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captainobvious 21st March 2010 02:54 AM

Building LM4780 amps
My (2) kits have been here wiht parts soldered for a while while I was waiting on the delivery of the rest of the items needed (Torroids, heatsinks, enclosures, etc)

I have a few questions about connecting up the transformers to the rectifier PS boards, and connecting the rectifiers to the amp boards.

I will use one 400VA 25+25 transformer to connect to one rectifier board. It will be connected to 2 lm4780 boards each run in parallel for a total of 2 channels audio (left and right midbass)

I will use one 400VA 18+18 transformer to connect to one rectifier board. It will be connected to 2 lm4780 boards each run in stereo for a total of 4 channels audio (left and right tweeters and midranges)

First, I'm unsure how to connect the rectifier output to two seperate boards. The diagrams Ive seen show connection of the V+/V-, and PG+/PG- to the same connections on a single amp board. What then when you use two amp boards? If there is a photo or diagram showing the connection from one rectifier board to two amp boards, that would be a great help.

Second, my transformers have 4 AC input wires. It is labeled as Red/Black 115v and Red/Black 115v. Should the 2 red wires be tied together and the 2 black wires be tied together for these?

Thank you

sangram 21st March 2010 05:41 AM

when connecting two amp boards to a single rectifier board, the rectifier has two runs of wire, one for each of the boards. So essentially the V+, V- and PG lines from each board run to the corresponding points on the rectifier board. If this is difficult to solder and you have a double-sided board, you may use the pads on either side on the originating rectifier boards. If it is a single-sided board and the pads are too small for two wire runs, you will have to fashion some kind of harness to connect it all together, or use PCB pins.

For USA mains the transformer primaries are tied in parallel. The red wires go together, the black wires go together.

captainobvious 21st March 2010 02:43 PM

Thank you sangram. This is what I ASSumed, but wanted to be sure.


Hmmm, this is an example of what I was asking...

In this post by Peter, It doesn't appear as though the PG+/- on the "amp board" side are connected. Just the V+/V- on the amp board to the corresponding locations on the rectifier board. Are the PG+/PG- on the amp board side not used? It appears as though the PG+/PG- from the rectifer board are connected into the power star ground on his photo...

Peter Daniel 21st March 2010 04:23 PM

Well, it's been explained in the post:


Originally Posted by Peter Daniel (
The important aspect about sharing a single transformer and common rectifiers per two amp channels is power star ground.

While you could simply run PG+ and PG- (ground) wires from rectifier board to each amp board, in some setups hum problems were reported and presently I always use star (power) ground in all my amps.

I simply achieve it by connecting output grounds (OG, on the opposite side of speaker wire connection) with a 14ga solid core copper wire.

The center of that wire is my power star ground and both PG+ and PG- grounds from rectifier board connect here. In the picture below the connections are so short that I used single runs of wire, normally I use two wires for both PG+ and PG-.

Please note that power ground is also directly connected to the chassis.

I specifically refer to it as power star ground, as signal wires from RCAs are not connect directly here. They connect through separate traces on PCB to OG (output ground) pads.

Both methods are fine, the second one is more reliable with regards to hum.

sangram 21st March 2010 05:21 PM

Sorry, should have mentioned - star grounding is required anyway you cut it for a stereo amp where the signal grounds will return to a common point (irrespective of how the power grounds are wired - so obviously at one point you will need to reference all the grounds to a single point.

I use two amp channels and two rectifier boards to run fully balanced, and after my conversations with Peter a few years ago I ended up connecting lots of grounds together to achieve freedom from hum - the power grounds are all tied together in my case, with 16AWG wire.

captainobvious 22nd March 2010 02:25 AM

Thank you gentleman.

littlerick 22nd March 2010 08:59 AM

I never connected any signal ground to chassis.. I have mains grounded to chassis and run a cable from chassis ground on on amp boards to chassis.

I gotta say I get no hum at all... I can turn amp to full volume and dont even get a hiss through speakers. I have never had such a good amp in my life. Hats off to Peter for such a great kit.

I am only running one transformer a 300VA 2x22v into one rectifier, serving both amps which are bridged to get the 2x 120 watt output.

captainobvious 28th March 2010 07:44 PM

Quick question before testing...

Does it matter what side of the fuse you place the light bulb for testing? What I mean is, does it have to be spliced into the live wire before it reaches the panel power input/fuse module or can it be spliced into live off of the rear of the power module?


AndrewT 28th March 2010 08:00 PM

the easiest way to use a bulb tester is to attach a plug top to the inlet cable and a socket outlet to the output cable.
This way you can take the bulb tester off the shelf every time you need it and plug it into the wall socket and then plug in the equipment to be tested into the tester's socket outlet.

No need to wire or re-wire anything.

captainobvious 28th March 2010 08:40 PM

That is a good idea actually.
In this case though, I currently have a bulb socket wired inbetween the live lead of the power module back and the transformer leads. Is this acceptable for testing?

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