xtian wrote:We should add to the Headphone Trick FAQ: The idea is to MINIMIZE the hum you hear, not eliminate it entirely, which is sometimes only possible by moving the OT off the playing surface entirely. YMMV!
thanks. i should add that those extreme locations eliminates it entirely. ill hook it up again tomorrow and see where i get minimal hum.
and i don't have it anywhere near the input jack, which is in the lower left corner of the pictures. you can see the POTs coming out the bottom and the input is on the far side of them. this is a ceriatone trainwreck chassis.
wsaraceni wrote:does this sound ever get amplified? could i just hook an actual speaker cab up to the OT to determine how much hum is coming through. It may be a lot less than what i hear in a set of headphones.
No, it won't get amplified. This will be a constant background level of hum even with the amp in standby mode (with mains power on) so you can plug in a speaker and hear how much hum you'll be getting induced. It won't get worse than that.
Yes, both transformers should be contacting the chassis, the chassis should be grounded like it normally would be through the ground of the power cord.
Yes, you can hook the OT up to a regular speaker and the hum you hear from that will be the hum that you get when the amp is built (provided you don't pick up more hum from the build itself).
Any hum generated in the OT at this point will NOT be cancelled out by the push-pull noise canceling nature of the power amp. The hum that gets picked up from transformer placement is there to stay.
Jana wrote:Yes, both transformers should be contacting the chassis, the chassis should be grounded like it normally would be through the ground of the power cord.
Yes, you can hook the OT up to a regular speaker and the hum you hear from that will be the hum that you get when the amp is built (provided you don't pick up more hum from the build itself).
Any hum generated in the OT at this point will NOT be cancelled out by the push-pull noise canceling nature of the power amp. The hum that gets picked up from transformer placement is there to stay.
yeah your right about the cancelation my bad.
However, the ot hum would have to be above the noise floor of any other hum sources to be a significant problem.
Jana wrote:Yes, both transformers should be contacting the chassis, the chassis should be grounded like it normally would be through the ground of the power cord.
Yes, you can hook the OT up to a regular speaker and the hum you hear from that will be the hum that you get when the amp is built (provided you don't pick up more hum from the build itself).
Any hum generated in the OT at this point will NOT be cancelled out by the push-pull noise canceling nature of the power amp. The hum that gets picked up from transformer placement is there to stay.
painted chassis. let me hook a ground wire up to it tomorrow and re-do the test. also i'll test through an actual speaker cab.
Can you swap the location of the choke and put the OT there instead? Did you try that spot for the OT, you can put the choke anywhere. or, just use a resistor instead.
I would put it where the choke is then and move the choke. JMHO...again, the hum from the OT is not amplified, so with the amp cranked the S2N ration will be huge vs the hum.
the headphones i had one connected to the black wire and one connected to the 16ohm tap. i thought that's what i read on the ax84 site. everythign else capped
the PT is totally wired in the chassis. but the only thing connected to anything, besides the fuse, is the CT to ground. everything else is either capped or soldered to an empty fuse socket.
i wasnt thinking last night, and didnt think about the grounding of the devices. since my chassis is painted on the outside, i'll install a ground wire and redo the test.
my wife and I are having our first baby and her due date is today. im a little scatterbrained this week because of it. i go to work on the amp to relax a bit, but then i do stupid things like this ground issue which i knew would be an issue and even thought about it when i was waiting for the parts to come in.
tonight i was going to hook up an actual 16ohm speaker to the 16ohm tap on the + side of the speaker, and the black ground tap to the - side of teh speaker. will that work for what we are looking to do?
In the beginning stages of the build, with a blank chassis, have only the PT partially installed (connected to AC) by having the PT connected to the power switch...and the power switch wired to the IEC socket. That's it.
The OT is not connected to anything in the amp. With the PT "on" have only the 16 ohm tap (or greatest impedance tap) and ground wire loosely connected to a pair of headphones via the headphone jack (you can hold each of the wires in your hand, 16ohm to the tip, ground to the sleeve). With the OT sitting on the chassis (and your headphones connected to it), to find the place to mount the OT that will omit the least hum, just move it around on the chassis and listen. When it's the quietest, mark it, drill it, done.
The only minor detail is that you've got to power up the PT. Tape any other wires with bare metal exposed, otherwise they can hang there as long as they are not touching anything!
ill remember that for next time. this build was my first attempt at doing something that wasnt looking at pictures and copying a layout (totally) since im doing an 18 watt version of a 36 watt amp with a smaller chassis. it also makes me realize if i buy another pre-drilled chassis, i'll only get the tube sockets pre-drilled. i hate all the extra holes. and since this is the 3rd amp i've built in this chassis, there are a lot of extra holes.
Not really, it's not very stressful at all on the PT to run them unloaded. I tend to test my non-NOS ancient transformers by "cooking" them (letting them run unloaded for a few hours) before I put them in a build if I don't trust them. If it gets anything past warm to the touch, meaning you can't touch the outside because it's so hot (losses eventually cause *some* heat buildup, but not a lot), then the PT is usually fine.
wsaraceni wrote:
my wife and I are having our first baby and her due date is today. im a little scatterbrained this week because of it. i go to work on the amp to relax a bit, but then i do stupid things like this ground issue which i knew would be an issue and even thought about it when i was waiting for the parts to come in.
Hah! B-Day. Congrats, and good luck. (has your wife been advised that this might hurt a little?)
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