Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Heavenamps on April 03, 2018, 08:11:30 pm

Title: HiFi output noise
Post by: Heavenamps on April 03, 2018, 08:11:30 pm

For my senior design project for my EE degree we are building a tube driven stereo HiFi amplifier. It's been quite a learning experience as we have made mistake after mistake. Our design consists of a single 12ax7 preamp tube and 2 el34 power tubes (1 per channel). I designed the amp in LTSpice and printed the circuit onto a PCB. The output on the simulation looks great. However, when we turn on the amplifier and look at the output waveform on an O-scope it is quite noisy. I can't seem to find the source of the noise and am wondering if anyone here can suggest some common sources of noise in tube amps. I tried moving the 6VAC filiment voltage taps to connect underneath the board instead of on top, but there was no change. The only other thing  that I can think of is that the output transformers are angled too closely to the Power transformer. Any advice is welcome and I can supply any additional information as needed. Thanks!

Title: Re: HiFi output noise
Post by: shooter on April 03, 2018, 08:45:43 pm

Quote

I can't seem to find the source of the noise

is it coherent, what frequency? or more like white noise, frequencyless  :icon_biggrin:

there are basically 3 usual suspect noise, 60hz, 120hz, or shshshshsh hiss

60hz is typically filament, transformer coupling, funky tube
120, typically B+ rail, AC ripple

hiss, crackly, typically bad connection, missing/bad ground.

pics help, schematic also
hang in there, you'll nail it

Title: Re: HiFi output noise
Post by: PRR on April 03, 2018, 10:32:55 pm

> look at the output waveform on an O-scope it is quite noisy

If you turn a 'scope up enough, you can always find noise. What does it *sound* like?

Title: Re: HiFi output noise
Post by: 92Volts on April 04, 2018, 02:51:29 pm

You need a serious power supply to keep SE amps quiet. This is true for guitar, but there are fewer sources of noise (the instrument) to mask whatever the amplifier's doing in a hifi application.

If you have a scope, check what's going on in the PSU. Also, how much noise was expected in the power supply based on your simulation? If it's not zero, some portion will pass to the speaker because SE amps don't cancel out power supply ripple. It might be an acceptable amount though. As PRR says, you'll always find noise if you look close enough.