Alpine head unit says 4V pre-out but only 40mV? - diyAudio
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Old 18th April 2005, 05:25 AM   #1
rth is offline rth  United States
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Question Alpine head unit says 4V pre-out but only 40mV?

The sensitivity of my Kicker ix2302 is 150mV-3V and Alpine says this head unit has 4 V pre-outs, which I assumed was either an RMS voltage or a peak voltage that I would see on the RCA cables, so I decided to measure it first to make sure I didn't exceed the amp's input range. I looked at each channel on the pre-out (subwoofer and regular) and at normal volume on the main speakers I'm only seeing 80 mV p-p and when turned way up I see around 300 mV p-p What exactly does "4 V pre-out" refer to if it's not even half a volt peak to peak? I'm having to turn my gain all the way up on the amp to get the level to match the main speakers (unamplified till i fix my ppi). The scope had a hard time triggering on this level so the waveform looked really noisy but looking at the output on the amp I was seeing a perfect sine wave at up to 160 V p-p I was surprised that it looked that good on the amp given how nasty it looked at the low-level input but I guess the scope was not giving me the true story.

Does 300 mV p-p seem low for a pre-out or is this common? I would think a level of several volts would be better to avoid noise being amplified along with the signal, no?
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Old 18th April 2005, 06:24 AM   #2
Stocker is offline Stocker  United States
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Turn down the gain on your amplifier so you do not deafen yourself.

Turn up the volume all the way, or until the preamp output hits 3V.

Are you using a test tone, sine wave or music as a signal? Looks like a sine wave.

What is the input impedance of the amplifier...could it be loading the preamp output too heavily?
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Old 18th April 2005, 06:36 AM   #3
Tim__x is offline Tim__x  Canada
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Many 'scope probes attenuate the signal, perhaps your probe is a 1:10 which would mean your measured 300mv was actually 3v.
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Old 18th April 2005, 09:07 AM   #4
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I did have to use a 10x probe to see the amp output due to scope clipping at ~120V but I used a 1:1 probe on the RCA cables and it surely was ~50mV at reasonable volume. My DMM also confirms this, which is why I got the scope out, because it was reading ~30 mV RMS before I cranked up the volume. I am using a 60 hz sine track off a bass test cd and don't worry about the spl on my eardrums because i have it turned up only for testing right now... sounds like booty without an amplified front stage anyway

One possibility is that the cd player is sensing that the rca cable is disconnected and shutting off that output so maybe I will have to put a load on it to see anything, though both the amp and scope should have very high input impedances. I can try turning up the volume more but these factory speakers don't like that 60 hz very much

Taking it back to the basic question: should I see 4 Vpp (or is it 8 Vpp, 4 V peak?) if Alpine says "4 V pre-out"? Have any of you ever looked at the output of your head unit?
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Old 19th April 2005, 04:11 PM   #5
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I think the Alpine is 4V peak (not peak to peak) but you will only get this with a sinewave recorded at 0dB and the volume turned all the way up. Also if you are using the sub out, that has a 15dB attenuator, usually if you press the function knob you can select the sub out, you will have to turn that up to +15 to get your 4V out of the sub out.

I had an old alpine and I couldn't get the outputs to clip it was awesome, full scale recordings with the volume up to max and the bass boost up full we could get about 11V peak out of it and it was still a clean unclipped sine wave!!!
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Old 19th April 2005, 04:53 PM   #6
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WOW

This is my brother's Alpine mp3 player that I'm using so I haven't messed with it much but my older Alpine cd player is still rock solid and I know it had a large pre-out signal because I could leave the gain almost all the way down on the amp and still go boom boom boom

As soon as this PPI amp is fixed (waiting on a new FET driver from TI) I'm going to clip these factory speakers and then I can turn up the volume and see what the hell is going on with this newfangled alpine thingamajigger
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Old 21st April 2005, 09:45 AM   #7
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i was always under the impresion that preamp output voltage was used as a carrier for the signal, the higher the voltage the better the signal has of staying clean, if thats not the case, please someone educate me... besides, im at the opinion that nothing beats cables with proper and/or excellent sheilding. this is something that is over looked in a lot of sound systems ive seen, home or car...
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Old 21st April 2005, 10:25 AM   #8
rth is offline rth  United States
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By carrier do you mean "4 V pre-out" would mean a 4V DC offset with the signal swinging around that voltage? That's possible, I wouldn't have noticed it with the scope on AC coupling. I'll throw the DMM on it tomorrow and see if that's the case.
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Old 21st April 2005, 11:27 AM   #9
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I was told that it was peak to peak, however Puggie seems to have practical experience to counter that. It won't be DC offset is 4V as that would be completely pointless and worse than the normal 6V/half supply.
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Old 21st April 2005, 04:21 PM   #10
rth is offline rth  United States
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An offset would help reject common-mode noise but you are right, a split supply with as large a swing as possible would be the easiest and best route. I don't guess there could be an offset anyway because there's no zero-adjust on car amps
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