Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 12:52:03 PM

Login with username, password and session length

Recent Posts

 

OCL power amp stages

Started by brianhujung, June 05, 2012, 12:12:26 AM

Previous topic - Next topic

brianhujung

So I found this OCL schematic yesterday and I'm thinking to use it as power amp for my guitar amp because the parts are easy to obtain here.
Now before I make the boards, here's a couple questions :
1. Does this schematic suitable for this purpose? or is there anything I can do to improve it for guitar amp?
2. If i run this amp with +/- 25V 3A transformer, how much power will I get?

Thank You.

phatt


brianhujung

Thanks Phil,
i think it'll enough for practicing.
I'll simplify the schematic and make the boards. hopefully it'll works fine for guitar.
oh, and I'll put my BD-2 Clone in front of it as the pre-amp.  :cheesy:

phatt

I'd be making sure the Schematic is a working unit before embarking on a project like that.

Try this if unsure http://sound.westhost.com/project27.htm
Plenty of help if you buy the PCB's from Rod.
Phil.

J M Fahey

+1 for getting (at least) the PCB from Rodd Elliott.
Inexpensive and trouble free to be sent by mail *anywhere*, plus it has short circuit protection.
The first one you posted has not.
No bic problem in a hardwired combo, but dangerous in a head/cabinet system, because regular guitar pligs are the worst connector ever designed.

Roly

#5
{Since this circuit has been posted...}

Actually it's fairly similar to my Twin-50 which uses a 25-0-25VAC tranny to give rails of +/-35V and just on 50 Watts into 8 ohms.

I had to laugh when I saw the "150W" title because anybody who thinks they are going to get 150 watts out of a pair of 2N3055's is fooling themselves.   :duh

The supply is shown as +/-42V which is fine as long as you don't actually drive the amp because at full swing each output transistor has to withstand 2 * 42 = 84 volts, while the Vcer rating is only 70V max - booffo!.

The speaker impedance isn't specified but looks like 4 ohms was intended, and again 2N3055's aren't happy with loads below 8 ohms because the current rises to a point where the Hfe drops quite seriously.

This is sadly typical of many circuits that are published on the web that somebody has drawn up but never actually built, leaving it to trusting readers to suffer the bruising experience.   xP

All the other transistors could be substituted by BD139/BD140's.

D4 and D5 should be thermally coupled to the output transistor heatsink (which even for 50W into 8 ohms needs to be quite generous).

Ra can be a 47 ohm 2W resistor and L can be as many turns of 2mm enamel covered wire as will fit on the body of Ra (about ten), or both can be replaced by a 0.22 to 0.5 ohm 5W wirewound resistor.

Rb can be 10 ohms 1W and C a 0.47uF 100V greencap.  Note that this should actually be connected to the join of R15 and R16, not as shown.
If you say theory and practice don't agree you haven't applied enough theory.