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cmoy amp and potentiometers (newbie)

post #1 of 16
Thread Starter 
Hi,

Im about to build my very first cmoy

Should I get a linear or log pot?
There's so many around. The ones on the cmoy site, I cant seem to find anywhere in Oz

I can get a plastic pot for about $25 oz which looks good.
The pots for $5 are carbon and have noise <100mV.

The bread board I got is untracked, so I got to make my own.
Is this hard to do? Im thinking of getting a piece of bare solid wire and run 3 of it along the breadboard (-ve, +ve and earth rails)

I got metalized polyster for the 0.1uF cap, I cant find any polyprop's.

I got 1 question, how do get the earth?
I see 1 +ve 4.5/9v rail and 1 -ve 4.5/9v rail but where do I connect the earth rail to?

Also does anyone have schematics for the cmoy
Im not sure what layout is the best on the bread board.
post #2 of 16
Thread Starter 
I have 100v greencaps metallised polyester or mylar caps.

There's also 100v MKT miniature polyester caps, mains 250v AC metallised polypropylent caps.

Which of these will be best for the 0.1uF (C1) cap in the cmoy?
post #3 of 16
log pot is probably your best bet. but you can use a linear pot if you put a resistor from the wiper to ground.

in my experience plastic pots are nicer than carbon.

breadboard or perfboard? i'm thinking it's probably perfboard. it's not too hard to work with.

connect ground to the centre of the resistor divider and all the other places where you see the ground symbol.

the opa134 is very stable so it'll work in practically any layout. i've done some messy stuff with it and it works fine.

and i'd probably go with the metallised polypropylene over the polyester caps.
post #4 of 16
Thread Starter 
So the mains 250v AC metallised polypropylene caps will do fine?

When you say "connect ground to the centre of the resistor divider and all the other places where you see the ground symbol. "

Could you explain further? Im new at this
Where is the centre of the resistor divider?
post #5 of 16
the polypropylene cap should be fine (what's the capacitance?)

take a look at the pic of the power supply for the pocket amp. there's a resistor from +4.5v to ground, and a resistor from -4.5v to ground. there's also a cap from +4.5v to ground and likewise from -4.5v.
post #6 of 16
Thread Starter 
Te caps in the figure 3 are 220uF 25V electrolytics.
Te caps in figure 1/2 are 0.1uF 100V polypropylenes.

Can I mix and match electrolytics with polypropylenes and vice versa?
post #7 of 16
Thread Starter 
help !!
post #8 of 16
i'd keep they electrolytics in the power supply, and the polypropylenes in the signal path.
post #9 of 16
Thread Starter 
Kool

Can I use 250V mains AC polyproplylene caps?

The cap that was recommended was 100v dc polyproplylene.

Thanks !!
post #10 of 16
it's always fine to use a cap with a larger voltage rating.
post #11 of 16
CTN,
You don't really need the input cap. (The .1uf) I have removed these from all my amps with great results. Almost every source available today does not have DC in the output which is what this cap is for. The amp sounds better without it.
I find it much easier to build the Cmoy amp with a dual opamp too. Look at my little Face amp here;
http://www.wilkes.net/~dwpumphery/
This is an older photo of an amp with small Wima input caps. I made this one really tiny by keeping the parts close together. The two Electrolytic caps have the two 4.7K resistors between them to form the resistor divider. The cap on the left has the negative power to pin 4 and the right cap has the positive power to pin 8. The back side of the caps go to ground + to gnd on left and - to gnd on right. Channel inputs are left to pin 3 and right to pin 6.
Pin #1 is left out, plus a 10K resistor to pin 2.
Pin #2 Has the 2K (or 3K for gain of 5)resistor to gnd plus the 10K from pin 1.
Pin #3 Input. plus 100K resistor to gnd.
Pin #4 - neg. Power. Left Cap - and 4.7K
Pin #5 (Across from pin 4)Input for right channel, plus 100K resistor to gnd.
Pin #6 2K resistor to gnd. plus 10K resistor to pin 7.
Pin #7 Right out, plus 10K resistor to pin 6.
Pin #8 + pos. Voltage. Right + cap and 4.7K

This is a good example of a dual opamp and how simple the layout is. You can spread it out some for ease of building. I find this much easier for power supply wiring. BBrown states a very low crosstalk between channels for dual chips. The 2228 used in the photo is in place of a 2134 or 2132 dual opamp chip. Do not use single opamps in this circuit. You will even save money on chips with duals. Headroom and other amp companies use even quad chips very effectively. The Grado amp uses a dual too.
You can build a complete Cmoy in about 10X10 perfboard holes if you want to, spreading it out a little makes it easier. This is so much easier than single opamps.

Dan
post #12 of 16
Quote:
You don't really need the input cap. (The .1uf) I have removed these from all my amps with great results. Almost every source available today does not have DC in the output which is what this cap is for. The amp sounds better without it.
Hi Dan,

Forgive me for stating the obvious here (I'm still waking up ) ... but you are saying that in the cmoy design the input cap C1 can be left out all together? When I built mine I substituted (as per the article) the .1uF caps with 1uF caps, so really they are redundant then.

If can ditch the caps I'll be able to make mine a lot smaller.

Thanks,
Ben
post #13 of 16
I guess can you just totally skip them without a worry when you use the amp out of a headphone-outpu of an mdp, right? no DC coming out of that for sure. (yeah I know it's not optimal, but just as a powerbooster, it should work. the mdp is very clean sounding as well).

going to skip em them. would need some acrobatics to fit it in my tin

and less points to solder sounds like a good thing
post #14 of 16
Thread Starter 
Hi Daniel,

Wow thanks for the great reply, it has helped me alot !!

Quote:
You don't really need the input cap. (The .1uf) I have removed these from all my amps with great results. Almost every source available today does not have DC in the output which is what this cap is for. The amp sounds better without it.
What sources have output other than DC?
Are all portables/component outputs in DC?

I just reliased something...
Call me stupid but where does the signal from the line in jack connect to on figure 2 and 3? and where does the power rails from the batt connect to? Where it says input/output hehe

Im really new at this
post #15 of 16
i guess since most sources have caps at the output or are dc coupled input caps aren't necessary in this case, but it's always safe to have them in.

Quote:
Call me stupid but where does the signal from the line in jack connect to on figure 2
in figure 2, one side of the pot is labelled 'input.' that's where the signal goes.
Quote:
where does the power rails from the batt connect to?
the +V goes to pin 7 of the opamp and the -V goes to pin 4.
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