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Old 12th December 2011, 10:19 PM   #1
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Default 20W Blameless Class A

Hello.

I was reading Mr. Self's book Self on Audio second edition just last night and saw an interesting circuit. It was a very sophisticated A-Class design with a Novel current control. It's located on page 305.

Has anyone built this amp? It seems quite efficient - at least, in terms of 'A' Class amps! Here is a quick description;
  • 24VDC Dual rail power supply.
  • Current mirror in the two long arms of diff amp.
  • Beta boosted VAS
  • 1.6A Iq
  • Low cost/high value components.
Here's a picture:
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File Type: jpg Blameless 20W Class A Amp.jpg (96.8 KB, 962 views)
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Old 13th December 2011, 09:04 AM   #2
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There's nothing wrong with such a design, the controller works just as claimed though Silicon Chip's 20W class A design is already physically worked out for the application and uses more modern parts with lower distortion, current and heat if that is what you seek.

The real problem is buying a transformer, around 150VA, of the precise voltage to give rails no higher than 24V but still allow the amp to deliver 20W (18V AC is too high IMO and regulation is a further waste)

I built a Stripboard version a few years ago and was quite happy until I heard SC's version which had the noise issues of constant high current solved, where I never got it quite right.

There is little point building high performance designs if hum & noise exceeds the capability of the design and Self's Trimodal design is as good as those parts get.You will find more refinements in later editions of his Manual covering lower noise input transistors among other developments.
IMHO, any good class A is down to critical power layout and thermal arrangements.
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Old 13th December 2011, 10:19 AM   #3
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Hi there Ian, Thankyou for your comments on the power supply and also the SC 20 watter. I have a couple of other irons in the fire at the moment but I will try to order a back copy of SC to read up on the mod's. Do you have any idea which issue the article is in?I have fond memories of Coffs Harbour. Back in 2000 I was part of a bike ride from Tweed Heads down to Sydney, we were clapped out by the time we got to the land of the big banana so camped at the show grounds and I really enjoyed the warm water and cold beer!Nice to meet you Ian.
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Old 13th December 2011, 11:04 AM   #4
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Likewise, farmerjack
Try 2007, August IIRC, in SC but you can see much of the detail on the their site archives.
I should say the mods were to Self's design in his subsequent Manual editions which increase the features to add a nifty class A/B or Winter/Summer switch for example.
(You may appreciate that in Devonport, Tas. - also a lovely part of the planet - Brits would feel as at home there as in Devonshire, I think. I could swear I was in "Tassie" when I drove the UK tourist routes there some years ago. You're even on the Tamar River! )

SC's is a straightforward CFP version which is not an issue in class A, just more efficient and linear with only 1,2A quiescent on 22V rails. Neither would be called 4 ohm designs, perhaps 6 Ohms as many small HT speakers tend to be.
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Last edited by Ian Finch; 13th December 2011 at 11:13 AM.
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Old 31st December 2011, 05:03 PM   #5
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Default Class A amp

Hello again Ian.


I wanted to ask if you might give me a little guidance on my above project?


I have read a chapter of Mr. Self's book; Self on Audio - but still don't undererstand the circuit completely.


The schematic I saw in the book (the one I previously posted) had a different output configuration to the two pole cfp configuration that I wish to use:- can you look at this picture now and tell me if I have actually drawn a '2 pole CFP'?


I still haven't seen the Silicon Chip schematic and that issue is proving difficult to get at the local library - I may have to lash out and by the article online!


As you can see from my schematic, I want to use the the original circuit but change the drivers for 669/649 pairs and the outputs for MJ15003/15004 pairs. (because I have them in my transistor box).


I have two of the Alltronics heatsinks H0545 (0.4 deg. Cels/W) and the local sheet-metal shop folded two 40mm by 40mm angles 300mm long and from 6mm aluminium. The angles simply bolt onto the machined face of the heatsinks and provide a platform to mount the TO-3 Output devices.


I recall what you said about 18V being a little high, BUT I actually have a 0-18, 0-18 transformer of 300VA and would like to use it on a good Class A beastie that also uses the above sinks.


If you can find time to look at the attached picture and make some constructive criticisms I would really appreciate it. Although I have been dabbling in electronics for years, I don't actually know all that much about amplifiers.


If you know of a better circuit, then perhaps you might be able to point me in the right direction [schematic].


Cheers Ian, sincerely, Phil Elliott aka farmerjack61
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Old 31st December 2011, 05:18 PM   #6
AndrewT is offline AndrewT  Scotland
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There are a few changes I recommend particularly for a beginner.
1.) delete the CFP output stage and replace it with a dual EF output stage. These are in the book.
2.) delete the shared CCS from the input LTP and the VAS. Dedicate a pair of CCS for the LTP and VAS.
3.) add a protection resistor to the collector of the EF VAS driver. It has 1k0 in the emitter, add ~ 1k0 between collector and audio ground.
4.) add degeneration to the VAS (emitter resistor).
5.) add provision for base stoppers (~2r2) for the output devices.
6.) add emitter resistors to the output devices (~0r22 to maintain the bias sensors).
7.) change the input LTP pair for devices that have much more gain.

D.Self's book shows an explanation of how tolerances around the input stage affect output offset and how currents/voltages change. This in effect is telling you that selected pairs of everything in and around the LTP should be used.

Quote:
folded two 40mm by 40mm angles 300mm long and from 6mm aluminium.
Make sure the mating faces are flat.
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Last edited by AndrewT; 31st December 2011 at 05:22 PM.
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Old 31st December 2011, 05:57 PM   #7
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Quote:
Originally Posted by farmerjack61 View Post
Hello.

I was reading Mr. Self's book Self on Audio second edition just last night and saw an interesting circuit. It was a very sophisticated A-Class design with a Novel current control.
Quadrature feedback. Hmmmm. Didn't know Self went there...
I have his book , why didn't I notice this before?

I can think a dozen ways to implement that same function with
vastly fewer parts. One or two transistors at most, and emitter
drop of the same can double for his mystery block of a voltage
reference... He over complicates this to the point of being silly.
I'll attach one example, how simple this all could have been...

Self made a mistake to put such a huge cap across his sliding
bias. When you shunt regulate in real time with a quadrature
feedback of output currents, slowing this down is detrimental.
Should not have cap'd any greater than to make his quadrature
control loop stable.

You only want big cap here (and thermal compensation) when
you drive blind, with no feel of the actual output stage currents.

---

OK, I've labeled incorrectly which MPSA is for NPN and PNP.
NPN should be the 06. PNP should be 56. Just switch them.
You know what I mean...
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Last edited by kenpeter; 31st December 2011 at 06:26 PM.
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Old 31st December 2011, 06:56 PM   #8
Bonsai is offline Bonsai  Taiwan
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Yes I also think this current control scheme is a bit complex. It's fairly easy to do it with a single transistor and its electrically and thermally stable.

I also agree that you need a decent supply for a class a amp. Means large filter caps and well filtered supply rails to the front end.
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Old 31st December 2011, 07:37 PM   #9
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Quote:
Originally Posted by AndrewT View Post
There are a few changes I recommend particularly for a beginner.
1.) delete the CFP output stage and replace it with a dual EF output stage. These are in the book.
2.) delete the shared CCS from the input LTP and the VAS. Dedicate a pair of CCS for the LTP and VAS.
3.) add a protection resistor to the collector of the EF VAS driver. It has 1k0 in the emitter, add ~ 1k0 between collector and audio ground.
4.) add degeneration to the VAS (emitter resistor).
5.) add provision for base stoppers (~2r2) for the output devices.
6.) add emitter resistors to the output devices (~0r22 to maintain the bias sensors).
7.) change the input LTP pair for devices that have much more gain.

D.Self's book shows an explanation of how tolerances around the input stage affect output offset and how currents/voltages change. This in effect is telling you that selected pairs of everything in and around the LTP should be used.

Make sure the mating faces are flat.
1) CFP is just fine, providing you have real time overwatch of collector
current. This one qualifies, once you get rid the cap on the sliding bias...

2) Yeah. I woulda just used bootstraps and shunt regulated them down
to the correct (not necessarily constant) current, its a lot fewer parts for
same or superior result...

3 & 4 ) If you had said a base resistor on VAS, I might have agreed.

5) Pointless waste of 2R2 stops nothing. Base itself is 26ohms min.
Ferrite bead on base leg if you worried. Ferrite can easily provide 1K
equivalent resistance against high frequencies.

6) Why? Superior bias sensing is already present in the collector.

7) Maybe the only thing we agree on completely.

Flat mating surfaces) OK, maybe that makes two things....
Sometimes TO92 pairs jam well together in TO5 heatsink.

Last edited by kenpeter; 31st December 2011 at 07:47 PM.
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Old 1st January 2012, 12:41 AM   #10
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Appreciate you comments, kenpeter
Hi Phil,
Lots of things are said in comment on Self's designs, but this is a work in progress to the Trimodal. I've built it in that form for others and it works just as described but I would also agree in warnings against heatsink brackets and their flatness if bent. The dissipation of the output transistors is constantly high and with 24V rails, the sinks will go over 60 deg.C in the centre. That hurts, I assure you.

The parts on the design are generics as befits a model but a final design would have selected inputs and output transistors at least. Your OPs will work OK, much as the design types but the inputs would likely be improved with matched low noise KSC1845/A992 or similar non-obsolete parts.

At the very least, space the OP transistors widely, >100mm, as the narrow sink needs every help to get the heat out to the ends to meet its rating. Surely, it's no issue to stand the Txs up and bolt or screw direct to those sinks?

It might be better not to attempt high quality class A unless you are confident of the right way, to design the board and layout as it is going to be expensive whether the result is as fantastic as it darn well should be or just so-so and something you will eventually bin or try to improve.

Yes its a bit lightweight for a reply but others covered specifics and this is New Year, man...recovery time---
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