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PostPosted: 28 Feb 2015, 10:32 
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Joined: 20 Feb 2015, 21:22
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Location: Central Scotland, UK
Encouraged by the interest shown here and on http://www.diyaudio.com for Mark Houston's, minimum component amplifier, I decided to re-hash a simple 3 transistor low power class A idea I'd come up with before. The original could only put out about 300-400mW into 8 ohms.

Having acquired a hefty heatsink with which to try out the ZCA and MCA circuits, I decided to try out a higher powered version of my 3 active device amplifier whilst waiting for parts for Marks circuits. If you really wanted to stick with the 2 device topology T2 and T3 could be substituted with a Darlington.

The output as you can see in the diagram is an emitter follower with resistive load. The slightly odd value (7.6 ohm) for the load resistor was arrived at by paralleling 3 of 22 ohm 10 watt resistors, though strictly that should be 7 1/3, they are only 10% resistors! (Wirewound VTM ceramic resistors)

BTW the name hot stuff was inspired by the fact that I burnt my finger on the output resistor whilst working on the circuit! :hot:

I would recommend you use at least one of those bolt on 50 watt resistors, either 7.5 or 8.2 ohm, you might have to tweak the bias slightly for maximum and symmetrical output swing, note the dissipation of the resistor(s) is nearly 24 watts.

I used the TIP3055 which is in a TO-247 package, instead of the TO-3 Packaged 2n3055, because it suited the heatsink I had and I only had fairly small TO3 style heatsinks. It might be advisable to use the TO3 Transistor as it will run cooler and require a smaller heatsink.

The first stage is a common emitter amplifier, which I had intended to have a gain of only x4, which was fine when I was testing with just the signal generator, however when I hooked it up to my Samsung tablet to provide a source of audio, the gain turned out to be way too low. I tried to bias it properly 15v @ tr1 collector, without the need for a bypass capacitor around the emitter resistor, the original circuit had a 250 emitter resistor and a1k collector resistor giving a gain of 4, this could be biased reliably for 15 volts at the collector. ( The choice of 15 volts was to ensure 13.5 v at the output, that I'd found to be the optimum with 1.75 A standing current and the 7.6 ohm resistor and an 8 ohm load.) Anyway, it turned out that I needed a gain of x34 as the output from the tablet was very low, about 300mv peak to peak maximum, I haven't tried it with a CD player yet, it's downstairs and the workbench is upstairs! I expect I wouldn't need all that gain and could go back to a resistor only at the emitter of tr1.

Another thought and I'd appreciate anyones views on this, is the addition of a resistor either from tr2 emitter to tr3 emitter or from tr2 emitter to ground, someone suggested elsewhere that it would help to linearize tr2 and hence lower distortion.

Summary, this was thrown together, see the spaghetti junction layout below, with parts I had lying around as a temporary use for the bid heatsink, I didn't expect it to be very good, but I was very surprised, I am sure there are many on here who have heard much better, Best amp I've owened was a Cambridge audio ( I forget which model ), so I can't really claim to be an audiophile ( yet :king: )
I am open to all suggestions for improvement and would enjoy a lively exchange of ideas on this matter.

In summary:

Topology: Single ended Class A with resistive load and no GNFB
Supply voltage: 22.0 volts @ 1.75 amps.
Output Power: 2.5 w just prior to clipping, about 1.5 watt with no visible distortion on scope.
Gain: X 34 ( 30.6dB )
Sensitivity for max output: 130 mV for 4.45V RMS out.
Distortion: unable to measure.
Output impedance: < 0.2 ohms.
Optimum load: 8 ohms

Note this was only a mono prototype, also the output cap was a 4700 uF electrolytic.

The 7.6 ohm resistor might not be the perfect choice, it was convenient with the parts I had available, I am open to alternative views on optimum resistor value, bias current and midpoint voltage, and any other thoughts you might have on it, be kind though, it's the first power amp I've designed and shared with the public ! :headphones:


Will add more pictures! Need to compress them to < 500k, Sorry about the crumby schematic, I'm working on my tablet just now and don't have access to ltspice or tinycad as my laptop is broken.


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PostPosted: 28 Feb 2015, 18:41 
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Joined: 06 Jun 2008, 18:23
Posts: 5143
Location: Australia
Well done. A few watts into efficient speakers can work quite well. My 1920s tube amp only produces 330mW and into 98db efficient Beymas plays more than loud enough.

Keep us informed of progress.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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PostPosted: 02 Mar 2015, 10:22 
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Joined: 20 Feb 2015, 21:22
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Location: Central Scotland, UK
Thanks Mark :beerchug:

:confused: First a couple of questions;

Would putting a resistor between the emitters of the driver and output transistor reduce distortion and improve linearity, I see that single Darlington's have resistors there, I've never quite understood why.

If I was going to replace the load resistor with a CCS what current should I choose to maximize output power, and why that value? I need to understand so that I can better design my own circuits.

The original version of this circuit only put out about 400 mW, it provided reasonable volume with a sine wave into a little crappy 3 watt speaker from a cheap stereo. I never got round to testing it with music and / or a proper speaker(s).

For anyone interested, the original circuit was broadly the same. A common emitter vas, with local feedback ( emitter r + c ), a bc547c as driver and a tip41 emitter follower with a 15 ohm load resistor, Vcc = 12 volts, midpoint at output was about midway, output cap was 2200 uF feeding an 8 ohm load. Was just doing a few calculations and the output power would have been a bit higher with the midpoint at about 7 volts, perhaps it would be OK as a headphone amp, I haven't tried making one of them yet.

There is a thread on diyaudio.com with some further discussion regarding my circuit:

http://www.diyaudio.com/forums/solid-state/270539-hot-stuff-2w-bipolar-class-amplifier-newbies-first-effort.html

The speakers I have can only manage 88db at 1w / 1m, so in reality I'm probably going to have to look at something a bit more powerful.


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PostPosted: 03 Mar 2015, 21:16 
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Joined: 20 Feb 2015, 21:22
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Location: Central Scotland, UK
Second draught of the class a amplifier;

Improvements

1. Common emitter VAS has been replaced with an ne5534 low noise opamp.

2. Both driver and output stage have been loaded with CCS.

3. Supply voltage increased to 30 Vdc.

4. Negative feedback has been taken from the output back to the input of the opamp.


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