How to use a DMM to set gains?

[shaneb 10+ year member]

Okay, so I want to get the most bass with the most clarity out of my subs, I have a mephis pr300.2 amp that is pushing 2 10'' kicker comps at150rms@4ohms.

i'm playin' with gains and turning on and off bass boost, basically my question is, is there away to use a dmm to check the voltage being sent through gains? my eclipse HU ha 5v preouts..

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[n2audio 5,000+ posts]

download the 50hz 0dB tone from realm of excursion and put it on disk - disconnect your rca's from your front amp if you have one -- disconnect your subs from the memphis. Play the tone on repeat - turn the HU up to about as loud as you ever listen to it (as loud as it will go before it begins to clip -- 75% or so if you're not sure).

Set the amp's bass boost to off - set the crossover to low pass and the frequency to 80 hz or so.

Measure voltage at either speaker output -

Using the formula V = sqr(PxR) you want to adjust the gain up to a voltage of about 24.5v

 

[CadillacMatt 10+ year member]

I had a question about this as well... how accurate is the DMM reading when no subs are hooked up? Does the lack of resistance affect the reading? Or is voltage unaffected?

 

[GlocksRock 10+ year member]

the formula is square root(power*resistance)

I too was wondering how accurate the reading is when the subs aren't hooked up. I would think it would have some effect, since the resistance would affect the power.

 

[rod_b 10+ year member]

The video on the elemental designs website shows the guy setting gains with a resistor attached to simulate load. I gotta imagine that's the only way you can do it correctly for any other load besides 4ohms.

 

[GlocksRock 10+ year member]

I don' think so

 

[n2audio 5,000+ posts]

You don't need a load. The amp should provide X volts into whatever load as long as the load is within the stability limits of the amp. Having no load connected is like having an infinite impedance.

Using a calculated voltage from the amp's known power capability at the target load is an accurate way to KNOW the amp won't clip.

The only drawback is that in most cases the amp ends up being set quite conservatively since very little material is actually recorded at 0dB.

 

[GlocksRock 10+ year member]

So then is most material actually recorded higher or lower than 0db? The vid of the guy at eD setting the gain has a load on the amp, but the gain setting tutorial on the JL website says to disconnect the speaker(s) when setting the gain. So either way should be good... I still haven't set my gain using the dmm, but I may do that very soon.

 

[Haunz 10+ year member]

You don't need a load. The amp should provide X volts into whatever load as long as the load is within the stability limits of the amp. Having no load connected is like having an infinite impedance.Using a calculated voltage from the amp's known power capability at the target load is an accurate way to KNOW the amp won't clip.

The only drawback is that in most cases the amp ends up being set quite conservatively since very little material is actually recorded at 0dB.

WRONG

You should most certainly have a load on the amp.. not having a load will affect rail swing significantly..

 

[James Bang 5,000+ posts]

See avatar and sig pls.

thx

 

[01 S 10 10+ year member]

what can be used as a load? something like a blown sub?

 

[GlocksRock 10+ year member]

Why not just use the subs you plan on using in your vehicle for the load?

 

[James Bang 5,000+ posts]

wipe that load off your face

 

[99grandprixGT 5,000+ posts]

Why not just use the subs you plan on using in your vehicle for the load?

i don't know if i want to send the test tone threw my subs. i accidentally left my speaker amp on when i did and the speakers blew lol.

 

[n2audio 5,000+ posts]

WRONG
You should most certainly have a load on the amp.. not having a load will affect rail swing significantly..

You know - I remember when you were the poor, borderline illiterate dork at Carsound that nobody paid any attention to.

Now that you've learned something I guess you've earned a little respect around here, and that's just swell.

It seems you take some enjoyment in picking on anything relatively technical I have to say, and I don't have the necessary background in electronics to go debating "rail swing" with or without a load, but when you convince JL that this anomaly is significant enough to effect a baseline gain setting, and have them revise their tutorial, I'll consider it applicable.

Until then, bloviate to your heart's content.