John Curl's Blowtorch preamplifier part II

[simon7000]

And this from the guy trying to show twisted pair coupling by dropping a chunk of metal in the middle instead of winding the DUT around it?

And then there was the turntable snyc to match phase?

If I didn't know you wore starched undies, I'd really dis you.

 

[scott wurcer]

Now that back energy must go somewhere or else Scott would be unhappy.

What is not so obvious is that the amplifier has to sink this energy. In oversimplified theory but not in practice as much energy as you put into the transducer can come back at you.

Yes I know, high end audio likes to not be subject to first principles. I doubt you will find a speaker to be 100% reciprocal unless the voice coil is superconducting. How much acoustic power is 90dB SPL on a 1 meter hemisphere and compare to 1 Watt in (random typical speaker)?

 

[jneutron]

And this from the guy trying to show twisted pair coupling by dropping a chunk of metal in the middle instead of winding the DUT around it?

Tell me you understood the testing I did..puhleese...

I kept it so simple, you had to understand, no???

If you recall, I used two metals, one with relative permeability of 1, and one with relative permeability of about 200... to demonstrate coupling and lack of coupling based on current cancellation and twist pitch...

Will I have to provide you remedial teaching?

And then there was the turntable snyc to match phase?

You got me on that. What in the world are you talking about??? Is this about my club DJ experience with phasing/flanging as a sound effect??

If I didn't know you wore starched undies, I'd really dis you.

Who said I wear undies?

jn

 

[simon7000]

Yes I know, high end audio likes to not be subject to first principles. I doubt you will find a speaker to be 100% reciprocal unless the voice coil is superconducting.

Would you believe "Near Superconducting?" Maximum efficiency is around 30% for a good quality pro woofer, last I paid attention. However one stunning artifact ignored by many is (hold on tight...) Outdoors the air moves by itself!

This is called wind and can actually create significant back voltages.

The other issue (True, tempting JN.) is that typical loudspeaker wiring is 10 gauge twisted pair with 2 -3 twists per foot. One stadium design had 100++ pairs all in the same 3" conduit. In the 1400' run how much woofer signal would the tweeter feedback design amps see?

 

[jneutron]

The other issue (True, tempting JN.) is that typical loudspeaker wiring is 10 gauge twisted pair with 2 -3 twists per foot. One stadium design had 100++ pairs all in the same 3" conduit. In the 1400' run how much woofer signal would the tweeter feedback design amps see?

None.



That's because a code inspector noticed you exceeded 40% fill, and issued a stop work order.

edit:

This is called wind and can actually create significant back voltages.

And yet, your keyboard continues to operate..

bada boom.
jn

 

[scott wurcer]

Would you believe "Near Superconducting?" Maximum efficiency is around 30% for a good quality pro woofer, last I paid

Reference? JBL Selenium 12" is speced at 2% same for their silly 15" car subwoofer. Answer to above - 100% efficiency is 112dB/W @ 1m

 

[Kindhornman]

As long as we are talking about phenomena that are normally not considered consider this. While you are looking at the phase lag of the back emf wave caused by stored energy in the device or from the box tuning how about the second speaker in a stereo pair that is being modulated by that same air and that speaker cabinets port resonating with the sound from the first speaker producing a second back emf voltage operating on that same amplifiers second channel. What would be the phase angle of that second emf source and how would that affect the amplifier at high spl output levels?

 

[a.wayne]

Dick,

I'll be glad to answer your question. 2 ohms for a JBL 8 ohm rated professional series cone transducer.

Now the background. JBL used to offer the same transducer for both pro and consumer use. It would be rated 4 ohms for the consumer and 8 for the exact same model sold to a pro. That was because pro amplifiers could handle the dip in impedance and consumer gear didn't.

Now as we are talking about transducers, it should be clear that the DC resistance of the voice coil contributes nothing to the actual output energy. So for a midrange or higher frequency driver you are misleading if you use an impedance value of 1,25 times the DC resistance as the impedance in the working frequency range is often higher than that,

There are two issues that raise the impedance, one that is well known is that there is inductance in the voice coil. The other that should be obvious but is often ignored is that the devices are transducers! That means they convert electrical energy into motion which moves the air and produces sound. As that we requires energy we see it as additional resistance. What is important is that in a transducer it works both ways, so motion energy will also produce electrical energy.

Now that back energy must go somewhere or else Scott would be unhappy. It goes back into the amplifier. Or in other words the amplifier provides damping to the loudspeaker. Now most know about amplifier damping and that it decreases with increasing frequency as the gain available for feedback diminishes.

What is not so obvious is that the amplifier has to sink this energy. In oversimplified theory but not in practice as much energy as you put into the transducer can come back at you. So in simple terms the 8 ohm loudspeaker when it dips to 4 ohms can reflect back energy and behave as if it were two ohms.

2245H? And Crowns , neezus they sounded bad in the 70's,80's,90's better today ..?

 

[Kindhornman]

a.wayne,
Crown DC300 Yuk!

 

[DavidL]

That's ok. Physics is pretty much the same. I do find however, your knee jerk bashing of everything JC says to be a bit tiring. Perhaps you could find within yourself the ability to tone it down a tad? I'd appreciate it.

Sorry but speaker design has changed "just a little" since the 70's. Sorry but I'm not here to bend to your wishes. When someone claims they know more about speakers than 90% of the people on here yet has shown us that they have next to zero experience about it in fact, then I will speak up. You don't have to read it by the way.So tone down your own rhetoric just a tad okay?

Perhaps. What does concern me is the plotting data of speakers which interact with the cabinet to achieve the impedance shown on the graphs. There is a detachment between the sine reactance and the step reactance when multiple energy storage mechanisms are at work within a cabinet. Case in point, reflex and tuned enclosures which introduce group delay, vs horns or overdamped infinite baffle. edit: In designing output protection, the step reactance is important.

So show us how to "really" do it then instead of complaining. I see nothing wrong with how Stereophile presents the data.

I understood that. However, as you stated, ""Of course speaker impedance load has to be taken into consideration"" (with amp design). Since JC is indeed an amp designer, I would assume he does do that.

As a buyer, I would expect a reasonable level of speaker load expertise on the part of the designer of any power amp equipment I would purchase..

jn

Designing an amp that can handle 2 ohm loads has been done for years in the car audio industry. It isn't that hard apparently. Most if not all amp manufacturers state what minimum load their amps can drive.If speaker manufacturers want to deviate from that norm then they should state what is required to drive their speakers.Wilson seems to have a "whatever" attitude about impedance and so buyer beware.

 

[scott wurcer]

As long as we are talking about phenomena that are normally not considered consider this. While you are looking at the phase lag of the back emf wave caused by stored energy in the device or from the box tuning how about the second speaker in a stereo pair that is being modulated by that same air and that speaker cabinets port resonating with the sound from the first speaker producing a second back emf voltage operating on that same amplifiers second channel. What would be the phase angle of that second emf source and how would that affect the amplifier at high spl output levels?

Folks always talk about back emf, it's the short circuit current that the other channel needs to eat. Very, very easy experiment play one channel and measure the other (probably why no one has done it).

 

[john curl]

80% then? I do not know YOUR background, because you refuse to expose it.
I have used Wilson speakers as my reference home units since the mid 80's. Maybe, I was able, from the experience, to make better amps, than I might have made in the 70's, from this.

 

[a.wayne]

So we need a 4 ch chip for stereo....

 

[jcx]

you need 2 measurements: open circuit V, and short circuit I

and with Z you can calculate max available power coupling

for two dynamic direct radiator drivers in separate R,L channel boxes, typically spaced in normal domestic room the electrical/mechanical/audio/mechanical/electrical number is going to start with a few zeros right of the decimal point

 

[a.wayne]

a.wayne,
Crown DC300 Yuk!

Ohh, there were many more after that one .....

 

[john curl]

Scott, it did it once with 2 K-horns, back in 1978, and showed to HK, when I was a consultant there. It is real.

 

[jcx]

real, real small - in the context of a conversation about how many amplifier power doublings you need from nominal rating? - 10s of amps versus 10s, even 100s of mA?

audio coupled "back emf"/current simply doesn't enter into significance for audio power amplifier output design given the magnitude being orders smaller than the uncertainties in loudspeaker load, even than just the nonlinearities of most speaker Z

 

[simon7000]

Reference? JBL Selenium 12" is speced at 2% same for their silly 15" car subwoofer. Answer to above - 100% efficiency is 112dB/W @ 1m

Oops typo 3.0%

120 db is defined as 1 Wa. For 1M Lw=Lp-10log(.08/1**2) or Lw=Lp+10 so I get 100% =110db. (Drats now those who read this will know what a Wa is.)

But http://www.jblproservice.com/pdf/thiele small parameters/theile parameters.pdf gives the efficiency of most of their product line. Some actually hit 6% over the rated FR. I suspect spot frequencies may be higher.

As long as we are talking about phenomena that are normally not considered consider this. While you are looking at the phase lag of the back emf wave caused by stored energy in the device or from the box tuning how about the second speaker in a stereo pair that is being modulated by that same air and that speaker cabinets port resonating with the sound from the first speaker producing a second back emf voltage operating on that same amplifiers second channel. What would be the phase angle of that second emf source and how would that affect the amplifier at high spl output levels?

When doing critical listening to loudspeakers it is best to only have the set you are testing in the room. A second unpowered set will resonate and change things.

Folks always talk about back emf, it's the short circuit current that the other channel needs to eat. Very, very easy experiment play one channel and measure the other (probably why no one has done it).

Electrical crosstalk is often measured that way. Take a look at the damping factor measurement test for another example of electrical tests.

Now as to acoustical crosstalk. If you have a woofer and midrange in the same box... But actually if you have a woofer excited with a unit impulse it will ring a bit and that energy needs to be damped. In music a cymbal or drum strike is as close as you get to a unit impulse.

Scott, it did it once with 2 K-horns, back in 1978, and showed to HK, when I was a consultant there. It is real.

Horns are misleading as they do not radiate into a full sphere.

 

[a.wayne]

When doing critical listening to loudspeakers it is best to only have the set you are testing in the room. A second unpowered set will resonate and change things.

You can shunt the load and turn them in ...

 

[simon7000]

None.



That's because a code inspector noticed you exceeded 40% fill, and issued a stop work order.

.
jn

40% fill is not applicable to low voltage wiring. It just isn't possible to fit that much wire into the conduit.

We did use two 4" conduits. Now allowing for the 6 db crosstalk increase that comes from the conduit, care to guess what the cross talk is?