AMB Laboratories DIY Audio

[Mullet]

I've been doing this for awhile now. I'm wondering if other people are hearing a slight channel imbalance when having two devices plugged in and the 2nd amp doesn't even have to be on. It also seems like the my headphones are slightly louder when nothing else is plugged in besides the main amp. On the other hand, I could be imagining things. Any thoughts?

[amb]

Not sure what "having two devices plugged in and the 2nd amp doesn't even have to be on" means...
What devices and what amps? Are you trying to drive a common headphone with two amps? That would be bad.

[Mullet]

To elaborate: I have two amps plugged into the Gamma 2 portion of the DAC at once -- one via RCA and the other via 1/8" jack. I then would A/B one pair of headphones by unplugging a pair of headphones from one and then plugging it into the other amps output jack. In this situation, I was plugging and unplugging the 1/8" jack that is the output of the Y2 and noticed it with a pair of Thunderpants. I don't notice it with other phones. Hopefully, that clarifies things.

[amb]

OK, so you're driving two amps with the γ2's outputs. So far so good.

Then you said you're A/Bing a pair of headphones, do you really mean two pairs of headphones? (i.e., one stereo headphone is usually described as "a pair" for the two ears).

And each of the two amps drives a pair of headphones? This part is unclear to me.

And this problem occurs only with one specific pair of headphones, and only when you have both amps plugged in to the γ2?

What amps are they? What happens if you swap the headphones between the two amps, does the problem move with the headphone, or does it stay on the same amp as before?

[Mullet]

One pair of headphones. I switch back and forth between the two amps which are both plugged into the Gamma-2 ie unplug the pair of headphones and move it from one amp to the next.

This issue is thus far only noticeable with one pair of headphones. I've only really tested it with my pair of Thunderpants, which are closed and tend to respond to transients to a greater degree compared to other headphones I own. I plan on testing this with other pairs to see if the same thing is noticeable.

I'm now noticing that this goes beyond whether I have two amps plugged in at the same time or not. Here are my findings: With my Thunderpants I'm noticing that the left side is louder especially with transients like a snare drum. I can feel the vibration on the left side, but not on the right side. If I flip the RCA jack then the right channel is more pronounced. So this tells me it's not the headphones. If I use the Gamma-1 portion of the DAC things are balanced again. So this tells me its definitely the Gamma-2. If I just go from my headphone out and bypass the DAC and go straight into the amp things are balanced. So what does this tell me? That there is more juice flowing from the left channel of the DAC in comparison to the right channel. Knowing that my VMIDL has a 2.264v reading and VMIDR has a 2.254v reading, would this make the difference? I have a 2nd Gamma-2 at work that I can compare to and see if it has the same issue. All of my other headphones are open with the exception of my in-ears so this might be harder to detect. I'll try checking resistor values and solder joints to see if that helps.

[amb]

The Vmid voltage differences is not the problem. But checking and reflowing the solder joints at U7 and its surrounding passive parts is a good start. Also make sure all resistors in that area are the correct values.

[MisterX]

Have you tried to measure the output to see if there really is any difference between the channels?

[Mullet]

Ok. I know this may come off as newbish and I know I should be beyond this at this point, but what is the best way of testing the outputs? I'm assuming I could hang a 1/8" to 1/8" cable from the Y2 and use a DMM from GND (sleeve) to + and GND (sleeve) to -. Is this the correct way? or is there a better way?

Another observation that I've not noticed before because I just never gave it much thought is that the Gamma-1 seems so much louder and more in your face compared to the Gamma-2 with the same volume level set on my Objective 2 amp -- same goes for any amp I have. Maybe it just how different the DACs are. The Gamma-2 has more separation between channels. The Gamma-1 sounds more centered with expected left and right separation. Again, I don't know if this has anything to do with the left side seeming to be louder than the right.

I'll have to wait until some measurements and A/Bing tomorrow between my two Gamma-2s.

[amb]

Ok. I know this may come off as newbish and I know I should be beyond this at this point, but what is the best way of testing the outputs? I'm assuming I could hang a 1/8" to 1/8" cable from the Y2 and use a DMM from GND (sleeve) to + and GND (sleeve) to -. Is this the correct way? or is there a better way?

Yes, except it's sleeve (G) to tip (L) and sleeve to ring (R). Play a mono test tone and measure the AC voltages of the two channels.
Here is 1KHz mono 0dBFS sine wave 16/48 file.

Another observation that I've not noticed before because I just never gave it much thought is that the Gamma-1 seems so much louder and more in your face compared to the Gamma-2 with the same volume level set on my Objective 2 amp -- same goes for any amp I have. Maybe it just how different the DACs are. The Gamma-2 has more separation between channels. The Gamma-1 sounds more centered with expected left and right separation. Again, I don't know if this has anything to do with the left side seeming to be louder than the right.

γ1 outputs 1.6Vrms at 0dBFS whereas γ2 is 1.4Vrms. It's less than 1.2dB difference but is audible by direct comparison.

[Mullet]

I knew I was hearing something messed up... These are my readings while playing the sine wave:

Gamma 2 Work:
S to T = .889Vac
S To R = .890Vac

Gamma 2 Home:
S to T = .900Vac
S to R = .547Vac

The second set of readings is the problematic Gamma-2. Looks like I'm going to dig a little deeper here. Maybe some reflow action and parts checks are in store.