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-   -   help needed with noise in phono preamp (http://www.diyaudio.com/forums/tubes-valves/102474-help-needed-noise-phono-preamp.html)

duderduderini 25th May 2007 06:45 AM

help needed with noise in phono preamp
 
Hi guys
Well we are getting there re the sound of this. The 6sn7 srpp line stage is fine but the phono stage is making a fair mount of noise.. some hum and induction noise? Would it be worth putting some form of feedback/noise cancellatiom circuitry in?
If so where.. i have been on tubecad site but would like some advice
Thanks in advance
nick

SY 25th May 2007 07:08 AM

I've removed the attachment- it's a copyrighted schematic.

If the noise you get is hum, you've got layout and/or grounding issues, not a design one.

duderduderini 25th May 2007 07:25 AM

oops
 
Sorry re that.
i got the schematic from these forums and i apologise to the rightful owner.
Nick

SY 25th May 2007 08:13 AM

Well, let's see what we can do to help. Describe the symptoms in some detail and if it's possible to get some photos of the layout and wiring, post them and something might be deduced.

duderduderini 25th May 2007 01:26 PM

Thankyou
 
Thanks for your help.
the noise I think has 2 components
1. Mains hum.... i am running a regulated psu based on a 6as7,12at7 and 5651..
2. a louder lower component (not motorboating i dont think) if i place my hand over the tube it exacerbates or increases it..
If i could email you photos as they will be around 100k
I may even be able to do an mp3 of the noise
Once again thanks for your help, I really appreciate it
Nick

SY 25th May 2007 11:31 PM

I think that the posting limit for photos is 105k, so if theyre under that, go for it. Otherwise, you can either shrink them a bit or zip them up and attach the zip file. The more eyes on this, the better (and Jim Hagerman, who designed it, is a participating member here).

What you're describing is screaming "grounding and layout." That's much easier to fix than design.

duderduderini 26th May 2007 02:35 AM

will get some photos up
 
Thanks for that.
I know it isnt design.. I can hear just how beautiful this phono stage sounds and I am very grateful to Jim for this design.. so i will get some photos up and we will take it from there.
Thanks Again
Nick

Jeb-D. 26th May 2007 11:19 PM

Phono pre-amps use a lot of gain and are followed by more gain in the amplifier before it gets to the speaker. So basically as mentioned the layout and grounding are critical and you have to extensively filter the power supply. I'd also consider using rectifiers with minimal "hash" such as Hexfred's or tube rectifiers. Usually for pre amps I ground the chassis to earth for safety, but leave the power/audio ground floating or connected to chassis through a resistor, so that when you attach it to an amp, it gounds to the amplifiers ground through the RCA shielding. This prevents system-level ground loops.

dvisves 27th May 2007 03:49 AM

Preamplifiers being the first links in the amplification chain are suspect to noise pick up .i would do the follwing things 1)use good shileded cable for the input 2)Ground the input jack ground to the power supply ground with thick copper wire 3) Use a three terminal regulator having a smoothing capacitor of roughly 47 microfarad for powering the preamp.This has helped a lot in my audio projects.Hope this helps

duderduderini 27th May 2007 09:06 AM

Many Thanks
 
2 Attachment(s)
Hi
I am posting a few photos for comments.
i will go through each suggestion to see if I cn reduce the hum.
Might I addd that the line stage does not have any hum at all. I know the phono amplifies many times more than the line stage but it just so everyone knows
thanks again to all for the help
Regards
Nick


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