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JLH Class A revealings

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    Posted: 10 Aug 2010 at 8:36pm
Back in a 1969 edition of Wireless World can be found John Linsley Hood's classic 10W class-A amplifier. It's basically a three NPN transistor stage having quite a fantastic squarewave performance. The man must have been an inventive genius, or was he?

The design can be found here http://www.tcaas.btinternet.co.uk/jlh1969.pdf see fig.2 page 7.

After doing much research on this NPN transistor triple including simulating and building a number of variations to suit my own requirements I landed on this page...

http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic

Transistor-transistor logic (TTL) of the 7400 series features this output stage...



...bearing a strong resemblance to the JLH class-A amp, and obviously dealing with fast logic signals it had to have fantastic squarewave performance too...

So was the class-A amp still a stroke of inventive JLH genius? The 7400 TTL was introduced in 1966 by Texas Instruments - that's three years before the JLH class-A was published in 1969. Did looking into TTL give him the idea? I would still say JLH was quite clever even if all he did was to recognise the 7400 TTL output stage could be made into a great audio amp output stage.

But apart from a slight redesign published in Wireless World in 1970 there hasn't been much if any news of its use since - or has there?

I found the design crop up again in the Signetics NE5534 op-amp (since made by Philips, New Japan Radio Co, Texas and On-Semiconductor) in its output stage! (obviously...)

It's difficult to see but if you remove the protection circuit from the schematic you'll notice that there's the addition of three diode junctions (one's a transistor), an emitter resistor and a collector resistor, along with a transistor current source replacing the capacitor bootstrap.

See data sheet here http://www.onsemi.com/pub_link/Collateral/NE5534-D.PDF page 2.

The natty bit being the diode just above the lower totem-pole transistor collector (cathode connected to collector) which mimics a Sziklai PNP transistor base whereas the input is to the Sziklai NPN transistor base.

OK, I won't be surprised if you haven't a clue what I'm talking about, but the point is that the JLH article was subject to thermal runaway - where bipolar transistors get warmer and conduct harder so get warmer and conduct even harder, and so forth until they blow. The only solution was to use a massive heatsink. Here in the NE5534 we find the answer to the thermal runaway problem - the 3 diode junctions and two emitter resistors!

Now we have a workable JLH output stage that can be tailored to do many kinds of amp stages from preamp functions to power amps, controllable regarding thermal runaway.

I can vouch having listened to both, that both the JLH and the NE5534 output stages sound very similar and as the JLH design sounds very musical you can read that so does the NE5534 in conditions where the input stage (which is the real downfall of this op-amp if there ever was one) influence is reduced. But as to how that's done will have to remain my secret recipe (a poor boy has to have some aces up his sleeve... Wink)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2010 at 10:23pm
 


Edited by Fatmangolf - 13 Aug 2010 at 7:12pm
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2010 at 12:55pm
Carrying on from my last paragraph above, I have some further listening test results which may be of interest.

The classic JLH stage, as I may have mentioned, relies on a rather large heatsink to prevent it going into thermal runaway. It is possible by placing a small value resistor in the emitter lead of the lower totem pole transistor to counter the rising current due to rising temperature. Even so, the "totem pole" transistor arrangement still warms until the equilibrium of transistor currents with heatsink mass is reached. The heatsink mass has a bearing on how it sounds. As the stage warms over several days of being powered up, it can go through sonic changes because a hot transistor's current gain reduces with increasing temperature, or to put it another way, a hot transistor gets very close to being a dead short.

Though my implementation, a much lower current design to drive a pair of headphones, sounded quite impressive early on, it went on to sound thinner and thinner over time, as the stage warmed up and the current gain became less. This is also a warning to those who equate the degree of hotness of an amplifier with sound quality. A hot transistor does not have much current gain and therefore the design will become more distorted as it gets warmer.

However, I do feel the stage is worthy of being mentioned in dispatches. And although I would not use it as a commercial amplifier - for headphones or speakers - members here may be interested in experimenting with it for their own use.

If sufficient DIYers reply to this I will publish my findings in greater detail with schematics to enable the constructor to have a go. I think it is a must hear for all DIY audio enthusiasts.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrarroyo Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2010 at 1:27pm
Graham, Alex (SandyK) is a big proponent of the JLH ripple eater. I hope he chime is.
Miguel
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tg [RIP] View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg [RIP] Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2010 at 3:28am

Miguel,

in my understanding the "ripple eater" is for input power filtration.

Since the amplifier builds a replica of the input signal from the input power, the cleaner that is the better the replica can be.

This circuit (or part thereof), if I understand correctly, is where that replica is built and is about providing a linear response to the stimulus of the input signal.

In other words, the two would be complementary parts.

Which is not to say that input from Alex would not be valuable.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2010 at 12:03pm
Unfortunately Alex has expressed the opinion that this being a commercial forum that it would not be the place for him to contribute.

I hope one day he will reconsider.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrarroyo Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2010 at 2:29pm
TG, Alex also has the Class A amp. That is what I was talking about, but as Graham states he has no interest since this is a commercial forum.
Miguel
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