information needed about low power amp topology

We all start somewhere
User avatar
Mike H
Amstrad Tower of Power
Posts: 17662
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

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
Image
Image
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

User avatar
Ami
User
Posts: 13
Joined: Fri Sep 14, 2018 6:06 pm

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

User avatar
Mike H
Amstrad Tower of Power
Posts: 17662
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

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.
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

User avatar
Ami
User
Posts: 13
Joined: Fri Sep 14, 2018 6:06 pm

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/

Ami.

User avatar
Nick
Site Admin
Posts: 11304
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

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/
Resistance isn't futile it's V / I.

User avatar
Mike H
Amstrad Tower of Power
Posts: 17662
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

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.
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

User avatar
Ami
User
Posts: 13
Joined: Fri Sep 14, 2018 6:06 pm

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.

User avatar
Mike H
Amstrad Tower of Power
Posts: 17662
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

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.
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

Post Reply