Bottlehead Forum
Bottlehead Kits => Mainline => Topic started by: HF9 on June 19, 2013, 07:30:35 AM
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Doc, PB, Paul, et al. I was wondering you had experimented with chokes in the CRCRC power supply in the Mainline prototype. If not, could you let us know the resistance values? I'd like to order up one or two chokes so I can have them to test when the unit arrives.
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We haven't messed around with chokes yet, mainly because choke to output transformer interactions in a headphone amp can cripple the circuit if not done properly.
The power supply is a single high voltage power supply with a 270 Ohm dropping resistor. Current is around 60mA.
The power supply choke could have a bit less resistance than 270 Ohms, but should be at least 2H.
Please do build the amp stock first.
-PB
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...choke to output transformer interactions in a headphone amp can cripple the circuit if not done properly.
Are you referring to flux interactions from close proximity?
Please do build the amp stock first.
That's the plan, thanks as always PB
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Would this choke be alright?
http://ca.mouser.com/ProductDetail/Hammond-Manufacturing/158M/?qs=HY6QdUiTBEzMutme2RvbIw%3D%3D
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That would be an appropriate choke for replacing the 270 Ohm resistor.
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Is there a point at which the measured resistance of the choke is too low (or too high) and could mess with (or worse damage) the amp? I'm ordering a Hammond 157M choke which has a resistance of 259ohm but a tolerance of +- 15%.
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You want something with a DC resistance near the resistor you are replacing. So a 1025 Ohm choke is good for a 1,000 Ohm resistor. I'd SWAG that +/- 10% is good. If the voltage is high then you can add a resistor. You still get the noise filtering of the choke.
You also want to be certain the current is covered.
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Is there a point at which the measured resistance of the choke is too low (or too high) and could mess with (or worse damage) the amp? I'm ordering a Hammond 157M choke which has a resistance of 259ohm but a tolerance of +- 15%.
Yes, too much resistance and the hybrid shunt regulator doesn't work. Too little resistance, and that hybrid shunt regulator works well, but will heat up from excessive voltage.
FWIW, the sonic benefits of a choke in the power supply drop significantly when the high voltage supply is regulated.
-PB
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Got the choke in, decided to place it on top of my Mainline, encased in a cast aluminum enclosure. Shielded the wires with both braid and foil. Wires on the underside of the Mainline are shrink wrapped to avoid shorts.
The Hammond 157M choke measured in at 247 ohms, so I placed a 22 ohm resistor in series. Voltages measured out fine.
PB was right, if there are sonic benefits they are pretty minimal. Though I don't have much of a trained ear for noticing subtleties. Not too much of a loss at a cost of $35 for the whole process. (Except that I bent some of the aluminum so that I could get the thick wires through :-\ )
P.S. I haven't finished staining the base, looks pretty ruff from sanding...
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I would go back and reread the power transformer mounting instructions. (this matters, a lot!)
You will need to pull yours off and correctly mount it, especially before moving on to modify the amplifier.
-PB
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I would go back and reread the power transformer mounting instructions. (this matters, a lot!)
You will need to pull yours off and correctly mount it, especially before moving on to modify the amplifier.
-PB
If you are referring to the nylon washers on top of the bell - those were extras that I had at my house. I have the original nylon washers between the power transformer and the chassis as per the instructions. I did however lower the bell top by removing the lockwashers that normally go between the bell and the top of the transformer. I placed these on top of the bell, then placed the extra nylon washers on top of those and secured them down. This was done as a means to remove an audible hum (not through headphones, but from the transformer itself) that I was experiencing. It was abolished by lowering the bell this slight amount and placing the extra set of nylon washers above.
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Having the end bell and stack floating is a safety issue.
If the transformer was vibrating, generally this is cured by tightening the screws up a bit.
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The transformer stack is currently floating, but the end bell is not :-\
How serious is the safety issue? Is it due to heat build-up, or another reason?
I don't believe the transformer hum was due to screw tightness, as I tightened the screws and it did not go away originally (the first thing I thought of was tightening the screws, but that didn't help). I tightened the screws until the sockets were almost stripping, but again that didn't help. I tried different heights of metal washers under the end bell, and the further away the end bell was from the transformer, the louder the hum became. Only when the end bell was fully touching the transformer did the hum disappear.
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If you put pressure on the bell with your finger does the noise stop? If so add a small square of acoustic damping to the bottom side to stop it, dynamat is ideal as it will take the heat. Its a common issue with Edcor transformers which i experienced myself until i stuck a 1" square of damping material inside the bell.
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We won't really be able to provide any support on a kit with these modifications.
-PB
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If you put pressure on the bell with your finger does the noise stop? If so add a small square of acoustic damping to the bottom side to stop it, dynamat is ideal as it will take the heat. Its a common issue with Edcor transformers which i experienced myself until i stuck a 1" square of damping material inside the bell.
Yep, strong pressure to the centre of the bell with my finger when the bell was raised using the supplied lockwashers did reduce the noise to a great extent. I'll give the dynamat damping a try and replace the lockwashers to under the bell end and see if that works.
Thanks for the replies everyone and all your help! ;D
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CB was serious. If the transformer primary or high voltage winding shorts to the core/laminations (not likely but possible) you could get a deadly shock. It would be best if you went back to the manual and followed the instructions closely.
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CB was serious. If the transformer primary or high voltage winding shorts to the core/laminations (not likely but possible) you could get a deadly shock. It would be best if you went back to the manual and followed the instructions closely.
Thanks for the explanation, I wasn't sure what the safety issue was, but hearing that I'll definitely return to stock power transformer config ASAP.
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I returned the power transformer to stock config and... no hum! I was experiencing the hum before in my other apartment building that I was living in half a year ago, but have since moved. I had originally dropped the bell end when I was living in my old place and had been running it like that for the 1/2 year, but no need any more! ;D
I'm suspecting that there was something to do with the power in the previous building I was in as it was extremely old, each apartment had multiple dimmer switches, and each apartment was using many fluorescent ballasted lights.