Automotive 12V to +-20V converter

[audioguru2]

Nice new project: http://www.electronics-lab.com/projects/automotive/012/index.html


Its 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
View attachment 37105

 

[JonathanFilippi]

You are right, but only at half.

I'm claiming 50W output power on

 

[Alun]

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.

 

[JonathanFilippi]

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

 

[Alun]

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. ;D

The 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.

 

[audioguru2]

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

 

[JonathanFilippi]

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

Of 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 ?

 

[audioguru2]

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

 

[MP1]

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

 

[audioguru2]

I made a simple challenge because the project severely under-powers a pair of the excellent amplifier IC's that the author mentioned:

View attachment 37114

 

[JonathanFilippi]

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

I'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)

 

[Alun]

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.

 

[JonathanFilippi]

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 

 

[audioguru2]

Thanks, Alun. ;D

Hi 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. 

 

[JonathanFilippi]

Thanks, Alun. ;D

Hi 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.

 

[audioguru2]

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

 

[Kevin Weddle]

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.

 

[audioguru2]

Kevin,
Which circuit did you build?
This project does have a center-tapped transformer.

View attachment 37299

 

[Kevin Weddle]

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.