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Common Emitter OPS?
Common Emitter OPS?
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Old 20th November 2016, 08:09 PM   #11
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by soundbloke View Post
As discussed in the thread I referenced, this is the case in the ATC implementation where the devices are sandwiched between the heat sinks and a ground plane. It does indeed have its benefits although the MOSFET pin inductances can require some attention...
OK, I reread the referenced thread and the distinction between the QSC and the ATC is now clear.
The ATC is a particularly neat implementation if you have cases connected to Source/ Emitter.
Most BJT aren't, of course.
What is the problem that requires attention to the MOSFET pin inductances?
I suspect this is the flip side of my concern over the power supply float capacitance.

Best wishes
David
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Old 21st November 2016, 03:31 PM   #12
soundbloke is offline soundbloke  United Kingdom
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Quote:
What is the problem that requires attention to the MOSFET pin inductances?
I suspect this is the flip side of my concern over the power supply float capacitance.
The lead inductance issue is that they need be somewhat longer than normal due to a 90 degree bend. The 90 degree band I suspect also has an influence here. But with MOSFETs particularly operating above about 30V drain-source all manner of parasitic oscillations are possible - and probable if not careful. A higher than recommended gate resistor helps, as can well placed RC snubber networks.

Nevertheless I believe the use of MOSFETs in this application where source resistors can be omitted is preferential to BJTs. But please check out the local feedback nulling arrangement that ATC apply if you get the chance - IMHO it ranks as one of the most simple and effective fixes available...

As to power supply capacitance, ATC appear to use a large E-I transformer specified to medical equipment grade - that is inter-winding screening. It does get warm, however. But then in a single channel amp case, a separate output stage transformer would likely not pose much of an overall cost issue?
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Old 21st November 2016, 08:00 PM   #13
PHEONIX is offline PHEONIX  Australia
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Default Schematics

Hello Dave ,

Can you post some schematics of the Cherry amplifier you are talking about and possible the article you are referring to. Have you done any simulations.

Regards
Arthur
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Old 21st November 2016, 08:33 PM   #14
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by soundbloke View Post
...As to power supply capacitance, ATC appear to use...
The floated supply makes the amp much more vulnerable to noise coupled from the mains, so presumably the transformer is screened to reduce this.

Quote:
Originally Posted by PHEONIX View Post
Can you post some schematics of the Cherry amplifier...
Have you done any simulations.
Hi Arthur
The amp is in an article in "Proc. IREE Australia, vol. 39, pp. 1-8, Jan/Feb. 1978" E.M. Cherry.
Not public domain but I will ask Dr Cherry what I can do.
No simulations yet, I don't plan to simulate the entire amp anyway, I mainly wanted to see what his OPS looked like.
The rest is fairly conventional LTPs for input and second section but with his favorite "nested feedback" scheme, that I am less keen to follow.
More later.

Best wishes
David
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Old 22nd November 2016, 08:35 AM   #15
kgrlee is offline kgrlee
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I haven't studied all the links in this thread but a quick glance seems to indicate the commercial amps are all more (or less) sophisticated versions of Vanderkooy & Krauel

AES E-Library A Simple Reliable Power Amplifier with Minimal Component Count

If I was actually designing an amp for production this Millenium (instead of doing the 1ppm THD20k wank) I would use
  • something like this
  • attempt to get THD20k etc below 0.01%
  • ensure good overload &
  • unconditional stability with load as I've pontificated at length in several threads on this forum.
I think these are the important criteria for good sound.
____________________

I've spent a lot of time dreaming up new fangled PA topologies but have always come back to refine the basic Lin/Dinsdale circuit. No pedantry about why the modern topology isn't Lin/Dinsdale bla bla please.

In my book (and also according to Self & others) Lin/Dinsdale is a 3-stage amplifier
  • Transconductance i/p
  • VAS
  • 'EF' output
The 'standard' compensation (Self et al) treats each stage separately and applies compensation around the VAS.

JLH & Cordell treat the Transconductance + VAS as one block for compensation.

Cherry treats the VAS + 'EF' output as a single block for compensation

I spurn the bastardised TMC versions (hope I got dem TLAs rite)
_____________________

In fact Vanderkooy's circuit isn't 'new' either. Circa 1980, Great Guru Baxandall proposed something just like it to instruct me in the 'similarities' between CE & CC .. but as a thought experiment.

The real problem in trying to abandon Lin/Dinsdale, is getting good Class B. I would like to run 50mA or less through each pair of devices. I know the performance above is possible with the 'standard' EF2 circuit (without Self's extra EF feeding the VAS) with 'modern' devices .. with low crossover distortion.

The technology which made this possible is pure Cherry compensation which I've done to death in other threads.

What pure Cherry allows is
  • 'current driving' the EF OP stage so they behave like CE
  • maximum feedback around the output stage which is always the biggest THD source
Need to look at how to apply this to Vanderkooy & Krauel.
________________________

On CFPs for the 'EF output stage, apart from the Thermal issues I highlight in Bob's thread ...

Self shows in his 4th (and IIRC earlier) edition Fig 5.40 that though THD at full power may be better than EF2, at 1W & below, his EF2 is a LOT better than his CFP output stage.

He has no explanation for this. Anyone with the latest edition of Self and can tell us if he has investigated this further?
________________________

I can't comment on Cherry's IREE, Oz paper, not having seen it.

And the rail2rail OPA crowd must have something useful on this topic.
________________________

Dave, I tried to send you a PM but your mailbox is full

Last edited by kgrlee; 22nd November 2016 at 08:51 AM.
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Old 22nd November 2016, 09:21 AM   #16
soundbloke is offline soundbloke  United Kingdom
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Quote:
The floated supply makes the amp much more vulnerable to noise coupled from the mains
Not in a closed loop for differential noise. Common mode noise doesn't appear in the load.
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Old 22nd November 2016, 09:28 AM   #17
soundbloke is offline soundbloke  United Kingdom
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Quote:
In fact Vanderkooy's circuit isn't 'new' either. Circa 1980, Great Guru Baxandall
The ATC amplifier has been in use since the mid 1980s and was based (I believe) on a PA amplifier that was manufactured well before that. The Vanderkooy implementation is very similar!

Quote:
The real problem in trying to abandon Lin/Dinsdale, is getting good Class B
Quote:
Need to look at how to apply this to Vanderkooy & Krauel.
Please see me earlier comments about the local feedback that is unique to the ATC design (I believe). It has the ability to null the transconductance differences between the two halves and renders significantly lower distortion with no compromise in stability.

Also to add is that in this topology MOSFETs behave particularly well and when high power and reliability are concerned, any on-paper advantages of BJTs are best left there...
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Old 22nd November 2016, 10:01 AM   #18
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by kgrlee View Post
...The real problem...is... Class B.
This is the core of my enquiry but I am not sure if it's a problem.
CE OPS with same size emitter resistors as a typical EF seems like it should work, yes ?

Quote:
... which made this possible is pure Cherry compensation which I've done to death in other threads.
I actually have some nice results on this issue since we last swapped ideas.
NOT what I expected intuitively, will discuss this more, this is just a quick reply to some of your points.

Quote:
On CFPs for the 'EF output stage, apart from the Thermal issues...
Self shows in his 4th (and IIRC earlier) edition Fig 5.40 that though THD at full power may be better than EF2, at 1W & below, his EF2 is a LOT better than his CFP output stage.

He has no explanation for this...
It is perfectly predictable from his own plots of the transfer functions of CFP versus EF.
"Sub-optimal", to put it politely.

Quote:
I can't comment on Cherry's IREE, Oz paper
I will scan it and at least put the circuit up for you and Arthur.
After almost 40 years I think this qualifies as "fair use"

Quote:
And the rail2rail OPA crowd must have
That's what I expected too but damned if I can find much.
They seem more proprietary than they used to be.

Quote:
Dave, I tried to send you a PM but your mailbox is full
I used to donate and had more mail space.
But donations are in US$ so it has become more expensive and the mail space shrunk when my donation expired.
Need to clean it out.

Best wishes
David
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Old 22nd November 2016, 10:16 AM   #19
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by kgrlee View Post
I haven't studied all the links in this thread...all more (or less) sophisticated versions of Vanderkooy & Krauel
I am not convinced about the wisdom of a floated supply in amp optimized for really low distortion.
Makes much sense for Public Address reliability and is probably adequate for Hi-Fi in practice (i.e. not detectable in DBLT).

But the use of a CE OPS makes sense to me.
Drive it with an AD797, how simple is that?
And even ultra low distortion <0.001 @ 20 kHz should be doable.

Best wishes
David

Last edited by Dave Zan; 22nd November 2016 at 10:18 AM.
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Old 22nd November 2016, 10:23 AM   #20
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by soundbloke View Post
Not in a closed loop for differential noise. Common mode noise doesn't appear in the load.
My expectation was that it could couple into the feedback loop but I haven't checked this.
I did read an advocate of floated supplies, who admitted that mains noise transmission was one of the downsides so it seemed plausible.

Quote:
Please see me earlier comments about the local feedback that is unique to the ATC
Do you have a link to study?

Best wishes
David

Last edited by Dave Zan; 22nd November 2016 at 10:27 AM.
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