Bose 701 Series II - Project Frankenstein

Charlie Hdez

Charlie Hdez

Member
I recently bought a pair of used Bose 701 Series II in pretty decent shape. The kind that have an amplified 8" (sub) woofer.

Bought them for cheap, so I wasn't too surprised when I hooked them up and the woofers didn't put out any sound. A quick direct test showed that the 8" woofer drivers are fine, so it's likely the built-in amplifier that's dead.

I’ve decided to try and revive them by getting rid of the amplifier and joining the low-frequency tranducer with the mid-hi driver series. I’ve already opened them up and put in enough detective work to figure out all the internal cable mappings, so I'm ready to begin patching.

My question to you all is: should I joing them in parallel or in series? Also, would the lack of a crossover between the woofer and the three mid-hi drivers be an issue? .

-CH
 
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Charlie Hdez said:
My question to you all is: should I join them in parallel or in series? Also, would the lack of a crossover between the woofer and the three mid-hi drivers be an issue? I guess I'll find out.
Click to expand...
Parallel would be the logical choice. Though I'd personally start by measuring impedance of the woofer and proceed with caution. Especially if it's 4Ω (or less!). Depending on how much bandwidth is shared by woofer and midrange in parallel, might risk low impedance across your amplifier.

Should pose no danger to the speakers themselves. The passive mid/high frequency drivers are designed to run off AV receiver set to "large" (full range), and also have built-in protection.

This system prefers a 150Hz low pass for the woofer, according to user guide. If you can get a sense of its impedance, an inductor suggested by an online calculator can be wired in series with the woofer for a rudimentary 1st order filter. Less than perfect, but affords some protection to the amp.

Then it becomes a matter of matching level between woofer and mid/high frequency. The stock speaker is bi-amped, with separate woofer level control to match high freq. output.

Which suggests the best solution, if you have a spare amplifier/receiver at hand. Connect the main receiver LFE output to second amp input and drive subwoofer off that amp, with its volume control tweaked to match high freq. level. No crossover work needed. Less low impedance worries.
 
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Thanks for the good information. I was going to try parallel first, but now I know it’s the better choice. Will check impedance and proceed cautiously…
 
If you have a digital AVR and a spare power amp (or receiver you can use as an amp) you can wire the LF drivers up as subwoofers (you'll probably need an RCA Y cable), setting the receiver's sub crossover appropriately. It's a little higher than the ideal crossover frequency (100hz is usually the maximum recommended) but will be a lot more flexible.
 
I've successfully connected the LF driver (8" woofer) in parallel with the HF drivers (midrange + 2 tweeters). The speakers sound great with my Crown XLi2500 amplifier. My one concern is that the midrange seems to be working just as hard as the woofer with the lower frequencies. I think I need a crossover to keep the frequencies below 150Hz from going to the midrange. It's my first time dealing with speaker internals on my own. I "sort of" know what I'm doing, but need some quick guidance. Any advice which component inserted in the right place will give me that result?
 
swechsler said:
If you have a digital AVR and a spare power amp (or receiver you can use as an amp) you can wire the LF drivers up as subwoofers (you'll probably need an RCA Y cable), setting the receiver's sub crossover appropriately. It's a little higher than the ideal crossover frequency (100hz is usually the maximum recommended) but will be a lot more flexible.
Click to expand...

What I'm trying to achieve is to turn them into a full-range speaker, rather than requiring two separate amplifiers. Thanks for your suggestion, though!
 
You'll need a high-pass filter on the midrange. A simple first-order (6dB/octave) crossover requires only a single capacitor, and there are online calculators to figure out what type. One problem, of course, is that you only know the nominal impedance, rather than the actual impedance at 150hz, so it's going to be a little bit of a guess.
 
Charlie Hdez said:
My one concern is that the midrange seems to be working just as hard as the woofer with the lower frequencies. I think I need a crossover to keep the frequencies below 150Hz from going to the midrange.
Click to expand...
701 II's already have a high pass filter on the midrange. It's connected directly to the driver. There's a thin resistor at the top and film capacitor below. Apparently wired in series with each other and, of course, in series with the mid driver. [ Edit -- this turns out not to be the case! ]

Mid xover.jpg
(pics from a Nguyên Bách Audio youtube video)

Tweeter filters are similarly mounted in close proximity to drivers, as seen behind this upper tweeter ...

tweet xover.jpg
... showing a series cap above and shunt coil below.

If you have a cable to connect a PC line or headphone output to the Crown inputs, you could disconnect the subwoofer and run a low frequency sweep using an online tone generator. That will give you an idea of how much low frequency energy the midrange is contributing below 150Hz. There will be some -- it's a first order filter with a gentle roll-off. But that would be by design.

I suspect the problem is excessive energy around the crossover point from the woofer sharing too much bandwidth with the mid. Woofer really needs a low pass filter, as it had when actively driven. That means at least a coil in series with the woofer. Figuring the appropriate value is the challenge.
 
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Stuart Pedaso said:
701 II's already have a high pass filter on the midrange. It's connected directly to the driver. There's a thin resistor at the top and film capacitor below. Apparently wired in series with each other and, of course, in series with the mid driver.

View attachment 2663811
(pics from a Nguyên Bách Audio youtube video)

Tweeter filters are similarly mounted in close proximity to drivers, as seen behind this upper tweeter ...

View attachment 2663824
... showing a series coil above and shunt coil below.

If you have a cable to connect a PC line or headphone output to the Crown inputs, you could disconnect the subwoofer and run a low frequency sweep using an online tone generator. That will give you an idea of how much low frequency energy the midrange is contributing below 150Hz. There will be some -- it's a first order filter with a gentle roll-off. But that would be by design.

I suspect the problem is excessive energy around the crossover point from the woofer sharing too much bandwidth with the mid. Woofer really needs a low pass filter, as it had when actively driven. That means at least a coil in series with the woofer. Figuring the appropriate value is the challenge.
Click to expand...

I connected a tone generator and ran it at 60 Hz and the mid range vibrated just as much as the woofer. I think your last point is correct.
 
Charlie Hdez said:
I connected a tone generator and ran it at 60 Hz and the mid range vibrated just as much as the woofer. I think your last point is correct.
Click to expand...

Here's a picture of the system board that takes the input from the receiver and feeds the internal subwoofer amplifier (as well as a straight signal to the HF drivers).
IMG_9821.jpeg
 
Charlie Hdez said:
Here's a picture of the system board that takes the input from the receiver and feeds the internal subwoofer amplifier (as well as a straight signal to the HF drivers).
Click to expand...
Well THAT'S interesting! A 100µF bipolar capacitor is roughly the right value for a 1st order high-pass filter at about 150Hz for a driver of approx. 8Ω.

Situated amongst speaker level input and output PCB traces places them, with high confidence, in the passive crossover circuit. Should look something like this:

701 II mid.png

It appears that cap and resistor in series are just strapped across the 6.5" midrange like an impedance-flattening Zoebel network. Image posted earlier is too fuzzy to discern. (Capacitor inside the dashed box for the 2" tweeter just denotes a generic high pass filter that hasn't been visually identified)

If all above is correct, the 100µF caps would block low frequencies from the mids when Speaker-In and Speaker-Out are connected normally. That can be verified with a tone generator sweep.

When connecting woofers, need to bypass those big caps or they'll get no low bass. Perhaps connect wires from inside of Speaker-In terminals J6 directly to middle pins of Right Speaker-Out J8 for the woofer. And similarly from J6 Left Speaker input to appropriate pins of output connector for the Left (internal) woofer. Which would yield:

701 II rework-R.png

Might be worth a shot as a place to start.
 
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I did exactly what you suggested on the last example. The sound is great, but I think it’s drawing too much from the amplifier. I think impedance is below 4 ohms. Why I think so? My crown XL I 2500 amplifier never used to turn on the fan, and now it does after a few minutes of low-volume playing. . But also the clipping light is flashing at way lower volume than it used to before.
 
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By the way, the midrange stopped reproducing low frequencies when I ran it back through the system board with the integrated 100µF bipolar capacitors. Right on the money.
 
Charlie Hdez said:
I did exactly what you suggested on the last example. The sound is great, but I think it’s drawing too much from the amplifier.
Click to expand...
Could be another possible issue here.

Mid and tweeters are of course connected directly from Receiver Input (through the 100µF cap). Woofer on the other hand, even though wired directly from Receiver Input to Speaker Output, is also still connected to the Bose bass amp via traces on the PCB. The Bose amp output stage circuitry might be seen by the Crown as an additional impedance load in parallel to both woofer and Mid/Tweeter circuits, dropping total load impedance even further.

Easiest way to isolate speakers from the Bose amp is probably to cut traces at the Right Speaker output terminal and just cut either the two wires from, or PCB solder leads to, Left Speaker output terminal woofer connections.

Rt LF mod.jpg

Left LF mod.jpg

This really should be done even if it doesn't solve the apparent low impedance issue.

If you have a multi-meter handy, try getting an Ohm reading for the woofer. Easiest done at the Right speaker's input connector. Just disconnect the cable and Ohm across the two terminal pins feeding the woofer. From a DC Ohm reading we might infer an approximate nominal impedance, lending insight into what the Crown is having to deal with.
 
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Stuart Pedaso said:
Could be another possible issue here.

Mid and tweeters are of course connected directly from Receiver Input (through the 100µF cap). Woofer on the other hand, even though wired directly from Receiver Input to Speaker Output, is also still connected to the Bose bass amp via traces on the PCB. The Bose amp output stage circuitry might be seen by the Crown as an additional impedance load in parallel to both woofer and Mid/Tweeter circuits, dropping total load impedance even further.

Easiest way to isolate speakers from the Bose amp is probably to cut traces at the Right Speaker output terminal and just cut either the two wires from, or PCB solder leads to, Left Speaker output terminal woofer connections.

View attachment 2665072

View attachment 2665073

This really should be done even if it doesn't solve the apparent low impedance issue.

If you have a multi-meter handy, try getting an Ohm reading for the woofer. Easiest done at the Right speaker's input connector. Just disconnect the cable and Ohm across the two terminal pins feeding the woofer. From a DC Ohm reading we might infer an approximate nominal impedance, lending insight into what the crown is having to deal with.
Click to expand...

I can do that! Will post results

-CH
 
Stuart Pedaso said:
Could be another possible issue here.

Mid and tweeters are of course connected directly from Receiver Input (through the 100µF cap). Woofer on the other hand, even though wired directly from Receiver Input to Speaker Output, is also still connected to the Bose bass amp via traces on the PCB. The Bose amp output stage circuitry might be seen by the Crown as an additional impedance load in parallel to both woofer and Mid/Tweeter circuits, dropping total load impedance even further.

Easiest way to isolate speakers from the Bose amp is probably to cut traces at the Right Speaker output terminal and just cut either the two wires from, or PCB solder leads to, Left Speaker output terminal woofer connections.

View attachment 2665072

View attachment 2665073

This really should be done even if it doesn't solve the apparent low impedance issue.

If you have a multi-meter handy, try getting an Ohm reading for the woofer. Easiest done at the Right speaker's input connector. Just disconnect the cable and Ohm across the two terminal pins feeding the woofer. From a DC Ohm reading we might infer an approximate nominal impedance, lending insight into what the Crown is having to deal with.
Click to expand...

Once again, your recommendation was right on point. After disconnecting the woofer outputs from the internals of the PCB (subwoofer circuits), the amplifier stopped clipping, the fan doesn't kick in any more, and I am able to hear crystal clear music at loud levels.

I am so happy with the results. The speakers really do sound great without the subwoofer amplifier.

This is the final result:

IMG_9834.jpeg

I allowed the midrange lines to go through the built-in capacitors on the system board.

I then bridged the receiver inputs directly to the woofer output terminals. (I didn't need to use plastic caps, but it made soldering and mapping a lot easier - Kudos to my friend Ron for his awesome soldering skills). And lastly I cut the link from the woofer output jumpers to the amplifier internal circuits by drilling holes on the printed wires.

to left woofer:

IMG_9834-L.jpg

and to right woofer:

IMG_9834-R.jpg

:rockon:
 
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Nicely done!

Your purchase turned out to be a good investment, yielding a usable pair of speakers and also a fun project in the mix.

:thumbsup:
 
hello i didnt the same on my set of speakers and it works, just one question the right speaker woofer sounds with less punch that the left?
any ideas, and thanks a million for the post i purchase the speakers in 2001
and just recently the amp died, for a long time there was a hum coming
out when there were not playing music, i kind new soner or latter the amp was going to Die
 
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