CMoy amp sounds bright
May 30, 2002 at 12:24 PM Thread Starter Post #1 of 10

millerdog

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I would like to hear from others who built this amp. Reading past threads it would seem the consensus is that the CMoy amp is smoother than the CHA47.
In comparing the two, I find the opposite. I don't know if my CHA47 was tweaked(I got it used).
I also do not have a volume pot on my CMoy. Would changing the gain values lower or increase the volume?
(edit) Holy ****! I was listening to a cd with a track that had some talking inserted to the intro. I was about to hit the post button when WHAAA? Scared the living **** out of me as I thought someone was behind me! Was listening to the CHA47 at the time!
thanks,
md
 
May 30, 2002 at 1:56 PM Post #2 of 10

andrzejpw

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lol, I think we've all had that experience. With the soundstage of my HD580s, sometimes I spin around in my chair to see what's behind me.
smily_headphones1.gif
 
May 30, 2002 at 2:21 PM Post #3 of 10

tangent

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Quote:

it would seem the consensus is that the CMoy amp is smoother than the CHA47.


I wouldn't put it that way. To me it's a difference of speed -- the CHA47 is faster. All else being equal, a CMoy will also have a rounder bass line, which I guess is just part and parcel of its slower sound. If your music is bright, a CHA47 may show this more, but the amp itself is not inherently bright.

A "CMoy" is just a topology. What components does it have in it? That may tell us why you're hearing a brightness to the sound. Also, what source and headphones did you use when making this judgement?

Quote:

Would changing the gain values lower or increase the volume?


If you raise the gain, the amplifier will amplify the signal more. If you lower the gain, it will amplify the signal less. A Cmoy can't attenuate the signal by changing the gain value -- you will always have at least unity gain.
 
May 31, 2002 at 3:28 AM Post #4 of 10

millerdog

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tangent,
source was my slimx, headphones grado sr325.
cmoy was built to your specs via tutorial. Previous CHA47 owner said amp was built for senn hd580.
The highs seem brighter and louder on the cmoy. Rim shots are more pronounced, vocals almost too loud.
Hiss from slimx also seems a bit higher in tone.
I am also getting a slight hiss on the cmoy at idle. Could there be a problem in my contruction?
I used the cmoy with some KSC35s and got a bombastic intermittent hum. I had it with the grados also, but went back and resoldered the outputs and it seemed to go away until I used the koss.
I am thinking I should start over.
md
 
May 31, 2002 at 6:09 AM Post #5 of 10

tangent

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If the op-amp is oscillating with some loads, it could explain this. The thing about oscillation is, it can sound like anything -- there is no single "oscillation sound". Your op-amp could be oscillating in ultrasonic frequencies, which will cause the chip to lose bass power -- the chip is too busy oscillating to put out good solid bass -- so you reflexively turn the volume up to get the bass level right, which brings the mids and treble up to piercing levels.

This is just a guess, mind.

You can tell if your amp is oscillating by putting a microammeter on the amp and seeing what it's drawing. It should be in the 10-20mA range. If it's 30-40mA, it's probably oscillating. You can also touch the chip with the back of your finger after it's been running for a while -- if you can't leave your finger on the chip for more than 5 seconds, it's probably oscillating, or you have a ground fault.

Also, what values of R2 and C1 did you use? If you changed these from 100K and 0.1uF for some reason so that the input filter's corner frequency is moved way up, you could cause a similar symptom to what I described above.
 
May 31, 2002 at 6:36 AM Post #6 of 10

millerdog

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tangent sensei,
perhaps I have a ground fault as you said.
The op amp is cool to the touch.
R2 and C1 were as your tutorial. I only ordered enough parts to finish one amp.
The freakin bombastic humm seems to come from the 1/8 females.
If i wiggle the inputs, the humm goes away. I put the board on my desk in the first sound test and got the same humm. I supposed it was because of the uh... virtual ground.
Okay, if I touch the positive voltage rail, the sound goes to ****.
I turn the amp off and on and everythings alright.
A grounding problem? Or perhaps I shouldn't be touching the damn things in the first place?lol
(edit) I can still get a humm when I touch the collar on the input jack. I still think it's the jacks. I ordered the ones from digikey.
thanks,
md
 
May 31, 2002 at 6:52 AM Post #7 of 10

tangent

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That's a ground fault all right.

Any place you have both a ground connection and either a signal or a power rail connection nearby could be the source of the problem.

For example, if you melted the innards of your 1/8" jacks when soldering the hookup wires on, that could cause this problem. If you've got a small solder bridge, you can get a problem like this. If something you think is insulated has a small hole in it, you can have this sort of problem. You get the idea.

To troubleshoot this, first try wiggling all the wires to see if the sound changes. If not, start desoldering panel components and then using alligator jumpers to hook things up temporarily. If the problem goes away when you "replace" one of the connectors this way, you've found the problem. Take the amp entirely out of the case while you're doing all this, if you're using a metal case.

Grounding is the second nastiest kind of problem to chase, after oscillation. I sincerely wish you luck. That's what caused me to spend 16 hours on my first CMoy, so I know your pain....
 
May 31, 2002 at 7:05 AM Post #8 of 10

millerdog

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Tangent sensei,
Actually I am listening to my CMoy amp right now with no problems. Works fine, just as long as I don't touch it! lol I think I fried something in the 1/8 inputs. The koss jack just won't work without problems.
I also caught the thing about the R5 resistors and the hiss in another thread.
I do have a ground problem as you surmised.(edit) That's also why I get some loud pops when the input comes up I guess.
I think I will rebuild, since my soldering skills have improved.
thanks,
md
 
May 31, 2002 at 12:26 PM Post #9 of 10

CaptBubba

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tangent, doesn't bandwidth go up as gain goes down? Couldn't this change the sound in a noticable way other than "louder" or "softer"?

just a little thought.
 
May 31, 2002 at 4:05 PM Post #10 of 10

tangent

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Quote:

doesn't bandwidth go up as gain goes down?


With normal op-amps, yes. (Last week I wouldn't have qualified that statement -- then yesterday I looked at the AD797 datasheet ....a strange little beastie, that one is...)

For normal op-amps, gain and bandwidth have a simple linear relationship described by the gain bandwidth product (GBP). The OPA2132 has a GBP of 8 MHz, which means that at g=1, the bandwidth is 8 MHz, at g=10 bandwidth is 0.8 MHz, etc. To get it down to 20 kHz, you would need a gain of 400.
 

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