[krisjan]

most people here probably know about this tut: How to Build the CMoy Pocket Amplifier

I've decided to give it a go, and just have a q re the parts - Part Lists and Suggestions. Is it worth it to go for higher spec parts? And if so what would make the most significant difference apart from the opamp - caps? resistors? in the tweaks section he mentions some ideas for better caps (not specific brands, model numbers, though): Tweaks. I quote Quote:

For the power caps, 220 µF is adequate, but bigger ones will provide a bigger current reserve, which can be useful in handling high instantaneous loads, like big drum hits. I've tried 470 µF caps in my CMoy amps before, and it does improve the bass handling significantly and they're not all that much bigger than 220s.

I see the BSG CMOY I have has a 1000microf cap?!
Anyway - will this make a big difference or is it not worth it for a first diy amp?

btw space is not a problem for me in terms of this build - i am actually not planning to use this protable - probably i'll just put a 24v supply jack in and use it in some box - it's really just a test/practice first build (if it's nice i'll definitely be using it though

)

k

 

[MisterX]

I would strongly suggest you make use of the forum search feature.
This topic has come up before and the answers are usually the same.

 

[krisjan]

yep - sorry - i did it the lazy way...have done some brief searching, and couldn't get sth very specific to the tangentsoft tut - maybe i didn't search that hard....never mind - drop this thread...

 

[fortney]

Something else--consider using ceramic (NPO best?) bypass capacitors of .1 or .01 uF--one lead as close as possible to V+ of the opamp and the other as long as it takes to go to ground. Then use another ceramic for the V-.

It probably improved the audio on my CMoy even though I use a "slow" OPA2227 opamp. I believe it did of course.

After admiring your work consider building the crossover circuit described in C Moy's library at the Headwize forum. Tangent sells complete kits. The wiring is tedious and be sure you have a good ground. I added a piece of wire between the ground points, as I recall, and deleted the in/out toggle switch since I put the circuit in a separate little box.

F

 

[millwood]

the amp as designed has some drawbacks:

1) it doesn't have supply decoupling. you may encounter oscillation due to poor layout or faster chips;

2) it doesn't do offset cancellation (though that can be partially addressed with a servo). R2 needs to be R3//R4.

3) the gain of 11 (=1+R4/R3) is just tremendous for a typical headphone amp. a gain of 3x - 5x is more likely.

4) the use of virtual ground: there are many alternatives out there.

5) the chip's current output capabilities: you want 200 - 300mw from a headphone amp. into 16 ohm earbuds, that means the amp has to output 150ma rms (for 300mw) or 110ma rms (for 200mw). opa2132/4 can output just 40ma max when shorted.

you can try many other chips, but to get that kind of current output, your choices had been mostly limited to CFB opamps (with their built-in buffers) bu they have their own bag of problems as well: poor dc performance, too fast, tough layout required, etc.

however, there are some new offerings. My favorites are ths4012 (fast VFB opamp with a diamond buffer, 110ma peak), and some in the opa350 family can delivery 100ma+ peak current and they are rail to rail, and are available in sot23(-5 or -6) packaging, making them almost perfect for a miniature headphone amp.

so many potential ways to improve upon the original cmoy design.

 

[rembrant]

Quote:

Originally Posted by millwood /img/forum/go_quote.gif 5) the chip's current output capabilities: you want 200 - 300mw from a headphone amp. into 16 ohm earbuds, that means the amp has to output 150ma rms (for 300mw) or 110ma rms (for 200mw). opa2132/4 can output just 40ma max when shorted.

Do you mind explaining how you came up with those numbers?

 

[rds]

P = R*I^2
Actually 137mA for 300mW.
Of course the assumption of putting 2.2Vrms across a 16ohm ear bud is absurd to say the least.

 

[mattcalf]

I have a question that is related to the tangentsoft tut, what resistor is meant to be used for R5. For the life of my I couldn't work it out and just chucked a 100K in there. Any ideas?


Thanks,
Matt.
(Sorry if it's too offtopic)

 

[rembrant]

OMG 100k? 20-50 OHM dude!

 

[MisterX]

Tweaks

 

[mattcalf]

Ooo, haha alright I'll fix that one up.

Thanks for quick replies!

 

[nsx_23]

What op-amps would swap straight into the tangent-style CMOYs?

 

[millwood]

Quote:

Originally Posted by rembrant /img/forum/go_quote.gif OMG 100k? 20-50 OHM dude!

it depends on a lot of factors, and can be as low as 0ohm and I have used as high as 1k.

but 11ohm - 120ohm is typical.

 

[millwood]

Quote:

Originally Posted by rembrant /img/forum/go_quote.gif Do you mind explaining how you came up with those numbers?

simple laws of physics.

Quote:

Of course the assumption of putting 2.2Vrms across a 16ohm ear bud is absurd to say the least.

worse than "bicmos" is "a bipolar bjt with cmos"?

 

[rds]

.
Quote:

“True knowledge exists in knowing that you know nothing.” - Socrates