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Old 30th October 2001, 07:12 AM   #1
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Hi

I would like to find out what everybody's opinion is about the Leach Amp. I haven't built anything yet (audio) and am seriously considering this project. How is this amp, providing it is correctly tested and constructed, compared to todays high-end standards.

I would also like to find out about a preamp circuit that was posted by Graham Maynard,

http://www.mitedu.freeserve.co.uk/Ci...udio/gmpre.htm

which has exceptional high-frequency performance. Has anybody used this preamp circuit or have any opinions about it? I wrote to Graham regarding building the preamp, and he gave me this response,

"The circuit is for hi gain 600 ohm working, you would need to reduce the gain for audio use, fit a slew rate limiting (dominant pole) capacitor, and insert a 560 ohm resistor in series with any cable coupled load to prevent oscillation."

As a rookie to amplifier building, I have college level electronics experience, but I don't understand what a slew rate limiting (dominant pole) capacitor is, what value to use, or where to put it. Also, does a 'cable coupled load' include a power amp?
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Old 31st October 2001, 06:38 AM   #2
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Hi,
I've built two of these over the past couple of years. The first sounded great, the second is a work in progress.
Soundwise, using mostly Prof. Leach's spec'd parts except for 1% resistors, slightly better (but still 'industrial' caps) it comapares well with with most mid-fi and even some 'hi-fi' commercial offerings. But... built it becauuse you want to build it. It costs $300 to $500 to build two channels and consideable time, so a descent used amp could be had if thats the ultimate goal. The nice features are that boards are available (I have not seen them, I wanted to etch my own), many folks have built it, and Prof. Leach has tons of documentation on the web and has always responded to any of my ?'s via email. As a first project it is ambitious, but doable if you've done some soldering, basic electronics, etc.(It was my first audio project) Debugging is much more of a headache, but so many of these have been built that the design is relatively foolproof.

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Old 31st October 2001, 07:50 AM   #3
bawang is offline bawang  Malaysia
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Thumbs up Leach Amp

Hi Kimblea. I have just recently completed a 3 channel version of the Leach Amp. To my tin ears, it sounded excellent even when directly compared to offerings from many so-called high end manufacturers. No turn on/off pops/thumps and extremely silent (with no input). Remember that you are paying a hell of a lot for cosmetics; brushed aluminium, stainless steel knobs, polished casing etc...... Be careful with the power supply section, a lot of energy is stored there.

Seriously, it is an excellent sounding project and the documentation at Prof. Leach's site is second to none. Be careful of counterfeit transistors though, esply the MJ15003/15004 output devices. I was caught in that trap....... until I replaced all of them.

Good luck!
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Old 1st November 2001, 12:20 AM   #4
bawang is offline bawang  Malaysia
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Lightbulb Leach amp alternative

Hi again Kimblea. I nearly forget. If you want a simpler amp to build and does not require all the power (and protection) of the Leach amp, try to build the Project 3A 60-100W amp by Rod Elliott at Elliott Sound Products. I built two of these. They're extremely simple to build (I built one in just half a day), high quality PCB is available from Rod and bridgeble. And used within it's power limits, it sounded excellent.

Use a good sized transformer though, and you won't regret it. 60W (8 ohms) may seem low, until you tried it out. Remember that this is not an IC amp, and therefore it is capable of much higher power into low impedance loads. Using this amp to try out Tchaikovsky's 1812 cannon shots on the Telarc label at quite high volume levels (enough to make the neighbours complain!) proved that this amp is a capable performer.

Have fun!
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Old 1st November 2001, 08:15 AM   #5
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Thumbs up Thanks

Thanks for the help. I got a lot of info regarding the Leach from past threads, which maybe I should've read first. I have decided to go ahead with the Leach Amp which to me seems the best DIY project to satisfy my creative desires, but not necessarily what I need. From what is sounds, the amp has a very solid sound.

I am thinking of building the BOZ preamp to compliment the Leach amp, but I am still contemplating. It also seems a very solid design, but I want to know more about the subjective performance of the BOZ and Leach combined. I have grown up listening to a NAD 3020i, which although is only a 25w design, is a well matured design that has proven itself. This is the amp I will be subjectively comparing my project to.

Leach/BOZ vs NAD 3020i...Any comments anyone?
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Old 1st November 2001, 09:02 AM   #6
bawang is offline bawang  Malaysia
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Talking BOZ + Leach vs NAD 3020i

Kimblea,

Exactly what I am using now, ie Leach + BoZ. Can say without hesitation, they sound great together. A bit of hum/buzz though, if you place your ears within a foot or so from your loudspeaker. At normal listening distances (> 3 feet), even in a quiet room, no noises can be heard. The Leach is completely silent by itself.

I also have the opportunity to listen to the NAD 3020i extensively. It's a nice integrated, but lacks the muscle. The Leach has more than enough power, and can drive even highly capacitive loads. BoZ, what can I say? Simple is BEAUTIFUL! I build the BoZ in 2 hours (minus etching the board which was done the day before). Look into my posting in the Bride of Zen thread for further info.

Have fun!
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Old 3rd November 2001, 12:38 PM   #7
ppl is offline ppl  United States
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I built my version of the Doub.Barr. Leach amp and i thought the sound was as good or better than a ML#23.5 The High frequency responce is outstanding and like someone elce posted No turn on Thump. While my circuit changes are quite involved like 2SK389 & 2SJ109 in place of the Bipolar transistors and DC Coupling Changing the Strange floating Cascode Second Voltage amp stage with a Fixed Biased Cascode stage Replacing the Output transistors with some 2SC1302's and it's Compliment Using 2N3501's and 2N3635 rather than the 2N5415 and 2N3440 and using MJE-15032 and MJE15033 as Driver transistors rather than the MJ-15001 and MJ-15002 as Leach used. Separate Voltage regulation for the Voltage gain stages using a custom made Plitron transformer and Discreet Component Dual tracking regulator. Boy can this still be considered a Leach Design?
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Old 3rd November 2001, 02:54 PM   #8
bawang is offline bawang  Malaysia
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Thumbs up Boz + Leach

Kimblea,

Managed to get my hands on a 'scope and function generator two days ago and just finished tweaking the BoZ preamp. As per the original design by Nelson Pass and from his feedbacks, there may be excessive gain if the input to the preamp exceeds about 1.5V (output of around 15V), which any CD player (2V) is capable of.

Since I want to minimise the number of potentiometers (at the input AND output of the preamp as advised by Nelson), and by further experimentation, I found out that with R104 (and therefore R204) at 600 ohms (2 nos of 1.2K, 2W resistors in parallel), it matches perfectly with the Leach amp.

Biasing is exactly as per Nelson's design, with a gate voltage of 8V for the IRF610. This new value of R104 also solved the unsymmetrical clipping of the preamp. Clipping voltage at the input increased to aroung 2.5V, gain of preamp around 5 without load. With a load of 10k ohms, input dropped to around 4.5. The Leach amp needs an input of 2V for maximum output.

Be careful of the signal and ground wire location as running the wires near the transformer may cause humming/buzzing. Try putting the transformer in another metal enclosure, separate from the preamp.

After extensive testing today, yes, it will outperform the NAD 3020i easily. And a hell of a lot cheaper them ANY commercial offering. A match made in heaven.

Happy building & listening!
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Old 5th November 2001, 06:31 AM   #9
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Talking BOZ / LEACH

Thanks Bawang and ppl for the info. I feel much better about the pre / power combo, as I can concentrate on the quality of construction, instead of worrying about whether or not the two will work together. I will be building the Leach Amp exactly to Leach's specs, As I have sourced all the components already. I found all the heat sinks in a scrap yard. I will write a new post about these...
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Old 12th November 2001, 08:37 AM   #10
djk is offline djk
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"Boy can this still be considered a Leach Design?" Umm, sorta.I changed the floating cascode on the second voltage amp to a fixed voltage and ran a high voltage tier for the front end and added a Baker clamp to prevent output stage saturation.For the upper, series connected outputs, I converted to a cascode.But added a +/- 40V supply and an or-ing diode.A Hafler XL600 transformer has the high voltage tier for the front end and +/-90V for the outputs.A 250VA 30-0-30 toroid provides the +/-40V.The amp runs like a normal Leach until it hits 100W.Above there it becomes a cascoded output stage (similar to the older Threshold gear).The amp delivers 600W per channel at 4 ohms.If you want to drive 2 ohms just add another pair of outputs and use two transformers, 1KW at 2 ohms.Because the worst case voltage swing on the 'off' transistor is only 40V + 90V SOA is very good.MJ21193/94 outputs.With three pair of outputs per channel running off the 40V supplies and another three pair off the 90V rail in cascode mode the amp may be biased into class A for the first 100W at which point it becomes cascoded class AB the rest of the way.I should point out that this is covered by a Nelson Pass patent http://www.passlabs.com/images/misc/pat_5343.jpg and should thank him for the idea.
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