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Old 21st July 2015, 05:40 AM   #1
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Default Double Barrel Leach underway

Hi Guys,

As many of you know I build a lot of amps. I first started back in 2005 and the second amp I attempted was the Leach Double Barrel. Try as I may, I couldn't make it work so I put it on a shelf and went on to other amps. A couple years ago I got interested in DIY audio again and with the help of a few of the guys around here I finally got it working. I built a big case for it and enjoyed listening to it for quite a while until one day I accidentally shorted the output while listening to it and blew up something. The boards were so tattered from all the repairs I had done earlier on and by then I had plenty of other amps so I just let it sit. I have contemplated many times of gutting it and using those large heatsinks an big power supply for something else. Well, Last week I was bored so I decided to etch some new boards and build another one. After all there aren't a lot of amp designs floating around that are happy with +-90V rails and the Double Barrel will do that without breaking a sweat.

So now I have the boards complete and ready for testing. Tonight I drilled the new heatsinks and I should receive the TO-3 sockets tomorrow. Here my question. I have plenty of the original type outputs, MJ15003/15004 but I also have plenty MJ21193/21194. Do any of you have an opinion as to whether I might gain anything from using the newer MJ21193/94 over the originals?

Thanks, Terry
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Old 21st July 2015, 07:17 AM   #2
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Either type will work reliably with this design, so you can't go wrong there.

I'd have a preference for the MJ21193/94 as they're supposed to be more linear over a wider current range and have a higher Ft. There may be a slightly greater risk of instability, but that's what base stopper resistors are for, and maybe a slightly higher Miller capacitor value.

I may need to obtain a few more of the 'regular' Leach boards.
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Old 21st July 2015, 07:32 AM   #3
owdeo is offline owdeo  Australia
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Hi Terry,

I've been enjoying music through my Leach amp for the last 15 years or so although it is somewhat underpowered for my B&W801 speakers. Building the Superamp/DB is on my list of projects...

I built my original Leach amp using the MJL21193/4s mainly because they were about the same price and much easier to mount, and it hasn't missed a beat. It measured about the same as the published specs and is totally stable and pretty much indestructible. I'm not sure whether there is any advantage to using these devices even with their higher Ft and better gain maintenance at higher currents, and in Leach's FAQ he mentions specifically that the output devices do not limit the speed of the amp. Also if you look at the SOAR curves the MJL21193/4s are not quite as rugged, so the current limiting circuit might in theory need to be adjusted to suite them. If I ever get around to finishing my PCB layout for the Superamp my plan is to add an extra set of TO-264 output devices in parallel to allow for their relative weediness compared to the TO3s. I don't like wiring and would much prefer to have as much stuff on the PCB as possible.

Having said that, if you don't mind wiring and already have the heatsinks drilled and sockets coming, I'd stick with the TO3s. I doubt there is any improvement in SQ due to the newer devices.

Why did yours blow up? Did you build it without the current limiting circuit?
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Old 21st July 2015, 07:52 AM   #4
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Quote:
Originally Posted by owdeo View Post
Hi Terry,

I've been enjoying music through my Leach amp for the last 15 years or so although it is somewhat underpowered for my B&W801 speakers. Building the Superamp/DB is on my list of projects...

I built my original Leach amp using the MJL21193/4s mainly because they were about the same price and much easier to mount, and it hasn't missed a beat. It measured about the same as the published specs and is totally stable and pretty much indestructible. I'm not sure whether there is any advantage to using these devices even with their higher Ft and better gain maintenance at higher currents, and in Leach's FAQ he mentions specifically that the output devices do not limit the speed of the amp. Also if you look at the SOAR curves the MJL21193/4s are not quite as rugged, so the current limiting circuit might in theory need to be adjusted to suite them. If I ever get around to finishing my PCB layout for the Superamp my plan is to add an extra set of TO-264 output devices in parallel to allow for their relative weediness compared to the TO3s. I don't like wiring and would much prefer to have as much stuff on the PCB as possible.

Having said that, if you don't mind wiring and already have the heatsinks drilled and sockets coming, I'd stick with the TO3s. I doubt there is any improvement in SQ due to the newer devices.

Why did yours blow up? Did you build it without the current limiting circuit?
I don't know why it blew. I never opened it up after that. Too busy building other amps. It can probably be fixed but I want to build one into a smaller case and I never really liked the way I mounted the outputs in the first one. I know it was really good sounding when it worked.

Blessings, Terry
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Old 21st July 2015, 08:01 AM   #5
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Careful , Terry. "double barreled" has local feedback loops within
the output stage. Much less tolerant of device substitutions.

When you see a warning like this -
Quote:
If you build this amplifier, you must keep the wiring between the heat sinks and the circuit boards as short as possible if you don't want oscillation problems.
... more attention must be paid to keeping everything original.

OS
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Old 21st July 2015, 01:09 PM   #6
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Good point. I have plenty of the original parts. I just wondered if I might gain anything by using the newer devices. I suppose I could just try them and see. I will be using sockets so they can be swapped fairly easily. I'm going to have to find a way to cover those outputs. The cases are at rail voltage. That would give you quite a whack if you touched both sides at the same time.
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Old 22nd July 2015, 07:49 AM   #7
owdeo is offline owdeo  Australia
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Sorry I just realised that you meant you have the TO3 version of the MJL21193/4, (MJ not MJL) hence being able to swap them in sockets. I was going to mention that another problem with using the TO-264 devices like I did is that hard wiring to them is fragile - there is no strain relief on the wires and if you move them, the device pins bend. That's why I wanted to design a PCB to suit for the DB.

Let us know if you try the 21193/4s, I'd be very interested to know if you think there's any difference in sound over the 15003/4s. But Ostripper makes a good point so maybe check for oscillation with a scope and capacitive loads before connecting to your speakers! (but you'd want to do that anyway, right? )

Maybe you had a wiring error or something. I've tried shorting the outputs on mine and the current limiting works as advertised. Also if you remove one rail fuse the output still sits at 0V - try that with nearly any other design!

You can get plastic cover caps for TO3. Try Digikey and Mouser - otherwise there are some Australian suppliers that stock them, but that probably won't be much help to you: http://www.altronics.com.au/p/h7290a...tor-cover-t03/
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Old 22nd July 2015, 08:28 AM   #8
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Hi owdeo,

Thanks for the tip on the covers. I will get some of those. It would be a bummer to be trying to move the amp while it was on and get 180vdc across your heart.

I suppose I should have investigated the issue but I have so many amps I just didn't need it and was having too much fun building different designs. I have probably built 30 or more amps this year. There is a lull in the Slewmaster and Apex sagas so I wanted to do a new one of these Superamps in a shorter case. My first one is 12" tall. This new one should be able to be 4U.I need to hit the metal supply shop and get some supplies to build a case. The old case will probably house a fire breathing Slewmaster. It's about time I have one of those set up for daily use.

Blessings, Terry
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Old 22nd July 2015, 11:32 PM   #9
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Terry,

Have you built any of the Leach amps that Jens Rasmussen published here on DYAudio?

I built a couple. They use newer parts and sound very good. I built a couple and still have them in regular use.
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Old 22nd July 2015, 11:47 PM   #10
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I wish I had downloaded the leach amp by Jens while it was still available. Seems it has been taken down and not readily available anymore.
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