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Old 24th May 2018, 06:16 PM   #11
THRobinson is offline THRobinson  Canada
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Is there a calculator or chart or something for the whole Ohm thing?

It's probably simple and just hasn't "clicked" in for me yet, but still not getting how to know what numbers go up/down when I have a speaker listed in Xohm and an amp listed in Yohms.

Seems that the speakers keep showing as 4, 8 or 16ohm, and amps 6ohm.

@PRR I hear what you're saying, and ya, I was getting things mixed around. Found a mono 60w6ohm amp, and in another forum, a few mentions of a Warehouse G12C 75w as being a good speaker for modelling, or possibly a Celestion Seventy 80 80w.

60w amp on a 75-80w speaker should be good. Now... how does the ohm factor in here? amp this time is 4ohm.

Sure Electronics' webstore 1 x 60 Watt Class D Audio Amplifier Board -TPA3118

@Gnobuddy - Saw that app for Linux, but not too impressive. Plus, Linux... I've never figured that thing out. Seems like the hard way to do stuff But sometimes you get some good IMG files for it. I have KODI now running off OpenElec for my media server, set itself up basically, same as RetroPie for arcade games. I tried Ubuntu on my Laptop, and didn't seem to matter which tutorials I used and how to-the-letter they were followed, nothing seemed to work right.
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Old 24th May 2018, 10:32 PM   #12
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by THRobinson View Post
It's probably simple and just hasn't "clicked" in for me yet, but still not getting how to know what numbers go up/down when I have a speaker listed in Xohm and an amp listed in Yohms.
First thing to know: in your example above, if "Y" is lower than "X", the amp will not fry itself.

In other words, a (solid-state) amp rated for a 6 ohm load will happily drive an 8 ohm load. But it may not be happy with a 4 ohm load; at best it will shut down to save its own behind, at worst, it will die a sudden death.

Second thing to know: As long as the amp is operating within its capabilities, power delivered to the speaker depends on both the DC voltage you supply to power the amplifier, and on the speaker impedance. More voltage equals more power; more speaker impedance equals less power.

The "100W @ 2 Ω" Sure amp you linked to earlier will work with 2,3,4,6,8, and 16 Ω speakers. Powered by the recommended 21V DC power supply, it will only deliver the rated 100 watts (at unusably high distortion) to a 2 Ω speaker. You can safely connect 4 or 8 Ω speakers, but, as printer2 said, you will get only half or a quarter as much power, respectively: around 25 watts RMS if you use an 8 Ω speaker with a 21V DC power supply, and still at unusably high distortion.

As an aside, the continual misrepresentation of the actual power ratings of these class D chips seems distinctly unethical to me. Again, as printer2 said, this is not really a 100 W amp, even with the rated 2 Ω load and 21V power supply; nobody in their right mind would want to listen to a solid-state amp clipping hard enough to reach 10% distortion. Class D amps sound particularly unpleasant when clipped.

Want some simple math to give you a good honest ballpark answer as to how much power you'll actually get from any of these amps (assuming, as always, that the amp is operating within its ratings)? Here you go:

1) Subtract 3 volts from the power supply voltage.
2) Square the answer from step 1.
3) Halve the number you got in step 2.
4) Divide this by the speaker impedance in ohms. The number you just got is the maximum RMS output in watts, give or take a few percent.

Let's try that recipe with 21 volts and 2 ohms specified by Sure for their "100 W" amplifier board:

1) 21 - 3 = 18 volts
2) 18 x 18 = 324 (peak) volts squared
3) Half of 324 = 162 (RMS) volts squared
4) 162 divided by 2 ohms = 81 watts RMS power to the speaker.

Now let's look at the numbers on the Sure website:
Quote:
Originally Posted by Sure
Output Power:
100W@2Ohm 21V DC THD+N 10% (nonsense!)
87W@2Ohm 21V DC THD+N 1% (almost reasonable)
Forget the 100 watts number: only a deaf person would want to listen to a solid-state, class D amp deep enough into clipping to produce 10% distortion.

Now notice that the 81 watts I calculated is very close to the 87 watts that Sure specifies (within 7%, in fact.) Add in the fact that the "87 watts" number will vary a bit from one 21V power supply to another, and our quick math estimate is quite accurate as far as real-world numbers go.

There you go, you will never again be fooled by misleading (or outright dishonest) marketing lies about class D output power!

Quote:
Originally Posted by THRobinson View Post
Gnobuddy - Saw that app for Linux, but not too impressive.
Fair enough. I've never tried it, but it's the only game in town for guitar + Linux.
Quote:
Originally Posted by THRobinson View Post
Plus, Linux... I've never figured that thing out. Seems like the hard way to do stuff
I've been all-Linux at home since 2001, so 17 years now. IMO it is vastly better than the $@#!! that Microsoft offers, but horses for courses, and all that. If it's not your cup of tea, that's all there is to it.

Incidentally, I mentioned Linux because you talked about using a Raspberry Pi with your guitar - I assume you know that Linux is the only real game in town for the Raspberry Pi. I know MS has some kind of largely useless watered-down version of Win 10 they offer for the Pi, but I've never heard of anyone actually being able to do anything with it. It's not the same as the Win 10 that people are happily using on their Intel and AMD PCs and laptops.
Quote:
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I tried Ubuntu on my Laptop, and didn't seem to matter which tutorials I used and how to-the-letter they were followed, nothing seemed to work right.
Sorry to hear that. If I lived closer to you, there is a good chance I could probably get your laptop running happily on Linux, but sadly, I'm a few thousand kilometres away...

-Gnobuddy
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Old 25th May 2018, 01:52 PM   #13
THRobinson is offline THRobinson  Canada
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Thanks for all the info... I'll have to figure out what I'm after a bit more and do some math this weekend I guess. Not in a rush luckily, just, when I buy all the wood for building my arcade cabinet I plan to get enough to make a speaker cab at the same time.

Linux... I tell ya, I dunno what it is about it but even the simple stuff never works. I installed Ubuntu I think it was, maybe it as Mint... I have tried both, pretty sure it was Ubuntu that came with LibreOffice and I didn't want it, I was setting up a media server that day and tried Linux and even immediately after a fresh install and running through a YouTube tutorial for uninstalling LibreOffice, was getting file not found and can't find something else errors etc. Simple uninstall from a fresh installation wouldn't even work. That's when I found OpenElec, a stripped down Linux with Kodi preinstalled. Installed the IMG from USB to a small 16GB SSD drive and done, no setup needed outside of Kodi. Plus, I have an iR attached to my media server for the TV remote and couldn't find anything to make it work with.

Never tried it on the Pi... same deal, downloaded RetroPie's IMG file and installed and done.

Use to be a MAC guy for years so, some similarities since I guess based off the same OS, but, dropping to a cmd line to type in stuff for installations and such, was too much a pain.

I do plan on getting another Pi, and making a high quality audio player some day. They do have some nice app for that. Volumio I think was the name, and Rune... both seem pretty well developed. Had hoped by now, there'd be an opensource app similar to GuitarRig out by now. Simple IMG file, install to SD card and done.

Oh that amp I linked to before, I asked the company for advice on what would work...


For a piece of AA-AB32189 (2 x 100 Watt Class D Audio Amplifier Board - TDA7498), its typ value of power supply is 36V.
If the impedance of speaker is 8 ohm, its RMS of output will be about 75W.
I have its manual attached for your reference, please check.
If you use this amplifier, we will suggest you kindly choose following item as its power suplly:
Mean Well MW 36V 9.7A 350W AC/DC Switching Power Supply NES-350-36 UL SKU: PS-SP11156
Sure Electronics' webstore Mean Well MW 36V 9.7A 350W AC/DC Switching Power Supply NES-350-36 UL
If you have any need, please feel free to contact with us.
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Old 25th May 2018, 04:07 PM   #14
THRobinson is offline THRobinson  Canada
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Contacted Warehouse... asked what they thought would be a good speaker to use for this application that was relatively flat/neutral.

WGS 12" Reaper HP - 50W 8ohm - Canada

Not 75w but unlikely need that loud indoors anyways... but kinda went from 2x12 75w, to 1x12 50w..

I also read that it's their version of a Celestion G12H, which were on my short list.... until I saw the price.

So... if I take that 50W 8ohm Reaper HP, and use this amp.... 1 x 60w Class D... using the math above... 20.25w RMS power to the speaker? Bit underpowered.

So, reversing the math if I am looking for let's say 40w from the 50w speaker. I'd need 28v PSU... for a 50w speaker at 8ohm?

Last edited by THRobinson; 25th May 2018 at 04:30 PM.
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Old 25th May 2018, 05:06 PM   #15
Printer2 is offline Printer2  Canada
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It depends what you are doing with your combination. 20W with a decent 12" will be more than enough as a practice amp at home. If you are going to jam with a hard hitting drummer then around the 50W mark would be in order. But it also depends how clipped your signal is. If you are playing in the fuzzy range 20 W can be loud.
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Old 25th May 2018, 07:22 PM   #16
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by THRobinson View Post
...kinda went from 2x12 75w, to 1x12 50w..
One thing to keep in mind - most of the class D boards I've seen are stereo, so you only get the full output they're capable of if you have two separate speakers to hook up to them.

For example, the "100 watt" Sure board you linked to earlier, in combination with a 24 volt DC power supply, will actually only deliver about 25 - 30 watts RMS to a single 8 ohm speaker.

Forget 30 watts, even 17 watts RMS used to be considered beyond extravagance not so long ago ( 17 Watts?: First 20 Years of British Rock Guitar, the Musicians and Their Stories: Mo Foster: 9781860742675: Amazon.com: Books ).

That said, class D amps need to be kept entirely away from clipping, unlike valve guitar amps, so you will never get the full benefit of the 30 watts. And 30 watts is quite affordable, so why go with less?

Quote:
Originally Posted by THRobinson View Post
.... until I saw the price.
Everything about valve guitar amps seems to have turned into an expensive luxury now. In Canada, $150 for an "affordable" speaker, $75 for a pair of 6V6 valves.

Quote:
Originally Posted by THRobinson View Post
So, reversing the math if I am looking for let's say 40w from the 50w speaker. I'd need 28v PSU... for a 50w speaker at 8ohm?
Yes indeed!

(You will also have a second channel, capable of another 40 watts output, sitting there forlornly doing nothing whatsoever. If you have a touch of obsessiveness, like many of us DIY types, the thought of this will be extremely annoying, like that loose tooth when you were seven years old...)

Incidentally, I had little luck looking for a 40 volt switching power supply at the usual electronics vendors (Digikey, Mouser.) There seems to be a gaping void between 24V power supplies, and 48V power supplies.

I just took a look at the Sure website, and found only one power supply between 24V and 48V. That one is rated 36V and 450 watts (which is a huge overkill for your application), and it costs $93 (presumably USD.)

-Gnobuddy
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Old 26th May 2018, 12:36 AM   #17
dotneck335 is offline dotneck335  United States
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The TDA7492MV is a mono chip that will deliver 30 watts@1% THD with a 24 volt supply. This board is only ~$12:
DC12V-24V TDA7492 Mono Channel BTL Output 50W Power Amplifier Board Amp Module 699902423545 | eBay
Here's a 24 volt 150 watt supply for $22:
Mean Well MW LRS-150-24 24 VDC 6.5A 150W Regulated Switching Power Supply
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Old 26th May 2018, 01:08 AM   #18
Printer2 is offline Printer2  Canada
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You got to love the Chinese. Or it might just be Google Translate that needs some work.

Click the image to open in full size.

That aside, BTL, 30W@24V. 1PCS DC 12V-24V TPA3118 BTL 60W Mono Digital Audio Power Amplifier Board Module | eBay
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Old 28th May 2018, 08:56 PM   #19
THRobinson is offline THRobinson  Canada
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I grabbed one last year off eBay for that Arcade Cabinet project I mentioned, which I decided not to use. Grabbed a pair of really good 2.1 Logitechs with all the electronics and such for super cheap... seemed a better option.

Saw a review on it once where the caps blew. Cheap fake caps, so, I have a bunch of Nichicon caps to put in with higher V ratings. Just never happened.

Ah found it
DC 8V-25V TPA3116 2X50W HIFI Digital Amplifier Board Dual Channel 2.0 Stereo AMP | eBay

I will admit the 2x100 was bugging me if I used it for a single speaker. When I liked it though, I was debating a 2x12 cabinet. I've gone from 2x12 75w, down to possibly a single 50w.

WGS 12" Reaper HP - 50W 8ohm - Canada

Company (Warehouse Speakers) said this was probably the best one to get for a neutral sounding speaker.

50w wise, this is a 60w amp but also says

60W@4Ohm 24V DC THD+N 10%
50W@4Ohm 24V DC THD+N 1%

So for a 50w 8ohm speaker, this would be a good one? Possibly?

WONDOM 1X 60W Class D Audio Amplifier Board Mono -TPA3118 AA-AB31211 | eBay

plus a power supply.. 24v2A50w?

AC 110V-220V TO DC 5V 12V 24V Switch Power Supply Driver Adapter LED Strip Light | eBay

Plus the pre-amp mentioned a few posts back...

Now... ignorant question time... I see some listed like, 2x50w or 100w mono... does that mean you can combine the channels on the stereo amps? or just if the board is designed that way... like a jumper or something.
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Old 28th May 2018, 10:06 PM   #20
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by THRobinson View Post
60W@4Ohm 24V DC THD+N 10%
50W@4Ohm 24V DC THD+N 1%

So for a 50w 8ohm speaker, this would be a good one? Possibly?
I think it should work fine, delivering about 30 W @ 10% THD into 8 ohms.

10% THD is obnoxious, so probably more like 15 - 20 W before clipping starts (i.e. less than 1% THD.)

I've never been able to turn either of my push-pull 6V6 (nominally 15 watt) Fender amps up anywhere near maximum volume for home use. If you're like me, 15 - 20 W into a guitar speaker is probably more than you'll ever need at home.
Quote:
Originally Posted by THRobinson View Post
plus a power supply.. 24v2A50w?
Sounds about right for one 8 ohm speaker.
Insanely low price. Wow.
Quote:
Originally Posted by THRobinson View Post
I see some listed like, 2x50w or 100w mono... does that mean you can combine the channels on the stereo amps? or just if the board is designed that way... like a jumper or something.
Some let you put the two amps in parallel, so they can deliver twice as much current to the loudspeaker...but this only works if you get George Simon Ohm's blessing, which means you have to use a loudspeaker with half the impedance. Usually, that means a 2 ohm speaker.

-Gnobuddy
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