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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
 Posted - Jul 10 2010 : 01:40:29 AM
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Usually when I think of audio amplifiers I imagine a varying voltage on the input being amplified to a larger varying voltage on the output. But what about converting the varying voltage input into an identical but amplified varying current output. Do any amplifiers use this technique and what are the problems with it? I can see an advantage in that there is short circuit protection on the output by the nature of the design, current is limited in other words. Also as the speaker temperature increases the loudness will stay the same as voltage is automatically increased to maintain the desired current output, since current is what directly causes the diaphragm to move.
All input is much appreciated. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Jul 13 2010 : 12:48:47 PM
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An amplifier with a voltage output has an extremely low output impedance (called its damping factor) so it damps the resonances of a speaker. An amplifier with a damping factor of 500 has an output impedance of 0.016 ohms when it drives an 8 ohm speaker.
A current source has a high output impedance so will not produce the needed damping.
A speaker has a wide range of impedance depending on its frequency. Speakers are designed to sound correct when driven by voltage. If they are driven by current then at resonance the current is low so the amplifier will blast a high signal to try to keep the current high. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 13 2010 : 11:37:50 PM
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Hi John,
I see. Do class D amplifiers generally have low output impedance for the full audio range? I'm thinking of making one but they are not DIY friendly that's for sure, but they are energy efficient and compact.
EDIT:
I've been doing lots of calculations and time intensive simulations with my own custom made simulators and I think I can make a mono Class D audio amplifier with no output filtering by having a very high PWM frequency, taking advantage of the speaker's inductance. I should be able to achieve a PWM frequency up to 2MHz with roughly 80% to 90% efficiency. I've been thinking and I can see why these amplifiers would have very low output impedance. I gonna work on it and hopefully get one made before the Uni holidays are over. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
Edited by - wasssup1990 on Jul 14 2010 01:33:39 AM |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Jul 14 2010 : 10:59:02 AM
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Class-D amplifiers have an output filter so that they can be measured using a resistor load, so that the speaker does not draw too much power at the very high frequency all the time, and so that audio-frequency negative feedback can be used to reduce distortion and reduce the already fairly low output impedance of the amplifier.
Discrete-parts class-D amplifiers do not use a frequency as high as 2MHz, but monolithic class-D amplifiers do because their wiring capacitance and inductance are very low. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 14 2010 : 12:05:58 PM
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After watching this video...
http://www.youtube.com/watch?v=iJXb7Yf_j5w
... I thought it might be possible to do away with the output filter in my design. I have seen simple class-D amplifiers without output filters but it seems like that might defeat the purpose if the idle current is going to be so high. I need to research into how negative feedback works in a class-D amplifier.
Basically what I want to achieve is a mono class-D amplifier "brick" with an inbuilt SMPS. Audio inputs are from a single 3.5mm socket that accepts optical-digital and electrical-analog signals. An output power of 100WRMS would be nice. Also with 24-bit 192KHz on the input SPDIF (optical).
I wanted to achieve a fully digital class-D amplifier with no triangle wave generator and comparator to produce the PWM signal. I thought of the "Capture Compare Register" (CCRs) method that is used in PICs but after some number crunching I found that is practically impossible to do as it requires a clock in the THz range. Then I thought of staggering clock signals to multiple CCRs but then I found that I would need to stagger the master clock thousands of times and direct them to thousands of CCRs to get sample rate of 192KHz at 24-bit resolution. Well both of those ideas are out the window. Do you know of any practical way to stay purely digital in the stages before the actual amplifier stage?
No doubt I'll think of yet another idea later. I would like to keep the signal as pure as possible (digitally) until the last road from the output of the switching MOSFETs to the speaker. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Jul 14 2010 : 8:59:30 PM
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| Texas Instruments have many class-D amplifier ICs. Most use a very high oscillator frequency and some do not need output filters. Most have a digital input. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 15 2010 : 12:43:35 AM
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| The TAS5706B is very nice but darn it, I wanted to make my own amplifier. Well at least I know I'll be able to get the sound quality that I want now. Now I will just design the support circuitry around it and the SMPS. Thanks for your help John. If I have any more questions "I'll be back". |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 16 2010 : 03:24:33 AM
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Oh yeah. These Texans have some really good chips. I think most of my components will be comming from them.
7.1 Surround sound with 100W per channel. Hows that sound? Super dooper. Well at least I'll still be able to hear this thing when I'm an old man.
I won't finish it before my Uni holidays are over. This is going to take a long time. Quality and power are my goals. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 16 2010 : 10:40:03 AM
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Okay I have a question.
Is there a reasonable power rating for a subwoofer that is placed in a lounge room? If so what power range should I aim for? I know that a power rating on a speaker doesn't tell you how loud the speaker is going to be but I'm just wondering. Thanks  |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Jul 19 2010 : 1:01:43 PM
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When I am alone I "turn up the volume" so that it is too loud to talk to anybody.
Then I feel the bass vibrate my car, home and body. My dog also likes the vibrations.
But a lounge is usually has people who are probably having a conversation so just background music volume is wanted (only a few Watts). |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 19 2010 : 11:55:45 PM
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| Okay thanks. I've decided that 100WRMS into each surround sound speaker is the maximum I need and 150WRMS into the LFE speaker. It's just a learning experience anyway. I'm not making this project to sell. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 20 2010 : 03:37:17 AM
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| So I'm gonna design a power supply that will satisfy those output power requirements above but the power amplifiers I'll be using will be rated at a higher power output. Since I have never designed a SMPS this powerful before I will start "small" then if I want to later I can either tweak the current SMPS or design a new SMPS to take full advantage of the audio amplifiers. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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wasssup1990
Nobel Prize Winner
A Land Down Under
2261 Posts |
Posted - Jul 20 2010 : 04:07:59 AM
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| In an attempt to keep PCB tracks and power wiring small I've decided that I will be powering all the audio amps from 50V rather than 30V. With 8ohm speakers this all means lower current than what I was originally designing for. I'd think that switching at lower currents will reduce EMI as well. I'll still be designing for a combined output power of 850WRMS but theoretically I can get this beast to push out 1280WRMS after upgrading the power supply and increasing PVDD limits per channel inside the MCU. |
When one person suffers from a delusion it is called insanity.
When many people suffer from a delusion it is called religion. |
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Audio Amp - Converting varying voltage to current
