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For tube RIAA MM preamp - Chassis = Steel, or Aluminum?
For tube RIAA MM preamp - Chassis = Steel, or Aluminum?
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Old 12th June 2019, 08:44 PM   #21
Lingwendil is offline Lingwendil  United States
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I've built a handful, and even in an aluminum enclosure hum wasn't an issue with Antek toroids. If you really care to, you can do the tubes and circuitry inside the case, with a separate power box connected via umbilical cable. More fussy to build, and no exposed tubes for looks, but with some adequate ventilation holes strategically placed it will last forever and be dead silent.
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Old 13th June 2019, 12:20 AM   #22
Troy Madden is offline Troy Madden  United States
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Quote:
Originally Posted by Eli Duttman View Post
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.
This MM/MC phono preamp is very quiet. AnTek toroid shielded with one of their enclosures, but upside down with an electrical box cover. . Lundahl step-ups in a steel project box. Power supply well away from signal pcb. Aluminum chassis. Click the image to open in full size.

Last edited by Troy Madden; 13th June 2019 at 12:40 AM.
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Old 13th June 2019, 05:16 PM   #23
rongon is offline rongon  United States
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For tube RIAA MM preamp - Chassis = Steel, or Aluminum?
Thanks for the ideas everyone. Rather than buy new parts like the Antec transformer, I think I'll try to use stuff I already have.

I have a few pairs of 5-pin BNC-type connectors (male and female) I can use to make an umbilical. I think they're these. Male connector on the audio circuit chassis (the load), Female connector at the end of a 1m pigtail coming from the PSU box.

For the audio circuitry, I bought a Bud aluminum chassis which measures 12" L x 8" W x 3" H (305 mm L x 203 mm W x 76 mm H). It fits neatly inside a wooden box with perf board base I bought fer cheep at a local crafts store (perforated bottom will serve for ventilation once rubber feet are installed). There's more space than needed for the PCB and jacks on this Al chassis, but I suppose I can use the extra space for adding a couple of boards with LR8 regulators on them for each channel's B+.

I have a couple of Radio Shack plastic project boxes with metal plate for mounting parts. One of those should serve for housing the power transformer, rectifier (UF4007), reservoir cap and first B+ filter section.

This way I can use this power supply with different versions of 12AX7-based RIAA preamps. Perhaps I'll make a version of Eli's RIAA with grid leak bias on the 2nd stage. EQ rolling!
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Old 13th June 2019, 05:56 PM   #24
Zeta4 is offline Zeta4  United Kingdom
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I recently built a valve headphone amp in an ali chassis. Initially I had hum problems
in spite of trying to mount the mains tx as far away from the signal circuitry as possible.

Eventually i got hum down to very low levels by minimising the power and signal lines loop area.
Because I needed to have the signal i/p on the back panel and near one of
the heater tx's I had great success with screening the i/p sockets and cables with some mu metal sheeting.
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