[Imperial]

Adding a conjugate network to the woofer section to stabilize high frequency impedance to a value below the characteristic impedance of the speaker wire would fix the problem.

If the vmps 60 is anything like the vmps 40 vmps, it's load to the amp is resistive above say 166hz.
So looking at the woofers in the speaker could be a thing to do here yes.

While I don't think one should go into personal attacks, like you do in some above posts in this thread, shame on your lilly ass for that, I think you have described the cure here rather apt, because this speaker is most likely purely resistive above 170hz or something like that.

However, the speaker is designed to have it's filter the way it is. So another speaker cable it is.. 

I've got a question also.
The restive ribbon, does that act as a drive resistor? (or what you call it, I'm not so good with terminology..)

I also would ask you Daryl, to ignore people that are trying to pick a fight.
Let them write what they want. Just answer to the best of your ability and leave it at that.
I think you've got your head on the right way! Include your mouth and it will be super!! 

Imperial

[*Scotty*]

It would be nice to see an impedance curve of the of the V-60, something might be learned from that. Paul, you might also contact Acoustic Zen and ask them what the capacitance per
foot is on their speaker cables and ICs. Unfortunately Imperials assumption about the resistive nature of the load fails to take into consideration the fact that there is a crossover with
both capacitive and inductive elements loading the amplifier.
Scotty

[opnly bafld]

If the vmps 60 is anything like the vmps 40 vmps, it's load to the amp is resistive above say 166hz.
So looking at the woofers in the speaker could be a thing to do here yes.

According to the VMPS website above 280hz.

[PLMONROE]

Even if the amp is not shutting down now, the excessive capacitive load is still doing very bad things to the linearity of the amplifier.  The internally oscillations are slightly less now, but still there.  Making a change at the input will not affect the bad effects of the capacitive load on the output, unless there is a big interaction in the internal feedback loops affecting the dynamic input impedance of the amp.  Then it is possible that the different input load could be damping the internal oscillations.  It is interesting to put an amp misbehaving like yours on the test bench, and move the scope probe from the output when it is oscillating to the input, and find the same oscillations showing up there!

Please please go back to simple two conductor speaker cables, available at Home Depot for about 25 cents per foot.

Regards,

Frank Van Alstine

Good point! 

Paul

[JoshK]

It would be nice to see an impedance curve of the of the V-60, something might be learned from that. Paul, you might also contact Acoustic Zen and ask them what the capacitance per
foot is on their speaker cables and ICs. Unfortunately Imperials assumption about the resistive nature of the load fails to take into consideration the fact that there is a crossover with
both capacitive and inductive elements loading the amplifier.
Scotty

Even better would be impedance and phase.  Low and resistive isn't nearly as difficult as low and reactive. 

[jneutron]

A couple of points.

1.  Daryl...I was pleasantly suprised to see you extend the t-line argument I developed years ago, to the issue of unloading a cable at higher frequencies.  I make the assumption that you read it in a post of mine, but if not, I'd love to find out if you derived it yourself, or the source you found it at.  I've never seen it other than within my writings.

2.  Daryl...The fact that John Curl made decisions with respect to his amplifier design that does not provide unconditional stability under every load known to man, is his decision to make.  He understands the tradeoffs quite well, and made a business as well as intellectual decision with respect to his design.  He wanted his product to do something specific, and chose not to "water it down" for all possible conditionals. That in itself, does not mean it is a bad design.   While John and I have a history of "animosity" that goes back years, I must state that I believe him to be an excellent designer of the product he creates.

 I make widgits that can blow up if their temperature rises half a degree during operation, but that is a design decision..not bad design.

3.  Art.  I would ask only a tad more tolerance.

4.  Dyohn...Your statement could have been worded nicer, but you made a nice point with respect to level of effect and audibility.  Given what Daryl stated regarding unloading at the high freq end, It may not actually be the capacitive lagging storage that is the audible part, but rather, what the amplifier does in response to it.  Certainly a question which bears investigation, eh?

Cheers, John

[opaqueice]

A couple of points.

1.  Daryl...I was pleasantly suprised to see you extend the t-line argument I developed years ago, to the issue of unloading a cable at higher frequencies.  I make the assumption that you read it in a post of mine, but if not, I'd love to find out if you derived it yourself, or the source you found it at.  I've never seen it other than within my writings.

Huh...  I read Daryl's explanation as a nice exposition of standard RLC circuit theory.  As far as I can see, everything he said follows immediately if you sketch the equivalent circuit, treating the cable's resistance and inductance as in series with the load and its capacitance in parallel.  That's why I liked it so much - it made perfect sense to me. 

I still fail to see why this has anything to do with T-line theory, but I'm a physicist not an EE and don't think about these things all the time - so I'm happy to learn.

[jneutron]

A couple of points.

1.  Daryl...I was pleasantly suprised to see you extend the t-line argument I developed years ago, to the issue of unloading a cable at higher frequencies.  I make the assumption that you read it in a post of mine, but if not, I'd love to find out if you derived it yourself, or the source you found it at.  I've never seen it other than within my writings.

Huh...  I read Daryl's explanation as a nice exposition of standard RLC circuit theory.  As far as I can see, everything he said follows immediately if you sketch the equivalent circuit, treating the cable's resistance and inductance as in series with the load and its capacitance in parallel.  That's why I liked it so much - it made perfect sense to me. 

I still fail to see why this has anything to do with T-line theory, but I'm a physicist not an EE and don't think about these things all the time - so I'm happy to learn.

Ewww. a physicist..   (seriously, some of my best friends are physicists...)

If the load matches the line, the amplifier will not see the capacitance nor the inductance of the line.  For that specific case, the lumped elements dissappear..that is exactly what t-line theory tells us.

If one were to be rigorous in the application of lumped elements, the line has to be modelled as a long string of LC's..  Otherwise, the models fall apart when the load matches the line.

Cheers, John

[jneutron]

Here's a good example:

Assume that you've built a cable.  It specs are:

19.2 nH per foot
300 pf per foot
#8AWG equivalent
10 meters long

The lumped element numbers are: 300 pf per foot times 32.8 feet, or 9840 pf...and 19.2 times 32.8, or 629 nH.

The cable impedance is:

Z = sqr(L/C) = sqr(19,200/300) = sqr(64) = 8 ohms.

The cable dielectric constant is:

LC/1034 =  19.2 * 300 /1034  = 5760 / 1034 = 5.57       (note, this equation uses L in nH per foot, and C in pf per foot..)gotta watch the units..

The cable propagation velocity is:

v = 1/sqr(dc) = 1/sqr(5.57) = 1/2.36 = .423 lightspeed

Now, terminate this cable with an 8 ohm resistor.

At the drive end, what does the amplifier see?

Does it see the 9.84 nFarads?   Does it see the .629 uHenries?

No to both.  The amp sees 8 ohms.

Now, remove that cable, and replace it with an 8 ohm impedance cable made with american superconductor's G-2 product, and make it a thousand miles long, using the same dielectric coefficient insulation, and run it at 77 Kelvin. (course at 40 dollars per meter, this is certainly a thought experiment..)

What does the amplifier see?  Does it see the 1.5 millifarad of capacitance and the 101 millihenries of inductance?  No.

It sees 8 ohms.

The only way to tell the difference at the amp end, is to mismatch the load to the cable.  In fact, of one wished, it is possible to plot the settling time at the load vs the ratio of line to load...at unity, the minima cusp of the plot reduces to exactly the transit time of the cable.

Cheers, John

[opaqueice]

Ewww. a physicist..   (seriously, some of my best friends are physicists...)

And some of mine are engineers  .

If the load matches the line, the amplifier will not see the capacitance nor the inductance of the line.  For that specific case, the lumped elements dissappear..that is exactly what t-line theory tells us.

If one were to be rigorous in the application of lumped elements, the line has to be modelled as a long string of LC's..  Otherwise, the models fall apart when the load matches the line.

OK, that sounds like a useful thing for me to try to understand.  You're saying if the impedance of the load equals the impedance of the cable, then I can't model the cable by a simple equivalent circuit (with one L, or C, one R) and must instead use a whole series of such elements, one for each bit of the cable?  Is that right? 

Sorry to be so slow...

[jneutron]

Ewww. a physicist..   (seriously, some of my best friends are physicists...)

And some of mine are engineers  .

Not that there's anything wrong with that

If the load matches the line, the amplifier will not see the capacitance nor the inductance of the line.  For that specific case, the lumped elements dissappear..that is exactly what t-line theory tells us.

If one were to be rigorous in the application of lumped elements, the line has to be modelled as a long string of LC's..  Otherwise, the models fall apart when the load matches the line.

OK, that sounds like a useful thing for me to try to understand.  You're saying if the impedance of the load equals the impedance of the cable, then I can't model the cable by a simple equivalent circuit (with one L, or C, one R) and must instead use a whole series of such elements, one for each bit of the cable?  Is that right? 

Yes.

Sorry to be so slow...

You're not.  E/m theory is one of the most difficult subjects taught as part of a core curricula.  Almost as difficult as the course that teaches one NOT to hook a 12 ounce claw hammer on the top rung of a 24 foot extension ladder just before you try to move it.......(don't ask, I failed that course..)...

But the voices did stop after that...

Cheers, John

[opaqueice]

You're not.  E/m theory is one of the most difficult subjects taught as part of a core curricula.

EM in physics (Maxwell's equations, radiation, some plasma physics) I'm very happy with - in fact I teach it sometimes .  But circuits, antennas etc. aren't a big part of that - we leave those to the EEs!

Anyway, when I get a chance I'll take your example above (which was perfect, thanks), replace my equivalent circuit with a long series of LC elements, and see if I believe you. 

[jneutron]

You're not.  E/m theory is one of the most difficult subjects taught as part of a core curricula.

EM in physics (Maxwell's equations, radiation, some plasma physics) I'm very happy with - in fact I teach it sometimes .  But circuits, antennas etc. aren't a big part of that - we leave those to the EEs!

Anyway, when I get a chance I'll take your example above (which was perfect, thanks), replace my equivalent circuit with a long series of LC elements, and see if I believe you. 

You teach that stuff??  Your as bad as me..

I'd love to see your model and the results..I remember the textbook back in '73 said the t-line model was lots and lots of lc's, but back then I took the author's word..

Here we have kicker magnets which require very high rise time flat pulses, 20 to 30 kA and 10 to 15 kV, rise at 100 nsec or so, flat for tens or hundreds of microseconds.. and they use what's called a "pulse forming network".  Some simply charge a long coax and use a thyratron to discharge it into the magnet, others use a bank of caps and inductors to simulate a t-line of the correct length.  You might be able to find some guidance there for how many elements are required to closely model a t-line.

Cheers, John

[Imperial]

Uhm, so what did the amp "see" that made it shut down then? 

Imperial

[jneutron]

Uhm, so what did the amp "see" that made it shut down then? 

Imperial

I believe the general consensus was that it was oscillating at some high frequency as a result of capacitive loading of the output terminals, and the fact that the woofer is incapable of holding it's impedance low at that hf.

My conjecture is that if the cable is loaded at the woofer end with a highpassed 8 ohm resistor, to force the speaker line to not go so capacitive, it may stop.  This seems consistent with what the OP described.

A theory at least.  I hope somebody can try that..maybe the OP could put an 8 ohm resistor/cap combo at the woof to load down the hf.

Cheers, John

[Imperial]

So Daryl came close in his assessment then?

Imperial

[*Scotty*]

So if I understand this correctly we are talking about a zobel network at the speaker terminals to stabilize the amplifier. A good way to do this is to put the components into a Pomona dual banana plug and pop it into the speaker terminals, in or out in five seconds. You can also use this to stop ringing when driving the speaker full range but you may notice the high frequency air to be a little reined in. The Goertz speaker cables now come with a compensation network built in to avoid the problem produced by their inherent high capacitance with some amplifiers.
Scotty

[jneutron]

So Daryl came close in his assessment then?

Imperial

Actually, what I've said is that daryl and I both are in agreement.

I did not say that we are correct..there is a difference...

If anybody could test the fix I mention, and it worked, then we'd be closer to being correct.

Scotty:  I believe the OP mentioned it was only a problem when he bi-amped, and the woof amp cutout.  So I don't think the hf's are so much an issue.

Cheers, John

ps..love the Z pinch pic.

[avahifi]

So why not just go back to Radio Shack and buy some 25 cents per foot 14 gauge standard low capacitance speaker wire?  The object is the music, the best music possible for the money.

This all kind of sounds like "how to make my Rolex keep exact time."  The answer of course is buy a Timex.

Not that I am knocking Rolexes, I have one, its jewelery, my only piece of jewelery, but I don't confuse it with WWV time.  Now I am going to hear bad things from those who "know" that Rolexes aren't really the best of jewelery.  I don't care.  

Best regards,

Frank Van Alstine

[TONEPUB]

Good one!