First Watt: current vs voltage

Hi All,

I'm interested in giving First Watt a try. I have Audio Note AN-E speakers, which are efficient, I like SET sound, and I'm intrigued by the approach of First Watt.

However, in man places I read that First Watt are current amps (in opposed to voltage amps) and thus they can only be used with single drivers speakers.

Is this the case for all of them?

Is there a model that will work well with my AN-E?

Thanks, Guys!!! I have read the 6moons reviews, but somehow missed that only F1 and F2 were current sources.

Liguy and bombay walla, your argument makes sense to me. However, I know very little about electrical engineer. :-) I trust people who actually know when they say the current source amp do not work with cross-overs.

F1 and F2 are the two current source amps. I would recommend reading the 6moons reviews to get a good feel for each model. A First Watt dealer can also be of great help. Mark at Reno HiFi is a great guy and has an excellent return policy so you can see what works best in your system. He usually has most models in stock.

The First Watt amps are great. I have tried most of the F models, with the current source models (especially the F2J) and the F3 being my faves. But of course this is with my speakers and my listening preferences...
Roscoelli is right, 6 moons has extensive reviews on every First Watt model and compares/contrast each one.
The current issue has a complete review of the latest S1(SIT) First Watt amplifier.
The equation for Power is Voltage times Current. Audio amplifiers are power amplifiers. Speakers only care about the power delivered to their load. Amplifiers are rated in a maximum power delivered into a specific load. The amplifier only cares about the load that it needs to drive it's power into. The First Watt amps could care less what type of speaker they are connected to as long as the load is 4 or 8 ohms and that the load is fairly linear. Most of the First Watt amps are lower power between 5 and 25 watts. What you should be concearned with is speaker efficiency whether it is a single driver speaker or multiple driver speaker. The First Watt amps are the solid state equivalent of SET Tunes amps and indeed they have a similar sound. Single driver speakers happen for the most part to be very efficient. Nelson Pass happens to enjoy tinkering with single driver speakers in horn configurations but make no mistake they can drive any 8 or 4 ohm speaker with an efficiency greater than 92 dB at pretty good levels.
However, in man places I read that First Watt are current amps (in opposed to voltage amps) and thus they can only be used with single drivers speakers.
this makes no sense at all!! How did you conclude the 2nd part of your sentence from the first part??
Liguy has basically hit the nail on the head - all First Watt amps are power amplifiers. The First Watt amps seem to be low wattage, high(er) current amps made with a certain type of speaker load in mind (& that speaker load seems to be what I would call a benign load i.e. load impedance remains fairly flat 20Hz-20KHz & the phase shifts in that same freq range are minimal & not wild excursions from positive to negative & vice-versa).
I have a feeling, like Liguy, that the exclusive single-driver application being touted comes from the fact that single-driver speakers (which have no xover if there is no super tweeter) are benign loads. These amps should be able to drive any benign load speaker.
Liguy and bombay walla,

Have you actually looked at the details and reviews of the F1 and F2? These are unique designs that do not work with speakers with crossovers. If you want to contradict Nelson Pass, fine by me. But take a look at the designs and the manuals and schematics that First Watt provides, or read the reviews which also explain this.
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:
It is not mandatory to use such these. You may get good results with other types.

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."

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.

Stan, sort of.

For a given input voltage an ideal voltage source will maintain a constant output voltage no matter what the load i.e. no matter how low the load impedance goes and no matter how much current the load draws. This requires the amp to have extremely low output impedance, ideally zero. Monstrous power amps like Krells are closer to these ideals than the Quad but they are both considered voltage sources.

Current sources are an entirely different beast.

For a given input voltage an ideal current source will maintain a constant output current no matter what the load. This requires the amp to have extremely high output impedance, ideally infinite. The First Watts are not that high with an output impedance around 80 ohms but that is 10 times most loads so they act more like current sources.


Good discussion.
I've seen a particular modder design series crossovers for Magnepan panels.
Makes me wonder if this would be the way to go with higher powered tube gear, like the ARC stuff, which I sometimes seen mentioned as playing well with Maggies.
The old / original crossover for the MMG (maybe the SMG) was a series crossover.

What about just putting a couple ohms in series with my amps outputs? Would that work? Carver did it a couple years ago......
What Carver did was supposed to make the amp sound like a current source but it didn't actually make it one if I remember correctly. I don't believe adding resistance will do anything except possibly change your crossover and burn up more power driving the resistors.
I guess I was misled by the 405 output pattern, which was, according to a test in HIFI Choice no. 14: 115/90/32 on right channel and 115/81/28 on the left at one Kz at one percent distortion for 8/4/2 ohms. The Sansui 919 in the same issue did 145/217/288 and 164/210/264. The 8 ohm power is fairly close but the pattern is quite different. They commented " Steady state drive into low impedences was very well maintained, and on toneburst approched the theoretical 'voltage source' ideal. Since the 405 was so different I assumed it was not a voltage source amp.