Work In Progress... Leach Based Amplifier - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 29th October 2013, 03:28 PM   #21
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
Spooky, Perhaps you should start a build thread to discuss your design choices. My questions: (intended as food for thought)

Why add an op amp to accept balanced input when you already have a differential pair input stage? Lift the feedback network ground and there's your inverted input. See the Pass A75 article for discussion about impedance matching the two legs.

Why TO-3 output devices? They may be more rugged, but mounting them on an angle adds another thermal junction, losing some of the advantage. You could fairly easily substitute 5 sets of MJL0302/1381 and be as rugged and simplify construction. MJ150034/35 have higher hfe and would be a better choice with higher numbers of output devices - more current available to drive the outputs.

Have you recalculated the output protection circuit resistor values? Those look like Leach V4.5 values, but things change with more output devices. You may find that you want to reduce R28 and R30 and/or increase the values of R41-43 and R60-62.

Have you considered higher hfe input devices? At 60V rails you could use BC546C and BC556C. The more available B types have higher hfe than the MPSA42/92. Not sure if they are the only reason my new Leach (Jens' 5 pair boards) sounds better than my V4.2 Leach, but it probably contributed.
  Reply With Quote
Old 29th October 2013, 06:05 PM   #22
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
Hi,
I'm glad you're dropping in.

Quote:
Originally Posted by BobEllis View Post
Spooky, Perhaps you should start a build thread to discuss your design choices.
I considered this and I might end up doing it, as this thread may not be the most appropriate and I may take time away from others.

Quote:
Originally Posted by BobEllis View Post
My questions: (intended as food for thought)

Why add an op amp to accept balanced input when you already have a differential pair input stage? Lift the feedback network ground and there's your inverted input. See the Pass A75 article for discussion about impedance matching the two legs.
I didn't know how to make proper use of that other side of the diff amp, so I didn't mess with it, but of course if we can rig it that way, why not? But then how do we handle the case when the line doesn't come in balanced on the XLR? There would be some need for a switch or jumper then, wouldn't it?

Quote:
Originally Posted by BobEllis View Post
Why TO-3 output devices? They may be more rugged, but mounting them on an angle adds another thermal junction, losing some of the advantage. You
I know, and I made that choice only because I have plenty of those devices on hand and I wanted to make use of them instead of buying even more devices while leaving the old stock alone. The fact that I added an extra thermal junction bothered me, but since I was out to overbuild and I chose rather large sinks, I preferred using my existing devices.

Quote:
Originally Posted by BobEllis View Post
could fairly easily substitute 5 sets of MJL0302/1381 and be as rugged and simplify construction. MJ150034/35 have higher hfe and would be a better choice with higher numbers of output devices - more current available to drive the outputs.
When I make the other versions that I planned to make, I will use other devices, but I want to use those in stock for now.

Quote:
Originally Posted by BobEllis View Post
Have you recalculated the output protection circuit resistor values? Those look like Leach V4.5 values, but things change with more output devices. You may find that you want to reduce R28 and R30 and/or increase the values of R41-43 and R60-62.
Not yet, and that's one of the big things that I still need to do now. I was also considering the V-I limiting, so I provided for the extra resistors on the pcb in case I was going in that direction.

I also made use of those non inductive MPC722 double resistors, very nice and space saving, plus since they have a common pin, that suits very well the output stage configuration.

I also have those MPC722 in 0.22 + 0.22 ohms, so the protection scheme definitely needs to be properly calculated.

I plan on doing a serious matching on the transistors, all of them, including the input ones, since I have all those in stock and sufficiently large numbers, no problem with making a selection.

With the matched transistors, the lower emitter resistor values will be less of a concern and will mostly serve for the protection.

Quote:
Originally Posted by BobEllis View Post
Have you considered higher hfe input devices? At 60V rails you could use BC546C and BC556C. The more available B types have higher hfe than the MPSA42/92.
True, and I was actually considering using the 2N5401/5551 types, since I also have those and they have a slightly better hfe than the mpsa. I also am reluctant, may wrongly, in using the BC546 because of their lower Vce0.

This current design with the MJ15003/4 is only planned for 60V rails (toroids provide around 43Vx2), since I do have the transformers on hand as well.

Later on other versions, I will use beefier transformers, actually I'm planning for 2 toroids per amp, which explains how the amp module is prepared for this with the room below the pcb support plate, with the mounting holes already there now.

I also have on hand a bunch of MJ15024/5, so that's going into the next upper version, keeping the same case design. And finally, I also have a few MJL21193/4 but not enough to allow matching. Those will make for an other version later.

Quote:
Originally Posted by BobEllis View Post
Not sure if they are the only reason my new Leach (Jens' 5 pair boards) sounds better than my V4.2 Leach, but it probably contributed.
Could be, as the transistors do make such a difference.

For now I want to make use of the ones on hands, and later I will get the better ones and alter the design for them.

I need a good number of amps anyway, many for myself, but also for friends around, so they will all find a good home.

I am continuing on my plans for a 4 ways active system. That's a bunch of amps!!!
  Reply With Quote
Old 29th October 2013, 06:09 PM   #23
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
Quote:
Originally Posted by BobEllis View Post
See the Pass A75 article for discussion about impedance matching the two legs.
btw: I'll go for that, but what is that article you're referring to? Very interesting I'm sure.

If I can properly make full use of the diff amp's inputs for a balanced input on the xlrs, then I would definitely consider removing that sym line receiver, which would remove quite a few parts at the same time, since I could do away with the extra power supply for it...

Thanks for dropping in!
  Reply With Quote
Old 30th October 2013, 04:11 AM   #24
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
Start here https://www.passdiy.com/project/amplifiers/a75-part-1 then read part 2
  Reply With Quote
Old 30th October 2013, 09:39 AM   #25
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
Thanks bob, I will read this...

In the mean time, my other renderings are now done, so here are a few more:
Attached Images
File Type: jpg amp module input end detail.jpg (612.0 KB, 350 views)
File Type: jpg amp module input end.jpg (458.5 KB, 325 views)
File Type: jpg amp module top all.jpg (634.6 KB, 318 views)
File Type: jpg amp pcb module top.jpg (545.2 KB, 301 views)
File Type: jpg ampli top no covers.jpg (754.4 KB, 290 views)
  Reply With Quote
Old 30th October 2013, 10:07 AM   #26
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
Now I think it's time to take a close look at the protection and all the calculations.

The fact that there are more than one emitter resistor with the "sensor" resistors on each of them, makes it more complex to calculate and I'm not comfortable enough with this. So that's what I'd like to work on.

Plus there is the option of doing it as V/I limiting, which I also want to explore.

This can be interesting for diyers who like me want to know how to do it and not count on others to do it for them without understanding how it's really done.

I have access to a mathcad setup for now, so I am trying to use this to make some graphs and calculations, but I'll need some help figuring this out properly, as what I'm doing may not be quite right. I'm on Mac, so no MathCad for me, but I'll touch that windows thingy anyway (yuk!) just to get the work done.

Here is a graph I managed to make with it. I plotted the SOA from datasheets for the MJ15003/4 in black. I think I may have this right, since it looks like it's matching the datasheet's graph. Then I plotted 2 sets of load lines, one for Vcc at 60V for the 60V rails that I'm using on this current design, and the second for 70V.

I don't know how to add captions on the graphs, so I'm just going to provide the legend in text.

For each set, I plotted for 8 ohms and 4 ohms for a pure resistive load, which I may have gotten right, and I also tried plotting a 100% reactive trace.

The green traces are the 60V Vcc for 8ohms resistive and that possible 100% reactive load situation. I need to make sure I get this right, as it's quite near the limits. The red trace is just the 4ohms resistive on 60V and the orange and brown are the 70V Vcc versions for the 8 and 4 ohms.

Then there is that blue trace, which exceeds the SOA, which is supposed to be the 100% reactive condition for the 70V Vcc on 4 ohms.

All these traces are for 3 pairs of MJ, and that is also something that I need to double check to make sure I got it right.

If this is correct, then the 8ohms 100% reactive situation on 60V rails passes with no problem, but the 70V rails would pose a problem.

On pure resistive there are no problems anywhere...
Attached Images
File Type: png MJ15003:4 3 pairs.png (95.5 KB, 120 views)
  Reply With Quote
Old 30th October 2013, 11:05 AM   #27
Boscoe is offline Boscoe  United Kingdom
diyAudio Member
 
Join Date: Feb 2010
Location: England
Very impressive layout and renders but will you actually make it?
__________________
I thought about it once, but then thought again.
  Reply With Quote
Old 30th October 2013, 11:22 AM   #28
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
Quote:
Originally Posted by Boscoe View Post
Very impressive layout and renders but will you actually make it?
Of course, that's why I'm doing all this, it's not just to fool around, I have many of the parts already, which is the main reason behind some of the choices, plus I am right now working with a contact of mine in china who can handle the sheet metal work, they have the equipment, including the laser cutting table.

This will be nice, and I plan on other versions, but keeping a good part of the case the same to limit costs.
  Reply With Quote
Old 30th October 2013, 11:24 AM   #29
Boscoe is offline Boscoe  United Kingdom
diyAudio Member
 
Join Date: Feb 2010
Location: England
Quote:
Originally Posted by spookydd View Post
Of course, that's why I'm doing all this, it's not just to fool around, I have many of the parts already, which is the main reason behind some of the choices, plus I am right now working with a contact of mine in china who can handle the sheet metal work, they have the equipment, including the laser cutting table.

This will be nice, and I plan on other versions, but keeping a good part of the case the same to limit costs.
Very nice. So your planning of selling them!

I just see a lot of projects that never get off the ground.
__________________
I thought about it once, but then thought again.
  Reply With Quote
Old 30th October 2013, 11:44 AM   #30
diyAudio Member
 
Join Date: Apr 2012
Location: Florida & France
I have 2 toroid transformers for this amp, 470VA and about 43V with no load, so my rails should end up close to 61V unloaded. I don't know how far they'll sag under load, but the toroids are pretty efficient and don't sag that much, so I'm expecting a power somewhere in the 180-200watts on 8ohms. We'll see how that goes when it's built.

The main heatsinks are Fischer SK479, the smaller heatsink for the pre-drivers is an SK546 and the one on the rectifier bridges is an SK597. The angled profile for the TO3s to link up to the main heatsinks is also from fischer, SWP55 anodized black.

The plans I'm making aren't just for show, they are precise and ready for manufacturing. I just forwarded the cad drawings to my metal worker in china and we're chatting about it as I'm writing this...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Work in progress... Asio4All Q pcb121055 PC Based 3 25th November 2011 05:59 AM
FE167E finally ! work in progress... ow31 Full Range 27 28th June 2006 10:03 AM
progress pic MMTMM based on HIVI B3N Tazz Multi-Way 1 18th July 2005 06:03 PM
BZLS work in progress till Pass Labs 10 26th February 2004 08:34 AM
Aleph 5 - work in progress SuppersReady Pass Labs 23 14th November 2003 10:40 PM


New To Site? Need Help?

All times are GMT. The time now is 02:27 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2