Choke placement

[pablogt]

I'm still working on a channel-linked plexi type of amp with a Trainwreckish" layout. I would like to install a heat sink to cool down the power scaling mosfets and the only place it can go is were the choke is on a Rocket. So, where do you think that the choke should go? I know that I can change it later but, since it would be somewhere above the main board, and I would have to remove it to change the choke placement, it would be nice to get it right the first time.

So these are the alternatives I came up with (big transformer is the PT, smaller one is the OT and the grey rectangle is the heat sink).

Having shorter AC lead runs would be a plus, so the further to the left, the better in that respect. Also, the further to the left, the least likely to disturb the first stages.

So, Which alternative do you think would be the quietest overall?

Pablo

[Cliff Schecht]

First or third one, preferably first because the choke leads won't be as long (not that it matters compared to all of that inductance in the choke!). I like for all of the transformers to be orthogonal to the PT but with a choke it doesn't matter so much.

[pablogt]

Thanks Cliff... I thought of that one too, for the same reason. I was wondering if being off axis of the PT and closer to the PI would have any consequences. I'll probably put it there and hold it with tapped holes so that I can move it without lifting the main board and check if the noise level improves


Pablo

[Roe]

connect the choke with long wires and move it around, listening for noise. use the spot with the least noise

[pablogt]

The problem with finding the right place after everything is mounted is that I would have to put the amp apart again to fit the choke in it's final position (the main board is just below). Or start drilling and tapping holes from above without removing the main board. That would generate quite a mess with metal filings.

I'll also use the "headphones trick" before everything's in place. That will only give me an indication on the influence of the PT's magnetic flux but will not catch any possible interferences with the OT or the PI, but it's better than nothing.


P.

[leadfootdriver]

connect the choke with long wires and move it around, listening for nice. use the spot with the least noise

I did this with another amp, and didn't notice any noise at all. But test your amp to make sure.

[Cliff Schecht]

Chokes are part of a filtering circuit and tend to not be very susceptible to hum induced by magnetic coupling. Also remember that the choke stage usually feeds a push-pull phase inverter and output stage which in itself can reject noise common to both parts of the PP circuit. I jam chokes up right into the power supply often, even though I still orient them orthogonally out of habit. I usually mount the choke and OT going in the same direction (as I think you have done) FWIW.

It's the OT that is sensitive to placement because it is just being used to transfer energy from the primary to the secondary, they aren't part of a power supply filtering network that removes the 60 Hz component. They typically have uneven windings due to interleaving and so both sides of the primary won't pick up the same amount of hum. Thus the CMRR of said stage can't help because the noise is common to both tubes in the pp pir.

[pablogt]

Hi there. I did the headphone trick to find the best place for the choke. I could not hear any difference in hum as long as the choke stayed within the limits of the PT bell. So I'm going to place it closer to the PT.

Playing around with the OT connected to the headphones, I got to the same conclusions as rj: as long as PT and OT stay in exactly the same ax and with the laminates exactly at 90º, the hum is almost minimal. Distance does not matter too much. On the other hand, if you put OT and PT on a 45º diagonal line from each other and with parallel laminates (!!!), it's quite silent as well, although not as good as in the "traditional" Trainwreck position.


Thanks all for your help
Pablo