9407 Tube MicPreAmp

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9407 Tube MicPreAmp

Post by PAiA-Scott »

Hello Stan.

It's good hearing that you have solved the trouble.

The main reason we have the power source connecting direct to the board is for more options when we buy them to go in the kits. Then, by wiring direct, the cost of a connector set is saved too. Since it was a kit 'repair' that alerted us to the use of a connector causing hum or a possible short, the cautionary note didn't make it into the kit manual.

Yes, the use of a split washer for the XLR connectors is a good idea. I'll save this message as a reminder to consider this for future similar instances and upgrades/revisions.

I like the idea of threaded stand-offs too, but there is a big difference between the price of these and the rolled-aluminum type we provide. The last time I looked, I didn't see them in the 3/16" size and this is a factor with the bolt length in some instances for the stand-off sets in some of our kits.

A crimp can be made in the rolled-aluminum spacers to make them work to thread up and down the bolt, but care must be taken to bisect the seam with the crimp to make it less likely the spacer will crumple as the nut is tightened on the set. Putting the spacer over the bolt and aligning the seam to meet the jaw of diagonal cutters closed down with just enough force to meet the bolt and make a crease is the key to getting it to work. Unscrew the spacer, push the bolt through the case bottom, hold it in place, screw the spacer down over to hold the 'stand-off' in place. Attached is a image of a bolt with a crimped spacer.

Thank you.

Sincerely, Scott Lee

Stanley A. Rubin wrote:
> Hi Scott,
> Thanks for your support and the "tips and suggestions."
> Indeed, I'm one of "those" that insists on mounting a power jack rather than
> directly attaching the power supply to the PCB. Therefore, the power jackat
> was touching the case. No "burn out," but hum. Simple fix: mounted the
> jack through a grommet that isolated it.
> Two small pieces of feedback on the kit's mounting hardware for PAIA to
> consider:
> 1. XLR jacks mounting hardware:
> Consider: adding split washers to the screw/nut assembly mounting the jacks
> to the case.
> Why? The XLR jacks get rugged use through a lifetime of putting the XLR
> plugs in and out. They are likely to loosen. The split washer reduces that
> chance. The cost for 4 split washers should be negligible. The difficulty
> in the user assembling the washers to the screw/nut assembly is negligible.
> 2. PCB mount standoffs.
> Consider: changing the standoffs to threaded standoffs (same height) and
> providing screws.
> Why? The PCB is an awkward/tight fit in the case (once all the components
> are added to the board and the controls on the case; it's easy in the bare
> case). Trying to maintain the supplied standoffs within the screw, and then
> getting the screw to go through the hole in the bottom of the case is
> difficult. If PAIA supplied threaded standoffs, they could be securely
> attached directly to the PCB before sliding into the case with a screw.
> Then sliding-in the PCB and alignment of the standoff holes to the bottom of
> the case, and attaching it through screws from the bottom of the case,
> becomes a snap. Obviously, threaded standoffs are considerably more
> expensive than that supplied with the kit. But, I think your customers
> would appreciate it.
> Thanks for "listening."
> Stan
> On 3/5/09 3:22 PM, "Scott Lee" <scott@paia.com> wrote:
>> Hello Stanley.
>> I have attached some tips and suggestions which you might find useful in
>> getting rid of any hum. I played the sound sample and could hear the
>> click, but the hum wasn't apparent, or was softer than the fan whirring
>> on the computer next to my monitor's built-in speakers and the volume at
>> Max. I can listen again closer, but I wonder...did you note the hum to
>> be louder or softer than the click of the power switch? Is the hum
>> apparent when the mic is connected and sounding?
>> Thank you.
>> Sincerely, Scott Lee
>> Stanley A. Rubin wrote:
>>> Hi Scott,
>>> You¹ve been very helpful/wise with some of the other PAIA kits I¹ve
>>> needed advice on. Please help me with my just completed vacuum tube
>>> preamp (9407K mounted in the rack).
>>> Problem: line hum.
>>> I¹ve enclosed a wav file sample clip that shows the problem.
>>> Setup for sound file recording: no mic input, tube drive/blend/output
>>> knobs all turned to minimum. Adding the mic/mic cable or changing the
>>> setting of the knobs does not make a difference to this line ³hum.²
>>> The sound clip attached: about 20 seconds in length. The first 5
>>> seconds is with the power switch ³off.² Then, you¹ll hear an audible
>>> ³click² as the power is turned on. At that time, you¹ll hear the
>>> ³hum.² I then turn off the power switch for the final 5 seconds of the
>>> recording. The ³hum² goes away.
>>> Can you help me with this?
>>> Thanks,
>>> Stan
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Scott Lee