Amplifier circuitry-4 ohm vs 8 ohm

The only place I know where this happens is in tube amps with transformer outputs.

The idea is to keep the power output the same, so the 4 Ohm tap has less voltage, more current, same power as the 8 Ohm.

Each tap is a secondary winding on the output transformer. The primary voltage is transferred to each of the 8-ohm and 4-ohm secondary windings equally. The impedance is set by the turns ratio of wires between the primary and the 4 ohm tap, and the primary and the 8 ohm tap. The relationship is the square root of the impedance so to get an 4 ohm impedance, you wind 1.44 times as much wire to the 4-ohm tap (square root of 2) as you do for the 8-ohm tap. This will set both the voltages the impedances the speakers the speaker sees when connected. 

      YEP (GS beat me to it)!

       Here are two references, that may illustrate/shed more light on the subject:

                                   and (under ’Output Tranny Types’, parameters):

                      https://education.lenardaudio.com/en/14_valve_amps_5.html

The single amplifier at some point splits the signal going into either one of these Ports. What differences are there in the circuitry?

Maybe this will help me better understand the difference between these two taps. I believe 4 ohms is a wider more open path for voltage to flow. So when you’re speaker attempts to go lower, which requires more power, the 4 ohm more easily allows this to happen with a better outcome. Or maybe I got this wrong.

@emergingsoul 

You did.

As pointed out, the one output transformer for each channel has taps in a single winding for 4 and 8 Ohms. If the 4 Ohm tap is used with an 8 Ohm loudspeaker, the power tubes will be loaded at too high an impedance (transformers get their name from the fact that they transform impedance, and that goes both ways) and so will not make nearly as much power as they are supposed to.

If the 4 Ohm tap is loaded with 8 Ohms the transformer will also ’ring’ which is to say it will make distortion of its own. If the amp employs feedback, it might be able to compensate for this. But it will be lower distortion if the transformer is simply loaded correctly- so if an 8 Ohm speaker put it on the 8 Ohm tap!

If a 4 Ohm load is put on the 8 Ohm tap, the transformer will again be improperly loaded and so will the power tubes- they will have a load too low, causing some of the power they make to be dissipated in the tubes themselves, causing them to run hotter! In addition, the output transformer will be rolled off in the highs, although feedback might be able to compensate for that. However the power tubes will make more distortion, so you can see this is a Bad Idea. If a 4 Ohm load, use the 4 Ohm tap!

@atmasphere 

What if the speaker has a nominal impedance of 6 ohms? Which tap (4 or 8) should be used since the load is right in the middle of the two windings?

Wilson has their trained representatives setup their speakers and the Sabrina X list nominal impedance as 4 ohms at 135 Hz and it spends much of the time around 5 ohms. When they set up the speakers they used the 8 ohm terminals and I asked if 8 ohms was the correct terminals and they said yes, but I didn’t understand why they chose 8 ohms. I’ve not changed to the 4 ohm terminals. The MA-12000 has a solid state power amplifier, so I’m guessing that the only downside to this would be the amplifiers working harder for a given SPL.

What if the speaker has a nominal impedance of 6 ohms? Which tap (4 or 8) should be used since the load is right in the middle of the two windings?

@dspringham Try it on both. If you have access to the impedance curve of the speaker, look at the impedance it presents to the amplifier in the bass region. That's where the energy is and will have the most effect on the interaction between the amp and speakers. If it seems more like 4 Ohms than 8, use the 4 Ohm tap...

I guess one has to be verse in understanding the role of current (amps), power (watts) and volts in relation to a 4 and 8 ohm tap.

When using a 4 ohm tap the level of resistance is reduced, which means that less current (ie amps) flows, however, a higher voltage level is needed in order to keep the power ( ie watts) level the same as what comes through an 8 ohm tap. An important concept is that the end resulting power (watts) passing through either the 4 or 8 ohm tap is the same (ie total watts remains the same).

What this means to me is 4 ohm taps are designed to deal with a higher voltage level being delivered to a speaker.

And then the big question is, when certain speakers are designed to handle a higher voltage level, this makes the speakers more efficient.

So if we’re talking about more efficient speakers, it means they are designed to handle higher voltage levels. with power levels remaining the same as less efficient speakers.

And with a higher voltage levels, how does this impact delivery of varying frequencies to a speaker driver?

hoping that I got this right. It’s a challenging concept for a mere CPA

@atmasphere

What you suggest makes sense. However,  I do recall that the late Roger Modjeski (RAM Labs Music Reference Audio) advocated “light loading” of amplifiers. He recommended using a tube amplifier’s 4 ohm tap to drive an 8 ohm speaker load. I’m not sure what the rationale was for this approach.

Charles

The concept of light loading of tubes and transformers would  seemed very important.  Can someone add some clarity to this concept, and how it relates to the overall delivery of information to a speaker driver and/or impact on the amplifiers Health relating to tube operation. Thanks

@charles1dad regarding light loading, which Roger felt benefited all of his amps. the following is taken directly from the Music Reference RM-10 manual:

"The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material. At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes’ rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. 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 some neat design features in the circuit, the way the output transformers were wound also contributed to the benefits of light loading as discussed in the manual. The only negative was a 7 watt loss of power, which is inconsequential if you also consider light loading doubled the class A range of the amp to a little over 20 watts.

@clio09 

Thank you for posting Roger’s explanation and rationale for light loading his tube amplifiers. So it seems the way his output transformers were wound made the difference. Ralph made a good case for using an 8 ohm tap for an 8 olm impedance speaker load. 
 

I suppose that with any given amplifier and speaker pairing one could try both the 4  and 8 olm amplifier taps , listen and decide which sounds better. I’m sure it varies from one scenario to another. My speakers are 14 olm impedance. My amplifier has 8 and 16 olm taps. I prefer the 16 olm tap.

Charles 

However,  I do recall that the late Roger Modjeski (RAM Labs Music Reference Audio) advocated “light loading” of amplifiers.

@charles1dad Roger's amps were designed so this technique could be used. Most amps are not. Roger's amps also used feedback and his recommendation relied on this. Your amps are zero feedback so it won't work with them. You will get less distortion from your power tube doing this, but the distortion generated by the output transformer on account of being improperly loaded will be far more than the gains you get from lightly loading the power tube.

AFAIK, the transformer output is so the amplifier sees a similar impedance, not for what the speaker sees.

Also, AFAIK, if the impedance ratio is changes, so is the voltage.

I'm sure @atmasphere can correct me, but I thought the point of an output transformer with multiple taps was to maintain output power, which means the voltage has to change with each tap.

@atmasphere 

Roger's amps were designed so this technique could be used. Most amps are not. Roger's amps also used feedback and his recommendation relied on this

Thank you Ralph. I realized this distinction after reading the reply from @clio09. As you note, light loading wasn’t the best choice for my amplifier-speakers. Now I better understand what I determined through listening.

Charles

Implied, but so far not stated specifically, is that the output tubes of an amplifier need to see a high load of several thousand ohms (the exact value depends on the tube and amp design.)  If you run an output tube straight into a speaker, whether 4 or 8 ohms, it looks like a dead short to the tube -- not good. 

And, straight from the tube, one is talking an output of up to hundreds of volts. Also not good for the speaker, which wants a lower voltage, but more amperage. 

Hence, the transformer's job is to "translate" the output signal from the tubes so that the tube sees the proper impedance for its high voltage, lower amperage output, and the speaker receives a lower voltage with higher amperage. 

Regarding which output tap to use -- usually either 4 or 8 ohms on modern tube amps (units from decades ago also often had a 16 ohm tap), keep in mind that very few, if any, speakers have a flat impedance value across the spectrum. Usually the value varies quite a bit from low bass to the highs. That's why the speaker impedance value is typically referred to as "nominal" which is an average of the lowest values the speaker presents.  Using either the 4 or 8 ohm tap still puts one in the needed ballpark, so there is no risk of damage to either the amp or speaker regardless of which one is used.  I believe the general recommendation among most knowledgeable people is to try both taps and use the one that sounds best to you. 

What does light loading of tubes mean? Is this concept specific to a certain type of amplifier or is it a universal concept.

Rogue Audio has told me in the past to try either 4 or 8 ohm and use whichever sounds best. 

Hmmm, I have two pair of Vienna Acoustics that are 6 ohm speakers and a simple EL34 amp with 8 ohm and 4 ohm taps. Thanks to the excellent posts above I am much better informed but more confused!  VA support told me to run off the 4 ohm tap, but the best advice seems to be try both and see which sounds better! 
 

Thanks for the science and advice. : )

The manufactures/distributors of these very very expensive products should be better positioned to answer these types of questions.

while above points help to clarify it, there’s still seems to be great confusion about relationship of voltage and its influence to amplifiers and speaker drivers.

It seems clear very few people understand these complex areas, and the communication of this understanding is very challenging. Kudos to those above who have been helpful.

Implied, but so far not stated specifically, is that the output tubes of an amplifier need to see a high load of several thousand ohms (the exact value depends on the tube and amp design.)  If you run an output tube straight into a speaker, whether 4 or 8 ohms, it looks like a dead short to the tube -- not good. 

And, straight from the tube, one is talking an output of up to hundreds of volts. Also not good for the speaker, which wants a lower voltage, but more amperage. 

We've been flying in the face of this for nearly 50 years making Output TransformerLess (OTL) amplifiers with a direct coupled output.

Somewhere along the way I realized that the distortion of the amplifier is also the 'sonic signature' that almost any amplifier has. So that meant if you could build a solid state amplifier with the same distortion spectra/signature, it would sound the same also. That proved out to be true.

So there are now solid state amps that allow you to leave the tube world behind without missing them for anything.

@atmasphere 

I've looked at many tube amplifiers. Interested to know which solid state amplifiers achieve the same goal as tubes?

@atmasphere -- I understand that OTL tube amps have been around for years, but they are their own thing, with specific design considerations and tube selection, typically needing multiple output tubes to get the needed current. You just don't take a tube amp with transformers and hot-wire around the output transformers. 

But, you already know all that! Many others don't. But, OTL amps weren't really the focus of this discussion.

I've looked at many tube amplifiers. Interested to know which solid state amplifiers achieve the same goal as tubes?

@emergingsoul In my experience, not very many! But I am playing a class D amp at home that has the same distortion spectra as you expect to see in a tube amp. It is at a lower level however. Most solid state amps look distinctly different when you look at their harmonic distortion.

@atmasphere 

And there in lies the challenge.

I believe tubes represent a more even profile of the harmonics, elevating the harmonics in the upper range better than solid state.

So the richness of tubes and benefits of improved gain at lower levels makes them very attractive.  However, powerful tube amplifiers generate a ton of heat and this is disappointing.  It's like having a portable heater unit set to low continuously while you operate a tube amplifier, as I have learned. Unfortunately most of us don't live in a room with very high ceilings which would be a great place for heat to be accumulated without impacting the humans.

Maybe A more efficient speaker would be more desirable when working with a tube amplifier that uses less watts. ie, Full benefit of tubes without all the heat.

Hopefully I got all this right

I believe tubes represent a more even profile of the harmonics, elevating the harmonics in the upper range better than solid state.

@emergingsoul 

This is incorrect. Tube amps tend to make more the lower ordered harmonics, the 2nd and 3rd, which is the source of their 'richness'. These two harmonics are at a high enough level in tube amps that they are able to mask the higher ordered harmonics, which are heard otherwise (if not masked) as harshness and brightness. The class D amp I'm playing at home does exactly the same thing, and since the distortion signature of any amplifier is the way that amplifier sounds its not a surprise that this amp sounds exactly like a very good tube amplifier.

I have ARC amps from 1995, 2018 and 2023, and they all have 16, 8 and 4 ohm taps. My ELS speakers impedance curve indicates about 30 ohms at 20Hz and 3 ohms at 20kHz. The 16 ohm taps sound excellent. The 8 ohm taps do too, but with a different balance and therefore dynamic.

@emergingsoul:

I doubt any two amps, especially a tube amp and a class D amp, sound "exactly like"....

@williambf

Very interesting, you make up above.

It seems the impedance curve of a speaker is very important in assessing Ohm range throughout the frequency range. And there by it seems selecting the proper tap somewhere within the ohm range on the curve would influence how each tap may perform.

Very important and I’m gonna get the impedance curve for my speakers which seems a good idea, because I like to think I’m somewhat smart about this sort of thing albeit very slow

@williambf Wrote:

I doubt any two amps, especially a tube amp and a class D amp, sound "exactly like"....

I agree 😎

Mike