SETTING GAINS with Ohm's law - *cheap amp reading low voltage output*

[LouieC]

Hey guys, today I decided to finally tune my amp correctly after having it at 8 o'clock on the bass knob for some time now.

According to ohm's law guidelines, I should be aiming for 33.16volts as a max threshold before 'clipping' occurs, with a 2.2 impedance and 500wrms amplifier.

The Gear:

1x MB Quart FX1.500 Monoblock (500wrms rated)

1x Alpine Type S-12" (500wrms rated)

1x Kenwood single din headunit

The Issue:

With my volume set at 31 (max 35 - no clipping occurs at this volume), playing a 50hz test tone, and the GAIN on MAX (yes my speaker cables were unplugged and all bass boosting/subsonic filter off) my multi-meter only reads 30.0volts.

According to ohm's law not even FULL gain produces 33.16 volts (500rms worth of power).

Even with MAXIMAL gain, the reading is still well under where it should be. What is causing this?

Is this just a typical case of cheap product (only cost me $130 off ebay), low output?

How should I set my gains accordingly????

Note:

• Constant power reading of 13.9v.
• Great ground.

Any help would be appreciated. Thanks!

[Stephen13]

Tune your amp "correctly"? IMO this is the worst and most unreliable way to set your gain. If you don't have access to an O-scope or a DD-1 or similar device you're better off just setting your gain by ear. The math is a shot in the dark at best since you're dealing with impedance (AC) and not resistance (DC). There's also the accuracy (or lack thereof) of the DMM to consider. Unless you have a high end calibrated DMM you have no way of knowing how close to being accurate your calculations will be. Your ears are far more reliable and don't require calibration.

[Broke_Audio_Addict]

I thought you said you decided to tune it correctly?

Nothing about that method is correct.

[bolanorthhighlands]

Turn your bass knob all the way up

[Jessica]

30v is pretty close to what you want, your amp is rated 500 at 2ohm not 2.2

31.5v is 500 at 2ohm.

since you have no load on the amp you dont have to worry about phase angle.

also, how do you know you arent clipping?

[zack_e89]

Turn your bass knob all the way up

then yo shit wil knocc

[LouieC]

Tune your amp "correctly"? IMO this is the worst and most unreliable way to set your gain. If you don't have access to an O-scope or a DD-1 or similar device you're better off just setting your gain by ear. The math is a shot in the dark at best since you're dealing with impedance (AC) and not resistance (DC). There's also the accuracy (or lack thereof) of the DMM to consider. Unless you have a high end calibrated DMM you have no way of knowing how close to being accurate your calculations will be. Your ears are far more reliable and don't require calibration.

I saw a comparison between a DMM and a DD-1 and both were very closely accurate when setting the gain to a specific voltage by ohm's law, hence why I thought this was the 'correct' method of tuning WITHOUT a DD-1 or Oscope. I've had it tuned by ear for a fair while now and just wanted to be sure I was safe as I don't want to hurt my equipment by trying to make it do something it's not designed for. Guess it'll be back to the ear tune.

I thought you said you decided to tune it correctly?

Nothing about that method is correct.

So I've gathered. Only trying to be safe without access to an oscope or dd-1 at this point in time.

30v is pretty close to what you want, your amp is rated 500 at 2ohm not 2.2

31.5v is 500 at 2ohm.

since you have no load on the amp you dont have to worry about phase angle.

also, how do you know you arent clipping?

Tested resistance of my subwoofer comes to 2.2ohms - hence that calculation.

Amp is definitely clipping since something about the math doesn't seem right, as when it's at full gain its not producing the required voltage to make 500wrms according to the equation.

Just going to retune by ear like I had it before until an oscope is available

[Triticum Agricolam]

Like strangeduck said, you calculate what the voltage should be based on 2 ohms, since that's what the amp is rated for. The impedance of your sub is irrelevant (as long as its still wired to 2 ohms nominal).

While I don't think the DMM method is as useless as some people do, there is still the problem of knowing if the output is clipping or not. If clipping is bad enough you will hear it, but if its slight you probably won't. If you are going to be pushing your system to the max, setting your gains using a method that can detect clipping (DD-1 or o-scope) is highly recommended. Using the DMM/ear-o-meter can work OK for more moderate usage though.

I'm surprised you aren't getting at least 30 volts out of that amp. You might measure what the output voltage from your deck is with the volume set on 31 and playing a test tone.

[LouieC]

Like strangeduck said, you calculate what the voltage should be based on 2 ohms, since that's what the amp is rated for. The impedance of your sub is irrelevant (as long as its still wired to 2 ohms nominal).

While I don't think the DMM method is as useless as some people do, there is still the problem of knowing if the output is clipping or not. If clipping is bad enough you will hear it, but if its slight you probably won't. If you are going to be pushing your system to the max, setting your gains using a method that can detect clipping (DD-1 or o-scope) is highly recommended. Using the DMM/ear-o-meter can work OK for more moderate usage though.

I'm surprised you aren't getting at least 30 volts out of that amp. You might measure what the output voltage from your deck is with the volume set on 31 and playing a test tone.

I see now, even at 31.6 volts it's only getting a flat 30volts on MAX gain. Oh so strange.

Had the sub wires disconnected so I wouldn't hurt the speaker, but after hooking everything back up last night my system isn't the same. Not knocking anywhere near as loud from the settings I last had with the ear-o-meter.

Think I may be in the market for a new amp, lol. Piss off when you were trying to do the right thing.

[DubNDodge]

biggest flaw in this "method" of gain setting: the assumption that the amp makes 500w clean.

If it only makes 300w clean, then you're clipping the fuck out of the signal to achieve the voltage required for 500w. Which is why this is flawed as fuck. Tuning by ear is a better method than this, unless you are 100% certain your amp can put of 500w clean.