[jcg0322]

Hello Lab,
I am repacing the xover network on a set of 1974 vintage Jensen 3-way speakers. It has a simple 1st order network and I will keep it that way. The speakers are a 15" woofer, 2 4" midrange and a dome tweeter. The tweeter uses a 4uF capacitor, the midrange an 8uF capacitor. These are easily identified and will be replaced. The problen is an inductor which is used by the woofer. I cannot identify the original value. It has no markinigs. I measured it with an lrc meter at .56mH. This can't be right, it would put the xover frequency at 2300 Hz. My speakers are 8 ohm and would like some music people out there to suggest a typical frequency for a 15" speaker.
My guess would be 200 - 300 Hz but since I have never done this before I would like to here some input from others. Thank you for the help.
Jack

[Tam Lin]

Try looking at it from the other end. Ask not how high the woofer should go but how low can the 4" mid-range go? The natural roll-off of the mid-range will define the crossover of the woofer.

What do you expect to gain by changing the inductor, especially if you arbitrarily change the crossover frequency in the process?

[Æ]

Try looking at it from the other end. Ask not how high the woofer should go but how low can the 4" mid-range go? The natural roll-off of the mid-range will define the crossover of the woofer.

What do you expect to gain by changing the inductor, especially if you arbitrarily change the crossover frequency in the process?

It would appear that he isn't interested in changing the midrange component values, which would entail a complete redesign of the crossover. Interestingly, he keeps asking the same questions over and over and has received several helpful responses. Apparently he would rather have someone "engineer" him a new crossover, save him all the trouble of having to figure it out for himself. If he only wants to copy the original, I'm sure he can locate an original schematic off the world wide web and confirm if indeed the original crossover used a .56mH low pass inductor.

I would be curious if the midrange drivers are closed back or open back, and if they are wired in parallel or series. Application of modern crossover design would certainly make his Jensen's sound even better. Then again maybe he isn't concerned with the sound and only wants to preserve their nostalgic value.

[jcg0322]

Not interested in nostalgia and I will change anything or everything if I have to. But I would like to keep it simple. The four speakers are wired in parallel.
Each branch of the parallel circuit contains a xover component in series with a speaker. The xover components are a .56mH inductor for the bass, a 4uF capacitor for the tweeter and an 8uF capacitor for the 2 midrange drivers.

[Æ]

Not interested in nostalgia and I will change anything or everything if I have to. But I would like to keep it simple. The four speakers are wired in parallel.
Each branch of the parallel circuit contains a xover component in series with a speaker. The xover components are a .56mH inductor for the bass, a 4uF capacitor for the tweeter and an 8uF capacitor for the 2 midrange drivers.

Well then, I have some solid ideas that would fit your requirements. I'd have to talk you through it, but it is quite doable and wouldn't cost you much. Secondarily, have you thought about buying/reading any of the build it yourself loudspeaker books that are currently available?

[jcg0322]

AE,
Talk me through it. I have a wife and kids so I don't have lots of time. I am not an audiophile but like things done right.
It doesn't have to be perfect. I just want something simple if that is possible.
I am a certified electronics technician but only for aircraft radios and navigation systems. Speaker systems are not my thing.....but I am learning!!
Thanks

[Æ]

AE,
Talk me through it. I have a wife and kids so I don't have lots of time.
Give me your best shot. I just want something simple if that is possible.
I am a certified electronics technician but only for aircraft radios and navigation systems. Speaker systems are not my thing.
Thanks

Ok, but I'll have to write it all up first. It is too comprehensive for me to finish it all in a few minutes. A lot to explain. Basically you are going to have to measure the loudspeakers by ear. Take notes and make some simple ohmmeter measurements followed by some simple algebra. Do you have a signal generator? If not do you have any test CDs? You can download individual test tones or a complete CD from Michael Knowles. I recommend the entire CD. http://binkster.net/extras.shtml#cd
Be sure and read the entire users guide, well at least enough of it to understand what the different tracks are used for. If you already have and or have used a test CD you are already off to a good start.
You'll be designing a first order crossover, a low pass on the woofer, a band pass on the midrange(s) and a high pass on the tweeter. You might need to add some attenuation to the mids and/or the tweeters if they are louder than each other or louder than the woofer. Simple series resistors will accomplish that. So basically two new inductors and two new capacitors and then maybe one or two resistors.
Hopefully you know how to solder well, soldered connections are best. Some decent wire, minimum 16 gauge lamp cord or better. Also, you might need, benefit from the addition of some fill material such as fiberglas or poly pillow stuffing to help damp any unwanted internal reflections, resonances or standing waves.
Why not go ahead and audition your loudspeakers and tell me what they do or don't do well. How do they sound? Are they lacking in certain areas? What about them would you improve if you could. What about them annoys you? Do you have an SPL meter? Radio Shack sells an affordable one.

[jcg0322]

AE
I have a signal generator and oscilloscope. I am currently bidding on an SPL meter on ebay. The midrange and tweeter already have a potentiometer in series........they were like that when new. I will look for some poly pillow stuffing, I know how to solder very well and nothing really annoys me about the speaker sound. But, always room for some improvement.
Thanks

[Æ]

AE
I have a signal generator and oscilloscope. I am currently bidding on an SPL meter on ebay. The midrange and tweeter already have a potentiometer in series........they were like that when new. I will look for some poly pillow stuffing, I know how to solder very well and nothing really annoys me about the speaker sound. But, always room for some improvement.
Thanks

Now is where I would start listening to the individual drivers disconnected from the crossover, but still mounted in the enclosure. Listen with music, then with a 20Hz-20KHz log sweep and finally listen with individual test tones. Use a SPL meter at 1KHz to reference the relative volume (perceived loudness) of the individual drivers. You have to be careful when feeding low frequencies to a tweeter, you either have to do it at a very (low amplitude) low amplifier setting or use a simple high pass to protect the tweeter from low frequencies. Since a tweeter cannot reproduce lower frequencies, the power gets converted to heat and can cook the tweeter (if you are not careful). Anyway, the idea is to listen and determine the usable range of each driver. Evaluate them individually, listen to the woofers, midranges and tweeters by themselves. Take notes, maybe even draw a crude chart on graph paper from the SPL measurements. A large woofer will beam as the frequency goes progressively higher. Same with a midrange and tweeter too, just at higher frequencies.
While you are checking out the woofer listen for undesirable enclosure resonances, boxiness, honkyness or one note bass emphasis. That is what the stuffing is for. If the enclosure needs more stuffing, then add more stuffing to make the bass smoother, mellow as opposed to BOOM, BOOM, BOOM or thump, thump, thump.

This is what you should listen for:

By Martin Colloms,  from his book
"High Performance Loudspeakers"

Characterization                                           Approximate Frequency                               
                                                                                     Range

Fizzy, gritty                                                              10 - 15 KHz
Sharp, metallic, sibilant                                                5 - 8 KHz
Presence, upper hardness, wiry                                   2.5 - 5 KHz

Nasal, hard                                                             1.8 - 2.5 KHz
Cup-like, honky                                                      700 - 1200 Hz
Tubelike, tunnelly                                                    400 - 600 Hz

Boxy, hollow                                                            150 - 300 Hz
Chesty, plummy                                                       100 - 150 Hz
Boomy                                                                      50 - 80 Hz

[Æ]

The midrange and tweeter already have a potentiometer in series......

Probably not a potentiometer, should be either an L-pad or a rheostat. It can be a wire wound potentiometer, but it will likely be wired as a rheostat.
A variable L-pad is preferred over a simple rheostat since it will maintain a more constant load resistance. If the combined driver impedance and attenuation resistance are constant (do not change) then the crossover point will not shift undesirably.
Do you know how to identify an L-Pad a potentiometer and a rheostat? Externally they all look pretty much the same.

[jcg0322]

I should have an spl meter by next week and then I will bypass the crossover and do the testing. In the meantime I was looking at the situation more closely and noticed that the two midrange are wired in series. I thought that was strange, but what do I know.
The pots or L-pads are wired as voltage dividers. They have three terminals and all are wired. Terminal 1 has 2 wires with 1 wire coming from the cap and the other wire goes to the speaker. The middle terminal (wiper) goes to the speaker. The third terminal has a wire going back to the amp.
Next week when I get this meter I will hook up the audio generator and do the testing you suggested. It all sounds pretty straight forward. Thanks.

[Æ]

I should have an spl meter by next week and then I will bypass the crossover and do the testing. In the meantime I was looking at the situation more closely and noticed that the two midrange are wired in series. I thought that was strange, but what do I know.
The pots or L-pads are wired as voltage dividers. They have three terminals and all are wired. Terminal 1 has 2 wires with 1 wire coming from the cap and the other wire goes to the speaker. The middle terminal (wiper) goes to the speaker. The third terminal has a wire going back to the amp.
Next week when I get this meter I will hook up the audio generator and do the testing you suggested. It all sounds pretty straight forward. Thanks.

Unless the potentiometer or whatever is clearly marked in ohms, you have to use an ohmmeter to determine if it is actually a potentiometer or an L-Pad. L-pads and potentiometers look the same and are wired similarly. A speaker L-Pad is a special configuration of rheostats used to control volume while maintaining a constant load impedance on the output of the audio amplifier. It consists of a parallel and series resistor in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction. To maintain constant impedance in both directions, a "T" pad must be used. In loudspeakers it is only necessary to maintain impedance to the crossover; this avoids shifting the crossover point.

If you were to wire the midranges in parallel, the output (SPL) would be 6dB greater for the same amplifier voltage. While you are at it, use an ohmmeter to measure the resistance of each individual driver out of circuit and record the readings. At least within a tenth of an ohm.

[Æ]

Do you have a digital camera? How about a few pics of your Jensen's? A few pics of the individual woofer, midrange and tweeter? What is the name or model number of your loudspeakers? Maybe a few pics of the crossover and attenuators too.

[jcg0322]

Moving along.......
The 15" bass speaker dc resistance is 5.3 ohms. The 2 midranges are 6.7 and 6.8 ohms and the teeter is 6.6 ohms. The L-pad is 0 - 50 ohms. I think it is an L-pad now.
I have seen hundreds of pots and this does look different. I looked at L-pads on the internet. The feel when turning it is different. There are 2 numbers on it 51-685 and 137-7539.
I can send you pics....send me your email. I will take pics tomorrow.
AE, I really appreciate the help.

[Æ]

Moving along.......
The 15" bass speaker dc resistance is 5.3 ohms. The 2 midranges are 6.7 and 6.8 ohms and the tweeter is 6.6 ohms. The L-pad is 0 - 50 ohms. I think it is an L-pad now.
I have seen hundreds of pots and this does look different. I looked at L-pads on the internet. The feel when turning it is different. There are 2 numbers on it 51-685 and 137-7539.
I can send you pics....send me your email. I will take pics tomorrow.
AE, I really appreciate the help.

Send me a Personal Message with your email address and I'll reply with my email address.

We really need to determine if they are potentiometers, potentiometers wired as rheostats or L-Pads. You'll have to figure it out. Typically L-pads are 8 ohms, so if there are no 8 ohm markings, then maybe its a potentiometer. Wire wound pots and L-Pads feel different than carbon composition pots. You should disconnect the pots/L-Pads from the circuit to check them.
If you take an L-Pad and connect it to an 8 ohm resistor in place of a tweeter, you can check the resistance between legs 3 and 1 while rotating the shaft. The resistance should remain pretty much the same regardless of where you turn the shaft. It's been a while since I've used an L-Pad. If I remember correctly you connect legs 3 and 1 to the amplifier, while legs 2 and 1 go to the tweeter. Leg 1 would be common to both the tweeter and amp. The resistance will vary but not too much. I had some inexpensive Radio Shack variety 8 ohm L-Pads and the resistance varied from about 7 ohms to 11 ohms, which really isn't very good, but hopefully you get what I'm trying to explain to you.
If it's a pot, check with an ohmmeter between the outside legs and regardless of where you turn the shaft the resistance between the outside legs will remain the same. The center leg is the wiper and divides, Place the shaft midway and you should get 25 ohms in either direction if it is a 50 ohm pot. I'm sure you know, understand how to check the function of a potentiometer.

I see no reason to change the tweeter crossover frequency, so go ahead and use 4uf capacitors, hopefully you'll purchase new capacitors. NP electrolytics are OK, but film are definitely better. Film caps will sound brighter, clearer and slightly louder than electrolytics. Once we've determined the attenuator profile we can figure in the additional resistance with the tweeter and calculate the high pass frequency which will then be the starting point for your midrange crossover. If they are pots instead of L-Pads, you might consider purchasing and using some L-Pads instead.

http://www.mh-audio.nl/spk_calc.asp