Hi Folks
This is my first attempt at amateur DIY project after a long time.
I have limited knowledge about Electronics and Electrical stuff, just what was covered in a single semester of college, as my domain was Computer Science Engineering. So I hope you guys would take my follies with gentlest of consideration and point me towards right path.
Objective:- Build a small 8 inch Active Subwoofer
Parts list:-
Like If I want a 50V 12A DC output then would that mean I have to use a Step Down transformer that converts 220V AC into 50V DC and provides 12A of current? Is it possible to have a smaller(thus cheaper and less heat dissipation) transformer, say a 12-0-12 step down transformer and then boost it's output to 50V DC and current to 12A ? If yes then which component in the circuit does that?
If I have got all of this wrong then please point me the direction where I can learn and have proper information.
This is my first attempt at amateur DIY project after a long time.
I have limited knowledge about Electronics and Electrical stuff, just what was covered in a single semester of college, as my domain was Computer Science Engineering. So I hope you guys would take my follies with gentlest of consideration and point me towards right path.
Objective:- Build a small 8 inch Active Subwoofer
Parts list:-
- Driver-> 8 inch 4 Ohm 50W Subwoofer (8"PA speaker(4))
- Low Pass Filter-> Dual TL084C based active low pass filter(Bassoon Sub Woofer Board: Amazon.in: Electronics)
- Amplifier-> 1 x TPA3118 PBTL Mono Digital Amplifier Board 1X60W 12V 24V Power Amplifier Module (TPA3118 PBTL Mono Digital Amplifier Board 1X60W 12V 24V Power Amplifier Module - Robu.in | Indian Online Store | RC Hobby | Robotics)
- Power Supply-> ???
- What kind of PSU would be suitable, a transformer based regulated one or a Switching PSU ?
- Should I go for DIY PSU or buy one off the shelf unit ?
- As both the amplifier and the filter require power, would a single unit suffice or will I be needing 2 separate power supplies?
- Finally, what rating PSU would be appropriate for the build?
Like If I want a 50V 12A DC output then would that mean I have to use a Step Down transformer that converts 220V AC into 50V DC and provides 12A of current? Is it possible to have a smaller(thus cheaper and less heat dissipation) transformer, say a 12-0-12 step down transformer and then boost it's output to 50V DC and current to 12A ? If yes then which component in the circuit does that?
If I have got all of this wrong then please point me the direction where I can learn and have proper information.
With any project like this begin with the datasheet for the device, http://www.ti.com/lit/ds/symlink/tpa3118d2.pdf Here we see on page 6 the device draws without any load 50ma from the supply
Perhaps you will be quicker than me finding current draw when it has a loudspeaker load
but it appears it is single ended, meaning you need just a positive voltage supply derived
from a full wave rectifier supply. the application data provides it has onboard regulation
for the various stages
So to power, no need to convert one voltage to another just use the right AC secondary voltage. I would suggest a fused toriodal providing a AC secondary of 18v @ 160VA which once
full wave rectified ( with suitable capacitance suggest 4700uf) will provide 25.45V DC
Perhaps you will be quicker than me finding current draw when it has a loudspeaker load
but it appears it is single ended, meaning you need just a positive voltage supply derived
from a full wave rectifier supply. the application data provides it has onboard regulation
for the various stages
So to power, no need to convert one voltage to another just use the right AC secondary voltage. I would suggest a fused toriodal providing a AC secondary of 18v @ 160VA which once
full wave rectified ( with suitable capacitance suggest 4700uf) will provide 25.45V DC
The output voltage of a simple DC pwer supply is determined by the transformer secondary voltage. The DC output will be roughly 1.4 times the AC secondary voltage.Rude1 said:
The output current is determined by the load applied.
The maximum safe current is usually set by the transformer. Typically the max current will be much less than people expect, for reasons which you may one day understand. Very roughly, the DC power draw can be half the transformer VA rating.
My understanding so far
So from the above two replies, this is what I have understood, but please do correct me if I am wrong.
So from the above two replies, this is what I have understood, but please do correct me if I am wrong.
- Go for Transformer based Regulated PSU, skip Switching PSU (?)
-
It's all about the Transformer, the bigger it is, more volts it gives out, more current it gives out. Did I get it right?
No, not quite. The size of the transformer determines the product of voltage and maximum current, not each of them individually. A 50VA transformer could be 50V x 1A or 1V x 50A, depending on how the secondary was wound.
You need to work back from your amplifier. What power does it require? How will you use it: real music with a significant dynamic range, or compressed music which is permanently loud?
The amplifier is TPA3118 PBTL one, below are it's specifications
- Working Voltage: DC 8-24V
- Power Output: 60W
- Applicable Speaker Impedance: 4-8 ohm
The amplifier needs a single rail only but the Low pass Filter needs Bipolar supply. As for maximum current draw, it wasn't mentioned on the seller's site but Chris Daly pointed out the datasheet of the amplifier and below is the pic of it's DC characteristics, which hopefully would answer your question and also allow me to make head and tail of what all is written there.
Attachments
Page 7 of the datasheet states it has a over current trip point of 7.5 amps
A VA calculator Volt-amps (VA) to amps (A) conversion calculator
helps to show the 160VA rated transformer I recommended earlier as being ideal.
As you also need a positive and negative DC rail for your low pass filter, which will have much less current draw a separate smaller fused transformer and bridge rectifier and capacitors observing polarity makes sense. You might be able to find a transformer with a separate 12v 0v 12V winding as well as the needed 18v AC with 160VA rating.
Overshoot?
Speaking in n00b terms, why would a 18V 160VA transformer setup be ideal? From what I've gathered so far on this thread, if the over-current trip point is 7.5 Amps then why a transformer rated for more than 8 Amps be ideal, more so why a 160VA(~160W), when the amplifier that is to be driven is rated at just 60W? Where would the extra 100W go?
Also for the the low pass filter, as no datasheet was provided by the manufacturer for it, so the source for its current draw rating would be the TL084C datasheet?
Speaking in n00b terms, why would a 18V 160VA transformer setup be ideal? From what I've gathered so far on this thread, if the over-current trip point is 7.5 Amps then why a transformer rated for more than 8 Amps be ideal, more so why a 160VA(~160W), when the amplifier that is to be driven is rated at just 60W? Where would the extra 100W go?
Also for the the low pass filter, as no datasheet was provided by the manufacturer for it, so the source for its current draw rating would be the TL084C datasheet?
The "extra 100W" does not go anywhere, because it never existed. As I said, you get less DC power from a PSU than you might expect.
A 60W Class AB amp needs a supply of at least 80W DC, which needs a transformer of 160VA. You could get away with a smaller transformer (say, 100VA) if you only listen to real music with a high dynamic range so you are mainly running at 5-10W with an occasional peak to 60W.
As a general rule, you cannot use a single PSU to feed both a single rail amp and a bipolar amp - at least, not unless you really know what you are doing and can properly arrange DC and AC grounds. You asked for our advice and have received it. You can simple follow our advice, or do lots of reading and then in a few months/years time you will understand why we gave this advice.
A 60W Class AB amp needs a supply of at least 80W DC, which needs a transformer of 160VA. You could get away with a smaller transformer (say, 100VA) if you only listen to real music with a high dynamic range so you are mainly running at 5-10W with an occasional peak to 60W.
As a general rule, you cannot use a single PSU to feed both a single rail amp and a bipolar amp - at least, not unless you really know what you are doing and can properly arrange DC and AC grounds. You asked for our advice and have received it. You can simple follow our advice, or do lots of reading and then in a few months/years time you will understand why we gave this advice.
I hope to learn and share with the community. As learning is going to be a big part of it, a clear understanding of why I am doing along with what I am doing will be an important of that process. Simply following up on an advice before fully understanding would lead to lot of mistakes/blown capacitors/burnt fingers. I intend to keep that to a minimum. Thus I do try to read up whatever I can and hope to clarify my doubts here.
Now the amplifier in question isn't Class AB, it's Class D which are supposed to have >90% efficiency so wouldn't it be delivering at least 50W ? If so then why to go for a Transformer based PSU? It becomes even less of an attractive option due to the fact that even the "cheapest" torroidal transformer available in local market, costs more than twice the total cost of all the components.
The PT can be 100VA.
Converting AC to DC is hard work for a transformer. We get 1.414X the voltage at DC, but the heating/sag effect of the AC current may be 1.8X the DC current. So the transformer VA must be 1.3X the DC load. Accepting the "60 Watts" number and 90% efficiency, we need 67 Watts of DC. 67W*1.3 is 87VA of transformer. You don't custom-design a just-right transformer; standard product is far cheaper than custom design and wind. 100VA is likely the next-up commercial part.
(Because un-clipped speech/music is VERY dynamic and averages 15dB down from test-tone power, it is very likely that a much smaller transformer will not get hot in everyday operation. However I have seen such "it's fine" amplifiers crap-out when put to Party Duty. The added cost of a large core is often much less than the value of any eventual aggravation.)
> you didn't say that it was Class D.
The OP mentioned a chip-number I did not recognize, and a blurb with the word "digital". That's inconclusive. But these D amps have become VERY common, and are exceptionally suited to "subwoofer" work (no midrange crap), so I kinda assumed a "D" was in the picture.
Converting AC to DC is hard work for a transformer. We get 1.414X the voltage at DC, but the heating/sag effect of the AC current may be 1.8X the DC current. So the transformer VA must be 1.3X the DC load. Accepting the "60 Watts" number and 90% efficiency, we need 67 Watts of DC. 67W*1.3 is 87VA of transformer. You don't custom-design a just-right transformer; standard product is far cheaper than custom design and wind. 100VA is likely the next-up commercial part.
(Because un-clipped speech/music is VERY dynamic and averages 15dB down from test-tone power, it is very likely that a much smaller transformer will not get hot in everyday operation. However I have seen such "it's fine" amplifiers crap-out when put to Party Duty. The added cost of a large core is often much less than the value of any eventual aggravation.)
> you didn't say that it was Class D.
The OP mentioned a chip-number I did not recognize, and a blurb with the word "digital". That's inconclusive. But these D amps have become VERY common, and are exceptionally suited to "subwoofer" work (no midrange crap), so I kinda assumed a "D" was in the picture.
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Narrowing down
For the Filter, these are the parts I'm looking at. Please let me know your thoughts on them
For the Filter, these are the parts I'm looking at. Please let me know your thoughts on them
- 12-0-12V Transformer (https://www.robomart.com/12-0-12-st...ampere?search=Transformer&dsearch=Transformer)
- Dual Rail PSU (Eight Audio Low Noise CRC 15V DC Regulated Dual Rail Power Supply)
You are not even close.
Chris calculated 18VAC; you propose 12V (or 24V).
You have a 50W speaker with a 60W amplifier, but propose a 12-0-12 1A (24VA) transformer ? ? ? Like filling the bathtub with a teaspoon.
You have a single-rail amplifier but propose a dual-rail supply board? With NO rectifier?
Depending how you connect mis-matched connections, yes it may smoke. The very best result would be it "plays" but craps-out on any heavy bass.
Chris calculated 18VAC; you propose 12V (or 24V).
You have a 50W speaker with a 60W amplifier, but propose a 12-0-12 1A (24VA) transformer ? ? ? Like filling the bathtub with a teaspoon.
You have a single-rail amplifier but propose a dual-rail supply board? With NO rectifier?
Depending how you connect mis-matched connections, yes it may smoke. The very best result would be it "plays" but craps-out on any heavy bass.
Please read my posts carefully. This dual rail setup is for the separate active low pass filter. Fir the amplifier itself, as DF96 also concurred , a 12V5A Switching PSU would do and I am keeping that as go to option as fits the power criteria and the bill. I repeat, the aforementioned dual rail setup is for active low pass filter that accepts + and - 12 to 24V.
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