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Calculating power consumption from iffy specs
Calculating power consumption from iffy specs
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Old 7th September 2017, 11:36 AM   #1
Stuntmunky is offline Stuntmunky
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Default Calculating power consumption from iffy specs

I am currently building a mono modular powered busking system, comprising of three boxes. Top, mid and bass, each have their own amp/power. The mid box will cover most of the shows I do and is the main workhorse, containing a microprocessor to mix the lines in and to split the signal out for the other two boxes.

I have a lot to learn about audio and electronics so would really appreciate a bit of clarification :-)

I already had some decent speakers and so I was trying to match amps to them, specifically the digital amp boards that are available.

For mid I have two options, an 8ohm beyma 605nd 125rms or a 4ohm ds18 pro neo slim 200w rms. So far I am digging the ds18 but the beyma is more efficient so I may wait and see what my battery life is like before the final decision.

I have bought two boards so far, choosing the suppliers who seemed to give the most comprehensive data. Making guesstimates on the power supply I would need.

This is the board I am using for the mid WONDOM 1X 400W Class D Audio Amplifier Board Compact T-AMP Module Subwoofer | eBay

I was hoping to get around 100w @ 8ohm but there seems to be a massive difference between the stated rms output and the current drawn. Am I missing something?

I am running the amp like this for testing: Car battery to amp/voltage display to 42v step up converter to amp. The supply is a 1200w dc-dc converter and I have left the current unlimited.

The output from the processor is at 3.16v p to p, I think I have to divide this by two to get the voltage of 1.58 . The amp can take 1.8v so I think I can drive a little harder here.

I am running at 42v because there seems to be a few datasheets around for this model stating slightly different upper limits but I might be able to squeeze it to 45v. The power supplies I have tried seem pretty steady but I would like to leave a little buffer just in case.

The most I have been able to get the amp to draw without clipping is around 2.2a @ 12.8v or 28.16w on the ds18 driver.

I am well aware that companies massage their figures to boost sales but this seems like a massive difference. Is there a ballpark formula to tell me what an amp will draw from its stated specs?

To be fair I am pretty happy with the output, it's clean and loud enough for my purpose but having to wait for the board to arrive and be tested before buying an appropriate power supply is slow and more than a bit annoying. I really would like to know how all this is calculated before buying the amp for the bass box.



Thanks to anyone willing to shed some light on this
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Old 7th September 2017, 12:28 PM   #2
Pano is offline Pano  United States
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Calculating power consumption from iffy specs
Welcome to the forum.

No, there really isn't a ballpark formula to tell you the current draw for live music. Average power for recorded music tends to be about 1/30th to 1/20th max power. For live is just hard to say, it tends to be higher. But you have done the right thing and measured the draw right at the battery which takes into account the converter, amp, speaker and any other load. You can calculate from there.

Are you looking for the current rating of the dc-dc converter?

One way to look at is is that at full power from that amplifier, you'd have a 7A draw. As you can see from your measurement, you didn't get close to that, you measured 2.2A which is 19.4 watts into a 4 ohm load. You'd chose a minimum of a 3A power supply, or a maximum of 7A. because that's between your measured draw and the max the amp can supply into a 4 ohm load.
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Old 7th September 2017, 02:09 PM   #3
Stuntmunky is offline Stuntmunky
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Thanks for the reply :-)

I have a power supply now for the mid box, based on the results from the testing.

It was more to match the bass speaker to an amp and power supply without making spendy mistakes and to have the components arrive at roughly the same time, to make layout design easier.

The first amp I bought based on the specs, turned out to be underpowered for the mid.TDA7498E 2X160W Dual Channel Audio Amplifier Board, Support BTL Mode 1X220W R6G0 | eBay

I'm not too fussed because it will do for the tweeters but the bass amp is likely to be more expensive and I really can't afford to make mistakes on that one. Pennies is righty tighty :-(

I guess testing is the best way, I just wanted to be sure I hadn't made a mistake regarding how rms is calculated.

The bass speaker is an eminence neodymium 8ohm 250w rms, I was thinking to match it to this amp


1X1000W 1KW Class D Audio Amplifier Board - T-AMP Module WONDOM Subwoofer | eBay It seems to be the most powerful board I can get with a regular single dc supply.

I do really like these boards though, they seem like decent quality and a fan instead of a huge passive heatsink is great for the design brief on this one, small and light as poss without compromising too much on volume and clarity.

One other question I have is about grounding. In the mid box I am running 3 separate power lines from the same battery. A 5v for the processor and input devices, a 12v for a bluetooth board and power out for my mic and the 42v line for the amp.

At the moment the boards are all mounted on plastic in a plastic box. Would it be a good/safe idea to run all the grounds to a common point?

I have got some isolators but I'm not sure they are up to much and it's taking a long time to mount/bypass them for testing and I'm not sure if I remember each time what the last test sounded like. I am getting some higher end windy/whistling processor style noise, I think this is down to the bluetooth board. It's not too much of an issue but I'd like to fix it if I can.

Thanks again :-)
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Old 7th September 2017, 03:40 PM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Stuntmunky View Post
I am currently building a mono modular powered busking system, comprising of three boxes. Top, mid and bass, each have their own amp/power.
Ain´t this way too complex and heavy for a busking system?

Quote:
For mid I have two options, an 8ohm beyma 605nd 125rms or a 4ohm ds18 pro neo slim 200w rms. So far I am digging the ds18 but the beyma is more efficient so I may wait and see what my battery life is like before the final decision.
A car battery, which is *very* heavy and will require some kind of cart or wheels to move around (besides what you may need for speakers and extra hardware) , if supplying, say, 150W power for a real 100W amp plus extra stuff will last some 4 hours full blast, *maybe* 8 hours "show time" if you make silences to collect money, talk to people or simply rest now and then, so that is the realistic maximum weight you can carry and time you can play.

That if amp is directly 12.6V powered; any converter included will lower system efficiency, personally rather than the amp you show plus an extra converter I´d much prefer to use a car type amplifier, which already has everything built in and properly tuned.

Any power increase or weight saving will proportionally reduce your play time.

Quote:
I have bought two boards so far, choosing the suppliers who seemed to give the most comprehensive data. Making guesstimates on the power supply I would need.
Lots of data, most of it questionable or irrelevant,and as you noticed, not what you want toknow

Quote:
This is the board I am using for the mid WONDOM 1X 400W Class D Audio Amplifier Board Compact T-AMP Module Subwoofer | eBay

I was hoping to get around 100w @ 8ohm but there seems to be a massive difference between the stated rms output and the current drawn. Am I missing something?
No, you are right.
The "400W" spec is for the birds, all you can get with very stable +44V (or so) single supply, bridged output, is 40V peak, or 28V RMS: 100W into 8 ohms, period.
Anything else they write comes from a Poet´s pen (or keyboard), , not related to any Engineering reality.

Quote:
I am running the amp like this for testing: Car battery to amp/voltage display to 42v step up converter to amp. The supply is a 1200w dc-dc converter and I have left the current unlimited.
The output from the processor is at 3.16v p to p, I think I have to divide this by two to get the voltage of 1.58 . The amp can take 1.8v so I think I can drive a little harder here.
3.16Vpp means 1.1V RMS .

Quote:
I am running at 42v because there seems to be a few datasheets around for this model stating slightly different upper limits but I might be able to squeeze it to 45v. The power supplies I have tried seem pretty steady but I would like to leave a little buffer just in case.

The most I have been able to get the amp to draw without clipping is around 2.2a @ 12.8v or 28.16w on the ds18 driver.
100W RMS should take about 150W from the battery (considering amp efficiency, converter and wiring losses) so at clipping into an 8 ohm load, you should be pulling 150W/12.6V=12A .

Very far from what you measure, proper procedure is to drive amp with continuous 1kHz (or 400/440Hz) tone, rise volume/signal until you reach clipping into the load and back down a little so it just stops clipping.
To get the >1VRMS drive signal, you´ll need some kind of preamp or mixer or get it from a DAC which often puts out up to 2V RMS but check specs.
Quote:
I am well aware that companies massage their figures to boost sales but this seems like a massive difference. Is there a ballpark formula to tell me what an amp will draw from its stated specs?
Massage indeed but I suggested an experience based formula (I commercially build and sell street busking systems)
Quote:
To be fair I am pretty happy with the output, it's clean and loud enough for my purpose but having to wait for the board to arrive and be tested before buying an appropriate power supply is slow and more than a bit annoying. I really would like to know how all this is calculated before buying the amp for the bass box.
See above
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Old 7th September 2017, 04:35 PM   #5
Stuntmunky is offline Stuntmunky
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Sounds like you da man I need to speak to :-)

I should probably explain the ins and outs of what I do and how I intend to use this system.

I am a street performer rather than a musician, my show lasts around half an hour and will only be playing one song during crowd build and setup and the rest of the time its just me talking. The additional boxes are for when I do night time booked fire shows where higher fidelity and longer play time are needed. Even then though there is probably only 20mins of full power audio during the show and I am unlikley to do more than 2 shows at a single gig.

The car battery is just for the test setup, I wanted to get a handle on how much power I am drawing before matching a lithium battery to the box. And yup they are damn heavy, I know that all too well, so are the sealed type.

This is my current system made from car parts with a 7.2ah sla battery and comes in at around 16 kg, it has a passive crossover for the mid and highs. The drivers are the same as the ones outlined above, a 6o5nd and a 12" eminence. Its not a bad system and luckily I had the bits for the new system without having to break it up.
https://i.imgur.com/os9YerG.jpg

It's more than I need for the most part though, and by ditching the heavy passive heatsink amp, 12" bass driver and switching to lithium I have it down to around 5kg and a backpackable size 22x22x15 cm
Imgur: The most awesome images on the Internet

I have enough cells to make up two batteries for this system. I also don't really want to be charging onboard with a diy pack, so it will be removable. Charging to be done with a decent balance charger under supervision.

Interesting about the input, I'll take a look into a pre amp, I do have a tone board which does amplify a little but I'll have to check its specs.

An experience based formula? sounds expensive haha but I guess I'm down the rabbit hole now.

Thanks for the info, and good to know about the test, I am reading learning and watching almost constanly at the mo, it's great to get a push in the right direction

Last edited by Stuntmunky; 7th September 2017 at 04:40 PM.
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Old 7th September 2017, 04:48 PM   #6
Pano is offline Pano  United States
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Calculating power consumption from iffy specs
I used to see guys in London and Paris busking all the time with little guitar amps run off small SLA or motorcycle batteries. They were plenty loud. Mostly they used those little fold up shopping carts to haul the gear. We used to busk with a large boom box, and that was plenty. But we weren't trying to blast around an entire city square - maybe you are.

You are on the right path by doing the testing and measuring. That's going to get you the closest estimates to real use. As Jim says, you'll get about 100 watts out of an amp running on 42V. Or about 200 with a 4 ohm woofer. From there it's Ohm's law to figure out the max current needed. Your tests so far show you using about 1/3 that full amount. That's your range - 1/3 to full on.
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Old 7th September 2017, 07:11 PM   #7
Stuntmunky is offline Stuntmunky
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Thanks guys that gives me a ballpark figure.

For the most part I won't need too much power but occasionally I get pitched next to a bouncy castle generator, etc. it would be useful to be able to go up to 11 on those occasions :-)
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Old 7th September 2017, 07:19 PM   #8
Stuntmunky is offline Stuntmunky
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Sorry to ask again as you guys have already been very helpful and I will read into it on my own but what are your thoughts on running the grounds together?
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Old 13th September 2017, 10:23 PM   #9
Pano is offline Pano  United States
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Calculating power consumption from iffy specs
Do you mean the negative speaker terminals? Generally this is not done on Class-D amps because the left and right oscillators aren't synchronized. But with some chips this is not a problem. Just remember that there is always a DC offset from ground on these single supply amps, so you can't tie the speaker negative terminals to ground.

You can try tying the negatives together, the chips have built in protection. Just be sure to monitor the temperature, to look for signs of stress.
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Old 14th September 2017, 07:40 AM   #10
Stuntmunky is offline Stuntmunky
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Hi Pano, I actually meant the dc side but I think I jumped the gun a bit with the question. I need to do some more testing and try to diagnose where noise is coming from. I am currently staying with my step dad for a few days, he is helping me design a small op amp to take my signal up a little for the main amp. I will be re-wiring and testing soon but it might be a little while as I will have to travel to make some more money :-( Hate it when real life gets in the way of projects
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