4 and 8 Ohm taps into 8 ohm speakers

[philsimm]

Hi all,

I'm hoping the collective wisdom of the 'wam can explain to me in laymans terms the differences between using 4 ohm and 8 ohm amplifier taps to run a set of 8 ohm speakers.

For instance should the 8 ohm taps sound objectively "better" or more "dynamic" because the impedences match between the two or is that a gross simplification? Would using the lower impedence tap on the amp produce more power output and would there be a perceived difference in "roll off" at the LF and HF extensions?

Sorry for the "noob" question but just looking for peoples none techy thoughts.

Phil

 

[HoopsOnToast]

Try it. :^

I heard no difference between the 4ohm and 8ohm taps when i had the Puresound A10, but that was with very easy to drive spekers.

 

[philsimm]

I have Rob and marginally prefer the 4ohm tap with the 8ohm Rogers speakers but don't know why and wanted to get at least a rudimentary understanding of "why". To me the treble sounds a little bit more rolled off if that makes sense but was just curious to understand the science behind it.

 

[SergeAuckland]

The main difference between 4 and 8 ohm taps is in the voltage they give out. 10 watts into 8 ohms is a voltage of just under 9 volts. 10 watts into 4 ohms is 6.3 volts. So, if you're running 8 ohm 'speakers from a 4 ohm tap, they will get rather fewer volts and so the maximum volume will be less.

However, the valves themselves only work efficiently if they see the right load, so running 8 ohm 'speakers on 4 ohm taps means the valves see the "wrong" impedance so their distortion will be higher. With pentode valves, especially if ultra-linear, this isn't very important as they are rather more load tolerant than triodes, especially if theres decent amounts of feedback applied.

Now consider the other possibility, that of running 4 ohm loudspeakers on an 8 ohm tap. As before, 10 watts into 8 ohms means a current of 1.1 amps, but 10 watts into 4 ohms is a current of 1.6 amps, so if you're putting 4 ohm loudspeakers on an 8 ohm tap, the 'speakers will try and take more current than the amplifier is expecting to have to provide, and so distortion, especially in the bass, will rise.

Whether any of the "wrong" settings sound better, is a matter for the individual, except to say that an amplifier designed for 8 ohms expects to see 8 ohms, so putting 4 ohms will make it work differently to the way it was designed.

One further consideration:- Loudspeaker impedances are very nominal. A 8 ohm loudspeaker shouldn't drop below 6.4 ohms according to the IEC recommendation, but many loudspeakers don't keep to the IEC recommendations and do have impedances that drop well below 6 ohms. Consequently, whatever the amplifier's design, it will be subject to varying impedances, so will operate differently to the way it would with a straight 8 or 4 ohm load. It should also be noted that a valve amplifier's own output impedance can be anything up to several ohms, and as that's in series with the loudspeaker's own impedance, the voltage across the loudspeaker, which is what determines what sound it makes, will be affected by both the amplifier's performance and it's own impedance characteristic.

Ultimately, with valve amplifiers, the only way is to evaluate the amplifier and intended loudspeaker as a system, as both will be different when driven by /driving other equipments.

S.

 

[Guest]

The main difference between 4 and 8 ohm taps is in the voltage they give out. 10 watts into 8 ohms is a voltage of just under 9 volts. 10 watts into 4 ohms is 6.3 volts. So, if you're running 8 ohm 'speakers from a 4 ohm tap, they will get rather fewer volts and so the maximum volume will be less.However, the valves themselves only work efficiently if they see the right load, so running 8 ohm 'speakers on 4 ohm taps means the valves see the "wrong" impedance so their distortion will be higher. With pentode valves, especially if ultra-linear, this isn't very important as they are rather more load tolerant than triodes, especially if theres decent amounts of feedback applied.

Now consider the other possibility, that of running 4 ohm loudspeakers on an 8 ohm tap. As before, 10 watts into 8 ohms means a current of 1.1 amps, but 10 watts into 4 ohms is a current of 1.6 amps, so if you're putting 4 ohm loudspeakers on an 8 ohm tap, the 'speakers will try and take more current than the amplifier is expecting to have to provide, and so distortion, especially in the bass, will rise.

Whether any of the "wrong" settings sound better, is a matter for the individual, except to say that an amplifier designed for 8 ohms expects to see 8 ohms, so putting 4 ohms will make it work differently to the way it was designed.

One further consideration:- Loudspeaker impedances are very nominal. A 8 ohm loudspeaker shouldn't drop below 6.4 ohms according to the IEC recommendation, but many loudspeakers don't keep to the IEC recommendations and do have impedances that drop well below 6 ohms. Consequently, whatever the amplifier's design, it will be subject to varying impedances, so will operate differently to the way it would with a straight 8 or 4 ohm load. It should also be noted that a valve amplifier's own output impedance can be anything up to several ohms, and as that's in series with the loudspeaker's own impedance, the voltage across the loudspeaker, which is what determines what sound it makes, will be affected by both the amplifier's performance and it's own impedance characteristic.

Ultimately, with valve amplifiers, the only way is to evaluate the amplifier and intended loudspeaker as a system, as both will be different when driven by /driving other equipments.

S.

Enjoyed that Serge, a few lightbulbs going on for me reading that, great to have things explained simply. Repped.

 

[philsimm]

Much obliged for the thorough explanation Serge.

:notworthy:

Cheers

Phil

 

[HoopsOnToast]

Cheers Serge :^

 

[awkwardbydesign]

As I understand it, the amps' output impedance will be lower from the 4 ohm tap, so the damping factor will be higher, giving tighter bass control. This is the theory, but as Serge pointed out, the output will be lower. I have tried it both ways with my SETs, and couldn't hear any improvement, but they feed my midrange units only, so bass damping is irrelevant. I put them back on the 8 ohm taps, as I don't want to lose power.

Anyway, many bass drivers don't want too much amplifier damping, they don't sound as good. As Serge said, suck it and see.

 

[philsimm]

As I understand it, the amps' output impedance will be lower from the 4 ohm tap, so the damping factor will be higher, giving tighter bass control.

So in theory the bass should be more tuneful, less flabby but slightly less of it?

Feel free to correct me if i'm wrong

 

[SergeAuckland]

As I understand it, the amps' output impedance will be lower from the 4 ohm tap, so the damping factor will be higher, giving tighter bass control. This is the theory, but as Serge pointed out, the output will be lower. I have tried it both ways with my SETs, and couldn't hear any improvement, but they feed my midrange units only, so bass damping is irrelevant. I put them back on the 8 ohm taps, as I don't want to lose power.Anyway, many bass drivers don't want too much amplifier damping, they don't sound as good. As Serge said, suck it and see.

Yes, the output impedance will be lower on the 4 ohm tap, but so is the load supposed to be, so the damping factor into a 4 ohm load will be the same. Driving an 8 ohm loudspeaker from the 4 ohm tap is likely to give a better result than driving a 4 ohm loudspeaker from the 8 ohm tap, but the voltage will be lower so the available loudness will be lower. Also, if the amplifier uses triodes, especially with little or no feedback, then the impedance mismatch will affect the sound too. It is indeed a suck-it-and-see exercise.

S

 

[Valvebloke]

Serge has already given a clear, concise explanation of the general principles of impedance matching. I can't add anything to that. So I thought I'd see what I could find out about your particular amp - the Prologue One. Since this is the technical section I was looking for technical stuff. Like so many manufacturers Primaluna tell you the basics, but not much more. Fortunately John Atkinson has done a substantial measurement job in this Stereophile review http://www.primaluna.nl/pdfmap/REVIEWS_PL1/Stereophile-Art%20Dudley%20(eng).pdf so it is possible to see the effects of presenting the amp with different loads.

It turns out that the results are a little surprising. The amp delivers less and less distortion as the load impedance drops. And since the distortion is quite high to start with (a good fraction of a percent) this improvement might be worth having. But the article doesn't go into the trade-offs, including such things as damping factor and power limits. The damping factor may be a real issue as the output impedance of this amp is unusually high - 5.8 ohms on the 8 ohm tap* - so the overall performance should depend quite critically on matching it to suitable speakers. I'd agree that it really will be "a suck-it-and-see exercise".

VB

*High output impedance and high distortion are both characteristics of low-feedback amplifiers. The designers have gone for a relatively high gain configuration (35W out for 230mV in IIRC) so perhaps with only three stages they didn't have enough raw gain available to 'spend' much of it accommodating a higher level of feedback.

 

[philsimm]

Thanks Valvebloke thats very enlightening. Thank you all for your help. Mucho appreciated.

:^

:^

 

[silencio]

Highly interesting and thanks to all for the explanations. Does it mean that the PL would have less distortion from its 4 ohm output, than the 8 ohm taps?

 

[Valvebloke]

It turns out that the results are a little surprising. The amp delivers less and less distortion as the load impedance drops ...

Durrr, sorry, I've just had another look at this and I mis-read the graphs :doh: . In figs 5 and 6 in John Atkinson's technical section the impedance falls as the curves go from bottom to top. So the distortion rises as the load impedance drops. Which is not surprising.

Highly interesting and thanks to all for the explanations. Does it mean that the PL would have less distortion from its 4 ohm output, than the 8 ohm taps?

For a given output power, yes. You can see from fig 6 that with an 8 ohm speaker on the 4 ohm tap the distortion at 1kHz is about 0.3% and the curve bottoms out at about 0.2%. Fig 5 shows that if you put the same speaker on the 8 ohm tap then you'll have closer to 0.4% at 1kHz and well above 0.3% at the bottom of the curve. You may find that the maximum power available to you is limited with the 8 ohm speaker on the 4 ohm tap though. Just one of the many trade-offs ...

VB