400 watt DIY amplifier

[ll81]

Hi there, got the itch to build a rack mount guitar head for use with a seperate pre-amp and this is the end result. Came across this forum and thought it worth while posting the end result and some schematics. About to start on the pre-amp but have spent so much time on the amp wanted to post some pics just cause I was pleased with the result so far.

Inside the case is a 300 Watt into 4 ohm (400 Watt at ~10% THD) triple EF output stage BJT (Bipolar junction transistor) amplifier with short circuit / overload protection. Fast latching DC protection that activates at less than +- 2v offset, active balanced line reciever signal input, mains soft start and fan controller.

The amp has two 325VA transformers each with a CT 70v secondary and a single 15v secondary. The main secondarys each goes to a BR then BR to caps allowing parallel operation without directly connecting the less than perfectly matched secondary voltages. One transformer 15v secondary goes to a full wave bridge rectifier then via an LM7815 15v reg then feeds the fan controller and soft start. The other 15v secondary on the other transformer goes to a voltage doubler dual 1/2 wave rectifier then via LM7815 LM7915 creates a dual 15v rail for the balanced line reciever and DC speaker protection.

Through carefull conection of the multiple 0v points the amp even with no attenuation network (volume controll) does not hum or buzz and can only be heared to hisssss pleasantly when an ear is pushed gently onto the speaker. Input is via XLR and output is via binding post or 4 pole Speakon. Case was a really badly built Skytec dodgy PA amp, don't have the fan controller schematic as I designed on the back of an envolope around components I had in the scavanged parts bin so as I can't remember what the values of those where can't re-draw it on the computer.

If anyone wishes to build the amp I am happy to give some pointers, am really looking forward to getting the hybrid transistor-valve pre amp built. Looking at profusions digital delay board (guitar optimised version) amongst other things to stick in there.






Main amplifier schematic


Essential DC protection


Active balanced line reciever


Soft start allows 1 amp of mains current for about 1 second then allows full current draw, prevents nuisance fuse blowing/circuit breaker tripping and protects caps and BR's from massive current inrush.

That's all for now folks, Look forward to your opinions, Cheers Liam.

[J M Fahey]

Excelent build.
Very impressive.  :tu:
Mouth watering.
I guess your bandmates will HATE you, let alone your neighbours. :lmao:
Keep posting and thanks. 8|

[ll81]

Thanks. I feel kind of weird saying this on a guitar forum but I never learned guitar  , drums was my thing. I do however know alot of peole who can play the skin off their fingertips who have been nagging at me to stop building PA gear and move over to musical instrument amplifiers. I love the sound of guitars and if any of my designs can help anyone else pluck, strum, thrash or wail away into the early hours then I am allways happy to help.

I must admit it is annoying having to wait until someone else plugs in to see what my creations sound like.

[E]

I'd strongly recommend learning it. Not necessarily to an advanced level, just the basics would suffice. You could get one of your friends to help you with fingering and stuff, and use this great resource for doing stuff in your own time:

http://www.justinguitar.com/