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Old 11th June 2014, 06:18 PM   #11
Philfr is offline Philfr  France
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Hi Tom,
Maybe you are right..
Sub is two Alpha 15A in H frame, so they give about 95 db (with what you call room gain, I prefer say "walls effect" because listening room is not a amp).
If I connect TA2020 , it does not work, TDA 2050 (17/0/17 volts) it does not work too. No bass magnitude, no bass dynamic, flat sound.
If I use mosfet amp (190 watts, weight 18 kg), it's works.
Tell me why ?
Thank's in advance.
Phil.
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Old 11th June 2014, 07:25 PM   #12
GoatGuy is offline GoatGuy  United States
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Contrary to other advice, I say "go ahead and build it" (the dual, parallel, integrated-chip amplifier)

ONE important thing though - it'll be critical to shim the gain of each half to be quite similar and symmetric. LOOK AT the PA-100 diagram. Note that there are 3 resistors in parallel (cheaper than buying a big wire-wound resistor) on the output of each chip-amp. These are called "current steering" resistors, since they work to decouple the load from the output of each opamp a bit. 3 resistors each, at 1/2 ohm is 1/6 ohm per stage. Two of those in parallel (the two stages), and you're now at 1/12th effective ohm of series resistance to your subwoofer speaker.

Since "ohms is ohms" and P = IČR ... with no squaring or other "power" fiddling with the R, then if R = R1 + R2, you can also say that P = IČR1 + IČR2 = P1 + P2, etc. Which is to say, power delivered will be shared by the impedances in series, in proportion to their values.

Therefore, for a nominally 4 ohm subwoofer, and a 1/12th ohm series current-steering resistor network, 2% of the output power goes to heat the resistors instead of powering the load. And, given our memory of decibels ( dB = 10log10( pA / pB ) ) then

dB = 10 log10( (1 - 0.02) / 1.00 )
dB = 0.088

So, you'll be losing a completely inaudible 0.1 dB (close) of sonic energy. Seems like a good trade-off.

NOTE though - all this assumes that you use precision resistors for R3, R4, R8 and R9 at the least. You'll probably want precision resistors for R2 and R7 as well. These are the resistor values that set gain for each half of the parallel circuit.

GoatGuy
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Old 11th June 2014, 08:02 PM   #13
Philfr is offline Philfr  France
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Hi GoatGuy,
Thank's for your advice. I appreciate your accurate help..
I think buy several Dale resistors, match it, and connected like in this schematic (PA100) :
http://www.shine7.com/audio/PA100_schematic.gif
For a 300VA 24 volts transformer, I think using 3x10000 uF per side.
Phil.
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Old 12th June 2014, 05:42 AM   #14
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The problem with an open baffle is that it is very inefficient at low frequencies. So you need a lot of power for a sub. Try one of those drivers in an enclosure to hear what is being lost at low frequencies.

One amp per driver and an active or dsp filter to boost the sub bass is what I would go for. You could place the amps and filters close to the speakers.
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Old 12th June 2014, 06:34 AM   #15
Philfr is offline Philfr  France
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Hi Mark,
Linkwitz use two driver with low Qts, so they need a boost and bass drop compensation..
In my cas I use Alpha15A with hight Qts, 1,26 in OB and near 1,5 in H frame.
The predict with MJ King sofware (at listening position 3,5 m) :

Click the image to open in full size.

The mesurement :

Click the image to open in full size.

Click the image to open in full size.

Where did you see I need a boost ?
Phil.
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Old 12th June 2014, 09:42 AM   #16
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Nice looking build. I had checked them out before posting.

I am convinced you know more about your speakers than I do.
I just don't see how they can output 20 Hz. The driver has a Fs of 41Hz, placed in an open baffle that acts as a high pass filter.

Even if you don't need the baffle compensation, I would still use an active or DSP filter and one amp per driver. This configuration simplifies the amp design with all the advantages of active filters.
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Old 12th June 2014, 10:46 AM   #17
Foxx510 is offline Foxx510  Australia
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Wouldn't you be better off with a class D module? I wouldn't go less than 150-200wpc even if you don't listen loud.
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Old 12th June 2014, 11:48 AM   #18
Philfr is offline Philfr  France
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I do not try 20 hz or 30 hz, cause cone displacement is 1,25 mm at 30 hz for only 1 watt.
Cone displacement is the big problem with OB at very low frequencies.

Class D ?
I knaw several chipamp, but I never heard class D amp, in doubt I prefer what I know.. Maybe I wrong.
Phil.
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Old 13th June 2014, 07:35 PM   #19
tomchr is offline tomchr  United States
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Quote:
Originally Posted by GoatGuy View Post
ONE important thing though - it'll be critical to shim the gain of each half to be quite similar and symmetric.
Yep! These days 0.1 % tolerance resistors are only slightly more expensive than dirt. Don't bother matching your own resistors. Just buy the 0.1 % resistors and use them in the few critical spots in the circuit. You only need two precision resistors per amp channel if I recall correctly.

~Tom
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Last edited by tomchr; 13th June 2014 at 07:37 PM.
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Old 26th June 2014, 07:47 AM   #20
Philfr is offline Philfr  France
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Hi all,
Thank's Tom..
I have found only this kit in my area :
Module Amplificateur - Module amplificateur Mono 100W Audiophonics LM3886
So I changed some values ​​and redraw the schematic like this :

Click the image to open in full size.

Do you see something wrong ?
Thank's..
Phil.
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