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My expectation was that it could couple into the feedback loop but I haven't checked this
Build a CE amp. Add a voltage source in series with the collector (preferably properly cascoded) and see what noise you measure in the output...
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Old 22nd November 2016, 11:02 AM   #23
Dave Zan is offline Dave Zan  Australia
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Originally Posted by soundbloke View Post
... the use of MOSFETs...
I have already stated that for MOSFETs with source connected to the case that the floated supply is a very neat solution, allows the case to be earthed directly to the heat-sink, no thermal washers, mechanically foolproof, safe.
If it's done with lateral FETs, as I expect, then the result should be very robust indeed.

I did not say that the floated supply is bad, only that I am not yet convinced.
Surely the supply capacitance to earth must appear as a load?
Usually capacitive loads are not desirable, especially as we try to push up the gain crossover frequency to reduce distortion.

Best wishes
David
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Old 22nd November 2016, 01:39 PM   #24
AndrewT is offline AndrewT  Scotland
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Whether it's a single ended amplifier runing from a single polarity supply or a push pull amplifier that is running from a single polarity supply, they both have to drive the ground connected load via a large DC blocking capacitor.

When you move to a dual polarity supply push pull amplifier driving a ground connected load, then you will find that the load currents come via the PSU capacitors. They have the same effect as the DC blocking capacitor of the single polarity supply versions.

In all of these topologies the load current effectively comes through big electrolytics.

Move to one of the grounded output topologies and you find the same current flow arrangement. The big electrolytic capacitors have to pass the load current.
Effectively all these apparently very different topologies ALL pass the load current through big electrolytic capacitors. It as if the load is always in series with the capacitors with respect to the amplifier.
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Old 22nd November 2016, 05:36 PM   #25
soundbloke is offline soundbloke  United Kingdom
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It as if the load is always in series with the capacitors with respect to the amplifier.
Stated perfectly - barring the "It is as if"
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Old 22nd November 2016, 06:59 PM   #26
PHEONIX is offline PHEONIX  Australia
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Default Distortion at 20Khz

Hello Dave,

>>And even ultra low distortion <0.001 @ 20 kHz should be doable.

Can I ask at what power level you expect this THD to be the case, is it 1W , 10 W, 100W into a 8 ohm load.

Regards
Arthur
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Old 22nd November 2016, 08:38 PM   #27
Dave Zan is offline Dave Zan  Australia
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Originally Posted by PHEONIX View Post
...Can I ask at what power level you expect this THD to be the case, is it 1W , 10 W, 100W into a 8 ohm load.
My usual case for simulations and comparisons is @ 100 W.
That's about nominal full power with just one pair of output transistors, makes it easy to draw.
The number is only a benchmark at the moment, no simulation yet, but I don't see why the AD797 + CE shouldn't do it.

Best wishes
David
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Old 22nd November 2016, 08:48 PM   #28
Dave Zan is offline Dave Zan  Australia
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Originally Posted by soundbloke View Post
Build a CE amp...and see what noise you measure in the output...
My concern is not with the CE amp, my initial impression is that it's an excellent idea, that's why I want to study it more closely.
My concern is with the floated supply implementation, that is the detail that looks vulnerable to noise coupled from the mains.
Is there a schematic of the ATC that we can both refer to in discussions?
Would make it easier to discuss which nodes couple to where, and so on.

Best wishes
David
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Old 22nd November 2016, 09:27 PM   #29
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by AndrewT View Post
... It as if the load is always in series with the capacitors with respect to the amplifier.
I think you have missed my point.
The issue is not about current thru the power supply capacitors, the problem is that the floated supply moves with respect to earth, so that now the amplifier load includes the stray capacitance of the supply to earth.
This does not happen when the supply is earthed.

Best wishes
David
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Old 22nd November 2016, 11:09 PM   #30
kgrlee is offline kgrlee
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Quote:
Originally Posted by soundbloke View Post
Build a CE amp. Add a voltage source in series with the collector (preferably properly cascoded) and see what noise you measure in the output...
Can you post a schematic of what you propose?
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