Here are some more guides to using and setting up the 300W power amp.


**  When setting the trim pot (2k2) variable resistor, first switch the amplifier off, remove the 5A fuses and solder a 10 ohm resistor across the fuse holder. Switch on and adjust the trimpot until 1 Volt is measured across the resistor. Check that it is approximately the same across both resistor/fuse holders, for +ve and -ve rails. If the trimpot cannot get the voltage up to, or down to 1V, you may need to change resistor R21 or R22 to a 5K6, depending on whether the range of the trimpot is too low or too high. I normally don't have this problem when using 1% tolerance resistors, but it usually happens with 5% devices. Finally the 50Hz buzz I mentioned in my earliewr visit to the site can be eliminated entirely by careful grounding. eg for an input volume control, connect the case of the volume pot to the power supply 0V. This amp usually likes 0V connected to Earth as well, dependent on what you're hooking up to the input. A rule of thumb here is to Ground the AMP, and leave all the input goodies floating (not earthed). Have fun, these AMPs deliver. I've also run these AMPS in bridged mode - Some advice here is to use screw terminals for the output connections, they can get so warm with this power they de-solder themselves!!

Shawn O'Neill

Hi Sam
The circuit diagrams on your site are exhalent, I have found some very
useful information.

I am building a full set of active loud speakers for my home Theatre system
including the active sub in the same box but in a separate compartments.
Your 300 Watt amp will work perfectly , I want to build 1 board to se if it
meets my needs as my theatre room is  rather large 6m x 8m and 2.6m  High.
Do you possibly have the PCB layout but separate views of top tracks and
bottom tracks of the double sided board. Then I can have a board made up.
Please let me know if it is possible, and once again your site is exhalent.
For amateurs like me it is a grate help!!!!

Ivan Cummings
Technical Adviser Electrical
Delta Motor Corporation

Dear Sam,
I built the 300Watt amplifier on your site ( Though the english description is not very clear, the circuit is almost self-explanatory. With minor modifications, built dual monoblocks without a single hitch in the assembly/fabrication.

My modifications are:
(a)designed my own single-sided pcb layout.
(b)used MJ15024/MJ15025 (2 pairs)as output transistors.
(c)eliminated the output filter (L1, C15, R46). I do not intend to use electrostatic speakers. 
(d)using individual torroid transformers (900VA). I first used a 350VA transformer, which was dropping some voltage at high volume.
(e)10,000 MFD X 4 capacitors for each channel.
(f)eliminated the soft-start components(Rx, Tx, RL1)on the mains-transformer primary. Also removed the associated circuitry on the protection board. The amps are running for more than a month, without any problem.
(g)added an MOV on the mains transformer primary after the fuse (voltage fluctuation is high in my area).

The final result: The sonic quality is superior to anything I've heard! My comparison is with 2 commercial amps (Adcom PA555 and a Marantz Integrated). Incredibly neutral, vividly trasparent and smooth. Handles any of my speakers without effort. Absolutely no switch-on thump, even without the delay circuit.

In my opinion it is a real high-end audiophile design - both for the ease of building/setup and the sound quality. My deepest thanks for sharing this design.

One question: Do you have a preamplifier design that is equally high quality? I am presently using it with an Adcom analog preamp, but wish to make my own.

With best wishes,