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Old 1st January 2008, 05:27 PM   #1
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Default Diamante -a discrete medium power opamp

Hello and happy New Year!

I wanted to post something for the forum on the first day of 2008 in the hope that we all have happy and enjoyable new year, and hopefully some of you can use it in your audio pursuits.

I have designed a discrete operational amp/headphone amp I have named "Diamante" mostly because it has a diamond buffer type output stage.

The design is pretty simple and not at all expensive to build. The cost/performance ratio is very high. The prototype was made with all BC550 and BC560 transistors but others with the same pin configuration can also be used. I chose BC550/BC560 because of their low noise and high gain. Many transistors have reverse pin configurations and they can also be used but you will need to put them in reversed of course.

The basic schematic as shown is configured as a 6db voltage gain headphone amp. But you can configure the board any way you like. The amp is unity gain stable with 10pf at C5. Choose RF and RG so as not to overly load the output stage.

The output R (R26) is a 3W type with magnet wire wrapped around it (15-20 turns is fine) to form an output inductor. This will prevent ringing into capacitive loads, but it is not required. The amp is stable without it. if you wish to omit it you can jumper it with wire or take your output at the resistor pad.

Rbias sets the bias for all stages. I should be a 1K pot, when initially setting up the amp set it to full 1K resistance. Then slowly adjust it downward until you get about 25mv across the emitter resistors (R1-R6). This is a safe bias point.

Once you have the bias properly set, then adjust Roffset to get 0V offset from output to GND.

Wait around 5 minutes and re-check bias and offset.

This amp is pretty simple to scale. As it is presented it should be about to do 100ma or so without much problem. If you change the output devices you could get even more output.

It drives headphones with ease and that's really the purpose I had in mind. I will easily drive even 32ohm cans. 300ohm types such as HD650s are cake for this amp.

Noise floor is very low with the transistors specified. I can't hear any noise through the cans - nada.

Distortion is very very low, though I don't have any pretty graphs to show you yet.

So happy new year to all. I am providing PDFs and the Schematic for those who want to etch boards for DIY use only. The amp can actually be built pretty easily on prototyping board too.

Here is a picture of a prototype (slightly different CCS) in use. Special thanks to Brian Donegan who etched stuffed the first boards and encouraged me to finish the project.

Cheers!
Russ
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Old 1st January 2008, 05:56 PM   #2
t. is offline t.  United Kingdom
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Happy new year to you too!

This looks a nice amp Russ, thanks for sharing
I've actually been looking around lately to find something to try with my AKG K701's

Soon as I can get a board done I'll post results
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Old 2nd January 2008, 03:32 AM   #3
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Looks nice, Russ. I have quite a few BC550 and 560's on hand, but probably not enough to do this up...I'll have to check it out at some point, though, when funds allow. I look forward to seeing any numbers you guys can put up.
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Old 2nd January 2008, 10:35 AM   #4
TzeYang is offline TzeYang  Malaysia
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are transistor matchings extremely crucial with this design?

Thank you
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Old 2nd January 2008, 11:13 AM   #5
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The prototypes have no matching done. The offset adjustment lets you easily eliminate any offset. Matching would not hurt, however.
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Old 2nd January 2008, 11:40 AM   #6
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Dear Russ White,

Do you accept international orders and international credit card payment?

Thank you.
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Old 2nd January 2008, 11:47 AM   #7
Nordic is offline Nordic  South Africa
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Thanks Russ, will try it this year....

Do you know the song Diamante? It is by Zucherro... realy nice song...

The word allthough pronouced diffirently is the exact spelling for our (Afrikaans) word for diamonds...
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Old 2nd January 2008, 02:38 PM   #8
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Quote:
Originally posted by Nordic
Thanks Russ, will try it this year....

Do you know the song Diamante? It is by Zucherro... realy nice song...

The word allthough pronouced diffirently is the exact spelling for our (Afrikaans) word for diamonds...

Hi Nordic,

I am not familiar with the song, but I will listen to it if I can find it.

The diamond structure is integral to the amp, that's why I used the name.

Cheers!
Russ
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Old 2nd January 2008, 02:41 PM   #9
Nordic is offline Nordic  South Africa
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http://www.youtube.com/watch?v=95uL3ag9T3c

I am more familiar with the english version of the song... but it sounds pretty much the same... sadly the video clip looses out to the quality of his recordings... they are in another league just in recording quality...
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Old 2nd January 2008, 04:21 PM   #10
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Default Some notes...

RBias = 1K multi-turn pot in rheostat mode. Adjust it to 1K before initial power up. If If you wish you can note the resistance of across Rbias and replace the pot with a metal film resistor after you have the bias current you want.

Roffset = 10K multi-turn pot.

Roffset can actually be omitted, the offset should still be very low, especially if you match transistors.

Transistor matching should not be required at all.

When choosing Qs especially for the input and VAS use high gain types (200 or better hfe if possible) for best results.

BC560, and BC550 B and C types will work very well.

The LEDs we used on the prototype were green LEDs which have a forward voltage of about 2V.

Cheers!
Russ
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