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Valve amp to speaker impedance matching?

2N3055

pfm Member
Thinking about experimenting with a valve amp driving my 6 ohm Sonus Fabers.

When one has an option of O/P impedances, typically 4, 8 and 15 ohm, which is the best O/P setting for speakers of 6 ohm? 4 or 8 ohm? And why?
 
Speaker impedances are not constant, they fluctuate with frequency, and often by a lot. It's worth having a search over on Stereophile to see if they've reviewed your SFs as they measure and plot the impedance, very useful as it gives a far better indication as to what's what then just a nominal figure. By saying that try both and as ever whichever sounds best is best! You'll not do any damage to anything with either the 4 or 8 Ohm setting, the 15 Ohm tap may put the amp or tubes under some stress in this context so is best avoided.
 
Great! Thanks for the super quick advice! Haven't got a valve amp yet. Just planning ahead. But good to know whats what.
 
To get the best with a valve amp your speakers need to present a constant impedance. The output transformer does what it says on the tin - it matches the speaker impedance to that expected by the valve. If you have speakers that have impedance that moves all over the place like mine 2 Ohm to 6 Ohm then using a valve amp will be similar to a stopped clock that shows the correct time twice a day.

In the case of impedance matching means that you introduce distortion sometimes considerable when speaker-valve impedance mismatch but sweet on those rare occasions when both match.

Cheers,

DV
 
Great! Thanks for the super quick advice! Haven't got a valve amp yet. Just planning ahead. But good to know whats what.

I asked what speakers as I had these amplifiers rebuilt for 6ohm impedance Harbeth speakers, using very large split core Sowter transformers. Just not powerful enough for the 86dB Harbeths, OK for most music, just lacking low level definition. Would be fine for speakers 89db or above. Hence the sale.

http://www.pinkfishmedia.net/forum/showthread.php?t=188163
 
Sonus Faber Concerto Home.

With small 86dB speakers you are going to struggle with a medium/low powered valve amp.

How about considering a hybrid like a Croft? Also very sensibly priced. Dave at Radlett Audio is your man for sensible advice. He sells lots of Croft and is also a Sonus Faber dealer, so will know all the matching issues. He started selling audio in the days of wax cylinders, so there's not a lot he doesn't know.
 
I'd agree in theory, but my Leak Stereo 20 seems surprisingly happy driving my JR149s (84 db or so, but an easy load), though I do listen in the extreme near-field and not very loud. To be honest I'm amazed sls4321 found 4 KT88s a side struggled with Harbeths, I drove both Compact 7s and SHL5s with a Prima Luna Prologue 2 stuffed with EL34s (30-35 Watts) and never felt it was struggling at all, though again I'm not a high volume listener (and neither are Harbeths high-volume speakers). I'm also the type of listener who hates small speakers driven loud by anything, I'm a firm believer that if you want loud you neeed big speakers as they just move more air with less effort. I'd have thought a good quality 30-40 watt push-pull tube amp would take a pair of 86db stand mounts louder than I'd want to be in the same room as them assuming no really nasty reactive loads or impedance dips. Sonus Fabre seem to often be partnered with tubes so I suspect all will be well.
 
I'd agree in theory, but my Leak Stereo 20 seems surprisingly happy driving my JR149s (84 db or so, but an easy load), though I do listen in the extreme near-field and not very loud. To be honest I'm amazed sls4321 found 4 KT88s a side struggled with Harbeths, I drove both Compact 7s and SHL5s with a Prima Luna Prologue 2 stuffed with EL34s (30-35 Watts) and never felt it was struggling at all, though again I'm not a high volume listener (and neither are Harbeths high-volume speakers). I'm also the type of listener who hates small speakers driven loud by anything, I'm a firm believer that if you want loud you neeed big speakers as they just move more air with less effort. I'd have thought a good quality 30-40 watt push-pull tube amp would take a pair of 86db stand mounts louder than I'd want to be in the same room as them assuming no really nasty reactive loads or impedance dips. Sonus Fabre seem to often be partnered with tubes so I suspect all will be well.

Its not about loud. My KT88s are set up in triode mode at about 40w and can make Harbeth go very loud. It's about dynamics. Most of the time they are excellent amps. Listening to very low frequency sounds, specifically double bass and tuba, there is a loss of dynamics. Also to some extent on symphonic music. They do rock music perfectly well. For many users they would be absolutely fine. For vocals and instrumental chamber music they are superb.

The OP does not state his taste in music - it may be that my amps are perfect - they are a bargain and I'd be delighted to sell them to him!!!
 
My Radford STA25 III has switchable impedance matching, 4, 8, and 16 Ohm, in to most of my speakers can't really tell much difference between settings, apart from it getting a little louder on the higher impedance settings. This can be confusing as louder can be mistaken for better, quite easily.
 
The transformer taps on a valve amp output transformer aren't primarily about speaker impedance, they are about output impedance. So the 4 Ohm tap has a lower output impedance than the 8, a higher maximum current but unfortunately also a lower maximum voltage. So 2.8vrms on the 8R tap into an 8R load might make 1W, but into a 4R load it won't make 2W. From the 4R tap you will still get 1W but closer to 2W into the 4R load. But the 4R tap might clip at 15W into 8R, whereas the 8R tap will manage 24W.

I've guessed the figures to try an illustrate a point. But I would expect the 4R tap to be best with nominally 6R speakers, assuming it goes loud enough.

Paul
 
There's also something about safe operation parameters too isn't there? I'm still very much grappling with the very basic stages of electronics, but as I understand it the output transformer is kind of a safe conduit/translation device that enables the output tubes to communicate with the speakers. Something about converting high voltage and low current into lower voltage and higher current or the other way round or something like that, but if the impedance mismatch on the outside (speakers) is too great the tubes the other side of the output transformer end up being forced to operate outside their design parameters and can overheat, wear too fast or otherwise fail. I'm kind of regurgitating this from decades ago with regards to hooking too many cabs up to a tube bass or guitar top, and in that context you also need to know about series or parallel wiring etc as it all impacts the impedance.
 
For best overall performance the lowest impedance dip of the speaker wants to be matched to the output impedance of the amp. Typically 6 to 8 ohm speakers will be best run from the 4 ohm tap.
There is a caveat, however.
Many modern high powered valve amps, especially foreign built ones, are running at or beyond the valve permissable parameters, so damage from flashover is a real risk, so under these conditions it is better to increase the loading. I.e. anyone with something like a CJ should run 6 ohm speakers on the 8 ohm tap
 
Sonus Faber Concerto Home.


I use a pair of these speaker with a Luxman SQ38 Signature , EL34 PP
amp rated at 30w / 6 ohms .

It works beautifully , there is plenty of bass (within the limits of the speakers) and dynamics .

Web wisdom will have you believe SF's need huge power , they don't SF's own Musica dispells this myth , what they do like is quality .

I would try them with the best valve push pull amp you can afford .
 
There's also something about safe operation parameters too isn't there? I'm still very much grappling with the very basic stages of electronics, but as I understand it the output transformer is kind of a safe conduit/translation device that enables the output tubes to communicate with the speakers. Something about converting high voltage and low current into lower voltage and higher current or the other way round or something like that, but if the impedance mismatch on the outside (speakers) is too great the tubes the other side of the output transformer end up being forced to operate outside their design parameters and can overheat, wear too fast or otherwise fail. I'm kind of regurgitating this from decades ago with regards to hooking too many cabs up to a tube bass or guitar top, and in that context you also need to know about series or parallel wiring etc as it all impacts the impedance.
Something like that.

A transformer can change the ratio of current to voltage, but the product, and hence the power, remains the same. Any amplifier, of whatever construction, will have a limit to both the voltage it can produce and the current it can deliver. Reaching either limit is a form of clipping. The transformer taps on a valve amp change the ratio between the two, so the 8R tap can make a higher voltage, but will limit at a lower current than the 4R tap.

If you run into the current limit, for instance by using an 8R tap into a 4R load to try and get more volume, then you may exceed the capacity of the output tubes to supply current, because the voltage limit won't be reached, and they (and the power supply) will do their best to deliver.

In the OP's case he has a speaker with a relatively low impedance, and a relatively low sensitivity. So a 4R tap would be safest, in general as a starting point, because if he turns it up to clipping the associated current demands shouldn't do any harm.

Paul
 
Valve amplifiers are damaged by higher than expected loads, causing voltage stresses.
SF speakers are mostly 4R so the 4R tap is the one to go for
 
<steals thread for a moment>

Ok, if one had a speaker with an impedance plot such as this (JR149):

8710147828_09b2df2288_o.jpg


Would the 8 Ohm or 16 Ohm tap be the more appropriate? I'm using the 16 Ohm tap at present and think it sounds better (bigger, freer), though I hate power-cycling a tube amp unnecessarily so have only really compared one listening session to another (i.e. hours later).
 
My speakers are a bit unusual, have a transformer on the input. Rogers BBC LS3/6.

I can vary the nominal impedance by moving a tapping on the transformer.
 


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