Chassis Design

[Steve Conner]

This is true, turning the transformers to the proper relative angles has the same effect as greatly increasing the distance between them.

However, you can make any layout work if you know what you are doing.


This amp actually has a little hum from induction between the PT and OT, even though they are at opposite ends of the chassis, because I put them in the "bad" orientation for looks. But luckily when the HT is turned on, the global NFB makes the hum disappear.

I had no trouble from capacitive coupling, even though the input jack is next to the OT and it has 5 gain stages before the PI.

[Ken Moon]

If this is a new design/build, why not orient the OT for lowest noise, rather than just putting it perpendicular to the PT?

For those not familiar with how to do this, you just apply power to the PT with all secondaries taped off, and listen (and/or look at the signal on a scope) to the OT output with the OT primaries also taped off.

As you rotate the OT, you'll hear a definite difference in hum in different locations. Just orient for lowest hum and mark your mounting holes.

It may look a little funny if the lowest noise position is at a 45-degree angle to the rest of the chassis, but it works.

For more in-depth discussions, search here or google 'headphone trick'.

[Ronsonic]

Yep...was already planning on the perpendicular orientation of the OT relative to the PT. Sounds like many have done it a variety of different ways with no issues which is good news.

I've also thought about twisting the primary and secondary wires (each twisted with their own only) to cancel any potential EMI into adjacent circuitry as well. In theory this should help as well (at least that's what my brain is telling me but I could be wrong).

Do twist wires with AC running in them. I recall a famous name bass amp where they ended up shielding some high voltage stuff, you'd never have expected. Ampeg did a bit of that in the V4s, etc with a channel in the chassis.

Mounting trannies perpendicular is an approximation. If you like, energize the power tranny and put a scope or ACVM on the primary of the OPT and move it around. You'll find that the best (lowest signal) condition is always some slight, goofy looking angle. The second best position is always perpendicular. Always. In some fairly exotic gear you'll see trannies mounted at an angle, but it's pretty rare.

[Chuck H]

If you read the rest of the thread you'll see the OP is orienting the iron on opposite ends for appearances, not to lower PT/OT coupling. But I will say that I have done the OT placed for for loest noise thing and it works very well. It's surprising really how you often can find a spot where the EMF's don't seem to couple at all... Just silence.