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B22 and distortion question

post #1 of 48
2/14/10 at 4:28am
Thread Starter 
I recently bought a 3-channel B22, which I've been using with my Denon D2000 (25 ohm) and Audio Technica AD2000 (60 ohm) headphones
The problem is, when I plug my headphones in, there is a faint humming in the background. Unplugging the DAC makes no difference, and it can be heard at any volume level.
The humming is especially loud with the Audio Technica AD2000s, and becomes unbearably loud at the lowest and highest volume setting
There is a gain of 8x, but I'm not sure if the noise is because of the high gain... Has anyone else had this issue before?
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post #2 of 48
2/14/10 at 4:39am
I would start by kindly asking the person from whom you purchased the amp if it exhibited this hum when they used it. If the answer is no, second step would be to make sure your outlet is grounded and try it in a couple different outlets. The noise could be induced by something else plugged into the same circuit. Only after ruling these two things out would I move on to examining the amp itself.
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post #3 of 48
2/14/10 at 4:48am
Is this by chance a single-box B22?
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post #4 of 48
2/14/10 at 4:50am
See if you can find a room without anything electronic interfering with the b22. Or move around devices nearby that may be the cause of the interference. What you're describing sounds like too much gain (8x is a lot for D2000, not sure about AD2000 but its impedance is pretty low too), or possibly a ground loop.
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post #5 of 48
2/14/10 at 4:56am
Thread Starter 
This is a single-box B22, but the trafo is shielded
HeadWize: DIY Workshop > β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3)

I tried moving the B22 to a different room, but the humming is still there... The amp itself is plugged into a Panamax line conditioner.
Its really strange that the distortion would be significantly more present with the AD2000 headphones. Could it have to do to their open design (vs. D2000's closed)?
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post #6 of 48
2/14/10 at 5:00am
Despite a shielded transformer, unless the wiring 100% perfect, the B22 seems to love to pick up hum
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post #7 of 48
2/14/10 at 5:15am
Even if it's a single chassis B22, if there was a hum present the seller should have disclosed this before selling it, obviously. Perhaps something got rattled loose in transit, or perhaps you have an issue with your power line.
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post #8 of 48
2/14/10 at 5:17am
Quote:
Originally Posted by ShinyFalcon View Post
See if you can find a room without anything electronic interfering with the b22. Or move around devices nearby that may be the cause of the interference. What you're describing sounds like too much gain (8x is a lot for D2000, not sure about AD2000 but its impedance is pretty low too), or possibly a ground loop.
Wouldn't the high gain cause hiss if anything, not hum?
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post #9 of 48
2/14/10 at 5:22am
Yep, wasn't giving the issue much thought. Thanks

Can you check and see if your volume pot is grounded properly? Are you able to take internal pics? Can you plug in your b22 directly to the wall (skipping the conditioner)?
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post #10 of 48
2/14/10 at 5:43am
boozcool, the default gain of the β22 is 8 which is good for a large number of headphones, in this instance the voltage gain is way too high for your Denon and Audio Technica headphones which are probably too sensitive for this setting, lowering the gain to 5 or even 2 should take care of the distortion or hiss.
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post #11 of 48
2/14/10 at 5:48am
Thread Starter 
Thank you for the replies!
Just to clarify, there is a hum at all times with both headphones
And with the AD2000, there is a hiss, which disappears at 12 o'clock and progressively gets worse as I increase/decrease the volume
I will lower the gain to 5 and hopefully that will solve the problem
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post #12 of 48
2/14/10 at 6:14am
Quote:
Originally Posted by boozcool View Post
Thank you for the replies!
Just to clarify, there is a hum at all times with both headphones
And with the AD2000, there is a hiss, which disappears at 12 o'clock and progressively gets worse as I increase/decrease the volume
I will lower the gain to 5 and hopefully that will solve the problem
A ground loop noise is usually hum not hiss, ensure there is a ground loop breaker and that’s it’s connected properly. Sounds likely that the gain setting and ground loop need to be checked.
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post #13 of 48
2/14/10 at 6:50am
Thread Starter 
I just checked the ground loop breaker, and it is connected
Aside from the gain setting, perhaps installing a zobel network for headphones will fix the hiss with the AD2000s, since the distortion becomes very loud as the knob is turned to the max highest or max lowest setting
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post #14 of 48
2/14/10 at 7:00am
Sounds like the amp might be oscillating. The Denon may be a highly reactive load which caused some instability. The cure would be to add a zobel network at the output jacks. See the β22 website "Wiring & ground" section.
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post #15 of 48
2/14/10 at 2:42pm
Thread Starter 
Ok, thank you!
So to confirm for my 3-channel B22, to lower the gain, I will need 3x 392Ω, 3x 1.5KΩ, and 12x 47pF capacitors
For the zobel network, how many 22Ω 2W metal oxide power resistors and 0.047µF 100V film capacitors will I need since they are being added at the output jacks?
Cheers!
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