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Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
I have a fairly decent 2 channel amplifier, with 2X35W outputs, with 2 35W (smallish) speakers to match, which is nice for my dorm room, but i also have a 215W bass guitar speaker (actually 2 10 inch speakers) and an amp for that. I have the input of the bass amplifier connected to the left channel of the recording out via a RCA to 1/4 inch cord. However, the volume level of this RCA plug doesn't change when the volume on the reciever is changed, so the bass is at a fixed volume no matter what the volume setting on the reciever. Im wondering if the best way is to put a small audio transformer in parallel with one of the speakers to match it to the 15kohm input impedance of the bass amplifier, or if i should just use some sort of resistive divider to do that. Suggestions anyone? This way i can set the bass to the correct volume and then just adjust the one volume control.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Does the bass amp already have a low pass filter in it? Because if it doesn't then you will will have to make your own (not too hard). But if it has one already a simple resistive divider should be fine...
I am going to take a guess that it is putting 35w into a 8r load, which is like 4v (although you should put a scope on there since amps power ratings never make sense) which would need like a 4:1 resistive divider. (for bass frequencies any old resistor is just fine). If you don't have a scope use a fullwave rectifier on the output of the speakers and put a small cap on the dc side of it. Measure the voltage across the cap at full voltage. Add 2v for the diode voltage drop (and a little safety zone). Then use a divider to get it down to to standard 1v p-p.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Since you didn't mention any hum from the bass/guitar amp a resistive divider will be ok (no need for galvanic isolation) I would also add a capacitor across the guitar amp input, and use both left and right speaker signals.
example, as above say 4:1 division, 100k Ohm from left and 100k Ohm from right speaker outputs straight to the guitar amp input with 100nF to 0V for 160 Hz hf cutoff.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
The amplifier does have a low pass filter- it is able to be BiAmped with an adjustable crossover- 300W RMS per channel at 2 ohms. Too bad I don't have an 18 inch speaker... However, the bass cabinet is rated down to 38Hz, so it works pretty well. And each 35W channel has its own 4ohm speaker, not 8 ohm.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I never did figure out how the bi-amp function on bass amps worked. It seems to vary depending on the make and model. Any time I've used one, the crossover knob either did nothing at all, or something totally different to what I expected.
As far as I could figure out, they mostly have an active crossover built in, but only one power amp, which is hooked up to the high-pass output of the crossover. Then I guess they have a line output to drive another power amp for the low-pass. So it normally functions as a woofer and you can add a subwoofer, if you can be bothered lugging another power amp and huge speaker cabinet. But I've seen some other brands with two power amps built in, one for the woofers and one for the tweeter.
I used to have an old Peavey 4x10" speaker with a 100 watt MOSFET amp that I made from a kit. (Maybe some of our UK members will remember the old Maplin MOSFET amp.) 100 watts isn't much, but bass guitar speakers are a lot more efficient than a car subwoofer or whatever. I lent it to my brother once and he hooked it up to his stereo and nearly got it impounded by the police.
Bah, I'm wandering off topic... I think a resistive divider that takes the average of the two speaker outputs from the stereo is a great idea. The method Sulaiman suggested is good, but you might want a lot more than 4:1 division ratio since the inputs of these amps are quite sensitive. Also, a first-order low-pass filter (which is what the capacitor thingie is) might not be steep enough to be any use as a crossover. Then again it might be, you never know.
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Adding Sub to 2 Channel Amplifier
However, the bass cabinet is rated down to 38Hz, so it works pretty well. And each 35W channel has its own 4ohm speaker, not 8 ohm.
