Small hum in Transcendent Sound phono clone

[Renton76]

Hi there,
I just finished the following phono stage and am really happy with its sound:



Psu:



The prob is a small 100hz hum, thats gets noticable at around 50% of maximum volume.
I think it comes from the hv supply, cause increasing the reservoir or the first filter cap to around 120 uf eleminates the hum.
By connecting the bigger cap, i think, this degrades the imaging of the phono pre. The stage gets slightly smaller, the heights are not so clear.
Can this be and would be the orig. hv supply the better option? How about going the regulated route?

Best regards,
Rents

 

[Ketje]

If it is volume related it comes from the input not the power supply.
(Not hum related)..Looking at the schematic, who got that stupid idea to put a resistor at the middle of dc heaters to ground ? Simply connect the minus to ground.Avoids capacitive coupling between cathodes.
Mona

 

[Renton76]

Thanks Mona,
the volume pot is between the phono and the power amp, not at the riaa's input.

 

[DF96]

By connecting the bigger cap, i think, this degrades the imaging of the phono pre.

Why should that happen?

How about going the regulated route?

A regulated PSU can be helpful for a phono preamp because it stops mains voltage variations from causing woofer flapping.

A few 22uF plus a 10H choke may not be enough smoothing for a phono preamp, especially one with a cascode input stage (PSRR approximately 0dB). I would replace the 22uF polypropylene caps with 100uF electrolytics - but I would also do a PSRR calculation to find what ripple is acceptable and then design the PSU to achieve this.

 

[Ketje]

Thanks Mona,
the volume pot is between the phono and the power amp, not at the riaa's input.

You are right, my mistake.Now that you put in the story about the extra C it's obvious the supply.
Since DF96 allready has given good advice I can rest now 😉
Mona