Converting Nikko Alpha 230 to Leach Low TIM

[BrianDonegan]

Yes, another Leach amp. I have an old Nikko Alpha 230 that has served me quite well for many years. Last year, I plugged it in, turned it on, and choked on smoke. Fried output devices lead to a burnup on the driver board. I could get replacement Toshiba and NEC devices, but they are quite expensive. I decided to reuse te chassis and some other parts to make a Leach amp.

The original output devices (4 pairs of 2sc2565/2sa1095) were running at +-58.4V, so that should work. There are also secondaries for the driver section which used to run at 63.1V, but I will probably just tape those off. The toroid is shielded, which should be nice.

I will probably add to the existing pair of Marcon 15000uF 71V power supply caps. Do I need to for a stereo Leach amp (4 device)?

I really like to layout of the amp, which is much different from most of the designs I have seen. Power supply is on one side, driver board on the opposite side, power devices and heat sinks in the middle (see pic).

There is plenty of room for four devices on each of the two heat sinks. I am planning on putting two devices from each channel on each heat sink. Possibly putting two of the diode from each channel in each sink as well. I'll add a couple other pics in the next few posts.

Let me know what you think.

Thanks-
WAH

[BrianDonegan]

Layout using the original boards:

Migt make my own, but will probably use the originals first to see how I like it.

[BrianDonegan]

And a pic of one of the heatsinks. They measure 245mm x 100mm x 55mm.

[anatech]

Hi WAH,
I have two Alpha 230's at home, and a 220. Let me know how yours turns out. All of mine still work and are in storage now.
-Chris

[BrianDonegan]

Question for folks who have built Leach's:

I am planning on moving all the passives after the the output devices to a seperate baord that will mount just below the heat sinks. This way I can cut down the number of wires between the two, and I can just run the outputs to the speaker terminal block directly, which is one the opposite end of the amp from the driver boards (behind the power supply). I will also route power to the output device board before it goes to the driver boards, so I will need to put the decoupling caps on that board.

I know I am supposed to keep these wires as short as possible to prevent occs, but how long is too long? I want to use some amp connectors between the two boards, and the wires will be about 2-3 inches long. I will use one connector for power, one for left, on for right. Think I will have a problem with the connectors?

Thanks-
Brian

[BrianDonegan]

Also, I have leads for +12, +15 and +18V, and some free space. Anyone have ideas for add on goodies?

-Brian

[acenovelty]

Hi WorkingAtHome,
The LowTim calls for 58V rails so you should be good. You don't say what the A is. Should be at least 6.
"I will probably add to the existing pair of Marcon 15000uF 71V power supply caps."
Considering their age, might be a good time to replace these with new caps. A pair of 25000uF/75V is a good investment in the future. Adding to the old ones might be a small problem. Lots of smarter people can tell you why.
You really must put the diodes on the heat sink with the transistors per the construction guide.
"wires will be about 2-3 inches long" is not going to be a problem.
Add on goodies:
Speaker protection circuit: http://sound.westhost.com/project33.htm
(altho I've never used one with the Leach amp 'cause it makes no pop on noises and Prof. Leach claims a dead transistor turns off the output of the amp to the speakers. Having intentionally shorted one out to find out, it seems to be true.)
Soft-Start Circuit from: http://sound.westhost.com/project39.htm
saves blowing fuses frustration.

Most all the transistors can be had as free samples from:
http://www.onsemi.com/site/MyON

When it's finished, you don't have to tell anyone that you replaced the boards.

Prosit

[BrianDonegan]

A pair of 25000uF/75V is a good investment in the future

Will do.

You really must put the diodes on the heat sink with the transistors per the construction guide

I plan on mounting them in the heat sink, similar to the way described, but since each pair of transistors will be on a seperate heat sink, I was wondering if I should arrainge them with all four between two tranistors, or two between each pair (wired the same of course).

Thanks-
Brian

[BrianDonegan]

So, for the connections between my driver board and Output board, I want to use a connector. I won;t be passing +VCC and -VCC though the connector, just the transistor and diode connections.

I was thinking of Amp AMPMODU MT connectors: http://www.mouser.com/catalog/620/860.pdf

Does anyone think I will have problems with noise/capacitance/etc?

I would be using 22gauge wire.

Also, I am reworking the original boards to have a single stereo driver board (sort of a dual mono). I want to pass power in a single connection for both boards, and plan on using a standard 4 pin disk drive power connector for the power (+VCC, gnd, gnd, -VCC). I have a layout with a single power connector for both channels. Any problem with having asymetric power trace lengths other than resistance (I can make the traces nice and fat if needed)?


Thanks-
b

[BrianDonegan]

One more question:

I was thinking of wiring the output devices like this:
http://www.ampslab.com/Images/bi240drv1.jpg

Looks like they are using solid wire for DC. Should I do this or use tinned stranded? If I use solid, I would most likely use 14ga.

Thoughts?

-b