Testing output transistors.

[miklos]

I am planing to replace my Parasound HCA855A with some low watt choke loaded amp for driving a set of Mid and High frequency horns.
The reason being that the power needed is less than a watt and the Parasound seem to be noisy ,plus it's a 85W/ch amp. The noise is somewhat less than a millivolt, but I can hear it on the midrange. I will use the inexpensive Hammond 159ZC 60mH 2A choke and use ca. 15V with it. This way I expect ca. 3W clean output by using not more than 30W/CH. To start I was to measure a few different output transistor candidates. I'm using an old HP analyzer ,which is able to measure down to -90dB and using for the signal source either a test CD or an old Bruel & Kjaer beat frequency oscillator's 600 Ohm's output. Here to follow a set of pictures for 1W output with different transistors in a source follower configuration.

[miklos]

The following transistors I tested:
Hitachi 2SJ50
Hitachi 2SK135
IRF240
IRF520
SemiSouth E120R100
Sony 2SJ28
Sony 2SK60
Sony 2SK82

[AndrewT]

What are each of the pics showing?
1W into what?
What else?

[miklos]

Pull your mouse pointer over the pics, the data is in the name of the jpg file.
15V ,2A ,1kHz, 1W, into 8 Ohm, the gates are driven from the oscillator's 600 Ohm output.
The pictures showing the magnitude of the harmonics of the 1kHz signal.

[richie00boy]

Does the oscillator have a 50 or 75 ohm output? I ask as I think the 600 ohm drive to the gate will cause worse performance with the higher gate capacitance devices. May be these higher capacitance devices can actually perform well when driven by a low impedance.

[miklos]

Has 6, 60 and 6000 Ohm's, tested 60 Ohm's, but no difference from 600 Ohm's.
600 seems to be enough for these transistors (source followers). I could later test the LU1014 which high input capacitance, on 60 and 600 Ohm's, I think I have some.

[magnoman]

miklos

Interesting plots. I'm sure you've done them can you post the source generator distortion plot only?

Many thanks
-Antonio

[miklos]

Here's it. I have also made a test tone CDR and played back the 1kHz track on a Philips CD921 player, as a low distortion source. Here's the spectrum of that ,the signal going through the LM4562 opamp and it's output at 3V. To start with I will use this IC to drive the output stage, so the last picture is the Semisouth E120R100 at 1W , 1kHz into 8 Ohms with the CDR source.

[richie00boy]

Nice and interesting work then. CD signal looks amazingly good.

[magnoman]

Miklos

Thank you for taking the time to post the source spectrum.
I had no idea that a Bruel & Kjaer beat frequency oscillator could get anywhere near these low levels. Sorry have to ask, are you sure those are 10db steps ?
Interesting stuff, thanks for the real data.
-Antonio