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Old 12th August 2011, 09:29 PM   #11
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The first one you made was clipping probably. I'm not sure of the opamp you were using for the ptp one but they usually don't have the output required for lower Z applications. Assuming you are using 25v components, try +9/-9v and not +4.5/-4.5. That should help a lot. I recently ran into that problem myself. A Cmoy like that one really benefits from a rail to rail opamp and true ground.

The new one you built looks better already.
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Old 12th August 2011, 09:56 PM   #12
tade is offline tade  United States
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When I get around to building a preamp, I think I ought to use this as my headphone section.

I bet you are right rembrant, about a real ground. I've got some 12V transformers that I ought to suck up and use. Maybe when these 9V batteries die.
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Old 17th August 2011, 11:14 AM   #13
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Uhgg.. I just noticed that you used the old Apheared's 47 amp schematic. While it is a decent idea, the resistors values used for the current matching are abysmal. Please tell me you did not use 47-Ohm as that schematic indicates. That is sure to make your output resistance very high. You could probably get away with 2-Ohm or even 1-Ohm.

I'm pretty sure I remember they guy saying that he used that value because he had no smaller values left at the time. It is important that the current matching resistors are at least 1%. I would hand match those to 0.1% even. While I understand the propensity to "use what you got", it's probably not the best idea to draw the schematic that way.

Last edited by Rembrant; 17th August 2011 at 11:19 AM.
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Old 17th August 2011, 11:46 AM   #14
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Rembrant, for opamps with small current drive capabilities, 47 ohms or even a litte higher is appropriate, the smaller values you suggest would overload the outputs and increase distortion. And because they are inside the feedback loop, the output resistance is low, the only pentalty is a slightly lower output voltage. I agree that using two 9 volt batteries is much better.

Mike
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Old 17th August 2011, 02:47 PM   #15
gareth is offline gareth  Wales
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Could the C-Moy be used to feed some PNP/ NPN' to give an increase in the current output of the amp ? If so could you then go further and end up with a workable amp in, say, the 100watt range ?

I have built one of these C-Moy's myself using the same parts and varying the op-amps and adding a better power supply. Personally I found it to be O.K soundwise, maybe a little closed-in sounwise, but apart from that, for what is a minimal amount of parts and effort - pretty good. Could I build further and extend the design out though ?

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Gareth
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Old 17th August 2011, 04:18 PM   #16
gareth is offline gareth  Wales
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Also, in reference to post 14, I went down the dual (+9v - 0v -9V) with dedicated earths using elna's as the main supply cap's bypassed with smaller wima's. Also I have built a regulated supply for when I use it at home
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Old 17th August 2011, 04:32 PM   #17
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Quote:
Originally Posted by Michael Bean View Post
Rembrant, for opamps with small current drive capabilities, 47 ohms or even a litte higher is appropriate, the smaller values you suggest would overload the outputs and increase distortion. And because they are inside the feedback loop, the output resistance is low, the only pentalty is a slightly lower output voltage. I agree that using two 9 volt batteries is much better.

Mike
Ok. I can't find any NF535n Opamp in searches so I can't look at the data sheet. The idea of parallel opamps is to increase current to an acceptable level. If the two of them parallel still doesn't have the current available to drive his phones, then those 47 Ohm resistors are just a crutch anyway and the phones are not still not being driven properly. Then what is the point of building it?

Doing the math, I get around 9-Ohm output impedance with the 47-Ohm resistors. If you add the battery impedance it is slightly more. With 2-Ohm resistors there, it drops to around .9-ohm plus battery. The feedback loop isn't some magical fairy that removes output impedance.

Anyone know what Opamp he is talking about so we can at least look at the data sheet? The OPA2132, like the schematic indicates, should have enough current in parallel for 32-Ohm phones. I have no idea what a NF535n is. Neither does Google for that matter.
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Old 17th August 2011, 10:43 PM   #18
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What math did you do?

Mike
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Old 18th August 2011, 03:22 AM   #19
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http://www.eecs.umich.edu/~aey/eecs210/lectures/amp.pdf

Obviously, I don't have the tade's amp here to make measurement's, so the value is a guesstimate at best. Note, the qualifier used was around.

I can't be the only person around here that remembers the actual thread at headwize where this exact topic was discussed and it was decided at the time that the 47-Ohm resistors were on the large side. That was quickly followed by the author's admission that those were the resistors that he had on hand and he did try lower values at a later date and they didn't sound as good to him as 47-Ohm.

I tried to find the original thread at headwize. I got about this far...HeadWize - Project: Apheared's Project Scrapbook by Michael Shelton If you try to click the forum links near the bottom they 404.. I didn't try to backtrack through the entire DIY section to see if I could find that thread. My memory isn't the greatest. Perhaps this is a fabrication of my imagination. I'm pretty sure it isn't though. This time.

Last edited by Rembrant; 18th August 2011 at 03:47 AM.
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Old 18th August 2011, 06:06 AM   #20
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Quote:
Originally Posted by Rembrant
I have no idea what a NF535n is. Neither does Google for that matter.
Maybe it should be "LF353N"?
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