audioguru Posted May 30, 2005 Report Share Posted May 30, 2005 Nice new project: http://www.electronics-lab.com/projects/automotive/012/index.htmlIts author claims that an audio amp can supply 50W to an 8 ohm speaker with a +-20V supply. I disagree.A typical amp can supply only 17.6W to an 8 ohm load at clipping when powered by +-20V. About the same amount of power as a bridged amp driving a standard 4 ohm car speaker without this complicated voltage converter.If this project was designed for a +-32V output, which is 60% more, then it could power a typical amp that would produce 50W into 8 ohms at clipping. ;D Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted May 30, 2005 Report Share Posted May 30, 2005 You are right, but only at half.I'm claiming 50W output power on Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted May 30, 2005 Report Share Posted May 30, 2005 How about noise?As with any switched mode power supply the output is likely to be very noisy.I know L1, C8 and C9 will do their bit but, the electrolytics have tend to have a significant ESL which can limit their performance at high frequencies.I'd recommended an inductor similar to L1 in series with each output and a 100nf capacitor connected to ground. Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted May 30, 2005 Report Share Posted May 30, 2005 Thanx for the advice. I will use the caps on the amplifier input (1000uf+100nF for each rail near the amplifier). Anyway even it is noisy because of ripple, the noise is in the ultrasound range (areound 50KHz), so it will not be heard Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted May 30, 2005 Report Share Posted May 30, 2005 I strongly recommend you use inductors too.Even though the ripple will be 50kHz there's a small risk it can be demodulated by non-linear elements in the amplifier and be converted to audio frequencies. Some people have told me they can hear a local AM radio station on their guitar amplifier. ;DThe TL494 controller also PWMs the 50kHz output according to the load so will introduce lower frequency components into the signal, if you're very unlucky and demodulation occurs in the amplifier it will make some very strange sounds and possible even oscillate since the PWM signal will depend on the amplifier load and if the amplifier is demodulating the output will affect the PWM thus causing positive feedback. As well as 50kHz there will be harmonics at 100kHz, 200kHz, 300kHz etc. possibly extending to 1MHz, these could be easily picked up on the AM radio band. Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 30, 2005 Author Report Share Posted May 30, 2005 Hi Jonathan,I am sorry that I misunderstood your load impedance. So your voltage converter project can power a 30W per channel into 4 ohms stereo amplifier. I wish it was designed for much more voltage and power output so it could drive National's Overture amplifiers to the much higher power that they were designed to produce.Have you seen Philips' TDA1562Q class-H amplifier IC? Its output is rated at 55W RMS into 4 ohms at low distortion with an ordinary car battery for its power supply. It has a voltage-doubler inside and operates as a 14.4V bridged amp most of the time to stay fairly cool, then switches at zero crossings of the input signal to using its doubled supply voltage for high power peaks. It also doesn't need your voltage converter circuit for much more power than your project.http://www.semiconductors.philips.com/acrobat/datasheets/TDA1562Q_ST_SD_2.pdf Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted May 30, 2005 Report Share Posted May 30, 2005 Hi Jonathan,I am sorry that I misunderstood your load impedance. So your voltage converter project can power a 30W per channel into 4 ohms stereo amplifier. I wish it was designed for much more voltage and power output so it could drive National's Overture amplifiers to the much higher power that they were designed to produce.Have you seen Philips' TDA1562Q class-H amplifier IC? Its output is rated at 55W RMS into 4 ohms at low distortion with an ordinary car battery for its power supply. It has a voltage-doubler inside and operates as a 14.4V bridged amp most of the time to stay fairly cool, then switches at zero crossings of the input signal to using its doubled supply voltage for high power peaks. It also doesn't need your voltage converter circuit for much more power than your project.http://www.semiconductors.philips.com/acrobat/datasheets/TDA1562Q_ST_SD_2.pdfOf course it can be designed very easily to output +/- 30VDC per rail. This can be accomplished varying two things. First the turn ratio (instead of 1:2 using 1:3) and downregulating to 30V via PWM (adjusting the voltage divired feeding the error amplifier accordingly). If you want to change the turn ration without changing primary turns (and so the switching frequency), you can use 5T /15T of litz or multiple wire (more wires in parallel to reduce skin effect). This design can also work at 35V if you keep the input voltage at 12V (though it may drop due to wire resistance/battery resistance). Since at 30V the power will be higher the core size must be increased accordingly, or the switching frequency (but this will higher the hysteresys and switching losses). You may try 3:9 or 4:12 with a higher switching frequency.The TDA1562Q is marvellous, probably it is used in most car amplifiers . But what does mean class-H ? Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 30, 2005 Author Report Share Posted May 30, 2005 Hi Jonathan,Class-H is a mode of amplifier design that switches between supply voltages on demand. I wonder if the switching is audible at very low frequencies, where a zero-crossing has already past and a very loud phrase of bass music occurs. An ordinary amplifier would produce max power for this half-cycle, but the class-H amp would clip this half-cycle then switch to the higher supply voltage for the next high power half-cycle without clipping.It would need a memory and be running in the past in order for it to switch its supply voltage at exactly the correct time. ;D Quote Link to comment Share on other sites More sharing options...
MP Posted May 31, 2005 Report Share Posted May 31, 2005 Jonathan,Thanks for your contribution to the site. It seems that every time you post a new project, you are challenged by someone who thinks you made mistakes (like when you posted the temp controlled fan).I hope it is not discouraging to you. Very good work!MP Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 31, 2005 Author Report Share Posted May 31, 2005 I made a simple challenge because the project severely under-powers a pair of the excellent amplifier IC's that the author mentioned: Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted May 31, 2005 Report Share Posted May 31, 2005 Jonathan,Thanks for your contribution to the site. It seems that every time you post a new project, you are challenged by someone who thinks you made mistakes (like when you posted the temp controlled fan).I hope it is not discouraging to you. Very good work!MPI'm not discouraged ;), answering questions and solving doubts is funny, also Audioguru started an interesting discussion.I made a simple challenge because the project severely under-powers a pair of the excellent amplifier IC's that the author mentioned:The power supply is easily scalable up. With those heatsinks the mosfets should hold up to 300W (but with fan), the important thing is the transformer core, that must be sized accordingly. Also the turns strenght must be increased or they will heat up and reduce efficiency, the diodes too must be be increased, maybe using higher current capablility shottkies. The reason that i scaled down the power to 30W is because i have 20W speakers on hand (and probably i cannot go farther than 20W without blowing them). I'm building a small amplifier for a small car, to be used with an mp3 cd player. (suggestions with the Lm3886 amplifier are welcome Audioguru) Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted May 31, 2005 Report Share Posted May 31, 2005 I'm not discouraged ;), answering questions and solving doubts is funny, also Audioguru started an interesting discussion.Indeed engineering would have never got to the advanced level it is today if it wasn't for people like audioguru challenging ideas and offering constructive criticism. :) Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted May 31, 2005 Report Share Posted May 31, 2005 Indeed engineering would have never got to the advanced level it is today if it wasn't for people like audioguru challenging ideas and offering constructive criticism. :)Costructive criticism is the key for technological improvement and learning :) Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 31, 2005 Author Report Share Posted May 31, 2005 Thanks, Alun. ;DHi Jonathan,Thanks for your project but I don't think I need your voltage converter for my car. My factory-equipped 4-channel radio/CD player puts out about 14W per channel at low distortion into my high-quality 4 ohm speakers.Without modification, your voltage converter at full output would power an amp to produce about the same power. :( Quote Link to comment Share on other sites More sharing options...
JonathanFilippi Posted June 6, 2005 Report Share Posted June 6, 2005 Thanks, Alun. ;DHi Jonathan,Thanks for your project but I don't think I need your voltage converter for my car. My factory-equipped 4-channel radio/CD player puts out about 14W per channel at low distortion into my high-quality 4 ohm speakers.Without modification, your voltage converter at full output would power an amp to produce about the same power. Quote Link to comment Share on other sites More sharing options...
audioguru Posted June 6, 2005 Author Report Share Posted June 6, 2005 Hi Jonathan,I deleted mentioning the very high power SMPS car power supply that I posted because it wasn't fair to you and your much lower-power project. I am sorry that I mentioned it.You don't have mains beside your tabletop so you can use a simple transformer to power your amp? ???We discussed it before but I couldn't find it in a search: do you believe that your car system actually puts out 52 X 4 Whats? ???My 14 Watts RMS per channel into 4 ohms at low distortion over the entire audio band system could also be called 52 Whats per channel:1) 28 Watts with square-wave output, usually called peak or maximum power.2) Measured only at 1kHz because it doesn't do as well at 20Hz nor at 20KHz.3) Power doubled again to nearly 56 Watts into a 1 ohm load, measured very briefly before it blows up!The car amplifiers marketing departments play a numbers game with power ratings. ;D Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted July 7, 2005 Report Share Posted July 7, 2005 I have a general question about the circuit. It appears the transformer combo will not work. I have searched for a mistake but I still cannot make it work. I think what is missing is that a rectifier bridge will not give you the desired results. You have to have a center tapped transformer to get both the positive and negative supply voltages. You can do this with a rectifier bridge, but not both positive and negative at the same time.Please tell me how I should evaluate this rectifier cirucuit if you feel that it works. Thank You. Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 7, 2005 Author Report Share Posted July 7, 2005 Kevin,Which circuit did you build?This project does have a center-tapped transformer. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted July 7, 2005 Report Share Posted July 7, 2005 Stick with me. I'm on to something. It is not a bridge rectifier. I was wrong. But the coil is playing a weird part in this. Follow the polarity.I am right. Follow both the negative cycle and the positive cycle. Try using one half of this circuit. Try the positve supply voltage. Notice that the polarity of the coil does not change on both the positive and negative cycles. Now apply the other half of the circuit. Notice that the polarity of the coil remains the same for both cycles, but it is opposite the polarity of the coil using the positive supply voltage. You can't have it both ways. Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 7, 2005 Author Report Share Posted July 7, 2005 It is not a bridge rectifier. I was wrong. But the coil is playing a weird part in this. Follow the polarity.Of course it is a full-wave bridge rectifier and with the seconday's center-tap grounded, produces positive and negative voltage outputs.L1 connects the center-tap to ground and doesn't have any current through it when the positive and negative load currents are equal. ;D Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted July 7, 2005 Report Share Posted July 7, 2005 Audioguru, read my last reply. It will explain it. It's the polarity of the coil. Trust me. If your looking for the positive supply, the polarity of the coil remains the same for both cycles. But if your looking for the negative supply, the polarity of the coil remains the same for both cycles, but it is opposite the positive supply.I have determined this not to be a standard bridge. The problem with the circuit is the coil location. Thank you. Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 7, 2005 Author Report Share Posted July 7, 2005 Kevin,Coil L1 is just a piece of wire to ground, with a small inductance to slow-down the fast charging current of the filter capacitors when the positive and negative loads are unbalanced. Since the center-tap is grounded, when one end of the secondary is positive, the other end is negative and they keep swinging with opposite voltages.The secondary rectifier bridge is a standard circuit for a dual-polarity full-wave output. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted July 7, 2005 Report Share Posted July 7, 2005 Can you see the polarity problem I am talking about? Just scribble it down on paper. The polarity of the coil for the positive supply is opposite for the negative supply.I am very skeptical about this situation. If it does not work on paper, there is something screwy going on. Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 7, 2005 Author Report Share Posted July 7, 2005 The polarity of the coil for the positive supply is opposite for the negative supply.It doesn't matter because the resistance and impedance of the coil L1 is very low. The current comes from the entire secondary winding, not from only half of it. Since one end of the secondary is positive and the other end is equally negative, the center-tap is at ground voltage. The center-tap is connected to ground through the coil as its 0V reference. If it wasn't connected to ground, the power supply would still work, but its output voltages would drift all over the place while still 40V apart. +20V might drift up to +30V, while -20V would drift up to -10V. Quote Link to comment Share on other sites More sharing options...
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