Wright Audio WPA 3.5 Humpot & schematics - diyAudio
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Old 9th January 2013, 09:03 PM   #1
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Default Wright Audio WPA 3.5 Humpot & schematics

Hi Y'all,

I recently acquired a pair of Wright Audio WPA 3.5 monoblocks, they sound really excellent in stock. However, there was some hum in one of the amps, just a tiny bit more than in the other one, so I decided to investigate...

There are no hum pots on these amps, and also no schematics available online (at least I couldn't find one). One drawing of WPA 3.5 schematic floating on the web is incorrect.
2a3 filaments in this amp are fed from center-tapped filament transformer with CT connecting to the cathode resistor/capacitor. I installed 10 turn 50ohm pot connecting the wiper to the cathode bypass capacitor, cutting the connection to the cathode resistor on top. This way I have the CT of the transformer running to the cathode resistor and humpot wiper - to bypass capacitor. After adjusting the amps are dead quiet, I could detect minute traces of hum with my ear to 95db speakers. This method of connecting the humpot with center-tapped filament transformer was described on this forum site by a kmeier.

I attached the humpot using hot glue - easy to reverse the changes. You can see the big Xicon 750ohm resistor and small blue 100uf cap with connection between them snipped and bent off.

Attaching pics before and after.
I will post the schematics of the amp as soon as I finish it on TinyCAD. I believe sharing the pics and schematics will serve the owners of these fabulous amps. Sadly Mr. Wright is no longer with us and service of his creations is left to the owners.
Attached Images
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Last edited by ClefChef; 10th January 2013 at 05:38 AM.
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Old 10th January 2013, 04:57 AM   #2
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The listening test results are in...Installing humpot solved problems associated with hum balancing but affected the sound quality in a negative way. The sound lost some of its brilliance and detail, sucking the soundstage into the speakers. Humpots - out.

For now I am keeping these excellent amps stock.
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Old 10th January 2013, 02:17 PM   #3
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You had the entire current for the 2A3 going through the wiper of the hum pot. See the attached schematic for an alternative method: the current from the cathode resistor goes through 15 ohm resistors to each side of the hum pot. These resistors carry most of the current. Another point is the hum pot itself. You don't need a 10 turn pot; what you need is a robust pot. I use a PEC 2W.
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Old 10th January 2013, 02:56 PM   #4
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Thanks Palustris, I will try padding resistors and a pair of Clarostats next.
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Old 10th January 2013, 04:08 PM   #5
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Replaced wirewound 10-turn pots with Clarostat 2W padded with 15 ohm resistors. Disconnected filament transformer centertap and connected the wiper from the pot. Used better OFC wire as well. The amps sound excellent. Hum adjusted to inaudible levels.

Thank you!
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Old 10th January 2013, 07:22 PM   #6
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You are a quick worker! I'm happy it worked out for you. Many thanks to G. R. for showing us how to do it correctly.
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Old 10th January 2013, 08:09 PM   #7
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There's definitely an audible difference in the way the cathode is wired and the AC is balanced. I suspect that long taper 10 turn pot is not suitable here, or maybe it's the way I used the pot together with the filament transformer center-tap, creating two different signal paths that resulted in signal smear. Whatever the case the G.R. method seems to be the best.
Another thing to try is Tentlabs DHT filament supply, but in this case those modules would not fit into the chassis. I used Tentlabs modules on my other amps with excellent results.
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Old 4th February 2013, 01:47 PM   #8
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I too have a pair of Wright WPA 3.5 Monoblocks and would love to see your schematic if you have had a chance to finish it. It would be very helpful.
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Old 20th February 2013, 03:19 AM   #9
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Replacing stock components with quality parts do benefit these amps a LOT. Just changing 2a3 cathode bypass cap to a quality component improves the sound quite a bit, not to mention better quality PS caps, resistors, AND the coupling cap...
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Old 21st February 2013, 03:00 AM   #10
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I totally agree. Replacing the stock attached cord with a better heavier removeable power cord helps also.

I don't suppose you ever finished your schematic. I woould love to compare it to my amp.
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