# Technical question: voltage level of amplifier signal input



## Thumper26 (Sep 23, 2005)

A little background info: I've been working on a project using my factory deck in a more daily driver install, and have run into a white noise hiss on every input from the factory deck. Signal goes through a David Navone LOC to a Genesis Profile 4 Ultra amp. I've troubleshot and tested everything imaginable trying to fix this, and am trying two last things this weekend before I have the factory deck modded to a deadhead and pull a signal before the onboard IC amp, which is running a 12 gauge ground from the ground soldered onto the factory radio board directly to the car chassis, and also seeing if for some reason zapco's balanced signal can kill it. Personally I don't see symbilink being a fix, but we'll see.

Regardless, I've had several conversations with David Navone regarding the issue, and told him about the deck mod. I mentioned we were going for 4v output from the deck and he suggested 30v for the sake of a cleaner signal.

I've never questioned why signal voltages were rated what they were, and had assumed delivering significantly more would burn something out in the amp. David said it wouldn't and so now I'm curious.

My question is, what is the risk in delivering 5x the signal voltage to a car amplifier's input? Is the rating based on the pots adjusting the gain levels? And what are the side effects of doing this? I'm assuming output would be louder, and with my very limited knowledge/experience of this aspect of 12v, I'd assume output would be louder, and potential risk would be overloading the internal circuitry designed to run on a much lower signal, but that's just a guess.

Thoughts?


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## Bluliner (May 16, 2011)

Input voltage is inversely proportional to gain settings. 

More voltage means the gains are set lower...which means less noise. Also, the greater the voltage going through the wires means lower noise induced into the signal. A few electrons here & there will make no audible difference, much like your speaker wires. However, a low voltage signal is more prone to noise which is why signal RCA's are often shielded and/or twisted. If the noise is entering the system through the signal wires or is a result of the amplifier gains being too high (or both) a high-level signal input should fix it. 

The only caveat is that some amplifiers cannot handle that kinda input. Check your owner's manual.


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## Thumper26 (Sep 23, 2005)

i completely understand the voltage/gain relationship, and the benefits of a higher voltage.

my question is WHY can't some amps handle higher input voltages? what does it do to the amp?


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## Thumper26 (Sep 23, 2005)

Nobody? I have to say I'm a little disappointed...


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## ErinH (Feb 14, 2007)

I don't see why an amp couldn't handle a higher input voltage unless the input voltage is so dang high already that the gain can't be properly matched. I've never seen/heard of this before so have no idea if it's plausible or not.


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## Gary S (Dec 11, 2007)

My first recommendation would be to change out the factory head for an aftermarket one. I know you don't want to hear this, but it's the truth.


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## Thumper26 (Sep 23, 2005)

Gain matching is all I could think of too. If an amp built for a 4v signal got a 30v signal, then would there be the same hiss as if the gain was up too high with a 4v signal?


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## Thumper26 (Sep 23, 2005)

I've got my stereo covered, I'm just curious about amp design and limitations on input signal


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## 14642 (May 19, 2008)

The maximum input voltage of the amplifier is set by the input to the preamp and an attenuation circuit before it. If it's and amplifier with a ADC, then the input attenuator is designed to set the voltage so the input to the DAC isn't clipped. If it's an analog amplifier, the same thing is true, but clipping the input of the preamp won't sound as bad as clipping the input of the ADC.

If the amplifier is noisy, then no amount of input atenuation will fix the problem. If the preamp in the amp is noisy, then turning the input sensitvity control down may help. You can check this by disconnecting the RCAs from the amp and turning the input sensitivity control all the way up. If you hear hiss, then setting it lower will help to eliminate the problem.

Huge output voltage can help to overcome noise that's radiated into the signal cables, because the level of the signal is likely to be much higher than the radiated noise. If you're planning to mod the radio to eliminate the hiss, it would be a good idea to determine if the hiss is caused by the amp in the radio or in the preamp in the radio. Just make a simple signal to noise measurement--you'll need a scope and a sine wave disc. If the ratio is different when you measure the output of the amp and the output of the preamp, then the mod may help. If it isn't and you mod the radio and then boost it from the 1V or so that you find in there to 30V, you'll just boost the noise too and you'll have spent a bunch of money and time on nothing. 

Sending 30V to an amplifier designed to attenuate signals from a max of 4V so threy don't clip the preamp will just clip the preamp and require that you use less of the rotation on your volume control.


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## Thumper26 (Sep 23, 2005)

Andy, thanks for he info, that's what I was looking for. For the record, I don't plan on sending 30v to the amp, that number was thrown out by Navone and was what got me thinking about this to stary woth. My goal is 4-8v max.

Great tip on measuring the deck ahead of time, I'll definitely do that before digging in. I've already confirmed the amp isn't the problem, so now it's just getting a solution for the source. I once saw a build where someone modded the factory face to relocate the factory display and ac controls so a single din deck could be placed up top. I'm trying to find that build, as that is another option I'd consider.

I know it seems like a lot of work just to not have a radio down below, but I really like the OEM look, and it's a headache dealing with anything that's a flip face due to the shifter. I don't mind the work for something custom that gets me what I'm going for.

If there was an easy solution to control the ac from an ODBC interface with a car PC, then I'd definitely consider that as well.

Thanks again Andy, you answered exactly what I needed to know!


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## Thumper26 (Sep 23, 2005)

Lol tried that. I've muted the amp, tried different rca's, removed the loc, ran a balanced signal via zapco symbilink, connected a zapco amp with symbilink, isolated power from the deck straight to the battery, isolated ground from the ground soldered onto the board to the chassis, put a 3sixty to see if it would kill it, and last night connected speakers to the rear channels to see if it fixes it. I'm going to try connecting a LOC to the rear channels and connect rca's to the amp to see if it fixes it, but im doubtful. I ran a crappy deck to it and no hiss, and I've competed with this car with no hiss at all. It's the deck. Google shows other people having the same problem, some with only upgrading he speakers, but nobody with a fix. I'm curious if putting a resistor in line before the LOC would fix anything, just on the thought that it's seeing a higher impedance from the factory speakers.

This has been a great learning experience and a major pain in my ass at the same time. I have a 15 and an IB wall that needs to be trimmed to fit just waiting on me. This would have all been completed weeks ago if it weren't for this issue.


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## Thumper26 (Sep 23, 2005)

As a side note, gains are all the way down on the amp, and when the LOC is fully attenuated tthere us nno hiss, but max volume is extremely low. Raising gains on the among just brings it back


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## Moon Track (Mar 10, 2011)

Measure the input impedance of your LOC. If impedance is high, you can try to connect 20-50 Ohm 25Watt resistors in parallel to simulate some load for HU. Possible this noise comes from feedback circuits. It will be enough to use 3-5Watt resistor at test stage or you can connect the speaker.
As for HU modification, it will work, but without preamplifier with low output impedance the cable will pick up some noise.


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