Gains with a DMM

[61ragtop]

Well I got my Audison AV5.1k amp up and running. I don't have an O scope or access to one so I tried to set my gains with a DMM. Now the 2*75w and 2*140w channels worked out to the watts times ohm load square route calculations but I had an issue with the sub channel. It is a 600 watt at 4 ohm mono channel. Or 1000 at 2 ohm.

I disconnected the speaker wires and tested. It should have been 48.8 volts but I was getting closer to 90 volts unless the gain was all the way off. Is there something I am missing with this being a mono channel or?????

[ca90ss]

Throw away the dmm. The best tool for setting gains is attached to the side of your head.

[chad]

I feel the "DMM approach" was made popular by someone who wanted to say that they used some sort of scientific measurement instrument to do something that is much more complex than the tool was intended. In order for this to even come close to working, especially in a system that is active, is to know every distance and driver sensitivity so you can get the appropriate power distribution between drivers. And even after that it's a very rough estimate.

I'll echo that your ears, combined with what is in between them is a much better tool than a DMM.

That being said, a time when I will/do use a DMM is, for example, when I'm swapping out an amp and want the same gain structure. I'll generate tones around midway through each passband, play the tones (does not have to be loud) and measure the output. Then set the replacement amplifier's gains to that level. Keep in mind that this method only works if the amplifier has the same type of gain structure, especially in professional audio. Some may be configured so that X volts = max output, while others have constant voltage gain regardless of max power output. I much prefer the latter.

[61ragtop]

Thank you both for the replies!! I did try to play a test tone and adjust the gain but I fund it really hard to hear when it started to clip. I was also informed by the local Audison rep that he didn't recommend tuning the gains on my Voce tweeters with a test tone as they are very easy to blow this way. So as a safety I resorted to test tones and the DMM. the only one that didn't seem to work out was the sub channel.

I have heard of a way with a small powered speaker that makes it easier to hear the clipping and won't blow speakers or make a ton of noise? But I was also unsure of how this was performed....

[ca90ss]

Throw the test tone cd away as well. Start with whichever drivers are the weakest link in your system. Throw in a cd and turn your hu up to 2/3 volume. Turn the gain up until you hear distortion and then back off until it goes away. Level match the rest of the drivers to those and you're done. No need to make things hard on yourself or over complicate things.

[pocket5s]

Throw the test tone cd away as well. Start with whichever drivers are the weakest link in your system. Throw in a cd and turn your hu up to 2/3 volume. Turn the gain up until you hear distortion and then back off until it goes away. Level match the rest of the drivers to those and you're done. No need to make things hard on yourself or over complicate things.

2/3rds volume? and what if your HU doesn't clip at all? what if it clips before 2/3rds? or just past it and now you've limited your system. and then throw in a DSP and your method really goes out the window.

DMM doesn't show you clipping, so it is generally used in situations like chad mentioned although I do use it for sub amp setting I admit. The small speaker method uses a radio shack self powered speaker (about $15) and an RCA adapter to plug it into the signal chain. using tones and your ears you can hear the clipping easily on tweeters and midranges. midbass (if you have separate mb) is much harder for me and sub nearly impossible for me. plug it into the HU, find the clipping, back down, then plug it into the next item in the signal chain.


and don't throw away that dmm, it is a useful tool for other things.

[chad]

If you are playing tones to listen for clipping, especially for small mids and tweeters, there is a decent chance that the driver will fail before you hear it unless the amp is really low power. Tones have the highest power density you will throw at a driver.

[chad]

Also, I see WAY too many people set a system up so that the amps all clip at the same point. This will lead to a rather unbalanced sounding system as the HF components will generally have a much higher sensitivity than the LF components. In a passive crossover, all those resistors you see, are generally used to pad that tweet down so the woofer can keep up with it. Granted, some are there for impedance correction, but most are used for padding in some way, shape, or form.

[pocket5s]

If you are playing tones to listen for clipping, especially for small mids and tweeters, there is a decent chance that the driver will fail before you hear it unless the amp is really low power. Tones have the highest power density you will throw at a driver.

Yep, that's why I was taught to use the small external speaker. If you pop a $15 radioshack speaker you might say "s*%t", but you pop your main speakers and you are likely to say far worse.

[chad]

Back in the 9's there was a little DIY thing you could use that had a piezoelectric tweet, those things are unique because as the frequency rises the impedance goes down, that's why they are touted as "crossover-less"

Basically it was padded down and may had some cap coupling in it. when the amp clipped you could hear it due to the harmonics generated form clipping. IIRC it was called the clip cop but I cold be totally wrong.