Anything special about the Threshold SA/1 "Limited Edition" ?

[coolnose]

Hi Everybody,

I have been offered a pair of Threshold SA/1 "Limited Edition"in black and was wondering if someone could give some more information about these amps .

Is there anything special about the "Special Edition", technically speaking ?

Thanks in advance to all,

Max

[RobertS61]

Originally Posted by coolnose

Hi Everybody,

I have been offered a pair of Threshold SA/1 "Limited Edition"in black and was wondering if someone could give some more information about these amps .

Is there anything special about the "Special Edition", technically speaking ?

Thanks in advance to all,

Max

No, all S/1000, S/1000 series II, SA/1 and SA/1 optical bias stated 'Limited Edition' on their fronts. Pure marketing but the price of 6 ~ 7K USD in the eighties made them pretty limited I guess.

[RobertS61]

My last statement was partially incorrect.
As far as I know now the S/1000 and S/1000 Series II stated 'Limited Production' on theire fronts. The SA/1 and all other Thresholds from that era stated 'Hand Crafted'.
The offer is/was an updated S/1000 Series II with the front- top and backplate also updated.
The S/1000 and SA/1 compare to each other as a S/1600 and SA/12 and a X-600 and a XA-160. Same hardware but outputstage bridged or in a massive parallel configuration.

[lcniii]

Hello all, Can anyone tell me in simple terms, how to reduce bias (temp) on one of my SA-1's? Where is the trim pot located, and which one is it? (top, bottom, left, right)

[Nelson Pass]

I responded to this in email, but I will share it here:

There are no more than two potentiometers in an SA-1. Definitely one
for bias, and if there is another one, it is for offset.

You can measure the bias current many ways – the easiest is to read
the DC voltage across one of the many power resistors next to each
output transistor – pick one in the middle. The other way is to use a
device which monitors the power draw from the wall.

If there are two pots, then you also will need to be able to read the DC
offset, as you may find yourself adjusting that. Depending on whether
the current draw moves or the output DC, that’s your clue.

Alter the bias in half-steps and then set the cover back on and wait an
hour, monitoring the draw and/or the temperature. The normal temperature
is about 50 deg C. on the tops of the sinks.



BTW, the owner reports success in the adjustment.

[RobertS61]

I've seen four pair of SA/1's and only one pot for bias on their frontendboards.
I got the best result with popping out one of the railfuses and connecting the leads of a multimeter in high ampere mode (usually 10A) instead.
With the proper connections inplace you start the amp and you can monitor the quiescent current for approx. two hours.
What you will observe is a rapidly ascending bias (for shortening the warmup time) over 4 Amps in the first couple of minutes and then after it peaked it will gradually taper of to the proper setting of approx. 3A.
To get the 160 Watts of Class A rating @ 8 Ohms you need 3.2 Amps of bias
(3.2 x 2) = 6.4 (because it's a complementary design not single ended one) then ((6.4 x 6.4) x 8)/2 = 163 Watts. If the heatsinks stay around 50 degrees Celcius in two hours you're basically okay.
And yes because of the thermistor on the frontendboard you need to measure with the topplate on the amp. Just carefully route the leads of your MM out of the amp (there's some space at the left and right topcorner) and put a temperature rod (you need another MM with temperature sensor) in one of the second holes (where the Allenscrews of the toplate go in) counted from the back.
Now you can monitor the amp for the full distance of two hours without ever needing to open up the hood. Any changes should be made only after two hours and carefully because those pots are one-turn devices and the bias reacts pretty wild when turning them.