[sdiy] semi ot: loud speaker for synths

[Czech Martin]

well, I learned that in fact it is more
difficult then most texts say.

An amplifier is needed that goes down almost to DC.
Real amps do not do this, with reason.
An signal generator with stable amplitude and vernier
frequency control is needed, most cheapos can't do this.
A multimeter is needed that is able to give precise
AC readings at 10 Hz -20Hz. Even mid prize DMM
have 20% error here.
And you need a known resistor in the 8 Ohm range.
Most DMM can not measure this (4 pole method, Thomson).

So the need for an enclosure for the Vas measurement
seems to be the easiest thing.
I used 20mm MDF (680kg/m^3) 47x47x47 cm^3, with heavy
bracing. Not glued, but 100 screws. Can be disasembled.
(Note which sides were together !).

You could overcome all the generator/amp stuff by
using correlation techniques with pseudo noise.
But: at resonance the speaker current can easily become
nonlinear, this is where the correlation is flawed.

So my guess: my old XR2206 generator with 10 turn pot
with some amplitude regulator circuit and/or 
IC op amp power amp.
If you can not measure the low ohm resistor, buy a couple
of 47 Ohm 1% metal films and parallel them.
Finally you need to build a precision rectifier with
low pass filter. I used 2x OP07, 1N4148, and 4 capacitors,
one in the active filter (summing amp) and three larger
in the passive filter (1k / 40u, 4.7k / 10u, 10k / 4u).
Lytics, but checked for frequency. Took me 2 hours to build.

If I fix the amplitude problem, TS parameter measurement
can be made fast and easy.

Don't believe data sheets too much, +- 15% is possible.

m.c.  




> -----Original Message-----
> From: Tom McClintock [mailto:tomm at mgcap.com]
> Sent: Montag, 24. November 2003 19:36
> To: Czech Martin
> Subject: Re: [sdiy] semi ot: loud speaker for synths
> 
> 
> Hey Martin,
> 
> Is there an *easy* way to get some Thiele/Small measurements of
> loudspeakers? Relatively speaking of course ;)
> 
> 
> tm
> 
> Czech Martin wrote:
> > 
> > well, this is really on topic, because you finally need
> > a loudspeaker in order to hear your synth.
> > Ok, you can use headphones, but not all the time.
> > 
> > You can use these cheap "hifi" boxes from the mall, but you 
> will notice
> > that the synth bass attack will cause serious problems, or even
> > break the speaker. The tweeters can suffer, too, but this 
> is a question
> > of bandwidth and filter steepness.
> > Synths can make sinoidal waves with full blast, which are 
> very dangerous
> > for speaker chassis. Mechanical instruments tend to make a lot
> > of harmonics , with some exceptions like church organ...
> > 
> > I have still 8 12" woofers in the cellar. Now I made the
> > Thiele/Small measurments:
> > 
> > Fs=25Hz
> > Qts=0.42
> > VAS=180 dm^3 (!!!)
> > 
> > uuuppps, I did not expect them to be that "soft".
> > Well, a closed box turns out to have a 43Hz corner, and 94 liters
> > volume, that is a cube of (45 cm)^3.
> > A vented box will have about double that volume 190 litres!!!
> > I planed to have 4 of them
> > in parallel, in order to have low distortion sub bass...
> > The spouse acceptance criteria seems not to be met...
> > 
> > mmmmmucho disappointment after all this measurment work...
> > 
> > During my experiments I learned that only 5mm piston movement
> > is allowed , because otherwise the action becomes non linear
> > and noisy. So, displacement limits are very soon hit with
> > very little amp power. Power efficiency is no problem.
> > 
> > Could I deliberately design a "wrong" enclosure, much too
> > small and repair the resonance and missing low end via
> > electronic means?
> > 
> > m.c.
> 
>