Well done Tad - pulling this off is quite an effort! 
Thanks Jens - sharing your design allows us to built a proven design.
Tad - have you checked the www.hsdissipadores.com.br link?
I will be in Brazil next week and hope to be able to buy some for me... 12 boards = lot$ of aluminium/aluminum!
Cheers!
Thanks Jens - sharing your design allows us to built a proven design.
Tad - have you checked the www.hsdissipadores.com.br link?
I will be in Brazil next week and hope to be able to buy some for me... 12 boards = lot$ of aluminium/aluminum!
Cheers!
Regulated PS for Leach Amp
Jens,
I recalled that there was some thread regarding a regulated PS for the leach amp. Was it ever completed?
I ordered a custom-build transformer and made some error on the specification --- I had specified the input as 110V, output at 65-0-65 VAC. It hasn't arrived yet, but it was shipped already. With the US voltage of 120V, the output would now be at 71-0-71 VAC. That's 100+VDC, an overkill for the amp. Do you have any suggestions?
Jens,
I recalled that there was some thread regarding a regulated PS for the leach amp. Was it ever completed?
I ordered a custom-build transformer and made some error on the specification --- I had specified the input as 110V, output at 65-0-65 VAC. It hasn't arrived yet, but it was shipped already. With the US voltage of 120V, the output would now be at 71-0-71 VAC. That's 100+VDC, an overkill for the amp. Do you have any suggestions?
I set up a Jens Rasmussen PSU 2.2 for 65V operation intending it for my Leach front end. I still haven't got around to installing it, though. It started reliably for me at 150 mA, but AndrewT had trouble with his at ~45V.
There have been several others who have documented Leach and other amp regulated front ends here.
As for your big transformer, how about BIG heat sinks?
Seriously, with appropriate attention to detail, the amp should scale to 100V rails. You may find it more cost effective to just reorder the correct transformer and e-bay the current one to someone building a Kilowatt amp.
There have been several others who have documented Leach and other amp regulated front ends here.
As for your big transformer, how about BIG heat sinks?
Seriously, with appropriate attention to detail, the amp should scale to 100V rails. You may find it more cost effective to just reorder the correct transformer and e-bay the current one to someone building a Kilowatt amp.
BobEllis said:
There's a saying overhere : pull a pole out of your own eye first before you see a splinter in someone else's.
Note : Jens is an EE and one of the minority of EE's at this gathering who actually worked for a pro audio company, a biggy at that.
It hasn't been that long ago when the layout of output stage and NFB take-off point as in this board was standard.
A lot of pricy serial manufacture power amps have been designed like that, there's also a benefit of laying out the output like that within the restrictions resulting from decisions made for the total layout of the board.
Some folks do modify boards and NFB take-off locations, but they're the sorry ones for whom amp fiddling became more tempting than building a good sounding power amp ages ago.
The 2.2 regulators i got from big Bob's GB operate beautifully at <45V out in my KSA50 cloink.
Another note : Jens offered to help out with the PSU2.2 design, merely as a favor to the gents who desired a universal voltage regulator board.
There's always the option of unwinding 6VAC from a transformer, lowering the nominal output voltage by less than 10% is not a problem.
I'm not sure I have read everything (I'm not worried, just curious) but the 6 pair pcb seems to be fixed, the NFB point is at the very output of the amp but I can't see it in pictures and pdf's (not checked the gerbers) of the 10 pair pcb.JensRasmussen said:
Have you or anyone else measured(!) any difference between the feedback points?
Distortion difference?
Output impedance difference?
I'm sure noone has heard any difference!
Bob Ellis
A comment about your earlier remark with this board. If the amp will work, with attention to detail, with 100 volt rail 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.
I had been thinking of bridging the Leach boards but with that output bridging will not be necessary. And thanks Tad
A comment about your earlier remark with this board. If the amp will work, with attention to detail, with 100 volt rail 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.
I had been thinking of bridging the Leach boards but with that output bridging will not be necessary. And thanks Tad
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.
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.
Hi Bob,
You are right about this...
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.
I think this may be the best choice...
You are right about this...
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.
I think this may be the best choice...
Re: Bob Ellis
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!
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!
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.
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.
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.
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.
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.
