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Old 15th April 2015, 03:18 AM   #1
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Default Mega Simple Class A Amplifier

Hi:

This simple design has all the ingredients to satisfy even the most demanding DIYers. Output power = 4 W at 8 ohms. THD is only 0,017 % at full power and 1 KHz. Voltage gain = 13,85 dB.

Only two stages. Three transistors. Very few components. Its performance is surprisingly good, despite its total simplicity.

High bandwidth. Low distortion. Input and output in single ended configuration. PSRR: 56 dB.

I want to hear comments from those who intend to build this simple amplifier.

regards

R4 is a 50K trimpot. C3 = 3 x 4700 uF. Iq = 1 A. R2 = 2 W. R3 = 1 W. R6 = 4 W.

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File Type: jpg 4W Pure Class A Amplifier.jpg (270.6 KB, 535 views)
File Type: jpg THD vs Frecuency at Full Power.jpg (164.4 KB, 524 views)
File Type: jpg THD vs Output Power at 1 KHz and 8 Ohms.jpg (151.1 KB, 521 views)

Last edited by diegomj1973; 15th April 2015 at 03:52 AM.
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Old 15th April 2015, 07:13 AM   #2
Calvin is offline Calvin  Germany
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Hi,

I don't intend to build it, but allow me a comment
You may want to simulate/test not only with a resistive load, but with a complex impedance load too.
Your circuit is similar to a SRPP/modulated current source which may react with oscillations on complex loads.
See if it remains stable.

jauu
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Old 15th April 2015, 07:41 AM   #3
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Default Stability

Im for the same idea of calvin .
But for 5-10W do you use a modulated current source try
the costant current generator and calculate it for 5 ohm
output load
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Old 15th April 2015, 08:09 AM   #4
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Firstly use preferred values, what is 37.83k all about?
Is it stable or haven't you tried it yet? When Q2 is turned on, will there be enough current, in the gate, to avoid distortion?
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Old 15th April 2015, 10:24 AM   #5
knutn is offline knutn  Norway
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I would use a trimmer for R1 and insert a capacitor in series with R3.
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Old 15th April 2015, 12:43 PM   #6
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I have recently designed. I have not implemented yet.

I read about their possible behavior with varying loads. I was unable to test this, yet.

The basic idea is to keep it as simple and small as possible, without impairing the quality characteristics. Including a constant current source instead of the modulated current source, seem to improve the parameters, but I've tried and it has not happened as expected (at least in the simulations).

The value of 37.83 K ohms is the setting that we must carry on a trimpot. The value of the trimpot is 50K ohms.

One could use a trimpot to replace R1 and adjust to the most suitable value.

A capacitor in series with R3: Why add more capacitors, deteriorating feedback, if it is impossible to reduce the asymmetry of the output with the existing modulated current source? C3 serves to preserve the speaker of this asymmetry.

Thank you all for the opinions and suggestions.

Regards

PD: It is very simple and economical... If there are any doubts, it can be built very quickly and you can test their operation.

Last edited by diegomj1973; 15th April 2015 at 01:08 PM.
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Old 15th April 2015, 01:31 PM   #7
Ketje is online now Ketje  Belgium
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Still rather simple
Mona
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Old 15th April 2015, 05:55 PM   #8
knutn is offline knutn  Norway
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A capacitor in series with R3 is to ensure DC stability (increasing the feedback at DC).
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Old 15th April 2015, 06:24 PM   #9
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Quote:
Originally Posted by Ketje View Post
Still rather simple
Mona
In a simple and smart way, you've eliminated that large output capacitor.

This allows adjustment of offset.

Still only 13 components, but now even cheaper and with great advantages in the frequency response.

It is a very promising circuit.
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Old 15th April 2015, 09:35 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by diegomj1973 View Post
...Still only 13 components,...
You'll need gate stoppers too.
Raise of distortion in high frequency region is due to very low current through Q1 - 0.5 mA is too low, 3-5mA will be much better
Also, IRFP240 or similar MOSFETs will work there much better than IRF610.
Sim is a a nice toy but reality is not so forgiving.

Nice thing about that design is that by varying R1 value you can change the amp's character - from single-ended (dominant second harmonic) to push-pull like (dominant third). That's why it's called SEPP...
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