Ta-daaa! New Mini Aleph

[MBK]

I finally finished (sort of) my Mini-Aleph monoblocks. Thanks to Pass Labs for publishing circuits and ideas, and to the many contributions on this forum. This allowed me to stitch together topologies, ideas and values from Aleph 3, Aleph 30 and the several Mini-A emulations.

Final circuit below, I hope this time I didn't make drawing mistakes - I build sort of free hand from various schematics, draw by hand, and only in the final stage I use Circuitmaker

Data: single IRFP140, PSU one 18-0-18 160 VA with corcom line filter per channel, rails at 22V, 3x3,3000 uF per rail (found Elna caps surplus at $0.40/piece).

Front end from Aleph 30, power stage using mostly Kristjan's board values but since I used different IRF's I had to adapt the AC current gain resistor in situ by measurement to 2k to get to 50% (initial current gain with 1.3k turned out much too high).

Bias close to 2 A and dissipation over 40W per device, I hope the IRFP's will stand the test of time under these conditions. Each power mosfet has its own heatsink. They get slightly too hot to touch.

Measured output Z 0.12 Ohms into 7.5 Ohm dummy load (wirewounds).

I use it in "pseudobalanced" mode ("cold" balanced in through equal impedance ground return, DIY quick fix common mode choke between the leads).

Case: DIY - riveted aluminium rails and panels, sanded, on wooden base plate with aluminium metal flake paint (using the dust from filing ). Blue LED through sanded glass window. All painfully done by hand except for the electric drill.

Sound: well, how can I give an objective statement? ... I find the sound "normal" in the way that it does everything right, and does not produce artifacts - very, very clean sound. It also seems to me that I hear more details, and that the amp sounds very "precise" - no mushyness or colorations to my ear. These opinions may change as I go along of course.

[MBK]

complete

[MBK]

sorry my electronic camera does badly in closeups...

[jleaman]

Right on. another aleph person SWEET..

[Jay]

Originally posted by MBK
Final circuit below, I hope this time I didn't make drawing mistakes - I build sort of free hand from various schematics, draw by hand, and only in the final stage I use Circuitmaker

I think R15 and R22 should be 1K5, right? And R24 doesn't need to be 5W, correct?

How much wattage the audio output produced, 20W? Have you tried BC550? BC550 has Vcb of 50V (quite close) but it is low noise. I'm going to build similar amp, soon...

[MBK]

Originally posted by Jay



I think R15 and R22 should be 1K5, right? And R24 doesn't need to be 5W, correct?

How much wattage the audio output produced, 20W? Have you tried BC550? BC550 has Vcb of 50V (quite close) but it is low noise. I'm going to build similar amp, soon...

Right on R15 and R22, should be "k".

R24 - I also don't think it has to be 5W, but in the A30 manual it is in there at 3W. I had a 5 W lying around and space on the board so I put it in.

I had BC549 before. By rating they should have blown but they didn't. Noise wise I can't hear a difference with current setup. The issues that really affected noise were:

- star grounding (and I mean it: 5 mm difference makes a *massive* differecne in noise and hum, by ear)
- input transistor network matching / fine tuning
- that 3.3 Ohm resistor did help in addition to the above.

After fixing that I am left with noise that's practically inaudible. On one channel I can hear some hum but only when the 89dB/W speaker literally touches my ear. On the other channel I can hear it at about 3 cm from driver.

[MBK]

Wattage, depends on speaker. With the ca. 1.8-2 amp bias depending on line voltage (variable in SG, so my rail varies fom 21.5V to 23 V or so) and 50% AC current gain I should be getting about 30 W. At 4 Ohms the current won't quite suffice to double that for 60W into 4 Ohms, but I don't need it in my system.