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Old 8th August 2003, 03:15 AM   #1
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Default Goal: Build a GC without killing myself!

OK, I've got my parts!

I am jumping in, thanks to the confidence-boosting help of many of you on the board. Muchas gracias, y'all. I am not a total newbie - I just built a CMoy headphone amp. But that is different - it is rather hard to kill oneself with a nine-volt battery, now isn't it? I suppose I could choke on it.

Anyway, I plan to ask what may sound like very dumb questions specific to my project, especially when being wrong could cause injury or deadness. I promise to search for generic stuff. But I want to be totally sure on the critical stuff, too.

The plan is to build the PSU first on a plank, then after successful testing, build an amp channel. No sense in making it easy - I wanna go P2P, with short-as-possible paths in the (LM3875TF) amp circuit. I think I have the chip-wiring puzzle worked out. (Inverted GC.) Pins 1 and 4 are pointing straight out for the caps and such. 3,7 and 8 are pointing down. That should keep me out of trouble. NFB has to cross 7 between 3 and 8, but that is about the only place I see a potential worry. NC pins are clipped.

After building the amp successfully (fingers appropriately crossed here), I plan to get a little creative on the enclosure. Heck, worst thing I can do there is lose a thumb to the table saw, right? I have two of those, anyway. My CMoy was built in a solid (butcherblock-style) chunk of red oak. That won't be feasible with the GC, but I have some ideas...

I will post pix as I go. I am a photojournalist for a metro daily paper in the Northeast US, so one would hope at the very least, the pix should be sharp.

Looking at my Digikey and Partsexpress goodie bags that just arrived in the mail, I think I have found my first problem. The 1uf cap for running across the rails (pin 1 to 4) is a 50v Panasonic cap. I am running a 25/25/330 toriodal trafo, so I think I have to get a higher voltage cap for the voltage across the rails after DC conversion. Am I right here? If so, should 63v do it, or do I need 100v?

Also, Peter, my first executive creative decision is to build the bridge on a piece of wood, similar to your latest fiberglass bridge layout. But I am heatsinking them in a cheap, Home-Depot-kinda way. I am (gently) screwing each diode into the wood using a sandwich of small-big-small washers between the mur860 and the wood. This should give me enough heat dissipation to be safe, I figure.

I can't tell you guys how excited i am about this. I probably sound like a 15-year-old walking around with a condom in my pocket, "just in case." I already love my CMoy. This should rock. I am even thinking about building a CMoy headphone amp (as a totally separate circuit) into the final enclosure. Ahh, now there's a headphone jack one can be proud of...



Thanks much,
GnD
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Old 8th August 2003, 03:36 AM   #2
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Default Re: Goal: Build a GC without killing myself!

Quote:
Originally posted by GrahamnDodder
The 1uf cap for running across the rails (pin 1 to 4) is a 50v Panasonic cap. I am running a 25/25/330 toriodal trafo, so I think I have to get a higher voltage cap for the voltage across the rails after DC conversion. Am I right here? If so, should 63v do it, or do I need 100v?

You are looking at close to 80V between rails with those secondaries, so 100V cap is a safer choice. You might not need it at all, anyway. I didn't find it useful in my circuit. So what exactly is your primary, 330V and you are using it with 110V?

You indeed sound like 15 years old with a condom in a pocket
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Old 8th August 2003, 03:47 AM   #3
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Transformer is a 25/25/330VA toroidal, I am using a single for both channels, running 115 v AC input. Two bridges, split the rails for each channel there. Normal (2-1000 uf) caps on each channel.

What, in theory, does the pin-1- to 4-cap do if it is not really needed? Reduce relative noise between the rails? When I was first, well, memorizing the circuit I just took it for granted. But now as I start to understand the amp more it, (the cap) doesn't seem to make a whole lot of sense.

GnD
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Old 8th August 2003, 04:12 AM   #4
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In some circuits the cap may help to reduce noise or oscillations, but if you go for a compact p2p layout, you might not needed. Try it out and see if it does anything to the sound. In my circuit, I noticed that highs didn't sound natural with a cap, so I didn't bother.

If you go for minimized GC, you actually need only 2 resistors, series and feedback, (you might use pot or coupling cap, if your source generates offset). Other than that, you need only 2 PS filtering caps and power supply itself. Pretty simple circuit and you can't go wrong with it.
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Old 8th August 2003, 04:47 AM   #5
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Default Re: Re: Goal: Build a GC without killing myself!

Quote:
Originally posted by Peter Daniel

You indeed sound like 15 years old with a condom in a pocket

Where did that come from?
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Old 8th August 2003, 04:49 AM   #6
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Default Re: Goal: Build a GC without killing myself!

I thought you past that age already, SkinnyBoy. I guess it came from here:


Quote:
Originally posted by GrahamnDodder


I can't tell you guys how excited i am about this. I probably sound like a 15-year-old walking around with a condom in my pocket, "just in case." I already love my CMoy. This should rock. I am even thinking about building a CMoy headphone amp (as a totally separate circuit) into the final enclosure. Ahh, now there's a headphone jack one can be proud of...

__________________
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“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 8th August 2003, 04:53 AM   #7
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Default Re: Re: Goal: Build a GC without killing myself!

Quote:
Originally posted by Peter Daniel
I thought you past that age already, SkinnyBoy. I guess it came from here:



Well, I didn't actually bother READING the guys post, that would make my post increments too slow..
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Old 8th August 2003, 12:33 PM   #8
halojoy is offline halojoy  Sweden
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Default GC without killing

Quote:
Originally posted by SkinnyBoy

Well, I didn't actually bother READING the guys post, that would make my post increments too slow..
Thanks Skinnyboy
for that hint.
Why haven't I thought of that myself?
Post without reading previous posting
and certainly not the first post (who cares of the subject ).

"GK without killing"
Is this a new "Peace-thread" ???

GrahamnDodder: OK, I've got my parts!

halojoy - think it real great to meet a guy
with some fresh and positive DIY entusiasm
too much "monkey.business" elsewhere:

- I know better than you do.
- No, you don't!
- Yes, I do.
- No, by the way, you stinks!
- You stinks yourself.

Moved to Texas by moderator X, with his little cherished Cap On Top
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Old 8th August 2003, 01:05 PM   #9
halojoy is offline halojoy  Sweden
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Default dont take my laugh - away from me

"don't take my laugh away from me
cause I don't want to live in misery"
/paraphrase from song "Breaking Up Is Hard To do"

* * * * *

I guess moderator
roddyamam
has not got same sense of humour as we have
probably a boring type

I have a feeling that GrahamnDodder himself
would have gotten a good laugh
by our comments
he seems to be such a guy

no harm done
he can read our posts in "Off Topic" Texas-thread
and
get the laugh we wanted him to get

halo - demands freedome of laughs
(see "Marketing Terms" -thread for additional info)
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Old 8th August 2003, 01:07 PM   #10
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Thats abit rude... if our posts were off topic then so was the first post in the thread (not that I read it in full, soemthing about "parts" and condoms..)
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