Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: BluegrassDan on April 12, 2022, 09:19:02 pm

Title: Univox UB-250: Taming the "hiss"
Post by: BluegrassDan on April 12, 2022, 09:19:02 pm

Hey all,

I'm working on a Univox UB-250 solid state amp. Just recapped it and added a new power cord.

The amp has a noticeable "hissy" noise floor. A friend of mine had one of these back in the 80s and 90s and I absolutely recall this same noise in his amp year ago.

Attached is the manual with schematic and layout. I wonder if there are specific components that may contribute to this such as the Germanium transistor.

Thoughts?

Title: Re: Univox UB-250: Taming the "hiss"
Post by: Williamblake on April 13, 2022, 04:39:50 am

Maybe you could loose some serial resistance at the input?

Title: Re: Univox UB-250: Taming the "hiss"
Post by: PRR on April 13, 2022, 05:43:02 pm

It's got a LOT of gain all behind the Volume pot. It's gonna hiss. All the time.

There is some chance the first transistor has been "broken-over" by extreme inputs. A lot of the Heaths get this way. The BC169 was a very fine transistor; still is if you can find one. A modern equal-or-better part is 2N5089. Even Stew Mac sells them, tho I would go to DigiKey or Mouser. NOTE the pinout is different, bend the legs to make it right.

Title: Re: Univox UB-250: Taming the "hiss"
Post by: BluegrassDan on April 13, 2022, 07:40:01 pm

I should have mentioned that the hiss is related to the volume pot. No volume, no sound, very little hiss. Increasing volume increases the hiss.

Title: Re: Univox UB-250: Taming the "hiss"
Post by: BluegrassDan on April 13, 2022, 09:17:20 pm

Voltages around the 2N5492 and 40544 are not quite right. Bridging some resistors to up the voltage has decreases the noise. I might just replace them and report back - perhaps all of the transistors.

Title: Re: Univox UB-250: Taming the "hiss"
Post by: BluegrassDan on April 16, 2022, 09:40:34 am

REPORTING BACK.

Replaced Q3, Q4, and Q5. All voltages remained the same with no changes in noise floor.

The voltages are lower than the schematic. For example, the junction between R14 and R15 is only around 20-22v, instead of 30v. The junction between Q4(B) and Q3(C) is down around 12v.

I adjusted down R14 and R15 to allow higher voltages closer to the schematic. Also adjusted R13 up and down.

In a nutshell, it seems to run and sound fine stock. When I adjust voltages up things start getting crunchy and distorted.

Could the schematic be wrong? Or could the difference be using the alternative part numbers (40544 vs 2N3053 for example)?

Title: Re: Univox UB-250: Taming the "hiss"
Post by: thetragichero on April 18, 2022, 09:36:54 am

i've spent some time on the vintage kustom amps forum and the general consensus from folks that played those kind of things back when they were new: they've always hissed, back then that's how we knew they were on and working

here's a good article on resistor noise: https://www.aikenamps.com/index.php/resistor-types-does-it-matter