sx-1250 bias incorrect?

native · Dec 19, 2014

I set the bias on my 1250 to 100mv per side and my variac is showing 100 watts consumption at idle! That seems really high to bias the outputs and and run a few bulbs. Seems to me to be a waste of power, creates a bunch of heat, and just generally be hard on everything. Is this a mistake in the manual? Mark, EW?

bktheking · Dec 20, 2014

It's not a mistake. They run hot at 100mv, set it to 60mv, that's what I run them at.

Watthour · Dec 20, 2014

I'm even more conservative (chicken?) since I started at 35mA after repairs, then after several hours burn-in only set up to 50mA idle. It may not provide the most optimal bias point for distortion control, but it certainly runs cooler on both amp boards. Since the replacement outputs have a different B-E on voltage than the original devices, the bias point in the manual may be even less appropriate.

BK's recommendation is probably better, and I'm guessing that will be substantiated by more of the experts at AK if you simply search the archives for "SX-1250 bias" and tabulate the results.

zebulon1 · Dec 20, 2014

From this thread:
Mark was addressing the idle relationship between the sx-1010 and the sx-1250. They use the same outputs;

http://www.audiokarma.org/forums/showthread.php?t=620106

markthefixer said:
I determined the individual SX-1010 idle currents by starting with the current arrows that read 100mA @54v on the main schematic.

Since the same supply (2 big caps) is shared by both channels:

Two channels knocks it down to 50mA per channel

TWO OUTPUTS PER CHANNEL makes it 50 mA split between the two output pairs for 25mA

25mA pair pnp / npn pair through BOTH emitter resistors of 0.5 ohms each for a total of 1.0 ohms means 25mA reads as 25mV, and across ONE emitter resistor of 0.5 ohms it reads as 0.0125v for 25mA.

Now to address the 1250 systematically:

The 1250 has FOUR big capacitors, meaning each channel is fed individually.

EACH pair/side/channel has a notation of "200mA @ 70v".

The instructions direct to adjust for 100mV between terminals 7(+) and 19(-).

terminals 7(+) and 19(-) is a special summed set of measuring points for the 2 pairs of output transistors coupled to each of FOUR corresponding emitters by a 22 ohm resistor, if bypassing the summing network and instead going directly to the individual pair's emitters should yield approximately 100 mV, being slightly over or under due to component tolerances...

sx-1250 bias incorrect?

native

Member

bktheking

Gitter Done!

Watthour

Electron Rancher - JS3600

zebulon1

Working on my own stuff. Finally

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