Hi everybody. I decided to slap together a cmoy from scavenged parts, and guess what? I've pretty much done it. I do have a question which is probably easy for several of you out there. I have chosen to build the cmoy on a 276-170 as opposed to the 276-150 in favor of a compact design with not as much jumpering.

Here is where my question comes in. Looking at the schematic on the 150 design leads me to believe that R5 (from the output leg of the opamp) jumps over to a blank pad not tied to anything else.

However when looking over the pictures from the 170 it looks like you jump R5 from the output leg to the pad with R4 and the - input signal. I just want to make sure this is ok before I power this thing up and watch it cook!

r5 should tie in with output and r4.
On the 150, a jumper connects three
"two pads"

Ok, while I waited for replies to come in I looked over the schematics a bit more and did some reading, and see where my confusion was. What I am wondering now is if I am right in assuming that R4 jumps from the pad with - input to a pad which is also connected with the output signal. That is what it looks like to me, but again, I just don't want to fire this thing up and watch smoke form.

Thanks for the help guys!

Well I got impatient. That's hard to believe eh Tangent?

I decided I was right and hooked it up. First with no load waiting for any heat. Nope. Cool. Not fried. Then, with the cheapo headphones and the MD turned to 0. Slowly I raise the volume and sweet cranking tunes emanate. Success.

Next I actually hold the phones to my ears and turn up some more. Oh yeah. This is what I have been missing. Now time for the KSC/35's. Sweet lovely music. First try no problems. I can't believe it.

So now it's time to place a volume pot, actual female 3.5 jacks, led, etc. However, I have one last question I'm hoping somebody can verify for me.

The caps I have on here are 16V 470uf. If I am correct, I really can't run two 9V in a series through these puppies anyway, so the other caps I have @ 10V 1000uf might be a better choice. Perhaps no noticable difference in operation, but they are slightly shorter and more aesthetically appealing. Would anybody like to let me know they agree or disagree with me and why?

Actually, depending on how much of a risk-taker you are, either of those caps would work with a 2x9V supply. If the ground is divided properly, each cap only sees half the supply voltage...the voltage between rail and ground. Of course, if your opamp destabilizes the virtual ground, or you accidently short a rail to ground, you're out a capacitor.

As for your second question, read through this, especially the last section on power supplies. It talks about different capacitor values.

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No, no. This is not right.

Output,r4 and r5 only are tied.

Input and c1 only are tied.

You don't want input tied to output.

Picts?

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I do intend to post pics, but they will be with a very bad quality webcam. I just don't have access to a digital cam anymore.

An update though:

I got everything wired up and it works fine. The sound is as good as expected, but I also think I goofed in the 3-4am hour it all came together and decided I could just wire the ground from my input/output 3.5mm jacks to the general ground rail of the board instead of to the pad adjacent to the - input lead of the opamp. Perhaps that will make a noticable difference.

Initial impression was very positive. However one of my MD's has a mostly audio clip on it where when turned up most of the way had what sounded to me a bit like skipping or something. Perhaps this is the phenomenon I hear of called clipping?

Also, as this was built from mostly scavenged parts I used some stranded wire I robbed from an old keyboard and while stranded was rather rigid, so I plan to replace it. In fact one wire already broke while fitting into my tin.

Which leads me to the tin. Perhaps no engineering award will be made, but I was happy with my first go of it. I remembered having a little slide measurement device with mm as the unit of measurement, and I could simply grasp such things as my pot, etc. to determine their measurement, and then scratch lines on the tin with an awl. Holes wound up working out just fine. I still have to make the one for the square led I scavenged. It is marked, but not yet drilled and filed.

Tips for making holes would definitely include stepping up the drill bit size. If you are unfamiliar, what this entails is using a relatively small bit first, then, going up to the next size, and the next until you have the proper sized bit for your final hole. Also, if you have bits with those tips made to keep the bit from wandering (those of you who have drilled much know about wandering bits on metal ) are a good choice for your first bit. They wont, however work well for stepping up.

Finally, I took the extra effort to drill the pot hole just big enough for the rotator initially, then stuck it through the hole and scratched with my awl around the shape of the pot which is like a circle with a square cut into the sides. This can only be accomplished with a file, and thankfully I had a small round file which worked perfectly. I was pretty close to right on the money with that and was very please about it.

I will try to post up some pics in the next couple of days if I can work it in.

Now I am all anxious to start my next project.

Is this the layout you used?

Yeah, that's pretty much it. Although I do have to admit that I never figured out how to figure out which posts are for the pot. I scavenged through threads with pics until I found a clear one of an exactly same pot and traced the wires in the pic.

I am still working on getting my very poor quality pics up too. I have had some serious computer issues at home in the past week or so.

Also, anybody want to help me clear up, where is best to pull my output ground from?

Right from the middle of that star... same as where the input ground will go. Hang in there.

Hey,

I make all my amps with scraped or scavenged parts. I get them from lab left-overs bought (with cash only! shady eh?) and scavenging in shading side of Tokyo. Only parts that is bought without begging, nagging, bluffing, or kissing-*^& is my board. I am soooo proud of it. Got double ground plane. Incredible. Cost me 50 buck for 5 large (~ 17 X 18 inch) board. Got me over ebay. (Ok, ok! Some begging.)

http://www4.head-fi.org/forums/showt...threadid=31425

Resistors are reused. Caps are super good; bulk purchase with a lot of begging. IC sockets are reused. Wires ... oh yeah ... I got these for 10~100th of price over ebay. (Oh yeah lot of bluffing.)

Man, I should get paid for this.

Tomo is not Mr. Anderson ( at Voodoochile)

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Thanks Tomo. It's good to know you aren't alone!

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Let me make sure I understand this.. cuz I am a noob after all.
As I began typing this I realized my point of confusion. I was thinking of the - audio output as the same thing as a ground audio output.
So, should I be connecting the - audio input and the - audio output to the "star" (which I assume is the resistor cluster between the PS caps)? If so, should I also still connect the - input to the appropriate leg on my opamp for - input A or B (L or R)? I guess the fact that the opamp has a place for - input but not - output throws me off.

Also, now I feel I have messed up the wiring of the pot in re-wiring the amp. Does anybody have either good instructions they don't mind posting or a link to a page or something to help me figure out which leg should attach to what? I do have a multimeter and am familiar with using it.

All help is very appreciated.

Input GROUND/common and output GROUND/common both to the star/one and the same. This presumes that the star is your virtual ground rail. Only the "+" signal is amplified in this amp. In a balanced amp (four amps), the "+" and "-" halves of the signal are differentially driven, each by an amp...X 2 channels.

The "-" input on the op-amp is part of the feedback loop. I'm sure you have seen this, but i'll post it anyway... great reading:

http://headwize2.powerpill.org/projects/opamp_prj.htm

Enjoy!

Ok, the pot thing is messing with my head, so I did some more testing. This is my rationale.
With the pot shaft pointing toward me and the points standing straight up off the desk from left to right:
pin 1 has connectivity to pins 5 and 6 @ 50k each with knob turned counterclockwise. Pins 5 and 6 have essentially ~0. I'm guessing I could use pin 1 with either 5 or 6 for one channel, (or 5 with 6 if I want reverse volume control?..)
Then, pin 2 has the same interactions with pins 3 and 4. So, if I'm not mistaken I could use 1 with either 5 or 6 for one channel, and 2 with either 3 or 4 for the other channel. Then, are both of the remaining pins for ground?

I am so confus - ed