A Question of Balance?

Housteau · « **on:** 11 Mar 2005, 08:29 pm »

No. Not the Moody Blues Album, but a question on level matching.

I am needing to lower the overall volume on one speaker in order to match another without using the balance control on my preamp. This level will be a fixed setting and so an adjustable pot is not necessary. I was thinking of something very simple such as possibly a resistor at the line level input of my mono amp for that channel.

What would be some good resistor values that I could try and experiment with? I may need to adjust that level as much as 3 db. Also, how would that be installed, hot to ground, or inline?

Dave

Housteau · « **Reply #1 on:** 13 Mar 2005, 07:52 am »

Come on guys. This shouldn't have been a hard question for those of you here in the lab forum. I am basically asking how to attenuate an audio channel with a fixed resistor inline to my power amp. What I am not sure of is how that resistor needs to be added. Does it need to be inline with the audio signal conductor, or across it to the shield.

Dave

Occam · « **Reply #2 on:** 13 Mar 2005, 01:00 pm »

An arbitrary resistor put in series with a voltage yields no attenuation, I believe what you are asking about is attenuation associated with a voltage divider, typically the attenuation assosicated with a series resistor followed by another resistor (or component with a frequency depedant reactance) which is usually a shunt to ground. You are concerned with the voltage at the junction of these 2 resistors.

For example -
Input via a 10k series resistor connected to a 50k shunt resistor to ground. The output voltage Vout = Vin( Rshunt/(Rshunt + Rseries))
and for this example Vout = Vin*(5/6)

You can convert that voltage ratio to dB with the equation
dB = 20*log(votage ratio) = 20*log(Rshunt/(Rshunt+Rseries)) =
20*log(5/6) = -1.583 dBv

You can get far better explanations by using google and entering 'voltage divider' which will offer discussion and calculators -
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

and doing the same for 'decibel calculator'
http://www.mcsquared.com/dbframe.htm

I'm sure there are calculators out there that combine both funtions, or its quite easy to do your own in Excel with the formulas above.

And all of the above is going to lead to yet more questions. And you don't want to use your balance control to do so.... Why? Just take your balance knob (assuming it has enough range) and adjust it on the shaft so what looks to be the 'balaced', equal. position has the degee of attenuation on the channel you require. We won't tell anyone. Mucking about with a set screw on a knob is a lot easier than adding attenuation.

Housteau · « **Reply #3 on:** 13 Mar 2005, 04:00 pm »

Thank you for your response. The reason I do not wish to use my balance control is because it is a 4 tower system, two mid/trebble units with twin bass towers. Using the balance control would also shift the bass which does not need to be shifted. I would just like to rebalance my upper range units by a passive attenuation on a single channel.

Dave[/url]

Housteau · « **Reply #4 on:** 13 Mar 2005, 10:43 pm »

By using your formulas I was able to get the ratio placing me in the ball park. Then, playing with a few more values I was able to get the balance very close, if not perfect. Later down the road when I have the chance to critically get the speaker positions correct in all 3 dimensions, I can fine tune any balance issues, if they exist. Thanks again.

Dave