On 10/9/06 6:38 PM, in article
, "Jerry"
wrote:
John Stone wrote 10/8/2006: On 10/7/06 8:40 PM, in article
, "Jerry"
Yes, I read that. It wasn't Chuck McShane that wrote this, it was his
nephew
writing what he BELIEVES Chuck McShane said. Hardly authoritative. The
context of this post is totally missing, and the meaning of the word
"remove" is misleading at best. He could easily have meant removing the L
pad aspect of the pots as opposed to removing the pots completely. I find
it
hard to believe Chuck McShane would have recommended just pulling them
out.
Well, John, it was published in the AR library and someone would have
complained vehemently if it was totally wrong.
Jerry, AR is long gone. The "AR library", as you put it, is nothing more
than a loose collection of posts and articles from DIY folks and others with
an interest in and nostalgia for the speakers. There's nothing official
about it and while there's lots of good stuff there, there's also some
really questionable stuff. Nobody polices it for accuracy, so it should not
all be taken as bible.
I think this is the part
that is most convincing:
"Even wide open, the pot reduces tweeter output by about 1db."
So, if the tweeter pot is set to max and the pot opens up, the net change in
the tweeter is 1 db. However, if the wiper in the pot is corroded the
reduction can be far, far more substantial than 1 db.
What a novel interpretation! Jerry, the term "wide open" is common
terminology to describe a pot being set to maximum, not a pot that is
electrically open. The 1dB simply refers to the insertion loss of the pot,
operating properly in circuit, at full setting.
No, my question is NOT how you can remove the pot and still maintain the
proper balance. It's how you can remove the pot and still maintain the
proper crossover frequency.
John, xover frequency is determined primarily by the reactive components.
Any filter network's turnover frequency is calculated through the values of
source impedance, the value of reactive components, and the impedance of the
load. Don't believe me? Then substitute a 100 ohm resistor for the tweeter
and run a sweep of the crossover. I'm sure you'll you'll see a pretty huge
change.
The coils and caps are all still there. The xover for the mid-range is
really a band pass and I have tested the xover to see whether it was in fact
working.
I did this by applying a full frequency signal to the mid/tweeter (did
exactly the same for the woofer). So a full frequency and full strength
voltage was applied to the mid/tweeter and I measured the current actually
going to the drivers. In short, the xovers were performing perfectly.
That is, they were "blocking" (little or no current) was flowing for the
frequencies NOT intended for the respective drivers.
I have read this paragraph over and over, and for the life of me, I can't
figure out what you are talking about. What kind of "full frequency and full
strength voltage" are you referring to? How do you accurately measure the
crossover response with a "full frequency" signal? Do you have a spectrum
analyzer? The simplest way to accurately measure the response is to run a
sine sweep and plot the voltage/frequency curve using an rms voltmeter at
the driver terminals. Do it with and without the pot in place an you'll get
an exact picture of what is happening.
You also keep using the term "current". How, and why are you are measuring
current (amps)?
Lastly, John, as you stated in your other post, the net resistance of the 16
ohm pot and the mid-range driver (with the pot at max) is still pretty close
to the resistance of the mid-range driver.
??? I never stated any such thing. Please provide a quote, and please show
me where I ever used the term "resistance" in reference to the drivers.
Not my rules. Function of the pots is to balance energy going to the
respective drivers to adjust for room conditions and RECORDINGS. I can
perform exactly the same function with the volume controls.
No you can't. You pulled out 2 pots and substituted them with 1 amplifier
gain control, driving the entire mid/tweeter section. You have removed one
of the pots' key functions, which was to allow shelving of the mid and
tweeter levels individually.
With a single amp, John, you have no where near the control over sound that
I have. And with harmonics riding on fundamentals, you have no where near
the headroom that I have.
I have no idea what this means.
As an aside, John, a totally flat response on AR-3a's is achieved with the
pots at max AND a slight increase in the treble volume control.
Anybody who has actually measured AR3a's as opposed to just reading the AR
literature will tell you this isn't true. Here's an actual real world
measurement of the 3a's:
http://murphyblaster.com/content.php?f=AR3a.html
These measurements are backed up in a post on 8/7 by Dick Pierce to you
regarding the frequency response of the AR3a:
"No, the reason for this is that Roy Allison and Ed Villchur at AR
decided on a balance that resulted in an overall downward trend
in the speaker's response toward the high end. That was their
choice, for whatever reasons. The midrange efficiency is about
2 db less than the woofer, and the tweeter is another 2-3 dB
less efficient than that. That's what the people of AR designed
it to be." (end quote)
A "slight increase" in the treble control will not achieve a totally flat
response as is clear from the actual response measurements.
...
Jerry, your ignorance of even basic loudspeaker design principles is
appalling. If I didn't know better, I'd swear you were joking. The tweeter
network in the AR3a consists of a single series element; i.e., a 6uf cap
feeding a load consisting of a tweeter and a 16 ohm tapped resistor. For
all
intents and purposes, when you run the tweeter pot at full, you simply
have
a tweeter and a 16 ohm resistor in parallel. The crossover frequency is
determined by the value of the series cap AND the impedance of the load.
Removing that 16 ohm resistor alters the load (increases the impedance),
which in turn alters the crossover frequency, moving it downward. I'm
quite
sure what you've done IS significant,
My ignorance, huh? John, you spouted a lot of "facts", but not a single
number. Are you lazy, John, or perhaps you don't know how to do the math.
Let's just see how significant.
DC resistance of tweeter is approx. 3.2 ohms
So with the 16 ohm pot full across - net resistance = 2.67 ohms or a
difference of .53 ohms
Sorry Jerry, but yes, your ignorance, or perhaps just lack of attention to
detail, as again demonstrated above. DC resistance is utterly meaningless to
this discussion, as the last time I checked, tweeters required AC to produce
sound. Yet you use resistance interchangeably with impedance as if they were
one and the same. The "nominal" impedance of a tweeter with 3.2ohm dcr is 4
ohms. If you take that nominal impedance in parallel with the 16 ohm pot
(the pot will have an impedance close to 16 ohms given that it is largely
non-inductive) the 2 in parallel yield a resulting impedance of 3.2 ohms.
With the pot totally out, you shift the impedance up by a minimum of 20%.
Real, real significant, John ... huh??
Um, yeah, I'd say by any measure a 20% shift is significant. And in a
previous post, you yourself said so:
As for the improvement in sensitivity, all I can say it's significant
and very noticable.
Those are YOUR words, Jerry. So which is it? Significant or not? 20% is way
beyond the impedance tolerance AR specified for the drivers alone, so the
crossover is indeed operating into a significantly different load than
intended, with the resultant shift in xover response as previously stated.
More importantly 20% is the MINIMUM shift. In reality, the tweeter (and
midrange) impedance curve is nowhere near flat, and deviates considerably
with frequency. Given that these drivers use no ferrofluid they will exhibit
a fairly high Q at mechanical resonance ( which I'd estimate around 300Hz
for the mid and 3kHz for the tweeter), yielding an impedance peak at
resonance perhaps double the nominal value. The driver impedance also rises
gradually with increased frequency, due to the voice coil inductance. So the
pots were there for 2 reasons: provide level adjustment, and to provide a
measure of impedance stabilization for the crossover. By removing them, you
have changed things a whole bunch.
Tell us, John, do you
think there is a human anywhere ... that could actually hear this
difference?
You said you could hear it yourself, so I'd assume others could as well.
Perhaps your arguments would have some weight, John, if you backed them up
with the math.
There's plenty of "math" above. And it least its based on correct parameters
(impedance rather than resistance), which makes your "math" less than
useful.
But let's just say that those are quite fragile and you've
definitely moved things in the wrong direction regarding the chances of
surviving your "improvement". If you still don't get this, here's a link
that demonstrates, in simple terms, the relationship between load
impedance
and crossover frequency:
http://www.carstereo.com/help/Articles.cfm?id=1
What in heck are you talking about?? What wrong direction? 0.5% more
current at 6000 Hz than at 3000 Hz!! Who cares??
Now, if you are talking about sensitivity in total .... YES, for the same
voltage we get approx. 11% more current going through the tweeters. We must
absolutely reduce the voltage in the mid/tweeter amp or the sound would be
too bright.
What in heavens name does that have to do with "fragile"?
Pushing down the crossover frequency puts more strain on the tweeter. Like
it or not, those tweeters were notorious for marginal power handling
ability. A.75in voice coil on a paper former with no ferrofluid cannot take
much power without burning out.
John, this is getting a little tiring. You have never quote a single fact
... just total supposition.
Jerry, you're the one making all the claims. It's up to you, not me, to back
your claims with facts. I and others smarter than me have provided plenty of
opposing facts, all of which you have chosen to ignore. And you think you're
tired?
Here is my charts and my graphs and facts :
http://www.classicspeakerpages.net/d...forum=3&topic_
id=10658&mesg_id=10658&listing_type=search
Nothing more than the same old stuff you've been saying here for the past
few months. I've read it all and there's nothing further to learn from it.
You have come to the conclusion that ANY amp will drive them
better with split x-over based on what? Using what even you admit is a
pretty marginal amp for those speakers? Many here have told you from their
own experience that using a single amp of adequate power and stability for
these speakers will get as much out of them as they are capable of giving.
Look over the impedance charts, John. Then let's see your facts. Your
charts. Your experiments ... or do we have an empty bag?
Jerry, as others here have told you countless times, nothing is to be gained
in your configuration compared to a single amp, as long as the driving
amplifier has adequately low output impedance and sufficient undistorted
power to drive the speakers to the desired level. Virtually all competent
amplifiers act as voltage sources, and within the limit of their power
output capability, perform largely independent of the load they see. The
AR3a designers knew this and designed the speakers accordingly. You have
never provided a shred of proof that your configuration solves any problem
that couldn't be solved with a single more powerful amplifier. To now
extend this argument to ALL amplifiers, regardless of power, is absurd and
based totally on unsubstantiated generalizations about power amplifier
behavior. Yes, by using 2 amps, you can now adjust the mid and tweeter
outputs to levels above the woofer, but that was never the intent of the
original design.
As for my amps, after looking over the facts ... I believe I have far, far
more voltage headroom than anyone with a single amp.
I know what you believe. Now, please explain the basis of this belief. Let's
see your experiments or charts showing your "biamp" configuration with
ancient low powered amplifiers of dubious quality having far more headroom
than a single 250 watt/ch amplifier with total stability down to 2 ohms.
Your claim, your proof.
Which brings up another question: If using separate amps gave such a huge
improvement, why didn't AR suggest it in the first place?
I don't believe at the time that the AR's were developed, people were even
experimenting with bi-amping for home use.
I don't believe back in 1968 ANYBODY was thinking about bi-amping for home
use, but please share a reference showing that I am wrong. Quote any
article from 1968 or earlier recommending bi-amping in the home.
Having lived during that era, I can assure you that biamping was indeed
being done in home systems well before 1968. Ever hear of the Marantz 3?
It's an electronic crossover from the '50s. In fact, even Heathkit offered
one called the XO-1 which also dates back well before 1968. There were
others.
I'm not so certain there are comparable drivers available today.
There aren't. Nobody buying a high quality driver today would accept the
performance compromises in power handling, sensitivity, or frequency
response.
It seems
that "quality" anything is harder to find. Just look at the products
offered today in terms of receiver and speaker systems.
Please look a little more closely. You will find plenty of examples of very
high quality loudspeakers that will outperform the AR3a by a wide margin.
Contrary to what you seem to believe, advances in loudspeakers did not end
in 1968.
Jerry, the last word is all yours. I won't be posting any further responses
to this thread in the hope that it dies the merciful death it so deserves.
Enjoy your speakers.