Using a passive subwoofer with old stereo receiver

[Pat24601]

Hello!
I have an old style Yamaha stereo receiver, and a passive subwoofer.
The sub is a relatively simple one judging from various posts i've read. It has 2 connectors (a + and a -. that's it that's all) and an impedance of 4 ohms.

I want to connect it to my yamaha system. the yamaha has the typical A / B systems of that era... to which i have my left and right 8 ohm bookshelf speakers on system A. What i'd like to know is what is the best way to connect my sub to the receiver? Can i connect it in parralel to something without damaging the system? can i put it on system B? can i connect both left and right of system B to the sub?

Finally, the specs of the system says: A or B 4 ohm max (per speaker i'm guessing) A & B 8 ohm max (each, per speaker).

Hopefully someone out there has enough info from this post to help me out?
THANKS A LOT EVERYONE!!

 

[Leprkon]

put the sub in the left side of the B speakers.

It's not going to benefit you much. If you only have a stereo receiver, it won't read the sub portion (also called LFE for low frequency effects) of the signal. You need a Dolby Pro-Logic or better (5.1 or better) to be able to take that part of the signal and send it to the sub..

Your only real benefit from the sub will be on music, and that won;t be great. Your sub will be getting the whole signal, which it really wasn;t meant to deal with. A modern sub uses a filter (called a crossover) to take the midrange and tweeter portion of the the signal out, so the sub can do what it does best, bass.

Sorry I can;t give you better news, but as you upgrade in the future, your sub will become more valuable.

 

[markw]

It's a 4 ohm sub?

I'd be very cautious about putting a 4 ohm speaker in parallel with anything. Gua-ron-teed it's gonna result in a very low impedance on the amp. Yammies are good, but I dunno how it would handle this. Plusm without a crossover, youi're only be able to drive it with one channel and it'll sound like drek 'cause the full spectrum will be going thru it.

If you really wanna use this sub, invest in an inexpensive plate (sub) amp from partsexpress, madisound or the like. It'll turn that sub into a powered sub with all the inputs and outputs you need to mate it with any system.

 

[gregz]

Like MarkW said, adding a 4 ohm impedance in parallel with anything will likely overload the amp. In addition, the speaker connected in parallel with the woofer will suddenly be quieter too.

Depending upon how much power your amp has, you CAN set up the system through impedance matching.

This would involve adding a 4ohm resistor to your sub, and adding 8ohm resistors (or more) to your mains. Not the most efficient, but it would work well if you have the power.

Some passive subwoofers were made with built-in passive crossovers. If not, you can build your own passive crossover with a couple of choke coils and a capacitor from parts express (we can give you the values if you need them).

Many speaker crossovers do just this - add resistors to match the load and allow the woofers to work harder. I'm doing this in my current system, which is still quite loud thanks to a solidly built 100Wx2 receiver/amp.

 

[Pat24601]

would i need to add resistors if i were to just put the sub on system B?
I am not clear on what you described in adding 4ohm resistors to the sub and 8 to the mains.. the two speakers are each 8 on A and the sub would be 4 on B, but the specs say that it "can't" be 4 ohms if i'm using both systems... but would the fact that its only one of the channels change the risk?

i'm a complete amature with only the most basic of knowledge in electronics... i hope it doesn't show tooo much..

 

[gregz]

Pat, system A and B are actually just separate jacks to the same channels. The only purpose of the A/B speaker system is to allow you to switch between two different pairs of speakers (for separate rooms) without having to disconnect them each time. If you turn on both A and B at the same time, they connect to each other internally.

Finding 4 ohm and 8 ohm resistors that can handle 75W+ is difficult and often expensive. Nichrome wire is a great substitute, and is dirt cheap. You can stretch it out and zig-zag it in a blob of modeling clay for high power applications. (Nichrome wire is what they use to make heater coils in toasters and space heaters). I got mine from either Parts Express or MCM.

Place a 4 ohm resistor in-line on one of the subwoofer wires (doesn't matter + or -), and start out with 8 ohms in-line in each of the bookshelf speakers.

Depending upon your sub, and your bookshelf speakers, you may or may not get anywhere but it costs so little to try it's worth it - especially if you end up with better sound than when you started. As an inexpensive substitute for a crossover on the sub, you can aim it away at a non-reflective surface (some subwoofers are down-firing - carpet will nicely attenuate the unwanted high frequencies). The low bass frequencies will still be loud, since they're non-directional.

Feel free to ask more questions. And have fun!

 

[Pat24601]

Okay, so just so i have this strait in my head, my goal will be to have:

A B
L R L R
- + - + - +
8 ohm res 8 ohm res 4 ohm res -
8ohm spkr 8 ohm spkr 4 ohm sub -

so basically 16 ohms on both left and right of A and 8 on left of B, right is empty?

Thanks

 

[Pat24601]

i see this takes the html habits of making multiple spaces into single spaces...
appologies for the buggy formatting

 

[gregz]

LOL! I thought you had typed it into a secret code!

Yes; the amplifier will see each bookshelf speaker at 16 ohms on A channel L and R, and the subwoofer at 8 ohms on B channel R. Doing this, you promptly dump half your amplifier's power into resistors as heat, but such is life on a low budget and it sure beats frying your amp

You'll have to let me know how this works out. Did you confirm whether your sub has a built-in crossover or not?

 

[Rob Babcock]

Even if your receiver can run both, I'm not optimistic about the results. You'll have no crossover nor any means to blend the sub with the mains, and no way to change their relative volumes. The sub will get the mids & highs, too, so you'll want to place them close to the mains.

Have a fire extinguisher hand in case!

 

[Pat24601]

I have no way to check this for certain but i'm am fairly sure it has no such mechanism.. did i mention this is a sub intended for a car sound system? yeah.. i am just trying to get more fun in my system and this was dropped in my lap

i took a look through partsexpress but had trouble finding the nichrome wire you mentionned.. could you help me out? and how does the resistance get regulated with this wire? i'm not sure how that works....

is it necessary to bring the A system to 16 ohms? i thought the issue was only on the system with the sub?

Pat

 

[gregz]

I did a test tonight and took the choke coils off the connections to my subs. YECCHHHH!!! I then turned my subs around and pointed them at the back wall, but the nasty muddy midrange howl reflected off the back wall. Things got a whole lot better when I placed pillows between the subs and the wall, but I could still hear too much mid playing through them.

The moral of the story is you WILL need at least one choke coil (10mH inductor) in line with that sub to tame those mid frequencies. You can order those through the aformentioned companies, or even possibly find one at Radio Shack if they still cary those.

As for the nichrome wire, I'll have to look through my catalogs some more. It must have been in one of the industrial type catalogs like Grainger... To know how long to cut the pieces of nichrome wire, you either use a voltmeter to measure out the right amount, or you stretch it out and calculate it based upon the resistance per inch (cold).

Now the A speakers don't have to be at 16 ohms, but if you don't add resistance to the bookshelf speakers, you won't hear the subwoofer. Woofers need more energy, and since it will be tied to the same amp as the bookshelf speakers, you can't turn its volume up without also turning them up.

 

[Pat24601]

Okay, i believe i have everything pretty strait in my head, and i thank you guys for all your input! I have 2 final questions and that should do:
- To be clear, it is definately not recommended to connect both left and right channels of B to the same single input of the sub?

- Can i simply buy a crossover addition to cut out the mids and highs from the signal entering the sub?

and i guess one more question is what would be a minimum power capability of an amp to do this? meaning, power the 2x8ohm bookshelfs on A and the sub, with what will probably end up being with a 4ohm resistor... so 8hm load, on B?

Thanks again!
Pat