Headphone amp issue.

[wanders]

I've got two pieces of diy gear that seem to be duke-ing it out.

I've built and have been using the HypnoToad AD-797, lomc phonostage; and, I recently completed the RJM Sapphire headphone amp. Both pieces, IMO, are superb performers.

Here's the problem. I have a Mac c-27 preamp which has four output circuits: Tape 1, Tape 2, Preamp 1, Preamp 2. One preamp out circuit controls the main amplifier and one Tape output controls my tape deck/recorder. (The AD-797 is connected through the Aux 2 input circuit.) When the headphone amp is connected to either of the the tape out circuits, it works fine until the phonostage is turned on then and the input selector is turned to Aux 2 (the lomc input circuit). Then, there is an audible hum. When it's connected to the preamp out, there is no hum, but both the preamp and the headphone amp volume controls must both be employed; and the sound is degraded a little.

It's not a question of proximity; I've positioned the Sapphire adjacent to and away from the AD-797. It makes no difference whether they share the same or different wall outlets for power. (They do share the same 15 amp circuit, however.)

Here's something mysterious, at least to me. For both tape output circuits, the Sapphire operates (produces sound) whether the tape monitor circuit is used of not. I expected that I would have had to engage the monitor circuit to hear output.

This is my first foray into serious headphone audio and, for me it's an eye-opener. I've had a set of ATH-M50 headphone for a couple of years and listened through the jack on the cd player, amp, or preamp. But the sound was pretty ordinary. But the new Sapphire is a great experience. And, I'd like to solve this issue.

Thoughts appreciated.

 

[Trio]

It could be a grounding issue- try flipping one power cord over if it is not polarized and see if that has effect. It is possible you have a grounding issue, or with the headphones in you have lifted a ground when it is making noise. These are the obvious things that come to mind.

 

[wanders]

It could be a grounding issue- try flipping one power cord over if it is not polarized and see if that has effect. It is possible you have a grounding issue, or with the headphones in you have lifted a ground when it is making noise. These are the obvious things that come to mind.

Any time I run into hum, it seems to be related to grounding or shielding, or both. Anything related to lomc wiring seems especially vulnerable. The relatively high gain needed to drive an lomc cart seems to expose any weaknesses. It always reminds me of balancing a heavy billiard ball on a drinking straw: it can be done, but everything has to be right.

Anyway, in this case, I could not find any issues with the headphone amp, but, going through the phonostage, I reinforced all grounding connections, especially the chassis grounding points to ensure that there were good contacts throughout. This resulted in a substantial reduction in competition between the two components. In fact, all that I'm getting now is an audible snap in the headphones when the stage switch is powered on. I'll keep working on it. (Who said people in this hobby are anal?)

 

[Binkman]

glad you got the hum solved. read where the power method used for the headphone amp. could be troublesome where a transformer is used, where a double throw switch should be used.

Didn't quite get the "stage switch" in any expose on the head ph. amp. wonder if a small value poly could arrest that 'snap'?

 

[Trio]

Any time I run into hum, it seems to be related to grounding or shielding, or both. Anything related to lomc wiring seems especially vulnerable. The relatively high gain needed to drive an lomc cart seems to expose any weaknesses. It always reminds me of balancing a heavy billiard ball on a drinking straw: it can be done, but everything has to be right.

Anyway, in this case, I could not find any issues with the headphone amp, but, going through the phonostage, I reinforced all grounding connections, especially the chassis grounding points to ensure that there were good contacts throughout. This resulted in a substantial reduction in competition between the two components. In fact, all that I'm getting now is an audible snap in the headphones when the stage switch is powered on. I'll keep working on it. (Who said people in this hobby are anal?)

I have had instances where flipping the cord over helped, potentially due to an imbalance of chassis to line cord lead capacitance or paths and flipping the cord corrected it. This is usually done after I have checked for external noise sources- Light dimmers are notorious, and flourescent lighting is also capable of causing inductive hum, a high gain circuit is especially prone-even through walls and floors.

Have you tried moving the devices from where they are physically? One may be picking up a power supply magnetic field.

As for the switch noise- it could be related to the hum if there is a bad solder joint.

 

[wanders]

The plugs involved, excepting the pre, are three wire. I flipped the preamp plug, but no difference.

The h/p amp chassis contains two toroidal transformers and the phono stage uses a walmart and a small power supply housed in a case separate from the pcb case. The two phono stage pieces are stacked and sit about 12" from the tonearm side of the turntable. For around six months, those three pieces and the cart have been harmonious. I tested the h/p amp under the two p/s pieces. There was immediate hum, so I moved the h/p amp about four feet away, on top of the system preamp. There was no change, so I moved the amp a few feet away from everything, no change.

After refreshing the chassis grounds in the p/s power case, the hum abated.

As to the switch, there's no noise except when the h/p amp is getting signal from the cart, through the p/s, and through the system pre. All other configurations, no switch noise.

 

[Trio]

You might try adding a 1 meg resistor so that when you flip the stage switch off- the circuit that is losing power by that action then has an alternate route to discharge to ground rather than through any path it can find (ie haedphones) and a reverse biased rectifier parallel to it to handle large spikes fast.

 

[shelly_d]

deleted

 

[wanders]

You might try adding a 1 meg resistor so that when you flip the stage switch off- the circuit that is losing power by that action then has an alternate route to discharge to ground rather than through any path it can find (ie haedphones) and a reverse biased rectifier parallel to it to handle large spikes fast.

Sound interesting. I'll give it a shot and let you know.