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. :)

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

Right on Al. Actually, what I had in mind was a speaker with low'ish impedance in the bass/lower midrange frequencies, with a peak at the mid/tweeter crossover point. As I'm sure most would agree, matching a high'ish output impedance tube amp to that kind of load would tend to attenuate bass and augment the mids and low treble.

I would add one more point. Current delivery capability, albeit a subset of my comment about an amp operating within its design parameters.

More specifically, many speakers not only have low'ish impedances in the bass frequency region, but also capacitive (negative) phase angles. In other words, tough loads. Hopefully, regardless of the tap used, the amp in question will have enough power supply reserve to muscle its way through the tough spots.

Best,

BIF

Thanks a lot for the explanation Al, you ARE the man!!!
I think you nailed it with the first line in your last paragraph: "The bottom line: It pays to experiment with the different taps, and no harm will result regardless of which tap is selected."

Cheers,
John

Thanks for the responses gentlemen. Interesting stuff. Glad to hear I'm not hurting the amp. It does sound much better on the 4 ohm taps.

I've read through most of the tap vs. tap threads and get the gist. But my results from trying 8 and 4 ohm taps would seem to contradict the predictable, at least as I hear it. Maybe just proves "The bottom line: It pays to experiment with the different taps, and no harm will result regardless of which tap is selected."

Just curious if any of you have insight on why I hear better base, maybe a little tighter, and less emphasis on highs using the 8 ohm taps. Here are my speakers' impedance and phase vs. frequency graphs: Minuetto, and my amp's measurements : RM 200. I've read that early SF speakers were developed using tube amps, but not sure if that's true or even applies, especially to the low-end ones.

The sound quality difference is not huge, but still there. True will all music types played at moderate volume in a medium-smallish room (so not pushed to clipping) Also, I haven't tried 1 or 2 ohm taps, but figured why bother if 8 is preferable. Any insight would be appreciated.

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

Wow Tmcclintock .... and I thought my speakers had wild impedance and phase angle curves. :)

Thanks for responding, Al. That's pretty much what I thought but wanted to be sure I understood the science. Also, 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? May try the 2 and 1 ohm taps just for grins.

Bifwynne, can you say "off the chart" - literally. Just one of the trade-offs, I'd guess.

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

Hey TMcClintock .... wild curves??? Who cares? If your rig sounds good, that all that counts. Enjoy!!

Al - Got it. Thanks again.

Bifwynne - Too true.

Later,
Terry