Yup, understood. But my goal as amateur was to get an acceptable
APPROXIMATION. What I will do next is to use a tone generator and adjust the
components in the PA feedback network to get something that at least looks
like a flat curve from 20-100 Hz. It will of course be very output power
dependent. Not sure I will have the patience for that. Anyway, I still
enjoy creating something from scratch all by myself. Bought a 5.1 system,
don't listen much to it. Of some reason, I don't like it. My homebuilt with
12 loadspeakers and three digital processors gives more possibility of
variations.
Inge
-----Ursprungligt meddelande-----
Från: Guy Ramsey [mailto:[log in to unmask]]
Skickat: den 20 september 2007 18:34
Till: 'TechNet E-Mail Forum'; 'Hfjord'
Ämne: RE: [TN] SV: [TN] NTC:subwoofer
It isn't that simple. It gets harder to keep up as frequency increases. Your
number is true for a 20 Hz pulse. Not a sine wave or worse yet a 20 Hz
square wave containing all odd order harmonics up to the crossover point of
your sub filter.
If you are crossing over at 80 Hz or higher the system will sound muddy and
slow unless you calculate using higher frequency.
The acoustic output of your speakers is real power, after efficiency loss.
The relationship between the demand of your audio source material and the
acoustic output of your speakers is far more complex than the RC time
constant of the caps in the supply. The demand on the supply can only be
calculated after you can account for the loss in the system. This is the
hard part of matching a sub to the main system.
Then to make it sound good, you need to adjust for energy that feeds back
into the acoustic experience from room resonance.
Not simple.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Thursday, September 20, 2007 11:51 AM
To: [log in to unmask]
Subject: [TN] SV: [TN] NTC:subwoofer
Thanks Guy,
According to your information:
RIAA curve @ 20 Hz is 7,500 us
My power supply Ri= 0.1 Ohm
Backup caps on supply rails is 100,000 F
Tau = R (Ohm) x C (F) > 0.1 x 0.1 = 0.01 s which is 10,000 us
Not too bad! Near RIAA spec. Fine! I'm satisfied.
Thanks again. Didn't realize it was that easy.
/Inge
-----Ursprungligt meddelande-----
Från: Guy Ramsey [mailto:[log in to unmask]]
Skickat: den 20 september 2007 17:00
Till: 'Hfjord'
Ämne: RE: [TN] NTC:subwoofer
On the output? You mean on the power supply rails?
You can calculate how long it will take to discharge the caps if you know
the output impedance of the amp and the speaker load. These act in series.
With the RC time constant.
http://en.wikipedia.org/wiki/RC_time_constant
Compare the output impedance of the power supply to the audio amp. How long
it will take to charge the caps.
Then you need to decide what kind of sonic compromise you can tolerate base
on the expected duty cycle imposed on the system by the music or sound track
content.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Wednesday, September 19, 2007 4:57 PM
To: [log in to unmask]
Subject: [TN] NTC:subwoofer
Audio techies,
I've built a subwoofer with a 15" 400 Watt loadspeaker (4 Ohms). Bought a
200 Watt low frequency 12 V amplifier. For that one, I built a 12 VDC power
supply with 15 A toroid, regulator, OP amplifier and a backup consisting of
totally 100,000 uF on the output. What is your experience? Will these caps
be enough to take the short power tops in Space War or the enormous sound
effects in Matrix or Jurassic Park? Any thumb rules or maths?
/Inge
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