MosFet Audio Amplifier (25W)

Parts:

R1,R4_________47K  1/4W Resistors
R2____________4K7  1/4W Resistor
R3____________1K5  1/4W Resistor
R5__________390R   1/4W Resistor
R6__________470R   1/4W Resistor
R7___________33K   1/4W Resistor
R8__________150K   1/4W Resistor
R9___________15K   1/4W Resistor
R10__________27R   1/4W Resistor
R11_________500R   1/2W Trimmer Cermet
R12,R13,R16__10R   1/4W Resistors
R14,R15_____220R   1/4W Resistors
R17___________8R2    2W Resistor
R18____________R22   4W Resistor (wirewound)

C1___________470nF  63V Polyester Capacitor
C2___________330pF  63V Polystyrene Capacitor
C3,C5________470µF  63V Electrolytic Capacitors
C4,C6,C8,C11_100nF  63V Polyester Capacitors
C7___________100µF  25V Electrolytic Capacitor
C9____________10pF  63V Polystyrene Capacitor
C10____________1µF  63V Polyester Capacitor

Q1-Q5______BC560C   45V 100mA Low noise High gain PNP Transistors
Q6_________BD140    80V 1.5A PNP Transistor
Q7_________BD139    80V 1.5A NPN Transistor
Q8_________IRF530  100V 14A N-Channel Hexfet Transistor
Q9_________IRF9530 100V 12A P-Channel Hexfet Transistor
R1____________3K3  1/2W Resistor

C1___________10nF 1000V Polyester Capacitor
C2,C3______4700µF   50V Electrolytic Capacitors
C4,C5_______100nF   63V Polyester Capacitors

D1__________200V 8A Diode bridge
D2__________5mm. Red LED

F1,F2_______3.15A Fuses with sockets

T1__________220V Primary, 25 + 25V Secondary 120VA Mains transformer

PL1_________Male Mains plug

SW1_________SPST Mains switch

Can be directly connected to CD players, tuners and tape recorders. Simply add a 10K Log potentiometer (dual gang for stereo) and a switch to cope with the various sources you need.
Q6 & Q7 must have a small U-shaped heatsink.
Q8 & Q9 must be mounted on heatsink.
Adjust R11 to set quiescent current at 100mA (best measured with an Avo-meter connected in series to Q8 Drain) with no input signal.
A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of R1, R4, R9, C3 to C8. Connect C11 to output ground. Then connect separately the input and output grounds to power supply ground.
An earlier prototype of this amplifier was recently inspected and tested again after 15 years of use

Technical data:

Output power:
well in excess of 25 Watt RMS @ 8 Ohm (1KHz sine wave)
Sensitivity:
200mV input for 25W output
Frequency response:
30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz:
0.1W 0.014% 1W 0.006% 10W 0.006% 20W 0.007% 25W 0.01%
Total harmonic distortion @10KHz:
0.1W 0.024% 1W 0.016% 10W 0.02% 20W 0.045% 25W 0.07%
Unconditionally stable on capacitive loads

Info

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.

Layout
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.


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.

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:

Use a high quality, film plastic potentiometer
Do not use output fuses or, if you would provide in any case some protection to the loudspeakers, use high quality fuse sockets, preferably gold plated
Gold plated input and output connectors are also warmly recommended
Use large heatsinks for the output transistors, as the output power is far higher than the nominal 25W/channel specified
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.

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