First Watt: current vs voltage

Liguy gives a great explanation of what the First Watt amps are about. Circling back to the original question, except for the F1 and F2, the big limitation with First Watt amps is their power output, which tops out at about 25 watts per channel, with no additional headroom, depending on model. I have an Aleph J, which is a wonderful amp, and which drives my very conventional, not-single-driver, loudspeakers to reasonable levels. Nelson Pass designs actually do sound different, and IMHO better, than many other amps. If you play music at moderate levels, you'll probably never notice the power limitation. I attached a power meter to the Aleph J and have never needed an output above 15 watts, peak. But note that my system is in a relatively small room.

I second the suggestion that you talk to Mark at Reno Hi-Fi, who can probably suggest the right First Watt amp to compliment your system.

Here is the recommended xover types for current amps:
http://www.firstwatt.com/pdf/art_cs_xvrs.pdf
It is not mandatory to use such these. You may get good results with other types.

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Liguy and Bombay, you are incorrect. There is a night and day difference in the way current and voltage source amps operate. They are not just power amps.

Some of the First Watt are current source amps and some are the more common voltage source amps. I think Mr. Pass screwed up to mix the two topologies in one line of amps since it leads to confusion like this.

The current source models will not work with most speakers. They are not the equivalent of SET tube amps as the latter are voltage source amps. The need to use them with single drivers has no relationship to the fact that most of these speakers are efficient even though being lower power does make a good match with them.

From the F1 manual

"This is a very unusual amplifier that will not sound good with about 98% of the loudspeakers on the market. It requires careful attention to loudspeaker loading to get the best performance.

It does not work well with ordinary passive crossover networks – the components and their locations tend to be reversed. It won’t break, but probably won’t sound good."

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A current source amplifier has a very high output impedance- usually multiples of the speaker impedance.

A voltage source amplifier has a low output impedance, usually a small fraction of the speaker impedance.

Either one can sound good, but as far as what speakers work with them has to do with the speaker manufacturer's intention- whether the speaker is designed to work with voltage source amplifiers, power source amplifiers (tubes) or current source amplifiers (usually the latter employ some sort of current feedback which increases the output impedance).

The issue is two-fold: first, the crossover frequencies may not work right as the way these amplifiers behave is very different (an example here is using horns with voltage source amplifiers often results on horns that honk, due to the crossover not working correctly with the voltage source amplifier), and then if there is a port or other means of dealing with the box resonance, that may not work right for the same reason.

This is why equipment matching between amps and speakers is an on-going conversation in high end audio. Now if the speaker is OK with SET amplifiers, it stands a good chance of also working with a current source amplifier. So in this case its worth an audition- see what you think!

I have always seen voltage source attached to amplifiers that are capable of a high output in amps and can increasing their output into lower impedence loads. IE Krell is a voltage source amp, a Quad 405 isn't.At least this is the usage in England that I have seen in reading HF mags the last 40 years.