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Old 5th March 2013, 01:36 AM   #1
mortron is offline mortron  Canada
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Default Finally bought an amp kit... LM4780.

Just received my Audio Sector LM4780 kit, and am looking to assemble all of my parts to begin.

I will be ordering heatsinks, wire and rca jacks, as well as pots. When I do so, I have several questions.

1) How would one wire mono pots into the circuit to control the levels of each channel independently?

2) When selecting heatsinks, what is the minimum requirement if I intend to run this with 4 or 6 ohm loads and running a 22v or 25v transformer and upto 160w @ 4ohm?

3) I have read the thread on assembling a power supply, but my kit comes with the rectifiers etc. Is it okay to fuse the transformer at the mains in and connect the transformer to the rectifier board and onto the amp boards! I noticed a "shaded" area in the Power Supply tutorial, but am not sure what I'm supposed to follow.

I intend to order heatsinks, and be a bit resourceful in finding an enclosure, rousing different materials. Other than that there is a lot of information in threads that address some questions, but I've had trouble finding answers to the ones above. Thanks
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Old 5th March 2013, 03:01 AM   #2
SoIL4x4 is offline SoIL4x4  United States
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Why do you want to control each channel with a pot instead of one for both?

I used a 22v antek for each channel to drive a pair of 4r and 6r speakers, every day for the last year. A 6" x 6" heatsink for each chip has never gotten past what I would consider warm. check out the data sheet for this chip, many questions answered.

I also used a fused iec with a power switch, the same one Peter Daniel used somewhere in here Commercial Gainclone kit- building instructions read the whole thing...or most of it anyway....

keeping the rectifier boards connected to the amp boards will simplify things, but again that is in the thread.

I enjoy soldering, but I really love building the enclosure...there are no rules.
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Old 5th March 2013, 03:35 AM   #3
mortron is offline mortron  Canada
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Ah yeah, I have a room issue, requiring me to adjust the balance of the amp to accommodate the bias and get a nice stereo image. It's a dirty job... but you know how it goes.

Thinking a 22V 300VA transformer as suggested by Peter is the way to go with one of those fused IEC sockets.

I am looking at the 10.080" profile heatsinks in a 4" height or something, but am not sure if the shape has anything to do with it's sinking ability. Need to read more.
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Old 5th March 2013, 10:58 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by mortron View Post
...........I am looking at the 10.080" profile heatsinks in a 4" height or something, but am not sure if the shape has anything to do with it's sinking ability..........
That sink size is probably big enough to keep 4 chip amplifiers cool.
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Old 5th March 2013, 12:18 PM   #5
mortron is offline mortron  Canada
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Quote:
Originally Posted by AndrewT View Post
That sink size is probably big enough to keep 4 chip amplifiers cool.
Could I use a single one for both channels? I don't understand sizing heatsinks, is there a tutorial?
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Old 5th March 2013, 12:49 PM   #6
AndrewT is offline AndrewT  Scotland
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tutorial in the 3886 datasheet
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Old 7th March 2013, 07:37 AM   #7
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For the pot, using just one channel as an example:

Remove R4, and wire a pot in its place (using the two non-wiper terminals).

Desolder the end of R7 that's not connected to the IC input, and connect the free end of R7 to the wiper terminal of the pot.

You should twist the bundle of three wires going towards the pot.

For the other channel, do the same thing with R0 and R6.

Don't mess around with a pot in the feedback network; a certain minimum gain is needed for the amp to work right.

The use of a single resistor to draw the muting current from both channels together looks weird to me... it's pretty much depending on both channels having the same tolerances, etc, for 50-50 current division.
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Old 14th March 2013, 02:22 AM   #8
mortron is offline mortron  Canada
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Thanks for the tips... I am not sure what you mean about the mute resistor as I am a little confused what exactly the mute function does... Still have a lot to learn.

As for pots, are linear or audio pots appropriate?

Thanks
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