Help me put this transformer to good use...

TheShaman

Member
2007-01-31 2:17 pm
Hey all,

I'm about to replace the linear PSU of my ClassT (Truepath) amp with a SMPS and I was thinking of building something entirely "different" with the left over parts.

These include a 3U case (no heatsinks - but can be attached on the sides), a rectifier board (upgraded with these diodes and 4 x 10000uF, 100V Epcos Sikorel caps), the matching softstart module and last but not least a beefy Amplimo 800VA, 2x40V AC transformer.
The Sikorels will, most probably, not be enough but I can live with buying bulk caps as well (even if they're of lesser quality).

Anyway, I'm looking for a SS amp that would work with the aforementioned PSU. As a minimum I'd like to be able to get my hands on PCBs and a detailed BOM for the amp (a kit would be a plus but not a requirement).

I would very much like to build an F5 since I read only good comments about it, it's a fairly easy build with PCBs and kits but, unfortunately, my rails are too high (~56V DC) and, moreover, the F5's power is too low for my loudspeakers.
Speaking of which, I'm listening through a pair of ATC SCM-40s which sound great but are not what you'd call "easy to drive".
I'd say 100W @ 8Ohms are a minimum. This puts any "pure" Class A amp out of the picture, I suppose. As much as I like it I wouldn't be able to afford 2x100 watts of it - not only in terms of build cost but also everyday use (see: power consumption).

So, I'm open to suggestions...
Some other design of Mr. Pass which would work well with 56V rails?
A Krell clone (KSA50?) perhaps? [Don't underestimate the "Look mom, I own a Krell"-factor!]
A Leach clone?
Concerning the latter, I found a really interesting post of AndrewT on a Leach clone thread:

There a number of ways to use this 12device PCB.

1.) as the standard Leach using a big 40+40Vac transformer that ends up able to drive down to 3ohms of severe reactance speaker. ~400W into 3r0

2.) almost Leach, but on higher voltage 45+45Vac, for severe reactance 4ohms speaker and above ~400W into 4r0. 50+50Vac for 6ohms and above, ~350W into 6r0.

3.) A high optimum bias on 40+40Vac or possibly a lower voltage that delivers a lot of ClassA before transitioning over to ClassAB for the big transients. This one will run pretty warm.
Try Re=0r1 and Vre~19mV for a ClassA output of ~2.25Apk giving ~20W into 8r0. ~175W ClassAB into 8r0 or 335W into 4r0.

That 3rd option in specific seems a very attractive solution.
A tried and tested design, with positive feedback from users, more than enough power to handle my speakers and the benefits of class A at moderate listening levels.

Any input would be appreciated!
Don't hesitate to recommend an amp I haven't mentioned here if you think it's worth a look.

Cheers! :)
 
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TheShaman

Member
2007-01-31 2:17 pm
Thanks Andrew,
I went through some of your older posts. Very informative.
The amp might not be "the best in the world" (as is popular in thread titles nowadays ;p ) but I suppose it's a good and proven design to start with.

Other than the Leach, is there another "popular" amp (JLH, Pass, Krell ?) that'd work with 40V AC rails?
 
A standard F5 runs 1.3A from +-25Vdc.
That gives the heatsink a dissipation load of 1.3 * [25+25] = 65W/channel.
This allows ~2.6Apk of ClassA into 8r0 for a maximum ClassA output of ~2.5^2 * 8 / 2 = 25W into 8r0.

If one cascades the F5 to use say, +-35V and wants maximum ClassA into 8r0 then the output bias goes up to ~1.9A.
The heatsink dissipation load goes to 1.9 * [35+35] = 133W/channel.
This allows a maximum ClassA of ~55W into 8r0
I would take that as the maximum a paralleled F5 output stage could reliably manage.
Any more than +-35Vdc and you are aiming for a 3pair or more output stage.