Topping L70, compatibility with current equipment, and ground loop hum

[grogzilla]

Hi, everybody. Sorry if these things have been asked elsewhere. I did try to search and couldn't find anything that was immediately applicable or easily understandable for me, so I hope this is okay to post. I asked some of this in the Topping L70 thread, but I don't know if I'll get any replies there.

I unfortunately don't know much about electronics, but I'm trying to understand things. From what I've gleaned, it seems like it's usually desired for the input impedance on a device to be at least 10-20 times the output impedance of the other device connected to the inputs. What happens if it's a 1:1 ratio between balanced connections instead of 10:1 or higher? Would this cause significant issues of some kind?

I currently have an older Roland Quad-Capture audio interface (https://www.roland.com/us/products/quad-capture/specifications/) that I would like to use for recording, and it has a specified output impedance of 2k ohms on the balanced outs (TRS), which would allow for that 10:1 ratio when connecting directly to some active monitors that I'm looking at that have an input impedance of up to 20k ohms (the Adam Audio T line), but not when trying to connect to something like the balanced inputs of the Topping L70 and some other Topping amps. It's been mentioned previously that the input impedance on the L70 is around the same 2k ohms as the output impedance of the Quad-Capture interface, so I'm just wondering if hooking the interface up to the amp through their balanced connections would cause problems. The headphone out on the interface has a relatively high output impedance of 47 ohms, so none of my headphones technically "match" well with it, either, but I could potentially run a 1/4" to dual RCA adapter from the headphone out of the interface into the RCA line-ins of the L70 if that would work better. Or is there some other way of doing this that would be more ideal, besides replacing the interface with something that has a lower output impedance?

Also, I recently bought an L70 to replace an old, original Schiit Magni that I have, because the Magni is distorting pretty severely above certain volume levels and because I wanted the extra outputs and inputs available with the L70. With the Magni in my setup, things seem to be generally fine besides the distortion above certain volume levels, but with the L70, there's a noticeable hum that I can hear when the amp is connected to the headphone out of my TV (an LG) using a 3.5 mm to dual RCA cable, and a quieter hum or buzz when connected to the RCA line-outs of my PC's X-Fi Titanium HD. If I unplug the RCA connections, then the hum disappears. If I flip the ground lift switch on the L70 to enable the ground lift, then the hum also disappears. Is there anything wrong with using the ground lift switch on the L70 to fix this? Do ground lift switches cause any issues that would be audible? Is it safe to use ground lift switches, or should I be looking into another solution to this problem for the long term? Everything is currently plugged into a UPS and the UPS is plugged into a single outlet.

Please let me know if I should provide more information. If anybody can help here, I would really appreciate it. Thanks.

 

[AnalogSteph]

The headphone out on the interface has a relatively high output impedance of 47 ohms, so none of my headphones technically "match" well with it, either, but I could potentially run a 1/4" to dual RCA adapter from the headphone out of the interface into the RCA line-ins of the L70 if that would work better.

This would be a standard approach for using a headphone amp with an interface like that. Probably the best one in this case given the combo of wimpy output levels and high output impedance on the regular output (which I presume is a purely impedance-balanced affair, with the analog stage running on +5V).

Is there anything wrong with using the ground lift switch on the L70 to fix this?

No. That's exactly what it's for.

Is it safe to use ground lift switches, or should I be looking into another solution to this problem for the long term?

It's more or less as safe as plugging headphones straight into the respective inputs. You may want to get the TV explicitly grounded e.g. via an antenna connection if you are concerned about mains leakage when touching the amplifier chassis, assuming that's not already taken care of otherwise.

 

[grogzilla]

Alright. Thanks for the feedback. So, I'm guessing there isn't any audible negative impact to the sound if I use the ground lift? Sorry if this is basic information. I haven't delved into this side of audio very much in the past.

If I don't have access to something like an antenna or cable connection where the TV is located, how would I go about grounding the TV? It's mainly being used as a monitor and display for my PC and game console through the HDMI connections.

 

[unpluggged]

It's more or less as safe as plugging headphones straight into the respective inputs.

That's what Topping L30 said

 

[grogzilla]

I decided to return the L70. I'm going to have to figure out if there's a better solution for my setup and if I should bother trying to use the audio interface for anything other than the recording inputs. My old Magni does distort above a certain volume level, but at least it isn't producing the ground loop hum that the L70 does without the ground lift, so I can make do with it for now. I also don't have a very full-featured DAC in any of my current equipment, so maybe looking for a combination DAC/amp would be a better idea for me.

 

[AnalogSteph]

If I don't have access to something like an antenna or cable connection where the TV is located, how would I go about grounding the TV? It's mainly being used as a monitor and display for my PC and game console through the HDMI connections.

Then the PC would already take care of this.

I decided to return the L70.

In your application an L50 would have been fine anyway, and if you don't have any cans with balanced cabling, even an L30 II (which actually sports higher maximum gain). These have a transformer plug pack much like your old Magni, so do not require a ground lift.

 

[grogzilla]

Yeah. The L70 was an impulse purchase, but I thought the preamp functionality could be useful in case I pick up some active monitors. I don't know what I'm looking for at this point. I'm trying to find a setup that would let me run my few pairs of headphones (which it sounds like most newer amps can handle) and possibly a pair of active monitors. It sounds like maybe I could do that with one of those other amps for the headphones and a DAC with extra outputs for the monitors, but I'm not sure if a DAC is even needed or if my current interface could handle the monitors. If I just needed a simple replacement for the Magni, then it sounds like the L30 II might be fine for that.

Edit: It seems like I could probably use my interface to run the monitors if I got a couple of TRS to XLR cables, which would remove the need to buy yet another piece of equipment that I don't currently need or a more expensive amp with the required XLR outputs.