Setting gains with Oscope vs DMM

[Ao1FTW]

How much of a diff is there in setting the gains on and amp with the oscope and dmm? Huge diff or little? And how would you do it with the dmm? Not sure if I have an oscope available for me yet. And which would would be easier to use? Any help would be great! Thanks.

-Justin

[Alex K]

oscope over dmm. dmm is kinda guessing, o scope shows the signal clip.

[Kranny]

oscilloscope is useful for making sure you're getting full unclipped power from your amp

dmm is useful if you want to use a portion of your amps power

you would find out how much power your amp is outputting by going to the amplifiers section and finding the amplifier gain control sticky up at the top and clicking the link that is posted by ChevyBoy near the bottom.

[nCOMP1337]

i was checking for some info about this myself and this is the info i found, it was posted by someone on some forum somewhere, i didnt catch his name, i only used the info, its definitely better to use an oscilloscope especially since you can actually watch for when the waves start to turn square, its easier to know when and whereto back it off at, besides that with the DMM and some math you can set it based on voltage being sent to the amp i believe, based on the RMS you plan to run and it is better than setting gain by ear, lol

_____________________________________________________________________________________

How to set your amplifier gain with a DMM:

There are a lot of tutorials on the internet talking about setting your amplifier gain using a DMM (Digital Multi-Meter). Here I will clean up the process as simply (but accurate) as possible.

Necessary Tools/Skills

1. A volt-meter or DMM with standard test leads.

2. Ability to do 6th grade math.

3. Screwdrivers and/or Allen Wrenches (to make amplifier connections).

4. A CD with test tones - 50Hz for subs, 1000Hz for speakers.

Start by making a quick list of your equipment and their output voltage (head unit, LOC, processor, crossover, eq, etc.) and input sensitivity (amplifier(s)).

Next, list your power output ratings (actual RMS power)

Then, list your speaker/subwoofer impedance(s).

You need to know how your speakers/subs are wired - what the final impedance is going to be at the speaker terminals. If you can't figure out these numbers, you shouldn't be installing amplifiers. Pay someone who does and save yourself the damaged equipment.

Those numbers are VERY important as it is needed for the calculations below:

We are going to use the equation solving for Voltage using Power and Resistance.

Voltage = SQRT(Power x Resistance)

Example:

I have a single subwoofer with 4 ohm DVC coils. I'll wire the coils in parallel for a 2 ohm load. The sub is rated for 500W RMS.

I have a single amplifier rated 500W x1 at 2 ohms. I want all of that power available (knowing that power will only happen for small durations).

Volts = SQRT (500W x 2 ohms)

Volts = SQRT (1000)

Volts = 31.6VAC

-----------------------------------------------------------

Determine the maximum head unit voltage you want to use as follows:

1. if you have a factory head unit and factory speakers and are using a LOC for your sub amp, use the highest volume level you do for music (without distortion to the speakers).

2. if you have an aftermarket head unit powering factory or aftermarket speakers and are using the HU preouts for your sub amp, use the highest volume level you do for music (without distortion to the speakers).

3. if you have a factory or aftermarket head unit feeding a signal to amplifiers for speakers and subs, you need to verify the maximum unclipped, non-distorted output - or just use a 75% volume setting if you are unsure if your headunit is capable of 100% unclipped volume. You can verify this with an oscilloscope.

When setting a subwoofer amp - if your head unit is powering speakers, you really want to disconnect them or use a high pass crossover on them. however, this may not be realistic for you. if not, then your max volume setting may not be possible without distorting your speakers. don't distort your speakers for this exercise. use whatever volume you can that doesn't distort your speakers. This is not the ideal situation, to do it right you should put high-pass crossovers on speakers when you have a sub.

----------------------------------------------------------

Once you have the head unit max volume determined, and the test tone CD playing on repeat. Set the EQ to flat, turn off processing. Make sure the fader is centered, balance is centered, sub level controls are maximum (sub level is just attenuation anyway).

You can go back to your amplifier.

Disconnect any speakers/subs not associated with the channels being adjusted. Set the gain to minimum. Turn off any bass boost. With the DMM set to VAC (Volts AC), touch the DMM positive leads on the speaker outputs. Slowly increase the gain until the DMM reads the voltage determined above.

While you're increasing the gain - LISTEN TO YOUR SPEAKERS/SUBS!!! Listen for any distortion - popping, scratching, noises that go away when you turn the volume back down.

You can use resistors to load the amplifier so you get a more accurate reading (like in the eD video), but the resistor load bank must have a power rating greater than the output power expected. Most of you reading this won't have a load bank on hand, especially for loads over 500W. Don't worry. You should be able to leave the speakers/subs connected - you're listening for clipping and distortion anyway.

Once you have set the gain on all of the channels separately, all of the amps separately. Hook up everything as it should be - all speakers and subs.

Now listen to your system with music. Listen for balance between all of the speakers and subs. If you desire a more balanced sound (i.e. subs overpower speakers) you TURN DOWN GAIN on the loudest speaker/sub. DO NOT INCREASE GAIN past your set points determined above. Most head units have sub level controls, fader, etc. that allow you to attenuate the signal from your HU.

Now you can start introducing EQ, boosts, etc. Note that any bass boost more than 6dB can result in clipping or damage. You've already set the amp gain as high as it should be. Once you get your EQ and boosts set, you should check your gain setting once again. Excessive bass boost settings will cause clipping.

If you get noises then reduce the gain at the point where there are no noises or popping as it may be an indication of amplifier clipping, or speaker/sub distortion.

If it's not loud enough - buy more or larger speakers/subs, more or larger amps, and start over. Don't force your equipment to be louder than it can, or you'll destroy it.

[Autruche]

Damn, I was just about to post my Head unit/amp tuning section from my car audio write-up I'm working on. Great find, kynto.

One more thing to know, though. When setting your subs you should use a tone that is the same frequency as the bass boost on your amp. That way you can use bass boost if you want, just know that you shouldn't go past 6dB of boost ever, and if you can, stay at 3dB or less. Using a tone the same frequency as the bass boost ensures that you are reading the power output at the peak of the boosting wave rather than at a point on the slope that is lower than your actual peak.

In the end though, it shouldn't even need to be given a second though on whether to use a scope or a DMM to set your gain, you will never know if you are giving clean power to your sub unless you can see the waves with an oscilloscope, but, as kranny said, if you are looking to get less than the full rms power out of your amp (IE: you have a 1000rms amp, but you only want 750rms from it) then you can just use a DMM.

[nCOMP1337]

are there places you cna rent an oscope, they seem bit too expensive for me to buy one, dunno if you can rent one or i suppose a local shop may let you borrow one for a price, i know best buy was gonna charge me 20 bucks just to see if my amp powers on, i know it works just want to set it properly, lol

[Autruche]

are there places you cna rent an oscope, they seem bit too expensive for me to buy one, dunno if you can rent one or i suppose a local shop may let you borrow one for a price, i know best buy was gonna charge me 20 bucks just to see if my amp powers on, i know it works just want to set it properly, lol

Try your local auto parts stores and stuff, they may have one to rent, you never know. I know in my town there used to be a place that rented out construction tools, and the would rent out electrician stuff, so you may get lucky.

[Ao1FTW]

Holy smokes! Thanks for all the info and help! Really appreciated! I guess i'm gonna be doing more researching about how to do this better then cause I really don't need to blow new subs and an amp. lol

[n8ball2013]

its astounding how easy it is to set up amps with an o scope. unreal easy,

[Kranny]

its astounding how easy it is to set up amps with an o scope. unreal easy,

because you turn 1 knob pretty much. LOL