Author Topic: Low Distortion audio power amplifier (Read 5682 times)

Adgj533 · « **on:** February 25, 2018, 12:16:00 am »

Hey guys, fairly new here but I have a project at school and I wanted to ask a few questions. My professor told me we can use PCB design or use breadboard. I am trying to simulate this in PSpice and its not going well. Also any tips would be appreciated. I have attached the schematic and I don't know how to proceed with this.
I circled the parts that confused me, any help is greatly appreciated
Cheers

GerryBags · « **Reply #1 on:** February 25, 2018, 01:38:03 am »

Only a noob myself, but I think the big circle on the left is the chassis ground, or the PCB ground plane, connection. All the other grounds are connecting back to this. The variable resistor leading to the base of the BC184 is the pre-settable type, I think, with the flat line on the wiper instead of the usual arrow. The other two that aren't wires (?) are a switch and an air-core inductor with 17 turns which is (again - I think) acting as a choke filtering out high frequency AC.

Adgj533 · « **Reply #2 on:** February 25, 2018, 01:59:29 am »

Heres is the preliminary pspice simulation, idk what the output is, i ran a transient analysis.

Audioguru · « **Reply #3 on:** February 25, 2018, 02:34:03 am »

It is obvious.

Adgj533 · « **Reply #4 on:** February 25, 2018, 03:00:54 am »

Quote from: Audioguru on February 25, 2018, 02:34:03 am

It is obvious.

Hey man thanks for the input, I fixed it and I think i got a stable output. My prof said I need to able to output to some kinda speakers so im thinking something small, what kinda speaker should i go for and what would be the R load I should put in for the schematic
Also the top of the output is kinda getting clipped, how can I change that? I want a smooth wave.
Thanks for all your help
Cheers

Audioguru · « **Reply #5 on:** February 25, 2018, 04:49:30 am »

R10 has a resistance that is 1000 times too high but has no effect with no load.
Your amplifier clips symmetrically at 3V peak when it should make at least 20V peak (50W peak into 8 ohms which needs a pretty big speaker, not a small one). Most home speakers are 8 ohms.
Maybe the sim program knows that the PNP transistors were destroyed when they were backwards?
I think 10mH for the output inductor is way too high, maybe it should be 0.1mH.

Adgj533 · « **Reply #6 on:** February 25, 2018, 04:59:38 am »

Quote from: Audioguru on February 25, 2018, 04:49:30 am

R10 has a resistance that is 1000 times too high but has no effect with no load.
Your amplifier clips symmetrically at 3V peak when it should make at least 20V peak (50W peak into 8 ohms which needs a pretty big speaker, not a small one). Most home speakers are 8 ohms.
Maybe the sim program knows that the PNP transistors were destroyed when they were backwards?
I think 10mH for the output inductor is way too high, maybe it should be 0.1mH.

I changed r10 to 1 ohm, and I deleted the old transistors and replaced it with new ones so i dont think PSpice lite is that smart. Also Im still only getting a gain of 15 V/V. you are saying I need a gain of 50 for it to be used with a normal speaker? (didnt really understand what u meant)
also I changed inductor, doesnt really make a difference. How can I smooth out the output? I keep changing values but its not helping, Idk y its clipped

Audioguru · « **Reply #7 on:** February 25, 2018, 05:00:09 am »

Hey! The amplifier voltage gain is 100k/6.8k= 14.7 times and the input signal is only 200mV peak. Then of course its output should be 200mV x 14.7= 2.94V peak which it is.
But the clipping looks fishy. The top clips into a triangle peak then the bottom clips with a left slant. Then the top clips with a right slant then the bottom clips with a triangle peak. I think the signal generator does that.

Audioguru · « **Reply #8 on:** February 25, 2018, 05:04:26 am »

Since the voltage gain is only 100k/6.8k= 14.7 times then for it to produce its maximum output of 50W into 8 ohms (20V peak) then its input must be 20v/14.7= 1.36V peak.

Adgj533 · « **Reply #9 on:** February 25, 2018, 05:08:35 am »

Quote from: Audioguru on February 25, 2018, 05:00:09 am

Hey! The amplifier voltage gain is 100k/6.8k= 14.7 times and the input signal is only 200mV peak. Then of course its output should be 200mV x 14.7= 2.94V peak which it is.
But the clipping looks fishy. The top clips into a triangle peak then the bottom clips with a left slant. Then the top clips with a right slant then the bottom clips with a triangle peak. I think the signal generator does that.

Should I try changing the source? I mainly use the V/sin source that I used here in class and in lab. idk what else to do
Btw thanks for all your help, really appreciate it EDIT: i added a new input vs output with 1.36V as input source but its not reaching that 20V out, its getting clipped
Also should the frequency be 1k? or should I go with normal 60hz since I need to implement this
Cheers

Audioguru · « **Reply #10 on:** February 25, 2018, 05:39:43 am »

Now it clips with an 18V positive peak so reduce the input a little so it reaches +17.8V without clipping. I simply guessed that it should reach a peak of 20V but more than 17V is fine.

Audioguru · « **Reply #11 on:** February 25, 2018, 05:42:12 am »

You asked if the 1kHz frequency should be 60Hz. Don't you know the frequencies of normal hearing?

Adgj533 · « **Reply #12 on:** February 25, 2018, 05:52:18 am »

Quote from: Audioguru on February 25, 2018, 05:39:43 am

Now it clips with an 18V positive peak so reduce the input a little so it reaches +17.8V without clipping. I simply guessed that it should reach a peak of 20V but more than 17V is fine.

Awesome, Thanks so much for your help. for the output would something like this work?
https://www.amazon.com/uxcell-Metal-Internal-Magent-Speaker/dp/B0177ABRQ6/ref=pd_sbs_147_1?_encoding=UTF8&pd_rd_i=B0177ABRQ6&pd_rd_r=RQZYH2E3JS0JDP1ACJ5P&pd_rd_w=kHSI0&pd_rd_wg=1ieVC&psc=1&refRID=RQZYH2E3JS0JDP1ACJ5P

Also how should i feed the input?
Sorry if Im bothering you with all these noob questions, EDIT: about the frequency, I thought the source should be same frequency as the wall but it was a stupid question.
your help is very much appreciated. Trying to get this project done professor is strict lol

ArdWar · « **Reply #13 on:** February 25, 2018, 06:31:27 am »

Quote from: Audioguru on February 25, 2018, 05:00:09 am

But the clipping looks fishy. The top clips into a triangle peak then the bottom clips with a left slant. Then the top clips with a right slant then the bottom clips with a triangle peak.

I suspect that's actually caused by the simulation timestep being too coarse

Adgj533 · « **Reply #14 on:** February 25, 2018, 06:35:58 am »

Quote

I suspect that's actually caused by the simulation timestep being too coarse

how can i fix that? I set the transient simulation for 3ms and started saving data after 0 sec. I didnt change any other options.
I can report back asap if you have any suggestions,
Thanks for your reply, really appreciate it

Audioguru · « **Reply #15 on:** February 25, 2018, 02:25:17 pm »

The cheap tiny 1.1" diameter speaker you found has a maximum rated power of only 2W. It would explode if you feed a peak of 20W into it. A 20W peak rating is 10W continuous rating if the manufacturer does not lie about it. A speaker rated for 10W continuously is probably at least 6.5" or 8" in diameter and must be in a proper speaker enclosure.

If you change the timebase of your oscilloscope to 2ms per division then it should show six sinewaves without the odd distortion.

Adgj533 · « **Reply #16 on:** February 25, 2018, 06:20:53 pm »

Quote from: Audioguru on February 25, 2018, 02:25:17 pm

The cheap tiny 1.1" diameter speaker you found has a maximum rated power of only 2W. It would explode if you feed a peak of 20W into it. A 20W peak rating is 10W continuous rating if the manufacturer does not lie about it. A speaker rated for 10W continuously is probably at least 6.5" or 8" in diameter and must be in a proper speaker enclosure.

If you change the timebase of your oscilloscope to 2ms per division then it should show six sinewaves without the odd distortion.

Hey how did you get the 20W value? Vout is 18V so using P=V^2/R ( R would be the speakers so 8 ohms)
i get around 40W
is my math wrong?
Also can u give me a link to a compatible speakers? I cant find any, the ones amazon has are all USB powered.

NiHaoMike · « **Reply #17 on:** February 25, 2018, 07:29:13 pm »

Quote from: Audioguru on February 25, 2018, 05:04:26 am

Since the voltage gain is only 100k/6.8k= 14.7 times then for it to produce its maximum output of 50W into 8 ohms (20V peak) then its input must be 20v/14.7= 1.36V peak.

And that's easily achieved from a circuit that runs from a 3.3V supply or higher. Back when that sort of amplifier design was common, even 3.3V wasn't very common in consumer electronics.

Adgj533 · « **Reply #18 on:** February 25, 2018, 08:04:53 pm »

[/quote]
And that's easily achieved from a circuit that runs from a 3.3V supply or higher. Back when that sort of amplifier design was common, even 3.3V wasn't very common in consumer electronics.
[/quote]
In my case (schematic posted above) what would u do for input and output, I have to show prof my amp works and have to play music thru it, I cant figure out if i should do like a 3.5mm from my phone as input and speaker as output, but I cant find 20W speakers, they are all USB. can i somehow cut the USB and turn it into breadboard friendly?
Thanks for looking at my post
highly appreciated
Cheers

NiHaoMike · « **Reply #19 on:** February 26, 2018, 12:14:44 am »

Just get some cheap stereo speakers from Goodwill and use any audio device as a source.

Audioguru · « **Reply #20 on:** February 26, 2018, 01:25:13 am »

Quote from: Adgj533 on February 25, 2018, 06:20:53 pm

Hey how did you get the 20W value? Vout is 18V so using P=V^2/R ( R would be the speakers so 8 ohms) i get around 40W.

Sorry, I said 20V peak awhile back. A problem with this website is that I cannot scroll back to see the schematic like on all the other amplifiers on all the other websites.
18V peak into 8 ohms is 40.5W peak which is 20.25W continuously just before clipping. Is this the amplifier powered from plus and minus 22V?

Quote

Also can u give me a link to a compatible speakers? I cant find any, the ones amazon has are all USB powered.

USB does not drive a speaker, it charges the battery in a low power portable speaker that already has a power amplifier in it. You need an 8 ohm speaker that survives 20W continuously.
Madisound has expensive speakers and Parts Express has a variety of speakers. Maybe you need a 20W 8 ohm 8" diameter guitar speaker. It must be mounted in an enclosure that matches its detailed spec's.

Adgj533 · « **Reply #21 on:** February 26, 2018, 02:03:15 am »

Quote

Is this the amplifier powered from plus and minus 22V?

Yes but if its too much Watts, i can put a few diodes in to cancel the negative part out?
Thanks for all your help btw, I learned a lot during these past 2 days ur amazing
Cheers

Audioguru · « **Reply #22 on:** February 26, 2018, 02:41:20 am »

Quote from: Adgj533 on February 26, 2018, 02:03:15 am

Quote
Is this the amplifier powered from plus and minus 22V?
Yes but if its too much Watts, i can put a few diodes in to cancel the negative part out?

No. This amplifier is designed to have the same negative voltage as its positive voltage.

Nobody makes a +22V and -22V power supply. You must design it and make it. If you wanrt less maximum power then use a 20VAC center tapped transformer which will produce plus and minus 13V. Then the output power will be about 7W continuously into 8 ohms.

Adgj533 · « **Reply #23 on:** February 26, 2018, 02:50:28 am »

Quote from: Audioguru on February 26, 2018, 02:41:20 am

Quote from: Adgj533 on February 26, 2018, 02:03:15 am

Quote
Is this the amplifier powered from plus and minus 22V?
Yes but if its too much Watts, i can put a few diodes in to cancel the negative part out?
No. This amplifier is designed to have the same negative voltage as its positive voltage.

Nobody makes a +22V and -22V power supply. You must design it and make it. If you wanrt less maximum power then use a 20VAC center tapped transformer which will produce plus and minus 13V. Then the output power will be about 7W continuously into 8 ohms.

I misread your question earlier, I do have a +22, -22 (max being 30v/5A i think)bench DC power supply, and the school has one too. so ill use that
Also, Im trying to put this on PCB using eagle software, any tips?

Audioguru · « **Reply #24 on:** February 26, 2018, 03:57:30 am »

Quote from: Adgj533 on February 26, 2018, 02:50:28 am

I do have a +22, -22 (max being 30v/5A i think)bench DC power supply, and the school has one too. so ill use that
Also, Im trying to put this on PCB using eagle software, any tips?

If you use two power supplies for the positive voltage and the negative voltage then adjust their voltages to be the same.
I have never used Eagle software.


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Author Topic: Low Distortion audio power amplifier (Read 5682 times)

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