I'm purchasing parts for a tube phono preamp build. I need to choose a chassis.
I like the look of the black powder coated steel chassis available from Hammond and Bud, but will there be a problem with hum pickup due to the magnetic steel chassis?
On the other hand, aluminum chassis of the same size are a bit cheaper. Is aluminum the better choice?
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I like the look of the black powder coated steel chassis available from Hammond and Bud, but will there be a problem with hum pickup due to the magnetic steel chassis?
On the other hand, aluminum chassis of the same size are a bit cheaper. Is aluminum the better choice?
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petertub said:
Thanks. I think the power transformer will be on top of the chassis, at the tail end, farthest away from the inputs (input end of PCB at front end of chassis, output end of PCB about 2/3 of the way back, transformer at back end of chassis).
The PCB dimensions are about 11 cm wide by about 20 cm long. I figure the power transformer will be located about 5 cm behind the back edge of the PCB. I hope that's enough distance from the output (highest level) part of the board to keep hum out of the output.
The total plate/anode current draw from both audio channels will be only about 10mA. I'm hoping the low current will allow a single-chassis build without hum.
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The transformer's radiated hum field is smallest in the plane of the laminations.
It's actually a PCB I bought that was laid out by someone who knows what the heck they're doing. I think it'll be OK. I'll hook everything up in the open and see what results I get.
Thanks for the advice.
Hmmm... Maybe I should hang the transformer off the back of the chassis, so that the laminations are parallel with the horizontal plane of the PCB? I'm thinking of using a PA-211 power transformer salvaged from an old Dyna PAS2 (770VCT about 15mA). It's small and not heavy, which should make it easy to locate.
Thanks for the advice.
Hmmm... Maybe I should hang the transformer off the back of the chassis, so that the laminations are parallel with the horizontal plane of the PCB? I'm thinking of using a PA-211 power transformer salvaged from an old Dyna PAS2 (770VCT about 15mA). It's small and not heavy, which should make it easy to locate.
If power trafo stray magnetic field is a worry, consider using a toroidal power trafo. The stray fields associated with toroids are tiny. Also, you could house a toroid in steel and use aluminum for everything else.
AnTek offers highly affordable 100 VA toroids that come with a pair of 3 A. 6.3 VAC windings. Various B+ rectifier winding voltages, starting at 150 VAC, are available. Look here.
AnTek offers highly affordable 100 VA toroids that come with a pair of 3 A. 6.3 VAC windings. Various B+ rectifier winding voltages, starting at 150 VAC, are available. Look here.
I wouldn't worry too much about the shielding aspect, but I certainly would be concerned about machining.
IMO, the Hammond Aluminum chassis are good for preamps and very small power amps. If you are going to build a big power amp in an aluminum chassis, the top plate really needs to be about 100 thou aluminum (0.1 inches) to avoid being overly flimsy.
For preamps? Not critical. I might go for steel only because I have the tools to machine it. It also takes paint better, and I think generally feels nicer.
IMO, the Hammond Aluminum chassis are good for preamps and very small power amps. If you are going to build a big power amp in an aluminum chassis, the top plate really needs to be about 100 thou aluminum (0.1 inches) to avoid being overly flimsy.
For preamps? Not critical. I might go for steel only because I have the tools to machine it. It also takes paint better, and I think generally feels nicer.
Well, yes. Very true. However, is this a big problem if the load is only drawing 10mA max?
I don't know if this is true, but I was under the impression that induction of hum into the audio circuits is a bigger problem if the power supply is delivering higher currents to the load. Some tube phono preamps are built with higher gm triodes and/or triode-strapped RF pentodes running at 5 to 15mA each triode, but I'm using pair of the humble 12AX7 (1mA and 500uA) with a 12AT7 cathode follower (3mA). Less than 10mA total.
I'm planning to use either a 12VDC 1A SMPS or a separate 12.6VCT transformer (DC rectified and LM317 regulated) for the heater supply, instead of using the PA211 transformer's heater winding. That should avoid the possibility of rectifier spikes being coupled from the plate supply to heater supply windings.
If I use a 12X4 for the rectifier then I'll use the PA211 heater winding for that, but not for the 12AX7 and 12AT7 heaters.
So that's the plan. Does that sound like it would require a separate psu enclosure with umbilical?
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I don't know if this is true, but I was under the impression that induction of hum into the audio circuits is a bigger problem if the power supply is delivering higher currents to the load. Some tube phono preamps are built with higher gm triodes and/or triode-strapped RF pentodes running at 5 to 15mA each triode, but I'm using pair of the humble 12AX7 (1mA and 500uA) with a 12AT7 cathode follower (3mA). Less than 10mA total.
I'm planning to use either a 12VDC 1A SMPS or a separate 12.6VCT transformer (DC rectified and LM317 regulated) for the heater supply, instead of using the PA211 transformer's heater winding. That should avoid the possibility of rectifier spikes being coupled from the plate supply to heater supply windings.
If I use a 12X4 for the rectifier then I'll use the PA211 heater winding for that, but not for the 12AX7 and 12AT7 heaters.
So that's the plan. Does that sound like it would require a separate psu enclosure with umbilical?
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Thanks for bringing this up. I think I'll have to make a new layout plan then.
I'm expecting delivery of parts for this project this evening. I think I'll do a trial run with the various parts connected and fastened to a wooden board, and try moving the transformers around to see where they need to be. That will be interesting.
I'm expecting delivery of parts for this project this evening. I think I'll do a trial run with the various parts connected and fastened to a wooden board, and try moving the transformers around to see where they need to be. That will be interesting.
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