dbx 286s Hum

Orson

Well-known member
Was at a friends place and when he turns on his dbx 286s there is a slight but noticeable hum through the ear phones and on the recording.

Anybody have experience of this?
 
Are you using balanced outs from the dbx to balanced ins where ever it is plugged into? It sounds like a ground loop hum is there.

Alan
 
Its just plugged into the mains and interface and mic. I have heard of ground/earth issues. Does this mean a separate new ground/earth wire should be installed connecting to all equipment?
 
Think back to school electronics - a circuit needs two conductors, out and back if you like. One of these could be a 'tube' of wire, surrounding the other. Call it a screen, or a shield if you like, trying to prevent electrical noise or interference being heard. Guitars are the most common musical bit of kit that uses this system. It's unbalanced because the screen is different to the centre conductor. Hi fif connections use the same system with different connectors - the usual smaller phono(RCA) connectors. It also means the shiled/screen is doing double duty because often it is connected to the ground connection - to make it even better at shielding.

The other system, used mainly in microphone circuits is to have the audio on two small conductors and then surround BOTH with the shield/screen connected to ground. This is a balanced connection. The basic idea is that external noise/interference lands on both identical conductors and fingers crossed, it cancels out like magic. You have in essence, 3 conductors in balanced connections (like XLR cables for mics) but only 2 in other connections. Here lays a problem. If two bits of kit are connected together with a balanced cable, even if, because of your electrical wiring the grounds are slightly different mains derived hum gets onto the screen, audio wise, nothing happens. The two audio carrying conductors are separate. However, if at some place there are unbalanced devices - this hum is being carried on half of the audio circuit and it gets in. 50Hz here in the UK and 60Hz hum in the US. A consistent, continuous low frequency hum. Nowadays those nasty cheap wall wart type power supplies make this worse by giving you potentially dozens of noise sources sticking rubbish on the audio ground. It leaks everywhere, carried by the audio cable shield/screen.

In your system, you have some devices with balanced connections and others without - if you have made up XLR to phono/jack type connectors, there's a chance you created the perfect path to let the noise in.

Worth remembering that we think of hum as that very low continuous hummmmmmmm. If you hear changing noise, this is not strictly speaking, hum. Screen displays, and even mouse movement sometimes cause different noise, that get wrongly described as hum - this is a similar process, but the noise gets carried on the screen of USB or Firewire cables generated inside the computer, but leaking into the audio devices we have connected.

The cures when going from balanced to unbalanced can be amzingly simply - often just disconnecting the connection to pin 1 in a XLR plug! It breaks the ground circuit.

For those living in the UK with our 240V mains power, we also bang on about safety. Lifting the ground wire in a 3 pin mains connector also often removes hum. It also removes the life saving purpose of the electrical ground, so here, the practice is considered stupid and dangerous and we just don't do it.

Grounding is the US of equipment is much less common as your standard two flat pin connectors don't include grounding. In a way - when you then DO have some kit grounded and others not, it's quite easy to have just a few items actually grounded to real earth, making it more difficult to work out where it comes from.
 
The cures when going from balanced to unbalanced can be amzingly simply - often just disconnecting the connection to pin 1 in a XLR plug! It breaks the ground circuit.

I remember last year someone talking about similar. Is the ground of the electrical system causing the problem and does it need a new independent ground?

---------- Update ----------

I should have also said if it’s not your setup, maybe you need to find out more about it so we can help you.

Alan

I will later tonight Alan. Thanks.
 
If the hum is pure, and always constant, then it's more likely you have accidental multiple grounds so look for devices in the chain that have ground connections and draw them on paper, and think how you could perhaps remove one of the audio grounds - easy in a balanced connection.
 
Balanced interconnection is a system where you have a second copy of the signal that's polarity inverted. At the receiving end it's re-inverted and combined with the normal copy. If noise gets into the signals it gets cancelled out.
 
It could be the power supply. I had a few dBx-266's that had hum problems. Both mechanical and a low but audible hum in the outputs.
Opened them up to check it out and found a pretty ordinary PSU. Once I replaced the PSU with a better design then the hum and noise went away.
Where I am the line voltage was high though (supposed to be 230VAC but was closer to 260VAC) and I just put it down to marginal PSU design (ie not designed for such extremes).
 
It could be the power supply. I had a few dBx-266's that had hum problems. Both mechanical and a low but audible hum in the outputs.
Opened them up to check it out and found a pretty ordinary PSU. Once I replaced the PSU with a better design then the hum and noise went away.
Where I am the line voltage was high though (supposed to be 230VAC but was closer to 260VAC) and I just put it down to marginal PSU design (ie not designed for such extremes).
Can you please recommend a specific better PSU to replace the one that comes built into the dbx 286s? This info would be greatly appreciated.

If I get a suitable PSU (right size, specs, etc.), I think I could find someone to help swap it out. Just want to get a "good" PSU that will work (better design, quieter, more forgiving of line voltage differences, higher quality components, etc.--whatever worked for you!). I have already tried upgrading the power cord and introducing two layers of surge protection.

(My dbx 286s emits what web searches reveal seems like the "typical" buzz/hum into the room from the unit. I like this mic processor except for the defect. The sound even seems to emanate from near the power cord end of the box. A replacement dbx 286s from the audio shop made the same buzz/hum--actually ever so slightly louder my original unit--so kept the original one, but still want to make it quiet. I definitely think it could be a power supply or power transformer noise.)

Thank you!
 
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I think we thought you'd given up after a pause of over a year. What Hydroflare suggested is impossible for somebody who is not into electronics and if you were, I guess you'd have already fixed it. You never really answered the connection question. The mic connection is fine. You have a ¼" jack that is balanced - the label explains it - ground on the sleeve, + on the tip and - on the ring. Your interface probably has balanced mic connection but unbalanced line input. Most modest ones do. This means the tip will be + but there isn't a ring and sleeve, just the sleeve - which does the ground and the return - connection. This is probably where the issue is. The solution could be a simple as making up a new cable - at the interface end, the jack tip goes to the jack tip at the DBX end, but the sleeve at the interface end is wired to the ring of the DBX end and the sleeve left unconnected - breaking the loop.

So probably a five minute job if you can solder, but another year if you can't.
 
I think we thought you'd given up after a pause of over a year. What Hydroflare suggested is impossible for somebody who is not into electronics and if you were, I guess you'd have already fixed it. You never really answered the connection question. The mic connection is fine. You have a ¼" jack that is balanced - the label explains it - ground on the sleeve, + on the tip and - on the ring. Your interface probably has balanced mic connection but unbalanced line input. Most modest ones do. This means the tip will be + but there isn't a ring and sleeve, just the sleeve - which does the ground and the return - connection. This is probably where the issue is. The solution could be a simple as making up a new cable - at the interface end, the jack tip goes to the jack tip at the DBX end, but the sleeve at the interface end is wired to the ring of the DBX end and the sleeve left unconnected - breaking the loop.

So probably a five minute job if you can solder, but another year if you can't.
Sorry for resurrecting a year-old thread, and thanks for the thoughtful reply! To clarify, (1) I am not the OP, but (2) hydroflare's post directly above mine was the closest I could find anywhere online as a good lead for solving the dbx 286s external buzz/hum that I have been trying to get resolved (for less than a year). My concern is regarding a pronounced buzz/hum into the room ("mechanical"?)--including when just the power cord is plugged in and all the dials are turned to off. I do not notice a problematic hum in the audio signal.

Totally, the soldering job is beyond my skills. I thought that if I could get the needed replacement PSU, then I could probably then hire someone locally to perform the PSU swap in the dbx 286s.

Truly sorry if my question was confusing or not in good form for HomeRecording. I am new here, but was glad to see the information discussed and even thought that the answer might be able to help people beyond myself (since the external hum is apparently endemic in these dbx units).
 
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I am the op and the cable/lead was a cheapo. A balanced with Neutrik was used as a replacement and all ok.
 
Ha! I see. The hum is physical hum, this I don't think was that typical but real transformers can often have real audible hums where the transformer lamination vibrates. if you look at these kind of transformers they often had an 'E' shape and were lots of thin separate items stuck together with a black sticky gunge and sometimes they just rattled - made worse by the 50Hz mains going through them. People would often stick lumps of foam on them so when the lid was stuck on it put physical pressure on the transformer and sometimes this worked - either permanently or until the sponge lost it's elasticity.
 
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