Tweet
Thanks Jon. No change to the hum doing that. Still looking for a cheap scope as might not get any further without one.
-
-
ok I picked up one of those cheap handheld DSO150 scopes as not much happening on the auction front for scopes in the UK at the moment.
Definitely 50Hz noise on the speaker output. +/-15v rails squeaky clean. Will try and work backwards from the speaker output to see if I can find the source.
edit: decided to work from vr13 instead. Looks like it's the TL072 (IC8) causing it. From what I can see the signals and power going into it are clean but 50hz signal on pin7.Last edited by Roob65; 05-22-2020, 09:54 AM.Comment
-
Originally posted by Roob65 View Post
If power supply and inputs are clean and you have 50Hz on one output of IC8 it is not likely it would start "producing" that frequency all by itself. Have you checked the power amp rail supply VE+/VE- for 50Hz?Comment
-
That totally makes sense Doom. I've checked the +/-15v for ripple and looks like a high of about 3mV peak to peak.Comment
-
It would be very hard to see the hum if it were at the inputs of that IC, which is probably the case. Try working your way back from there.Originally posted by Enzo
Comment
-
Thanks for that G1. I've been probing around and get a lot of noise from the outputs of IC9 and IC10 too (goes quiet again after R104/105 etc.) The rest of the op-amps seem quiet. Must admit I'm getting a bit lost. What would be acceptable ripple (if any) in the DC rails?Comment
-
Measurements from power amp +/-VE rails:
+VE, 39.14dc, 0.25vac
-VE, 39.25dc, 0.15vac
100Hz frequency.Comment
-
OK progress update for anyone following....
Massive thanks to Glen from Mars Amp Repair!
I found a rather old thread where Glen had a similar issue with a 8080 (essentially the same amp): https://music-electronics-forum.com/...alvestate+8080
I reached out to Glen and he gave me some great advice RE signal tracing. Armed with this I managed to obtain similar results to him and reduced the hum/buzz by around 50%. My solution wasn't quite the same whereby the best result I could achieve was jumping the ground trace between the star point (where R119 meets ground) and the ground trace at the line out jack.
There is still a bit of buzz (def more buzz than hum) which seems to have been being masked by the hum and I'm going to keep at it but this may be the best the amp can achieve. I'll update the thread again if I get anywhere or give up!Comment
-
IMHE often when you can decrease hum by jumpering two grounds it's because you've actually grounded a ground loop. Not that it could ever be easy in that amp, but locating any possible ground loops and correcting them would be a more ideal solution. If you have some sense of isolation for the hum you need to look for shared ground traces and circuits that might be grounded redundantly at more than one ground point. Even via components. If, say, a voltage divider is grounded at one point and then before the next amplification stage another voltage divider or mixer stage is grounded at another then any voltage differential between the two ground points can inject itself into the signal chain. So that's the sort of thing you'd be looking for.Originally posted by Roob65 View Post"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
-
What is the difference between "correcting" a ground loop and curing one? It would seem to me that if connecting two grounds together stops a hum, then it was what was needed in the first place.
And 50Hz hum (rather than 100Hz) means a grounding or radiated hum, power supply ripple would be at 100Hz.Education is what you're left with after you have forgotten what you have learned.Comment
-
As I wrote in post 6.Originally posted by g1 View Post
Doing that will remove any possibility of hum on the input of the voltage gain IC.Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
-
In that case the hum is not coming into the IC. It is either ripple from the +-15volt rails or the main HT rails.Originally posted by Roob65 View PostSupport for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
-
Hi Jon, thanks. I'm not knowledgable enough to know how to effectively get rid of the remaining buzz bar throwing bigger caps at it? I was reading this (https://robrobinette.com/Tube_Guitar...m#Hum_and_Buzz), which suggests a 'noisy' rectifier. Do you think it's worth trying to add caps as he suggests?Comment
-
Bigger caps will, in my experience, do practically nothing. Ground routing is the issue.
Your link is mainly for valve amplifiers. Slightly different approach for SS is required.
It is all down to grounding issues on a poorly designed stage amplifier. Crate and Trace Elliot are the same.
To make a studio amplifier, requires that different approach and attention to detail with the grounding is required.
I have repaired a number of similar amplifiers using this circuit and all are very noisy. Budget I suppose.Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
-
ok think I'm gonna put this thread to bed. Thanks to everyone who commented and gave advice. I think I've got a pretty decent result.
The best result I could achieve was by adding one more additional ground 'jumper' between R120 and R23. It's possible I could have gotten further noise reduction by adding more but I have a feeling it would be deminishing returns and no magic bullet! Suppose it's possible the ground traces are fatigued with age which is why we see more posts about hum from these amps.
Anyway, thanks again to all concerned! Cheers!Comment
Comment