inguz amplifier

[inguz]

The inguz amplifier is another variation on LM3886 topologies:



Gain ~18dB; output impedance ~8 ohms. Inductor keeps ZOut high as Avol drops. Deliberately unbalanced Howland bridge, meaning no need for resistor tolerances beyond 1%.

The first build is a four-channel version (bridgeable, but mainly to drive bi-amped Linn floorstanders); pictures to follow over the next couple of days when it's cleaned up a bit.

I see a little instability (RF pickup) with open-circuit input. A few pF across input should cure that, but this build will be permanently plugged in to my Squeezebox, so that's not a concern.

So far this has only been running into cheap, bad speakers. But it makes cheap, bad speakers sound good.

[inguz]

Power supply is +/- 24V, C-R-C filtered. I wanted to build C-L-C but didn't really have room for large chokes (as you'll see from the internals photos, there's barely room for 4x15000u capacitors).

PSU schematic

The enclosure is approx 8x11x3 inches, copper, acrylic, brass, mahogany. Here's a "rough assembly" version -- the final should look quite fine. Weighs a ton.

Enclosure (rough)

Internals: cramped. One word of advice to anyone else thinking of building four channels point-to-point: "reconsider". That transformer is pretty darn huge too.

Internals (1), Internals (2)

Enjoy :-)

[inguz]

More words (and the same pictures) here (pdf).

[Netlist]

This looks very neat. I'll work my way through the .PDF tonight.
The Point to Point approach is nicely done as well. Congrats.

/Hugo

[inguz]

My little amp has moved up from the basement; home now. At least, in place for a few test-tweak-test cycles...

[inguz]

Some first impressions about how this thing sounds. I only spent a couple hours yesterday evening, trying it with a wide mix of material. The speakers aren't bi-amped yet, so this was running two of four channels.

Bridging should "just work", but it's unstable. I already moved the 33pF compensation caps to 47pF. Some tweaking definitely needed there (eg reducing the series 4k7) -- I'd like to have bridged-mode work properly.

Anyway. Sounds. First impression: just a giant open soundstage -- more detail, more space, than I've ever heard at home before. Nothing localized to the loudspeakers; just sound all around the room, firmly positioned from snares to bass to horns and guitar. Bud Powell (Vol3) came alive. Mingus (Ah Um) was incredibly "real" - cleanly separated. Wes Montgomery (Smokin' at the Half Note) you could really feel the club setting; atmosphere, clear-as-a-bell guitar, and absoultely "right here" drumkit. TM Juke (Wilderness) I could listen to all night, now.

Some little harshness on vocals. India Arie was beautiful, but felt like she was singing in the recording room and me listening outside an open door.

The damping factor is really low, so I expected bass to be boomy and flabby. It definitely doesn't have the typical solidstate "tautness", but instead a very live-music sound. Even the heaviest music I tried (Keith Hudson - Depth Charge; Fugazi - Ex-Spectator, with two drummers) had no sense of boominess, just lots of presence, and much more "reality" than I'm used to.

Already, across the board, even with known-not-good recordings, it opened up details and atmospheres I've never heard before. So far then, I'm pretty happy. :-)

[doggy]

Congratulations on a beauty build!

I will be building the Mauro Penasa- twisted pear audio version when the parts arrive in a month or so, 2 Channel .

I use all single driver diy speakers; the ones I have running now are fostex fe126e, fairly efficient.

Your description of the sound of this amp has my attention. I have not yet built point to point so that may be a challenge. The parts count is low on the amp;helps keep it simple.

Can I ask you where you sourced the big p.s. 15000uf caps and the transformer?

nice amp!
doggy [pic. of my fe126e]

[inguz]

Thanks! Nice speakers. I want to try this on my friend's Gallo A'Diva fullrange spheres - I think he'll jump out his skin. "-)

The transformer was just lying around. In a previous life it powered a Quad405 (at 240v -> 40+40), but it was something else before that. The power supply caps came from Vetco Surplus in Bellevue WA (ten for $15, just couldn't resist...)!. But the powersupply for this amp really isn't anything "special" -- it doesn't need all that snubberized C-R-C thing to work -- I just built the best I could with available parts, in the hope that if everything turns out good I'll be able to resist opening the box up for tweaking. If you're building Mauro's Ref3, or a BrianGT, this one would work off the exact same main power rails.

Point-to-point is doable if you build each segment separately (I soldered all the decoupling caps segment into a little lump first; then the compensation R-C leg and the series feedback resistors; then wired up to the chips after mounting the chips on their heatsink/support). The first channel was a mess, and 2/3/4 went much more smoothly. Also I didn't think too hard about which direction the power leads would come in from, and now they're in the way a bit. So really I can't recommend it as a construction method... it would be much easier to put together using Veroboard or such, if (like me) you never etched a PCB before.

Hope you like the Mauro Ref. I fully expect it to be stunning. (Mr Penasa's been doing this stuff waaaay longer than me, so I would trust his design to sound at least as good as mine :-)

-ing

[doggy]

I live near Vancouver B.C. so I will give Vetco a call and see what they have.

Building stuff is is as enjoyable if not more so than listening to the finished product! And better yet is bringing your own design to life.

I hope to build a 2 channel version when I can get most of the parts together; most likely from digikey.

cheers
doggy

[inguz]

Thanks doggy - it's a bit scary having someone else actually plan to rely on my designs :-)

I'd recommend trying a 4-ohm ZOut -- see the attached spreadsheet for some calculations.