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Old 4th May 2008, 10:26 PM   #1
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Default What is wrong with this?

It should not be a surprise to anyone that there is more output voltage on the speaker leads of an aftermarket deck. For an Alpine entry-level deck that says 45 x 4 max (16 x 4 RMS) that should work out to be 8 V, according to this this calculator:

http://www.sengpielaudio.com/calculator-ohm.htm

That is four times the output voltage on those preamp outs.

This is where it gets interesting, if you haven't already guessed. Is anything wrong with using those speaker leads as preamp outs on male RCA jacks, and making a direct connection to the next stage in line, eq, xover or amplifier?

Is this a good or a bad idea?
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Old 4th May 2008, 10:30 PM   #2
defect9 is offline defect9  Ireland
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Are you sure its a good idea to run 2.0 amperes at your input stage? I smell barbecue.

-Jared
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Old 5th May 2008, 12:56 AM   #3
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Sometime last year or a couple years ago when I had an experimental entry level deck on my test bench, I made a direct connection of RCA jacks on the the speaker leads (by soldering them on) and observed over 5 volts on those RCA jacks connected to those speaker leads. No 8 volts and "no barbecue."
However, it was at a lower volume setting - 19 I believe down from 29 of 35. I remember taking them out, now sure why?

I also know that there is d-c on those speaker leads and not sure it a good thing to do, without using a suitable interface and adequate isolation.
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Old 5th May 2008, 03:12 AM   #4
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Not to mention noise, distortion, etc. from the crappy amplifier. Oh, and the fact the outputs are probably BTL and floating at half the supply voltage.
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Old 5th May 2008, 03:23 AM   #5
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Half the supply voltage is correct. Off 0V on 6V.
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Old 5th May 2008, 04:03 AM   #6
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I think 8V into a line level circuit will clip.

The line level input is high impedance so no high current will flow.

Personally, without knowing the output driver schematic, I'd put some kind of realistic load on the amplifier output, say 10-50ohms and then resistively divide down the 8V to a more sane 1V line level.

...or turn the volume down.

There shouldn't be DC on the speaker leads. Yes, the speaker leads could well be at Vcc/2 (6V) relative to the chassis but both speaker leads should BOTH be at 6V. Make sure you stick a DC blocking cap in the input to your circuit. A differential input looking at both speaker wires is probably best to minimize noise.
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Old 5th May 2008, 04:25 AM   #7
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Quote:
Originally posted by defect9
Are you sure its a good idea to run 2.0 amperes at your input stage? I smell barbecue.

-Jared

Current is limited by impedance. An amplifier will have a input impedence around ~28k-80k ohm so won't be an issue.

You could very likely damage the equipment still. The amplifiers signal ground is at ground but the deck output isn't as a aftermarket deck is bridged. You would be putting DC into the amplifier input stage, The deck will likely have built in protection but its still nothing you should try.
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Old 5th May 2008, 07:42 AM   #8
defect9 is offline defect9  Ireland
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Quote:
Originally posted by ocool_15


Current is limited by impedance. An amplifier will have a input impedence around ~28k-80k ohm so won't be an issue.

You could very likely damage the equipment still. The amplifiers signal ground is at ground but the deck output isn't as a aftermarket deck is bridged. You would be putting DC into the amplifier input stage, The deck will likely have built in protection but its still nothing you should try.
I had forgotten about the input impedance. thanks for clearing that up in my brain
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Old 5th May 2008, 04:26 PM   #9
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Quote:
Originally posted by ocool_15



Current is limited by impedance. An amplifier will have a input impedence around ~28k-80k ohm so won't be an issue.

You could very likely damage the equipment still. The amplifiers signal ground is at ground but the deck output isn't as a aftermarket deck is bridged. You would be putting DC into the amplifier input stage, The deck will likely have built in protection but its still nothing you should try.
I hear you brother, but what do you make of this:

"Here’s a thought on those big internal amps, use them as preamps instead - 27 watts into 4 ohms equates to about 10 volts. If you drive a higher impedance, such as that found at the input of an amp (with balanced inputs) or in a LOC, you’ll get even more. When’s the last time you saw 10-volt+ preamps in a head unit? This approach should prevent the unit from becoming overly hot and provide excellent performance from the noise perspective due to the amp’s voltage capabilities."

This is what started the whole thing.

Here's the full review, but ONLY the above pertains to this thread.

http://www.carsound.com/reviews/hd_units/cda-7875.html
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Old 5th May 2008, 04:53 PM   #10
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I don't think Brian Smith understands amplifiers very well. He seems to think that increasing the load impedance increases the voltage the amplifier puts out. This is not true.

An amplifier can only put out voltage to the limit of its voltage rails. In a car amp, unless there is a voltage booster/step-up circuit, you are limited to ±12V peak best case (bridged mode) That's 8V rms regardless of the load impedance (>4ohms).

I still think this will overload any other amplifier input stage (typically expecting 1Vrms) and cause severe clipping.
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