Hi,
Short version: this WOT preamp had 220r resistors on the secondaries even though every schematic I've seen shows 5k-10k. I swapped for 3.3k when putting in an L pad and I'm wanting to make sure this isn't a problem for some reason I'm not appreciating.
I've got a relatively new to me Welborne Labs Ultrapath preamp. You can see a schematic and assembly manual here: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/http://4tubes.com/2-SCHEMATICS/BY-BRAND/Welborne-Labs/ultrapath_1.pdf
Mine is one of the later, fancier versions in a nice wood box that uses Welborne's proprietary remote stepped attenuator and switching. (as shown here: https://hometheaterhifi.com/volume_12_4/show-report-rocky-mountain-audio-fest-10-2005-page-29.html) It was a pretty neat idea before the MUSES chip (and others) came along. Mine also appears to have been factory assembled.
The attenuator doesn't have a huge range and I was getting too much gain. There was supposedly a gain setting option, but I don't think this one was equipped with that. When I flip the only DIP switches that I think would do that, I get no output. I can't find a good schematic or manual for the attenuator nor are the individual boards or front accessible because it is all glued together (a good reason for not installing those parts).
To deal with the excess gain, I'm trying an L pad on the output (partially because the output transformers, like other WOT preamps I've had, can pick up some hum if this is close to an amp). 6.8k/3.3k works well. But in installing them and replacing the factory R8 (resistor across the secondary of the OPT), I noticed that the resistors were 220r. The only schematics I've found show 5.11k or 10k, which is more what I'd expect. 220r seems like a pretty big load on the output of a pre.
The rest of the oarts values are consistent with schematics I've seen and the selection is consistent with what Welborne would have done at the factory (Caddock). This isn't using R4 or the LED, but not all ultrapath pre schematics do.
Is there some reason it needed more of a load? I can't think of any, but the 200k input impedance it is driving now isn't adding much to those output resistors. I could always source lower value resistors for the L pad if needed. For all I know, there were different transformer versions used and different parts values and I don't want to put these big Electraprints in harms way.
And finally, if you've got a schematic or information on the Welborne remote attenuator/selector set up, I'd love to see it.
Paul
Short version: this WOT preamp had 220r resistors on the secondaries even though every schematic I've seen shows 5k-10k. I swapped for 3.3k when putting in an L pad and I'm wanting to make sure this isn't a problem for some reason I'm not appreciating.
I've got a relatively new to me Welborne Labs Ultrapath preamp. You can see a schematic and assembly manual here: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/http://4tubes.com/2-SCHEMATICS/BY-BRAND/Welborne-Labs/ultrapath_1.pdf
Mine is one of the later, fancier versions in a nice wood box that uses Welborne's proprietary remote stepped attenuator and switching. (as shown here: https://hometheaterhifi.com/volume_12_4/show-report-rocky-mountain-audio-fest-10-2005-page-29.html) It was a pretty neat idea before the MUSES chip (and others) came along. Mine also appears to have been factory assembled.
The attenuator doesn't have a huge range and I was getting too much gain. There was supposedly a gain setting option, but I don't think this one was equipped with that. When I flip the only DIP switches that I think would do that, I get no output. I can't find a good schematic or manual for the attenuator nor are the individual boards or front accessible because it is all glued together (a good reason for not installing those parts).
To deal with the excess gain, I'm trying an L pad on the output (partially because the output transformers, like other WOT preamps I've had, can pick up some hum if this is close to an amp). 6.8k/3.3k works well. But in installing them and replacing the factory R8 (resistor across the secondary of the OPT), I noticed that the resistors were 220r. The only schematics I've found show 5.11k or 10k, which is more what I'd expect. 220r seems like a pretty big load on the output of a pre.
The rest of the oarts values are consistent with schematics I've seen and the selection is consistent with what Welborne would have done at the factory (Caddock). This isn't using R4 or the LED, but not all ultrapath pre schematics do.
Is there some reason it needed more of a load? I can't think of any, but the 200k input impedance it is driving now isn't adding much to those output resistors. I could always source lower value resistors for the L pad if needed. For all I know, there were different transformer versions used and different parts values and I don't want to put these big Electraprints in harms way.
And finally, if you've got a schematic or information on the Welborne remote attenuator/selector set up, I'd love to see it.
Paul
Here is the signal path schematic. (Not the attenuator, switching, etc).
I had another thought. Some or all of these OPTs had taps for headphones. This preamp doesn't have a headphone jack and the only outputs are RCAs. The OPT has blue and red primaries. The secondaries that are hooked up are yellow and black. There are unused white leads. I wonder if this one has the headphone tap connected to the RCAs for some reason, and if in that case, 220r helps ensure the correct load?
Update: I realized the manual would tell me, and yes, the unused white leads are the headphone taps. So that isn't it.
Paul
I had another thought. Some or all of these OPTs had taps for headphones. This preamp doesn't have a headphone jack and the only outputs are RCAs. The OPT has blue and red primaries. The secondaries that are hooked up are yellow and black. There are unused white leads. I wonder if this one has the headphone tap connected to the RCAs for some reason, and if in that case, 220r helps ensure the correct load?
Update: I realized the manual would tell me, and yes, the unused white leads are the headphone taps. So that isn't it.
Paul
