Lower transconductance parts have proportionally lower capacitances and a better high frequency response but conversely they have a weaker bass.
For a ZEN amp with only one single device a higher gm of 16 would be better and 6 is really on the low side. Too low and the ZEN amp may sound too bright, too high and it may sound dark and rolled off. This is off course subject to personal taste. In my ZEN V4, I am using a MOSFET with a gm of 32 (all at the much higher "rated" drain currents off course) and still find it pretty well balanced.
For a ZEN amp with only one single device a higher gm of 16 would be better and 6 is really on the low side. Too low and the ZEN amp may sound too bright, too high and it may sound dark and rolled off. This is off course subject to personal taste. In my ZEN V4, I am using a MOSFET with a gm of 32 (all at the much higher "rated" drain currents off course) and still find it pretty well balanced.
MRupp said:
Thank you for your reply. Oddly one of the more popular devices used by Nelson Pass is the IRFP240 which has a transconductance of 6.9.
While reading the various articles on the PASSdiy site He (NP) speaks of the right values but does'nt mention what they might be.
I have a choice of two devices for a MINI-A;an irf231 which is tagged with a value of 6. The other is the irf044 which springs to a value of 16.
As you, noted the device with the lower value has an input capacitance at 800pf. thje other is twice that value.
This is probably in the nit pickin' category but since I have a choice , I just thought to inquire.
I am quite happy to get corrected by Nelson Pass. However, if I understand this correctly, he uses IRFP240 in designs where he parrellels several devices, probably even more than he needs purely from, say, power dissipation considerations, to the point where the design is optimally "balanced"?
P.S. I stand by my recommendation of the "16"er for a ZEN amp. If you have several devices in parallel - and I don't know how many that would be in the Mini-A - you may consider the "6"er device.
Man, you're quick. Yes, I think total transconductance increases with parallelled devices. The tranconductance curve is not linerar, it is pretty steep towards the lower current end where you are usually using those devices (they are rated in the tens of amps) and flattens out with increasing current. Consequently, 4 parrelleld devices running at 1 amp bias current each will have a higher combined transconductance than just one such device run at 4 amps.
MRupp said:
The MINI -A is a scaled down Aleph which has been Nelson Pass' flagship design. It is a two stage setup with a special CCS and in most instances has paralleled devices. Unfortunately as drawn the MINI has a single device in the output. I chose the device with the 6 rating because of its low input capacitance and its TO-3 configuration(I happen to have some nice sinks drilled for TO-3s but they only carry two devices each,the number per channel for the MINI). One of the devices is for gain the other for the CCS. Therefore paralleling is out of the question.
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