Phono Hum, even when just RCA plugged in...

[BM0127]

I know, I know...another phono / turntable hum troubleshooting post. In this case, however, I am rather certain that my turntable is not the issue. I think it is the Phono "Equalizer" / Stage on my MCS 2250 amp -- which is essentially a Technics SU-Z400 (manual here). This hum only occurs on Phono input, not an Aux or Tuner, etc.

How have I come to this conclusion? Well to begin, the hum is there when I spin a record, of course -- it is faint, but can definitely hear it when volume is high (granted higher than I normally would listen). Next I unplugged everything from the amp except for my speakers. Hum is gone. Then I start to just plug RCA cables into my amp and BOOM there is a loud crashing noise -- on both the right and left speakers. When I finally plug all the way in, the hum is back. There is nothing on the other side of these RCA cables, I have not even plugged the turntable in yet.

As an aside, I took some of the cabinet off to have a look and see if there was anything obvious wrong. I turned the amp on while the cabinet was off, and the hum was there with nothing plugged in. Does the cabinet somehow function as ground? Again when the cabinet is on, and nothing is plugged in, there is no hum. Then reintroduce a cable (with nothing on the other end), hum is back.

Am I diagnosing this correctly that the amp is the issue? I have tested different cables. Is there any recourse here with my amp? Is it the phono stage itself, or maybe just the RCA connections? I am only a beginner here, I am good with a soldering iron but don't have a multimeter. But maybe now is the time.

Thanks in advance.

 

[Doug G.]

A cable plugged into the phono input and not connected on the other end will pick up hum from the environment.

Doug

 

[BM0127]

@Doug G. thank you for the clarification!

Well in that case...back to the turntable?

Does anyone see any problems with any of these connections inside the turntable? Would it be worth re-soldering?

 

[aldena007]

Quite often you will see the cabinet used as an RF shield. With the cabinet off you will experience hum if there is RF [radio frequency noise]. Units with wooden cases sometimes have metal grills for heat dissipation and often the grill is grounded someway. You did not mention if your turntable has a dedicated ground wire/lug. Most turntables will hum noticeably without that ground. As "Doug G." said a cable connected to one end of the phono input will hum as you have the cable converted into an antenna. Is your turntable grounded with a dedicated ground wire from the turntable to the SU-Z400 ground lug?

 

[Doug G.]

The thing to remember is that radiation of frequencies that cause hum is always present in the air/environment and any opening into a circuit (disconnected leads, etc.) will provide a way for these frequencies to enter, be amplified, and be heard at the speakers.

Phono circuits are much more susceptible to this than the other inputs because of the greater gain required to bring the cartridge output. which is low, up to a satisfactory level to be fairly equal to the other inputs. The hum is amplified right along with the signal from the cartridge. The hum isn't as obtrusive with the other inputs as there isn't as much gain required and the hum isn't amplified as much.

Conquering hum involving phono inputs can be difficult and frustrating and it may not be possible to completely eliminate it some circumstances. The goal is to get it low enough so it can't be heard above normal record surface noise and not worry too much if you can hear some with the stylus off the record and the volume turned up high.

As far as the connections shown in your pictures, they don't look that great but if the wires are secure, not loose or in danger of bridging to other wires, resoldering won't really do anything regarding hum.

Doug

 

[DesertRatt]

1. Is this a NEW hum, that suddenly appeared? Or, you just started using this amp and it hums?
2. The spec for S/N on phono is NOT very good to begin with.
3. With speakers connected and no cables at all connected and the selector is on PHONO how is the hum? If there is hum, put a pair of shorting plugs into the PHONO INPUT -- is there still hum?

 

[trinhsman]

I’ve had hums before in my turntables.
Solutions that have worked.
1. Change RCA cables to a shielded cable designed for turntables.
2. Change the ground wire.

I wish you luck.

 

[Jefft91]

The reason the phono picks up hum and noise where other inputs may not is the high gain of the phono stage. To avoid this, my amp came with phono shorting plugs that short the floating RCA inputs to ground . When these are plugged in, I can select that phono channel and turn the volume all the way up without hearing hum. Without, there is hum.

These shorting plugs can still be purchased, if you are bothered by hum on your floating phono inputs.

 

[BM0127]

Hello everyone -- just wanted to provide an update.

I ended up replacing the RCA cables coming out of my turntable, and everything seems to be working really well now! No hum at all.

I want to thank you all for your responses. I learned a lot, and appreciate the wisdom from the community. It is an invaluable resource when diving into this hobby as a beginner.