Adjusting the BIAS for Threshold SA3 - diyAudio
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Old 28th January 2007, 04:16 AM   #1
sonic22 is offline sonic22  Canada
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Default Adjusting the BIAS for Threshold SA3

I have been using the Threshold SA3 and FET10/hl for more then 15 years and they keep bringing me continuous leisure and joys on listening to music throughout that period.

It is not until recently I have to convert the SA3 and Fet10 from 220V to 120V (as I have to move to another place) and that couple works fine for another month. Afterwards problems have arisen and I have replaced some of the components for the SA3, they were 2 X 41,000uf caps, 14 X MJ15022, 14 X MJ15023, 2 X 0.15uf caps, 2 X 2N5566 and 2 X multi-turn 2K trimmer respectively. Everything was back to normal again, except Iím not sure what is the recommended data for adjusting the bias for this particular power amplifier.

I've searched around this forum and got some ideas on how to adjusting the bias. At the moment I have adjusted the bias such that the voltage across the emitter resistor is 0.135V and the steady temperature on the heatsink is 40.5C (at room temperature 20C). When comparing with what was before I can remember that the heatsink should be hotter (something like 50C) after being switched on for an hour. It brings to me that the bias might not be enough and can anyone tells me the proper bias voltage setting across the emitter resistor? Thanks a lot!
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Old 28th January 2007, 07:10 PM   #2
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The 20 degree C. rise above ambient is close to the bias which
gives the "best measurements", but there is a strong consensus
that a higher bias at 30 degree rise sounds better. Try 160 mV.

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Old 29th January 2007, 02:54 AM   #3
sonic22 is offline sonic22  Canada
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I'm so excited and can't imagine that the designer of my Threshold SA3 power amplifier, Mr. Pass was answering my first question in this forum. Thanks so much!

I am working on it now and at the same time listening to the Cannonball Adderley's "Somethin' Else", its awesome!
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Old 29th January 2007, 06:16 PM   #4
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I like to pick all the low-hanging fruit.

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Old 19th August 2010, 08:04 AM   #5
omniex is offline omniex  France
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Default THRESHOLD SA.3 bias control and upgrade

hello, I bought recently a THRESHOLD SA.3 and after some tries with differents preamplifier, it works nicely.

I would like to control the differents things that must be done, especialy the bias current.
Can some one explain to me how to do, what voltage on what resistor (scheme, pictures ?), and also if there is some parts that can be change to upgrade this amplifier.

thank you for your help

marc
Fontenay, France
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Old 20th August 2010, 02:58 AM   #6
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First, the current through the output stage must be carefully
monitored at all times. You can do this by placing a DC
voltmeter across the emitter resistor of one of the outputs.

Pick one near the middle, so that you don't accidentally pick
the driver transistor. In any case, with the amplifier operating,
confirm that there are at least two others with the same voltage
in that row. Be careful not to get a shock!

This voltage will be proportional to the bias, so keep an eye
on it so that you don't blow up the amp.

Alternatively, you can monitor the AC current draw of the
amplifier as a whole - the main thing is not to accidentally
crank the bias up way too high and not know it.

There are usually two potentiometers on each channel. One
for the DC offset and the other for bias. If you turn one pot
a little bit up or down and there is no change in bias current,
then that is the DC offset pot. You can go back and adjust
that later after bias is adjusted.

Ideally the bias should be adjusted so that the heat sink tops
are 20 deg C. above the ambient temperature, or about 50
deg. This is the temperature where you can keep your hand
on the sink for about 10 seconds.

If your amp already is about this temperature after an hour's
warmup, then don't bother with it.

Only adjust the bias in small baby-steps and then let it sit,
monitoring the current and temperature, as it will usually
drift upward from where you set it. You can trust a value
after 2 hours with the amplifier top in place.

Any questions?

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Old 21st August 2010, 08:17 AM   #7
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I need to rebias my S-150 II. It runs at room temp even after being on for an hour or more. Any idea how high I can crank the bias on it?
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Old 22nd August 2010, 05:02 PM   #8
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Like I said above, you increase the bias in baby steps unless
you really know what you're doing.

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Old 23rd August 2010, 09:14 AM   #9
omniex is offline omniex  France
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thank you for these precisions.
I will do it as soon as I can, and will tell you how things are

cdt
marc
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Old 16th May 2011, 12:38 PM   #10
omniex is offline omniex  France
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hello, sorry to be so late
so I did change the BIAS value of my TRESHOLD SA/3.
It was about 300 mv on the 1 ohm resister, and I put it at 150 mv.
thinks have change no, particulary the bass are more clear and precise.
My next step is know to upgrade the supply, and I would like to change the capacitors for new ones, put electrolytique in parallel, and change also the diode rectifier.
Is there some people to help me to do the correct choices ?

THANK YOU
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