Checking ohms on my 1.2tl's

Toolfan66

I thought i would check my ohms again, don't ask why just something to do.

Well the lower post show 4ohms, and 2ohms to the SDA pin from the lower post's.

This may seem like a dumb question, but why is there no ohms on the upper posts? without the jumpers that is.

Curious,,


I also have another question on the tweeter on my RTiA6,

Why is it a 3ohm tweeter if it is a 8ohm speaker?

Like I said these may just be dumb questions, but I would like to understand them better.

Thanks again,

Larry.

shadowofnight

The crossover for the high frequency posts has capacitors in series with the drivers ( Tweeters ) ....if you have an autoranging dvm on ohms and it reads nothing ( Infinity ) ...reverse the leads and you will see the caps as they discharge/charge with the voltage from the meter. Perfectly normal.

The low frequency posts do not have any caps in series with the drivers ( Woofers)...they have inductors in series with them...you are reading the DCR of the inductor and the resistance of the voice coils.

You are reading 3 ohms on your tweeter ? Most raw drivers dc ohm reading is 75% of their rated ohm rating.....so your tweeter is most likely a 4 ohm driver....meaning nothing to the overall ohm rating of the entire speaker system....its a melding of the low freq/high freq crossovers/drivers that give the full frequency rating ( And you really need to see a frequency/ohm plot at all frequencies to see the peaks and dips in the overall speakers reading to see what load it actually presents to the amp.

shadowofnight

This schematic shows it perfectly... http://www.polkaudio.com/forums/attachment.php?attachmentid=26687&d=1186346402 .... Look at the HF positive input....it goes through the 12uF ( Bypassed with the 750pF Mica ) before being distributed to the individual crossover arrays for all of the tweeters............if you look at the LF positive input...it only has inductors in series with the driver arrays ( The caps are in parallel and their purpose is to make it a second order ( 12db per octave ) low pass filter. )

TECHNOKID

In order to measure a specific component in the circuit one side must be lifted out otherwise you will read the sum of everything else that is in series and in paralell. In the case of capacitors, your multimeter meter battery will charge the capacitor and giving you weird reading (going up or down while the capacitor is charging and then giving you an open reading once the maximum charge of the capacitor is attained). For example, the RM series speakers has 8 ohms impedance but due to the Xovers High Z you will not be able to read the speakers Z unless you unplug at the Xover and do your reading directly at the speakers leads.

Cheers!
TK

shadowofnight

TECHNOKID wrote: »

In order to measure a specific component in the circuit one side must be lifted out otherwise you will read the sum of everything else that is in series and in paralell. In the case of capacitors, your multimeter meter battery will charge the capacitor and giving you weird reading (going up or down while the capacitor is charging and then giving you an open reading once the maximum charge of the capacitor is attained). For example, the RM series speakers has 8 ohms impedance but due to the Xovers High Z you will not be able to read the speakers Z unless you unplug at the Xover and do your reading directly at the speakers leads.

Cheers!
TK

I dont believe he was interested in individual component measurement ( That sounds like something for us to be concerned with...well me for sure by designing my own crossovers and reverse engineering some I want to duplicate )


After reading his question, all he wanted was a quick and easy explanation as to why he was getting certain DVM readings....it really wasnt a technical question.

Hell, I even thought my telling him to reverse his meter leads on the HF input and watch the DVM indicate the cap was charging/discharging was going a little overboard pertaining to his simple question.

TECHNOKID

TOOLFORLIFEFAN wrote: »

... why is there no ohms on the upper posts? without the jumpers that is.

Curious,,


I also have another question on the tweeter on my RTiA6,

Why is it a 3ohm tweeter if it is a 8ohm speaker?

... I would like to understand them better.

Thanks again,

Larry.

shadowofnight wrote: »

I dont believe he was interested in individual component measurement ( That sounds like something for us to be concerned with...well me for sure by designing my own crossovers and reverse engineering some I want to duplicate )


After reading his question, all he wanted was a quick and easy explanation as to why he was getting certain DVM readings....it really wasnt a technical question.

Hell, I even thought my telling him to reverse his meter leads on the HF input and watch the DVM indicate the cap was charging/discharging was going a little overboard pertaining to his simple question.

In reference his question, the answer is actually technical... Reading the component in circuit will give him reading he will not understand... As pointed out, in order to read the driver impedence he must isolate from the actual cct. How can we properly answer his question without a technical answer?? Both his reading concerns are more likely reading some in paralell and through a cap, right?

Cheers!
TK

shadowofnight

TECHNOKID wrote: »

In reference his question, the answer is actually technical... Reading the component in circuit will give him reading he will not understand... As pointed out, in order to read the driver impedence he must isolate from the actual cct. How can we properly answer his question without a technical answer?? Both his reading concerns are more likely reading some in paralell and through a cap, right?

Cheers!
TK

He wasnt concerned with reading a component in circuit....his question was simply why do I not get a reading on the HF input with the jumpers removed ?

The simple answer is with that 12uF cap in series with the entire tweeter array ( Forget about the many crossover components, as well as separate whole individual crossover networks filtering each individual tweeters output ) .....all he is going to see when putting DVM leads onto that HF input is that cap charging and discharging ( When reversing the meter leads back and forth ) .

Thats all he needed to know and all he asked....that there was nothing wrong with his speakers and it was perfectly normal .


Now with his his separate question about his tweeter reading 3 ohms while out of circuit( This is a separate question and a different pair of speakers ) ...That just needed a simple answer as well....most raw drivers dc ohm reading is around 75% of its reading used pertaining to crossover design ( Most 4 ohm rated raw drivers read close to 3 ohms on a dvm.....most 8 ohm rated raw drivers read close to 6 ohms....most 2 ohm rated raw drivers read close to 1.5 ohms....etc ) .

The actual load a speaker system presents ( This can be 4 tweeters and 8 MW in the larger polks ) is done during the crossover design stage. Polk likes to use multiple woofers in a series/parallel alignment to get the load presented to the amplifier not too low or too high ( Also , since we are working with a AC signal here....reactance....both inductive and capacitive...Xl and Xc...have a huge impact on final load to the amplifier...also allowing a tweeter circuit that you cant measure with a DVM in circuit...can still provide a substantial load to the amplifier. Take a look at an impedance plot for a speaker system to see this effect )


Actually the simplest answer would have been is many different ohm tweeters ( 4...6 and 8 ohm tweeters are very common...his is a 4 ohm tweeter ) are used in 8 ohm nominally rated speaker systems.

That was really all that was needed.