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Old 19th February 2013, 12:21 PM   #1
hiktaka is offline hiktaka  Indonesia
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Default How to scale up this SE class A ?

Hello,

I'm interested to build this amplifier. It is Geoff's from here:
Simple amp circuit obsevations

Click the image to open in full size.

because of it's characteristics as:
- Pure class A SE
- no CCS, no quasi, just passive load
- can be driven with average preamps

But I want to scale it up to at least about 5W to 8 ohm.
It is good to up 150mW but start to distort at 500mW.

Click the image to open in full size.
Click the image to open in full size.

Question:
Is it possible, only with changing component values?
Or circuit must be modified?
Or that 2N3055 just won't go that high?

Thanks
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Old 19th February 2013, 01:14 PM   #2
DUG is online now DUG  Canada
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The 2n3055 will do just fine.

BUT:

To get 5W you would need 6.32Vrms into the 8R load

This is 8.94Vpeak and 1.11Ap

R7 would have to be a maximum value of (12V-8.94V) 3.05V / 1.11A = 2.73R

At no signal conditions this would mean an Iq of 4.4A (min)

The Hfe (current gain) of the 2n3055 is about 33 at this current.

This makes the collector current of the 2n2222 about .133A and at 11.4V would dissipate about 1.5W

That will not last very long as they are rated for 0.625W.

Now, just to be silly and say it could be done:

If you keep the case temperature of the 2n2222 at -57 deg C then it will dissipate 1.5W without destruction.

To get 5W you need to change R7 and Q2. (And possibly R4)

IMHO

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Old 19th February 2013, 01:50 PM   #3
DUG is online now DUG  Canada
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OOOPS

forgot about heat sinking:

you would need a really big heatsink for the 2n3055

Somewhere between 0.8 and 0.2 deg C / W

2n3055 datasheet is a little wide on J-C thermal impedance.

And of course a 4.5A (min) supply
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Old 19th February 2013, 03:35 PM   #4
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hiktaka:

It is impossible to reach an output of 5W in pure class A, with that configuration.

I give an example: with the circuit as it is, you get about 148 mW into 8 ohms with significant distortion. The only way to increase the power output without changing the original scheme is to reduce the resistance R7 from 50 ohms to 8 ohms (similar to the impedance of the speaker), to increase efficiency. But we have to check the power dissipation in the 2N3055 transistor. We also need to verify whether the distortion continues aceptabe yet. We also need to see if the rest of the circuit properly handles the 2N3055 transistor.

The emitter degeneration contributes to lower distortion, but also reduces the efficiency of the amplifier.

With R7 = 8 ohms, the power reaches about 1.77 W approximately. Never reaches 5W.

For maximum efficiency VCQ3 should be adjusted to about 6.66 V.

The quiescent current would be about 2.16 A!. The R7 power would be around 37.55 W!. The R9 power would be around 2.34 W!. 2N3055 dissipation would be about 12.09 W!. The power dissipated by the power supply would be 52 W!. The efficiency would be 3.4%.

While it is a more sophisticated and accurate, you can assemble the amplifier "PerfectA", which reaches to 5.1 W per channel into 12 ohms.

regards

PD: DUG: Do not forget the resistor R9!

Last edited by diegomj1973; 19th February 2013 at 03:50 PM.
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Old 20th February 2013, 04:02 AM   #5
DUG is online now DUG  Canada
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"PD: DUG: Do not forget the resistor R9! "

I did...morning rush, had to go to work.

Yes, R9 needs to go to 0.1R or so.

I still think it could go to 5W but only with a very big heatsink and new Q2.

If I was a complete sucker for a challenge (I usually am), I would design it.

But I don't have time right now.



However, I do agree that if you wanted a good 5W class A it would be better served with a new design.

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Old 20th February 2013, 04:03 AM   #6
DUG is online now DUG  Canada
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"PD: DUG: Do not forget the resistor R9! "

I did...morning rush, had to go to work.

Yes, R9 needs to go to 0.1R or so.

I still think it could go to 5W but only with a very big heatsink and new Q2.

If I was a complete sucker for a challenge (I usually am), I would design it.

But I don't have time right now.



However, I do agree that if you wanted a good 5W class A it would be better served with a new design.

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Old 20th February 2013, 04:08 AM   #7
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moderator please remove one of these posts...i don't know what happened.
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Old 20th February 2013, 10:04 AM   #8
hiktaka is offline hiktaka  Indonesia
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Thanks

How about replacement:
Q3 >> 2SC2922 (higher power?)
Q2 >> BD139
R7 >> 5 ohm
R9 >> 0.1 ohm
and how to set VCQ3? which resistor to adjust?

But I'd like to keep :
- BJT only, no FET
- Passive load, no CCS
- very inefficient is fine

I will use an SMPS with 24V/100Amp rating + enough filtering, so no issue on PSU.

Again, thanks ..
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Old 20th February 2013, 10:21 AM   #9
lineup is offline lineup  Sweden
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Quote:
Originally Posted by hiktaka View Post
Thanks
---
and how to set VCQ3? which resistor to adjust?
---
I will use an SMPS with 24V/100Amp rating + enough filtering, so no issue on
All those changes are good.
You should add one more diode in the input.
Makes 3 diodes for the first transistor.
This will make the VCQ3 around 10 Vdc.


You can expect output more than 1 Watt RMS with a distortion less than 1.0%
My SPICE simulation did actually gave 1.7 Watt at THD 1.0%
I used 4.7 Ohm and 0.1 Ohm + BD139
and 3 diodes for input.
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Old 20th February 2013, 10:25 AM   #10
lineup is offline lineup  Sweden
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Here you can see my circuit.
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File Type: jpg ScaleUp_1a.jpg (59.1 KB, 158 views)
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