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Old 23rd January 2013, 08:22 PM   #1
CraigP is offline CraigP  United States
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Default Simple DIY Amp

I'm looking to build a simple DIY two channel amp so I can run my computer sound card through a pair of 8Ohm Realistic Minimus 77 speakers.

I'm doing this mostly to learn more about building amps, but also in order to have a better sounding computer speaker than I currently have.

I've attached a datasheet on my computer sound card's output. I've highlighted the two sections I think I'm interested in with a red box. My question to the group, if the output Z of the sound card is 100 ohms what would I want my input Z of my amp to be? Or more specially what rule of thumb, or formula do I use to calculate that? I know I don't want to "load down" the sound card with too low a impedance.

I've also attached a drawing of a circuit I'm considering using for this. Though my other option is a simpler circuit based on the LM386. The part I like about the attached circuit over the LM386 is as I learn more, I can upgrade and tweak the circuit to make it better.

Thoughts?


- Craig
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Old 23rd January 2013, 08:56 PM   #2
JMFahey is offline JMFahey  Argentina
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Get some 10+10 to 15+15W amp kit, using a TDA2030 or TDA2050 or LM1875 chip amp per channel, plus the corresponding power supply.
Nice power, matches your speakers, is easily driven by a PC soundcard and easy/inexpensive to build.
An LM386 is less than 1W, not worth the effort.
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Old 23rd January 2013, 09:24 PM   #3
jaycee is offline jaycee  United Kingdom
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opamp plus transistors can work, but save yourself a lot of hassle and use a symmetric supply rather than a single voltage source.

Soundcards - it varies. To be safe stick with 47K impedance or higher.
For driving speakers you will want something better than 2n5401/2n5551, such as BD139/140 or possibly some TO-220 devices. Better still use some combination to make Sziklai output pairs and you'll get pretty good performance.

This project, designed for headphones but can also easily drive small speakers, may give you some clues: Headphone Amplifier
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Old 23rd January 2013, 09:36 PM   #4
selim is offline selim  Turkey
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Hi Craig,

With the component values you have shown, you won"t be loading your
sound card anywhere near its limits.

I once built an amplifier very similar to yours, except that mine was running from dual supplies.The sound quality, to say the least, left a lot to be desired.
The lesson I have learned from that experiment is that non-biased output
transistors MUST be avoided.There are a few steps you can take to make
the amplifier more 'listenable'.

1.Connect the output of the opamp to the base of the upper transistor.
2.Bias the output transistors (A string of diodes, a Vbe multiplier or any other
method of your choice.) Don't forget the emitter resistors after biasing.
3.Connect an R form the base of the lower transistor to the ground.This will
keep the opamp in Class A.Perfectionists may prefer a CCS.

Hopefully, you will have a better amplifier now, albeit a little more component
count than when you started.

Happy soldering
Selim
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Old 23rd January 2013, 09:41 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

The rule of thumb is greater than 10 times, here > 1Kohm.

However 10Kohm is a more typical amplifier input impedance.

rgds, sreten.
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Old 24th January 2013, 01:02 PM   #6
CraigP is offline CraigP  United States
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Thanks for all the ideas and suggestions!
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Old 24th January 2013, 01:03 PM   #7
CraigP is offline CraigP  United States
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Do you have a schematic for something like this? Or perhaps can you link one?




Quote:
Originally Posted by selim View Post
Hi Craig,

With the component values you have shown, you won"t be loading your
sound card anywhere near its limits.

I once built an amplifier very similar to yours, except that mine was running from dual supplies.The sound quality, to say the least, left a lot to be desired.
The lesson I have learned from that experiment is that non-biased output
transistors MUST be avoided.There are a few steps you can take to make
the amplifier more 'listenable'.

1.Connect the output of the opamp to the base of the upper transistor.
2.Bias the output transistors (A string of diodes, a Vbe multiplier or any other
method of your choice.) Don't forget the emitter resistors after biasing.
3.Connect an R form the base of the lower transistor to the ground.This will
keep the opamp in Class A.Perfectionists may prefer a CCS.

Hopefully, you will have a better amplifier now, albeit a little more component
count than when you started.

Happy soldering
Selim
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Old 25th January 2013, 02:19 AM   #8
routhun is offline routhun  United States
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Location: Sunnyvale
Hi Craig

Save yourself sometime. Check the amp HalOpB by OnAudio in the same forum. All the ideas suggested here were incorporated in that amp. Someone already started PCB for this amp.

Goodluck
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Last edited by routhun; 25th January 2013 at 02:21 AM. Reason: spelling mistake
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Old 25th January 2013, 01:59 PM   #9
JMFahey is offline JMFahey  Argentina
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Sorry but searching this entire Forum for "HalOpB" yields only one result: this very post.
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Old 25th January 2013, 03:44 PM   #10
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Yeah Craig, that's a terrible circuit for audio. It might be appropriate for a DC servo buffer.

Try an LM1875 chip. It's simple and you can build a circuit with a bare minimum of components. You can experiment with tweaking it too without getting too deep. You can use single or dual supply. A google search will yield hundreds of circuits. You might be surprised with the results.
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