Since I lost all my post on my transconductance amplifier projects, that ended up in an all N-channel MOSFET version of Nelson Pass First Watt F2 amplifier, I will summarize them here.


The above mess is the first prototype of a transconductance amplifier like the First Watt F2. It came to life after numerous SPICE simulations and chewing through OTA datasheets, Pass papers, and forum posts on DIYAudio.

It all started when I began experimenting with open baffle speakers, and got hold of some vintage SEAS full range drivers. It turned out Nelson Pass had been experimenting a with this kind of driver, and had written a paper on the subject.

Current Source Amplifiers and Sensitive / Full-Range Drivers

In short it seems that some full range drivers will benefit from being driven by a transconductance amplifier. A transconductance amplifier is a voltage to current converter with amplification. Our standard power amplifiers, are mostly voltage amplifiers, and will vary the voltage to the load, according to the input voltage. A transconductance amplifier will vary the current to the load, according to the input voltage. For a purely resistive load, this makes no difference as I=U/R, but a loudspeaker is not a purely resistive load. It is actually the current through the voice coil, that controls the force of the generated magnetic field, not the voltage. Because of this, a variable current source seems the most sensible way to drive a speaker. There is a catch though, since most amplifiers, are voltage amplifiers, most n-way speakers have their crossover designed for voltage drive, and will behave wrong, when driven by current. Nelson Pass has written a paper on how to design filters for transconductance amplifiers instead, I have not studied this very hard, since I am building this for full range drivers.

Current Source Crossover Filters

Nelson Pass had designed both his First Watt F1 & F2 as transconductance amplifiers, since I have a bag of IRF640 N-channel MOSFET's. I ended up modifying the F2 (First Watt F2 schematic), to use only N-channel MOSFET's and added a simple regulator, from Nelson Pass ZEN series.

The resulting sound, was good. Despite the two fans needed to keep the amp from burning a hole in the table, that it was lying on (class A, silver, custom made mains cord, 300B, nuclear reactor in the kitchen, mumble mumble). Despite the crude boxes the SEAS drivers had to put up with. Despite the insane amount of distortion, compared to most amplifiers. I have not tested this, but believe I can hear a change to the better, when driven with this amplifier. I have not yet tried correcting the speaker response as per Mr. Pass papers. I have simply decided that it sounded so well I want to finish it, and play with it some more along the way.

Generated on 2018-05-03 01:14:21.898153