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Pass A40 Power Amplifier
INTRODUCTION
This is the amplifier I am using now to drive my Magneplanar MGLR-1's. Some have emailed me regarding the substitutes and available options. The MPSL 01/51 can be replaced by MPSA 42/92. I have also used the TIP 142/147 complementary darlingtons for the output stage as they are cheap and easy to obtain. This amplifier is quite easy to construct. It does not require any soft-start circuitry nor output relays, nor any other form of protection for that matter.
DESCRIPTION
The amplifier is built in a single enclosure. A 330VA toroidal transformer from Avel feeds four 12000uF electrolytic capacitors from Mallory. The rectifier is a conventional 35A bridge rectifiers. A thermistor is used to lower inrush current.
The pcb is relatively easy to assemble. I have replaced the JFET current source with a 10 kohm resistor as suggested by Nelson Pass. The voltage rails are about +/-33v, this translates to about 60 wpc. The output stage is built on a seperate pcb mounted onto the heatsinks. I have modified my Vbe multiplier to make it adjustable, as the original circuit runs extremely hot. I made each channel idle at 60 watts, appropriate for my power supply and heatsinks, not to mention electricity bills! The amp need at least 1 hour to warm up fully, longer if in a cool environment. The amplifiers are running flawlessly, and the fuses are properly sized to prevent my treasured Maggies from damage, at the same time large enough to prevent current hogging.
PERFORMANCE
These amplifiers really excel at transparency in the mids and highs. So good, in fact, that I am able to hear the preamp's residual noise through the speakers at close range! They are balanced more towards the bright side, so I used a resistor to damp the Maggies' tweeters. They also like to be driven hard, but at higher volumes into the current-hungry Maggies, begin to sound aggressive. I wouldn't say they are a perfect match with the Pass Bride of Zen preamp either, as the input impedance is relatively low at 40 kohms, some of the amps' character is influenced by the preamp.
PICTURES
Power transformer & thermistor
Front end boards mounted on back
If there are any questions or comments, please feel free to email me.
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