300W Power Amp

[bostjancek]

Hello !

I Found this schematic in an older Croatian electronic magazine. What do you think about it ? It is suitable for PA or home use ?

[Nigel Goodwin]

From it's complexity I would suggest it's probably intended for home use?, but there's no reason you can't use it for PA - but make sure you use suitably sized heatsinks and power supply components - both of which could really do with being substantial for prolonged high power PA use.

[bostjancek]

I was thinking the same, but the article is saying that this amp is intended for PA use. I'm very curious about the sonic of this amp. Maybe I will build it for test purpose.

Cheers

[jacco vermeulen]

I've seen PA amplifiers circuits more complex than this one.
The output stage has a 1500 watt power rating, with 60 volt rails that is more than 7 times the continuous power in 8 Ohms.
That and the topology of the output stage with a driver for each output device makes it a circuit designed for PA purposes.
Input filters say so too.

[CBS240]

Overall, looks like a solid design to me, you should go for it. It would be interesting to see what the difference would be if Ic in first diff. was increased to 1mA, and 2mA per leg, and compare the results. Might need a resistor on the collector of current source to absorb some heat and change the two 1K resistors accordingly though.

As for the circuit, any improvements are a plus. You might add a resistor between the emitter of drivers and the base of outputs. Or you could change the three smaller drivers for 1 larger one and use base resistors on outputs and use less parts. I don't see where that would change the operation but then you might need a small heatsink for it. Degeneration resistors on the emitters of the first differential would be OK. With Ic 0.5mA in each leg, maybe like 200 or 300 Ohms?

Oh yeah, a Zobel net wouldn't hurt either.

Anyway, good luck!

[AndrewT]

Hi,
the 39K on the input seems a bit large, try 10k to 20k for an RF filter of 80kHz to 160kHz (-3db).
The gain @ 71times (+37db) also seems quite high for domestic use and very high for PA use.
The output stage should be good for 4ohm use IF you keep the heatsinks cool.
The input time constant @ 73mS is a lot longer than the NFB RC @ 47mS. These could usefully be reversed.
Yes, I agree, add emitter resistors to the LTP and compensate by increasing LTP current. Readjust LTP load to balance the LTP halves.
How about adding a cap to ground for the bases of the input cascodes.
The 100pF compensation cap may need some adjusting to optimise stability, possibly a lot smaller.

The circuit looks promising.

[bostjancek]

Thanks for your responses. So you think that this amp with some mods could be used for home amp ? I think that I will build one for test but with +/-50V power supply and with suggested modifications.

Cheers

[bigpanda]

Any alternatives for the BF470, BF469, BD239

[amplifierguru]

Hi bostjancek,

You cannot get 300W into 8 ohms from this - the supplies are too low for 69V output. You cannot get 300W into 4 ohm out of this into a reactive 4 ohm load at the protection circuit will trigger at half current (1/4 power) or on only 30deg phase, so would be unsuitable for a sub.

The protection circuit restricts the outputs to 120W each. Also the undegen first and second differentials mean higher THD and the topology looks weak on PSRR.

Whereas my GB300D amp will do 300W into 4 or 8 ohm from +/-55V or 75V respectively.

http://members.dodo.com.au/~gregball/guru_007.htm

Cheers,
greg

[AndrewT]

Hi,
three pairs of MJ15003/4 are OK for 4 ohm duty.
Does it matter that 300W into 8ohm is not available?
Recalculate the protection component values and ignore the advertising coming from amp guru.
At least Amp guru also agrees that adding Re is better.