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AN214Q 1st April 2012 04:05 AM

Classic 150 watts for Modification
1 Attachment(s)
Dear DIY fanatics,

New here, I've got this schematics from school days in the early 90's. Its been long time using this amp at home and planning to upgrade same schematic and power for outdoor use up to 2 ohms.
Original supply was +- 50v . Want to make it +- 75 volts.

DIY experts are invited to join, comment and revise this circuit.


audiomachines 1st April 2012 05:26 AM

the ouput stage could definitely benefit from an upgrade MJ15016/15, MJ15003/4. Wow a 2N3055/ MJ2955 on an 88VDC supply?, those must be originals OP's lucky you.

latala 1st April 2012 08:14 AM

this is an old maplin circuit sounds ok but the output transistors are over run as far as the voltage is concerned you can use mj15024/5 with good results

jaycee 1st April 2012 09:18 AM

You cannot simply just boost the supply with a few tweaks. Look for an amp circuit which is designed for this.

sreten 1st April 2012 10:05 AM


There is no point upping the rail voltages if 2 ohms drive is intended.

The circuit has no SOA protection, so it will work into 2 ohms,
for how long probably depends on how heavily its used.

rgds, sreten.

AndrewT 1st April 2012 11:16 AM

A 2stage EF is unlikely to perform well into 2ohm speaker loading.

I would not even recommend it for 4ohm speaker loading.

Start afresh with a design suitable for +-80Vdc into 2ohm loading. You are targeting ~120Apk. That is ambitious.

AN214Q 1st April 2012 01:20 PM

can we put SOA protection to this circuit? How would it look like, do you think it works?


AN214Q 4th April 2012 12:17 PM

1 Attachment(s)

I have revised here old circuit, DIY experts are welcome to advice.


jaycee 4th April 2012 01:18 PM

It won't work. The BD139/140 will not withstand the voltage. You will need more than 2 pairs of output transistors to run 500W RMS into 4 ohms - at least 6 pairs of TO-3 devices to be safe.

AndrewT 4th April 2012 01:38 PM

Yes, with the BDs there, the rails should be <=+-40Vdc and this would give a maximum power output of ~70W into 8r0.

That would be just right for a single output pair.

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