Any idea why a 50hz tone is recommended when setting gains using a DMM?

[RSDXzec 10+ year member]

Been running some test tones lately and the lower the frequency the higher the voltage reads from the amp outputs. I always use a 0db 50hz tone to set my gains and I'd like to know if anyone here has any idea why a 50hz tone is always recommended when setting gains on a DMM.

Cheers.

Click to read more...

 

[Kangaroux 5,000+ posts]

Sorta just like the industry standard I guess. The more consistent things are the less chance of someone doing something dumb and blowing up stuff.

 

[goingdef 10+ year member]

Been running some test tones lately and the lower the frequency the higher the voltage reads from the amp outputs. I always use a 0db 50hz tone to set my gains and I'd like to know if anyone here has any idea why a 50hz tone is always recommended when setting gains on a DMM.
Cheers.

I wondered the same thing a while back and if you compare rockfords amp output graphs around 50hz is the peak of it's output under a given ohm load so while your meter reads higher with lower tones in all reality your amp is struggling with that in a real life situation.

 

[Rich B 5,000+ posts]

Most all subs can play a 50hz tone with little strain?

 

[hispls 5,000+ posts]

In theory an amp should give you constant voltage into any load into which it is stable. In real life 50hz is a frequency that's almost certain to show over DC resistance by a good bit, so I can't immagine why anyone suggests that except the fact that that's about the point where you're becoming much less likely to break the woofer mechanically from over excursion.

 

[goingdef 10+ year member]

Most all subs can play a 50hz tone with little strain?

not the sub the amp, it does it's best output wise at around 50hz(at least according to the lit that came with the birthsheet)

 

[Rich B 5,000+ posts]

not the sub the amp, it does it's best output wise at around 50hz(at least according to the lit that came with the birthsheet)

Duh, I read it wrong!

 

[RSDXzec 10+ year member]

I wondered the same thing a while back and if you compare rockfords amp output graphs around 50hz is the peak of it's output under a given ohm load so while your meter reads higher with lower tones in all reality your amp is struggling with that in a real life situation.

so even though it reads a higher voltage it's using the same power? As in it should show less voltage or has less resistance?

In theory an amp should give you constant voltage into any load into which it is stable. In real life 50hz is a frequency that's almost certain to show over DC resistance by a good bit, so I can't immagine why anyone suggests that except the fact that that's about the point where you're becoming much less likely to break the woofer mechanically from over excursion.

If it shows over resistance then would that mean it should have a lower voltage at other frequencies? I use V=sqrt(P*R) to set my gains.

 

[hispls 5,000+ posts]

Almost all solid state amplifiers are constant voltage sources. This is why power doubles as impedence halves. At the same level your output voltage should be the same at any frequency. Actual "power" at any given frequency will vary greatly depending on impedence at that frequency.

 

[RSDXzec 10+ year member]

Almost all solid state amplifiers are constant voltage sources. This is why power doubles as impedence halves. At the same level your output voltage should be the same at any frequency. Actual "power" at any given frequency will vary greatly depending on impedence at that frequency.

so is that pretty much saying my amp is garbage?

the amp is a ****** cheap class A/B amp so i wouldn't be surprised.

 

[SomeguyinPDX 10+ year member]

I may be wrong but I was told by a tech that most DMM's are most accurate around 50-60cycles. The farther above and below that you get the bigger the error is in most meters, except high end meters.

 

[RSDXzec 10+ year member]

I may be wrong but I was told by a tech that most DMM's are most accurate around 50-60cycles. The farther above and below that you get the bigger the error is in most meters, except high end meters.

oh... my meter cost me about $10 so it isn't the best lol but it does what it needs to do. Only reason I use it is because my good meter died a few months ago...

Anyone else heard of this? I haven't seen that anywhere before.

 

[SomeguyinPDX 10+ year member]

I also read it somewhere on this site:

Basic Car Audio Electronics

not sure where but its on there somewhere.

Something to do with the fact most home wiring operates at 120vac @ 60hz and that is why most cheap meters are most accurate in that range.

 

[subzero 5,000+ posts]

I had a burp box. when I set my gains with the DMM and oscope, we used the resonate freq of my peak. which was 47hz. and set the gain to that.

 

[keep_hope_alive Premium Member]

It is possible an amp or processor or head unit has a bass boost or bass restoration circuit which can increase bass at some frequencies.

Yes, most DMM's are accurate at 50-60Hz since we usually measure AC voltage at that frequency. The meter should have a tolerance and frequency range listed, if not, you can't really trust it outside that range.

Note that there is an impedance spike at the woofer fs. That can affect the amp output voltage if the subs are connected and the tone is within an octave of fs.