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sashaw 9th September 2011 12:12 PM

Newbie, needs suggestion
 
Hi all, first post here. After reading few of other's posts and some document. I copy and combine them to have the design like the one in the graph. I know it is very simple to most of you. I would like to have some suggestions and corrections. I want to try the opa637 and buf634 combination and also parallel the buffer. I will power them with +-12V inside a cmoy like candy can.
http://yuwei.net.nz/images/637_634.png

sgrossklass 9th September 2011 04:48 PM

First of all, better persuade your CAD software to draw some dots where lines are connected, as it's usually done. Anything else gets confusing.

What do you intend to use in terms of cans and power source to warrant parallel BUF634s? You'd need <30 ohm loads driven at full swing for a single one to break into sweat.

Also, wouldn't that potentially get a little warm under serious loads? And are you sure that you could fit 2 channels of that onto a board small enough to fit something the size of a classic Altoids tin?

You have a single stage concept with a 11x (21 dB) gain there, hmm. First of all the gain setting resistors R8/R5 seem a bit high in value (I'd drop these to 1k/10k or so). I would also consider ditching the 2nd buffer in favor of a two-stage concept (10 dB before volume control, 10 dB after), for lower noise and better distortion performance.

In any case you should mention the intended performance goals and signal sources.

sashaw 10th September 2011 01:54 PM

I am using EAGLE under mac osx, I have no idea how to turn the dot thing one. Can someone help. Thanks sgrossklass, here is an update from your suggestions. http://yuwei.net.nz/images/637_634_v2.png

discrete 10th September 2011 03:12 PM

Quote:

Originally Posted by sashaw (Post 2704618)
I am using EAGLE under mac osx, I have no idea how to turn the dot thing one. Can someone help.


You have to place the dots one-by-one, yourself. Look for it on the toolbar in Eagle, somewhere close to the line tool.

Your circuit is basically this:
PIMETA v1 Headphone Amplifier
(don't go with the 3-channel/virtual ground idea though)

I would leave space to put a 50 to 100 pF local feedback cap on the op-amp in case of instability, like shown in the schematic in this relevant article:

http://waltjung.org/PDFs/WTnT_Op_Amp_Audio_1.pdf

I would also add a zobel network on the output, maybe 47 ohm in series with 100 nF, output to ground.

For the buffer you can use somewhat bigger bypass capacitors, maybe 1uF//470uF.

sashaw 11th September 2011 12:50 PM

Quote:

Originally Posted by discrete (Post 2704667)
You have to place the dots one-by-one, yourself. Look for it on the toolbar in Eagle, somewhere close to the line tool.

Your circuit is basically this:
PIMETA v1 Headphone Amplifier
(don't go with the 3-channel/virtual ground idea though)

I would leave space to put a 50 to 100 pF local feedback cap on the op-amp in case of instability, like shown in the schematic in this relevant article:

http://waltjung.org/PDFs/WTnT_Op_Amp_Audio_1.pdf

I would also add a zobel network on the output, maybe 47 ohm in series with 100 nF, output to ground.

For the buffer you can use somewhat bigger bypass capacitors, maybe 1uF//470uF.

Thanks for the tip on eagle.

Here is an update from your suggestions. One thing I don't understand is the bypass capacitor. Do you mean C6, C7, C8 and C9?
http://yuwei.net.nz/images/637_634_v3.png

discrete 11th September 2011 07:28 PM

Quote:

Originally Posted by sashaw (Post 2705603)
One thing I don't understand is the bypass capacitor. Do you mean C6, C7, C8 and C9?

Those are the ones.
Bigger is better, but with rapidly diminishing returns above some level, depending on your power supply, power feed layout and headphone impedance.

sashaw 11th September 2011 09:53 PM

I don't think I will use it to drive my HD650, so the highest impedance will be 150ohm yamaha hp-2. and maybe RE0.

sashaw 16th September 2011 10:52 AM

One more question, What type of capacitor should I use for the bypass capacitor? electrolytic or tantalum?

Mooly 16th September 2011 11:24 AM

Quote:

Originally Posted by sashaw (Post 2712129)
One more question, What type of capacitor should I use for the bypass capacitor? electrolytic or tantalum?

There is no absolute right and wrong here.

An electroylitic should be used of around 10 to 47uf (these are bypass, and local decoupling remember, not reservoir caps) and these should have a small film/polyester type in parallel with them of around 0.1uf

There are some circumstances where that is actually the wrong thing to do and the film cap should also have a small resistance of around 1 ohm (carbon/metal non inductive) in series with it.

This is because if there is a specific problem such as spikes from an SMPS then just paralleling the caps can actually make the problem worse if the frequency sets up some resonance with other inductive components on the PCB such as print or component leadouts.

In your case you won't go far wrong just paralling a film cap with the electros.

sashaw 16th September 2011 11:30 AM

Quote:

Originally Posted by Mooly (Post 2712147)
There is no absolute right and wrong here.

An electroylitic should be used of around 10 to 47uf (these are bypass, and local decoupling remember, not reservoir caps) and these should have a small film/polyester type in parallel with them of around 0.1uf

There are some circumstances where that is actually the wrong thing to do and the film cap should also have a small resistance of around 1 ohm (carbon/metal non inductive) in series with it.

This is because if there is a specific problem such as spikes from an SMPS then just paralleling the caps can actually make the problem worse if the frequency sets up some resonance with other inductive components on the PCB such as print or component leadouts.

In your case you won't go far wrong just paralling a film cap with the electros.

Thanks for making things clear for me.


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