information needed about low power amp topology

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Mike H
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Re: information needed about low power amp topology

Post by Mike H » Mon Sep 17, 2018 2:05 pm

Nick wrote:
Sun Sep 16, 2018 7:49 pm
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Re: information needed about low power amp topology

Post by Ami » Tue Sep 18, 2018 10:19 pm

Mike H wrote:
Mon Sep 17, 2018 2:05 pm
Nick wrote:
Sun Sep 16, 2018 7:49 pm
Image
Image
There's a gent on another forum who won't be pressing your button Mike!

Here's what he says of the, "sliding junior" circuit by sir Douglas Hall.....
Man from Chicago wrote: It's actually high gain PNP driving a NPN emitter follower. I spent more time on this circuit since it's a sliding bias that I am interested. The problems I mentioned still stand. This is my analysis:

What the circuit tries to do is to vary the current through the transformer according to the input signal. The larger the signal, the higher the current through the transformer to keep it in class A. But this is where the problem begin:

1) in order to vary the DC current through the transformer according to the signal, you need to drive the base of TR2 more positive with the amplitude of the signal.

2) Somehow, a voltage is set up across C2 which is the integrate capacitor that control the sliding bias.

3) D1 and D2 set up +1.6V for VR1. The circuit has to be ACV coupled. When the signal first goes positive, D3 turns on, the output see lower resistance of parallel of VR1 and R1 which is 1K//1.5K = 600ohm. This load down the positive going signal and clip the positive part of the signal.

4) When the negative half of the input signal. D3 turns off, C2 is discharged through R2. So the voltage across C2 decrease. This make the voltage at the emitter of TR2 increase, thereby increase the current through the transformer T1. This will increase the bias and keep the amp in class A. When the signal goes away, C2 charge back up through D3 and R2 and VR1.

Now, ignore the concerns I have in the last post. Just assume it all work. Still, in order for this circuit to work, the output impedance has to be high, so it will clip by the sliding bias circuit on the positive half of the signal. But the impedance through C2 and R3 can load the of the driving stage down.

You need an output transformer to block the DC current. That is one more component that is more expensive than transistors. Also, the big point of transistor amp is to do away with the output transformer that not only limit the frequency response, it also create more distortion.

You might get away with two transistors, but you end up having a more expensive circuit, sound quality is hand held transistor radio at best. Using transformer is so so old!!!

The circuit is so badly design, you have to tweak and tweak to get it to work. I bet when room temperature change, it will go hey wired.

I don't know about the author, but like Raj telling Sheldon in Big Bang Theory " Not all the stuffs come out of you are gold, some are GAGA!!!". This is engineering, it's science. You can analyze the circuit. Looks like some kid wiring it up in the garage.
Well, don't that make you wanna build it, and see Hu? I particularly like the comment "sound quality is hand held transistor radio." Well yeah, that's kinda what I'm building!

Oh, and the repanco people haven't answered the email yet....

Luv, Ami

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Re: information needed about low power amp topology

Post by Mike H » Tue Sep 18, 2018 10:51 pm

Yeah, I think he hasn't made one. Also I don't think it was ever meant to be super hi-fi or anything, just work.

He's right tho what's missing is a series coupling cap at 'Input', if that's large enough value then it's that that charges up thru D3, then can apply an increased Voltage on TR1 base, for the mid and negative going parts of the waveform, turning it on more.

Question, can the receiver part drive this audio input. I note circuit shows 300R ear phone as a load, so if we substitute with 270 or 330R resistor, possibly.

Only one way to find out! Shame there aren't any values shown.
 
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Re: information needed about low power amp topology

Post by Ami » Wed Sep 19, 2018 12:00 am

Mike H wrote:
Tue Sep 18, 2018 10:51 pm
Shame there aren't any values shown.
Mike.
Full article Radio & Electronic Constructor June 1973. http://www.spontaflex.free-online.co.uk/

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Re: information needed about low power amp topology

Post by Nick » Wed Sep 19, 2018 12:03 am

I think the point is these circuits are from the time when transistors cost more than transformers and the gaal was a hand held radio that could be powered by two 1.5v cells.

Values here

http://www.spontaflex.free-online.co.uk ... /page2.jpg

Other circuits here

http://www.spontaflex.free-online.co.uk/
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Re: information needed about low power amp topology

Post by Mike H » Wed Sep 19, 2018 2:48 pm

Ami wrote:
Wed Sep 19, 2018 12:00 am
Mike.
Full article Radio & Electronic Constructor June 1973. http://www.spontaflex.free-online.co.uk/

Ami.
Ah! Right, thanks. And Nick.
 
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Thanks guys!

Post by Ami » Sun Sep 23, 2018 4:51 pm

Hello peeps!

Thanks to a member here, who knows who they are ;) I was kindly sent two transformers that were for a small push pull transistor amplifier. The driver one was the most suitable as the sound from the output one was noticeably muffled, teach saying because it was "not gapped core" (I will look into the significance of that as I've heard the comments before).

Anyways, Sir Douglas Halls circuit does work, with reasonably unobjectionable quality tho definitely not HiFi. Different examples of transistors did need the biasing experimented with to get the low level output distortion to a minimum, the optimum sound quality seems to coincide with a current in the op transistor collector of between 10 and 15mA, raising to an average of 50mA before distortion begins again on loud passages. At 50mA Incidentally, the BC337 op transistor became very hot when driven by a 400Hz sine wave from a signal generator. The transistor seems to cope OK with a music program when araldited down to the copper laminate it was mounted on. The design could do with a higher power transistor in this position.

It does demonstrate that its current consumption is proportional to its output power, so it will meet the design specific.

This amplifier doesn't seem able to be fully driven by that radio circuit that I'd intended using, but, radio Caroline (1Kw @ 70miles) can be heard at room volume during the daytime with just its ferrite aerial, so good enough. All there is to do now is tidy it up and box it.

Thanks to my transformer benefactor, (swaps in the post :D) and all who took the time to reply.

Luv, Ami.

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Re: Thanks guys!

Post by Mike H » Sun Sep 23, 2018 6:09 pm

Very interesting, and congratulations. :thumbleft:
Ami wrote:
Sun Sep 23, 2018 4:51 pm
The driver one was the most suitable as the sound from the output one was noticeably muffled, teach saying because it was "not gapped core" (I will look into the significance of that as I've heard the comments before).
Yep, and when you "get it", you'll probably be like Doh! Of course! Image
At 50mA Incidentally, the BC337 op transistor became very hot when driven by a 400Hz sine wave from a signal generator. The transistor seems to cope OK with a music program when araldited down to the copper laminate it was mounted on. The design could do with a higher power transistor in this position.
It's worth noting bigger "more proper" amplifiers can have the same result, if they're the archetypal push-pull variety, will have a low quiescent current at no signal, but then they will get hotter with bigger output because the current goes up in proportion. And especially so with a constant amplitude signal such as from a sig gen, and even organ music.
 
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Project redux

Post by Ami » Fri Nov 23, 2018 11:59 pm

....So peeps, I presented my work to Teach, in the sure and certain knowledge that I was going to be praised for not only being the first to finish the assignment, but also for the quality of my method exposition and practical skills. I was wrong. Very, very, wrong! :(

My written work was criticised for "plagiarising" the writing of sir Douglas Hall, (who had according to the Teach, made a number of inaccurate claims about his design), If I were to use his design, I was to at least have to be able to explain correctly how it worked.

The practical realisation was also criticised for two reasons, one being of distortion that Teach said was caused by incorrect bias at idle, that varied the gain of the amplifier depending on instantaneous input level. The second reason he said was that the amount of output power available was limited by the mismatching caused by the output transformer.

G.O.M.

Back to the drawing board of despair :( :(

I decided to contact a gent who had offered to work through the problem with me in the spirit of a tutoring session. The gents name is Norman, he is a member of the DIY audio forum, and was a designer for a company in London in the fifties an sixties called "Radiomobile".

The key points he raised was that:-

1. At 3volts the transformer isn't required as a low impedance speaker can be driven directly through a blocking capacitor. An inductive load for the output transistor was still a useful, if expensive, addition, he says, as it means that the output voltage swing could be nearly TWICE the supply voltage, (I still don't understand that).

2. With Sir Douglases circuit the input signal also has to supply the power for the bias, Norman suggested that the bias power be supplied by the circuits output by using it to "bootstrap" ( :? ) the bias through a diode so as to clamp the input to ride upon a level of bias that maintains the amplifier (hopefully) in class "A". He also says that given the transistors available to the design, it should be possible to get 200mW or more from it with reasonably good audio quality.

Norman sent as an example a circuit of an amplifier that was used in an early transistor car radio using this method, it was promoted as a set that could be used all day without the engine running and not flatten the battery, (he said a normal radio in those times would drain 2-3amps continuously).

Norman, (who, it turns out, was 90 years young this April!) has been a real sweetheart and has made and sent me an inductor, (he called it a "choke") to use in my circuit, still based on what I have already built, but with his suggested modifications.

I will try the new ideas this weekend and post a circuit if its successful. :?

Luv, Ami. :)

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Re: information needed about low power amp topology

Post by Mike H » Sat Nov 24, 2018 8:57 pm

Wish I could say something comforting. :(
 
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Re: Thanks guys!

Post by Ami » Mon Nov 26, 2018 1:48 am

Well peeps, I built it and it works :D

And it's all good! :P

So good that to do it justice in listening tests I'm feeding it with a program of music from the line out on my CD player, (instead of the crappy MW tuner), driving a "vintage" 7" elliptical probably from an old TV set that is mounted in a plywood box.
Mike H wrote:
Sun Sep 23, 2018 6:09 pm
organ music
As I type this I'm listening to a recording of J.S.Bach Toccata & Fugue in D minor played by Eberhardt Kraus on the Mozarteum Organ in Salzburg. Nothing a girl likes more than a man who gives a satisfying performance with his big organ :shock: :wink:

Ahem...anyways Norman obviously knows what he's talking about, (and a member of the DIY audio forum to boot, who'd have thunk it!), I'm almost tempted to try building a stereo version and run it at 12volts with a hefty output transistor, he said the car radio that used the principal, could drive 3 ohms at about 2 to 3 watts, though the topology was not the same as this, as I've kept Sir Douglas Halls basic design.

The present version needs 25mA at idle, and draws about 200mA at the point it starts to clip on a 1KHz sine test....I very much doubt the BC337 could survive much more, it gets too hot to touch.... And, by simply working from the peak voltage of the output, I'm confident that Normans predicted 200mW is about right for a 3volt supply, that's 30% efficiency if my maths is correct, not bad hay?

If anyone is interested in recreation of 50 years out of date audio circuitry, I will get the details on that inductor for them.

All I have to do now is work up some plausible B.S. on how this works to impress Teach with...

My biggest lesson from this has been that there is a hellavalot to consider when trying to get electronics to do the shit you want and not to do the shit it wants.

Attached is the diagram of hopefully the final version of the amplifier.

Luv,
Ami.
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Re: Thanks guys!

Post by Mike H » Mon Nov 26, 2018 2:43 pm

All looks jolly spiffing, congrats. :thumbleft:

Ami wrote:
Mon Nov 26, 2018 1:48 am
As I type this I'm listening to a recording of J.S.Bach Toccata & Fugue in D minor played by Eberhardt Kraus on the Mozarteum Organ in Salzburg. Nothing a girl likes more than a man who gives a satisfying performance with his big organ :shock: :wink:
Image
Steady on some of us have delicate constitutions. :D

My biggest lesson from this has been that there is a hellavalot to consider when trying to get electronics to do the shit you want and not to do the shit it wants.
Not half. In my early days I spent a lot of time building stuff from books that didn't work. Or not as expected. Most often it was because some vital bits of information were missing. In the case of many of the books (from the local library, believe it or not), this was on purpose, precisely to stop people building copies of real manufacturers' amplifier cicruits. E.g. there'd be no component values, or if there were, no transistor type numbers, that sort of thing. And of course the power supply was invariably missing, which consequently was like a dark and mysterious unexplored territory.
 
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Re: information needed about low power amp topology

Post by IslandPink » Mon Nov 26, 2018 3:50 pm

I must point out that some men can give a satisfying performance with a small organ - for instance Al Kooper on some of Bob Dylan's songs.
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Re: information needed about low power amp topology

Post by Mike H » Mon Nov 26, 2018 4:02 pm

:D
 
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Re: information needed about low power amp topology

Post by shane » Mon Nov 26, 2018 4:29 pm

Not to mention the likes of Emerson and Wakeman who manage to satisfy thousands with an electric organ.







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