Impedance matching - woodoo magic?

[selfaddict]

This is something that has bothered me for some time now :scratch:. Is impedance matching important when it comes to amplifiers and speakers? I recently bought a pair of lovely Bastanis Mandala open baffle speakers, which I will keep for sure :eek:. These speakers are working between 8-16 ohm range for my understanding. In the future, when I get another pair of Bastanis wideband drivers, they would be mostly in the region of 16 ohm.

My amplifier, Art Audio Diavolo, has only one pair of speaker tabs and they are currently 6 ohm. I have tried to contact Tom at Art Audio to find out if Diavolo can be changed to have 16 ohm speaker tabs, but have not managed to talk to him yet.

What are the benefits and is there any real difference sonically if and when I could get the amplifier's speaker tabs changed to 16 ohm? Sorry about the question, most probably it is an obvious one, but I cannot get my head around it :confused:

[Reid Malenfant]

Not voodoo magic no, but as it happens very important for both valve & transistor amplification ;)

In the case of valves you need to make sure that the speaker is at the rated impedance or lower than stated to ensure amplifier stability ;)

Transitorised kit needs the opposite :doh: The speaker impedance needs to be at or higher than the lowest impedance stated that it can drive so not too much current is drawn...

[DSJR]

I'd be inclined to leave things as they are and forget all about it.

Solid state amps generally have a far lower output impedance when compared to transformer coupled valve ones, so unless there are 4 Ohm tappings on a valve amp, I'd be inclined to stick to 8 Ohm upwards nominal, which is what you have. Using a low impedance speaker with an 8 Ohm tapped valve amp shouldn't do any physical harm I understand, but the crossover frequencies in the speaker may be shot to heck I understand. As it is, the presence of a passive crossover, the inductors in particular, will have far more of an issue on damping.

I can't be a*sed to do the maths, but Labarum kindly (?) sent me this link :) -

http://sound.westhost.com/lr-passive.htm

[lurcher]

In the case of valves you need to make sure that the speaker is at the rated impedance or lower than stated to ensure amplifier stability

Not universally true, and certainly not in this case. A SET or any triode output stage (especially with no global feedback) is happy with a higher (to infinity) load, it will result in lower power output, but likely to also result in lower distortion. So if you have enough power out of the amp with a 16 ohm speaker on a 6 ohm tap, then its likely all good.

If you look at this chart of power output against load resistace and distortion levels you can see how higher impedance gives lower power (higher number) and lower distortion (higher number) for a given B+

http://tdsl.duncanamps.com/dcigna/tubes/sheets/misc/4300b-2g.gif

[Clive]

Bastanis in twin wideband config are 14 ohm, they are nominally spec'ed at 16. It's quite possible your amp will be fine with a 14 ohm load.

[Reid Malenfant]

<snip> Not universally true, and certainly not in this case. A SET or any triode output stage (especially with no global feedback) is happy with a higher (to infinity) load
Cheers for the addition, i hadn't considered the amp in question i'll be honest :doh:

[magiccarpetride]

This is something that has bothered me for some time now :scratch:. Is impedance matching important when it comes to amplifiers and speakers? I recently bought a pair of lovely Bastanis Mandala open baffle speakers, which I will keep for sure :eek:. These speakers are working between 8-16 ohm range for my understanding. In the future, when I get another pair of Bastanis wideband drivers, they would be mostly in the region of 16 ohm.

My amplifier, Art Audio Diavolo, has only one pair of speaker tabs and they are currently 6 ohm. I have tried to contact Tom at Art Audio to find out if Diavolo can be changed to have 16 ohm speaker tabs, but have not managed to talk to him yet.

What are the benefits and is there any real difference sonically if and when I could get the amplifier's speaker tabs changed to 16 ohm? Sorry about the question, most probably it is an obvious one, but I cannot get my head around it :confused:

I can only assume that woodoo means wooden voodoo?

[selfaddict]

This is what Nick told me earlier as well :respect:. I am sure I will be OK with Diavolo driving Bastanis, I just wanted to get my head around why some valve power amps have different impedance value speaker taps and what is the thinking behind this. I am sure this is nothing to do with woodoo/voodoo magic :lol: but just to make those electrodes flow more fluently :eyebrows:

I have also read that best thing is to believe your own ears and try each speaker tap and find the best sound for your liking this way.

Clive thanks for the info re 14 ohm impedance when used in twin wideband configuration. I assume the impedance curve have to be relatively stable throughout, just as valve gear likes it to be :cool: . Gemini tweeters are rated at 16 ohm.



Bastanis in twin wideband config are 14 ohm, they are nominally spec'ed at 16. It's quite possible your amp will be fine with a 14 ohm load.

[selfaddict]

Hi again.
I thought to resurrect this thread because I have new information and more questions :doh: sorry.

I was talking with Tom from Art Audio about having an additional pair of balanced outputs installed to my Diavolo. I have tried to live with active preamp, but this was not to be and it needed to go. There wasn't really point to have 10db gain from the active pre and then use 30db attenuators to be able to use the volume pot at any reasonable level .

He said there is no problem with having balanced outputs added, there is plenty of space for this inside Diavolo, so all well. Then he said that my Diavolo SET power amp actually most likely has 4 ohm speaker tabs. I thought that it had 6 ohm ones, but this isn't the case as it seems. It would be possible to change the output transformers to having (4 and 8 ohm tabs) or (8 and 16 ohm tabs) for additional £200 per transformer.

I know now that Bastanis Atlas are 14 ohm, thanks Clive. So does this new info regarding my Diavolo's output transformers being 4 ohm change anything. If I got it right, what Tom said, is that as long as I feel there is enough power available to drive the speakers, there should be no problem. I don't know if I have understood this right but first signs of not having enough power to drive the speakers would be that one looses the dynamics on the music. Would this be most obvious when on classical music, music transforms from quiet to really loud/complicated suddenly?

[Reid Malenfant]

<snip> Would this be most obvious when on classical music, music transforms from quiet to really loud/complicated suddenly?
Quite possibly, but it doesn't mean you might have a mismatch. Classical has huge differences between quiet passages & crescendos, vastly more than just about any pop music. I know 20Db isn't out of the question, probably higher is also realistic & out there somewhere when listening to a very quiet passage.

A 20Db increase is equivalent to 100 x the power, so unless you happened to be listening to less than 0.25W on a quiet passage, you'd need a 25W RMS amp to get the crescendo accurately without clipping.

[selfaddict]

A 20Db increase is equivalent to 100 x the power, so unless you happened to be listening to less than 0.25W on a quiet passage, you'd need a 25W RMS amp to get the crescendo accurately without clipping.

This is great piece of information that even I can remember, thanks Mark. So with 13w/channel I have no hope in hell :eek:. Does one hear amplifier clipping clearly(like on-off type), or can it happen more unnoticed?

[Reid Malenfant]

With a transistorised amp with loads of feedback it'd be as harsh as all hell if it clips. Valve amps tend to use vastly less though so clipping isn't quite so bad. If you happen to have an amp with zero feedback it'd be even less obvious. This is simply because there is no feedback loop attempting to force the amp to put out more voltage than it's capable of, so hard clipping can't occur :)

I guess it's why some valve amps sound very good indeed even when they are actually being overloaded. Not that it may harm them, just that they don't get too ragged round the edges when things get tough :eyebrows:

[selfaddict]

This is great information, many thanks Mark.

I have been thinking this might have been the case, without knowing the basic of the reason, but now it all seems to be coming together finally :). Lately, well since I installed second pair of Bastanis wideband drivers, I have been missing dynamics from my music. The clarity and integration is better that I have ever heard anywhere, but there has been something missing, something I could not put my finger into.

Diavolo is zero feedback SET amplifier, so perhaps it has been clipping without me really realizing it :scratch:. Only thing what I have noticed is that the mains transformer is running much hotter than it previously did.


With a transistorised amp with loads of feedback it'd be as harsh as all hell if it clips. Valve amps tend to use vastly less though so clipping isn't quite so bad. If you happen to have an amp with zero feedback it'd be even less obvious. This is simply because there is no feedback loop attempting to force the amp to put out more voltage than it's capable of, so hard clipping can't occur :)

I guess it's why some valve amps sound very good indeed even when they are actually being overloaded. Not that it may harm them, just that they don't get too ragged round the edges when things get tough :eyebrows:

[Reid Malenfant]

It does sound like you may well be running out of headroom. The hot mains transformer could be a bit of a clue, the thing is with valve amps is you can play about with the impedance matching due to the various taps on the output transformer. If you have a lower impedance connection i'd suggest you try it out & see what you reckon as it sounds like your drivers are now in parallel.

At the end of the day though you can only get so much power from any amp & you might need to consider an upgrade to something more powerful (just to reproduce those transients more accurately).

As you can imagine it's a tad difficult diagnosing a problem if you can't hear it for yourself. From what you have said so far though i feel i'm on the right track & a bit more power (along with the same quality & sound) may well see you in some kind of audio rapture :)

[selfaddict]

Currently my Diavolo has only 4 ohm speaker tabs. If I have understood this right, if I get output transformers changed to ones with 8 and 16 ohm speaker tabs, this should make the amplifier work less hard and by doing this increase headroom. Am I right with this?

I don't believe that I would need more power because Bastanis are rated at 100db sensitivity and even with 13w/channel I should get volume levels well beyond my needs.

[Reid Malenfant]

Maybe with 14 ohm drivers (you might have mentioned this & correct me if i'm wrong!) you'd be better off using an 8 ohm tap. Assuming what i mentioned is correct then it should result in a higher power output :scratch:

14 ohm divided by two is 7 ohms which may give a much better match.

In all honesty though valve amps are not my normal field. Please get a second opinion on this, if not a third :eyebrows: I do stand by what i said about feedback though ;) Lots gives hard clipping, less gives softer clipping & none results in even less harshness if you push things too far...

[selfaddict]

Your input and information given has been great, many thanks Mark :thumbsup:.

I just need to figure it out if there is anything to be gained by changing the Diavolo's output transformers to transformers which would have 8 ohm and 16 ohm speaker tabs on them. If this would give me more headroom and make my system more dynamic again, it is really a no brainer and I will do that for sure.




In all honesty though valve amps are not my normal field. Please get a second opinion on this, if not a third :eyebrows: I do stand by what i said about feedback though ;) Lots gives hard clipping, less gives softer clipping & none results in even less harshness if you push things too far...

[selfaddict]

I got reply from Robert Bastani. Nominal Impedance of Bastanis Atlas is 16ohm, minimum impedance 12.5ohm, max impedance 22ohm. So problem solved, I will go output transformers with 8ohm and 16ohm speaker tabs.

I wonder if I could get output transformers with 4ohm, 8ohm and 16ohm tabs? Better ask the man himself.