STC002 Trimodal Power Amplifier Module (Mono) - CML Audio

STC002 Trimodal Power Amplifier Module

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STC002 Trimodal Power Amplifier Module

The STC002 Trimodal Power Amplifier Module is a unique design that is instantly switchable between Class A and Class B. There are no compromises of any kind in either mode.

Available as a fully assembled and tested unit, PCB with test notes only or as a kit of parts, PCB and build notes.

The Trimodal PCB has now been updated so that the latest MT-200 package output transistors can be used. The supply-rejection has been improved so that power supply reservoir capacitors of 10,000uF can be used if required, rather than the 20,000uF originally recommended. The noise performance has been improved, and there have been many other minor improvements.

  • Exceptionally low distortion into low impedance loads.
  • Unique design offers operation in three modes: A, AB and B as required- hence Trimodal Amplifier.
  • Very low distortion in all modes- barely measurable even with top-flight test equipment in Class-A mode.
  • Very low noise, due to innovative feedback design.
The Trimodal PCB is produced to the highest quality standards:

  • Double-sided plated-through-hole fibreglass PCB.
  • Gold-plated pads.
  • Fixing holes and output device positions provided for direct mounting to a heatsink.
  • Solder mask both sides and full screen print.
  • Board size 138 by 130mm.
  • Fully RoHS compliant.



  • The Class-A mode does not (unlike some designs) clip abruptly if faced with an unusually low impedance load. It instead moves seamlessly and without audible disturbance into Class-AB operation.
  • One of the disincentives to building a Class-A amplifier is that a mistake in the quiescent-control circuitry can let through a huge fault current that destroys all the output devices in an instant. The same applies to bias-generators in Class-B, but these are normally much simpler and the chances of getting something disastrously wrong is much less.
    With this in mind the Trimodal amplifier includes a safety-circuit to limit the current flowing even if the Class-A controller is completely non-functional. The safety-circuit consists of very few components so it can be meticulously checked in very little time.
  • The radical change in internal conditions from B to A is accomplished completely silently, while the amplifier is operating. Direct comparison between the two modes is extremely simple.
  • The Trimodal principle allows you to experiment with Class-A with confidence. Many constructors of Class-A amplifiers find that the resulting unit gets worryingly hot, even if the heatsinks are enormous compared with a Class-B design of the same power. The Trimodal allows you to experience the highest grade of Class-A reproduction without committing yourself to it. The alternative of superb Blameless Class-B operation is always available at the touch of a switch.
  • Optional input bootstrapping to raise input impedance.
  • Full overload and short-circuit protection.



  • Class A power output: 25W into 8 Ohms with +/-24V supply. Assumes adequate heatsink.
  • Class B power output: from 25W to 100W into 8 Ohms, depending on supply voltage. Assumes adequate heatsink.
  • Power supply: +/-24V in Class-A mode, up to +/-40V in Class-B mode*
  • Distortion: Class-A. Less than 0.0003% at 1kHz, 20W/8R. (20kHz BW) Less than 0.0015% at 10kHz, 25W/8R. (80kHz BW)
  • Distortion: Class-B. Less than 0.0006% at 1kHz, 25W/8R. Less than 0.004% at 10kHz, 25W/8R. (80kHz BW)
  • Noise out: Less than -95 dBu (22-22kHz bandwidth, RMS, source resistance 50R)
  • Output DC offset: less than +/- 25mV
  • Gain: 23 times (+27.2 dB)
  • Frequency response: +/-0.1 dB, 10Hz – 20kHz
  • Input impedance: 2.2k without bootstrapping, 10k with bootstrapping option.
* The Trimodal PCB will actually operate correctly in Class-B mode from supply rails of +/-3V or lower, so initial testing can be done at low risk by slowly winding up a variable transformer. It will not of course meet its full specifications at this voltage.

Frequency Response

This graph shows the amplifier THD when running from the minimum supply rails. Performance at higher supply rails is slightly improved.
Measurement system Audio Precision SYS-2702. Measurement bandwidth 80 kHz




01 » STC002 – FRONT VIEW

  • STC002 Tri-Modal Power Amplifier Module FRONT


02 » STC002 – REAR VIEW

  • STC002 Tri-Modal Power Amplifier Module FRONT


03 » STC002 – PCB

  • STC002 Tri-Modal Power Amplifier Module FRONT


Product Order Details

STC002 Trimodal Power Amplifier Module Options and Price List
Reference Quantity Price (£) Description Order
STC002PCB 1+ £27.50 PCB and build notes only – MONO. Contact CML Audio »
STC002KIT 1+ £125.00 Kit of parts, PCB and build notes – MONO (excludes case). Contact CML Audio »
STC002BLD 1+ £185.00 Fully assembled and tested unit – MONO (excludes case). Contact CML Audio »
STC002UNIT 1+ £900.00 Fully assembled and tested unit – STEREO (includes case). Contact CML Audio »

E&OE: While every effort has been made to ensure all information supplied is accurate, no liability is accepted for the consequences of any errors or omissions. All products are subject to availability. All prices are quoted in (£) Sterling and unless otherwise stated, are exclusive of VAT. CML Audio reserves the right to change the product range and prices without prior notice.



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