http://ampslab.com/vifa62.htm

I have been insipred by PMC to create my own Trasmission Line design. I say inspired, as clearly I'm not going to match their acheivement on a private scale!

So to date I have chosen my drivers, the Vifa PL18 WO09 and the SEAS 27TFFC(H0881).

I'd love to hear if any one has had any experience with these drivers!

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)

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.

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)

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.

Hi, I hope I have this right- I am new to this! I am dusting off my electronics by rebuilding an old technics se-m100 power amplifier (1989) as a practice run (before upgrading my marantx cd-80). I bought this very cheaply, but it sounds rather thin compared to my old sugden's! I have had difficulty getting exact capacitor matches for the electrolytics in terms of voltage (not capacitance). I have ended up having to use 10v or 16v capacitors, in place of originals rated at 6.3v. Equally I have had to replace capacitors rated at 25v with those at 35v. They are nearly all in the signal path, and so I have used elna cerafines or silmics. Is this going to be OK?- I couldn't see why not, but, am I missing something obvious? In principle it is quite a nice amp (dual mono, big power supplies), so even though this is a practice run, I don't want to break it! Apologies for the ignorance of the question and thanks for any help!

Maybe i should have thanked Her,

Sooner, than i have,

for those Unexpected Presents.

Of Things, I really liked.

the kind that make life worth living ...





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