Looking for simple, proven, low power AB amp design for testing.

[mkiijam]

Hi Guys, Ive just spent the last few hours poring over the Webernet looking for a simple AB design to build.

I just wanna play with and do some testing on various VCC / VEE levels.

So... can anyone recommend a design?

1. Less than 1 watt needed on output / think "LINE LEVEL"
2. Push /Pull
3. Class AB(?) Can number 4 happen with Class A?
4. Bi Polar supply
5. discrete

What else do I need to spec? It almost seems like more of a preamp?

[RoGeorge]

Once I've seen a funny 4-transistors amplifier.  The topology looked intriguing, so I've asked if there is a name for that:
https://www.eevblog.com/forum/projects/(circuithology)-symmetric-4-transistors-(audio)-driver/

Turned out it is called 'diamond buffer' amplifier, so if you search for that term, you should find plenty of schematics.  Ignore my hand-draw schematic from that link, because I didn't built it yet.  It's on my project lists, but I didn't try it in practice.  I know it works because I've seen it in a few ham builds.

See if the diamond buffer amplifier suits your expectations.  Seems easy to make and has no adjustments, so it should just work no matter what.

[Zubeneschamali]

I just wanna play with and do some testing on various VCC / VEE levels.

1. Less than 1 watt needed on output / think "LINE LEVEL"

Not sure why you want to test with various levels for VCC and VEE unless you are also playing around with the load resistance while maintaining your 1 W specification. In general, you need a peak voltage into the load (assuming a speaker) that is sqrt(2*power*load). So if this were 8 Ohms and 1 W, then 4V peak. If it's not bridged (and I'm assuming its not) then you would need 8 V peak to peak. And that implies about another 2-4 V added. So a +12 V and ground or else +/- 6 V would be enough. Not much need to play with those values. Using larger voltages would be wasteful and using less would remove headroom you may need.

2. Push /Pull
3. Class AB(?) Can number 4 happen with Class A?
4. Bi Polar supply

Output stages driving speakers at 1 W are always push-pull (2-quadrant.) Attempting to use a passive resistor for one of the quadrants is... not advised.

Bipolar supplies can be used with class-A and with class-AB. The circuits don't care. But class-A is very very inefficient as a power stage. (It's just fine as a voltage gain stage, though.) So you probably want a class-AB output stage with a class-A voltage gain stage. Negative feedback will also be a necessity, I think, for good THD as well as setting the gain and/or volume.

5. discrete

That can be done. In the class-AB case, you may need to include provisions for setting the quiescent current using a VBE-multiplier with a potentiometer (or some other means.) For relatively simpler designs, anyway. (Transistors vary quite a bit, one to another. And so does the operating temperature. So you may need to play around a bit to get it set.)

What else do I need to spec? It almost seems like more of a preamp?

You mentioned "line level". That's almost a spec. (The meaning varies a little bit based on application domain. But close enough for horse shoes.)

If you intend on using electrets, you may need a pre-amp stage. But I think you are just saying "line level" so I'd guess you can avoid any specialized pre-amp stage and go straight to a nice class-A voltage amplifier stage followed by a class-AB power stage and some NFB (negative feedback.) 1 W isn't too hard, though it's not entirely trivial either. It's a good target for learning.

Line level is around 700 mV rms, I think (I'm no expert on it.) And an 8 Ohm speaker at 1 W would need about 2.9 V rms. So the voltage gain is only 4, if my math isn't failing me. That's actually pretty easy to achieve. So a lot of negative feedback can be used to correct many errors (thermal drift and so on) while also achieving the desired voltage gain, without stretching things. The complexity will be more at the class-AB end. Though even here the 1 W spec means a lot of otherwise difficult problems (dissipation, for example) are far less problematic.

[pqass]

For anything audio see http://sound-au.com/.
It's a projects and articles goldmine for discrete and opamp designs with full explanations.

[Conrad Hoffman]

I'd think any typical AB discrete design would do the trick. Anything from the simple designs in the back of the RCA transistor book of the past, the Motorola app notes (people hate darlingtons, but the one if their app note works just fine, to a low power version of Doug Self's Blameless amp.
https://conradhoffman.com/papers_lib/AN483B.pdf
http://www.bitsavers.org/components/motorola/_appNotes/AN-0485_High_Power_Audio_Amplifiers_With_Short_Circuit_Protection.pdf
http://www.douglas-self.com/ampins/ampins.htm
https://www.cordellaudio.com/#
https://www.rsp-italy.it/Electronics/Databooks/RCA/_contents/RCA%20Transistor%20Manual%20SC12%201966.pdf (see pg 424)

[Zubeneschamali]

For anything audio see http://sound-au.com/.
It's a projects and articles goldmine for discrete and opamp designs with full explanations.

Seconded!

I'm including something based in part on a design found at Elliot's site.

[David Hess]

Once I've seen a funny 4-transistors amplifier.  The topology looked intriguing, so I've asked if there is a name for that:
https://www.eevblog.com/forum/projects/(circuithology)-symmetric-4-transistors-(audio)-driver/

Turned out it is called 'diamond buffer' amplifier, so if you search for that term, you should find plenty of schematics.  Ignore my hand-draw schematic from that link, because I didn't built it yet.  It's on my project lists, but I didn't try it in practice.  I know it works because I've seen it in a few ham builds.

See if the diamond buffer amplifier suits your expectations.  Seems easy to make and has no adjustments, so it should just work no matter what.

Diamond buffers are very useful and ubiquitous, but not so visible.  Current feedback operational amplifiers use one for their input stage, and high slew rate voltage feedback operational amplifiers use two of them in place of a differential input stage.  Many later operational amplifiers use a diamond buffer for their output.  But without voltage gain and a feedback loop, a diamond buffer by itself is not really suitable as an audio amplifier because of excessive distortion, at least compared to an amplifier with feedback.

Below is the simple model amplifier that Douglas Self uses.  Remove Q9 and replace Q8 with a diamond buffer, and you will be all set.

Also below is the "diamond buffer triple" that Bob Cordell suggests, but I think that is overkill for 1 watt.  Just use a standard diamond buffer.