I tried that in 1971 and it didn't help.
I used an old coax and recycled the screen.
Farnell sell screen in 10m lengths @ 3 & 6mm diam.
A thread gives the parts numbers.
I used an old coax and recycled the screen.
Farnell sell screen in 10m lengths @ 3 & 6mm diam.
A thread gives the parts numbers.
Hmm, any recommendations Andrew, it seems like the first place to start is the input wires since the hum gets worse the longer they are.
All my grounding is connected to a single earth block which is earthed to the chassis and the mains ground input.
Cheers
Dean
All my grounding is connected to a single earth block which is earthed to the chassis and the mains ground input.
Cheers
Dean
Hi Dean, Have you done some isolation tests?
first thing I'd suggest is to remove the input wiring from the furthest amps and check that the noise is gone. If it is then its a pretty sure bet your input wiring is picking up the noise. It looks like you have used twisted pair wiring, you might want to try some plain old shielded hookup wire something boring like this Single Screened Audio Cable - Jaycar Electronics would be a start.
if you wanted something a bit better then this Single Core OFC Screened Audio Cable - Jaycar Electronics
or this FIG 8 - OFC Shielded Audio Cable - Jaycar Electronics
might fit the bill.
If the noise is still present after disconnecting the input wiring, you will need to look elsewhere.
Tony.
first thing I'd suggest is to remove the input wiring from the furthest amps and check that the noise is gone. If it is then its a pretty sure bet your input wiring is picking up the noise. It looks like you have used twisted pair wiring, you might want to try some plain old shielded hookup wire something boring like this Single Screened Audio Cable - Jaycar Electronics would be a start.
if you wanted something a bit better then this Single Core OFC Screened Audio Cable - Jaycar Electronics
or this FIG 8 - OFC Shielded Audio Cable - Jaycar Electronics
might fit the bill.
If the noise is still present after disconnecting the input wiring, you will need to look elsewhere.
Tony.
Hey Tony.
Do you mean remove the input wiring from the furthest amp then check the noise again on the nearest one? To see if one is inducing hum through the whole system?
I can turn off the furthest set completely, would this achieve the same thing electrically? The amps nearest the back panels also have some buzzing when they are the only ones powered up also.... What else might I check???
Dean
Do you mean remove the input wiring from the furthest amp then check the noise again on the nearest one? To see if one is inducing hum through the whole system?
I can turn off the furthest set completely, would this achieve the same thing electrically? The amps nearest the back panels also have some buzzing when they are the only ones powered up also.... What else might I check???
Dean
Hi dean, I just mean disconnect the input wiring from the noisiest channels, and power it up and listen for noise. If it is indeed the input wiring that is picking up noise then this should eliminate it for the channels where it has been disconnected.
The channels towards the front are more in line with the transformers, so it is possible that they may be picking up some emi from the transformers (though with the orientation of the transformers this seems unlikely (as they radiate most through the hole).
Tracking down hum issues can be difficult and a methodical approach is required. matter of eliminating things one at a time.
Tony.
The channels towards the front are more in line with the transformers, so it is possible that they may be picking up some emi from the transformers (though with the orientation of the transformers this seems unlikely (as they radiate most through the hole).
Tracking down hum issues can be difficult and a methodical approach is required. matter of eliminating things one at a time.
Tony.
Ahh ok, got ya now, I will start by removing the input wires and see what happens.
Cheers again.
Dean
Cheers again.
Dean
don't just "listen" to the noise. Measure it.
The results of trying different wiring can be compared, if you look at the annotated results. Otherwise you will get lost and learn nothing.
The results of trying different wiring can be compared, if you look at the annotated results. Otherwise you will get lost and learn nothing.
good point Andrew! Dean Pull out the BWD and take some measurements on the output, it's perfect for this, and even small changes will be evident that may not be able to be distinguished by ear. A good starting point would probably be 10ms/div and 5mv/div.
Tony.
Tony.
Been so busy with work I haven't had time to look at this issue until now.
Just to confirm a testing procedure.
1. Hook my Oscilloscope up to the outputs.
2. Set to 10ms/div and 5mv/div to start
Should I normally see any output or will I only be looking at the hum artifact?
Just trying to create a distinction between the two so I know what to chase.
Cheers
Dean
Just to confirm a testing procedure.
1. Hook my Oscilloscope up to the outputs.
2. Set to 10ms/div and 5mv/div to start
Should I normally see any output or will I only be looking at the hum artifact?
Just trying to create a distinction between the two so I know what to chase.
Cheers
Dean
Hum on a scope if lowish will not show up.
If you have 1mVac of hum that is approximately 3mVpp.
Will your scope trigger on a 3mVpp signal?
You could try line triggering. That should hold the mains hum steady enough that you can pick out the ripple amongst the noise.
If you have 1mVac of hum that is approximately 3mVpp.
Will your scope trigger on a 3mVpp signal?
You could try line triggering. That should hold the mains hum steady enough that you can pick out the ripple amongst the noise.
Hi Andrew, Dean has the same scope that I do and I can see hum on the output which is less than 2mv. The triggering should be fine. I'll try and dig up the thread where I posted some measurements.
Dean once you have it on 5mV/div you can increase it to 2mV/div buy turning the inner switch and pot on the control around fully clockwise.
On my chipamp I get a fairly wide scope trace (wideband noise) with little blips present above and below the trace at 100Hz intervals, that being the hum component from the rectifiers.
Tony.
Dean once you have it on 5mV/div you can increase it to 2mV/div buy turning the inner switch and pot on the control around fully clockwise.
On my chipamp I get a fairly wide scope trace (wideband noise) with little blips present above and below the trace at 100Hz intervals, that being the hum component from the rectifiers.
Tony.
Its kinda funny, I did exactly what You guys suggested I dont do and replaced the input cables without checking on the scope first..... 
What I did test was replacing the longest input wires on one channel with a shielded cable. This left me with the opposite channel wired as normal and humming and the new one with zero audible hum at all. Fixed thought me..... wrong.
I did this test with the new shielded cable not running to the back panel but simple hanging out the front of the amp.
This cable was very similar to the double shielded one you mentioned from Jaycar Tony.
I then went ahead and rewired all the inputs with the new cable.... Ready to go, music time thinks me.....
It now looks like the problem might be that all these input's are within a few inches of the mains section at the back of the amp and that this may be what is causing all the noise. But I am actually going to listen this time by testing everything as is. Then placing an earthed alloy box/screen over the PSU section as the scope is running. Everything is heat-shrunk and sealed but I will be very careful.
Project for tomorrow, work today.
Best wishes all.
Dean
What I did test was replacing the longest input wires on one channel with a shielded cable. This left me with the opposite channel wired as normal and humming and the new one with zero audible hum at all. Fixed thought me..... wrong.
I did this test with the new shielded cable not running to the back panel but simple hanging out the front of the amp.
This cable was very similar to the double shielded one you mentioned from Jaycar Tony.
I then went ahead and rewired all the inputs with the new cable.... Ready to go, music time thinks me.....
It now looks like the problem might be that all these input's are within a few inches of the mains section at the back of the amp and that this may be what is causing all the noise. But I am actually going to listen this time by testing everything as is. Then placing an earthed alloy box/screen over the PSU section as the scope is running. Everything is heat-shrunk and sealed but I will be very careful.
Project for tomorrow, work today.
Best wishes all.
Dean
I might also try desoldering one of the inputs from the back panel and connecting the scope to the loose wire, then moving it around to see what happens...
Trying to take a Photo of my scope so I can get your feedback. Having a bugger of a time getting the trace to show up in the photo. Has anybody had any luck with a particular setting on a camera???
Hi Dean, I set my camera up on the tripod, put it in shutter priority mode, and (I think) set the shutter speed to 1/10th of a second.
Here is a post where I did some scope measurements. This was PS ripple not amp output but shows the general jist 🙂
http://www.diyaudio.com/forums/solid-state/195240-pre-amp-oscillate.html#post2690773
Tony.
Here is a post where I did some scope measurements. This was PS ripple not amp output but shows the general jist 🙂
http://www.diyaudio.com/forums/solid-state/195240-pre-amp-oscillate.html#post2690773
Tony.
Here is a photo of my scope (thanks Tony - 1/10th works) connected to the output of 1 channel in my amp. The other channel is still connected to the speaker and producing hum.
The scope is set to 2mV/5ms DIV.
To me it looks roughly like 100Hz@8mV, is this correct and can I assume that is related to mains interference.
Hopefully somebody know's what to make of this and can provide some feedback.
Cheers
Dean
The scope is set to 2mV/5ms DIV.
To me it looks roughly like 100Hz@8mV, is this correct and can I assume that is related to mains interference.
Hopefully somebody know's what to make of this and can provide some feedback.
Cheers
Dean
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Yes definitely looks like 100Hz. somewhere between 6 and 8 mv.
100Hz is post rectification, if it was mains interference I think it would be 50Hz. So my guess is that it is power supply and earthing related.
Draw out all of your earth connections on paper and see whether you can see anywhere where there are two separate paths back to earth (possibly through the input wiring).
Tony.
100Hz is post rectification, if it was mains interference I think it would be 50Hz. So my guess is that it is power supply and earthing related.
Draw out all of your earth connections on paper and see whether you can see anywhere where there are two separate paths back to earth (possibly through the input wiring).
Tony.
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