A tribute to the late Prof. Leach (LX amp / voltage stage) - diyAudio
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Old 5th December 2010, 06:49 AM   #1
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Default A tribute to the late Prof. Leach (LX amp / voltage stage)

When I found out the good professor passed away , I was sorry that I never got his "creation" beyond a crude prototype. The original amp is truly a classic , I'm sure more have built it and more have survived than all the other DIYA amps combined.

Simulating the original (LT .asc file "leach 4.5.zip"-below) to see what's going on is a good first step. You might have to "fix" that LT file , get models .. etc. BUT , it simulates well ...but is nothing to "write home about" ,performance wise. Many hobbyist's love the sound and durability of the design and have written many nice things about it even as it is "normal" in its performance. Below in the (first picture attachment) I have beta enhanced it. I ran this simulation with the original VAS configuration , beta enhanced (BE) , and BE with TMC.

I got .01% across the audio spectrum in normal mode , .004% with BE and .001% or less with BE and TMC. All tests were done at 40v p-p/ 8r load -800K UG point. "Normal" had 60 db loop gain and the BE's had 80+DB.

This topology , on first impressions... seems to be very linear - independent of frequency and load (triple is used) ,it is easy to keep the slew very fast and symmetrical , and the design seems to tolerate a wide range of devices/ values. We have a lot of blameless type designs .. TOO many symasym types , why not a good "leach type" design to round out our collection of projects.

The second LT sim file, (LX.1.0.zip) is the one on the schema with BE and TMC , it DOES work with included models (.txt in zip)

Initially , I will build and run this as a voltage board in EF2 mode then design a "all in one" Triple / complete project with 4 pair plastic OP's (1 OP pair driver -njw0281/0302/ 3- NJW21193/4 as main OP's) .. all to run on 55-65v rails.


ANY ideas are VERY welcome and appreciated.



OS
Attached Images
File Type: gif leach_LX.gif (31.0 KB, 1104 views)
Attached Files
File Type: zip LEACH_AMP_4.5.zip (2.4 KB, 172 views)
File Type: zip LX1.0.zip (9.1 KB, 161 views)
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Old 5th December 2010, 07:35 AM   #2
taj is offline taj
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Dr. Leach is gone? Very sad. Very nice guy. I have a set of his last boards, so I'd better finish populating them and apply some power, and some speakers as a token of my respect.

..Todd
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Old 5th December 2010, 07:43 AM   #3
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Originally Posted by taj View Post
Dr. Leach is gone? Very sad. Very nice guy. I have a set of his last boards, so I'd better finish populating them and apply some power, and some speakers as a token of my respect.

..Todd
Here it is , I thought all knew of it , it is akin to D. self passing away.

W. Marshall Leach, Jr. 1940 - 2010 - Obituary - Tributes.com

I intend on making a "leach amp" that will be smooth-musical and cheap so many will build economically.
I will use his 3 diode Vbe and as many of his original design choices (split ac/dc NFB) as can be useful. I won't diverge too far (except using the excellent NJW TO-3P's for triple OP stage )
OS
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Old 5th December 2010, 08:19 AM   #4
Bonsai is offline Bonsai  Taiwan
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Wow - that's sad. RIP Prof. Leach - your amps are awsome.
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Old 5th December 2010, 11:12 AM   #5
djk is offline djk
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I wonder how long they will leave his gatech.edu/mleach site up?
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Old 5th December 2010, 11:54 AM   #6
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Originally Posted by djk View Post
I wonder how long they will leave his gatech.edu/mleach site up?
I found Prof. Leach's site one of the best learning tools for amp design, so I grabbed a copy a few days ago when I heard the news. My first thought was the IT department will just scrub the site one day during routine maintenance.

RIP
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Old 5th December 2010, 12:29 PM   #7
jaycee is offline jaycee  United Kingdom
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I mailed Georgia Tech about it and apparently they've had lots of mails and messages from alumni of Dr Leach and others, requesting that his web site be kept up in his memory. They seem to be taking the requests pretty seriously, so here's hoping.
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Old 5th December 2010, 02:06 PM   #8
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Originally Posted by ostripper View Post
When I found out the good professor passed away , I was sorry that I never got his "creation" beyond a crude prototype. The original amp is truly a classic , I'm sure more have built it and more have survived than all the other DIYA amps combined.

Simulating the original (LT .asc file "leach 4.5.zip"-below) to see what's going on is a good first step. You might have to "fix" that LT file , get models .. etc. BUT , it simulates well ...but is nothing to "write home about" ,performance wise. Many hobbyist's love the sound and durability of the design and have written many nice things about it even as it is "normal" in its performance. Below in the (first picture attachment) I have beta enhanced it. I ran this simulation with the original VAS configuration , beta enhanced (BE) , and BE with TMC.

I got .01% across the audio spectrum in normal mode , .004% with BE and .001% or less with BE and TMC. All tests were done at 40v p-p/ 8r load -800K UG point. "Normal" had 60 db loop gain and the BE's had 80+DB.

This topology , on first impressions... seems to be very linear - independent of frequency and load (triple is used) ,it is easy to keep the slew very fast and symmetrical , and the design seems to tolerate a wide range of devices/ values. We have a lot of blameless type designs .. TOO many symasym types , why not a good "leach type" design to round out our collection of projects.

The second LT sim file, (LX.1.0.zip) is the one on the schema with BE and TMC , it DOES work with included models (.txt in zip)

Initially , I will build and run this as a voltage board in EF2 mode then design a "all in one" Triple / complete project with 4 pair plastic OP's (1 OP pair driver -njw0281/0302/ 3- NJW21193/4 as main OP's) .. all to run on 55-65v rails.


ANY ideas are VERY welcome and appreciated.



OS
Hi OS,

Nice work and a nice tribute to Marshall. We all miss him and cannot believe he is gone. Over 35 years, we have all learned a lot from Marshall Leach. In addition to being certifiably smart as a PhD, we had an incredibly good ability to communicate ideas and was very practical. These latter traits are not always found in PhD's.

Cheers,
Bob'
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Old 5th December 2010, 09:22 PM   #9
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Originally Posted by Bob Cordell View Post
Hi OS,

Nice work and a nice tribute to Marshall. We all miss him and cannot believe he is gone. Over 35 years, we have all learned a lot from Marshall Leach. In addition to being certifiably smart as a PhD, we had an incredibly good ability to communicate ideas and was very practical. These latter traits are not always found in PhD's.

Cheers,
Bob'
Thanks , Bob. Marshall's style was similar to yours (not strokin' any ego's here). Just easy to understand yet informative. I wish I could afford your book , I'm driven to spend on parts instead (mouser is my "dealer" ).
I'm not done yet , I still have to decide how much of the good doctor's work to leave original and what changes are necessary to make the newer devices perform at their optimum.

The new NJW21193/4's are "in -between" they have almost the beta (most exhibit Hfe 60-80) of the 0281/0302 pair but have almost the SOA of the original TO-3 devices. I am running them (only 1.80/2.70 ea.) on my 60w amp and CAN NOT burn them out (shorted on purpose) I'm impressed. Ft/Cob is 4mhz/500pF , close to the 15003/4. The njw0281/0302 is a great choice of driver for 3 pair of the 21193/4 and has some "muscle" for ultra low imp. loads.

The input stage- The professor used mps8099 which is similar to the 2n5401/5501 but with less Vceo. For ease of construction/sourcing / higher rails, the 2N-xxxx pair would be a good substitute. Since we are cascoded , low noise ss9014/15 or any other suitable input pair can be used. The LED current sources are a simple and cheap improvement worthy of addition as well ...

VAS - This is the "great decision" , Leach's original voltage stage works ... it even works real good. This might be the source of all the favorable reviews , untouched - as is. I know this will disallow the addition of TMC and keep my loop gain low but I am after the classic's "flavor" (sound).

Feedback Compensation - "split" compensation (AC and DC - taken from both the driver and final OPS). I will reread the professors explanation of this to refresh my memory. I also want to experiment in LT to see how this method is better or different than straight global NFB.

Vbe - thermal comp. , from what was said in the professor's article and the fact that DIY'ers everywhere have reported minimal issues , the doctor's approach is a good - (practical)method , even as it uses a simple 3 diode string to bias the main Vbe.

I read the leach amp article before I even found this site (DIYA). At first , looking at the design , it "blew my brain" ... I wondered how college students could build this thing ?? It seems so simple now , not quite as simple as a "blameless" , but close. A lot of different "varients" can be derived from this design ... cascoded VAS (like the luxman) , beta enhanced cascoded VAS , even a folded cascode symmetrical VAS. As tempting as all this is , it would not be "classic". NO thermaltrack designs either.

I am a little uninformed as how does one correct any DC offset in a complimentary differential ? , better put .. what is the BEST way to correct/minimize it ??

Any comments - idea's from leach amp owner's or builder's would be greatly heeded/acknowledged. (original below)
OS
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Last edited by ostripper; 5th December 2010 at 09:25 PM.
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Old 5th December 2010, 09:31 PM   #10
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I was just looking at his site 3 weeks ago and was thinking of sending him an email.
This is very sad news.
It looks like I will have to wait before I can ask him any questions.
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