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Class D Switching Power Amplifiers and Power D/A conversion

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Old 13th November 2012, 03:50 AM   #21
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Tubes work just fine for PWM -- downsides are mainly that it's a hassle to use a lot of tubes, so you don't usually get as nice waveforms. Example: instead of building a comparator out of a diff pair, CCS, mirror, VAS and optional output, you might short-cut to just the diff amp (with resistive tail), and maybe a cathode follower if you need it to drive things. Even so, it's easy to get performance comparable to a similar transistor circuit; I've measured 150ns edges from a sweep tube under simple conditions, which isn't too bad.

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Old 13th November 2012, 07:31 PM   #22
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Yea.. I think its good they're challenging us but I feel like there was no build up of classes to warrant this project..

Anyway, I'm not sure why my input for that circuit outputs no sound but a function generator outputs noise. :/ I'm guessing it's not enough power lol but isn't that the point of a power amp @_@
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Old 14th January 2013, 11:59 PM   #23
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This might be a stupid question but, I bought a 3.5mm jack that I have as an input (coming from my computer) and the output going to an 8Ohm speaker. I start playing music, should my speaker be outputting sound? It seems my input voltage is too low because plugging this straight to a function generator at 4Vpp I can hear the frequency noise properly. I'm so lost ): This is the worst project I've ever been assigned

Any help would be appreciated ):
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Old 29th January 2013, 02:58 AM   #24
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Default It's Alive!!

So I've still been working on this project (lol no choice but to work on it to graduate!) and I've finally got it to the point where I can actually hear music through the speakers!

What I noticed before is that audio from headphone jacks give very little voltage so I had to throw in a pre-amp in there and mix matching the right caps finally got something going!

Attached are my up to date schematic and here's a video of it in action (here)

Sound quality wise, it seems to sound nicest at about 5V. When the music becomes more heavy the higher tones seem to sound less appealing (symbals specifically on the drums) Any advice to make this better? To be honest, I'm just happy it sort of works lol
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Old 29th January 2013, 10:09 PM   #25
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It looks like your M3 does never turn ON properly.
In order to get symmetric signal magnitudes to the your input of the switching stage you should chose R5<R47.
Background: Q40 is turned ON from +8V through R5+R47, while Q43 is turned ON from -8V through R47+UcedropofQ1.
Give it a try and check if something turns better when you change R5 from 100k to 18k.
In fact you have chosen very high values for R5 and R47 anyway, so in a second step slowly reducing their both values might be a good idea.
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Old 29th January 2013, 10:28 PM   #26
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Quote:
Originally Posted by darkamikaze View Post

@Soldermizer
My professor actually allows vaccum tubes to be used in my project but they're quite outdated aren't they? Here's his specifications and his mention of vaccum tubes for reference (here)

Thanks everyone for the advice!
So you are only allowed to use BJT, FET, or VACUUM TUBES for Class D amp? I mean, what else could possibly be used to make a Class D amp?
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Old 30th January 2013, 07:21 AM   #27
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Quote:
Originally Posted by ChocoHolic View Post
It looks like your M3 does never turn ON properly.
In order to get symmetric signal magnitudes to the your input of the switching stage you should chose R5<R47.
Background: Q40 is turned ON from +8V through R5+R47, while Q43 is turned ON from -8V through R47+UcedropofQ1.
Give it a try and check if something turns better when you change R5 from 100k to 18k.
In fact you have chosen very high values for R5 and R47 anyway, so in a second step slowly reducing their both values might be a good idea.
Wow thanks for the feedback! I'll try this out tomorrow at school. Helps a lot. PSPICE simulation already looks nicer. I don't want to trouble you but how do you conclude these things X_x? If it's too long to explain I'd understand.

Quote:
Originally Posted by cotdt View Post
So you are only allowed to use BJT, FET, or VACUUM TUBES for Class D amp? I mean, what else could possibly be used to make a Class D amp?
Haha, I'm not sure but everything needed to be a discrete component. I'm pretty sure this would have been a lot easier if I had the chance to use an IC for the comparator and the triangle wave form generator X_x. Vaccum Tubes, I don't even know how they work to be honest.
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Old 30th January 2013, 07:41 PM   #28
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Quote:
Originally Posted by darkamikaze View Post
...how do you conclude these things X_x?
In your SIM results I saw that the signal behind the switching stage is not going high. So I concluded M3 never turns ON properly.
Consequently I started to look what in the circuit is different of driving M3 vs M4. This leads you to Q40 and Q43 and their base drive.
Q40 is turned ON from +8V through R5+R47, while Q43 is turned ON from -8V through R47+UcedropofQ1.
If you want to have the available base drive currents similar you have to fullfil:
abs( 8V / (R5+R47)) similar abs (-8V + UcedropofQ1) / R47)
abs means 'absolute value of'
Now guess Ucedropof about 0.5V
==> 8V / (R5+R47) similar 7.5V / R47

This would lead you to an even larger ratio of R47 vs R5,
but I did not want to have R5 to low, because also the gate drive of Q1 is weak and its capability to pull down R5 is limited.

Furtheron when looking to typical properties of small signal BJTs, you will find that your drive impedances are pretty high for driving them at reasonable speed, that's why I additionally proposed to reduce both in second step.
How much you can reduce R5&R47 is limited by the pull down capabilities of Q1. If necessary - in a third step you could reduce R63 to extend amount of possible impedance reductions.

A fourth step might be a DC adjustment.
Without that you most likely will have a high DC component at the speaker.
Adjustment is possible through variation of R77 and/or R117

A fifth step could be asking your tutor why he expects that you would able to debug his circuit. ...just a guess that the circuit is from him..
In case step5 bears risks for your graduation, skip step 5 or schedule it after you have passed your graduation.
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Old 31st January 2013, 05:13 AM   #29
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So I've tried your suggestions and I've lowered majority of the resistors you had mentioned.

Trying it out on the board it sounds really poppy and sometimes non existent X_x.

I actually made this whole circuit ): excluding the amplifying stage which I borrowed from a fellow user here :/. I had a mind set that a class D involves a triangle wave, (middle of the circuit) and a differential amplifier (left of circuit). The triangle and the input are then compared and the result is placed in the amplifying stage. :/ ): So if anyone is to blame it's me lol but I'm trying!
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Old 31st January 2013, 06:09 PM   #30
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...hm, if you are able to do the circuit and even calculate some values, I am wondering why are so lost in debugging it. Normally if some has an opinion how to settle the schematic, he also has an opinion how the signals should look and is able to measure, see deviations, dig deeper and find the reason for the deviation (at least in lucky situations like your's, where the circuit does not immediately blow).
Anyway, in order to go on in a reasonable way, you have to measure and analyse the signals on the real board.
And check after every mod, if the mod had the intended effect.
It must be done in small steps.
Typically one starts with checking the supply rails.
Then you should check the triangle.
Then the output of the comparator.
Then the signal which you are feeding to your gate drivers.
Then the output of the half bridge and gate signals.

Where ever the signals do not look like they should - dig deeper.
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