Dual LA-2A like it should be, pictures

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Kingston

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dual-la2a-front1.jpg


Some of my best panel work this far. Many important features brought to front panel using on-off-on switches and relays. true 600 ohm output pad for example. The 'preamp' setting takes out all series resistance before the first stage (68k, and the "limit" resistor). Becomes a brilliant medium gain (something like 60dB) preamp with very low noise.

Sidechain high pass filter brought to front panel (0ohm, 500k, 1meg). Indispensable, highly interactive. I initially experimented if the Arco 50-380pF trimmer from sidechain should also become a series of switches on the front but it was too subtle. Couldn't hear any "de-essing".

Mostly PIO caps and carbon comp resistors, more than double the usual filtering capacitance, some russian alternative tubes (rewiring under the PCB) and DC heaters. The DC heater regulator (overkill, but what the heck) also controls B+ safety relays. If regulators die, B+ comes off, tubes will live instead of eating destructive B+ voltages.

Completely noiseless. I actually did not expect this, as I've read that LA2A is generally "noisy", whether it's tube hiss or hum from transformer etc.. Well those can be completely avoided so lets put an end to that myth. The circuit itself, and silent-arts PCB layout are capable of close to theoretical noise floor. With unity gain medium compression usage noise is around -100dBu. Max gain (about 60dB), it's still below -80dBu.

I come from the school of paranoid wire shielding. The transformer is a custom way over specified number, well shielded. Generally lots of attention was put to shields and faraday cages. It pays off.

dual-la2a-front2.jpg


dual-la2a-internals.jpg


Don't laugh at the "handbreak" aluminum pipe, the "Kingston-duct". It's a grounded duct to safely take mains VAC to front panel. Mains VAC likes to leak everywhere, and I'd rather brute force it out, than live with hum.

I already noticed the gain reduction VU zero likes to move as the unit heats up. It's those neons, old school regulators. I'll replace with zeners to stabilise metering.

a couple of last nuts to tighten, then this beast starts active service, hopefully will keep at it for the next 30+ years.

Mike
 

guavatone

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May 21, 2005
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Nice solution to non standard power locations.  Why can't se all just have AC on the right side of our enclosures people?
  :p
 

Kingston

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guavatone said:
Nice solution to non standard power locations.  Why can't se all just have AC on the right side of our enclosures people?

There's no such standard. And in this case, it would have looked ugly and broken the symmetry.
 

mitsos

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May 4, 2007
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Damn dude.... Just magnificent....  (Only loses to your mila cuz that was P2P ) :p

seriously you're doing some really amazing work.
 

Kingston

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There are two steel brackets below the duct serving two functions: anchoring it and grounding it.

The pipe is very lightweight and even one bracket would have been enough.
 

tv

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Feb 22, 2006
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But hey, I really think you should try to make The Duct look more badass. Like drilling some hole-pattern in it, adding some wire mesh - something.

(in your next build)

shall we christen it "The Duct of Silence" ?
 

Kingston

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tv said:
drilling some hole-pattern in it, adding some wire mesh

Yes!! I have always pondered what exorbitantly overpriced thingymagadget I will sell to the ever-gullible audiophile community.

The final product will also feature directional installation with NASA grade kevlar-rigid insulation. And no longer your everyday aluminum pipe, we now have gold sputtered steel with silver wire mesh.

Will work wonders with your directional IEC cables and really there's no point listening to music without it because it will otherwise have no 3d.
 

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