I built myself a BB3 phonostage, initially in a relatively small box, but suffered a bit of hum from time to time.
Recently, I decided to rehouse it into a bigger case, which is pretty much finished now - see below.
I am wondering if there would be any benefit from fitting a 5mm thick aluminium “shield” Between the BB3 PCB and the mains related side of the case. It will not be expensive to do, or take long, but will there be any benefit?
Recently, I decided to rehouse it into a bigger case, which is pretty much finished now - see below.
I am wondering if there would be any benefit from fitting a 5mm thick aluminium “shield” Between the BB3 PCB and the mains related side of the case. It will not be expensive to do, or take long, but will there be any benefit?
That looks a very neat build 🙂
Hum that comes from a transformer radiating its field will be at the mains frequency fundamental, a deep pure tone. Buzzes and that kind of thing that are genuinely intermittent and variable could be caused by grounding issues and they can become more prominent depending on the purity of the mains (which can vary hour to hour).
You mention moving it all to a bigger box but don't say if it still has the same hum issue as in the smaller one 🙂
If you do have specific issues then maybe as a temporary measure try rotating and moving the transformer and if necessary move it quite a way out of the box as a test. Also equipment on top of below the box can radiate its field into the preamp as well.
Meaning as in not all the time...
Hum that comes from a transformer radiating its field will be at the mains frequency fundamental, a deep pure tone. Buzzes and that kind of thing that are genuinely intermittent and variable could be caused by grounding issues and they can become more prominent depending on the purity of the mains (which can vary hour to hour).
You mention moving it all to a bigger box but don't say if it still has the same hum issue as in the smaller one 🙂
If you do have specific issues then maybe as a temporary measure try rotating and moving the transformer and if necessary move it quite a way out of the box as a test. Also equipment on top of below the box can radiate its field into the preamp as well.
I have yet to hear the intermittent hum in the new box, but it is still early days.
I "finished" wiring it up on Friday, and intend to bolt the lid down soon. I figured that the additional shield would not make things worse, and may help lower the noise floor?
My issue I believe was/is mains quality. The local electricity board have recently got in touch to ask permission to trim some of my trees to allow for the mains cable replacement, and are fitting a new mains transformer.
I "finished" wiring it up on Friday, and intend to bolt the lid down soon. I figured that the additional shield would not make things worse, and may help lower the noise floor?
My issue I believe was/is mains quality. The local electricity board have recently got in touch to ask permission to trim some of my trees to allow for the mains cable replacement, and are fitting a new mains transformer.
If it really is OK now in its bigger box then I'm tempted to say leave well alone and particularly as you seem to have known mains issues that should get resolved quickly. Although adding a shield shouldn't make things worse you just never know for sure meaning could it alter any circulating (magnetic) currents that are present.
Ideally you should try and look at any hum residual on a scope (even if its inaudible now) and see if placing a shield reduces it to a lower level or not. Also anything like that is only valid when you do screw everything down as that changes things as well.
Ideally you should try and look at any hum residual on a scope (even if its inaudible now) and see if placing a shield reduces it to a lower level or not. Also anything like that is only valid when you do screw everything down as that changes things as well.
Al does not make a very good shield as it is non-magnetic.
dave
dave
A 5 mm shield as proposed by OP would offer some attenuation at 50/60 Hz. I seem to recall that steel would be about 3x better but could be wrong on this. µ-metal is, of course, the best. One thought could be to make an aluminum bracket that runs the length of the chassis and then glue a sheet of µ-metal onto it. You can buy µ-metal on eBay for not that much money.
I would have rotated the two boards that connect to the mains by 90º so the wiring is away from the phono stage. I'd also scoot them as close to the side as possible. Move the grounding point if necessary. Spacing beats shielding on price any day...
I'd also measure the mains hum, or rather, measure the harmonic spectrum of whatever noise is on the output of the phono stage. That'll give you a lot of clues as to where the hum is coming from. Then we can decide what to do about it, if anything.
Tom
I would have rotated the two boards that connect to the mains by 90º so the wiring is away from the phono stage. I'd also scoot them as close to the side as possible. Move the grounding point if necessary. Spacing beats shielding on price any day...
I'd also measure the mains hum, or rather, measure the harmonic spectrum of whatever noise is on the output of the phono stage. That'll give you a lot of clues as to where the hum is coming from. Then we can decide what to do about it, if anything.
Tom
Steel is better than Al.
I have some VERY good, very pricey Siemens stuff that i was gifted. Haven’t found a use for it yet.
dave
I have some VERY good, very pricey Siemens stuff that i was gifted. Haven’t found a use for it yet.
dave
This says the skin depth in aluminum at 60 Hz is 10.5 mm versus 1.5 mm for mild steel. So a 7x ratio. But note that "aluminum" and "steel" can mean many things. There're many alloys. I'd think twice before assuming that stainless steel offers the same attenuation as mild steel, for example. Maybe do the math...
https://incompliancemag.com/skin-effect-and-surface-currents/#:~:text=So we see that at,reduces to about 1.5 mm.
Tom
https://incompliancemag.com/skin-effect-and-surface-currents/#:~:text=So we see that at,reduces to about 1.5 mm.
Tom
If the hum were coming from the build, it would be constant if the component placement does not change. A varying him suggests some sort of outside source to me.
Thanks all.
The noise was in the region of 100 and 200 Hz, but at the moment, there is no noise problem.
The original build is below. The new build is a big improvement on it.
The noise was in the region of 100 and 200 Hz, but at the moment, there is no noise problem.
The original build is below. The new build is a big improvement on it.
I'm in the process of building a phono stage and my circuit boards will be in a seperate copper plated box inside the enclosure, away from the power supply etc
Are the heaters powered by DC or AC?
Tom
It sure would be nice with a measurement. For 50 Hz mains, I'd expect to see rectifier hash around 100, 150, and 200 Hz. That can be dealt with ... if that's indeed the root cause.
Tom
Copper won't be any more effective than aluminum. A 1 mm thick steel sheet would do more.
Tom
I dealt with this same issue until I was nuts. I tried just about any shield that you can imagine (even a Tri lamination of copper, mu, and steel). Finally, I came to answer of making a separate chassis for the power supply. You won't get much better than that.
I did some advice about transformer covers thought that I should share. In your photo, the opening for the wires coming out of the transformer cover are actually pointed toward the phono board and are close too. John Broskie of Glassware audio had mentioned this to me. Also the idea of turning the transformer facing different directions sounds like a good idea.
Now, seeing how you have already solved the problem, leave 'er right there. You won.
I did some advice about transformer covers thought that I should share. In your photo, the opening for the wires coming out of the transformer cover are actually pointed toward the phono board and are close too. John Broskie of Glassware audio had mentioned this to me. Also the idea of turning the transformer facing different directions sounds like a good idea.
Now, seeing how you have already solved the problem, leave 'er right there. You won.