I have a very old set of speakers (Pioneer Elite TZ-7), 3-way, dual woofers, label claimed cabinet impedance is 4 ohm).  The drivers were custom mfg by pioneer and are not explicitly labeled with their impedance.   I have measured the RESISTANCE of the woofer(s) and midrange, and they come up as ~6.7 and 4.7 ohms respectively.

Here is the question: looks like one cabinet one of the crossovers (there are two) is dead (no signal to bass).  I would like to replace all crossovers with new ones, but I am not sure what to do in terms of impedances. 

First - the drivers.  My expectation is that the actual impedances should be HIGHER than the measured resistance.  So ... given the measured resistance, my initial belief was that the impedance of these drivers is 8 ohm (built back in ~1995 when most speakers were 8 ohm I believe????).   But (again, my understanding) is that it is bad practice to design crossovers with different imput and output impedances - would Pioneer really have done that?????   

Given my resistance measurements, is there any reason to believe that these drivers really are 4 ohm, or is it much more likely that they are 8 ohm?

Second - if they really are 4 ohm drivers ... so far in my search for off-the-shelf drivers, I have only seen circuits with 8 ohm output ratings (assume the drivers are 8 ohms).  Do any of you know of 3-way passive crossovers that accept 4 ohm output loads? (I know - I have the option of parallel or series wiring for the woofers - but I am stuck with the single midrange with its defined impedance).

I have been soldering circuits since I was about 8 ... so I am not against building my own crossovers (given a circuit design) ... but would rather go off the shelf.   Hopefully that is enough info to drive this thread for now.  let me know if you need/want anything more?

(I know I could  potentially repair the existing crossovers - but between the four cabinets there are two crossovers, and the wiring is extremely badly marked.  I dont trust the caps all the way around in circuits this old ... and I might never get the things wired back up the way they need to be  :-(

Thank you for your help!

rchamlen wrote: But (again, my understanding) is that it is bad practice to design crossovers with different imput and output impedances - would Pioneer really have done that?????

No such thing as input and output impedances for a crossover only what speaker load it is designed for, and the amplifier just sees 1 combined load of drivers and passive filters. Additionally the low frequency section dominates so it almost doesn't matter what impedance the mid and hi drivers are,  and that means there is no problem using different impedance drivers for the upper frequencies as long as the passive crossover is properly designed for them.

rchamlen wrote: (I know I could  potentially repair the existing crossovers - but between the four cabinets there are two crossovers, and the wiring is extremely badly marked.  I dont trust the caps all the way around in circuits this old ... and I might never get the things wired back up the way they need to be  :-(

Post up some pictures, they may not be as complicated as it first appears.

The crossover values should be to suit the impedance at the crossover frequency.

Otherwise a zobel network can be used to stop the impedance seen by the crossover from rising with frequency.

A cheap Inductance & capacitance meter can be very helpful when trying to service old crossovers.

Yeah - the impedance of the input to the crossover is the result of the entire circuit (including speakers) ... sorry, stupid question based on some quick reading on the web  :-(

Understood that the low pass section dominates overall impedance.  My concern is that if the midrange is indeed 4 ohm, though I can find crossovers out there that allow selection between 4 and 8 ohm woofers, they all seem to assume 8 ohms on the mid and high range.  Maybe that is what you are getting at when you say "4 ohm crossovers are not that common"??

I mis-spoke in the first post: each cabinet has TWO "crossovers" - one board (card really) appears to three simple crossovers wired up in parallel.. Interesting though that the circuit for the front woofer is a traditional LC "L" circuit, whereas the circuit for the rear woofer is different - just a simple series L low pass only (not sure whey they designed with two different circuits - phase  concerns maybe?  (that is beyond me with base knowledge).   the high pass for the tweeter I cant quite trace without pulling the card - it ALMOST looks like it could be a basic RC high pass, but does not seem to be quite wired right for that - as the load is not clearly wired across the resistor (unless there is a wire I am not seeing). The midrange bandpass filter is a totally separate board - and is tucked up high enough I can not get a good look at it.  Picture of the low/high pass section is below.  Front an center is the electrolytic cap I have been thinking is the failed component ... but now that I am analyzing this circuit in more detail - I wonder if it is the cap that is bad - as both woofers do not work, but the filters appear to be in parallel to each other.....   I did try to check the ground wire from  board to connector, and it seemed ok.  But now I am thinking I need to pull the board and more accurately check those connections.  MY main concern so far has been that with one cap failed (IF that is indeed the case), then the others will remain suspect - and I would prefer not to go through this tear-down at random during the next several years!  Anyway - lots of thoughts ... will post the picture for now.

(QUESTION: tried to paste in a photo, but could not do so.  does this forum only allow uploading of links???   I do not maintain a photobucket or similar account.  any way to just paste in a photo??)

 Looked up the speakers on Google and found a couple of pictures.

The speakers have a front and rear facing bass driver so that probably accounts for the 4 ohm nominal rating of the cabinet.

The speakers look to be fair quality items. I would probably advise you to service the existing crossovers rather than try to replace them, or find all the values used & make upgraded clones.

Even then the crossover circuit may even account for the resistances of the coils and the ESR values of the capacitors and changing the components can change the balance of the speaker.

Passive crossovers from scratch are not easy, You have to consider the crossover frequencies and slopes to suit the drivers, level matching, any EQ to suit the drivers response, & the mechanical roll off frequencies of the drivers.

The crossover circuit will be made to suit the impedance of each driver at the crossover frequency not at the nominal impedance.

I have restored a couple of old speaker crossovers recently and usually any electrolytic capacitors are what fail. Having said that you say you have no output from the bass drivers which would not usually be caused by a capacitor failure.

I used a cheap LC meter bought on ebay and verified against some known value capacitors & inductors to measure the crossovers I rebuilt.

From your description the tweeter might be a first order cap with the resistor in series to reduce the level to match.

https://imgur.com/a/PQymLyo" rel="nofollow - https://imgur.com/a/PQymLyo

No luck ... all I get is a "insert image" button, and when I click on it, I only have an option to choose a file from /uploads/public (I have no idea where that goes - and it has photos that have nothing to do with my computer) - and no option for changing the path.  I dont see any way to change the path to reflect choosing a file on my computer.

I am using Chrome - do you folks find that compatible with this site?????   Sorry if I am just doing something stupid....

Insert image button,

Browse button - select local image file

Upload Image button.

Image should appear in your image properties list and should be preselected, or can be selected from the list to appear in the preview window.

Click On OK to put the image in your post.

So, you have no output from either woofer, but the mid and tweeter work. 

Next possibility is that the cap for the front woofer has gone short. Lift one leg and check for continuity, it should read infinite impedance. If your meter has the function, check the capacitance value. It should be within 10% of what is printed on the cap.

If all those components pass these checks, then your box contains devils. Call a priest.

Good luck :)

rchamlen wrote: Oops??????

You know ... you might actually be pretty close on this one.   This was bugging me this morning, because what I could see and measure of the circuits just did not make sense (in terms of the failure I was seeing).  the two woofers are driven by entirely separate low pass filters, and only one of them has a capacitor.  Now that I understood how the circuits are laid out better, I  was able to take multiple measurements with my multimeter.  Speakers checked out (you saw that before), the cap is not shorted, I do have continuity between the input jacks and the hot output to each of the woofers (you are right - the inductors measure about 1.4 ohms), and I have continuity between the neutral for the woofers and the neutral on the input jack on the back of the cabinet.

Huh???

so I hooked the speakers up to a stereo, measured and got AC output at the wires leading to the woofers.   Hooked up the woofers (one at a time, alternating front and back connections) .... AND THEY WORK.  (I know from personal inspection that they did not previously)

Grrr.  The best I can figure (and here is where your "devils" come in ... is that the connectors from the crossover outputs to the speakers (the flat, slide on/off type) had corroded and lost continuity at the interface.  Likely one went a while ago, and was not noticed.  The speakers was thought to have "died" when the second woofer connector lost continuity.  (duh .... I have seen this on so many other type of connectors - from ethernet to coax, to computer drive cables, etc).  Not definitive, but that is my best guess).   so, probably my best course right now is to go through all of the drivers, and re-seat all of those connectors.

Question though: since I have the box opened, and I have (finally) traced the actual circuits .... I know those caps will eventually die (they are, what, at least 25 years old now???   should I go ahead replace all those caps on all the crossovers (four boards total .... maybe 4-6 caps per speaker) .... or should I just re seat the connectors and put the speakers back in to service.  (since these will be with my son, there is no saying I will be in a position to service them after a year or two.....).  What do you guys think?   

Thank you for your help in thinking this through - your comments did help guide me a lot!

The electrolytics are usually the components that will go out of specification.

Usually it makes sense to replace them with more electrolytics if the values are large. You could maybe look at higher voltage rated parts, or plastic caps if the values are small.

Once this is done the speakers should be working properly if all the drivers are OK, and should stay that way for years.

Most other component replacements would be to try to give the speakers a bit of an upgrade which would be down to you.

Yeah - I know about the degradation of electrolytics ... which is why I ask the question.   Sounds like you are voting in favor of replacing them while I have at least one of the speakers opened up?    given the age, I dont think I will even bother with checking their values - just replace them with equivalents.  The rest of the components I will leave just the way they are  .   Thank you all for your thoughts and insights!

thanks.  all connectors on the board itself are soldered - the connectors are only to attach to the speakers (I might keep those for ease of attachment/removal of the drivers).  They did actually use several polypropylene caps on the original build, but have that one electrolytic you see, and two electrolytics on the midrange bandpass.  Not sure why they would have used electrolytics when polypro were available?  I am not a EE - so I do not know enough to know if there was a solid reason (other than cost...)

There's a theory that if you connect a 1.5v battery across the electrolytic to maintain a polarising voltage, it sounds better. JBL do it in some of their expensive domestic models! Adding the battery holder is still probably cheaper than using the non-polar cap...