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Old 8th Dec 2021, 12:57 am   #1
Ewan Penkey
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Question Transistor power amp design confusion.

Hi , I hope you good people can help clear up a little confusion for me.
The attached schematic is for the power amp section of a 1989 Peavey bass amplifier I'm doing some servicing on. The amplifier in question is working fine, it just needed some caps replaced and some solder joints re-flowed. It's in remarkably good shape. I've been thrown into a bit of confusion whilst studying the schematic though. Transistor amps aren't my strongest suit, but I can usually get my head around what's going on well enough. I must admit defeat on this one though. I'd really appreciate it if someone with more knowledge than me could explain what's going on with the speaker output and power transistor collectors being connected (via resistors R27 and R28) to the centre-tap of the power transformer. I'm sure I've never come across this concept before. I've stared at the drawing a number of times until my head hurts, but I can't fathom why the output would be anywhere near the secondary of the power transformer. I get that the centre tap is floating, and thus the DC rails will presumably ride up and down with the output, but why would you want that? What's the advantage? Isn't there interference from the AC power? (Obviously not in practice, but I'm not sure why not......)
I'd love to understand a bit more, if someone would be so kind as to enlighten me a little.

Thanks in advance.

Ewan
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Old 8th Dec 2021, 2:00 am   #2
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Default Re: Transistor power amp design confusion.

It means that the base voltages of the driver transistors never have to go very far from ground, making the design of the stage before your snippet easier.

There are penalties. Your power transistors are no longer just emitter followers, and are used to make voltage gain. Their voltage gain wanders all over the shop with changes in speaker impedance, and the feedback is left to sort it all out and to hopefully still be stable.

There's a lot of reliance on capacitive isolation of those transformer secondaries from coupling into anything else, and being well balanced.

So this isn't a configuration which has caught on.

The output stage has taken on duties as the VAS, as well as their role in current amplification.

To understand amplifiers with common-emitter topology output stages, have a look for the Blomley design in wireless world. This too didn't catch on. It used small signal commutation for class B with a small class A region.

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Old 8th Dec 2021, 8:33 am   #3
peter_sol
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Default Re: Transistor power amp design confusion.

Some Fender amps do the same thing.
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Old 8th Dec 2021, 9:47 am   #4
Keith956
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Default Re: Transistor power amp design confusion.

The output stage makes perfect sense if you replace all the earth symbols by a common connection, and assume the centre tap of the transformer is the earth.

Then you have the emitters driving the speaker, whose other side is connected via L1/R30 to the 'ground' i.e the CT, just like a normal amp.

Why they did that though, it's hard to fathom.
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Old 8th Dec 2021, 10:12 am   #5
Ewan Penkey
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Default Re: Transistor power amp design confusion.

Ah, yes the penny has dropped a little
So it's upside down and backwards - now it makes sense!
I kind of get how it works now, or close enough to understanding that it won't irritate me like it did yesterday.
Thanks very much all - I knew I could rely on this forum.

At least I won't spend too much time scratching my head today. I've little enough hair left as it is.

Cheers

Ewan

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Old 8th Dec 2021, 11:05 am   #6
lesmw0sec
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Default Re: Transistor power amp design confusion.

Not seen that arrangement before. I'd reckon the designer also had shares in diode manufacturers...
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Old 9th Dec 2021, 2:22 pm   #7
mhennessy
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Default Re: Transistor power amp design confusion.

That configuration is actually very popular, just not so much in the domestic world. ATC have used it for decades in their active speakers, for example. It's probably the most commonly chosen topology for PA amplifiers, in my experience, at least.

Having all of the circuitry running at a lower voltage is really useful today, as all the decent small-signal-but-high-voltage transistors we used to use are going obsolete, sadly.

There are some challenges with stability, as there would be in any topology that uses an output stage with non-unity gain, but these are not insurmountable, and there are lots of examples of these sorts of amps with really very decent performance.

The main downside is the fact the beefy power supply rails are bobbing up and down with the audio signal - this means that each channel needs its own rails, so a stereo amplifier needs a pair of chunky windings. Plus the low voltage, low power windings for the pre-stages, of course. Not a big problem for quantity manufacture, but less good for the home builder - custom-wound transformers in single or small quantities are usually prohibitively expensive, so instead you might end up buying 3 standard types for a stereo amp - but at least you can make audiophile claims about dual-mono power supplies and a dedicated pre-amp supply And actually, larger off-the-shelf toroidal transformers tend to make a lot of mechanical noise, so doing the job with 2 smaller ones can neatly side-step that problem.

In an active speaker, this is much less of a problem, as the 2 or more drive units invariably require different powers (from memory, the ATC SCM20A is 200W for the woofer and 50W for the tweeter), so you were probably already looking at different rails for the two power amps.

I agree it is confusing when you first meet it. The simplest way to get your head around it is to see that all they've done is moved the earth connection from one end of the speaker to the other. Of course that has other ramifications which can be addressed subsequently, but for anyone comfortable with standard layouts, it's not that much of a conceptual leap really...
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Old 13th Dec 2021, 1:32 pm   #8
mhennessy
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Default Re: Transistor power amp design confusion.

Here's an example of such a design in a transistor radio - the Grundig RF110. The extract from the manual probably isn't terribly readable, but as the data is available from "up top", that's probably a good thing. I've done a simplified sketch which hopefully is clearer.

The battery on the left represents the supply that also powers the RF and IF stages. This is zener-regulated to 12V. The battery on the right is floating, and moves up and down with the signal. The main difference between this and other amplifiers with floating power supplies is the need for the capacitor joining the junction of the emitters to ground - normally the floating supplies are split, so the mid-point is where we take the output.

Not shown on the simplified diagram is the DC biasing for the first transistor, which comes via a 1M resistor from the junction of emitters/top of the cap.

The interesting thing about this is the date: 1967. This is the earliest example of this technique that I'm aware of - I'd be interested in any other examples.

I've no idea why the designer did this though - in this context I'm not sure there's any advantage over the conventional arrangement. Perhaps they were trying to keep the low power rail (for the RF/IF stages) free from modulation by the AF stages, but they could have simply used the two windings to power conventional circuitry. A later table model in my collection - the RF111 - used a single transformer winding and didn't regulate the supply to the signal stages.
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