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Power amp biasing/input level adjustment

iplay5strings

iplay5strings

Aug 10, 2007
1,177
707
Waterford, MI
www.adamsaylorbass.com
#1
So not too recently had a power amp replaced in my SWR SM900. I know there are those plastic dials on the circuit board for both power amps as well as in the limiter circuit. My question is how do you know if they're set to the right level? Do you need to measure them with a current meter or is there a way to audibly adjust them? I'm noticing some volume differences/oddities between the two amps (not distortion or anything like that so I don't believe anything is blown) as well as more noise than usual from the eq section, specifically with the limiter engaged at any level. So I figured maybe the bias' were a little off but I don't really know how to check measure that. Any help would be great thanx!
 
#3
A bias adjustment will not make a difference in the output level, it's unrelated.

Well, I take that back, actually it can make one difference. Run the bias too hot and the output transistor will fry. That channel's output level will then be zero. So setting the bias can change the output level...to zero... :)

Bias adjustments must be done with the proper equipment according to the proper method as indicated in the service manual.
 
#4
Hi.

Checking and setting the bias is one of the most important periodical service points in SWR amps, so if the tech has an internet connection, he has most likely run across the instructions on how to do so.

It's not rocket surgery, and can be performed with just the basic measurement equipment any tech has in their disposal, but since the adjustment can be a bit finicky and an accidental damage is quite likely, it's IMHO a job for a seasoned tech.

Transistors and other components do wear out, and there's probably been revisions of the amp board so it's possible that two amps of a different vintage doesn't sound the same.

DO NOT bridge them in that case though.

Regards
Sam
 
#5
One thing that is difficult in the SWR procedure is the the 2 Ohm dummy load per side. Most techs around have a 4 Ohm dummy load at best and the ones that do have a 2 Ohm dummy load have a month+ turn around time which just won't work. Would it be ok to run the amp in bridged mono with a 4 Ohm dummy load so that each amp is seeing 2 Ohms and then check the bias?
 
#6
Search "swr bias setting procedure" or something like that on here. I know somebody posted links to it a while back. I had them saved on a computer that went belly up a while back but they are on TB somewhere.
 
#8
will33 said:
Search "swr bias setting procedure" or something like that on here. I know somebody posted links to it a while back. I had them saved on a computer that went belly up a while back but they are on TB somewhere.
Click to expand...

I have the biasing instruction pdf file. It calls for a 2 Ohm dummy load when biasing each amp. I was just wondering if it would make a difference if I ran a 4 Ohm dummy load in the bridge mono out creating a 2 Ohm/amp load. My tech only has down to 4 Ohm dummy load, that's why I'm asking.
 
#9
It's really 2 amps in the power section like a stereo PA poweramp. Each one should be adjusted individually to get it right and not bridged together. Your tech shouldn't have any problem throwing together a 2 ohm dummy load, resistors are cheap.
 
#11
carvinbassplyr said:
Well long story short...we took my techs 4 Ohm load and chained it with a 4 Ohm speaker load to achieve the 2 Ohm load. The bias was quite a bit off on both amps so I'm glad I got this done b4 putting my amp through a 12 hr/day 4 day workload cause that could have ended badly.
Click to expand...

Now you know why the manufacturers instructions MUST be fallowed, not worked around? I hope it is now close, but a speaker is not a purely resistive load and the combination of the two may not actually suffice.;)

You CAN NOT make the adjustments "by ear".
 
#12
Well we turned the trim pot on the amps up till we saw the crossover notch disappear on the scope per what the instructions said. I know that a 4 Ohm dummy load daisy chained to a 4 Ohm speaker load wasn't ideal, but it accomplished the 2 Ohm load SWR requires for proper biasing and I believe it was sufficient enough to get the bias w/in a safe operating range.
 
#13
carvinbassplyr said:
Well we turned the trim pot on the amps up till we saw the crossover notch disappear on the scope per what the instructions said. I know that a 4 Ohm dummy load daisy chained to a 4 Ohm speaker load wasn't ideal, but it accomplished the 2 Ohm load SWR requires for proper biasing and I believe it was sufficient enough to get the bias w/in a safe operating range.
Click to expand...

Yes it may be close enough to be safe. But let me elaborate, the resistor load imparts the same DC and AC Resistance (AC Resistance is known as Impedance). The speaker should have a DCR (DC Resistance) of about 3.2 ohms, the AC Impedance changes with the frequency going into the speaker. Your tech should know this and purchase a second non-inductive 4 ohm resistor to do the job properly. :)
 

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