Simple Amp question.. 101

[Chromatic]

Thanks for the link. I watched several of the series.. spent a while learning the relationships and formulas.

But.. I need your help here.

My amp doesn't "add up" based on these rules.

Here's the amp link:

Ct Sounds 125 4 Amp Amplifier 125W RMS 250W Max Class A B 4 Channel 2 Ohm | eBay

This amp has 125watts RMS at 4 ohms (not bridged).

The amp has 4 channels.

Bridging two channels = 380watts RMS.

Ok, that doesn't add up. I'm trying to figure out what voltage to set the gain at when I have the multimeter on the (+) output of the amp.

So,.. Voltage of output = Sqrt x (RMS x Impedance).

Well,.. on this amp that puts the single channel for fronts at 100x4 = 500 sqrt(400) = 20v (Front speakers are rated 100w RMS)

For the sub (Bridging channel 3 and 4).. it would be = 300x4 = 1200 = sqrt (1200) = 34.64v (Sub is rated 300 watts RMS)

But.. if I calculate what the actual Bridged mode power should be given a 125watt per channel amp.. it isn't 380watts RMS, it's is RMS per channel x impedance.

Thus the bridged power should be 125watts RMS x 4 ohms = 500watts bridged RMS.

The amp is 380watts RMS,.. when bridging two channels.

So reversing the calculations.. To get the impedance to make this amp that is 125w per channel rms going to a 4 OHM sub, to equal 380 watts RMS . The Impedance value comes up to 3.04 ohms.

IE:

125w RMS x 3.04(ohms) = 380watts RMS.

With that 3.04 ohm value.. the voltage on the positive rail for MAX 380watt power would be 19.493v.

However, if I go with what I'm reading and watching.. which is 125w x 4ohms.

It wouuld be 380w RMS (bridged output) x 4ohms (1 sub at 4 ohms) = 1520. Sqrt(1520) = 39.98v.

So which is it?

I need to know whether to calculate based on 4 ohms,.. or the 3.04ohms that makes this 380watts in bridged mode RMS work in the formulas...because it's a big voltage difference.

Calculating the voltage is not too hard.

I unhook the positive terminal on amp. -Turn gain on amp to zero/min - Turn EQ/Bass/Treble settings to zero. Turn volume of headunit to 85-90 percent,.. Then, I put meter in AC voltage on positive terminal of amp,.. and turn the gain up slowly until I hit the voltage I calculated.

The 85-90 percent volume.. gives room to adjust the EQ/bass/treble and so on which will change the values a bit.. And volume won't ever be up at that level.. so it will help give a little headroom in the sine waves.. keeping them from going off the "Rails" positive or negative. As far as I understand it.. Speakers push out on positive current, and back in on negative current.

Any help on figuring out how I calculate the Voltage Power Output with this amp? If this amp was rated at at 500watts in bridged mode it would all add up, but it's 380watts RMS in bridged.

I know a lot more than I did an hour ago,.. still learning. but understand the clipping, resistance impact on the voltage output,.. The upper + rails and lower rails - from an amp,.. and why you want to set gain to stay within the amps voltage to prevent the wave from going beyond the upper or lower limit of these rails (clipping).

**Edit:** -- I was thinking.. The front voltage to set by sqrt(125wx4ohms) = 22.36v (looks right). On the sub I came up with another Voltage.. I took the 380watts RMS for bridged mode, divided it by 2 .. for 190. So,, sqrt(190wx4ohms) = 27.568v

AND -- I read that for amps with "TWO" gain controls.. you need to take the RMS power and divide by 2. So would I calculate voltage for fronts by taking 125/2 = 62.5,.. thus, Gain = sqrt(62.5w x 4ohms) = 15.8v (For fronts) [but that seems really low to me.]

And Sub ermm.. I don't know. In bridged the RMS all changes -- so maybe still stick to the 190, for the aforementioned 27.36v ?

Using the value of the speakers themselves, which are lower than the amp rating.. Fronts at 100watts RMS,.. and Sub at 300watts RMS.

The voltages to use I get are:

Front = 20v

Rear = 34.64v

Using the full rating from the amp stats.. The fronts gain should be set to 22.36v,.. and the bridged, sub, voltage set to 38.98v. Which coincides with this chart:

http://img9.imageshack.us/img9/452/amplifiersettingchart.jpg

Thanks for any help.

 

[So Low iT Hz]

Just measure the out voltage, and turn up the gain until the voltage stops increasing. Then turn it down until it barely drops below the maximum value. You don't have to do estimations to figure out what the voltage will be. And you can measure the voltage across the bridged channels for both the front and rear channels to set the gains. Cuts the total time in half for setting the gains.

 

[n2audio 5,000+ posts]

The way people more commonly blow speakers is because of distortion. The more distortion you introduce, the more unwanted heat dissipation you will have and the less control you have over the movement of the speaker. If you get an amplifier with triple the rated rms power of your speakers, and set the gain to a level that introduces less than .1% distortion, you won't blow the speakers. If you get an amplifier rated half the rated rms of the speaker and you set the gains at 10% THD you are going to blow your speakers. And it'll sound like crap.

I understand why people say this all the time, but to say "distortion kills speakers" isn't true.

That implies that a 100w speaker receiving a fully clipped signal from a 10w amp is certain to be damaged.

If it's a legitimate 100w speaker there's no way that would happen. It wouldn't happen with a 20w amp, or 30 -- and probably not 40.

A fully clipped amp wouldn't begin to threaten the thermal power handling of a 100w speaker unless its maximum linear output were around 50w rms.

It's not distortion -- it's power, and a clipped signal has more of it than a signal of the same voltage that isn't clipped.

 

[Chromatic]

Just measure the out voltage, and turn up the gain until the voltage stops increasing. Then turn it down until it barely drops below the maximum value. You don't have to do estimations to figure out what the voltage will be. And you can measure the voltage across the bridged channels for both the front and rear channels to set the gains. Cuts the total time in half for setting the gains.

lol.. I come back with very precise measurements,.. and you just say forget all that.. Stick a DMM on the amp.. crank the gain till the voltage stops increasing, then turn it down slightly and you're done.

Do you not understand what I'm asking?

Amps have a very precise formula for calculating with a simple calculator the voltage readings for X amount of power RMS you want to set the gains for for X rated speakers RMS.

The fronts come out to proper voltage with my "math" at 22.36v (Fronts will be on Chan 1 and 2) -- The amp is MORE than what the speakers are rated RMS. I'm going to give the speakers their max RMS power off the amp by setting gains with a single tone playing and dial the gain up till it hits the appropriate voltage.

My only question really was the Bridged RMS output from the manufacturer is 380Watts on Channel 3 and 4 bridged. The math for the amp doesn't add up to me.. as the bridged power should be closer to 500watts.. So do I just plug in the 380watts and ignore that.. and get my voltage requirements? Or ?

I was linked this theory and learned it.. it's quite simple actually, few formulas .. plug in data and you have exact figures to run by. Regardless of what I do.. The gains are going to be around 80-90 percent.

If I just put the amp at 90 percent on both gains.. wired it up.. and never put a meter on it, it'd sound fine. I just don't want to damage speakers.. and more-over this is a new skillset I've learned, might as well put it into practical application.

Using a meter on a terminal (or 2) takes a few minutes at best.. it certainly isn't the difficult part of the install.

Thanks

 

[So Low iT Hz]

lol.. I come back with very precise measurements,.. and you just say forget all that.. Stick a DMM on the amp.. crank the gain till the voltage stops increasing, then turn it down slightly and you're done.
Do you not understand what I'm asking?

Yes, I know what you're trying to do. I didn't say anything out of courtesy but if you want to learn ill just say it. Your math is all wrong...

Amps have a very precise formula for calculating with a simple calculator the voltage readings for X amount of power RMS you want to set the gains for for X rated speakers RMS.

It's not so simple actually. The algebraic approximation you just stated actually has little to do with speakers at all. It's a myth propagated by people without further knowledge of AC power. What you are calculating is a imaginary value, a figure that only has a theoretical importance.

The fronts come out to proper voltage with my "math" at 22.36v (Fronts will be on Chan 1 and 2) -- The amp is MORE than what the speakers are rated RMS. I'm going to give the speakers their max RMS power off the amp by setting gains with a single tone playing and dial the gain up till it hits the appropriate voltage.

Even assuming that you are using the correct math, the voltage across any varying resistance in parallel is the same so you can go based on that constant. On another note, your average impedance for say a 4 ohm 6.5" speaker between 120Hz and 6000 Hz is probably about 8 ohms. So if you are doing any math at nominal impedance you aren't using the right figures.

My only question really was the Bridged RMS output from the manufacturer is 380Watts on Channel 3 and 4 bridged. The math for the amp doesn't add up to me.. as the bridged power should be closer to 500watts.. So do I just plug in the 380watts and ignore that.. and get my voltage requirements? Or ?

Yea don't worry about doing the math. It's easier to have a machine do it for you.

I was linked this theory and learned it.. it's quite simple actually, few formulas .. plug in data and you have exact figures to run by. Regardless of what I do.. The gains are going to be around 80-90 percent.

If I just put the amp at 90 percent on both gains.. wired it up.. and never put a meter on it, it'd sound fine. I just don't want to damage speakers.. and more-over this is a new skillset I've learned, might as well put it into practical application.

Using a meter on a terminal (or 2) takes a few minutes at best.. it certainly isn't the difficult part of the install.

Thanks

No problem glad to help. Let us know how it sounds.

 

[calebkhill 10+ year member]

Wowww

 

[VWBobby 10+ year member]

edit: dblpoast

 

[VWBobby 10+ year member]

This is what happens when people try to apply science to an art form...... You are way over thinking it, and not only that, your theoretical math is way off.

Gain controls are not volume controls. You can't calculate what to set your gain, based on your output power. Gain is adjusted to match the output voltage of your head unit to the input of your amp, while keeping the headunit volume below the point of clipping. A head unit with a 4V output, might only have 3.5V at the end of the RCA's... The gain will be adjusted a lot lower than a 2V output head unit. This is something that will be shown in real world testing and not usually be calculated properly on paper.

Most headunits will clip at 1 - 3 volume numbers lower than max volume (38 out of 40, for instance). This is why using an oscilloscope or DD-1 is important for setting the gain to the maximum unclipped signal.

Your theroetical headroom is around 20%, so you really don't need to worry about blowing your speakers... Chances are you will not turn up your head unit volume more than 70% anyway... I know I normally listen around 50% volume or less and I'm running about the same amount of power to my comps as you.

Regarding the 380 watts verses the calculated 500 watts.... Amps will react to loads differently and power output will not be a linear equation, most of the time. For instance, the power output will not always be double at a 2 ohm verses a 4 ohm load. Some amps are also drastically overrated or underrated and power output will vary greatly from advertised specs. Another reason an o-scope or DD-1 comes in handy for finding the point of clipping and actual power output.

To summarize: Find your max unclipped volume level on your head unit with a test tone. Set it to this. Play test tone. Adjust gain on amp to max unclipped signal. Optional: If you like, turn down the gain to the max RMS voltage your speakers will handle. With impedance rise, you will be well below RMS power of your speakers.

 

[Chromatic]

No problem glad to help. Let us know how it sounds.

MMmkay.

 

[Chromatic]

This is what happens when people try to apply science to an art form...... You are way over thinking it, and not only that, your theoretical math is way off.

I was linked the "science/math" of it by the So Low it hurts.. on youtube. I spent the few hours to watch the series.. Then I come back and apply it, and it's said it's all junk.. you can't put on paper and apply it. Well why the heck did you link it then? Sigh..

Anyways.. I analyze everything,.. It's my personality and why I know so much about so many things.. because I tear them apart and teach myself.

Gain controls are not volume controls. You can't calculate what to set your gain, based on your output power. Gain is adjusted to match the output voltage of your head unit to the input of your amp, while keeping the headunit volume below the point of clipping. A head unit with a 4V output, might only have 3.5V at the end of the RCA's... The gain will be adjusted a lot lower than a 2V output head unit. This is something that will be shown in real world testing and not usually be calculated properly on paper.

I get that.. a 4v rated pre-out.. at the end of the run is likely not going to be a solid 4v.. thus the amp is well amplifying that 4v (to put it simply).. if it's 3.5v the figure it going to differ drastically..

Most headunits will clip at 1 - 3 volume numbers lower than max volume (38 out of 40, for instance). This is why using an oscilloscope or DD-1 is important for setting the gain to the maximum unclipped signal.

Agreed. I've never turned a headunit to max anyways,..

Your theroetical headroom is around 20%, so you really don't need to worry about blowing your speakers... Chances are you will not turn up your head unit volume more than 70% anyway... I know I normally listen around 50% volume or less and I'm running about the same amount of power to my comps as you.

This is part of the reason why I want a little more amp than I need. With the 125watts x 2, and 380 watts x 1 -- That gives me 10-20 percent headroom over my 100watt speakers and 300watt sub. It allows the amp to run a touch bit cooler than running at full tilt.. but it gives a little forgiveness in dialing it in.

Regarding the 380 watts verses the calculated 500 watts.... Amps will react to loads differently and power output will not be a linear equation, most of the time. For instance, the power output will not always be double at a 2 ohm verses a 4 ohm load. Some amps are also drastically overrated or underrated and power output will vary greatly from advertised specs. Another reason an o-scope or DD-1 comes in handy for finding the point of clipping and actual power output.

Yup.. I've learned this. Each amp is unique. And what on paper makes sense, doesn't mean an amp can pull it off. So while theoretically in a perfect world the amp would do 500 watts x 1 bridged.. it just doesn't. And frankly I don't need it either.

As for an O-scope.. or DD-1 -- well,.. I'm not going to spend the money on those to dial in one amp one time //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif --

To summarize: Find your max unclipped volume level on your head unit with a test tone. Set it to this. Play test tone. Adjust gain on amp to max unclipped signal. Optional: If you like, turn down the gain to the max RMS voltage your speakers will handle. With impedance rise, you will be well below RMS power of your speakers.

I've got the test tones already.. running a 40hz tone for the sub .. and 1khz sine tone for the front stage.. headunit at 75%. And.. I'll just dial in the gain with a DMM at the set voltage for 100watts at 4 ohms, and 300 watts at 4ohms.. (these are set values) -- and the voltages on the DMM will be hit for these values before the gain goes to max. Should be fine this way.