The TIP42 in bridged class ab amp is hot.

Audioguru again

Joined Oct 21, 2019
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The difference in hFE current gain shown in the datasheet between a sensitive TIP41 and a weak TIP41 is 5 times. Then your two amplifier halves probably do not match, then the sensitive one has much less base current than the weak one that needs 5 times more base current. Also the TIP42 has the same specs, then your TIP41 and TIP42 also probably do not match.
An amplifier IC has the transistors in the same chip so that they match very well.

In this simulation showing maximum undistorted output level, I changed the resistance of the collector resistors for Q1. Then I also needed to change the base resistor for Q1 and change the input signal level for the simulation that has perfectly matched transistors that have "typical" hFE current gain:
 

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mike_canada

Joined Feb 21, 2020
239
I'm curious as to why the bootstrap capacitor in your circuit is only 470uF.

Ok and lets say the hfe is actually different by 10x in the transistors I use. would external small valued capacitors connected somewhere around the transistors help with the Hfe problem along with the higher valued resistors you circled out?
 

Ian0

Joined Aug 7, 2020
9,667
I'm curious as to why the bootstrap capacitor in your circuit is only 470uF.

Ok and lets say the hfe is actually different by 10x in the transistors I use. would external small valued capacitors connected somewhere around the transistors help with the Hfe problem along with the higher valued resistors you circled out?
There's a whole section on bootstrap capacitors in Doug Self's book
http://www.douglas-self.com/ampins/books/apad.htm
which I suggest that you read back on August 7th.
 

Audioguru again

Joined Oct 21, 2019
6,673
I'm curious as to why the bootstrap capacitor in your circuit is only 470uF.
A simple calculation shows the frequency that has its output level reduced -3dB when a capacitor feeds a resistor.
-3dB is a small loss of level.

I calculated 470uF for the bootstrap capacitor feeding the two 82 ohm resistors in parallel (41 ohms).
The -3dB frequency is 8.3Hz.

I show a simulation of the max undistorted output level at 50Hz using a huge 3300uF capacitor and show the tiny reduction in level when a 470uF capacitor is used. You might notice the small drop in max level at 25Hz.
 

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mike_canada

Joined Feb 21, 2020
239
So you're describing a filter I take it? I never thought of it like that before.

Anyways, It looks like I may have to bias the heck out of the thing because I'm trying to achieve the best sound over a draining battery. I tested values in the sim and it seems I can get 3V waveforms with a 5V supply. I probably can't go much higher than that. Then I also did current measurements in the sim and it turns out all along that I need heatsinks on the power transistors. And god help sayal electronics. They charge $1.95 for 1 heatsink and online at digikey they charge between 30 cents and $1 (plus $8 shipping). I'll visit another local electronics shop and pray they have some in stock. I probably don't have the best of the best resistor values in my new amp but whoever has better values to suggest, (audioguru?) can I get some math so I can calculate values myself in case the certain value resistors that are required are not available in store?
 

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Audioguru again

Joined Oct 21, 2019
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Because your amplifier circuit is so simple using only 3 transistors in each amplifier, the clipping is not symmetrical when the supply voltage is reduced. IC amplifiers use at least 4 transistors then they operate symmetrically over a wide range of supply voltages.

The max output with fairly low distortion of the bridged amplifier driving a 6 ohm speaker when the supply is only 5V is 1.02W.
Each output transistor has max heating of only 1/3W. Then heatsinks are not needed.

Where will you find a 5V rechargeable battery? 2 cells of a Li-PO produce 8.4V when fully charged then the max undistorted power of the bridged amplifier into 6 ohms is 4.2W and each output transistor heats with 1.3W max, then small heatsinks are needed in the tropics on a hot day.
 

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mike_canada

Joined Feb 21, 2020
239
Because your amplifier circuit is so simple using only 3 transistors in each amplifier, the clipping is not symmetrical when the supply voltage is reduced.
It seems with my experimentation that if the input signal has a low enough amplitude then it will work through a wide range of voltages and the waveforms would look nice. then again, the low amplitude is too low for my liking sometimes.

The max output with fairly low distortion of the bridged amplifier driving a 6 ohm speaker when the supply is only 5V is 1.02W.
Each output transistor has max heating of only 1/3W. Then heatsinks are not needed.
I think I measured something like that with a 5V supply in the sim (or was it 1.2W I forgot).

Where will you find a 5V rechargeable battery? 2 cells of a Li-PO produce 8.4V when fully charged then the max undistorted power of the bridged amplifier into 6 ohms is 4.2W and each output transistor heats with 1.3W max, then small heatsinks are needed in the tropics on a hot day.
The reason why you see a 6 ohm resistor and 600uH inductor in series as the output load is because ltspice doesn't have a speaker component and when I asked about something better than an 8 ohm resistor to use as a speaker, someone suggested a 6 ohm resistor and 600uH inductor in series. Should I use something different to simulate a speaker or is that person right?

Also, the battery I'm using is 7.2V rechargeable. But the other day when I used it on an older amp circuit for 90 minutes (and heard distortion after about 40 minutes), I measured the voltage after use with a voltmeter and it showed 5V. So if I can get a perfect waveform and decent volume for 5V and up, I'll be happy. and so far with my simulations I can achieve that, but I'm guessing I'll need a heatsink if I take my perfect 5V circuit and change the supply to a fully charged 7.2V battery.
 

Audioguru again

Joined Oct 21, 2019
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Inductance of a speaker causes its impedance to rise at higher frequencies. The rise at 1kHz of your 600uH is only 3.77 ohms or 1.51 ohms at 400Hz.
Your speaker is probably rated at 8 ohms at 400Hz.

What chemistry is your 7.2V battery? Lithium or Ni-MH? Ni-MH is similar to Ni-Cad.
A Lithium 2 cell battery is 8.4V when fully charged and is 7.2V or 7.4V when half-charged. It must have its load disconnected when its voltage has dropped to 6V.
A Ni-Cad or Ni-MH battery has 6 cells but is also 8.4V when fully charged and is 7.2V when half-charged. It produces only a low current when its voltage has dropped to 6V.
 

Audioguru again

Joined Oct 21, 2019
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The little 4" speaker's inductance is 200uH. The datasheet shows the speaker's impedance is 7.8 ohms at 400Hz, 8 ohms at 2kHz, 10 ohms at 5kHz and 18 ohms at 20kHz. Its output level drops steeply below 100Hz so it produces poor bass.

Instead of fiddling with your poor amplifier circuit, maybe you should buy a cheap but good one:
 

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Ian0

Joined Aug 7, 2020
9,667
The little 4" speaker's inductance is 200uH. The datasheet shows the speaker's impedance is 7.8 ohms at 400Hz, 8 ohms at 2kHz, 10 ohms at 5kHz and 18 ohms at 20kHz. Its output level drops steeply below 100Hz so it produces poor bass
Visaton is quite a reputable manufacturer, but if it were intended for hi-fi there would be Thiele-Small parameters in the datasheet.
86dB@1W@1m is hardly the paragon of efficiency - equivalent to about 0.25%
 
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mike_canada

Joined Feb 21, 2020
239
Instead of fiddling with your poor amplifier circuit, maybe you should buy a cheap but good one
Didn't audioguru called things "cheap" when they were no good?
And also, when I do my fiddling, its all done through software. One thing for certain based on everything is that I'll be increasing some resistor values and looking into heatsinks.

That part on ebay... there's no heatsinks and I could imagine the quality of the sound.

I remember buying from a chinese seller on ebay a complete ISD1760 chip recorder circuit with buttons on-board and it didn't sound any better than my own circuit that used that chip. plus the chinese version broke down after having it for about a month or so.
 

Ian0

Joined Aug 7, 2020
9,667
I remember buying from a chinese seller on ebay a complete ISD1760 chip recorder circuit with buttons on-board and it didn't sound any better than my own circuit
If you can't design a circuit that sounds better than a voice recorder with 6kHz bandwidth, then there's not much hope.
 

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mike_canada

Joined Feb 21, 2020
239
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I may settle for the above circuit with the above values since with 5V I can make waveforms of at least 3V.

The reason I asked for the parts to build a simulated speaker is because the results vary. With just an inductor as a speaker, the waveform is more triangular, whereas a 6 ohm resistor and inductor in series looks more normal whereas an 8 ohm resistor makes the sound appear distorted at the bottom.
 

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mike_canada

Joined Feb 21, 2020
239
7.2 ohms means more power output as opposed to 8 ohms. I won't complain that I don't get super deep base and I probably never will but I can hear the sounds as intended through those speakers nice and loud to about 2 meters when the 7.2V battery stays at 7.2V. It's not deafening loud, but loud enough to hear the music lyrics.
 

Ian0

Joined Aug 7, 2020
9,667
View attachment 277521
I may settle for the above circuit with the above values since with 5V I can make waveforms of at least 3V.

The reason I asked for the parts to build a simulated speaker is because the results vary. With just an inductor as a speaker, the waveform is more triangular, whereas a 6 ohm resistor and inductor in series looks more normal whereas an 8 ohm resistor makes the sound appear distorted at the bottom.
If your amplifier circuit were any good, there wouldn't be any difference between them. A decent amplifier design can handle different output impedances without any changes in distortion performance.
 

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mike_canada

Joined Feb 21, 2020
239
I just think the issue here is that I'm trying to make my unit give decent sound without distortion happening as the battery dies off. If I wanted something ultra loud and I had the money, then I'd invest in speakers of different categories, large heatsinks and more power, but this box is portable. I will change a few part values, and I'll see what happens when I take it for another run later this week.
 

Audioguru again

Joined Oct 21, 2019
6,673
The inductance of your speaker is only 200uH which should have no effect at the 30Hz on your simulation. The inductor is simply a short piece of wire that is a dead short circuit and the simulator is wrongly driving it.

The simulator is also showing a wrong effect of the "piece of wire" in series with the 6 ohms of the speaker.

The simulator is driving your 6 ohms speaker with two amplifiers then the voltage and current in the speaker are doubled which is like one amplifier driving a 3 ohms speaker.

Here is your unsymmetrical clipping and my corrected biasing to correct it:
 

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