Facts and Figures

The 25 Watt MosFet Audio Amplifier re-tested recently

after 15 years of use

25 Watt MosFet Audio Amplifier
One of the first prototypes of the 25 Watt MosFet Audio Amplifier
Complete Stereo version built in 1992

Undoubtely, the 25 Watt MosFet Audio Amplifier page, is by far the most visited of this site, and is on-line since March 1999. The circuit has been built by many amateurs all around the world and is still very popular: as a logical consequence, in these years this small amplifier was frequently debated in Audio forums and rumors arose about its quiescent current stability and other topics.
Eventfully, I was now able to get and carry to my laboratory one of the first prototypes of the complete Stereo Amplifier I built in 1992 for a friend, so I was very anxious to perform extended tests on this amplifier after 15 years of use.
The tests results, with some added comments are shown below.

25 Watt MosFet Audio Amplifier interior
Overview of the 25 Watt MosFet Audio Amplifier interior


As shown, a single board with 2.54mm spaced holes was used to host the parts of both channels and power supply, except mains transformer, switches and input/output sockets. All part values correspond exactly to those given in the Parts list, but a 3.15 Amp fuse was added in series to each amplifier output (for problems possibly caused by these fuses see below).
A 6.3mm stereo headphone socket and a loudspeakers excluding switch was also added, plus an input switch array formed by two DPDT toggle switches, allowing to choose from three audio reproduction devices, namely: Moving Magnet Phono Cartridge, CD player and Tape Recorder.
Left to right on the front panel there are: Power switch - Pilot LED - Headphones socket - Loudspeakers excluding switch - Moving Magnet Phono Cartridge/CD switch - Tape monitor switch - Volume control.
The rear panel hosts four 4mm "banana" sockets for loudspeaker outputs and eight RCA audio sockets, six for the various inputs and two for Tape output.
The small board directly fixed to two input RCA sockets visible bottom right in the above photo, hosts the Phono Preamplifier also described in these pages.
The four Mosfets were bolted to a single heatsink, obtained from a U-shaped 2mm tick aluminium profile of 195x30x20mm bolted to the board and to the case bottom. Plenty of silicon grease was spread on the heatsink and case bottom surfaces in contact to facilitate heat dissipation.
Q6 and Q7 were not fitted with heatsinks, but the addition of small, U-shaped types, is recommended.

25 Watt MosFet Audio Amplifier interior: front detail
25 Watt MosFet Audio Amplifier interior: front detail

Output power

The original specifications rated the Output power of the amplifier well in excess of 25 Watt RMS into 8 Ohm (1KHz sine wave). This was a rather pessimistic value, as measured power output for this prototype was about 45W (left channel) and 42W (right channel) respectively.
This very good performance is mainly due to the oversized 120VA power transformer: in fact, the dc supply voltage (±36V in stand-by) is still ±33V at 45W output.
In any case, when both channels are driven simultaneously, the dc voltage supply rails fall to ±30V at maximum output and each amplifier is still able to deliver about 40W.
Measured max output power into 4 Ohm load (one channel driven): 72 Watt.

1KHz sine wave into 8 Ohm
1KHz sine wave into 8 Ohm
10V/div Vert. - 0.5mSec/div Hor.
52V pk-to-pk - Output power: 42 Watt

Total Harmonic Distortion

After the first THD measurement (1KHz @ 1W) was done, a consistent difference was noticed between channels: the left channel showed about the same figure specified in the Technical Data section (0.006%) but the right channel measured 0.03% (5 times higher and almost only third harmonic). A brief investigation led to the discovery that the responsible for this worsened performance was the output fuse. Therefore, both output fuses were bypassed and final results show a small improvement also in the left channel, particularly at high power outputs. Complete THD measurements are shown below and are very close for both channels.

Total harmonic distortion @ 1KHz:
0.1W 0.015% 1W 0.006% 10W 0.004% 20W 0.005% 30W 0.008% 40W 0.015%
Total harmonic distortion @ 10KHz:
0.1W 0.02% 1W 0.014% 10W 0.018% 20W 0.023% 30W 0.058% 40W 0.09%

10KHz square wave
10KHz square wave into 8 Ohm and 10KHz square wave into 8 Ohm + 2µF
5V/div Vertical - 50µsec/div Horizontal

Square wave performance is good and only a small ringing is present when the 8 Ohm load is bypassed with a 2µF polyester capacitor.

Quiescent current stability

The quiescent current of each channel was measured at first. Its behavior was as follows: after power was applied, the quiescent current raised in a few seconds to about 300 - 350mA, then stopped and started to decrease slowly. After about 10 - 15 minutes the current locked to about 100 - 105mA: this value was kept firmly and was always measured when the amplifier recovered to stand-by status after prolonged use and testing at different power outputs. Therefore, no quiescent current re-adjustment was necessary.
Measured dc output offset voltage never fell below -30mV or raised above +20mV for both channels.

Conclusions and advices

After 15 years of operation the Stereo version of the 25 Watt MosFet Audio Amplifier was found still in good health. Some small (and inaudible, I think) THD worsening was measured in one channel, but the cause was quickly detected and removed.
On the other hand, some audible scratches and unexpected noise due to ageing was generated by the rotation of the (carbon type) volume control. This problem was remedied with spray contact-cleaner oil.
Therefore, in order to build a high quality apparatus, please follow these directions:

All these improvements can be seen in the photo below, where a Stereo version of the 25 Watt MosFet Audio Amplifier, built by the German Audio enthousiast Friedhelm Prinz using the LedeAudio Kit is shown. As can be seen, large heatsinks, an excellent film plastic stereo potentiometer and gold plated connectors were wisely employed.

25 Watt MosFet Audio Amplifier Kit built by Friedhelm Prinz
Stereo version of the 25 Watt MosFet Audio Amplifier built by Friedhelm Prinz
Parts and pcbs come from LedeAudio Kit

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January 2007 ©Flavio Dellepiane