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My build blog for my active 3 ways.
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3 way post #4 - power supply

Posted 9th November 2009 at 09:27 PM by relder (Rob's active 3way with transconductance chipamps build blog)

Here's the power supply brick for all amps. A toroid with a CLC. -35V-0V-+35V

Note the color coded terminals on the right so I don't roach another LM4780! (see last post)
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3 way post #3 - the circuit boards

Posted 9th November 2009 at 09:20 PM by relder (Rob's active 3way with transconductance chipamps build blog)
Updated 9th November 2009 at 09:28 PM by relder (Update title)

Here are two of the assembled boards. A mono one for the woofer on the left and a stereo on the right for mid and tweet. I designed the PCB to do both.

Originally I did two in point-2-point, which I found tedious, especially for the LM4780 which has tiny pins with little room for error. And after I blew up the LM4780 board (I hooked up the power supply backwards - poof!) I decided to go PCB, since there was no way to replace the chip given how close the pins were to each other.

The ExpressPCB software is pretty easy to use. Eagle, for how often I attempt PCB's, had too many annoyances for me when creating new objects, etc. The service is pretty quick too, ordered on a Tues and received them on a Fri. They give you 3 boards in their low cost service - so I'll use the one P2P LM3886 and 3 PCB boards.
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Old

3 way post #2 - the amp circuit

Posted 9th November 2009 at 07:59 PM by relder (Rob's active 3way with transconductance chipamps build blog)
Updated 9th November 2009 at 09:29 PM by relder (Update title)

Here is the basic circuit for the amps. Turn the pot in one direction and it acts like a current source. Turn it the other direction and it behaves like a voltage source -- almost, you still have that 0.5 ohm series resistor there, so output impedance will always be at least that.

It's stable on the bench with a fixed R and with real speakers.

The adjustability will allow me to:
  1. Tone things down at woofer resonance (you'd have a big peak at Fs if driven totally like a current source)
  2. Act somewhat like an adjustable Linkwitz transform for the woofer (around that resonance peak again)
  3. Control phase somewhat (if I recall right there is a 90deg phase change between current and voltage driven)
  4. Adjust frequency response somewhat, depending on the impedance plot
  5. Adjust distortion (less is better but compromises with the above must be made)
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Old

3 way post #1 - intro

Posted 9th November 2009 at 07:34 PM by relder (Rob's active 3way with transconductance chipamps build blog)
Updated 9th November 2009 at 09:30 PM by relder (Update title/cat)

I figured I'd do this as a blog rather than a new thread somewhere.

My goals were:
1) Play with the distortion reducing effects of transconductance amps (voltage in - current out.) I think it was likely Nelson (via the F1/F2 papers) that really turned me on to Hawksford's paper that described the benefits of driving voice coils with current rather than voltage.
2) More bass than the 2 ways I built, so 3 way w/ a 10" it will be.
3) Given #1 and #2 building a FirstWatt F1/F2 per driver is a bit much, and given this is an experiment we'll go with chip amps (LM3886/LM4780)
4) I've always wanted to play with a ribbon, despite Zaph's warnings.

I've been working on these slowly for quite a while - so a flurry of posts on the various parts will follow quickly.
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