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rhysh 23rd November 2008 06:22 PM

Intrest Check: 120w @ 8ohm Tube Buffered LM3875 Amplifier
I have came up with this pcb design for a tube buffered LM3875 amplifer, it puts out just over 120w into a pair of 8 ohm speakers (stereo).

Click here for full size image (high res)

The PCB is not currently finished, but i am wondering if anyone is intrested? The pcb itself should not run more then $15 each, i am hoping.

Basicly, it is a standard gainclone fed by a tual triode tube, such as the ECC88. 3 Seperate power supplys are used.

1. Main Gainclone Powersupply (400va transformer)
2. Tube power supply (40va transformer)
3 Tube heater and digital power supply

What you can not yet see on the board, is a 555 timer delay circuit, that will connect to a external relay to give the tube time to heat up before the 2 main power supplys are switched on.

Some additional tydying and more sensible routing needs to be done on the board, but i would be delighted if anyone is intrested in some sort of group buy.

Please note - as the pcb is not yet finished there will be some random pads / incorrect routing that i have not yet took care of.

Also - Im aware of the 6v running thru the AC.

This board is about 2/3 of a A4 page wide.


danielwritesbac 23rd November 2008 06:47 PM

Score! (interest, that is). ;)

AndrewT 23rd November 2008 07:02 PM

do you mean >120W into 8r0 or 56W into 8r0?

rhysh 23rd November 2008 07:08 PM

56w per channel. It is a stereo board.

udailey 23rd November 2008 07:08 PM

Looks like he probably means ~120 into a pair which would mean ~60 each.

ps, Yes that is interesting!

rhysh 23rd November 2008 07:21 PM


Originally posted by udailey
Looks like he probably means ~120 into a pair which would mean ~60 each.

ps, Yes that is interesting!

120 into a pair is what i mean ;)

rhysh 23rd November 2008 07:39 PM

Just realised i have uploaded a earlier version in my original post. Find newer version below.

Click here for full size image

Puffin 23rd November 2008 08:05 PM

Yep. I'm in for two.

Tripmaster 23rd November 2008 08:28 PM


Originally posted by Puffin
Yep. I'm in for two.
Two please

Destroyer OS 24th November 2008 12:11 AM

Ok so you mean that you are going to have 25v or less on the secondary of the transformer right? I was going to say you are going to burn up the chips....

Can you do a schematic? From what I can tell you might have (a few) extra things that you may not need. If you are driving it with tubes simplicity is going to give the richest sound, I am pretty sure.

Also the spaces on what I think, but am not sure, are capacitors that carry signal might be a little close together for a modification I have recently tried and found amazing results with. All it requires is that you have capacitors for the output (or any carrying information, if you have more, or REALLY want to make some sweet sounding stuff with the effort involved) have isolated cans (the metal around the electrolytic element inside).

If you are not star grounding everything via p2p, which from what I can tell you are not, a modification to the board would be to have a ground PCB hole next to each capacitor to run a solid piece of copper around the capacitor (jacket removed) and then soldered into ground.

If you do not believe me that this can make a difference like between Teflon V caps and some harsh no name electrolytics, try it in something and be amazed.

There is no real reason to do this to non-electrolytics because there is no induction that is measurable to the plastic or paper etc.

The voltage does not seem to matter as long as you have solid wire pressing or some thing that holds the grounding wire on. The amperage is next to nothing. Also for the sake of electromagnetic field of induction it is important to do a loop around the capacitor. The same wire can be used for both output caps for the one side.

Oh and unless you are going to have two transformers for the supply to the LM3875s I doubt having two sets of capacitors and diodes is going to do any good. You could save money and cut to one, or put in some bypass capability so people would have the option to use two transformers and make it dual mono.

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