One opinion that I have frequently heard on forums is that older amplifiers used to increase their noise ratio when gains were increased. For that reason, the preference used to have high voltage pre-outs so that the gain can be set low on the amplifier. I have no idea if this argument still holds. Another argument I have heard is that some amplifiers do no have enough gain headroom to work with very low voltage inputs.
For those interested in hooking the JBL MS-8 processor, it may be worth nothing that the unit supports only up to 2.7V inputs on the low-level inputs. If the voltage exceeds 2.7V, then the input signal is "clipped". So, having high voltage pre-amp outs may be useless with this processor. In real life this is probably not a big deal. Having 4V pre-amp means that the pre-amp signal voltage is 4V when your volume level is at max setting playing a 0dB test tone. On a well recorded tracks, peaks rarely exceed -10dB, while a less carefully recorded pop track may have sound peaking at -5dB.
When I install a car sound system and I think that something is wrong with RCA outputs, low gain, I try with one of this;
New Boss B65N High Level to Low Level Converter RCA Input Sensitivity Control 791489170062 | eBay
Mine is other model, I think that is a scosche, the sticker was lost time ago, with this high to RCA you can have a idea of the difference...
I have a question guys, hope to clear my thoughts with your help. I have a case where I have 5 volts RCAs for the head unit. The amp voltage gains goes from 6 V minimum Counter clockwise (from left) to something like 2.5 mv max (0.25 v ) or close turning the dial to the right.
To set the gains with 3/4 volume using a wavetek digital voltmeter I had to go clockwise ( turn dial to the right) near the max. Does this sound right? That basically tells me, if the HU has 5 volts, instead of turning the gains low on the amp , they have to be turned high. Meaning high equals less voltage on the amp? Less V on the amp equals more volume? Again to get to 29V for a 220w sub at 3/4 vol, using a 5 volt rca HU I had to twist the dial to the right near the end or max close less than 1 volt.
Does this sound right or make sense?
First, the 5V rating on the preouts is maximum level with a 0dB source in the CD and volume control at max. If your source is not 0db you get less voltage. If your volume control is at 3/4 you get less, and it's not going to be 3/4 x 5V. Volume controls are logarithmic and the preout voltage drops rapidly as you back away from maximum.
The sensitivity control is as marked - lower voltage (more sensitive) clockwise. What you're seeing is that your HU is no where near 5V with the source and volume setting you're using. My 2.5V preouts are only 0.38 vrms at 65% volume where I tuned my system.
I wouldn't ever buy an HU with 2v preouts simply because the ones that have 2v preouts ****. How many decks w/ 2v preouts have 16 band eq or time alignment? HUs are pretty cheap as it is, might as well spend a few extra $$ to get the features you want.
I used a 50hz sinewave tone at -10db and set the voltage with maximum volume, just because it seemed odd to have the amp gain turned near maximum level. Volume goes from 1-50, 33 seems loud and make my ears hurt even if I listen to it for less than an hr.
Is this setting Ok now? or should I set the voltage with the 3/4 volume instead? Would not that be much louder at lower volume if I set the voltage at 3/4 volume? Also, is that 50hz sinewave at minus 10db better or good to use compared to a 60hz. 0db sinewave? Any suggestions or comments?
The only thing higher preouts does is allow you to use less gain. If you have a crappy amp then it will introduce more noise with higher gain and lower preouts. However if your using quality equipment it doesnt matter between lower preout or high preout voltages.
Eclipse 8V HU's + old school a/b power + headroom + gains at bare minimum = FTW.
Well I experimented today, this may help some or others probably already knew it.
1.With the car on, I got like 4-5 volts higher reading than with the car off. Is this how you set it?
2.For the 6 channel amp, it only gave me a reading with internal switch set to internal ( using the rear low signal to convert it to a 5th channel for bass). No reading with ext set despite everything is off and my HU will use externals since it is a 6 ch, 5 volt RCA HU. Is this the typical case?
In either case I tried to use 3/4 volume to set the voltage. Either my amp is under rated and puts out double the power or who knows at 15 volume my fronts rated at 80 watts were clipping or popping. I mentioned I set the voltage with the volume at max (50) and 30 volume gave me chest pounding bass, no clipping, using 50HZ -10 db sinewave and a 1k hz for the fronts and rears (I don't think it makes a big difference because it measured the same voltage with both tracks 50 and 1k Hz although they recommend the 1K hz track)
I even played a CD with loud low bass ( same cd tested once speakers and sub was hooked up), and after setting the target voltage of 30V for the Sub at 3/4, in fact I went to like 80% on the volume. When playing the CD it read up to 50 Volts peaks, once I saw that I lowered the sub amp gain to 43 V max at high peaks even though the voltage remained way over the 30Volts target set with the 50Hz -10db signal. The Sub was not even noticeable, probably because I need to increase its volume and maybe I lowered the gains based on the CD signal.
I did everything right, EQ flat, crossovers off on the HU, loudness off. I guess my issue has to do with the amp being way under rated.
Any thoughts other than I am going to have to go back and set the gains with volume at max level to be where I was before? is that -10 db sinewave responsible for having to set it with the volume at max instead of 75%?
Sad thing is when I set the gains on the full range channels the first time, I calculated it on the power rating of just one channel, I was supposed to do the total power of the 2 channels, I did 60X4=240 square root =15.5 volts, when it was supposed to be 21.5 volt based on 120W. Even like that gains set at max volume at 60% volume it was loud, now at 21 volts at just over 20 it is going to sound as loud as before.