Speakerplans.com Homepage
Forum Home Forum Home > General > Amp Forum
  New Posts New Posts RSS Feed - matching amp&drivers, 4vs8 ohm
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

matching amp&drivers, 4vs8 ohm

 Post Reply Post Reply Page  12>
Author
Message
jngggggggg View Drop Down
Registered User
Registered User


Joined: 27 April 2017
Status: Offline
Points: 88
Post Options Post Options   Thanks (0) Thanks(0)   Quote jngggggggg Quote  Post ReplyReply Direct Link To This Post Topic: matching amp&drivers, 4vs8 ohm
    Posted: 08 February 2022 at 5:36am
I am trying to understand what feels like extremely basic concepts that I am having a tough time wrapping my head around. The topics although widely discussed on many forums, don't quite address my concerns. i'm very much a numbers guy and am trying to put it all on paper without any luck. This is after countless hours of combing threads, articles and reading my sound reinforcement handbook (davis&jones). i appreciate everyone's help!

I am build a system and spec'ing out the subs right now. music is dubstep, dnb. I have 6 sub cabinets that I plan on loading with a single 21SW152s each. i am considering a 2-channel amp capable of:
2ohm  7530w
4ohm  5380w
8ohm  3160w

I am trying to understand 2 parts:

1. power matching these 6 drivers with 2 amps
so these 21sw152s (4ohm) have 2000w nominal power handling. since the tests were done using sine wave pink noise (6dB crest factor), that should equate to 500w 'rms' and 2000w 'peak' or 45v rms and 90v peak (using sqrt(pr)). the plan would be to run two in parallel on channel-1 and one on channel-2. so that equates to:
channel-1 sees 1000w average and 4000w peak demand (2ohm effective)
channel-2 sees 500w average and 2000w peak demand (4ohm effective)

does the amp in question have enough headroom? ie: is it properly sized?
4000 & 2000w peak demand from an amp capable of providing 2 channels with 7530 & 5380w respectively should be enough right? i've read that the rule of thumb is 2-3x the continuous power handling (termed nominal for B&C) so this configuration technically falls short. can someone explain why there is so much headroom required?

how do i calculate how close i am to blowing a breaker? keeping in mind that 99% of venues i deal with will only have 120V 15A service. 
is it voltage limit? current limit? kirchoffs current law apparently gives me a current draw of 33A rms for this one amp (66A peak?!?). i know this isn't being pulled out the wall as amplifiers have capacitance storage etc, but without being able to put numbers on paper, i start to get lost in it all. how does one know if the power supply is being pushed to its limit? do i simply rely on the 45v rms and 90v peak draw as a starting point for limiters, run the system at war volumes, check magnet temperatures and raise the limiters to my liking while still ultimately remaining below 120v? 

ive read in various places that 'amps can be assumed to be constant voltage sources' - does this mean that the 45v rms is continuously demanded by the power supply since that value is is the average voltage required for a given program signal? 




2. i am also considering whether it makes more sense to purchase 8ohm drivers for the above configuration instead of 4ohm - again factoring in 120V 15A service here in Canada.
this is again where i cant quite seem to make sense of the numbers. if i instead go with 8ohm drivers in the same configuration as above, the power demand remains unchanged but the channels operate at higher load and therefore gives me even less headroom than before but still acceptable? is this configuration now a poor match between drivers and amp? 
channel-1 sees 1000/4000w rms/peak demand (4ohm effective - amp providing 5380w)
channel-2 sees 500/2000w rms/peak demand (8ohm effective - amp providing 3160w)

in this situation, what are the design tradeoffs between 8v4ohm drivers assuming all else equal (beside damping factor obviously)

Again, this is where my previous lack of understanding of how the power supply constrains the amp really hinder me. under 8ohm loads, p=iv tells me that although the amp would require less current in exchange for more voltage, wouldn't i be drawing 63v rms and 126v peak from the amp and therefore exceeding the limits of the 120v power supply?


If it's of importance, there may be situations where i would only run 2 subs per amp (one sub per channel) if the venue had plenty of independent outlets to spare.
apologies for the long winded write up - i'm just doing my best to intentionally plan and spec this equipment out before committing the $$.
Cheers
Back to Top
Elliot Thompson View Drop Down
Old Croc
Old Croc
Avatar

Joined: 02 April 2004
Location: United States
Status: Offline
Points: 5011
Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 08 February 2022 at 9:03pm
Originally posted by jngggggggg jngggggggg wrote:


how do i calculate how close i am to blowing a breaker? keeping in mind that 99% of venues i deal with will only have 120V 15A service. 


A 120-volt, 15 Amp Breaker will trip around 24 amps when the current is constant within one minute. Bear in mind since you are using a standard household receptacle, there will be other receptacles within the chain connected to the 15 Amp Breaker pulling a load. So you can expect the breaker to trip lower than the 24 amp tripping point.


All in all you do not have enough line source current available at your disposal to achieve what you are aiming for to feed the amplifiers properly.


Matters are worse if you are eyeing the K30/K35/AD42 amplifiers. The power factor correction on those amplifiers will fluctuate between 0.65 – 11.46 amperes continuously until you power off the amplifier. That is without a load, no signal passing through. The above fluctuations is based on a K30 amplifier.


For the amount of power those amplifiers offer, you require a distro. Preferably offering a 240-volt line source, or at a minimum, 208-volt single phase line source. This will reduce the current demand significantly in which, will allow you to take full advantage of the amplifiers performance.


Best Regards,



Elliot Thompson
Back to Top
Conanski View Drop Down
Old Croc
Old Croc
Avatar

Joined: 26 January 2006
Location: Canada
Status: Offline
Points: 2260
Post Options Post Options   Thanks (0) Thanks(0)   Quote Conanski Quote  Post ReplyReply Direct Link To This Post Posted: 09 February 2022 at 12:15am
I can't say it much better than Eliot did... you simply cannot run those amplifiers on 120v circuits. Well actually you probably can but the amp will only be able to fully power 1 driver.

I'd vote for setup #2 with 8ohm drivers as well as I feel that will give you more options for powering multiple cabs, in fact it will deliver just under 1900w to four 8ohm drivers per channel if the ratings are for real. As for how much headroom you need in an amp, as long as they cleanly cover the peaks you plan for you're good, no need to have thousands of watts unused. 


Edited by Conanski - 09 February 2022 at 12:18am
Back to Top
jngggggggg View Drop Down
Registered User
Registered User


Joined: 27 April 2017
Status: Offline
Points: 88
Post Options Post Options   Thanks (0) Thanks(0)   Quote jngggggggg Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2022 at 4:11am
lots of questions in my first post so appreciate everyone helping me address so many concerns..

would it be possible for someone to walk me thru the numbers/calculations to determine that this amp/driver configuration would overload the power supply? im sure it isn't as simple as kirchoffs current law i stated above? (for example, 33A RMS with three 4ohm drivers). how do i know if i'm current limited? voltage limited? 
i trust your advice, but would prefer to understand the thought process.

as well, how would a PDU help? they cannot create voltage out of nowhere? - so i would still be limited to the 120v power supply feeding it? unless we're referring to rental power like gens etc.


Edited by jngggggggg - 11 February 2022 at 4:13am
Back to Top
Elliot Thompson View Drop Down
Old Croc
Old Croc
Avatar

Joined: 02 April 2004
Location: United States
Status: Offline
Points: 5011
Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2022 at 10:17am
Originally posted by jngggggggg jngggggggg wrote:


as well, how would a PDU help? they cannot create voltage out of nowhere? - so i would still be limited to the 120v power supply feeding it? unless we're referring to rental power like gens etc.

Calculate how much wattage you want under a 120-volt line source. You will see, no matter what you do, your limiting factor will be the 15 A Breaker offering a line source voltage of 120.


Best Regards, 
Elliot Thompson
Back to Top
Conanski View Drop Down
Old Croc
Old Croc
Avatar

Joined: 26 January 2006
Location: Canada
Status: Offline
Points: 2260
Post Options Post Options   Thanks (0) Thanks(0)   Quote Conanski Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2022 at 3:06pm
Originally posted by jngggggggg jngggggggg wrote:

would it be possible for someone to walk me thru the numbers/calculations to determine that this amp/driver configuration would overload the power supply? im sure it isn't as simple as kirchoffs current law i stated above? (for example, 33A RMS with three 4ohm drivers). how do i know if i'm current limited? voltage limited?
Well... you can't get 33A continuous out of a 15A circuit.. that should be obvious, and it's not legal or safe to connect an amp to multiple circuits at once so the ONLY solution is to connect this amp to a 230v circuit. The math says a single 120v/15A circuit can supply about 1800w continuously, but that doesn't translate 1:1 into audio power for several reasons. 
1. Music isn't a sine wave it is usually more dynamic so the continuous average current delivered to the speakers is lower than the math suggests.. anywhere from 1/3 to 1/8 of peak. 
2. The dirty little secret of AC grid supply systems is that there is an unofficial short term peak current supply capacity over and above the circuits rating, breakers and fuses are slow blow devices that require a sustained over current condition to trip.
3. No amplifier is 100% efficient so there is always some power lost in heat, Class D amplifiers are very efficient though at 95% or there abouts while class AB are only about 50%.

So all that together means that a 120v/15a circuit might be capable of producing 2500w-3000w of music power, but it depends on the music genre and the relative efficiency of the amplifiers. A 230v/50A circuit might support 15kw of audio. Also note that the output voltage produced by the amplifier is not directly related to the input voltage, an amplifier is an electrical power conversion device so it's output voltage can be higher or lower than the AC mains voltage.


Originally posted by jngggggggg jngggggggg wrote:

as well, how would a PDU help? they cannot create voltage out of nowhere? - so i would still be limited to the 120v power supply feeding it?
  No, a PDU will connect to a high voltage/high current source... usually a dedicated 230v outlet, and then break that out into multiple 120v circuits. These exist in many venues you just may not have noticed them before. 


Edited by Conanski - 11 February 2022 at 3:26pm
Back to Top
jngggggggg View Drop Down
Registered User
Registered User


Joined: 27 April 2017
Status: Offline
Points: 88
Post Options Post Options   Thanks (0) Thanks(0)   Quote jngggggggg Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2022 at 8:06pm
Originally posted by Elliot Thompson Elliot Thompson wrote:

Bear in mind since you are using a standard household receptacle, there will be other receptacles within the chain connected to the 15 Amp Breaker pulling a load. So you can expect the breaker to trip lower than the 24 amp tripping point.

All in all you do not have enough line source current available at your disposal to achieve what you are aiming for to feed the amplifiers properly.


i should clarify that 120v 15a is the standard for most receptacles in clubs and bars here in Canada. that being said, my understanding is that you can usually find multiple independent 120v 15a outlets to spread your amps across - not sure if that makes the bottom line any better though. perhaps there are 220v circuits i can tie into in the back of house - but majority venues ive encountered here dont have this (or if present, is purposely made inaccessible for us ..unfortunately)

Back to Top
jngggggggg View Drop Down
Registered User
Registered User


Joined: 27 April 2017
Status: Offline
Points: 88
Post Options Post Options   Thanks (0) Thanks(0)   Quote jngggggggg Quote  Post ReplyReply Direct Link To This Post Posted: 11 February 2022 at 10:30pm

Originally posted by Conanski Conanski wrote:

Also note that the output voltage produced by the amplifier is not directly related to the input voltage, an amplifier is an electrical power conversion device so it's output voltage can be higher or lower than the AC mains voltage.

 

the amp i am considering has the following specs.

RMS Output Voltage 185.0
Input Operating Voltage: 90 VAC - 270 VAC, 50 Hz or 60 Hz

 

can someone break down what this 185v rms output means given say 90v rms input for example? is 185v simply the max rms value the amp can continuously put out (therefore expect <90v rms output when sending 90v rms into the amp) or is that 185v rms constant?

 

 

as for regarding 8 vs 4ohm drivers. if i understand it correctly, the power demand is identical between the two drivers so the real difference between the two comes down to whether the amp requires more voltage or current from the wall (p=iv). for example, to feed 500w continuous / 2000w peak per driver:

4ohm drivers each demand 11A(rms) / 22A(peak) and 45v(rms) / 90v (peak)

8ohm drivers each demand 8A(rms) / 16A(peak) and 63v(rms) / 126v (peak)

 

i get that peak demand is not continuously required by the amp, but wouldn't 4ohm be better suited for 120v 15a since peak voltage and peak amprege are exceeded by the 8ohm?

Back to Top
Conanski View Drop Down
Old Croc
Old Croc
Avatar

Joined: 26 January 2006
Location: Canada
Status: Offline
Points: 2260
Post Options Post Options   Thanks (0) Thanks(0)   Quote Conanski Quote  Post ReplyReply Direct Link To This Post Posted: 12 February 2022 at 5:56am
Originally posted by jngggggggg jngggggggg wrote:

the amp i am considering has the following specs.

RMS Output Voltage 185.0
Input Operating Voltage: 90 VAC - 270 VAC, 50 Hz or 60 Hz

 

can someone break down what this 185v rms output means

The 185v output rating is the maximum it can generate and the only control you have over that is with the audio signal you feed it. Again, the AC mains voltage the amp is connected to does not directly influence or correlate to this in any way, it will still be capable of generating 185v output even on a 90v supply.


The input voltage range indicates that it has a universal power supply that can operate on all global electrical grids.

 

Originally posted by jngggggggg jngggggggg wrote:

as for regarding 8 vs 4ohm drivers. if i understand it correctly, the power demand is identical between the two drivers 
No you got it all wrong, a lower load impedance equals more current drawn which means more power dissipated.


Here is the thing... a power amplifier is fundamentally a voltage booster, you inject a 1v signal and it might produce a 50v output signal(depends on the voltage gain of the amp), it doesn't matter how many speakers are connected or if there are any connected at all, you still get the same 50v at the output. So for example wth no speakers connected no current flows and no power is dissipated. Connect an 8ohm speaker 6.25A flows and 312.5w is dissipated, connect a second identical driver in parallel for a 4ohm load, 12.5A flows for 625w dissipated. Connnect a 3rd driver in parallel, the load is now 2.67 ohms and 18.7A flows for 930w dissipated. See how that works?


Edited by Conanski - 12 February 2022 at 6:03am
Back to Top
Elliot Thompson View Drop Down
Old Croc
Old Croc
Avatar

Joined: 02 April 2004
Location: United States
Status: Offline
Points: 5011
Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 12 February 2022 at 11:57am
Originally posted by jngggggggg jngggggggg wrote:

Originally posted by Elliot Thompson Elliot Thompson wrote:

Bear in mind since you are using a standard household receptacle, there will be other receptacles within the chain connected to the 15 Amp Breaker pulling a load. So you can expect the breaker to trip lower than the 24 amp tripping point.

All in all you do not have enough line source current available at your disposal to achieve what you are aiming for to feed the amplifiers properly.


i should clarify that 120v 15a is the standard for most receptacles in clubs and bars here in Canada. that being said, my understanding is that you can usually find multiple independent 120v 15a outlets to spread your amps across - not sure if that makes the bottom line any better though. perhaps there are 220v circuits i can tie into in the back of house - but majority venues ive encountered here dont have this (or if present, is purposely made inaccessible for us ..unfortunately)


This is why many have smaller systems (B, C, D Rig) to accommodate venues that offer such access limitations.  

Best Regards,
Elliot Thompson
Back to Top
JonB67 View Drop Down
Young Croc
Young Croc


Joined: 22 April 2016
Location: UK
Status: Offline
Points: 1365
Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 12 February 2022 at 4:24pm
Im surprised we dont see more guys on your side of the pond building for more efficient output? Everyone seems to be racing for bigger amps and drivers that suck up the most power. Cant be the best plan on limited supplies of electricity.
Back to Top
jngggggggg View Drop Down
Registered User
Registered User


Joined: 27 April 2017
Status: Offline
Points: 88
Post Options Post Options   Thanks (0) Thanks(0)   Quote jngggggggg Quote  Post ReplyReply Direct Link To This Post Posted: 12 February 2022 at 6:58pm
Originally posted by Conanski Conanski wrote:

a power amplifier is fundamentally a voltage booster, you inject a 1v signal and it might produce a 50v output signal(depends on the voltage gain of the amp)

ok this clarifies things a bit - although leads to more questions on my part. i recognize that i was getting caught up in the 45v-rms / 90v-peak being delivered to the driver and forgot that the amp will amplify a given input voltage, so this 45/90v is not demanded from the wall necessarily. So how does one relate this amp output voltage back to input voltage required from the power supply? does it simply come down to measurements in real life and power factor? not much else available online from the manufacturer it appears. perhaps i can send an email out to further inquire.

Originally posted by Conanski Conanski wrote:

a lower load impedance equals more current drawn which means more power dissipated.


So for example wth no speakers connected no current flows and no power is dissipated. Connect an 8ohm speaker 6.25A flows and 312.5w is dissipated, connect a second identical driver in parallel for a 4ohm load, 12.5A flows for 625w dissipated. Connnect a 3rd driver in parallel, the load is now 2.67 ohms and 18.7A flows for 930w dissipated.

i think that we may be saying the same thing here, but just different? (you assume constant output voltage from the amp). in the example i had quoted below, if choosing 4ohm instead of 8ohm (less resistance and therefore more current demand) wouldn't one just lower the voltage output of the amp from 63v in this example to 45v rms to keep the total power dissipated the same at 500w average? ie: a give and take between voltage and amps.  unless my thought of adjusting amp voltage output to maintain the same power dissipated is flawed thinking? my logic is based off trying to feed the driver its stated 2000w continuous power handling rating; or 500w average

Originally posted by jngggggggg jngggggggg wrote:

to feed 500w continuous / 2000w peak per driver, the amp must provide:

4ohm drivers each demand 11A(rms) / 22A(peak) and 45v(rms) / 90v (peak)

8ohm drivers each demand 8A(rms) / 16A(peak) and 63v(rms) / 126v (peak)





Edited by jngggggggg - 12 February 2022 at 6:59pm
Back to Top
 Post Reply Post Reply Page  12>

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.05
Copyright ©2001-2022 Web Wiz Ltd.

This page was generated in 0.096 seconds.