[Zaphod]

I completed my first CMOY and it actually works! This is my first DIY project and I really enjoyed building it. I built it as a home amp with 12v wall wart, gold rca inputs, and a much larger case than the mint tin. The sound is great. Now my goal is to figure out HOW it actually works before I move on to a CHA47 or a META42. I do have one question. I'm using it with Phillips HP910's which are low impedance cans and my source is the line-out on my sony dvd player. I cannot seem to get the gain low enough to get any use of the pot. I kept R4 at 10k and put in 4.7k for R3 for a gain of 3.1 and I still only get about 1/4 turn. I also added R5 with a value of 100 inside the feedback loop, which sounds like it muted the highs a bit. I also tried adding 100k resistors in series before the input caps, which did absolutely nothing but introduce a hum. If I put in 10k for R3 would that drop the gain to 2 or do some other bizarre electro-math? Any input would be appreciated.

Thanks to CMOY and Tangeant for the great plans and to everyone who has contributed to this vast knowledgebase.

 

[Sol_Zhen]

Post some pics!

Most of us built a CMOY as a first DIY amp and we never tire of seeing pics of other people's work.

 

[Zaphod]

Once I get it tuned and tucked neatly into the case I'll post some pics. It's still spread out on my desk for testing.

 

[fiddler]

I think you may have a linear pot... does the volume get loud quickly, then stay about the same loudness for the rest of the turn?

Congrats, by the way.

It's a really fun hobby once you're into it.

 

[Zaphod]

I used the panasonic audio pot in Tangeant's parts list. Even with the volume completely down I still get a little sound.

 

[tangent]

Quote:

Now my goal is to figure out HOW it actually works before I move on to a CHA47 or a META42.

Read "Op-amps for Everyone" by Ron Mancini et al. It's an application note from TI.

Quote:

I kept R4 at 10k and put in 4.7k for R3 for a gain of 3.1 and I still only get about 1/4 turn.

The amp may well be operating normally. Let's crunch some numbers here.

As a random guess, let's say your DVD player is putting out 2V RMS for a full scale signal. I measured a log pot here, and at 1/4 turn, the signal would be 0.04x or so its original amplitude, or 0.08V RMS in your case. Multiply that by a gain of 3, and you get 0.24V -- about normal for efficient headphones.

Don't take these numbers too seriously, just the method. Plug your own values in here, and you may find that you're getting what you should.

To fix it, by all means, set gain to 2 or 1. (If you don't know how to do 1, see the app note I mentioned.) If that's still not enough, you can put a resistor inline with the signal right before R2 equal in value to R2 to get an additional 2x drop in voltage.

Quote:

I also tried adding 100k resistors in series before the input caps,

Just in case it isn't clear, this isn't the same thing as putting the resistor after the cap as I suggested above.

Quote:

Even with the volume completely down I still get a little sound.

That's normal. A pot is an attenuator, not a mute control; it never completely silences the music. At the lowest setting, the signal will be at -80 dB to -90 dB below the full scale value, and as you've said, you can't go anywhere near full scale with your current setup; 80dB below "deafening" could easily be "plainly audible". Once you get the system gain under control, -80dB will be plenty to effectively mute the sound.

 

[sonichead]

Pseudo O/T: While you're talking about resistors and and stuff, is it OK to just leave out the R5 as I'm building the cmoy? (making 150 vers. if that matters)

Thanks

 

[meat01]

Yes you can leave it out, but make sure you jumper it (ie. adding bus wire where the resistor should be).

 

[Zaphod]

Thanks Tangent (I spelled it right this time). Where can I find that app note? I just picked up a basic electronics book the other day that should help me figure this stuff out. Are there any audio diy books that you would recommend?

I will try reducing the gain again and moving that 100k behind the input cap.

Is it worth the money to build an uber-cmoy with high end components or should I simply move on to a different amp? My real question is, will a $5 resistor sound that much different than a $.30 resistor and the same with the caps?

 

[Zaphod]

I almost forgot. It seems a little redundant to reduce the incoming signal and then amplify it again. Aren't you unnecessarily tampering with the signal? I'm probably way off here but it seems illogical.

 

[meat01]

I would say a different amp would sound a lot better than a decked out CMoy. One of the higher end op-amps with buffers will sound better than higher quality resistors. The op-amps and buffers really give you the sound signature, while the resitors just reduce the amount of current in your circuit. Once you have built a CMoy or two, try a META with an AD8620 and see what you think.

As far as the input resistor, I always put a resistor in R5, because Tangents says to do so if you have low impedance headphones.

 

[guzzler]

it does seem pointless, yes, to reduce and then amplify then signal, but the pot and opamp also help to balance the impedance of the system, because the line out of your average player will not take kindly being put into a low impedance load such as headphones...

also, a pot is the most convinient method of varying the output volume, and since the input has to be reduced anyway, this isn't a problem...

g

 

[sonichead]

Quote:

Originally posted by meat01 Yes you can leave it out, but make sure you jumper it (ie. adding bus wire where the resistor should be).

Thanks, that's just what I did!