Nigels new power amp design.

[nigelwright7557]

Looking for constructive comments on my power amp design.

Most of the transistors are MPSA42 with a TIP127 darlington driver stage. I have IRFP240 and IRFP9240 on the output stage.

See attached file for schematic.

Regards Nigel

[nitrate]

Hi there,

Glad to see your having a go at your own design. At a quick glance it seems your differential front end needs more work, mainly to balance things up a bit. Also if R1 and C2 is supposed to be an input filter then it needs a quick re-wire. Have Fun!! Hope you get it going

Just my two pence worth

Regards
Leigh

[CBS240]

Hi Nigel

Why would you offset the input nodes (+&-) by 2 diode drops if you are using global feedback for DC bias? If your output stage is an equal follower between the rails, you shouldn't have much offset with a simple differential, unless the input stage isn't balanced correctly because of R3 value. VR3 is 1.2V as per Vbe of Q5. Q1 and Q2 should have equal 2.5mA through each as set by Q3 5mA CCS. 1.2V/2.5mA = 480R....or 470R for a common value. For offset control, you could use a small trim pot as emitter degeneration on Q's 1 & 2, wiper to collector of Q3. This would let you slightly mis-balance the differential so as to achieve closer 0VDC output. Personally, I don't like that method, but that doesn't mean it doesn't work. Perhaps a trim pot in series with R3, say 20% of R3? However, there are better, less simple ways to control offset. Also, I would think you'd get better linearity from using a discrete buffered VAS instead of a Darlington, JMO.

My two cents worth .....(even less than two pence)

[nigelwright7557]

Originally posted by CBS240
Hi Nigel

Why would you offset the input nodes (+&-) by 2 diode drops if you are using global feedback for DC bias? If your output stage is an equal follower between the rails, you shouldn't have much offset with a simple differential, unless the input stage isn't balanced correctly because of R3 value. VR3 is 1.2V as per Vbe of Q5. Q1 and Q2 should have equal 2.5mA through each as set by Q3 5mA CCS. 1.2V/2.5mA = 480R....or 470R for a common value. For offset control, you could use a small trim pot as emitter degeneration on Q's 1 & 2, wiper to collector of Q3. This would let you slightly mis-balance the differential so as to achieve closer 0VDC output. Personally, I don't like that method, but that doesn't mean it doesn't work. Perhaps a trim pot in series with R3, say 20% of R3? However, there are better, less simple ways to control offset. Also, I would think you'd get better linearity from using a discrete buffered VAS instead of a Darlington, JMO.

My two cents worth .....(even less than two pence)

The 2 diodes bring the output DC offset back to close to zero.
I relied on feedback to get rid of crossover distortion but there is a very slight amount still. It does need a trim pot in place of the diodes. Will probably be the next job.

The amp sounds fine despite not being perfect on the scope.
It was designed as a disco amp and dumps a good few amps into the speakers. The output MOSFETS are on a huge heatsink which also has a 120mm fan to keep it cool. Probably way over the top but better safe than sorry. A disco amp needs to be reliable.

The only problem in practice I found was that perhaps the 48 volts is a bit on the low side to cope with transients.

The darlington driver was used to get plenty of gain.

[nigelwright7557]

Many thanks for your replies.