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Old 19th September 2011, 17:50   #1
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Default Setting amplifier gain without DMM or Oscilloscope

My ICE setup consists of Pioneer GM 6400-F

Continuous Power Output (2 Ω, ≤ 1% THD+N) 75 W x 4
Continuous Power Output (4 Ω, ≤ 1% THD+N) 60 W x 4

Pioneer 4050UB head unit with 4V preout

JBL GTO 607C Components at the front with
RMS: 70 watts each at 2 Ohms

Infinity KAPPA693.9I with
RMS 110 Watts each at 2 Ohms

Now the tricky portion is how to set the gains correctly, the installer had set the gains, but I was worried if it was going to blow the speakers off.

Anyways from lot of readings what I understand is that amplifier gain is not a volume control knob but a setting to match the preout voltage of the Headunit with amplifier input voltage. My amplifier has markings that start from 6.5v and ends at 0.2v which means that somewhere inbetween these 2 markings falls the 4v which matches with my HU preout to extract the maximum amplification or gain its capable of. The normal marking in the amplifier is said to match the 2V preouts, but I am surprised here why does the Pioneer HU itself is rated at 4V , which is ubnormal in terms of Pioneer manual itself.

Anyways, since most of the HU would only provide clean undistorted signals upto around 3/4th of the volume or so they say (3/4*4=3V), what I thought was to set the rear Infinities close but lower than the normal gain setting mark, which can easily consume the 75W max output the amp is capable of and set the front channel gain a tad lower than the rear channel gain setting to extract only 70W to match the max output supported by the jbl components.

Can anyone out here confirm or reject my layman's way of setting the amps , that it would be safe and extract the maximum power from the amplifier to power the speaker?

I had tried the pink noise, 1 khz test signal at 0db to identify the volume at which distortion starts to kick in, but to my dismay, it never distorted or I was not able to pick it up with ear even after turning the HU volume up to the maximum. The amplifier distortion was also not audible to me after setting the HU volume at 3/4th and turning the amp gain way above and beyond the normal markings. Only thing I could make out was that LOUD music was paining my ear drums. That's when I thought about the above maths to solve my problem. Or DDM/Oscilloscope is the only way out?

PS: I have lost touch with all the logrithmic stuff that I learned during my graduation, so there is a good chance that I am wrong! All I know lately is google, ctrl+c and ctrl+v... !!!!

Last edited by glidealong : 19th September 2011 at 18:04.
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Old 19th September 2011, 19:52   #2
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Default Re: Setting amplifier gain without DMM or Oscilloscope

Simple way to do it is turn the Volume on HU at 75 - 80% & calculate the voltage to set it via DMM. HU might give 2.5 or 2.8V you can't be sure its 3V exactly at 3/4 volume.
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Old 20th September 2011, 10:22   #3
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Originally Posted by abhibh View Post
Simple way to do it is turn the Volume on HU at 75 - 80% & calculate the voltage to set it via DMM. HU might give 2.5 or 2.8V you can't be sure its 3V exactly at 3/4 volume.
Thanks abhibh, I dont have a DMM, so I will have to go with approximation.
As you mentioned if it is 2.5 or 2.8, then I have more leeway to turn the gain closer towards the normal(2v) marking. I think I would set the rear channels to normal gain, even if the HU output goes above 2v, the amp seems to have some function by which it cuts out the output if clipping happens. Moreover, most of the time I don't have any body occupying the rear seats and I set the fader to F2-4, which means the infinities are running at lower levels, so should be safe.

The front channel gain could be set to a tad lower than the normal marking and hopefully it wont blow off my 607c even if i turn the volume knob to 3/4th of max, at times when I feel like a metal maniac.

Last edited by glidealong : 20th September 2011 at 10:25.
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Old 20th September 2011, 16:00   #4
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Default Re: Setting amplifier gain without DMM or Oscilloscope

Your amplifier power is close enough to the speaker power handling that you won't have a power overload problem even at maximum amp output. Hopefully the amplifier is internally protected for thermal conditions, but your speakers (if not lying) should be fine.

There are two scenarios to watch for. One is voltage clipping and the second is over-excursion.

Clipping would be when the voltage output of the amplifier is made to exceed its voltage power supply level. In which case the tops of the signal are chopped off and results in large signal levels feeding directly into the speaker. This should be cured by keeping an enthusiastic hand in control, and you should be able to hear the amplifier overloading well below this point.

Over-excursion is likely if your speakers are unable to handle the mechanical excursion at low frequencies. A typical 6" woofer goes into excursion overload at about 30 watts in any ported enclosure, give or take a few watts. Above this it mainly produces distortion, and again this is usually audible.

The scenario you do not need to protect against is thermal overload, where the speaker is fed more power than it is capable of handling. This does produce distortion (which may be slightly less audible is using a crossover, for example), but melts the coils through prolonged heating. Since your amp and speaker power are comparable, you should not have this problem.
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Old 21st September 2011, 10:49   #5
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Default Re: Setting amplifier gain without DMM or Oscilloscope

Quote:
Originally Posted by glidealong View Post
Pioneer 4050UB head unit with 4V preout
My amplifier has markings that start from 6.5v and ends at 0.2v
Anyways, since most of the HU would only provide clean undistorted signals upto around 3/4th of the volume or so they say (3/4*4=3V),

I had tried the pink noise, 1 khz test signal at 0db

Only thing I could make out was that LOUD music was paining my ear drums.
1. If the HU has a 4V pre out this is a nominal number. It wont put out 4V all the time even if the volume control is maxed out. 4V is usually the nominal peak voltage above which the HU's internal preamp (if it can be pushed that far) is liable to clip.

2. at 3/4th rotation your HU will be putting out 90% of it's rated output becuase the volume pot is LOG not Linear. But this difference is really nto something to be concerned about anyway.

3. Pink noise is not just a simple 1kHz test tone. Pink noise is a signal that has equal energy (not power - that woud be white noise) ascross the frequency spectrum. Pink noise for example will have the same power comparing say 40-80Hz and 400-800Hz. White noise will have the same power comparing 40-80Hz with 400-440Hz. That said I prefer to use A-weighted noise as this is closer to how the humar ear hears that either pink or white noise. A-weighted noise gives the "perception" that it is equally loud at all frequencies - Dr. Bose (yes the same Dr. Bose) has penned a few white papers on this. Here is the A-weighting filter schema
A-Weighting Filter

Also a 1khz test tone can get irriating to listen to so I prefer 400Hz as my test tone (those who are musicians will note that instrument tuners are also calibrated to around 400hz - 440Hz to be exact). Not that this is significant (all meters are weighted with 1kHz and their center frequency) but it is just a personal preference of mine.

Quote:
Originally Posted by cranky View Post
There are two scenarios to watch for. One is voltage clipping and the second is over-excursion.

Clipping would be when the voltage output of the amplifier is made to exceed its voltage power supply level.

Over-excursion is likely if your speakers are unable to handle the mechanical excursion at low frequencies.
Exactly. 30W at 100hz is no problem for a typical 6" woofer. 30W at 20hz usually is - Disclaimer: I know a few 6" drivers that can manage 30W@20Hz without damange but these are exceptions not the rule.
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Old 21st September 2011, 11:10   #6
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Originally Posted by navin View Post

Exactly. 30W at 100hz is no problem for a typical 6" woofer. 30W at 20hz usually is - Disclaimer: I know a few 6" drivers that can manage 30W@20Hz without damange but these are exceptions not the rule.
Thanks Navin,
Well I guess that's ok for me as the components have crossover setup and the front channel amplifier is put in HPF mode with the specs as below

High pass filter:
Cut off frequency ........... 80 Hz
Cut off slope ..................... –12 dB/oct

So 20Hz should not make it to the 6" component woofer.

But on another note, you mentioned 30W at 100hz but in my case it would be 75W at 100Hz against a maximum power rating of 70W for the 6" woofer. Do you think 75W fed to a 70W speaker occasionally would blow the coil? I guess not, as I think speakers are mostly built to handle slightly higher powers than they are rated at, especially when they are from reputed manufacturers like JBL and Infinity?
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Old 21st September 2011, 11:15   #7
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Default Re: Setting amplifier gain without DMM or Oscilloscope

Quote:
Originally Posted by glidealong View Post
But on another note, you mentioned 30W at 100hz but in my case it would be 75W at 100Hz against a maximum power rating of 70W for the 6" woofer. Do you think 75W fed to a 70W speaker occasionally would blow the coil? I guess not, as I think speakers are mostly built to handle slightly higher powers than they are rated at, especially when they are from reputed manufacturers like JBL and Infinity?
Oh 75W should be fine too. 30W was just an obvious example. People dont think of 30W as damaging power but it can be at say 20Hz.

As Cranky mentioned earlier, do not worry about thermal ratings (the ratings manufactures use are all thermal ratings) worry about excursion limits (this depnds on the box size and type so manufactuers have an excuse not to disclose these). Usually a 6" woofer will be ok to about 4mm and will start ot distort badly by about 5-6mm of cone movement.
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Old 21st September 2011, 15:30   #8
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Default Re: Setting amplifier gain without DMM or Oscilloscope

Quote:
Originally Posted by navin View Post
3. Pink noise is not just a simple 1kHz test tone. Pink noise is a signal that has equal energy (not power - that woud be white noise) ascross the frequency spectrum. Pink noise for example will have the same power comparing say 40-80Hz and 400-800Hz. White noise will have the same power comparing 40-80Hz with 400-440Hz. That said I prefer to use A-weighted noise as this is closer to how the humar ear hears that either pink or white noise. A-weighted noise gives the "perception" that it is equally loud at all frequencies

Also a 1khz test tone can get irriating to listen to so I prefer 400Hz as my test tone (those who are musicians will note that instrument tuners are also calibrated to around 400hz - 440Hz to be exact). Not that this is significant (all meters are weighted with 1kHz and their center frequency) but it is just a personal preference of mine.
I did try both the pink noise and 1 khz 0db test tones, both didnt give me the 'distortion point of HU' by listening with my ears. 1 KHz test tone gave the feeling to my neighbors that I was tuning into old DD signals before the sunday evening movie which was still floating around in the atmosphere. well i know, that's a PJ.


As you mentioned may be i should try A-weighted noise and see if i can find the 'distortion point' of my HU. Any idea where can I download one that suits my requirement?

Thanks again!

Last edited by glidealong : 21st September 2011 at 15:36.
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Old 21st September 2011, 15:35   #9
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Originally Posted by navin View Post
As Cranky mentioned earlier, do not worry about thermal ratings (the ratings manufactures use are all thermal ratings) worry about excursion limits (this depnds on the box size and type so manufactuers have an excuse not to disclose these). Usually a 6" woofer will be ok to about 4mm and will start ot distort badly by about 5-6mm of cone movement.
Would it still be the 30W limit for my situation as I am fitting these to the doors with an HPF configured at 80hz with 12db slope?

Anyways, the distortion does kick in at around 1/4th of the HU volume with all the vibration and noise courtesy the swifts door panels. I seriously need some damping. Probably damping would help me to push the limits higher. But I guess with the rates of 2500/- a door at my neighborhood dealer, it has to be a DIY with the yoga mats!

Last edited by glidealong : 21st September 2011 at 15:38.
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Old 21st September 2011, 15:36   #10
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Originally Posted by glidealong View Post
As you mentioned may be i should try A-weighted noise and see if i can find the 'distortion point' of my HU.
Why do you even want to do this? Even if your HU puts out 2V clean it is sufficient to drive most car amplifers to their max rated power (usually 1V is enough).
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Old 21st September 2011, 15:38   #11
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Would it still be the 30W limit for my situation as I am fitting these to the doors with an HPF configured at 80hz with 12db slope?
Since all the LF is filtered I dont see why the JBL 607s should not handle 70W leave alone 30W.

Let put it this way, I have 6" woofer in my car too. I HPF these at 63Hz/12db. I dont have a problem at sane levels.

Since my HU can store multiple XO settings I have a second HPF setting 80Hz/18db for when I am alone and want to listen at louder levels.

Last edited by navin : 21st September 2011 at 15:40.
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Old 21st September 2011, 15:42   #12
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Why do you even want to do this? Even if your HU puts out 2V clean it is sufficient to drive most car amplifers to their max rated power (usually 1V is enough).
Yes, but I would be in a better position to know till what percentage of my HU volume does it give the clean signal and from that percentage I can approximate the HU preout voltage to set my gains.

So I am safe and I get clean signal. I know experimentation is costly and doesn't work very well.

DMM/Oscilloscope would be the perfect way out. But I am not yet ready to shell out the money just for my ICE.
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Old 21st September 2011, 15:47   #13
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Yes, but I would be in a better position to know till what percentage of my HU volume does it give the clean signal and from that percentage I can approximate the HU preout voltage to set my gains.
Most HUs are ok till well past 2V. Most amps dont need more than 1V (for max rated output).

When you say your HU clips at 3/4th volume what is the gain setting on your amp?

At what point are you measuring (or getting) this distortion? By this I mean are you measuring this distortion at the output of the HU, the output of the amp or just by listening to the speakers?
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Old 21st September 2011, 15:53   #14
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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When you say your HU clips at 3/4th volume what is the gain setting on your amp?

At what point are you measuring (or getting) this distortion? By this I mean are you measuring this distortion at the output of the HU, the output of the amp or just by listening to the speakers?
I haven't detected(or was not able to identify) any clipping or distortion in my HU preout with the amplifier gain setting at minimum - 6.5 volt and HU at maximum with either the pink noise or 1 KHz 0db test tones on one speaker at a time by listening to it by ear.
The 3/4th of the volume was the assumption I picked up from various DIY forums like
A tutorial on gain setting and amplifier power usage - DIYMA.com - Scientific Car Audio - Truth in Sound Quality

I dont have a DMM/Oscilloscope, so I am trying to identify the distortion/clipping by ear only.

1. Set the gains of the amp to the lowest, in my case 6.5v.
2. Turn off all the EQ, LPF,HPF,Bass Boost, SRtrv, Src Lvl Adj... in HU
3. Set the Fader Balance levels to listen to only speaker at time, say Front Right or Rear Right
4. Turn up the HU volume till i detect distortion/clipping

This is how I am trying to identify the 'clipping/distortion point' of HU.

Last edited by glidealong : 21st September 2011 at 16:03.
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Old 22nd September 2011, 10:27   #15
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Default Re: Setting amplifier gain without DMM or Oscilloscope

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Originally Posted by glidealong View Post
I haven't detected(or was not able to identify) any clipping or distortion in my HU preout with the amplifier gain setting at minimum - 6.5 volt and HU at maximum with either the pink noise or 1 KHz 0db test tones on one speaker at a time by listening to it by ear.
If the HU clips the amp will only amplify the clipping even at it's lowest setting. so since at the amp's lowest setting you cannot hear any clipping whne the HU's output is at max the HU is working within it's limits. Which is expected.

The clipping you heard would be the amplifier clipping since the amp is rated to produce max output at 1V and if you put 3V in the amp will try to produce more than it's rated output.

So if the amp was rated to poduce 20W with a 1V signal it will prouce more than rated output with a 2V or 3V signal. Which is why the amp has a gain control so you can set it to produce it's rated output when the HU's volume is at max. if your amp is distorting before the HU is at max reduce the gain on the amp so that it stops clipping/distorting. This will give you maximum control over the amp's volume levels.
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