Hey, I installed an amp in my car today with RCAs. The impedance of the speakers is 4ohms, and the amp is rated at 50W@4Ohms, 70W@2Ohms
so...
sqrt(50x4) = 14.14
But, when I plugged in my front channel RCAs and started turning up the gain the voltage was going up very LITTLE. At "Nominal" on the amp, I was getting about 1.3V, at the VERY END of the gain I got 13.2V
I plugged in the RCAs from my subwoofer line, and the voltage went up and down normally with the gain.
It's not the RCAs, I tried two pairs. Not directional RCAs either.
It's a Pioneer 6700MP, do I have to turn up the preouts somehow?
Help.
Replies (6)
swez on 06/24/2006 05:02:45
This HU is spec'd at 2.0 volts preamp outs. It has 2 pair. (Front & rear/sub)
Check your HU fade controls. If the fader is not on center, you can expect a deviation on RCA output voltage.
The 13.2 volts is = to: Voltage x Voltage/Ohms = 13.2 x 13.2/4 = 43.56 watts. Depending on amplifier B+ voltage available, that's about right for a 50 x 4 amp. (RMS)
What amp are you running here?
Swez
ttocs on 06/24/2006 09:36:45
where are you guys getting your formulas from?
"sqrt(50x4) = 14.14"????????? the square root of the total power = the voltage?
"The 13.2 volts is = to: Voltage x Voltage/Ohms = 13.2 x 13.2/4 = 43.56 watts. Depending on amplifier B+ voltage available, that's about right for a 50 x 4 amp. (RMS)"??????? V squared/impedence = ????
Also you need to know that if you wan t to acuratly measure the output of the amp, you will need an O'scope. A meter reads in rms voltage on an ac signal, this is not the same rms that we use in our power ratings.
swez on 06/24/2006 11:26:53
We are using Ohms Law derivations here. Many/most meters read RMS voltage in AC voltage mode. Voltage sq'd/impedance = Watts. Depending on the meter, this could be RMS, Peak or Peak to Peak wattage. Straight Ohm's Law calculations is all. (And no, I am not a math wizz either) We just know the Ohm's Law pie chart well enough to use it.
As I read your post again, it seems the amp gain on the non-sub channel is not working properly. It should also change voltage values as you adjust the gain from min to max gain. You may have a dirty gain pot on this channel set. A shot of WD-40 will often clean and lubricate a dirty pot.
The only other variable I can think of, if you use a speaker as the load for voltage measurments, speaker impedance changes with frequency. That could easily account for the variation. A 4 ohm, non-inductive load will be more accurate.
As mentioned before, if the BAT power to the amp/HU is 12.5 volts, you'll get a higher number when the engine is running and the ALT is putting out the typical 13.4-14.4 volts DC.
Swez
PS Ttocs is correct about using the O-Scope. It is a very accurate tool for calibrating and testing distortion levels and very precise voltage wave forms. That means a signal generator and non-inductive dummy load as well. Now, you have a dead-nutz reading of peak and peak-peak AC voltage numbers. To get the RMS value on this set up, we use Peak voltage x 0.707 = RMS power.
Happy now ttocs? SMILE
Victor on 06/24/2006 14:56:59
http://www.svconline.com/mag/avinstall_sines_times/index.html
Some interesting , detailed reading..
Victor...
BradSk88 on 06/24/2006 18:33:24
Okay, I'll give more information.
I hooked up speakers while the Gain was set at the amp's NOM setting (nominal,) and the volume changes as I turn the gain knob and as I use the volume on the head unit.
I was measuring the voltage using a 200V AC voltmeter while playing a 70hz test tone recorded at 0dB. And I'm doing all of my readings with the car running (14.4V) I was also reading the voltage with the speaker negetive terminals unplugged.
I'll try your suggestions, but first: questions.
What am I WD-40'ing?
swez on 06/24/2006 19:48:11
Test tone 70 Hz. ... OK... good point to mention. Other factors may be involved too.
If the amp channels tested had an HPF (High Pass Filter) activated and was set above 70 Hz., we could expect at lot of signal attenuation at the amp output. The filter, depending on its design and setting value, would block most of that signal, regardless of gain settings.
Now, on the other channel, if the LPF (Low Pass Filter) was activated, we should expect the signal to change voltage values as we adjust the amp gain control. It seems that this channel behaved normally and as expected.
Based on the above info, if you think a dirty gain pot (Potentiameter) is the culprit, then pulling off the outer casing of the amp is needed to get to the PC board(s) inside.
There, you will find 2 gain pots. One for the Front (L&R) channels and one for the Rear (L&R) channels. If the amp is old and been lying around in a dusty environment, the pots can collect debris and behave erratically. (These are the pots you mentioned in your earlier test.)
Here, we look for a slot in the potentiameter where we can inject a shot of WD-40 to clean away the debris, run the pot back and forth between min and max, to clear the internal wiper and contact surfaces of the pot. In the case of a very dirty pot, you may need to repeat this step a few times and retest the amp again.
FYI: WD-40 is both a cleaner and lubricating chemical. I have used it on many electrical/electronic components with no detrimental side effects.
Swez
PS Victor, "we have TEF and all that other good test gear."
What's a TFE? Thremo-Fusion Evacuator??? Interesting reading? "Yawn".... that was a $50.00 answer to a $0.50 question. What does that article mean, (have to do with) in the layman's terms to this topic??? I didn't get it. LOL "Splain it to me Lucy" On 2nd thought, never mind...
so...
sqrt(50x4) = 14.14
But, when I plugged in my front channel RCAs and started turning up the gain the voltage was going up very LITTLE. At "Nominal" on the amp, I was getting about 1.3V, at the VERY END of the gain I got 13.2V
I plugged in the RCAs from my subwoofer line, and the voltage went up and down normally with the gain.
It's not the RCAs, I tried two pairs. Not directional RCAs either.
It's a Pioneer 6700MP, do I have to turn up the preouts somehow?
Help.
Replies (6)
swez on 06/24/2006 05:02:45
This HU is spec'd at 2.0 volts preamp outs. It has 2 pair. (Front & rear/sub)
Check your HU fade controls. If the fader is not on center, you can expect a deviation on RCA output voltage.
The 13.2 volts is = to: Voltage x Voltage/Ohms = 13.2 x 13.2/4 = 43.56 watts. Depending on amplifier B+ voltage available, that's about right for a 50 x 4 amp. (RMS)
What amp are you running here?
Swez
ttocs on 06/24/2006 09:36:45
where are you guys getting your formulas from?
"sqrt(50x4) = 14.14"????????? the square root of the total power = the voltage?
"The 13.2 volts is = to: Voltage x Voltage/Ohms = 13.2 x 13.2/4 = 43.56 watts. Depending on amplifier B+ voltage available, that's about right for a 50 x 4 amp. (RMS)"??????? V squared/impedence = ????
Also you need to know that if you wan t to acuratly measure the output of the amp, you will need an O'scope. A meter reads in rms voltage on an ac signal, this is not the same rms that we use in our power ratings.
swez on 06/24/2006 11:26:53
We are using Ohms Law derivations here. Many/most meters read RMS voltage in AC voltage mode. Voltage sq'd/impedance = Watts. Depending on the meter, this could be RMS, Peak or Peak to Peak wattage. Straight Ohm's Law calculations is all. (And no, I am not a math wizz either) We just know the Ohm's Law pie chart well enough to use it.
As I read your post again, it seems the amp gain on the non-sub channel is not working properly. It should also change voltage values as you adjust the gain from min to max gain. You may have a dirty gain pot on this channel set. A shot of WD-40 will often clean and lubricate a dirty pot.
The only other variable I can think of, if you use a speaker as the load for voltage measurments, speaker impedance changes with frequency. That could easily account for the variation. A 4 ohm, non-inductive load will be more accurate.
As mentioned before, if the BAT power to the amp/HU is 12.5 volts, you'll get a higher number when the engine is running and the ALT is putting out the typical 13.4-14.4 volts DC.
Swez
PS Ttocs is correct about using the O-Scope. It is a very accurate tool for calibrating and testing distortion levels and very precise voltage wave forms. That means a signal generator and non-inductive dummy load as well. Now, you have a dead-nutz reading of peak and peak-peak AC voltage numbers. To get the RMS value on this set up, we use Peak voltage x 0.707 = RMS power.
Happy now ttocs? SMILE
Victor on 06/24/2006 14:56:59
http://www.svconline.com/mag/avinstall_sines_times/index.html
Some interesting , detailed reading..
Victor...
BradSk88 on 06/24/2006 18:33:24
Okay, I'll give more information.
I hooked up speakers while the Gain was set at the amp's NOM setting (nominal,) and the volume changes as I turn the gain knob and as I use the volume on the head unit.
I was measuring the voltage using a 200V AC voltmeter while playing a 70hz test tone recorded at 0dB. And I'm doing all of my readings with the car running (14.4V) I was also reading the voltage with the speaker negetive terminals unplugged.
I'll try your suggestions, but first: questions.
What am I WD-40'ing?
swez on 06/24/2006 19:48:11
Test tone 70 Hz. ... OK... good point to mention. Other factors may be involved too.
If the amp channels tested had an HPF (High Pass Filter) activated and was set above 70 Hz., we could expect at lot of signal attenuation at the amp output. The filter, depending on its design and setting value, would block most of that signal, regardless of gain settings.
Now, on the other channel, if the LPF (Low Pass Filter) was activated, we should expect the signal to change voltage values as we adjust the amp gain control. It seems that this channel behaved normally and as expected.
Based on the above info, if you think a dirty gain pot (Potentiameter) is the culprit, then pulling off the outer casing of the amp is needed to get to the PC board(s) inside.
There, you will find 2 gain pots. One for the Front (L&R) channels and one for the Rear (L&R) channels. If the amp is old and been lying around in a dusty environment, the pots can collect debris and behave erratically. (These are the pots you mentioned in your earlier test.)
Here, we look for a slot in the potentiameter where we can inject a shot of WD-40 to clean away the debris, run the pot back and forth between min and max, to clear the internal wiper and contact surfaces of the pot. In the case of a very dirty pot, you may need to repeat this step a few times and retest the amp again.
FYI: WD-40 is both a cleaner and lubricating chemical. I have used it on many electrical/electronic components with no detrimental side effects.
Swez
PS Victor, "we have TEF and all that other good test gear."
What's a TFE? Thremo-Fusion Evacuator??? Interesting reading? "Yawn".... that was a $50.00 answer to a $0.50 question. What does that article mean, (have to do with) in the layman's terms to this topic??? I didn't get it. LOL "Splain it to me Lucy" On 2nd thought, never mind...
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