How to get a Univox U-45B so quiet you can't even tell it's on: A Tutorial

[SoulFetish]

DDiodes (including LEDs) are as non-linear as it gets. Nothing is more linear than a resistor (meaning that dV/dI = constant, ensuring zero distortion).

As explained above, it is desirable that the cathode voltage increases in class (A)B operation to avoid overdissipation, especially if idle dissipation is above 70%. Diodes/LEDs might not allow for sufficient increase of bias voltage.
Otherwise you will need lower idle currents, i.e. something like 70% plate dissipation or lower at idle as typical with fixed bias.


Have you seen problems with heater elevation using the cathode voltage with a bypassed cathode resistor?

Sorry it took me a while to get back to this thread, I've been meaning to respond.


When I brought it up, I never meant to imply those LEDs were a suitable analog or replacement to a cathode resistor. It is most certainly a a form of fixed biasing. Keep in mind, I mentioned very specific part, with a "fairly" linear operating region from 10 to 100mA (the case could be made that it's "quite" linear in that region, but you won't hear it from me). This was only in the context of potentially elevating the heater supply in the U-45B as a means of improving noise performance. I mentioned it as an alternative to cathode biasing, which in a similar manor, would establish a stable positive voltage to achieve a bias, as well as elevate the heater supply.

If you want to continue a larger discussion on diode biasing, I'm happy to do it in a separate thread. I actually still have quite a few of these LED in a parts bin and we can run some tests. But, we're drifting off topic on a point which, quite frankly, is moot. There is no need to elevate the supply. The amp is unrecognizably quiet grounding the heater supply in the way I described above.

[The Roving Sign]

Hey there, I have a couple of these great little amps. I see you're in Boston, as am I, so I'm wondering if you do repair work. If so I'd like to discuss the amps with you. Thanks.

[SoulFetish]

I do. Send me a private message and ill give you my contact info.

[The Roving Sign]

Thanks I'm new to forum and can't figure out how to private message. When I click on your profile, or any profile, it takes me to page saying "Sorry, you are not authorized to view this page. For assistance contact the site administrator." Will try to figure this out, but in meantime if you can send me a private message please do. Thanks.

[E.L.S.]

...when I searched for information about this model, there are a few specific problems that owners and technicians experience which seem to come up over and over again relating to "noise".

SoulFetish you just made my day! I found this old thread last night, and now my U-45B is much quieter. It was a ground issue, which your layout solved.

Thank you for spending the time to create the drawings and for sharing.

Cheers!

[SoulFetish]

SoulFetish you just made my day! I found this old thread last night, and now my U-45B is much quieter. It was a ground issue, which your layout solved.

Thank you for spending the time to create the drawings and for sharing.

Cheers!

Awesome! Glad to help

[bsco]

very interesting repair....and excellent documentation....