power hum when recording from tape reel machine

I’m using dad’s good ol’ Revox A77 reel tape machine to transfer some old Pete Seeger material, but experience power hum (50 or 100 Hz) when connecting the A77 to the line input of my desktop PC. When A77 is powered off => no hum. When A77 is powered on, hum comes on after 4 secs. Originally A77 and PC was on different wall outlets, so I put them on the same but no change. I lack protective earth (3 lead) outlet in this room, which I believe the PC would “prefer” to have (since its power cord is a 3 lead). Should I try to connect the entire setup to a 3 lead outlet from another room, which would effectively earth the PC chassis. And would there be any point in trying to connect both devices chassis to each other? Obviously I dont want to inflict any damage on any of the circuitry. The A77 has only a 2 lead power cord.

I suspect that my current wall outlet is not 100 “balanced”, since when I touch the A77 chassis with one hand and the grounded heat radiator with the other hand, I can feel a small current flowing through my body. (Thats no fault of the A77 since its the same thing with the casetted deck). Now if the A77 chassis is not at ground level and the PC chassis neither?, could that produce the hum effect? Or alternatively the A77 has always had this “built-in” hum and only I havent thought about it before?

If it’s a desktop/tower computer you should definitely always use a 3 pin socket. Apart from any “hum” issue, one small fault on the computer power supply could be “BANG” and you thrown out through the window. Appliances that are equiped with 2 pin power leads should be double insulated to prevent that from happening. Full size computers are not double insulated and rely on an Earth connection so that they will blow the fuse in the event of a fault. Without an Earth connection, a fault could cause the chassis to become live, and when you touch it, you become the Earth conductor.

It may also cure the hum.

you have ground loops. if you can feel it then it is bad.
i would have an electrician advise you as safety is involved here if you try connecting things together with additional wires between teh boxes.

that said, DAK sells an isolator (guessing $20 ballpark) that would eliminate the hum. easiest fix. fast & cheap too. rca in rca out. you might need an adapter for rca to pc3.5mm mini plug.

you really need 3wire plugs and connect at the same outlet.
if you have two wire try reversing one of them 180degrees.

you might have bad caps in the decks power supply if it is old.
replacing them might help. but since it happens on that otehr device too i suspect deeper issues.

I’d concur with Steve. No workarounds for the computer. It has to go into a three-pin socket (or three-pin adaptor). If you are in an area that uses two-pin plugs as well as three-pin, it should be easy to get adaptors both ways (2 > 3 and 3 > 2). So you want to plug the computer into an adaptor that has three inputs but plugs into a two-pin socket, like this.

Think about the hum on A77 when you have the computer correctly plugged in and not before…


Here is a sample of the digitization from the Revox machine.
Can you hear any hum? I’m not sure if I can, its pretty low down.


I tried to connect the PC to a 3-lead power outlet - no difference.
I by-passed the PC and connected the Revox directly to my computer speakers - hum yes.
Just maybe I’m a little fooled by my computer speakers (Logitech Z2300) which are nice but heavily geared against gaming and “action cinema” rather than hifi,
in other words the subwoofer and the setup as a whole gives a substantial bass boost, even when the bass control is at minimum, so any hum there is,
seem to get magnified. I checked connecting to my hifi stereo, and the hum is there too, but on a pretty low level. Cant say if the Revox has always sounded
like this, or if it might be a case of “dry caps”, as someone suggested.

I can’t hear mains hum on your Pete Seeger interview, but I see there is a peak at 49 not 50Hz …
Peter Seeger Interview, Peak @ 49Hz not 50Hz.png
A possible explanation is the 50Hz hum is on the recording and the tape is running a little slow.

It is possible to notch-out the 49Hz peak, (using the attached plug-in @ Q=11) , but the improvement is barely noticable …

Notch filter 2 , Audacity Plug-in.zip (630 Bytes)

Thanks for looking into this, Trebor, what a fancy frequency response graph, I suppose its generated by audacity?
With my lousy hearing I cant hear any difference between the clips.
The hum I have reported earlier comes when I power on the tape recorder, not when I playback it, which makes me believe it emanates from the playback amplifiers,
and if so, should be exactly 50 Hz. There might be hum on the tape as well but the capstan speed is servo controlled and very stable but 50Hz +/- 1 deviations is probably well within the accuracy limits. Overall I think the recording sound perty nice taking into account its age, but the compressed youtube sound obviously cant do it justice.

assuming it is grounded correctly then
electrolytic capacitors in the power supply may be leaky.
have a good repair shop check out the amplifier power supply.

Yes frequency analysis is on the “Analyze” drop doown menu, then click on “plot spectra” (which produces a graph of the selected bit of the track).

The improvement is visible (and just audible) in the quiet bit: between “I’m not sure” and “one of my own songs”.
the red areas are sound on the “before” which is not on the “after” 49Hz notch …
Pete Seege,  before after 49Hz notch, comparison of spectra.png

Youtube formats have changed since I last used it but IIRC if you select the higher visual quality setting on youtube you get better sound too,
(much less compression artifacts).

Wow, you really know your subject, Trebor! I like techie stuff too, but in a face-to-face shoot-out I wouldnt stand a chance :slight_smile: