Class A, 4 Watt, No Feedback, Simple Circuit, Great Sound

Thanks thanong.
Here is first the simplified idea. See my attached diagram!

Q1 is the voltage amplifier.
The voltage gain is ruffly R1/R5 = 1800/270 ~6.6

After the voltage gain is one buffer follower = U1
The follower is biased by a CCS constant current source = I1

The final circuit uses heavy local feedback and so the distortion is rather low.
The circuit is very easy to setup and try and gives good Class A sound.

I show the complete circuit in my next post.
I am very happy for anyone who try this simple experiment.
It may sound better than you think - with only Class A 4 Watt ...
:)
 

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Here is the full circuit.
It delivers 4 Watt RMS into 8 Ohm with max 0.1 % distortion.
The output big transistors can be 2SC5200, MJL3281 or MJ21194.

The output DC voltage should be adjusted to V/2+0.8 Volt. Where V is the supply voltage.
Suitable transformer is 2x15 VAC. You use one winding for each channel.
One winding will give like 20-21 Volt DC supply voltage.

Enjoy :)
 

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Thanks lineup,I will try.I very love small class A amp.I'm not listening to lound I guess maybe 1-2 watt (my room is small and I don't want to disturbing other poeple in my renting house.)I have a small tube single end class A receiver(Silvertone) that I took from console.Its sound nice but a little bit hum and I don't know how to fix.My system is pioneer cd player and alesis monitor one speaker.I listening to mostly jazz,blues,60's rock and country.Hope your circuit is not to hard to try ( I have background about electronics but not fluent in tube honestly)
 
Thanks lineup,I will try.I very love small class A amp.I'm not listening to lound I guess maybe 1-2 watt (my room is small and I don't want to disturbing other poeple in my renting house.)I have a small tube single end class A receiver(Silvertone) that I took from console.Its sound nice but a little bit hum and I don't know how to fix.My system is pioneer cd player and alesis monitor one speaker.I listening to mostly jazz,blues,60's rock and country.Hope your circuit is not to hard to try ( I have background about electronics but not fluent in tube honestly)
Class A with a few Watts would be perfect for you.
My PC speakers are only a few Watt and I can listen perfectly to Winamp mp3 here in my little room.
 
Lineup,

A very nice circuit. Elegant driver. Do you think you could almost double efficiency with an inductor in place of the CCS?

You have added something very worthwhile and good sounding to DIYaudio! Congratulations!

Hugh
I am always very glad when AKSA think it is worthwhile to comment.
You are probably very right about to make some coil load.
But for simplicity we can use the circuiit as it stands.
There are always ways to make slightly more complicated circuits,
but there is not always a possiblity to keep it simple.

Thanks again AKSA.
 

matze

Member
2012-09-12 5:45 pm
At 1 Watt RMS out, the distortion is only THD 0.006%.
That is very good for such a simple circuit.
We can almost call it HIFI :)
Hi lineup,

thanks for posting the nice idea. My impression from an own small class A amplifier, many years ago and much simpler, was: Whow, what does it sound smooth!

At which frequency do the 0.006% hold? For room-listening levels, a good performance at 1W is usually enough. But it really has to be good.

Maybe, you already know my hobby: are there any stability issues with the triple darlington? I'm just wondering about the 47p cap.

Cheers,
Matze
 
Hi lineup,

thanks for posting the nice idea. My impression from an own small class A amplifier, many years ago and much simpler, was: Whow, what does it sound smooth!

At which frequency do the 0.006% hold? For room-listening levels, a good performance at 1W is usually enough. But it really has to be good.

Maybe, you already know my hobby: are there any stability issues with the triple darlington? I'm just wondering about the 47p cap.

Cheers,
Matze

1. the THD 0.006% is simulated SPICE at 1 kHz.
The upper MHz is of course very high for this circuit.

2. the 47 pF cap is what my SPICE shows to be stable.
CFB, in this case triple, shows such a cap is a good idea.

If there is any interest I would be able to present 20 kHz figures, too.

I have not built this amplifier, but I have no doubt it will work for real.
 
I like to see that people are not afraid of capacitor coupled outputs, they got bad press often because of the past (decades ago capacitors were much worse than today) and bad implementation (using too small capacitor).

I guess that there is feedback though, just no global - looking at the simple diagram there are two transistors. The output is in emitter follower topology so feedback is implied but local. The input device has the base biassed from the collector not from the V+ rail so it has feedback from collector to base. But both are local and so phase shifts are minimized. By having the first device do the voltage gain and the second device do the current gain it keeps it very clean.

I hope you get to hear this some day.