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Simplest, least number of components, amplifier design.

jack121

New Member
Hi Thanks for reading. Okay I want to build a simple audio amplifier with absolute minimum components.

So far with my amateur attempts i have built a single stage amplifier using a 2sc5200 transistor, with a 1uf capacitor and 1000 ohm resistor as an input stage, but it sounds like a glorified headphone amp, very little output about half a watt.

Do you have any minimal component designs which can put out about 5 watts up to 10 watts, or if this is not possible a two stage amplifier consisting of a driver transistor followed by an output transistor, again minimal components. I have 2sc5200 transistors so will appreciate any designs incorporating these.

Thanks
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You can not build a practical power amp for more than a tiny output power with just two transistors.

This is about the simplest design that may give a reasonable power level at acceptable quality; the exact transistor types are not critical as long as they have somewhat similar characteristics.

The possible power output will depend on the supply voltage (and speaker impedance).
Edit - capacitor values corrected.

Schematic_Basic Audio Amp- Stage 3_2021-09-13.jpg
 
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Pommie

Well-Known Member
Most Helpful Member
Love the fact you're using easyEDA for you schematics. Good, easy to use software. Oh, and free.

Mike.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You can not build a practical power amp for more than a tiny output power with just two transistors.

This is about the simplest design that may give a reasonable power level at acceptable quality; the exact transistor types are not critical as long as they have somewhat similar characteristics.

The possible power output will depend on the supply voltage (and speaker impedance).
Edit - capacitor values corrected.

View attachment 133680
A couple of comments on the circuit:

1: Connect the slider of the pot to the bottom of the pot, otherwise it will go 'boom' is the slider loses contact.

2: Add a PNP transistor at the front, with the feedback going to it's emitter. This makes biasing more stable, gain more controlled, and gives a better input impedance. For a smallish power amplifier it also allows you to make Q1 a bit more powerful, and drop the extra drivers on the output stages (move the outputs the other way, as emitter followers) - so you actually save a transistor as well.

3: (I lied about a couple) - miller capacitor on Q1, C to B.

4: Zobel network on the output is always handy.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Add a PNP transistor at the front, with the feedback going to it's emitter.
You'll have to explain the configuration you mean for that a bit better?
Most of the more complex amp circuits I'm familiar with use a long tailed pair as a differential amp between input and feedback.

it also allows you to make Q1 a bit more powerful, and drop the extra drivers on the output stages
I can't see that working well without also massively increasing the bias circuit current / lowering it's impedance, to compensate for the reduced gain of the output stage?

The outputs are effectively already in emitter follower config, with the additional transistors of the Sziklai pairs providing a much higher overall current gain.

OK on the bias slider connection & zobel network, I totally agree with those two.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You'll have to explain the configuration you mean for that a bit better?
Most of the more complex amp circuits I'm familiar with use a long tailed pair as a differential amp between input and feedback.

It was the standard configuration before long tailed pairs appeared - such as the old Leak examples here:


I don't recall the schematic you posted (nicely drawn :D ) ever been used commercially?, and only ever seems to appear on non-working, or barely working, Internet sites.

It's very lacking in gain, which is what the prior PNP transistor provides - so specs would be poor.

As far as I'm aware, the standard configuration with PNP then NPN (or the other way round originally) came for a very old Mullard application note.

Here's a Hacker example (from a radio I still own), no extra drivers here - but a rather peculiar extra transistor at the front to give bass and treble controls - but still the basic NPN in, PNP driver (at the top), and NPN/PNP outputs. It even has a Vbe multiplier for bias compensation.


It's always a bit confusing when circuits are 'upside down' like that one, but it's from back before it was decided how circuits should be drawn - and Hacker were the best radio manufacturer in the world, amazing radios.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I don't recall the schematic you posted (nicely drawn :D ) ever been used commercially?
That is my own theoretical third stage "fixed up" version of the nasty internet distortion generator "amp" circuit, as posted in the OPs other thread.
If it has ever appeared elsewhere, someone has stolen it from this site!

I keep meaning to build it and see how it actually works, but I just have too much else work-wise and too many other projects queued up at the moment...

The two amp circuits you linked to use two rather different configurations, the Leak design uses the same type transistors (as in PNP/NPN) and the Hacker uses opposite types.

The next-stage improvement I mentioned on the cct above was going to be a long tail pair input, and the miller cap (frequency response limiting), in the previous thread..

The pre-any-impovement internet version, from the other thread:

3-gif.132406
 

Nigel Goodwin

Super Moderator
Most Helpful Member
That is my own theoretical third stage "fixed up" version of the nasty internet distortion generator "amp" circuit, as posted in the OPs other thread.
If it has ever appeared elsewhere, someone has stolen it from this site!

I keep meaning to build it and see how it actually works, but I just have too much else work-wise and too many other projects queued up at the moment...

The two amp circuits you linked to use two rather different configurations, the Leak design uses the same type transistors (as in PNP/NPN) and the Hacker uses opposite types.

Perhaps you're too young? :D

The Hacker is fully complementary, and the Leak is quasi-complementary - the reason is dead simple.

Essentially they are VERY similar circuits - but (the age thing) you couldn't get decent (or any?) high power PNP transistors back then, the 2N3055 was an exceptionally high powered NPN, but there was no PNP equivalent. The quasi-complementary configuration is there simply to give an NPN darlington, and a PNP darlington.

Once high power PNP's appeared (many years later) at reasonable cost, quasi-complementary designs tended to fade away.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I was referring to the transistor driving the bias line (Q1 in mine), that you advised adding an extra stage prior to; that's what we were discussing in previous posts.

That position and the one before it, in the circuits you linked, not the output setup.
eg. Leak T4 & T5, both PNP; Hacker T2 & T4, NPN and PNP.

[ps. I'm old enough to remember 33rpm LPs appearing and transistors being introduced to consumer products].
 

audioguru

Well-Known Member
Most Helpful Member
With its upside down bias diodes, the Hackaday amplifier worked as a class-A heater and amplifier. Without any negative feedback its distortion must have been awful.
On the other forum I simulated it with and without the diodes backwards and I fixed it by also adding some negative feedback.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I was referring to the transistor driving the bias line (Q1 in mine), that you advised adding an extra stage prior to; that's what we were discussing in previous posts.

That position and the one before it, in the circuits you linked, not the output setup.
eg. Leak T4 & T5, both PNP; Hacker T2 & T4, NPN and PNP.

[ps. I'm old enough to remember 33rpm LPs appearing and transistors being introduced to consumer products].

Sorry, we're looking at different diagrams :D

I was refering to the leak Stereo 70, which is the normal NPN/PNP - third diagram down.
 

audioguru

Well-Known Member
Most Helpful Member
The sales sheet and datasheet of the PAM8320 20W (actually only 10W or 15W into 4 ohms) class-D amplifier say New Product but are dated Oct 2013. Digikey has none (broken supply chain) and say lead time is 20 weeks.
 

unclejed613

Well-Known Member
Most Helpful Member
It was the standard configuration before long tailed pairs appeared - such as the old Leak examples here:
it was also used in Dynaco amplifiers as well as thousands of others, one prime example being the Acoustic 370 bass guitar amplifier...

a single-ended class A amplifier will always have a lot of distortion, but there are ways to reduce this, such as a class AB double ended (push-pull) output stage, or the use of a large current source to linearize the output device (which will LOOK a lot like a push-pull output stage)... either way, it adds some complexity. there's really no way around it if you want good sound... the most "down and dirty" designs i have ever seen that sound ok, are what's used in about 90% of the entry level amplifiers and receivers on the market, and consist of about 6 or 7 transistors (with the output devices being darlington transistors), with the following transistors, a differential pair (2 transistors), a voltage amplifier stage (1 transistor), a bias transistor (1 transistor), and the output darlingtons (physically, 2 transistors) for a total of 6. if you add a constant current source to feed the diff amp, that adds one, and if you linearize the voltage amplifier stage with another current source, that adds another.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
it was also used in Dynaco amplifiers as well as thousands of others, one prime example being the Acoustic 370 bass guitar amplifier...

Like I said, that's how it was done - after years of development and testing - fairly low component count, excellent results.

Remember, back in those days transistors were incredibly expensive, if a manufacturer could save one or two it made a big difference to price.
 

schmitt trigger

Well-Known Member
To the TS:
Building your own discrete audio amplifiers is one of the best ways to learn about electronics.
How come? Because you will be tinkering with it for a long time. You’ll try this idea and then another, all the time listening to the results.

It is also fascinating to watch the audio signal jump through the different stages with a scope.
 

unclejed613

Well-Known Member
Most Helpful Member
Remember, back in those days transistors were incredibly expensive, if a manufacturer could save one or two it made a big difference to price.
that's also why quasi complementary output stages were so popular... PNP silicon output devices were very expensive compared to NPN silicon output devices at the time (like a 5:1 ratio in price if i remember correctly back in 1970.... 2N3055 devices were about $2.00, and 2N2955 devices were $10.00).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
that's also why quasi complementary output stages were so popular... PNP silicon output devices were very expensive compared to NPN silicon output devices at the time (like a 5:1 ratio in price if i remember correctly back in 1970.... 2N3055 devices were about $2.00, and 2N2955 devices were $10.00).

I don't even remember the 2N2955? - I remember the much later plastic versions, usually labelled MJ2955 and MJ3055, 90W instead of 115W.

Silly money back in the early days, so no one used them? - but as you needed individual drivers anyway, it made little difference to make it quasi-complementary.
 

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