Class D amp current 5x less than expected - Page 1
Forum section: electronics / beginners

author: Mark_M
author_rank: Newbie
timestamp: April 11, 2025, 08:25:14 am
content: Hello all, first post on here.I am sourcing a new 24v DC power supply for a speaker amplifier. The measured current is 500mA, which is far lower than the unit's input current label of 2.5A.The class D amplifier is an AV Gear AVG-MA3. A 40w 70/100v mono speaker amplifier with inputs for mic (48v phantom, preamp, line-in), RCA, 3.5mm, optical in.https://avgear.com.au/shop/audio/avg-ma3/When I measured current from my bench power supply it drew ~100mA at idle and max I've measured is 500mA (12w). The max current draw was measured while using 3.5mm input, volume/treble/bass to max and a 30W 100v speaker connected.The specifications (website manual) say "5W". I assume this must be 5W for the unit itself. 45W with the speakers connected. That equates to 1.875A [45w / 24v].But the unit says "MAX 2.5A", which would be a 60W power supply.Supposedly the included power supply would have been 2.71A (65W).Is there some 'magic' with Class D amplifier to measuring their power draw? I sadly do not own an oscilloscope to measure through a current shunt.I've got plenty of 24v 2.1A PSU I can use it with. But don't know for the long run if that would slowly overload/overheat/die.This unit appears to be a rebrand, also known as these:RTI AMV-340:https://www.rticontrol.com/amv-340-single-channel-70v-100v-mixer-amplifier.htmlIntelix AUD-340: (I cannot find a manufacture web page), only pdf manuals on other sites.KanexPro AP3DBL:https://kanexpro.com/item/AP3DBLThanks,Mark.

author: Whales
author_rank: Super Contributor
timestamp: April 11, 2025, 08:40:31 am
content: What is the impedance of your speakers? The manual specifically states "40W @ 8 Ohms", which means 8 ohm speakers. Higher impedance speakers will significantly reduce the maximum power output.Do the speakers sound like they are clipping when you are driving them at max volume? If they are not clipping then you are probably not feeding in a strong enough signal. Max power specs are usually only obtainable at a notable distortion level.> I've got plenty of 24v 2.1A PSU I can use it with. But don't know for the long run if that would slowly overload/overheat/die.If you don't run the amplifier at max power then they will likely work fine.EDIT: Just be wary of some amplifier designs that expect isolated power suppliers. It's not common, but it does happen, especially if the old power supply was a heavy old-fashioned transformer. I blew one up once after replacing its power supply with a (ground-referenced) switchmode type and then plugging it into my (ground-referenced) computer.

author: temperance
author_rank: Super Contributor
timestamp: April 11, 2025, 08:55:19 am
content: 2.5 A input current for 40 W output power when powered by 24 V sounds about right to me.https://www.redbackaudio.com.au/understanding-100v-line-distributed-speaker-systems/QuoteIf you measured this when playing music, you will never reach the stated 2.5 A because of the peak to average ratio or crest factor of music.https://www.izotope.com/en/learn/what-is-crest-factor.html?srsltid=AfmBOorxdVU4M_tX-7kKo_veXH0Ojoxgn9V2HO9VAkMQSeZJbFZCcGRlIf you would do the test again with a signal generator you will be able to reach 1.6 A continuous or about 2.5 A peak. However, the speaker might go up in smoke before you reach 1.6 A because speakers aren't rated for continuous power.

author: Mark_M
author_rank: Newbie
timestamp: April 11, 2025, 08:55:52 am
content: I've tested so far with a single TOA BS-1030W connected. With the dials set to 100v, 30W it is 330 Ohm.https://www.toaelectronics.com/en-us/products/detail/bs1030wIt seems my speakers do not have the option of 8 ohm while also being at 100v level. Attached image from speaker's manual.I did get to a level of distortion while testing. Brief distortion, not constant to truly measure.

author: temperance
author_rank: Super Contributor
timestamp: April 11, 2025, 09:10:21 am
content: To be on the safe side, just buy some 40 W power supplies and you will do fine. You could get away with a smaller power supply because of the crest factor of music. But in that case you must verify if the power supply can handle the required peak current without going into overload. (some power supplies can do that)Edit: note that you can not simply measure the impedance of the speaker on the 70 V or 100 V line by just measuring resistance. If you do so you will the DC resistance of the transformer winding.

author: Mark_M
author_rank: Newbie
timestamp: April 11, 2025, 09:12:50 am
content: Quote from: temperance on April 11, 2025, 08:55:19 amThank you. I'll take that as okay enough to try.40W RMS ~= 56.5W peak [PeakW = WattRMS / 0.707]. I was dumbfounded by it drawing ~12w and distorting a 30w speaker.I'll see how I go for a while with the 2.1A PSU I got. It's an Espson receipt printer PSU [Epson PS-180].But I'll look to find a 60w 24v DC adaptor online.It's only being used 30 nights a year for my Christmas lights.Thanks guys.

author: Mark_M
author_rank: Newbie
timestamp: April 11, 2025, 09:15:59 am
content: Quote from: temperance on April 11, 2025, 09:10:21 amMakes sense. Inductance/impedance happens with AC having a frequency component.I've been measuring the 24V DC input to the unit. Not the speaker output.Internally the unit has a step up transformer, and a step up voltage conversion for a 48v phantom power microphone.

author: bson
author_rank: Unknown
timestamp: April 12, 2025, 12:11:43 am
content: Quote from: Mark_M on April 11, 2025, 09:12:50 amThis is true for a sinusoidal, but it's NOT true for anything else. Most definitely not for music! Instead 1.414 you need to multiply by the music's crest factor, as mentioned. 7 is a reasonable music crest factor, but 10 is better in that it maintains some minimal headroom, which reduces distortion (due to gain compression); once you get closed to absolute maximums linearity drops quickly.

author: bdunham7
author_rank: Super Contributor
timestamp: April 12, 2025, 12:41:38 am
content: Sorry if I missed this somehow, but how (exactly) are you measuring the current?

author: temperance
author_rank: Super Contributor
timestamp: April 12, 2025, 01:35:27 am
content: Quote from: bson on April 12, 2025, 12:11:43 amYou might want to read what other people wrote.

author: Mark_M
author_rank: Newbie
timestamp: April 12, 2025, 03:25:52 am
content: Quote from: bdunham7 on April 12, 2025, 12:41:38 amMeasuring the amplifier's 24v DC input current draw.Powering off my (very old) Wonix YW-APS305 transistor power supply. Measured current with my Fluke 23 and a UT133A Uni-T multimeter.Bench power and the two multimeters all showed current 100mA - 500mA, at 24v DC.