4ohms mono versus 16ohms mono

Drew_TNBD

Can someone clear this up for me once and for all? What is the difference? Less resistance with 4ohms hence more volume??? Or is that bullshit??

No I haven't tried it. :P

digitalscream

For a solid state amp, yes - voltage stays the same, so less resistance = more power.

For valve amps, less resistance (EDIT: less resistance than the output you've plugged into) = bye-bye output valves, and possibly output transformer too

However, using the 16 ohm output into a 16 ohm cab will use all the windings on the output transformer, thus allowing the transformer to have more of an effect on the sound. 4 ohm output into a 4 ohm cab only uses a quarter (EDIT: half, I'm an idiot and forgetting basic physics) of the windings on the transformer, so less of an effect on the sound.

Or so the theory goes, apparently...I've never really been able to tell the difference at volume.

Drew_TNBD

My amp has 1x4ohm, 1x8ohm, 1x16ohm ... so I don't think I'd kill the OT. Just wondering if I'd get more volume really.

digitalscream

Hmmm...I might have misunderstood the question.

4 ohm output -> 4 ohm cab will be the same volume as 16 ohm output -> 16 ohm cab.

However, 16 ohm output into a 4 ohm cab will very likely damage the amp at volume.

jpfamps

If you are using a valve amp connected to the correct impedance tap then there should be no theoretical differences in power output, although in practice you will get slightly less power on the 4 ohm setting due to the higher current draw causing more power to be wasted in the wiring.

Regarding sound, there may well be differences for two reasons:

1) The 4 ohms setting will usually be 4 x 16 ohm speakers in parallel and the 16 ohm will be parallel/series or series parallel. On 4 ohms each speaker will see a lower impedance and thus be damped better, which in practice means the bass won't be a flappy as the 16 ohm setting.

2) Depending on how your output transformer is wound you may only be employing half the secondary windings, and thus coupling from primary to secondary may not be optimal (there are some sophisticated winding patterns that employ all the secondary on all impedance settings but you are unlikely to find these in a guitar amp).

With a solid state amp, the lower the speaker impedance to more power is delivered, however if you go too low you will destroy the amplifier in short order (as possibly your speakers as well).

You will also get more distortion as you lower the load impedance on a solid state amp.

Solid state amps will have a minimum impedance rating and it is wise to follow this.

For PA purposes (the only time I really use SS amps) I personally prefer to run solid state amps rated for 4 ohm into 8 speakers (which can have impedances so low as 5-6 ohms at certain frequencies) as this is safer.

ICBM

Or vice versa... different rules for solid state and valve. A 16-ohm valve amp going into 4 ohms may fry the valves quite quickly, but not the transformer - the other way round is actually more risky for both. But in general it's best to avoid more than a factor of two mismatch in either direction.

Even with the amp and the cab correctly matched, running at 16 ohms will sound different from running at 4 ohms - it's more due to the difference in damping and inductance than whether you're using "the whole winding". (Which is only half for a 4-ohm tap on a 16-ohm winding anyway, not a quarter - since the impedance ratio is the *square* of the turns ratio.)

digitalscream

ICBM said:

Even with the amp and the cab correctly matched, running at 16 ohms will sound different from running at 4 ohms - it's more due to the difference in damping and inductance than whether you're using "the whole winding". (Which is only half for a 4-ohm tap on a 16-ohm winding anyway, not a quarter - since the impedance ratio is the *square* of the turns ratio.)

Duly noted. I think I need to do physics GCSE again.

That said, I've still never really been able to tell the difference between 4R and 16R in terms of sound. Probably my cloth ears.

ICBM

It certainly isn't huge - just one of those things which will tip the tone slightly in one direction or the other, rather than change it a lot. Like a lot of power-amp effects it's more noticeable with power amp overdrive as well. You should find that 16 is a bit brighter, looser and more 'present' and 4 is a bit deeper, tighter and 'chunkier'.

Drew_TNBD

Guess I'll just have to try it! My amp and cab support both, but I've always just used 16ohms for both.

ToneControl

The Fender Tonemaster was designed unusually: you can use the impedance to vary the sound/feel:

 

D. IMPEDANCE SWITCH - This is labeled LO, MED and HI. This switch serves two functions.

One, to set the output impedance of the amplifier to best match the speaker impedance for maximum power. Second, to enable the amplifier to better achieve the cranked-up sound at a lower volume. On the B CHANNEL, with a single Tone-Master enclosure, this switch should be set according to the setting of the B CHANNEL volume knob: For volume settings of 3.5 and lower, LO would be the appropriate setting. For settings of 3.5 to 7, MED is best. For settings above 7,
HI will produce full power and tone. This switch will not damage tubes or reduce tube life at these settings. What it does do is increase the gain in the LO and MED settings and thereby better
approximate the Full volume sound at a lower level than by adjusting the VOLUME alone. With a single 16½ enclosure, LO will produce 30 watts, MED will Produce 60 watts, and HI will
produce a full 100 watts. With two 16½ enclosures, (an 8½ load), LO will produce 60 watts, and MED will Produce 100 watts. In the A CHANNEL, the switch has the same function, although
blues players may like running the switch on LO all the time to get that great crying tone.
Conversely, a player looking for a New Wave Strat® tone might like the sound of the A CHANNEL on HI.

 

...

 

POWER OUTPUT: 100 Watts R.M.S. minimum into 4, 8 or 16 ohmsOUTPUT IMPEDANCE: 4, 8 or 16 ohms 

 

 

   

ICBM

Mesa recommend experimenting with impedance as well, and say it won't damage anything - although I think they do warn about valve life in some combinations. You're certainly more likely to damage the valves than the amp in a well-made amp.

But the idea of running the amp at 4 ohms into a 16-ohm cab to get "a better tone" (ie more distortion) at lower volume was an occasionally recommended 'trick' with old Marshalls when I stated repairing amps, and that isn't a good idea... either old Marshall transformers are inherently weaker or there's some other factor, because it quite often led to a blown OT.

Running a vintage Fender head (4 ohm output) into a standard vintage 16-ohm Marshall cab is a bad idea too.

jpfamps

ICBM said:

Mesa recommend experimenting with impedance as well, and say it won't damage anything - although I think they do warn about valve life in some combinations. You're certainly more likely to damage the valves than the amp in a well-made amp.

But the idea of running the amp at 4 ohms into a 16-ohm cab to get "a better tone" (ie more distortion) at lower volume was an occasionally recommended 'trick' with old Marshalls when I stated repairing amps, and that isn't a good idea... either old Marshall transformers are inherently weaker or there's some other factor, because it quite often led to a blown OT.

Running a vintage Fender head (4 ohm output) into a standard vintage 16-ohm Marshall cab is a bad idea too.

I had a customer last week how thought it was OK to run an Orange OR120 set on 4 ohms into a 16 ohm cab.

The repair bill was £204.

Incidentally running the amp on too high an impedance results in increased screen grid currents.

This is much more of an issue with pentodes (EL34 etc) which have higher screen grid currents than with beam tetrodes (6L6), so in my view it's really not a good idea to mismatch impedances with amps running pentodes.

Regardless, I much prefer the sound of a valve amp running at the "correct" impedance.

If you have to monkey around with your loading to get a sound you like at the correct volume you've got the wrong amp.

jpfamps

ICBM said:

Even with the amp and the cab correctly matched, running at 16 ohms will sound different from running at 4 ohms - it's more due to the difference in damping and inductance than whether you're using "the whole winding". (Which is only half for a 4-ohm tap on a 16-ohm winding anyway, not a quarter - since the impedance ratio is the *square* of the turns ratio.)

The winding topology will have an effect on how the different setting work.

In the most basic transformer all the primary is wound, then all of the secondary; I can't imagine how using less of the secondary would have much effect, assuming that you are using whole layers.

A classic transformer design as used for example by the VOXAC30 (JMI era) and the Orange Tiny Terror, has 5 sections:

1/4 primary, 1/2 the secondary, 1/2 the primary, 1/2 the secondary, 1/4 the primary.

This is a very clever design as 1) it gives the most bang for buck regard reducing leakage inductance with the smallest number of sections and 2) by connecting the two aouter sections of the primary as one side of transformer and the middle section as the other side you can balance the resistances using only one gauge of wire..

However if you have a 4 ohm tap you will be only using half the secondary and thus with now only have a 3 section transformer, with corresponding increase in leakage inductance.

Whether this is audible is another matter, but leakage inductance is not a "good thing" when the amp is driven hard into clipping it's the energy stored in the leakage inductance that causes voltage spiking. Also a marginally stable amp may start ringing with increased leakage inductance due to reduced HF role off.

ToneControl

btw I'm not recommending running other amps' 4 ohm outputs into 16 ohms speakers. I hear the Tonemaster had an enhanced transformer or something to make this work 

jpfamps

ToneControl said:

btw I'm not recommending running other amps' 4 ohm outputs into 16 ohms speakers. I hear the Tonemaster had an enhanced transformer or something to make this work 

It's not hard to design a transformer that will cope with mismatching impedances, you simply need to ensure that there is enough insulation between the layers to cope with any voltage spikes, and the wire needs to be thick enough to cope with any extra current.

This of course costs money......

 

ICBM

jpfamps said:

Regardless, I much prefer the sound of a valve amp running at the "correct" impedance.

If you have to monkey around with your loading to get a sound you like at the correct volume you've got the wrong amp.

Agreed on both counts.

Dave_Mc

jpfamps said:

Regardless, I much prefer the sound of a valve amp running at the "correct" impedance.

Same here. A couple of times when I got confused I was running my amps at a mismatch, and I had the feeling that something I couldn't put my finger on was "not quite right". Once I realised and corrected the mistake, it sounded fine again.

I also tried once different impedances with the same cab into the same amp (it's a cab with series/parallel wiring for 4 or 16 ohms and the amp had multiple impedance taps), and while I thought there was a very subtle difference, it was within "is it just the placebo effect?" range and I figured it wasn't worth worrying about. Obviously that's not a scientific test, maybe it makes more difference with other amps and speakers (not to mention, going from series to parallel wiring for the speakers could also affect the tone), but I figured it was subtle at best.

ToneControl

jpfamps said:

ToneControl said:

btw I'm not recommending running other amps' 4 ohm outputs into 16 ohms speakers. I hear the Tonemaster had an enhanced transformer or something to make this work 

It's not hard to design a transformer that will cope with mismatching impedances, you simply need to ensure that there is enough insulation between the layers to cope with any voltage spikes, and the wire needs to be thick enough to cope with any extra current.

This of course costs money......

I knew it needed a more expensive transformer - I wasn't recommending it, I just thought it was interestingly unusual