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macs_lex
Joined: 13 Sep 2009
Posts: 9
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 03 Oct 2009 7:21 audio power amplifier |
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i just want to ask how to make an audio power amplifier (class B) on final stage to drive a load of 4 or 8 Ω to output power of 30 W? thank you. it's for a project. i'm quite confuse with the circuit i have. thanks again!
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03 Oct 2009 7:21 Ads |
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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04 Oct 2009 19:44 audio power amplifier |
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A class-B audio amplifier has crossover distortion. You want a class-AB audio amplifier.
30W RMS into 8 ohms is a signal that is 15.5V RMS or 43.8V p-p. Then the power supply must be about 54VDC minimum at 1.1A.
The amplifier would be an IC. Try an LM3875 or LM3886.
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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05 Oct 2009 9:29 audio power amplifier |
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so it wouldn't be possible for a class B amplifier? thank you so much for the reply!
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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05 Oct 2009 15:55 Re: audio power amplifier |
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| macs_lex wrote: |
| so it wouldn't be possible for a class B amplifier? thank you so much for the reply! |
Audio amplifiers use Class-AB because a Class-B circuit has crossover distortion that sounds horrible.
Here is a sine-wave without and with crossover distortion:
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pauloynski
Joined: 06 Oct 2004
Posts: 277
Helped: 34
Location: Brazil
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05 Oct 2009 16:32 Re: audio power amplifier |
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As posted above, class-B amplifiers sound horribly. However, they are used in some PA systems. There is one exception that comes to my mind: the current dumping technology developed by Quad uses a true class-B output and a small class-A driver which is used to drive small amplitude signals, resulting in a very good quality amp.
It is not an easy made project so, again as posted above, try using a LM3886 or similar and avoid having a headache.
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pauloynski
Joined: 06 Oct 2004
Posts: 277
Helped: 34
Location: Brazil
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05 Oct 2009 16:47 Re: audio power amplifier |
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Attached find the schematic of the Quad405 amp. As you can see, it is not an easy DIY project.
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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10 Oct 2009 11:12 Re: audio power amplifier |
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whew! yeah it's hard. 
but i'll try the dumping amplifier. we really have to make a class b. yeah, it's a pain really. but i have to make it. <laughs> thank you so much! thank you for the replies.. now i have a better idea..
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pauloynski
Joined: 06 Oct 2004
Posts: 277
Helped: 34
Location: Brazil
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10 Oct 2009 12:38 Re: audio power amplifier |
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Please find attached the original article from the Wireless World magazine (1975) about the current dumping amp (a nasty design, but it works!!!).
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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13 Oct 2009 7:38 Re: audio power amplifier |
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| Do you have a pre-amplifier circuit than can be connected to a class b power amplifier?thanks a lot!
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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13 Oct 2009 14:19 Re: audio power amplifier |
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| macs_lex wrote: |
| Do you have a pre-amplifier circuit than can be connected to a class b power amplifier?thanks a lot! |
We don't use a class-B amplifier because it has horrible crossover distortion. Instead we use a class-AB amplifier.
What do you want the preamp to do? Microphone input? Phono inputs?
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pauloynski
Joined: 06 Oct 2004
Posts: 277
Helped: 34
Location: Brazil
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13 Oct 2009 14:24 Re: audio power amplifier |
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Many audio amplifiers are designed to deliver full power to the load when the input voltage is in the range from 0.7 to 1V. And, obviously, many pre-amplifiers are designed to to deliver this same voltage. Each kind of sensor (e.g. a microphone) has its own characteristics and need different amplification and equalization. Apart from this, some tone controlls are desiderable. You can find lots of circuits on internet.
http://www.hobbyprojects.com/T/tone_controls.html
http://www.electronic-circuits-diagrams.com/audioimages/audiockt13.shtml
http://www.aaroncake.net/circuits/tone.asp
http://www.zen22142.zen.co.uk/Circuits/Audio/t-ctrl.htm
http://www.hobbyprojects.com/A/preamplifiers.html
http://www.tkk.fi/Misc/Electronics/circuits/micamp.html
http://www.epanorama.net/links/audiocircuits.html
Regards
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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15 Oct 2009 6:41 Re: audio power amplifier |
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pre-amplifier with a sound signal input coming from a player(mp3, cd, mp4, etc.) then the output would be connected to the input of the class b amplifier. the output of the class b amplifier is a speaker (8Ω).
we are to use lm741 in our pre-amplifier.
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pranam77
Joined: 22 Apr 2008
Posts: 1071
Helped: 94
Location: Mangalore(India)
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15 Oct 2009 8:01 Re: audio power amplifier |
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Find here an audio preamplifier with single transistor for auxillary Input. This is tested and used by me for my personal audio amplifier. Hope it helps you. Cheers
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pauloynski
Joined: 06 Oct 2004
Posts: 277
Helped: 34
Location: Brazil
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15 Oct 2009 12:50 Re: audio power amplifier |
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| These equipment can be directly connected to the input of the amplifier and you can use their internal volume control if you wish but the circuit posted above is also recomended. You can use the LM741 (TL071 recomended) with unity gain followed by a logarithmic potentiometer too (doing the same as the circuit posted above). It has not been clear to us why do you need to use a class-B amplifier since modification to class-AB is very simple.
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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15 Oct 2009 14:18 audio power amplifier |
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The single transistor preamp circuit is odd.
It has loss instead of amplification.
Its input is a divide by 48 times attenuator then the transistor has as much gain as possible to make up for it.
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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16 Oct 2009 10:44 Re: audio power amplifier |
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thank you. it's a project that requires a class b amplifier. how we wish to make a class ab, but it is a must to make the class b. maybe our teacher wanted us to learn first from class b before jumping to class ab. thank you very much for the circuit. i'll try it. i'm just having a hard time with the pre-amp. we are not being spoon-fed in our school that's why almost everything is self-studied. thank you very much to your replies. it helps me a lot!
how about for BJT used in push-pull amplifier?
what do you think is the best to use?
thanks again!
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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16 Oct 2009 14:06 audio power amplifier |
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You can make a very simple class-B amplifier with an opamp, an NPN power transistor, a PNP power transistor and two resistors to set the gain. It will have crossover distortion that makes sounds like a buzzer.
The two power transistors are complementary emitter-followers and drive the speaker push-pull.
The opamp has a max output current of about 30mA and the power transistors will have a minimum current gain of 25 so the minimum peak output current is about 750mA or 530mA RMS. Then the power into an 8 ohm speaker is 2.2 Watts. The power supply will need to be at least 16VDC.
I am not going to sketch the simple schematic because it is your project.
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macs_lex
Joined: 13 Sep 2009
Posts: 9
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18 Oct 2009 11:15 Re: audio power amplifier |
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| yeah. i got it already. thanks a lot!
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mr_byte31
Joined: 19 Oct 2005
Posts: 198
Helped: 1
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20 Oct 2009 8:13 Re: audio power amplifier |
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| pauloynski wrote: |
As posted above, class-B amplifiers sound horribly. However, they are used in some PA systems. There is one exception that comes to my mind: the current dumping technology developed by Quad uses a true class-B output and a small class-A driver which is used to drive small amplitude signals, resulting in a very good quality amp.
It is not an easy made project so, again as posted above, try using a LM3886 or similar and avoid having a headache. |
I tried to simulate this circuit on pspice
it doesn't work at all
i don't understand why u put transistors on the +V and -V of the opamp!!!!
did u simulate it before?
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FvM
Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany
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20 Oct 2009 8:31 Re: audio power amplifier |
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| Quote: |
I tried to simulate this circuit on pspice
it doesn't work at all |
Which circuit? Are you refering to the "dumping-amplifier-100w", which seems to be an ancient Elektor circuit?
Last edited by FvM on 20 Oct 2009 8:35; edited 1 time in total |
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mr_byte31
Joined: 19 Oct 2005
Posts: 198
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20 Oct 2009 8:34 audio power amplifier |
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| this one http://www.edaboard.com/files-eboard/dumping-amplifier-100w_1380.gif
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FvM
Joined: 22 Jan 2008
Posts: 5161
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Location: Bochum, Germany
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20 Oct 2009 8:45 Re: audio power amplifier |
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There are some oddities, e.g. TIP142/147 aren't shown as darlingtons, also with worst case LM741 parameters, cross conduction may occur. But it should basically work, although I don't want to predict the sound quality.
I know, that this structure has been popular among DIY audio guys some decades ago, and I guess, it's better than a basic class B. I have also seen similar structures in professional products, but they always used class AB output stages.
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