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Old 4th March 2014, 02:08 PM   #1541
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Quote:
Originally Posted by Ryan_Mc View Post
Thanks for the replies. Some good info and good ideas but I still have some questions remaining. As per the SOAR circuit... I've got a good chunk of (re)reading done on the Sloan book I have and studied Leach's write up and I think I will modify this aspect to match Sloan's writings.
As per the DC SOAR curves of the c5200/a1943, they should be able withstand 1A at 70V. With 8 devices, that makes it 8A - into a 4R load, it drops 32V into the load (different if it's highly reactive). It still seems to give ample margin with +/-75V rails - Vce is only 43V at 1A/per device into a 4R load.

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Was still wondering about rail voltage, as I'm still considering the 50V transformer to give me ~ +/-70V rails. Seems completely do-able component selection wise but some of the traces on the PCB look pretty close to be using those voltages. Anyone have any insight on this? Second I still haven't figured out if this board requires dual supplies or is designed that is 'can' use dual supplied or supply the front end from the main rails... or requires both. OK well I know it doesn't require both to operate but what is optimal?
There are two traces that go between the 7.5mm pitch pins of the large filter caps. There is more than 20 mils Cu-Cu clearance at all those locations, and the traces are covered by soldermask. You can always spray on a shot of conformal coating at that location if you want more insurance. Clearances at all other locations are significantly more - it can't get better than the pin-to-pin clearance of a TO126 device at any rate (in this case, 2sb649a/2sd669a pre-drivers or the bf470/471 VAS devices).

It requires only +Vs/GND/-Vs, i.e. dual rails. Depending on the value of Vs, two biasing resistors for the zeners/front-end have to have their values adjusted to keep the bias current through the zeners at a nominal 3 mA (?) as per the online notes for the original Leach schematic. I think you'll be fine with just 4 pairs for +/- 70V rails, the additional outputs are mainly for insurance with slightly higher rails (up to +/- 85V is within reach, but I'd stay with +/- 75V even with 8 pairs).

I've seen some late '90s Sherbourn/Emotiva monoblocks with only 4x c5200/a1943, and they're bulletproof even after 15 years with +/- 70V rails. Of course, it depends on the kind of loads they drove during that period.
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Old 5th March 2014, 12:06 AM   #1542
Ryan_Mc is offline Ryan_Mc  Canada
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@Bob... Figured that. Thanks for the confirmation!
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Old 5th March 2014, 02:07 AM   #1543
Ryan_Mc is offline Ryan_Mc  Canada
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Linuxguru, what I had reservations about is when Vce would exceed 70V (-'ve portion of wave), the leach version just limits current in this region. If the output rail was at -50v, Vce (for c5200) would be 120V. If the C5200 turned on at this point the SOAR says it can only take about 150mA per device which the standard SOAR circuit would not protect the devices. I agree a highly unlikely situation but still possible with a highly reactive load. Less than $2 of additional components could protect against this so I don't see a reason not to investigate it, even if it is likely never to be needed. Could be me being over paranoid as well but thanks for the reality check.

As a side note, I see you mentioned differenct part #'s than what are in the BOM I have. 5200/1942 pair vs. 3281/1302, I didn't realize the latter were renumbered. Probably explains why I have noticed limited availability of the 3281/1302's. Thanks for that. I also see you mentioned the 2SB49A/2SD669A and BF470/471's vs. the MJE340/350 that are in my BOM. Are these replacements as well or just preferred devices? Are the BC556/546 still the preferred devices for the diff. pair?

Thanks for your help.
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Old 5th March 2014, 05:45 AM   #1544
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Quote:
Originally Posted by Ryan_Mc View Post
... the standard SOAR circuit would not protect the devices. I agree a highly unlikely situation but still possible with a highly reactive load. Less than $2 of additional components could protect against this so I don't see a reason not to investigate it, even if it is likely never to be needed. Could be me being over paranoid as well but thanks for the reality check.
Jens' current limiting circuit differs from the Leach (v4.5), but I have no idea if it protects against the scenario you mentioned. If you're able to retrofit the Sloan or similar upgraded current limiter, please post the updated circuit here.

Quote:
As a side note, I see you mentioned differenct part #'s than what are in the BOM I have. 5200/1942 pair vs. 3281/1302, I didn't realize the latter were renumbered. Probably explains why I have noticed limited availability of the 3281/1302's. Thanks for that. I also see you mentioned the 2SB49A/2SD669A and BF470/471's vs. the MJE340/350 that are in my BOM. Are these replacements as well or just preferred devices? Are the BC556/546 still the preferred devices for the diff. pair?
c5200/a1943 are easier to obtain from multiple sources (ST, Fairchild, Toshiba, etc.) - they're the commodity TO-247 of choice, but have worse SOAR compared to some of the On Semi NJW series. The driver is also upgraded to TO-247.

I think MJE340/350 should be fine for the pre-drivers, just check the pinout carefully when mounting them - ECB vs. BCE, etc. The VAS devices are BF470/471 or 870/871 (same die, different package). Again, other TO-126 choices are possible for the VAS, like some of the Sanyo (now OnSemi) Chroma pairs, but verify pinout as above.

For the input pair, BC546B/556B with hFE ~= 300 seems to be the most widely available pair, while the alternatives like MPSA42/A92 seem to have lower hFE. There are numerous Japanese small signal pairs (like c2240/a970) that are arguably better, but with BCE pinout instead of CBE.
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Old 5th March 2014, 06:15 PM   #1545
AndrewT is offline AndrewT  Scotland
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Leach's IV protection is single slope.
Jens' version is a two slope.
The Leach can be converted to the two slope by adding one resistor.

I don't know why Leach missed out that resistor.
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Old 5th March 2014, 06:54 PM   #1546
pooge is offline pooge  United States
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Quote:
Originally Posted by linuxguru View Post

For the input pair, BC546B/556B with hFE ~= 300 seems to be the most widely available pair, while the alternatives like MPSA42/A92 seem to have lower hFE. There are numerous Japanese small signal pairs (like c2240/a970) that are arguably better, but with BCE pinout instead of CBE.
I can vouch for the c2240/a970 pair. They are very quiet and their very high hFE can really minimize DC offset. They do have a different pinout to consider, though it was absolutely no trouble to arrange the pins to fit.
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Old 5th March 2014, 07:05 PM   #1547
pooge is offline pooge  United States
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Quote:
Originally Posted by AndrewT View Post
Leach's IV protection is single slope.
Jens' version is a two slope.
The Leach can be converted to the two slope by adding one resistor.

I don't know why Leach missed out that resistor.
IV protection was not Leach's forte. He claimed he never put it in his originial amp, and never had a problem. But he was appreciative of Michael Kawanuka's SOA Protection article I sent him.
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