4 ohm taps on an 8 ohm speaker

I've also noticed the same thing on my VAC PA 35/35 amp. My speakers are a nominal 8 ohm load, but they sound so much better attached to the 4 ohm taps. Kevin Hayes of VAC has always been very encouraging of experimenting with the different taps. He says that many times the amp will sound best when set to the speakers minimum impedance, and not their nominal impedance. He doesn't seem to have any concern as to how this may damage the amp or speakers. I am glad that I experimented, because the improvement in sound is certainly a nice payoff.

I have been told that the lower ohm taps will drive more current, whereas the higher ohm taps will drive more voltage. I don't know if there is any truth to that though. Maybe our local electrical hero Al (Almarg) will drop in and enlighten us on this phenomenon.

07-03-14: Jmcgrogan2
I have been told that the lower ohm taps will drive more current, whereas the higher ohm taps will drive more voltage. I don't know if there is any truth to that though. Maybe our local electrical hero Al (Almarg) will drop in and enlighten us on this phenomenon.

Thanks, John :-)

That's true, although there are a number of other factors which also come into play when comparing the two taps with a given speaker.

Usually a tube amp having 4 ohm and 8 ohm taps is designed such that its maximum power capability when driving an 8 ohm load via the 8 ohm tap is the same or similar to its maximum power capability when driving a 4 ohm load via the 4 ohm tap. For a resistive load:

Power = Voltage x Current = (Current squared) x Resistance

= (Voltage squared) / Resistance

From that it follows that delivering the same amount of power into an 8 ohm resistive load connected to the 8 ohm tap as into a 4 ohm resistive load connected to the 4 ohm tap requires 1.414 (the square root of 2) times as much voltage on the 8 ohm tap as on the 4 ohm tap. And it requires 1.414 times as much current flowing into the 4 ohm load connected to the 4 ohm tap as into the 8 ohm load connected to the 8 ohm tap.

Also, since the output transformer transforms impedance in proportion to the square of its turns ratio, while transforming voltage in direct proportion to the turns ratio, it follows that the output impedance of the 8 ohm tap will be approximately twice what it is on the 4 ohm tap. Which means that for a **given** speaker impedance the damping factor (which is inversely proportional to output impedance) will be twice as high for the 4 ohm tap as for the 8 ohm tap.

To address Grimace's questions:

1)No, you will not hurt the amp by using the 4 ohm tap with your nominally 8 ohm speakers. That is often done, and as John indicated it generally pays to experiment. And the comments by Kevin Hayes which he referred to are of course correct.

2)I couldn't find an impedance curve for your speakers, but of course the impedance of most "8 ohm" speakers varies significantly as a function of frequency. So the 4 ohm tap may be a better match for the amp at some frequencies than the 8 ohm tap.

Also, as I indicated above, damping factor will be greater when using the 4 ohm tap with a given speaker than when using the 8 ohm tap. That figures to be a major reason for the better defined bass you noted.

Also, tonal balance will be affected by the interaction of amplifier output impedance with the speaker's impedance variations as a function of frequency. That can be thought of as a voltage divider effect occurring between the amplifier's output impedance and the impedance of the speaker at a given frequency. If the speaker has been voiced with the expectation that it is likely to be used with a solid state amp (nearly all solid state amps having negligibly small output impedances, amounting to a tiny fraction of an ohm), the effects on frequency response of that interaction using the 4 ohm tap would come closer to the interaction that was intended by the designer than would be the case with the higher output impedance of the 8 ohm tap.

Finally, distortion introduced by the amplifier will be affected by how well matched or mismatched the tap and the speaker impedance are at various frequencies. How that nets out subjectively, in cases where speaker impedance varies widely as a function of frequency, can be expected to not have a great deal of predictability.

The bottom line: It pays to experiment with the different taps, and no harm will result regardless of which tap is selected. Assuming, of course, that volume levels are not turned up to the point that the amp is asked to deliver significantly more power than it is capable of, in which case it would most likely clip and distort very noticeably.

Best regards,
-- Al

Roger Modjeski explains his reasoning behind this in the RM-10 manual. He refers to it as light loading. You essentially are increasing the damping. It also reduces distortion, and improves tube life as the output tubes do not have to work as hard. In the case of the RM-10 it will reduce power about 20% but given that there are 35 watts to start with (first 15 in Class A) the trade off is worth it and Roger recommends this approach with all his amps. The quote below is from Roger:

Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB.

In addition to the RM-10 I tried it with a set of VAC Auricle Musicblocs and had similar results. So I would encourage anyone with a tube amp that has multiple taps to try it.

Hopefully Al **will** chime in. But in the interim, much has been written about this subject. Before I go off the techno-babble deep-end here, let me say that I am dubious that trying different output taps will harm a tube amp. In most cases the conventional wisdom is to use the taps that sound best ... to you.

Al ... I started to explain my understanding of nominal tap output impedance, back impedance matching, output voltage regulation, and so forth, but started writing a magnum opus. So please ... chime in.

Let me just say that if a tube amp is able to produce clean power off the 4 ohm taps while it is driving a speaker whose impedance likely varies, possibly quite considerably as a function of frequency, then I would expect that bass will be tighter and extended, and higher frequencies less augmented.

The reason is that in most cases the "apparent" output impedance of the tube amp will be lower off the 4 ohm taps, thus resulting in less output voltage variation. I want to emphasize that this assumes the amp is being operated within its design parameters, i.e., not being over-driven.

In other words, the tube amp will perform somewhat like a Voltage Paradigm amp (almost always a SS amp). Most SS amps have extremely low output impedance and present a constant voltage source to the speakers if operated within their operating parameters.

Now ... what I just wrote assumes that the speakers in question were designed and voiced to be driven by a SS amp. In such cases, tube amps having lower output impedances and therefor tighter source output voltage regulation will possibly do a better job driving such speakers off the 4 ohm taps ... **IF** the amp is able to deliver clean power.

By contrast, if the amp is overdriven, the impedance mismatch between the speakers and the amp will result is attenuated power delivery and increased distortion which will likely sound bad. Unless, one likes to listen to tube amps while they clips and produce lots of funky distortion. :)