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Several newbie CMoy questions

post #1 of 12
Thread Starter 
hi all-

first off, let me say that i'm a complete electronics and DIY newbie, so if i ask anything laughably simple or obvious, try to avoid openly making fun of me. chuckling behind my back is fine, just as long as it's not overt.

in any case, i just built my first CMoy, following Tangent's specifications and instructions (which were excellent, by the way). i stuck with pretty much all "default" parts. the amp worked right off the bat, but i'm curious about how i can improve it.

that leads to my questions - i have a bunch. don't feel the need to answer all of them at once (or at all) - responses to any of them will be appreciated. some of these are specific to the CMoy design, and some are general amp-related questions.

1. (this has been asked in another recent thread, but i thought i'd add it to this set of questions as well), how important is matching feedback loop resistors? an important corollary to this is, what is a feedback loop, especially in the context of an op-amp? what function does it serve, and why is it necessary in this case but not in others? i should note that i'm picking my way through Horowitz and Hill, but i haven't made it very far yet.

2. while Tangent's basic specs show a single 9V power supply, all of his examples in supporting text deal with 2x 9V, and his finished amp appears to run on 2x 9V. what's the advantage to doubling the power supply, i.e., what physical difference does it make within the circuit? how will i know if my amp is under-powered (short of adding a second battery in series and seeing if it improves the sound)? what's the best way to add a second battery in series, if i already have the amp built with a single 9V clip?

3. in relation to (2), does increasing the size of the supply and input capacitors produce a need for greater input voltage? i'm thinking of swapping in 330 or 470uF supply caps and 0.22uF input caps, but i might just build a second one if it will require power supply changes as well.

4. i think i've read (perhaps even on one of Tangent's pages) that some amp designs use output capacitors as well - what's the advantage to this?

5. in his section on the CMoy virtual ground circuit, Tangent diagrams a TLE2426-based buffered virtual ground circuit, but then goes on to mention the weaknesses of this implementation and describes more complex buffered virtual ground circuits. is the TLE2426 implementation sufficient for a basic headphone amp? is it better than the basic CMoy's unbuffered virtual ground in most/all respects, and will implementing it improve sound quality? last, does the quality/tolerance of the 1uF capacitor between the TLE2426 and V- make a big difference, or will a basic +/-20% electrolytic do the job just fine?

thanks in advance for any answers you can give.
post #2 of 12
1) All resistors should match as close as possible so that left and right has the same gain.
2) The feedback loop is formed by R3 and R4. Gain is set R3/R4 + 1
3) 1x9volt for low impedance phones 2x9 volt for high impedance phones.
4) Going to larger capacitors has no effect on the supply voltage. Larger capacitors C1 can give more power reserve (Better Bass) and C2 will lower the crossover frequency.
5) The only thing to be aware of with higher voltages is the voltage rating of the capacitors and the max voltage of the opamp.
6) Output capacitors Typically are unwanted. they are usually used on single sided amps to block output dc
7) The tle2426 is better than a resistor divider. Its weakness is its low current carrying ability. It can be improved by installing c1 on output of tle2426.
8) The noise reduction capacitor is usually ceramic.
9) read these:
http://headwize.com/projects/showfil...=opamp_prj.htm
http://headwize.com/projects/showfil...=cmoy2_prj.htm
post #3 of 12
Thread Starter 
thanks for all the answers. i did know that the feedback loop is composed of the R3 and R4 resistors, i just don't know exactly what it does or why it's there. i haven't finished reading the two links you recommended, so perhaps i'll find the answer in there.

another question, in addition to the above: i haven't put my amp in a case yet, but i'm planning on just installing it in a standard Altoids tin. what are good ways to insulate the bottom of the case and secure the battery clip and circuit board within the case?
post #4 of 12
you could line the inside of the case with electrical tape, that'd probably be the easiest solution.
post #5 of 12
As Soymilk said, electrical tape is good. If you ordered from Digikey, you can use their labels like I did; they fit really well. Also, remember to put something over the tops of the capacitors to isolate them as well.

As for securing the battery clip and everything, it will secure itself with the wiring. It is a pretty tight fit, so most people do not use anything to hold it in place. Just pack the wires over the top of the board.
post #6 of 12
Thread Starter 
thanks - i have plenty of those digikey labels, so maybe i'll go that route.

with respect to low current output of a TLE2426-buffered power supply, will this be a problem if i'm only planning on using this amp with a pair of low-impedance IEMs, or only if i use it with larger, less efficient cans? is there a downside to putting C1 on the output of the TLE2426, rather than on the input, as Tangent had it drawn? also, is there a difference between the TLE2426CP and the TLE2426CLP? both are DIP-8 form factor, but they're sold under two completely different part numbers on Digi-Key, and i'm curious whether one is more suitable for audio applications than the other.
post #7 of 12
TLE2426CLP is TO-92. If you meant TLE2426CP vs IP...I can't find it in the datasheet anywhere...Commercial and Industrial . I've not heard of anyone using the IP.
TLE should be fine for current capabilities (check the opamp working conditions in the data sheet to make sure).

Another way to improve the amp is to buffer the channels and/or the virtual ground. This is what is done in pimeta/ppa.

heatmizer: I thought that all opamps were happier with higher current PS and more voltage from the supply would solve some problems (less likely to oscillate, etc). Is this not true in general? Why should there be a guideline for high/low impedance headphones? This doesn't affect the voltage of the ouput (that's what the gain does).
post #8 of 12
Thread Starter 
ahh, gotcha. didn't realize that TO-92 was a different, 3-pin IC package - for some reason i had assumed it was also a DIP-8 chip. i've already ordered the TLE2426CP, which sounds like it should work just fine. i'm relieved. the $5.00 surcharge on orders below $25 at Digi-Key puts a lot of pressure on to make the most of each order... out of curiosity, are there any good suppliers that DON'T have such a surcharge in place?
post #9 of 12
I agree that having as much current available for the ground and the opamp is a good thing. Where did i say it was not?
1x9 and 2x9 volt just seems to be a rule of thumb. Running at 2x9v all the time would not be a problem but is sometimes not needed or desired. Running at 18v does not increase battery life or amount of available current. An opamp might be happier at 18 volts instead of 9 volts and some cannot be ran at 18 volts. A cmoy in a altoids tin you might as well go with 2x9 volts because they will fit. But say a small hammond enclosure only one 9 volt would fit.

Mouser does not surcharge

more reading:
http://colomar.com/Shavano/intro_opamp.html
post #10 of 12
To answer your question about 'what does a feedback loop do'....

An opamp, by design, multiplies (amplifies) the difference between the two input pins (+ and -) by a lot. I designed an op amp with a gain of approximately 1.3 million. Running open loop (IE no closed feedback loop) would cause any tiny signal to be amplified so huge that the op amp's output would rail against the power supplies (even the tiny input offset voltage would be far to big).

What we do is use 'negative feedback' to tame this ridiculous amplification factor. A certain amount of the signal is sent back into the negative input of the amplifier and, in effect, subtracted from the input so that the amplifier is amplifying a smaller signal.. The output is constantly 'fed back' into the input.

If you care, I'm sure someone can show you the math. I could.
post #11 of 12
Thread Starter 
Quote:
Originally Posted by perfectturmoil View Post
To answer your question about 'what does a feedback loop do'....

An opamp, by design, multiplies (amplifies) the difference between the two input pins (+ and -) by a lot. I designed an op amp with a gain of approximately 1.3 million. Running open loop (IE no closed feedback loop) would cause any tiny signal to be amplified so huge that the op amp's output would rail against the power supplies (even the tiny input offset voltage would be far to big).

What we do is use 'negative feedback' to tame this ridiculous amplification factor. A certain amount of the signal is sent back into the negative input of the amplifier and, in effect, subtracted from the input so that the amplifier is amplifying a smaller signal.. The output is constantly 'fed back' into the input.

If you care, I'm sure someone can show you the math. I could.
if you'd like to, and can do so easily, i certainly wouldn't object... if it's a lot of trouble, though, don't worry about it. thanks for the info.

also, heatmizer, thanks for that op-amp introductory link - i think that, in combination with perfectturmoil's explanation of the feedback loop, is exactly what i've been needing to clarify what's actually going on in the CMoy circuit.
post #12 of 12
Quote:
Originally Posted by perfectturmoil View Post
What we do is use 'negative feedback' to tame this ridiculous amplification factor. A certain amount of the signal is sent back into the negative input of the amplifier and, in effect, subtracted from the input so that the amplifier is amplifying a smaller signal.. The output is constantly 'fed back' into the input.
Haha, wow. That sounds like a simple "reflection," as is seen in a video signal which decreases resulting output. Is this in fact the case?
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