Altec Biflex for OBs?

[iansr]

I have probably got this wrong but I thought the first watt doctrine was based on the argument that with reasonably efficient speakers one watt was all that was being used for most of the time; all those extra watts that you might have at your disposal only come into play fleetingly on high energy transients etc. The more dificult the load the more watts you might (momentarily) need. Rot or not?

Nelson, are you listening? No, thought not.

[steve s]

thanks Paul.. after years of practice and you realise you need to do something when nobody like the sound of your system...

so.. thanks to you all....

interestingly, I read a bit about the difference between high power magnets and lower powered ones...
you will all know this i suppose..

lower power motors need current 'source' to control them as they have less self control?... yes i know you know that..


but higher powered motors only need voltage source..so is ideal for you low wattage/ low current amp.. .
sorry...

steve

mmm i'm supprised.. i thoght i was writing what you all new.

but i read it form a reputable source


I'm still getting my head around the concept

damping factors interests me.. because i don't really understand them
..it controls the speaker? so better damping gives a stronger field from the speaker coil..?

they say a triode amp has more..
but it has less output ( watts)

a pentode has more output watts, but needs feed back to give it (enough?)more

the ratio between the current used by the valve vs the actuall watts output seem to be what its about?

a pentode give more watts for the same current.. so you put feed back in to help .. and reduce the rising distortion from them if you did not use it(feedback)
otherwise theres not enough control for some speakers

does that make sense or am I off track somwhere ?


steve

[Nick]

damping factors interests me.. because i don't really understand them
..it controls the speaker? so better damping gives a stronger field from the speaker coil..?

Yes, sort of, if you take a speaker without the coil connected to anything, the only thing preventing the cone moving is the spider and surround, But if you clip the terminals together, then when teh cone moves, the coil moving in the field acts as a generator, and the EMF generated then acts to resist the motion of the coil, as the current is shorted because of the clip leaded terminals. If you cliplean a resistor insteda of the wire, the force acting against random movement of the cone is reduced.

There is a interesting article I found somewhere, I think it was Crowthorn, but it might have been Hafler, talking about what was then the current trend of having amplifiers with variable speaker damping. In effect the feedback was adjustable to provide selectable damping, But what was very interesting (to me anyway) was by using current feedback, they could produce negaive as well as positive damping factors.

[Paul Barker]

yes your right I knew that bit. Not allowed to say that I posted that statement on WAD some 5 or so years ago.

But latterly you have demonstrated really deep useful knowledge about speakers respect for what you have achieved and how you have gained the knowledge.

You have shown us a really great set of speakers which if I may say so are sweetest with your low powered amp.

Looks like I'm back on Lowthers.

Just ordered the supravoxes,

Wish I could afford the visaton super horn tweeters 16 somats. then I'd get the scisors out again, watch out whizzers, Paul is about!

[steve s]

[Yes, sort of, if you take a speaker without the coil connected to anything, the only thing preventing the cone moving is the spider and surround, But if you clip the terminals together, then when teh cone moves, the coil moving in the field acts as a generator, and the EMF generated then acts to resist the motion of the coil, as the current is shorted because of the clip leaded terminals. If you cliplean a resistor insteda of the wire, the force acting against random movement of the cone is reduced.

There is a interesting article I found somewhere, I think it was Crowthorn, but it might have been Hafler, talking about what was then the current trend of having amplifiers with variable speaker damping. In effect the feedback was adjustable to provide selectable damping, But what was very interesting (to me anyway) was by using current feedback, they could produce negaive as well as positive damping factors.[/quote]


thanks Nick
i understand the motor of a speaker can created voltage.. thats roughly how a mike works .. or a cartridge for that matter and when that happens there is a impeadence or physical resistace created..

but what it in the signal to the speaker, that makes the damping factor more or less..

i assume the speaker is doing the same sort of movement where a pentode or a triode (or any thing else) is driving it...

i know i am missing something..

i am under the impression its something to do with the voltage to current ratio at the speaker terminals

but not at all sure..? any ideas



paul those are very kind words above
a bit of luck and good judgment and having a good combination of speakers to hand.. and reading some of gilbert Briggs old books.. no more than that .. well scott gives some sound advise..

but my thanks.. are to nick for inspiring me

is that too much back slapping??

cheers

[Nick]

for inspiring me

And James me.

I think you have to consider what happens if you take a amplifier, and feed a signal into it. Then connect the output of that amplifier, to the output of a second amplifier. Thats in effect what we have here, the damping factor, is a measure of the ability of the second amplifier in the previous description to remove any signal on its output.

It may sounds like a odd example, but (In my understanding) the back EMF from the speaker, is acting as if there was another source driving the output terminals of the amplifer.

Now if we have negative feedback in the amplifer, the voltage applied to the outout of the amp, will be coupled back to the input, and the amplifier will try and drive its output in the other direction from the applied voltage, trying to cancel it out. Its this action, that makes a speaker connected to this amp, think its connected to a short circuit. Any voltage it applies to the amplifier termonals, is turned into zero volts, as if it was a short circuit.

Of course, you can imagine how the amplifier is reacting to the voltage on its output, its doing a lot of work, and the net result is nothing, or near to nothing.

A zero feedback amp, is a bit different. All any signal sees if applied to the amps output, is the Ra of the valve, modified by the ratio of the transformer. I don't think there is any feedback going on (other than the mythical feedback in a triode that some love to bring up, and I will pass on that) to oppose the applied voltage, so its just a simple resistance that the speaker sees. Not perfect damping, but no feedback nasties.

[Darren]

for inspiring me

I think you have to consider what happens if you take a amplifier, and feed a signal into it. Then connect the output of that amplifier, to the output of a second amplifier.

I did that once when I was comparing my 6b4g SE to a SS amplifier. Both connected to the speakers at the same time. I was switching between them and did a bit of listening.

Mark J came round to have a listen an stated that I shouldn't be connecting two amps at the same time.

I have to admit when I then moved over to connecting just one amp at a time I couldn't hear any change even though I was expecting to.

[ed]

I did that once when I was comparing my 6b4g SE to a SS amplifier. Both connected to the speakers at the same time. I was switching between them and did a bit of listening.

Mark J came round to have a listen an stated that I shouldn't be connecting two amps at the same time.

I have to admit when I then moved over to connecting just one amp at a time I couldn't hear any change even though I was expecting to.

There is a similar discussion on Diyaudio at the mo and somebody recommended Niles DPS-1. It looks like the biz for ABing amps.

apologies for the OT

[steve s]

thanks for that Nick.. i think i followed it

so what your saying is that a triode will cope the the back generated feed singnal and see it as extra impedance.. and it can cope ok with that

a pentode cannot cope with this back wave so to speak. and the feed back in the amp acts to provide a vent for it, and depending on the amount of the feed back designed in the circuit.. or as many older high end amps had an ajustable amount, to cope with different types of speakers

i wonder, and to take that one stage further.. do speakers with a lower q and a smaller voice coil travel.. make less back voltage than one with say 5 times the travel..
as again, they say that high eff drivers need less damping, and thats down to the magnet strength holding them.. do you thing they would in turn give out less signal 'out the back door'.. if you follow..

i may just put a meter on a couple of speakers and see what happens
there may be something to measure
steve

[steve s]

to take what you've said maybe a stage further..

on a feed back amp...

on the speaker wires you have the source signal..
and the back emf from the speakers movements

i know most/ all speakers don't reproduce the signal accurately, as they are not really fast enough to do that ..
but they are all at different levels...

so you then get 2 differing signals on the speaker wires
and some of that is fed back into tha amp along with the original signal from the source..

if i've got that correctly
that would better explian to me the devastation feed back 'can' cause
and may be explain another side effect of poor quality speakers..?

does that make sense?

so the better quality speaker that start to come close to reproducing the actual signal would produce a better overall result.. because its getting a cleaner signal from the amp beacause its less 'dirty'..

or... would the back wave just be at the level of the speaker regadless of its quality and not affect things any more than the driver does.., but because of a poor quality speaker, the amp is not giving such a clean signal as it would with a better speaker..

don't the amp/speaker combo now look rather improtant if the above is true..

steve

[Nick]

Don't know, but I would think speakers with lighter cones, will have less stored energy so take less power to stop moving, so generate less back EMF.

[steve s]

I'm supprised now


i just put my meter on to an 'average' speaker in tems of magnet strengh

small movements fo the cone around 1 mil as fast as i could by hand gave around a 10th of a volt..

moving the cone again at the same speed, but at around 5 mil travel gave peaks of over 1/3 volt ac


bearing mind that is not anywhere near as much movement as when they are in use..
i would guess speakers are generating nearly as much voltage as they are using.. and a long throw low powered speaker motor would be generationg quite a lot, and using more voltage than than say a lowther.... and in that, the reflected voltage waveform is not the same as the signal ..because of the mechanical issues with all speakers.

although i would guess this secondary voltage will have much less current about it.. but some of it will be fed back to join another low current in the amp..that is then all amplified together..

if what we've said before is correct, that would relly mess up a pure ish signal from any amp that includes a fair amount of feed back..

if this is correct of course..

sorry to hi jack this thread.. nick is it worth spliting it off?

steve

[ed]

ah ha
so what you need to sleep soundly in your bed every night is no mass and no motor....

I think I know where to find such a device....

[steve s]

So If all the above is correct the current does explain the damping factor

i've update this below as its not right



triode stage/ high eff speaker

= high electrical current from the valves vs voltage + low watts output but a current rich signal..high damping factor... does that make sense
hi eff speaker gives less back emf so needs less damping the ratio of current in the signal vs the current from the back emf is low so theres less effect from it ?

class a triodes have a low damping factor.. not a high one so the current is not in the equation what ever damping factor is

a pentode stage/low eff speaker

= lower current vs same voltage (as above), give more watts output
less current in the signal for a given output vs voltage= lower damping factor..
wrong.. higher damping factor

the current controls the speaker not the voltage, the voltage is how loud not the control on the speaker coil ?
the low eff speakers gives off more back emf voltage, but at a low current
but because they are operationg at a low current it has more effect ?
so the feed back in the amp gives it an escape route, but it really goes back into the signal and distorts the output of the amp.. beacuse its waveform is not the same as the source..
so the feed back/ damping factor and the end quality result is dependant on the speakers..
mmm

this may well be crap.. but it might not be far off..

steve

[ed]

Sounds very plausible Steve....

..but have you explained what makes the motor inefficient in the first place, perhaps I've missed it...or are you assuming that the inefficiency is just big power in with little movement and the same would work in reverse, i.e little movement big power out...because if the inefficiency was in the motor itself it wouldn't necessarily work that way...would it?

perhaps a side by side test of an efficient and inefficient driver would prove that point....i.e how much relative power is generated.