Tube RIAA corrector - hum issue

[almasiz63]

Firstof a short introduction of myself...I am from Hungary and I've been bulding electronics since the 80s, but haven't touched them in the past 20 years. Anyway, one of my friend who is collecting old radios, recently brought me a diy tube preamp without any documentation. So, I had to do a little reverse engineering, since I wanted to finish the product. It was so unfinished, that even the power supply was not complete. Finally I have managed to make a regulated power supply using the schematics from this page (figure 4) : https://audioxpress.com/assets/upload/files/bicknell2890.pdf

After some modification I was able to get the RIAA corrector working too, but there was a "small" problem with it. It hummed like crazy. I finally was able to minimize the hum (50 Hz) to a lower level, using several techniques, like the star grounding, virtual filament ground and DC filament heating, but the hum is still there and sometimes quite annoying . Am I missing something here to eliminate the hum or it's unavoidable at the this low signal level ? Fyi, here is the schematics of the RIAA preamp :

 

[ruffrecords]

If you still get hum with dc heaters then it must be getting in via the HT supply or poor layout. R2 is doing you no favours either and is completely unnecessary. Check HT ripple with a scope with the probe set to 10X and coupling to ac. Can you post a picture of the actual build?

Cheers

Ian

 

[almasiz63]

Hi,

I was also scratching my head on R2, but it has no effect on the hum. Currently it's a bit of a mess because I removed the RIAA units and placed it on the top of a chair together with the power supply for easy access. The first picture shows how I received the preamp, the other how the RIAA unit (one channel only) look from the bottom. The two copper bar on the left side are for the supply voltage and the ground.
The whole unit supposed to have tone control and some additional amplifications, but the tubes are missing, so I will only use the RIAA correctors...if I can get rid of the hum. I have just returned from a long journey, so will check the ripple later.

Thanks,
Zoltan

 

[Walter66]

Good approach for the circuit! When it comes to hum, build it in a good style means point to point wired on a tag board.
Decouple both active stages with separate caps, thats good practice.
Look at how low the ripple is on the PSU line, that is most important beside the DC heating and good grounding scheme.
It's not so easy to build a MM phono stage without any hum.

 

[almasiz63]

I think I have found the main culcprit. Yesterday measured the ripple on the HT supply, as you call it, and it was quite high (>100mVpp) and asymmetrical. This has indicated a problem with the rectifier diode (EZ80). I swapped it and voila the ripple went down to around 5mVpp and now it's within spec. The hum was also vastly reduced at the ouput of the preamp. Not completely absent, but a lot less annoying and blends perfectly into the vinyl noise. Attached are the screenshots from the scope.

 

[ruffrecords]

That is a lot better. Probably as good as it will get. There may be small gains to be had by nproving layout but it looks like you are basically there.
Cheers


Ian

Cheers

Ian

 

[MidnightArrakis]

Shouldn't some of the non-electrolytic capacitors be either a film, polycarbonate or a polystyrene type? Would anybody here know about this and could specify which caps need to be of what type?

/

 

[bepone]

can be even better.. Ub+ line is easy to filter out, just add more RC RC stages..easy to build also mosfet regulator for Ub+ and for DC heaters..

 

[ruffrecords]

Shouldn't some of the non-electrolytic capacitors be either a film, polycarbonate or a polystyrene type? Would anybody here know about this and could specify which caps need to be of what type?

/

Indeed they should. Opinions amongst the hi-fi fraternity differ on the dielectric type, construction, tolerance and cost of the RIAA capacitors but any close tolerance film type will do.

Cheers

Ian