Current amp vs Voltage amp

I always tell owners of ARC to try the 4 ohm tap. Some like it better some not so much. Note that there aren't many true 8 ohm impedance speakers especially just past the woofer impedance peak. Woofers usually dip to around 3-6 ohms. You might even get better bass control and definition off that tap.

Hi Bruce (Bifwynne),

I've looked at the references you provided, and read your post, and I think that everything you've said is essentially correct.

Basically, your VS-115 has a lower output impedance than many or most other tube amps, on a given tap. The significant amount of negative feedback it uses is presumably a major reason for that. And it can be presumed that the output impedance of its 4 ohm tap is approximately half of the output impedance of its 8 ohm tap.

The lower the output impedance of a tube amp, the more closely the effects of the interaction of that output impedance with the impedance vs. frequency characteristics of the speaker will approach those of a solid state amplifier (which will have near zero output impedance, in most cases).

The one slight qualification I would make to your post (which you probably already realize) is to emphasize that JA's reference to obtaining a flatter frequency response with the 4 ohm tap than with the 8 ohm tap applies to his "standard simulated loudspeaker load," which has impedance characteristics that are quite different than those of your (and many other) speakers. As you probably already realize, for that and other reasons a generalization should not be drawn that 4 ohm taps will inevitably provide flatter frequency response than 8 ohm taps. It depends on the speaker.

In this particular case, though, given that your speakers were voiced for solid state amps, it can be expected that lower output impedance will result in a flatter response, everything else being equal.

Best regards,
-- Al

Thanks Al. One thing that the reports I cited answered for me was the magnitude of the FR variance. I didn't know how to convert the impedance and phase angle data into useable information.

As you can see from both reports, the ARC Ref 150 and VS-115 use roughly the same amounts of NF and have similar output impedance stats. Further, the amps were tested by different people at difference times which gives me some confidence that the data is reliable.

As I mentioned above, it appears that using the 4 ohm tap should yield a better FR result. I think it just might be doing that. Short of having an acoustician do precise measurements, I'll go by my ears. Just have to get use to a different acoustic presentation off the 4 ohm tap.

This post and others like it provides important information for folks who want to use tube amps. I think the take-a-ways are the White Paper Paradigms present the extreme cases. As we can now see, my tube amp doesn't neatly fit into the Power Paradigm category. The other take-a-way is that folks looking to match tube amps with speakers should be somewhat mindful about the speaker's impedance and phase angle swings and their tube amp's output impedance. Of course, there's no substituting for a serious listen.

My thanks to Al and Ralph for hanging in there with me.

I hear better sound quality with useing a current amp versus a voltage amp!, cheers!

A note on terminology: Unless there are some extremely unusual exceptions out there, all audio power amplifiers amplify both voltage and current, so the terminology in the subject line of the thread is a bit misleading.

Solid state amplifiers having near zero output impedance are sometimes referred to as being voltage sources, meaning that for a given input voltage their output voltage will remain constant regardless of load impedance (within the limits of their capability). Referring to them as being voltage sources is accurate terminology, to a very close approximation. (The reason it is a very close approximation, rather than being exact, is that no real world amplifier can have an output impedance that is exactly zero).

Tube amplifiers, having significant output impedances, are sometimes referred to as being current sources, which strictly speaking would mean that for a given input voltage the current they output would remain constant regardless of load impedance (within the limits of their capability). That is NOT accurate terminology, except in the extremely loose sense that their output current will vary LESS as a function of load impedance than in the case of an amplifier having near zero output impedance.

It may be helpful to read this Wikipedia writeup on the voltage divider effect. In the first figure, consider Z1 as representing the output impedance of the amplifier, and Z2 as representing the impedance of the speaker, which will be different at different frequencies.

Regards,
-- Al