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Cigar box Apheared 47 - Page 3

Okay now I understand. I'm going to re-read the tangent virtual rail article and see if it makes more sense now. Thanks again!

Thanks to you Avro I'm starting to read about audio and audio electronics. I have a great (old) book called the Audio Cyclopedia which is helpful.

I've ordered all the parts except for the pot. I have one that I can use, but I plan on getting a better one. Is there a way to implement a 50K pot when the schematic calls for a 10K one? Are there other resistor values I can change?

With a 50k pot you will have to change the 100k input resistor to 500k

I think I am going to build an A47 now...

So just put in a 500K resistor and the 50K pot will work fine? That sounds good. The A47 will be perfect for me because all of my headphones are <100Ohms.

Thats right, the resistor should be 10 times the value of the pot

cheers

FRED

FWIW, Here is my version of the Apheared 47.

I'll draw up a board for it some other time.

Quote:
Originally Posted by T.B.N.

So just put in a 500K resistor and the 50K pot will work fine? That sounds good. The A47 will be perfect for me because all of my headphones are <100Ohms.

Yep, your pot can be seen as source impedance while the input resistor can be seen as input impedance. The general rule is that input impedances should be at least 10x the source impedance. Something about damping factor which I need someone else to explain in greater detail for me.

So to better understand why the values are such, your mp3 player's output impedance may be somewhere around 5 ohms, from there it'll go into the pot which was originally set at 10k ohms. This more than meets the 1:10 ratio.

Then pot to input resistor is 10k : 100k (or 50k : 500k for your example), which boils back down to 1:10.

Finally from that resistor to the opamp which has input impedances into the megaohms.

Why the designers decided to use 10k instead of 50k is probably because they wanted to reduce Johnson Noise.

With a gain of 100, that might be a problem, but most opamp headphone amp designs stick to somewhere between 5 - 15. So I wouldn't worry too much.

And finally, yes, I can attest to the fact that the A47 handles low impedance loads remarkably. Because of its added current ability, my friend could hear a difference in the bass region even with his 16 ohm earbuds.

Great starter's circuit along with the CMOY.

Have fun!

I soldered together the power section of the board, with the TLE2624 and the BUF634, and decided to "test" it. I plugged the wall wart into it and something started smoking! I unplugged it immediately. I don't know what happened. When I wired the socket for the DC wallwart, I just wired the tip (positive) and the sleeve (negative). There was another post on the socket but I wasn't sure what it was for, so I didn't solder it.

What did I do wrong?!

I think that pin has to do with the ground of the AC/DC adapter, but not your virtual ground on the power supply of your A47. If I'm correct from my research, you shouldn't need to solder that pin. Maybe you have the wrong pins soldered for + and -. Of course I could be wrong so someone will eventually chime in and correct me.

Anyways, I'm also building an A47 using this layout that I found in another post... I believe the different between this one and Ben Feist's version is that this one has the 2 opamps in series with the 2nd opamp providing more current. Where as Ben's is all about having a double opamp for the left channel and a double for the right channel. I'm wondering which one sounds better.

I still haven't figured out what part to use for my DC jack. I see the DC jacks have voltage ratings, but I'm not sure if I go above the rating if it will cause trouble. Like you I want to use a 18-24v regulated power supply and don't want to mess with a Tread because I'm already using a BUF634 and TLE2624.

Finally, I'm going to try and add an LED. Not sure where it would fit in my layout. At this point it's all still a guessing game, but still fun none the less.

Without knowing which socket you used, the "extra" connection could be an internal switch

our just an extra soldering point for added strength. One way to tell is while the socket

is not soldered in, check and see if any of the connections are shorted to one another

using your meter on it's lowest resistance setting. Next, plug the wall wort into the socket

and see where the voltages come out and in which polarity.

Were any of the three connections shorted before you plugged in the jack?

Were they still shorted afterward?

Was voltage present on all three connections or just two?

After testing you should have a pretty clear idea of what the connections are.

It was probably the TLE2426 that had the magic smoke come out of it.

The BUF634 is pretty tough and hard to let the smoke out of.

I'll check that out. I think that the extra pin is a switch like you said. One thing I realized is that I forgot to put in the diode and the LED+resistor, I was planning to put them in later. Also, I think I might snip the legs off of the connections I don't need on the TLE2426 (it's the 8 pin version) to make it easier on the bottom of the board.

The extra pins on the DIP TLE2426 are not connected internally.

Feel free to solder to them or just leave them unsoldered.

I like to use them as extra connections to my ground plane

for heat sinking purposes.

Quote:
Originally Posted by T.B.N.

I'll check that out. I think that the extra pin is a switch like you said. One thing I realized is that I forgot to put in the diode and the LED+resistor, I was planning to put them in later. Also, I think I might snip the legs off of the connections I don't need on the TLE2426 (it's the 8 pin version) to make it easier on the bottom of the board.

I checked the connector. The extra pin is a switch; it is connected to the negative side when not plugged in, and unconnected when the plug is in.

Could the smoking problem be that the LED or Diode weren't connected when I plugged it in?

DOH! it was the LED. I soldered in the LED and there's no smoke. Live and learn. If you don't mind I'll share what my measurements are:

V- to V+: ~15.50V

Vground to V- ~16.25V

Vground to V+ .625V (I got a negative number when I had the positive end matched up with V+ for some reason, so I switched it around.

How are these voltages? Close enough?

Quote:
Originally Posted by T.B.N.

I checked the connector. The extra pin is a switch; it is connected to the negative side when not plugged in, and unconnected when the plug is in.

Could the smoking problem be that the LED or Diode weren't connected when I plugged it in?

Avro-Arrow -

A question for you...

What is the OpAmp in the schematic in Post #36???

Thanks,

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