2 power supplies for a amp

[vol.2]

Hello,

I am looking for a power supply solution to an audio amplifier board which requires +/-24VDC. Most symmetrical power supplies seem to be very expensive, but I thought maybe I can buy two of these cheaper ones?

https://www.digikey.com/products/en?keywords=lrs-100-24

One interesting note about this amp: although it is a single output amp, it seems to have four output terminals. They are two for ground and the other two are labelled +24V and -24V. I looked at the data sheet, and they are definitely single output, so I wonder also why they have this option.

Thanks,

[Zero999]

That power supply will probably do the job, but you forgot to post the data sheet for the amplifier, so we don't know how much current it requires.

Another option is to use two 24V supplies in series and take the 0V from when they join. Connecting a couple of diodes in parallel with each of them is a good idea to prevent either supply from seeing to higher negative voltage, if it current limits or shuts down. The diodes should be rated to a bit more current, than the power supply's rating.

Having more than one 0V connection is pretty common.

[soldar]

It is a single output PSU and you would need two.

[vol.2]

That power supply will probably do the job, but you forgot to post the data sheet for the amplifier, so we don't know how much current it requires.

(Attachment Link)

Thanks for your reply.

I am sorry I forgot the amplifier! This is what I see, there is a circuit schematic in the pictures. https://www.ebay.com/itm/20W-HIFI-Mono-Channel-LM1875T-Stereo-Audio-Amplifier-Board-Module-DIY-Kits/182588636277?hash=item2a83217c75:g:6OYAAOSw42JZJAU4

I plan to use 2 of these together and use some decent heat sinks. So I can use two of the 24VDC meanwell and wire two of these up?

Thanks,

[mariush]

You have another option, to actually buy a toroidal or regular transformer with two secondary windings or one winding with center tap, then use a bridge rectifier and two capacitors to get your +24v and -24v.

Your amplifier probably works with less than 24v and -24v but naturally, will output less audio power.

[vol.2]

You have another option, to actually buy a toroidal or regular transformer with two secondary windings or one winding with center tap, then use a bridge rectifier and two capacitors to get your +24v and -24v.

Thanks for the idea, but I'm kind of new to this, so I think that may get too confusing.

I think what I want to do is hook up two of these small amp boards:

https://www.ebay.com/itm/20W-HIFI-Mono-Channel-LM1875T-Stereo-Audio-Amplifier-Board-Module-DIY-Kits/182588636277?hash=item2a83217c75:g:6OYAAOSw42JZJAU4

To two of these power supplies:

https://www.meanwell.com/Upload/PDF/LRS-100/LRS-100-SPEC.PDF

To power two small amp boards using 2 PSUs, can I split the +24VDC from one supply to both amp board's +24VDC input, and then take the -24VDC output from the other supply to both -24VDC inputs?
Is this safe and correct?

[mariush]

Ok, cool.

So let me give you some more information.

That LM1875 is a class AB audio amplifier chip, which means it's kind of inefficient and therefore will produce A LOT of heat. It MUST be connected to a big heatsink. If you want to get 20 watts per channel, you're going to need a CPU style heatsink with a fan.

Here's the datasheet for this chip : http://www.ti.com/lit/ds/snas524a/snas524a.pdf

Look at page 4, figure 5:



This tells you that with a 8 ohm speaker, the amplifier can function with as little as +/- 10v (the x axis), but the maximum output power will only be around 5w (the y axis). Following the line, you can see it does ~10w at +/-15v, a bit more than 15w at +/- 20v and nearly 25w at +/-25v.
With 4 ohm speakers, the output power will be slightly higher.

So what you need to get from this is that you don't necessarily need two 24v power supplies, especially if you don't have 15-20w speakers.

On the same page, you have figure 8 : Device Dissipation vs Ambient Temperature and this picture must be used together with Figures 9 and 10 on the next page

 

So let's say you want to power with +/- 24v ... we'll look on the +/- 25v curves.
If you have a 4 ohm speaker, you'll look in the Figure 9 chart - you can see that to produce 15w of audio power, there's gonna be a bit over 30 watts of heat produced.
If you go with 8 ohm speaker, you'll look in the Figure 10 chart - you can see that to produce 15w of audio power, there's gonna be nearly 20 watts of heat produced.
This heat (20..30w) must be dissipated safely... and that's what Figure 8 shows you.

For safety, assume the ambient temperature (the temperature of the box where the amplifier will be) will go up to 40 degree Celsius, so you go on the x axis at 40 degrees C and then you go up.

You can see that the bottom two heatsinks (rated for 10c/w or 5c/w) are not good enough.
The next curve shows a heatsink rated for 2c/w and it's just barely good enough if you'll do 15w of audio power with 8 ohm speaker (which dissipates nearly 20w)
To safely dissipate the 30w of heat you'd produce to get 15w of audio with 4 ohm speaker, you can't even use a 1c/w heatsink on all its own - you'll need to supplement it with a fan to blow air over it and help cool it.

As an idea, this is the kind of heatsinks you'd need for 10w+ of heat dissipation : https://www.digikey.com/short/p2mv1q
The link above shows heatsinks with < 3c/w natural convection (<1..2c/w with fans)

So my advice would be to first decide on the speakers you're gonna use, and ideally use 8 ohm speakers as these amplifiers will dissipate less heat with such speakers.
If you're gonna have a couple of 5w or 10w speakers, there's no point to power this amplifier with +/- 24v, because you can produce 10w with just +/- 15v, and you're gonna produce less heat.

Making a power supply using a transformer is very easy, here's a schematic :

 

The unlabeled resistors should be something around 4.7kOhm - 10 kOhm and rated for 1w ... basically, they're there to discharge the big capacitors when the amplifier is not in use.
The bridge rectifiers should be something like 6A or higher... they're very cheap...here's a 8A one for 1.25$ : https://www.digikey.com/product-detail/en/micro-commercial-co/GBJ806-BP/GBJ806-BPMS-ND/8554715

So let's say you decide you want 15w of audio power and you have 8 ohm speakers. In that case, +/-20v is gonna be enough but we can go with +/- 25v curves.
Looking at Figure 10 chart, at 15w of audio, the +/-25v curve tells you there's gonna be nearly 20w of heat produced. So, in total, the audio amplifier needs 15w audio + 20w heat = 35w of power to achieve what you want.
For stereo sound, you'll need double this, plus some reserves... so you need something that can give you minimum 70w...

Now you can go look for a transformer with either two secondary windings or center tap, which outputs 15v AC or more on each secondary winding, or 30-36v AC if transformer has a single secondary winding with center tap

Here's some examples with center tap

21$ : 28v AC 100w : https://www.digikey.com/product-detail/en/triad-magnetics/F8-28/237-1722-ND/4878651

21$ : 36v AC 100w : https://www.digikey.com/product-detail/en/triad-magnetics/F8-36/237-1723-ND/4878652

32$ (with wires) 35v AC 140w : https://www.digikey.com/product-detail/en/triad-magnetics/F-191U/F-191U-ND/7318079

With dual secondary windings :

21.4$ 2 x 12v 80w : https://www.digikey.com/product-detail/en/triad-magnetics/VPS24-3300/237-1276-ND/666162
21.4$ 2 x 14v 80w : https://www.digikey.com/product-detail/en/triad-magnetics/VPS28-2800/237-1280-ND/666166
21.4$  2 x 18v 80w : https://www.digikey.com/product-detail/en/triad-magnetics/VPS36-2200/237-1285-ND/666171

28$ 130w
2x12 https://www.digikey.com/product-detail/en/triad-magnetics/VPS24-5400/237-1277-ND/666163
2x14 https://www.digikey.com/product-detail/en/triad-magnetics/VPS28-4600/237-1281-ND/666167
2x18 https://www.digikey.com/product-detail/en/triad-magnetics/VPS36-3600/237-1286-ND/666172

36$ 175w
2x12 https://www.digikey.com/product-detail/en/triad-magnetics/VPS24-7300/237-1278-ND/666164
2x14 https://www.digikey.com/product-detail/en/triad-magnetics/VPS28-6250/237-1282-ND/666168
2x18 https://www.digikey.com/product-detail/en/triad-magnetics/VPS36-4800/237-1287-ND/666173
 

[wraper]

That power supply will probably do the job, but you forgot to post the data sheet for the amplifier, so we don't know how much current it requires.

Another option is to use two 24V supplies in series and take the 0V from when they join. Connecting a couple of diodes in parallel with each of them is a good idea to prevent either supply from seeing to higher negative voltage, if it current limits or shuts down. The diodes should be rated to a bit more current, than the power supply's rating.

Having more than one 0V connection is pretty common.
(Attachment Link)

You need to be cautious with that. Not every PSU will be fine with that, some may have resistor to earth from negative output or even direct connection. Connecting such PSUs in series may cause magic smoke escaping.

[Audioguru again]

You selected "switching" power supplies that will probably produce low frequency beats and high frequency whining interference. For audio use "linear" supplies instead.

[vol.2]

Thank you so much for all of the help! Maybe I can do a transformer after all.

So what you need to get from this is that you don't necessarily need two 24v power supplies, especially if you don't have 15-20w speakers.

The speakers I have installed in that room are Bose 141. https://assets.bose.com/content/dam/Bose_DAM/Web/consumer_electronics/global/products/speakers/141_series1/pdf/owg_en_141_series1.pdf

They are widely believed to be 4-6 Ohm, but you know how Bose is about specs! They claim to 'handle' 10-80 watts, but can only deal with 40 watts continuous power. (it says on the back of the case) I picked them up at a yard sale cheap like 20 years ago. I was pairing them to a Aiwa P22 which outputs 30 watts per channel (which I also got at a yard sale) but it broke. I generally never cranked it up past half-way because it was so loud, but it never distorted or clipped at higher volumes.
So, as far as power goes, maybe 25 watts per channel isn't crazy?

Unfortunately, I'm not sure 100% about the impedance because I was using a Mission M2Sas subwoofer that has a cross-over inside of it. I plug in the speaker output from power amp into the subwoofer and there is speaker output after the cross-over. I don't know how this effects impedance, but I would like to keep using it. If I have to, I could get some kind of pre-power amp cross-over, if the advice is not to use the subwoofer crossover?

The next curve shows a heatsink rated for 2c/w and it's just barely good enough if you'll do 15w of audio power with 8 ohm speaker (which dissipates nearly 20w)...

So my advice would be to first decide on the speakers you're gonna use, and ideally use 8 ohm speakers as these amplifiers will dissipate less heat with such speakers.
If you're gonna have a couple of 5w or 10w speakers, there's no point to power this amplifier with +/- 24v, because you can produce 10w with just +/- 15v, and you're gonna produce less heat.

Okay. It took me awhile, but I think I follow the graphs mostly. The one thing I don't understand is why the better heatsink is a lower temperature. I would think that 1C/watt would be less heat dissipation than 10C/watt, but the chart shows it the other way around. The temperature value gets smaller until you reach "infinite heatsink." I think I need to better understand the metric to properly order one, right?

Making a power supply using a transformer is very easy, here's a schematic :

Okay, I think I can follow this, but I need time to digest it a little. Also, I think I should figure out what I'm aiming for before I run completely through to spec out the transformer.

Thanks again for your help! 

[mariush]

If you are willing to go with a class D amplifier, you have plenty of choices on eBay.

For example, this one is ~25$ : https://www.ebay.com/itm/Mini-Class-D-Power-Amplifier-HiFi-Stereo-2-0-Channel-Digital-Audio-Amp-50W-50W/262315996089

It uses a TPA3116D2 : http://www.ti.com/lit/ds/symlink/tpa3118d2.pdf

which can do :

2 × 50 W Into a 4-ΩBTL Load at 21 V
2 × 30 W Into a 8-ΩBTL Load at 24 V
2 × 15 W Into a 8-ΩBTL Load at 15 V

Has the added bonus of a volume knob, and you can power it using a plain wallwart adapter. I think it has a standard 2.5mm barrel jack... either 2.1mm or 2.5mm

19$ gets you an 18v 60w adapter: https://www.digikey.com/product-detail/en/mean-well-usa-inc/GST60A18-P1J/1866-2151-ND/7703714
34$ gets you 24v 90w : https://www.digikey.com/product-detail/en/mean-well-usa-inc/GST90A24-P1M/1866-2157-ND/7703720
44$ gets you 24v 120w: https://www.digikey.com/product-detail/en/delta-electronics/DPS-120AB-3-B/941-1788-ND/6561830

[vol.2]

If you are willing to go with a class D amplifier, you have plenty of choices on eBay.

For example, this one is ~25$

Thanks for your alternative suggestion.

I also considered a class D unit. I watched some various tear-downs of the cheap ebay stuff on youtube.
About the particular unit you linked to, this one review claims that the choice of output filter coils is optimized for 8 ohm loads and that lower impedance loads experience roll-off in higher frequencies. If you see at 12:33:


However, there is this other board here: https://www.ebay.com/itm/392072722443?ul_noapp=true

That board seems to be a little better and draws about 1.75A at 20VDC:



I could probably use a laptop supply with it. I have an orphaned toshiba laptop PSU that does 19VDC 6A. Maybe I should just get two of those mono D boards and split the laptop PSU to both of them?

Thanks,

[Zero999]

That power supply will probably do the job, but you forgot to post the data sheet for the amplifier, so we don't know how much current it requires.

Another option is to use two 24V supplies in series and take the 0V from when they join. Connecting a couple of diodes in parallel with each of them is a good idea to prevent either supply from seeing to higher negative voltage, if it current limits or shuts down. The diodes should be rated to a bit more current, than the power supply's rating.

Having more than one 0V connection is pretty common.
(Attachment Link)

You need to be cautious with that. Not every PSU will be fine with that, some may have resistor to earth from negative output or even direct connection. Connecting such PSUs in series may cause magic smoke escaping.

Yes, that's very true. The original poster should check the power supplies are floating first, before connecting their outputs in series.

[vol.2]

Yes, that's very true. The original poster should check the power supplies are floating first, before connecting their outputs in series.

Thanks. I checked the data sheet and it does look like the - side is tied to ground.

Can you tell me if i can split a 19v switching laptop supply into two so I can power two mono amp boards like the one I linked above?

[Zero999]

Can you tell me if i can split a 19v switching laptop supply into two so I can power two mono amp boards like the one I linked above?

I'm not sure what you're asking.

When you mean split, a 19V supply, which would give +/-9.5V, or use two 19V supplies for +/-19V?

Splitting a single 19V supply is possible with a virtual earth/ground circuit, but if you want to connect it to external devices, with grounded chassis, you need to check the 19V supply is floating.

Connecting two 19V supplies in series for +/-19V is only possible if they're floating. Laptop power supplies vary. Some are floating, others are not. One thing you can do is check what sort of mains connector, the power supply uses. If it's three pole, it will have an earth connection and might not be floating. If it's two pole, then there's now way it's grounded and has to be floating.

[dazz]

Yes, that's very true. The original poster should check the power supplies are floating first, before connecting their outputs in series.

Thanks. I checked the data sheet and it does look like the - side is tied to ground.

Can you tell me if i can split a 19v switching laptop supply into two so I can power two mono amp boards like the one I linked above?

If you're referring to the class D TPA3116s that mariush suggested, I think that's the best way to go. They're a lot more efficient and you will save a lot of trouble, money and energy with those by simplifying the power supply. Not sure what you mean by splitting the power supply to feed two boards, what you would do is to use a singly power supply for both boards, but a laptop brick will probably not  have enough output current for two TPA3116s with 2 ohm speakers.

Do you really need that much power? Because a single stereo TPA3116 board can output 40W+40W with 4 ohm speakers and a single 20V 5A laptop brick

Keep in mind that 100W is not twice as loud as 50W.

[vol.2]

If you're referring to the class D TPA3116s that mariush suggested, I think that's the best way to go.

I'm referring to this mono board: https://www.ebay.com/p/13016014274

I would two of them for stereo.

Not sure what you mean by splitting the power supply to feed two boards, what you would do is to use a singly power supply for both boards, but a laptop brick will probably not  have enough output current for two TPA3116s with 2 ohm speakers.

I have Bose 141 speakers, they are ~6ohm.

I mean I want to use my 19V power supply to feed both mono boards. I've got a few around, one of them says 4.74A. There was this review that tested the TPA3116 mono board on youtube, and he shows that the boards take about 1.75A each at 20VDC under a 4 ohm load. I am thinking I could get away with splitting the power from the one 4.75A laptop supply to both mono boards. Does this sound right, or am I missing something?

Thanks,

[vol.2]

I meant to split the one 19VDC laptop supply to power 2 of the mono TPA3116 boards because I need to power 2 of them for stereo. I don't know if that's safe.

Thanks,

[dazz]

I have Bose 141 speakers, they are ~6ohm.

Couple of things:
1. Those are 40W speakers, so having a 100W+100W amp makes no sense, unless you want to blow your speakers.
2. If you're gonna be using 6 ohm speakers, you'll never get 100W from one of those boards. The 100W output assumes 2 ohm loads. So you're much better off with a single stereo (BTL, not PBTL) board and a single power supply.

[mariush]

It would be safe.

Note that class D power amplifiers like these TPA ones are more efficient, let's say up to 85-90%... compared to class AB that are ~50% or even less.
So if the guy measured the power consumption at 20v 1.75A, that's around 35w in, so assume around 25 watts of actual audio output. Basically, he ran the amplifier at around 2x12-15w.
How much power the amplifiers will use will depend entirely on how much you amplify everything, on your volume knob position, how per-amplified your audio signal is and to some degree depends on the audio speakers and their impedance...

Yes, there's no reason why you couldn't run two mono amplifiers on a single such power supply. Just keep in mind that at very high volumes the two amplifiers may draw high current for brief moments of time and that could cause the power supply to enter in "over current protection" or restart itself ... so you may get "hiccups" (power supply turning on and off)

edit :  I recommend going with my version because you get it in a nice package, with volume control, dc in jacks, input and output connectors and everything.
It's also in the standard configuration of 50w + 50w max, so chances are you're not gonna damage your speakers if you accidentally set the volume to maximum ...
With two of the other (same chip but configured in BTL mode, mono) you may accidentally damage your speakers and you'll also have to do extra DIY (case, add volume control etc etc)

[vol.2]

Thanks for your input. 

It's also in the standard configuration of 50w + 50w max, so chances are you're not gonna damage your speakers if you accidentally set the volume to maximum ...
With two of the other (same chip but configured in BTL mode, mono) you may accidentally damage your speakers and you'll also have to do extra DIY (case, add volume control etc etc)

I have a passive volume control with four inputs that goes into the power amp. This will be replacing the power amp in a system I've been using for awhile. Yeah, I guess I'd need a case eventually.

having a 100W+100W amp makes no sense, unless you want to blow your speakers.

According to the review anyway, this actually maxes out at 50W at 4ohms. Maybe they are only using one channel?If the 141's are 6ohm, then at 19VDC, it should be more like 27W or so? Or is my logic flawed here? Am I crazy?

[dazz]

Those figures are pretty close to the spreadsheet graphs at 4 ohms, probably at 0.1% THD or something like that.
At 19V and 4 ohms he's getting about 32W so at 19V and 6 ohms you would get up to 21W if we extrapolate those figures. That's for a single channel.

The way that chip works, you can have it configured in BTL stereo mode so that it's a two channel amp that you can drive at 2 x 4 ohms for a maximum output of 50W + 50W, or PBTL mono that combines both channels in a single one that you can drive at 2 ohms, 100W. But again, I don't see the point in getting PBTL mono boards if you're going to be using 6 ohm speakers since the whole point of PBTL is to lower the load impedance to increase the output power. There are also dual PBTL boards like this one: https://www.ebay.com/itm/TPA3116D2-2x120W-Digital-Power-Amplifier-Board-DIY-Kit-Class-D-Dual-Stereo-Audio/202786866259. That's two TPA3116's in a single board for a maximum of 100W + 100W.

That might be a good option if you want to take advantage of PBTL in the future with 2 ohm speakers, but be careful not to blow your current ones.

[vol.2]

I ended up purchasing the stereo version of the TPA3116 board. I'll use a 19VDC laptop supply and it looks like the max I can push with my 6 ohm speakers will be around 25W a side. I am thinking it will be just about right.

I'll post once I get it in the mail and set it up in case anyone who read this thread  wants to know what happened.

[vol.2]

I got the little stereo version of the TPA3116 board installed and working in my setup.
https://www.ebay.com/itm/TPA3116-TPA3116D2-120W-120W-Dual-Channel-Stereo-Digital-Audio-Amplifier-12V-24V/392072658051?hash=item5b4959fc83:g:5mgAAOSw3ChbMrPQ:sc:USPSFirstClass!20905!US!-1

It was pretty easy to get going; I used a 19V 4.8A laptop supply and I jumped the little gain pots on the board because they looked really cheap and I'll never use them. (I also tied the wiper back to the left side of the pot with a resistor the value of the pot in order to keep the impedance identical)

It sounds good and clean, but it seems to me that my sound now lacks the energy in the mid range that my setup had with the previous power amp. I'll be repairing the old amp and put it back, but for now this will work.

One last question here, in case someone can help me! When I turn the stereo on or off (I use a power strip) there is a pop. Is there some way I can remove this pop by adding some components?
This is the board in question:

Thanks,

[fourfathom]

1. Those are 40W speakers, so having a 100W+100W amp makes no sense, unless you want to blow your speakers.

It's not uncommon to use an amplifier capable of delivering more power than the speakers can handle.  When running a lower-power amp near its limits you can get clipping of any transients which can produce tweeter-destroying amounts of high-frequency power.  Obviously trying to run 100W into a 10W speaker will also cause problems, but it's not a bad idea to give yourself some head-room when choosing an amplifier.