Leach Low-TIM amp with less power/lower voltage rails - diyAudio
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Old 30th May 2010, 03:22 AM   #1
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Default Leach Low-TIM amp with less power/lower voltage rails

I'm interested in a Leach Low-TIM amp with less power/lower voltage rails. WML recommends minimum +/-50VDC rails and the Low-TIM design is +/-58VDC (rated 120W/8 ohms). I'm targeting around 35W at 8 ohms. Just tired of the chip-amp (quasi-complimentary) sound proliferating in my TV, PC, portables etc. and thought of a decent say 35W solid-state amp. Apologies to the quasi crowd here. I built my double-barreled/Leach super-amp almost 30 years ago and love it.

The main changes I see are re-biasing the input diff-amp cascode and one or two pair of output transistors. TO-220 package looked good (compact) but On-Semi doesn't have much for TO-220 audio power transistors (which are basically the Low-TIM driver transistors MJE15030/MJE15031 $0.80). The NJW0281/NJW0302 (lower power 150W versions $1.96 of the NJW3281/NJW1302 200W are $2.38 parts at Newark. My point is you don't save anything other than space, so maybe leave the TO-264's alone? Has anyone scaled down this design?
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Old 30th May 2010, 06:55 AM   #2
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In the beginning, like 30 years ago... I built three Low TIM Amps, one that was about 75 wpc and two more with 8 large heat sinks each at about 30 wpc. Both worked fine, but I had to crank the bias up on the lower power unit and blow a big fan on it, guess it ran about 10 wpc into Class A. I forget what happened to one of them, but one is deep in my closet.
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Old 30th May 2010, 08:23 AM   #3
h_a is offline h_a  Europe
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Downscaling ususually does not save much money or space. IMHO it's most usefull if you want to cutdown the heat & power of a ClassA-amp or taylor it to your needs (high current/low rails for low impedances). For everything else you save maybe some outputs and that's it. But that's up to you

Most well designed amps do not rely on the rails to define working points, in other words don't need any readjustments for lower or a bit higher rails. On a quick glance this seems also to be the case for the frontend of the lowTIM, but please check.

Have fun, Hannes
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Old 1st June 2010, 05:46 AM   #4
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I'll take it that no one has downsized a Leach Low-TIM amp. The Wall Street mantra, more power is better, lol. For some applications, I just don't need a lot of power and I thought noobs might like something less explosive.

I figure 60%-70% of the costs in a power-amp project are the power transformer, filter caps, heatsinks, chassis and downscaling definitely saves $$$ there. But on the amp PCB's, you wouldn't save more than a few bucks.
The Low-TIM input cascode has 40V (zener) Q-point, so rails less than +/-50V need changes to the input diff-amp and driver stage bias which is some design work. I'll try it on my next build and see how it turns out.
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Old 1st June 2010, 10:39 AM   #5
AndrewT is offline AndrewT  Scotland
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The standard LO TIM is designed for ~+-60Vdc.
Reducing to +-50Vdc will require the resistors feeding the Zeners to be resized.
35W into 8r0 will work with 1pair of To247, To264, To3 or two pair To220.
35W is just 23.76Vpk into 8r0. You would need ~+-27Vdc or ~20+20Vac transformer. This will require the Zeners to be changed from 40V to ~20V and then the resistors feeding the Zeners must be resized and the resistors feeding the Tails must also be resized.

There are many much simpler and cheaper discrete designs that can put out between 35W and 100W that can compete with the Leach if they are competently built.

I run a pair of 60W chipamps. They cannot play loud. They become aggressive at much lower SPLs than a discrete amp delivering 100W. I normally listen @ <1W and it's the +20dB peaks that cannot pass through the amplifier without clipping that causes the sound quality problem.
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Last edited by AndrewT; 1st June 2010 at 10:54 AM.
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Old 2nd June 2010, 08:54 PM   #6
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Quote:
Originally Posted by AndrewT View Post
There are many much simpler and cheaper discrete designs that can put out between 35W and 100W that can compete with the Leach if they are competently built.
AndrewT, can you mention the alternate designs? Bipolar class AB 35W-100W ballpark would be okay. I'll take a looksee.
Over the years I've built a handful of discrete power amps and most were disappointing in terms of stability and fidelity, they ended up in the junk pile

Although the Leach design uses many transistors (over two dozen), perhaps twice as many as other designs, I don't worry about those costs.
I have the same experience with chip-amps, they "blare" at higher SPLs. Massive filter caps (>10kuF) and oversized power transformer help. I feel some of the chip-amp sound problem is due to the absence of PNP output transistors. That is a semi manufacturing limitation, something you cannot mix on a chip and why a discrete design can do better.
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Old 3rd June 2010, 04:32 AM   #7
Bonsai is offline Bonsai  Taiwan
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The Leach Amp is probably the all time #1 great DIY Amp - Kudos to Prof Leach - and will live on for another 30 years but probably morph into using more modern semiconductors. Its been going for over 30 years and there are a few things that would be done differently today, but it is a fine amp. Period.

I think AndrewT mentions some of the things that need attention, but I'll re-summarize here again:-

Use 1 single pair of TO-247/TO-264 Devices. Since the Leach Amp used fairly slow output devices, you can use 1 pair of MJL21194/21195. Because these are high voltage devices that you will be running on much lower rails, you get the benefit of reduced 2nd breakdown possibility.

Transformer - 20-0-20. Use 100VA component - should make for a very compact little 35W amp that can drive most domestic speaker loads.

Resize the input current source circuit. On 27-30VDC rails, you will need to use 18V-20V zener references to ensure under absolute worst case supply rail conditions you continue to get good current flow through them. Simply select the tail resistor (this is the resistor feeding each of the diff-amps) for the same current value as in the standard Leach Amp. Easiest way is to use only 1 1N5250 (20V 0.5W zener) per diff amp and then to put 2 12K resistors in parallel to get your 3mA tail current. A mismatch in the tail currents of 5% between the two diff amps only leads to a very slight increase in distortion (few PPM), so critical matching is not necessary.

Change the resistor that feeds the 1N5250 from the associated supply rail to 680Ohms (it is 2.2K in the standard Leach Amp). This gives a Zener current of 10mA - only slightly higher than the original 9mA.

For the small signal devices, you can go for BC557/556 across the board, but keep the VAS and the drivers the same.

I doubt there will be any compensation issues or stability issues.

Good luck
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Old 3rd June 2010, 10:51 PM   #8
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Bonsai, thanks for your help! I'll start with 20V zeners, 1/2 the tail bias resistors and aim for around +/-28VDC rails.
I have to re-think if 4ohm loads would be alright with one pair of outputs. I found this on WML's website FAQ:
"Removing 1/2 the output transistors will not reduce the output power. It reduces the maximum output current by a factor of 1/2. For an 8 ohm load, the power output would not change. For lower load impedances, the protection circuits would kick in at a lower output current than with all 4 output transistors. If you want to operate the amplifier with 1/2 the output transistors, Q18 and Q19 are the ones to omit. You can also omit R37, R38, R41, R42, R45, and R46. These changes will reduce the maximum output current by a factor of 1/2. If you wish to decrease the output power, you must reduce the rail voltages. I do not recommend reducing them to less than +50 V and -50 V."
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Old 3rd June 2010, 10:56 PM   #9
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I think the Leach Amp is the #1 DIY power amp. When the Audio Magazine article came out in 1980, it was a landmark. I think it should have been published in the AES Journal. DIY'ers were still burning after the SWTPC Tigersaurus disaster.

It's 30 years later and still worth all the effort building one. I'm going to start a thread on Leach Amp transistor alternates/substitutes. The TO-39 package is fading and I have been reading Doug Self's book on power amp design which has good info on the transistor requirements. I was wondering if the changes over the years are the best.
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Old 3rd June 2010, 11:36 PM   #10
mlloyd1 is offline mlloyd1  United States
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prariemystic:

that new thread sounds good. hopefully, there will be some good discussion on compensation changes that would be likely needed due the semiconductor changes.

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