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gain questions (cmoy) choosing the gain by a switch(schematic)

post #1 of 18
5/18/07 at 4:00pm
Thread Starter 
what gain is perfect for earphones of 11ohm(super fi 5 eb)i really need to know this because i want to make sure i dotnblow them up or something
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post #2 of 18
5/18/07 at 5:03pm
You will probably not blow them up with the provided gain; however, you want to be able to turn the knob a descent amount before the volume is unbearable. I have a gain of 3 on my 30ohm phones and my normal listening volume is when the potentiometer is about half way turned. I would try a gain of 2 or possibly even 1 if it doesn't cause any problems. Just get several resistors and see which one you like the best.
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post #3 of 18
5/18/07 at 5:10pm
Thread Starter 
1 will be great i think thx for your help your awsome


edit gain of 1 is not possible it seems (calculator..)
then 2 it will be
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post #4 of 18
5/18/07 at 5:26pm
I forget the CMOY, but isn't it just a difference amplifier, with a gain of
G = Vin(R2/R1) ?
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post #5 of 18
5/18/07 at 5:57pm
Thread Starter 
its like G=(R4/R3)+1
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post #6 of 18
5/18/07 at 6:34pm
Thread Starter 
so ive changed the schematic a bit. i made it so i can choose witch gain i want it to be (11 or 2) by using a switch. does this makes any sense at al? will it work?

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post #7 of 18
5/18/07 at 7:18pm
You could certainly use a switch to change gain but IMO, it's not a good idea. Ideally you'd want to keep the feedback loop short. When did you feel you might need a gain of 11? It might be too high for most uses.

Also, you may find OPA2132 insufficient current capability to drive low-z loads and these loads make a ground channel even more beneficial.
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post #8 of 18
5/18/07 at 7:29pm
Thread Starter 
what do you mean by "keeping the feedback loop short" ? and the gain of 11 you never know when its usefull, its not for the 11ohm EB's only. what opamp would be good then?
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post #9 of 18
5/18/07 at 7:49pm
That will just be one more switch to try to mount. Most people use the CMOY for portable and use something like a Pimeta for at home. Switchable gain is doable, but there is no need to do it in a CMOY. With 11ohm headphones you will only need a gain of like 1 or 2. A gain of 1 is achievable by jumpering the pin that choses gain instead of using a resistor. There is no possible need for a gain of 11 unless you are working with a cranky opamp. If you are thinking that you might go up to something that is like 600ohm, chances are that the CMOY would not sound good to you no matter what gain. I say that there is no reason to do it.

-Edit- I believe that mobo is referring to the fact that the small electronic parts (especially opamps) prefer to be directly after the resistor. The longer leads, especially as long as you would need to hook up a switch, can possibly throw your amp into oscillation. Some amps are more lenient than others, but some, like the AD8066 for example, are so cranky that mine oscillated when I tried to use a dip8 adapter.
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post #10 of 18
5/18/07 at 7:55pm
Thread Starter 
ty ill change it
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post #11 of 18
5/18/07 at 8:38pm
It's not just oscillation but change in feedback up to that point as well, potentially effects output. Remember that in the feedback loop (the pair of resistors forming the voltage divider from output to inverting input) this is how the opamp "knows" what it's doing (not really, but for our purposes it's true enough) and corrects itself. The ideal is infinitely short feedback like soldering resistors direct to the opamp pins and to ground, but for practical purposes we like to lay it out nicely on a board but still short.

Use of other opamps with inherantly higher current tends to mean more work to keep them stable. Tangent has written a bit on his website. As for what opamp you want, that's a whole 'nuther can of worms since many have their own sonic signature, particularly directly driving loads. Basically, to achieve more current is the starting point to move away from the basic CMOY and onto more elaborate amps. Truth be told, some sources already have as much current capability as the opamps being used in CMOYs. Then again I don't know how loud you need it, certainly some experimenation is in order if you had multiple opamps and socket it.

I doubt you will ever need a gain of 11, except if you had a passive crossfeed or other kind of passive circuit in front of the amp like an input selector.
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post #12 of 18
5/18/07 at 10:17pm
If you feel that you will be using it with a higher impedance headphone then you could go with a gain of 4; that might suit you well.
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post #13 of 18
5/19/07 at 7:26am
im using 2132PA for my CMOY. if i set gain at 11, there is a big noise, so i have to reduce gain to 2 (R3=R4=10k).
can you tell me more abit about function of R5? what is the feedback loop? thank you.
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post #14 of 18
5/19/07 at 7:42am
An opamp is designed to output the differnce between it's positive and negative inputs. The feedback loop is just taking what it's outputting and (logically thinking) how it needs to change the output, what is different about the output that what was the input.

That can be useful, for example if you want to add a bass boost you can reducej the signal level below a certain frequency with a capacitive filter. It can also be harmful in that whatever you add to the feedback loop, including resistance and inductance, the opamp "thinks" it needs to change it's output to overcome this difference. Problem is, if this difference only exists in the feedback loop, it alters the resultant output, Further, if the opamp has gain, even if that difference is present in the output, the difference is magnified by X * the gain set by the resistive divider.

I might be overstating the difference, in the end we only talk of ideals and it's up to the designer to listen and decide if they hear a difference. Even so, if too many ideals are abandoned it is expected the cumulative result is worse sound.
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post #15 of 18
5/19/07 at 9:03am
can you linkify the schematic pic..getting annoying sliding my bars left and right to read this post.
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