Unusual GC amplifier

[Algar_emi]

Hi. I came across a very unusual GC amplifier, doing some mods for a friend. It is built into two HP router slim enclosures. The power supply is the bottom one, the amplifier in the top one.

The supply is unusual because it is using eight toroidal transformers
It weights a ton. Each transformer has two 13V/2A secondaries. The secondaries are then connected in series for 26Vac total. Each 26V is rectified by a separate full MUR860 discrete rectifier bridge, with two WIMA MKP4 0.22uF snubber caps each. Everything is mounted point-to-point. That gives four 39Vdc supplies, two per channel. They are connected to the amp using Neutrik 4 pins XLR connector, one per channel. Here a top view of the supply, a monster

[Algar_emi]

Here the amp. It is using separate regulators, LT1083. Since they are floating from the power supply module, it is using only positive supplies, that are connected only at the amp IC, producing +/-V supplies. Again everything is point-to-point.

The Reg IC are adjustable using 5K pots. Vin=39V, Vout=25V. Supply Input capacitor banks are using five (5X) 1500uF, 50V, Panasonic FC caps per rail.

The amp topology is LM3886, non inverting, DC coupled. Dale CPF resistors at the input, Feedback resistors are 14K3 CRC 0.01% and 1K Vishay S102K. Local IC supply pins have 47uF,63V FC caps directly to the pins. A simple turn-on mute circuit, using the LM3886, pin8 (mute) function is implemented using a RC circuit on the supply caps (47uF,50V FC cap and 10K RN60D resistor). There is a small 150pF, Mica cap between pin 10 and 9 (inputs).

The reg and amp IC are mounted on small heatsink. The layout may be nicer, but it is functional, with a lot of attention to the GND layout. The Supply caps GND rail is connected together near the amp IC, then continue on to the speaker NEG binding post.

[Algar_emi]

The supply caps and input resistors. I modified the input to add a 50K variable volume pot, so my friend can match the amp to its Korneff tube preamp. We can see the mute RC cap and resistor on the caps filter bank.

[linuxworks]

do you have metal plates on top of the toroids? is that a 'shorted turn' by any chance?

[VictoriaGuy]

Originally Posted by linuxworks

do you have metal plates on top of the toroids? is that a 'shorted turn' by any chance?

That was my immediate reaction as well......

[6L6]

Those don't look like they go all the way around, so they probably are not shorted turns.

Regardless, that is a fascinating build, I love the use of salvaged parts. I bet it sounds wonderful!

[AndrewGM]

Good point - how does it sound ?

Andrew

[6L6]

Can you make a guess as to the black material that is damping the torroids?

[Andrew Eckhardt]

The way the dice are situated in the multiwatt packages leaves half of them overhanging the sinks in this installation. You don't really look to have the upper part of the tab doing most of the cooling via conduction through the cross section. If you look underneath you should see none of the thermal pad. Those sinks look little light duty to begin with, although at 25V rails and played at low volume you shouldn't have any problem.

[linuxworks]

Originally Posted by 6L6

Those don't look like they go all the way around, so they probably are not shorted turns.

if the return is the 2nd bolt, I'm pretty sure that meets the def of S.T.

in one bolt, thru the metal plate and down the other core to the chassis.