A simple quasi-complementary 50 watts amp

[gaetan8888]

Hello

Here's a simple quasi-complementary 50 watts amp that I've done. It's nice sounding and use easy to find parts.

You adjust the offset using R4, for minimum dc output. Adjust the R6 trimpot for 70ma of bias.

All capacitor should be rated at 50 volt. All resistors (except R21 and R22) are 1/4 watt. The resistors R21 and R22 are 5 watts.

It's a fairly simple amp to buid, you can do it using aerial copper wires to build it.

Use 4 amp fuses for each rail supply voltage inputs, it's good to put a 100 ohm 3 watt resistor across each of the fuse holder connections, this resistor will keep a voltage on the rail if only one side fuse blow.

Just be carefull NOT to put the input and vas transistors close to the output transistors or to the rails, and you need to use a star ground.

Don't use the BD140 to replace Q3 (2N5551) in the vas, the BD140 are slow and have a quite high COB for a vas, so it would reduce the slew-rate and the quality of the sound.

It sound better if you matched the two LTP input transistors for HFE and VBE.

After adjusting the offset, you can do a fine adjustement of the LTP transistors balance by measuring the voltage drop across R1 and R2 to get the same voltage by adjusting the R4 trimpot. It will give a better soundstage.

This amp have .01% of distortions at 40 watts output in 8 ohm. Thd numbers do not show very much about the sound quality of an amp, only listening will do it.

Bye

Gaetan

[AndrewT]

It does not look particularly Quasi Complementary to me.
The drivers (q4 & q7) seem to have very different loads presented to them and thus the "complementary" benefit is lost.

R3 may need to be changed to 750r or even 680r, if VR4 runs out of adjustment range.

[stratus46]

Originally Posted by AndrewT

It does not look particularly Quasi Complementary to me.
The drivers (q4 & q7) seem to have very different loads presented to them and thus the "complementary" benefit is lost.

R3 may need to be changed to 750r or even 680r, if VR4 runs out of adjustment range.

Look again. The main difference from '70s quasi amps is the polarity of the VAS and input differential pair.

G²

[AndrewT]

Originally Posted by stratus46

Look again.

look again at the loadings on the drivers. The loads on the emitters are completely different.
Look also at the CFP resistor, why is it double the value of the EF resistor? Could it be because 888 thinks that halving the emitter current will bring the drivers into balance?

[kenpeter]

R23 should be 50 ohms.
R19 needs a series pair of (Diode + 26R + (0.47R * beta(MJ15003))) in parallel with it.
If we assume beta of 50 (based on OnSemi curves 1Amp @ 25C), figure about 49.5R?

So picture this existing R19 of 50 ohms in parallel with new (DiodeDrop + 50ohms)

Then R23 (corrected to 50ohms) in parallel with Q6 and its emitter resistor
should drop nearly this exact same voltage at the same current

[nigelwright7557]

C15 looks a bit small, you could get a little oscillation on the output with that value.
I had a similar problem with my own quasi design.

[gaetan8888]

Originally Posted by kenpeter

R23 should be 50 ohms.
R19 needs a series pair of (Diode + 26R + (0.47R * beta(MJ15003))) in parallel with it.
If we assume beta of 50 (based on OnSemi curves 1Amp @ 25C), figure about 49.5R?

So picture this existing R19 of 50 ohms in parallel with new (DiodeDrop + 50ohms)

Then R23 (corrected to 50ohms) in parallel with Q6 and its emitter resistor
should drop nearly this exact same voltage at the same current

Hello

I've redone the schematic, I've added a diode and resistor, I've lowered R23 to 50R, and I've raised C15 value, does I've done it right ?

Thank

Bye

Gaetan

[nigelwright7557]

Originally Posted by gaetan8888

Hello

I've redone the schematic, I've added a diode and resistor, I've lowered R23 to 50R, and I've raised C15 value, does I've done it right ?

Thank

Bye

Gaetan

You havent increased c15 however I would leave it until you get the amp running then look for oscillation on the output, if there is then increase c15 a little until it goes.

[kenpeter]

Diode and resistor look OK. I don't pretend to know anything about C15.

[nigelwright7557]

Originally Posted by kenpeter

Diode and resistor look OK. I don't pretend to know anything about C15.

Becasue the lower driver has voltage gain and the top one doesnt things are out of balance. Getting C15 right balances things up again and stops any oscillation on the lower output transistor.