Jens Rasmussen Leach clone group buy

[BobEllis]

I haven't crunched the numbers, but watch dissipation in Rs and transistors. Leach provides a formula for calculating the front end resistor values that scale with rail voltage.

250 watts output doesn't directly translate to 25W in 10 outputs. Check out the safe operating area threads for how to determine if your output stage is safe. My gut tells me that MJL4281/4302 will be fine at 85 volt rails if your speakers don't go significantly below 8 ohms, though.

[soltari_knight]

Hi Bob,

You are right about this...

Seriously, with appropriate attention to detail, the amp should scale to 100V rails.

However, I intend to use the amp as PA for my band. And as we are aware of, the 5 pair output has SOA @ 80-90V with an 8-ohm load. It only takes someone connecting 4-ohm loads, an hour of fun.... then were smokin' leaches

I've also been thinking about this but I am pretty sure the transformer is E-I core.

There's always the option of unwinding 6VAC from a transformer

I think this may be the best choice...

e-bay the current one to someone building a Kilowatt amp.

[soltari_knight]

...then an 85 volt rail, approx. 250 watt, amp is Well within spec and safe to build. Allowing for 25 watt power draw per device does not seem to unreasonable. The heatsinks I have constructed are rather HUGE so I should not worry.

Tad,

At 85V rails, the amp will be at 450W @ 8ohms. Also, only half of the output transistors are ON at a time, meaning 5 transistors. You are looking at ~100W per transistor. However, note that the transistor's power handling would degenerate due to heat...

Btw, the PCBs look great Tad! Thank you very much for pulling this GB!

[tryonziess]

Yeah and guess what . Your boards are in the MAIL. Have fun Tad

[soltari_knight]

Originally posted by tryonziess
Yeah and guess what . Your boards are in the MAIL. Have fun Tad

Awesome!!!

[BobEllis]

sir knight -

Don't forget that the amp can only swing to within about 5-10V of the rails. Chances are at high output there will be a few volts of ripple on the rails, so the amp will probably barely swing 75 V or so with 85 V unloaded rails, around 350W.

Also, just because an amp is putting out 350W doesn't mean that the output devices are dissipating 350W. If you take a perfectly resistive load, at +80 V output the current in the 8R load is 10A. The P channel outputs are turned off, so the N channel devices pass 10A. But, with 85V rails, they only have 5V across them, so they carry 2A, dissipating 10W each.

Of course real world loads are not purely resistive. Take the worst case - a perfectly reactive 8 ohm load. In this case the current is 90 degrees our of phase with the voltage - the current is at its peak when the the voltage is 0. This means swinging 80V, the current is 10A at the zero crossings, and the outputs dissipate 400W per rail.

With a real word load the dissipation is somewhere in between the two extremes. And, yes, the output devices must be derated at elevated temperatures.

If you look at the standard Leach and Jen's version you'll notice an implication that 3 plastic output devices can handle about the same load as 2 TO-3s. This was thoroughly hashed out in a few threads, and seems to be a decent rule of thumb.

I have a couple of commercial amps that use 3 pairs of TO-3 outputs with 85-90V rails. (A Hafler DH-500 and a Yamaha M-2 rated 250W and 275W respectively). Seems to me, that using the 3/2 equivalence that Jens' 10 output amp ought to be safer than the commercial amps on 80V, with the caveat that the load impedance should be reasonable and not too reactive.

Another consideration is that in home use, you will rarely if ever approach the capabilities of an amp with 80V rails. and even when you do, it won't be for long, so you're probably a lot safer than it would first appear. Of course for pro audio use, you must design to the worst case. (which for class AB amps tends to be around 65% of maximum output.)

Sorry for rambling.

[soltari_knight]

Sorry for rambling.

No worries Bob, thanks for explaining further. I was simply stating the fact from a perfectly theoretical perspective of 85V swing. But you are right though, there's voltage drop across the transistor + resistor combination (depending on the transistor and the load value) which reduces the swing, and ultimately, the power delivered across the load.

[AndrewT]

Hi,
5pair of MJL4281/4302 on +-85Vdc supplies with +-40mF smoothing fed from a 4% regulation transformer can output upto 380W into 8ohms.
But to allow driving a 45degree phase angle load, the transistors must be kept below 55degC meaning about <=45degC heatsink.
This takes the devices half way between 1second and 100mS SOAR. The devices are passing 6.9Apk @ 82.5Vce (about 114Wpk /device).
Increasing the load phase angle to 54degrees takes the devices out to the 100mS SOAR (about 131Wpk/device).
The output stage will survive resistive load testing to 4r0 (53Vac), but only as long as the heatsink maintains Tc<60degC.
This is not a specification that is anywhere near PA duty.
+-75Vdc to +-80Vdc is a more sensible target for 8ohm duty and for 4ohm PA duty a safer +-67Vdc would be a better target.

[rolandong]

4ohm PA duty

that is why i still prefere the super leach...
the series outputs enhances SOA by half.

[tryonziess]

I want a couple myself. Where are the board layouts for the Super?

A pair of 50-0-50 trannys in the regular Leach should be perfect for my 6 ohm ribbons. Thanks for all of the information. It has norrowed my search. Now to shop for some affordable iron. Thanks Tad