Audio Amplifier, Current Limiting Voltage Supply problem - diyAudio
 Audio Amplifier, Current Limiting Voltage Supply problem
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 Class D Switching Power Amplifiers and Power D/A conversion

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 10th January 2013, 07:23 PM #1 diyAudio Member   Join Date: Jan 2013 Audio Amplifier, Current Limiting Voltage Supply problem Hello everyone, In the analog electronics project, we are required to build an audio amplifier. The restrictions are: - IC's will not be used. - At least 6 watts will be delivered to 8 ohms speaker. I have made 2 designs, one class ab and one class d amplifiers, in simulation everything works fine, i can get pure sinusoids and upto 100 watts on speaker, with +-25 double supply rail and, bridge configuration to double the speaker voltage if necessary. However there is one little problem: The power supply that i use is Agilent E3631A, which gives max + - 25 volt limited by 1 A current for each of supplies. This limited current here means that (i suppose): The power cannot give more than 1 Amps in any instant of time, which will clip the current seen on power supply, and on the rest of the circuit at 1 Amps, if the project is not designed considering this fact. Therefore, in order not to clip the audio signal, I need to pass at most 1 amps sinusoid across speaker, which will give (1/sqrt(2))^2*8=4 watts. Now since i use +-25 supplies the 1 amp dc means 50 watt is driven from the supply. Where the rest of power goes is : 1. the driven current is not dc but ac with lower effective value, which means the driven power is much less. 2. in class ab the practical efficiencies are around 60-70% if and the rest is dissipated on the rest of circuit. I think class ab cannot do much about this current limiting. I am not sure about class d. I would be very thankful if you could have some comments. Am i correct on these, and may there be anyway to get through these? In addition, i cannot model the power supply such that it limits currents over 1A, in spice. How can i do that? Thank you for reading
 10th January 2013, 08:08 PM #2 diyAudio Member RIP   Join Date: Nov 2003 Location: Brighton UK Hi, Why use the power supply if its clearly not suitable ? Why use a supply with current limiting when clearly what you want with an audio amplifier is high peak current ? rgds, sreten. (Look up crest factor for music signals.) Last edited by sreten; 10th January 2013 at 08:21 PM.
 10th January 2013, 08:33 PM #3 diyAudio Member   Join Date: Jan 2013 it is because there is only this supply in the analog electronics labaratory, we can only use this. Therefore i am wondering if i can give more than 4 watts to the speaker with this supply..
 10th January 2013, 08:46 PM #4 diyAudio Member   Join Date: Dec 2003 Location: Munich ...you could consider to tease the E3631A with huge rail caps...
 10th January 2013, 08:51 PM #5 diyAudio Member   Join Date: Jan 2013 sreten, Because, it is the only available power supply in labaratory. I don't want to buy high watts adaptor, if i can find a solution to the problem. chocoholic, Do you mean bypass capacitors connected between supplies and ground, by "teasing the supply" and "huge rail caps". Can you explain more or can you give a website's link for me to search?
 10th January 2013, 10:29 PM #6 diyAudio Member   Join Date: Mar 2005 Location: Quebec With a class d amplifier, you should reach near your 100W... Output current is always more than supply current.
 12th January 2013, 06:54 PM #8 diyAudio Member   Join Date: Jan 2013 Thank you very much chocolic, I was planning to use the large caps for giving the very high shoot-through current impulses reflected to the supply in class d amp design. Since these are impulses(very short time discharge of capacitors), i thought that large enough caps can effectively deliver shoot through currents, and maintain circuit operation (although to say this is odd , since shoot-through currents are unwanted in class-d design, and has to be kept small as possible). I did not think that these caps can be used in class ab stage to provide much longer discharge currents to the amp. However considering that supply does not give current for one half cycle(charging the capactior fastly), and since that at the other half cycle the capactior will begin its duty only after source cannot give 1A, this idea is very reasonable. I made the roughest estimates and reasonable sizes of capacitors can do the job. And in the simulation it does not cause problems dor audio frequencies(20-20k). However thinking that i am not going to give a sine wave after finishing the design but a complete acoustic signal, i do not know about its effects yet. However i put two sinusoidal sources with differenct frequency and delay in series, and results seems okay. Thank you very much again
 12th January 2013, 10:03 PM #9 diyAudio Member   Join Date: Jan 2003 Location: mississauga ontario canada Based on your amplifier using 100mA for Iq and everything else, and a capacitor big enough to supply 1.8A for the top half of the lowest frequency, and a peak output swing of 21.5V @ 2.7A, you should be able to get 28.9W before supply sag or clipping. If the cap could only supply 0.5A for that period of time you would still get 8W Your target is 6W No problem __________________ Doug We are all learning...we can all help "You can't stop the signal, Mal. Everything goes somewhere..."
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Join Date: Dec 2003
Location: Munich
Quote:
 Originally Posted by efevi I made the roughest estimates and reasonable sizes of capacitors can do the job. And in the simulation it does not cause problems dor audio frequencies(20-20k).
...now I am curious which cap size / how much voltage sagging you are planning.

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