4 ohm taps on an 8 ohm speaker

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

Bruce (Bifwynne), looks like you and I and Anthony (Clio09) were all responding at the same time. Great minds think alike! The one very slight qualification I would make to your post is that the reference to higher frequencies being less augmented relative to the bass when the speaker is driven by an amp having lower output impedance depends, of course, on how the impedance of the particular speaker varies as a function of frequency. Although the impedance behavior implicit in your statement is a common one.

Best,
-- Al

Tmcclintock, thanks for including the links in your post. My suspicion is that the main reason you have found the 8 ohm tap to provide the best results is simply that it is the tap that is the closest match to the speaker's impedance, at all frequencies. Therefore it is probably the tap which results in the lowest amount of amplifier distortion, and that also maximizes the amp's power capability.

I note that the speaker's impedance, while varying widely as a function of frequency, is always at least 6.5 ohms. And the amplifier's output impedance on the 8 ohm tap is not much over 1 ohm at any frequency, which is low for an amp having a tube output stage.

Therefore the effects on tonal balance of the interaction between amplifier output impedance and variations of speaker impedance as a function of frequency, which I referred to earlier in the thread, figure to be relatively insignificant in this case, because the speaker's impedance, while varying considerably, is always much higher than the amplifier's output impedance.

And the relation between the relatively low output impedance (for a tube amp) of the 8 ohm tap and the speaker's impedance at bass frequencies apparently (based on your findings) results in adequate bass damping for the particular speaker. Presumably helped by the fact that the speaker's impedance rises to very high values in the 60 to 120 Hz area.

So I don't find it surprising that you've found the 8 ohm tap to work best with your particular combo.

Regards,
-- Al

Does it make sense that because the amp's output impedance starts relatively low on the 8 ohm tap and doesn't have a lot of room to go lower on the other taps, the sound quality would vary less from tap to tap relative to higher output impedance amps, for a given speaker?

Yes, with respect to the sonic effects that may result from the interaction of amplifier output impedance with variations of speaker impedance as a function of frequency. And yes, with respect to damping factor. Especially in the case of speakers such as yours where the impedance does not reach low values at any frequency.

However, if a mismatch between the speaker's impedance and the load impedance a particular tap is designed to work into is severe enough there are still likely to be adverse effects on the distortion characteristics and the power capability of the amp. In general, what constitutes "severe enough" is probably hard to predict with any precision.

Best regards,
-- Al