John Curl's Blowtorch preamplifier part II

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DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
SY said:
Let's take the example of a 20Hz variation in Ed's box (.8 x 3.5 x 2.4 M). The wavelength at 20Hz is 15M. So along the longest dimension, you have less than 1/4 wavelength. The transit time is 11 milliseconds, compared to the period of 50 milliseconds.
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A rule of thumb sometimes used in RF is that you can use low frequency approximations (i.e. circuit theory) provided that the longest relevant dimension is 10% of a quarter-wave or less. 22% of a full wave means the full wave theory should be used. I doubt if sound is that different.
 
jneutron

jneutron

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Joined 2004
SY said:
True if you place the restriction that the sound must be completely absorbed within the box volume and that the absorption need be 100%.
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From wiki: hilite mine

An anechoic chamber (an-echoic meaning non-echoing or echo-free) is a room designed to completely absorb reflections of either sound or electromagnetic waves. They are also insulated from exterior sources of noise. The combination of both aspects means they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results.

Edit: btw, I take exception to wiki's definition. It is not designed to completely absorb reflections per se, it is designed to completely absorb the initial impinging wave so as to prevent reflections. In actual practice and at the microscopic level, the absorption spikes work best because they reflect multiple times, the reflections continuing towards another wall of absorption. At the macro level, there are no reflections.


DF96 said:
A rule of thumb sometimes used in RF is that you can use low frequency approximations (i.e. circuit theory) provided that the longest relevant dimension is 10% of a quarter-wave or less. 22% of a full wave means the full wave theory should be used. I doubt if sound is that different.
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It cannot be simplified in that way. If the piston volume is 10% of the cabinet, the backpressure on the driver will be significantly more than if the piston volume is .1% of the cabinet.

It is a simple case to measure the driver compliance after it is mounted to the enclosure. Effectiveness of the "anechoic" aspect can be see by how far the mounted driver's compliance varies from it's infinite baffle mount compliance. edit: another way is to look at the variation in compliance as a consequence of the driver's amplitude. At small signal, it will have one compliance, and at high amplitude, it will become stiffer. A true infinite baffle will maintain constant compliance over all amplitudes (neglecting suspension parts of course).

jn
 
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a.wayne

a.wayne

Member
Joined 2008
hhoyt said:
I am hoping you are being sarcastic, I certainly didn't state there were no differences to be heard. I stated that Richard Clark's specific test, for whatever reasons didn't indicate people could hear a difference. I was merely fleshing out what Demian had referenced. Please read what I wrote carefully if you though otherwise.

My own experience leads me to think that a lot of the perceived differences between amps do indeed have to do with performance in clipping. Especially with dynamic material, having 14-20+ dB crest factor (not that much music is anywhere near that), very little average volume would be required to induce clipping.

Viewed from a different perspective, unless you use fantastically efficient speakers, or huge power and speakers that do not suffer from power compression, when one turns the volume up to the point where you can hear deep into the mix, you will will most certainly have clipping. I have proven that to myself by using limiters and repeatedly enabling and disabling them. When clipping is occurring, the sound most definitely changes to the better by enabling the limiter.

Another amp characteristic which causes sonic issues is DC stability. There have been amps made which offset when fed asymmetrical signals, like a lot of orchestral music. During these events the woofers slew in and out, causing weird image shifts. Maybe this is mostly cured these days, I don't know. My experience in this is years old.

I am sure there are a multiplicity of other factors...which is why I joined this forum, to learn from those who design the circuitry.

Cheers,

Howie

Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
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IMO, it's usually due to the loudspeakers when one cannot hear the differences between amplifiers. Some speakers exhibit there own coloration and tone more so than others and in doing so will not change much from amp to amp. I had discovered this years ago, the better we got the speaker , the more the differences in the chain showed.

Impedance mag/phase plays big here too...

Nelson Pass said:
There is of course no technical reason why this can't be done. Being a
manufacturer, it is easy for me to provide a listener with two amplifiers with
identical exteriors and differing only in subtle ways. The amplifiers are
delivered blind, and the listener labels them to avoid confusion. They do not
get to see inside, and information regarding the differences is not provided.

The amplifiers are used in the listener's known system for as long as they
like, and upon returning them they tell me what is heard. Later, I can check
that using their labels. The test can be repeated, but care is taken that
the units cannot be identified by external scratches and such.

Anecdotally I can tell you that some people are reliably sensitive to things
that do not stand out in the "usual measurements". Not being very objective
myself, I try to keep these people around.

:cool:
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Agree, still waiting on adjustable feedback and bias knobs one on each side of the meter would be fantastic ...

:)


john curl said:
Well, that's the PROBLEM, Ray. You might not hear much difference at first, but over time, something gets annoying. Happens to me all the time.
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Experience this all too well, I have tried amplifiers which had a big powerful sound, very alive , very dynamic, all's well when playing single instrument recordings, really, really fantastic sounding. The problem really shows itself when trying to listen to music, after 5 songs, you are left bored from the dry uninvolved sound.

:(

Nelson Pass said:
You don't fool me SY. I was there when you correctly identified the sound
of one of my amplifiers as "Pentode" on music I wouldn't make my dog listen to.

:D
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Keep an extra eye on Sy, he's been known to use boutique silver cables from time to time...

:)
 
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john curl

john curl

Member
Joined 2003
Wayne, this is a SIMPLE schematic of a Transient Perfect Crossover.
It was first pointed out in the AES by a Dr. Ashley in the 1960's. I learned about it from John Meyer (of Meyersound today), back in 1974, and we made a successful 3 way all horn loaded speaker with one, in 1974. It DOESN'T WORK for just any set of speakers, we can talk about where it really fails. However, IF you sum the hi pass and the low pass network back together, you will get the ORIGINAL SIGNAL.
It is SALLEN-KEY, not Salen-Key
 

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SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
hitsware said:
I made one like that and someone sniveled
because the out from the subtractor wasn't
precisely 12 db/oct
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It can't be, otherwise the summed output waveform would be different than the input. Subtractives always give you asymmetrical slopes except for first order.

ezavalla said:
That circuit isn't a substractive xover because it has an adder at the output, so it isn't substracting anything... :confused: :confused:
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Someone forgot to invert polarity. :D
 
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