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Date:         Wed, 29 Jan 2003 01:20:10 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: NVC:  measuring impedance of a speaker?
Comments: To: Ben McCafferty <ben@VOLKSCAFE.COM>
In-Reply-To:  <BA5C701E.E2C9%ben@volkscafe.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 07:36 PM 1/28/2003, Ben McCafferty wrote: >Any of you stereo buffs: I need to determine if my home speakers are 4 or 8 >ohms, so I can set my receiver properly. Unfortunately, there is no marking >on the case or actual speaker anywhere, except "75 watts, magnetically >shielded, design for t.v."

Speaker manufacturers tend to make 4-ohm speakers because it's cheaper...

If you really want to measure the impedance, you'll need to use a sine-wave test signal of 1000 Hz frequency, and put a non-inductive power resistor in series with the speaker; then measure the ratio of the voltages across the speaker and the resistor; the impedance will be in the same ratio as the voltages, i.e. Zspkr : Zresistor = Vspkr : Vresistor. With a non-inductive resistor Z (impedance) should be the same as R at any reasonable frequency, so multiply that by the ratio.

Since the speaker is an inductive* load, impedance will be frequency-dependent, but 1000 Hz should give you the rating number.

*actually resistive + inductive and very likely capacitive as well, given the crossover circuits and all.

david

-- David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85 GL "Poor Relation"


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