Newsgroups: rec.audio.tech
Subject: Re: Connecting subwoofer with speaker wire
References: <[email protected]> <[email protected]>  <[email protected]> 
[email protected] (GregS) writes:

> In article <[email protected]>, [email protected] wrote:
> >Sorry if I wasn't specific enough. It is a powered subwoofer and the
> >speaker wire would be carrying a signal, not driving the speaker.  I
> >called Yamaha (it is a Yamaha receiver) and their tech support guy told
> >me that my solution might possibly work if I get some kind of line
> >conditioner to remove any noise from the signal.  He could not give me
> >any more detail on what I would need.  Can you tell me more about what
> >you mean by "balanced conversion" if that still makes sense now that
> >I've described the situation more completely.
> 
> Radio Shack part number 273-1374 is a 600 ohm isolation transformer. You
> attach the input to the drive, normally a speaker output. A line level output is probably not
> going to have a low enough impedance to drive it. 

True. 

> So you get another maybe 10 watt
> amp and attach the amp input to your sub out. The amp will attach to the transformer input.
> Its better to have the transformer near the sub. The output will feed the sub. 

True for the idea. The problem could be that the technical specifications 
of that Radio Shack transformer do not look very good for subwoofer application

From http://support.radioshack.com/support_supplies/doc9/9888.htm

1:1 Audio Transformer  (273-1374)    Specifications    Faxback Doc. # 9888

Coil Turns Ratio:......................................................1:1
DC Resistance:
   Secondary:..............................................60 Ohms +/- 10%
Impedance:....................................................600-900 Ohms
Frequency Response:........................................300 Hz to 5 kHz
Insulation Resistance:.....................More than 100 megohms at 250VDC

The frequency response is specified from 300 Hz to 5 kHz. 
That transformer is not expected to work well with subwoofer 
frequencies (20 Hz to 100-150 Hz typically). 
Attenuation of the lowest frequencoes and with strong input signals
(amplified line signals) I would expect serious distortions at lowest 
frequencies. 

Ttransformer approach works as descrubed above, but I would look 
for a better transformer with more suitable specifications. 

> The gain on the 
> auxilarry amp may have to be fiddled with, and will otherwise have no bass
> or treble boost. The polarity may also have to be reversed if the aux amplifier iverted the
> signal. 

> Since the 600 ohm transformer feeds a higher impedance input, its response will be
> much better than its specs indicate. This will give noise free performance.


The specifications on frequiency response would be with high 
inpedance load on output and low impedance input better than 
listed on the specs. That's true. But still might not be adequate 
for hifi subwoofer. And then there is that risk of transformer 
satutation at low frequencies at high signal levels. 
This is quite tiny transformer, it can't handle high signal 
levels at low frequencies. 

> I have used this system, 

Do you have any specifications to give how well it worked. 
Especially at lowest frequencies. 

> however you might want to check out part # 270-054
> which is a RCA ground loop isolator from Radio Shack. I have never used this, but
> if you connect the ends of the wires to the RCA input, it will feed directly into
> your sub. There are several products for car stereo which you can adapt to your situation
> if you do a little searching.

I have never used this particular products. But I have used 
similar devices from several manufacturers (some look pretty much the 
same as the one in the Radio Shack web page picture). 

This kind of RCA isolators generally work much better than 
those small 600:600 ohm transformers in audio applications. 
When connected to normal RCA line level sources, they typically 
attenuate the lowest frequencies somewhat, but not very much. 
You can get frequency rensponse from 40 Hz and up wery well 
on units I have worked with... with very low impedance signal 
source and not too stong signals lower than that to 20 Hz or so. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/