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Old Today, 12:27 PM   #1
juma is offline juma  Germany
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Join Date: Nov 2005
Location: Berlin
Default Cubie3

This project started few months ago when I found a nice R-core transformer in a pile of electro junk at the local flea-market (60V CT secondary, 200VA). With such a voltage/power rating the logical choice was 2 x 50W AB class amp. Lateral MOSFETs (connected as Source followers) were chosen as output devices this time too.

The input stage needs to provide more gain than in Cubie2 so I chose BA3-like topology with cascoded JFETs and current mirror load on them. Q5/Q8 still make a base-driven transconductance gain stage which enables GFB/NoGFB games. I chose the loop.

Vbe multiplier (Q13,R20,P1) should not be in touch with the heatsink (Laterals know how to take care of themselves), its only job is to keep the Vgs (and Id consequently) of the output stage at fixed value no matter what are the temperature/current conditions on BJTs but it can be replaced with LED or any other Vref of your choice (for Id of 200mA I need total OS Vgs of 1,3V which roughly equals the voltage drop on two 1N4148 diodes connected in series).

The current through Q1,Q2,Q9,Q12,Q3,Q6 will depend on JFETs' Idss and it can be anything from 3 to 10 mA. The current through Q4,Q5,Q7,Q8 shouldn't be higher than 4-5mA and it can be set with value of R4,R6 (the higher their value, the lower the current - and vice versa).

P1 sets the Iq in the output stage (my heatsinks can't take more than 200mA per channel comfortably, but you can go as high as your heatsinks permit you - you'll have wider cl. A regime) and the P2 sets the DC offset at the output - start adjusting with both pots at half of their value.
The gain of the amp is about 23dB, but it can be altered by simmetricaly changing the R13/R14 and R11/R22. C1,C2 will allow for about 100kHz bandwidth.

Signal-ground and power-ground are led to star-ground by separate wires.

Final result sounded good enough to retire the excellent Cubie2 (all the finesse is stil there but Cubie3 wins when it gets loud).
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Old Today, 01:53 PM   #2
Berny is online now Berny  Belgium
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Join Date: Feb 2006
Location: Brussels
Nice, so an AB beating the Cubie2?! Allthough I don't believe I need the extra watts, it certainly is the amp I have been looking for! Girlfriend is not going to be very happy, my Cubie2 is barely finalised... it will be more an end of the year project with some luck.
I do like what you did for the offset regulation with P2, can I do this too? (replace R12 =120ohm in Cubie2 by maybe 100ohm + trimmer) The layout of my PCB already forsees fixed resistor at the sources of 2SK170/SJ74, not yet used (as I do not like so much the trimmer in the feedback loop). A correction concerning my Cubie2: it is actually running at 860mA /channel @ +/-19V so in the end it is in total a little 65W stove.
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Old Today, 01:57 PM   #3
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Join Date: Feb 2012
this is what i call progress...(from cubie2 thread)
excellent juma.

maybe the pcb layout need a better arrangement.

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