help needed with noise in phono preamp

[duderduderini]

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]

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]

oops

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

 

[SY]

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]

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]

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]

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.]

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]

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]

Many Thanks

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

 

[duderduderini]

and another one

Heres another. The phono section is to the top. the fist uppermost socket is the 6sl7 with the lower of the 2 the 6sn7
Further below that is the 6sn7 in srpp topology.
This is my second amp ever, so please excuse any mortal sins i may have committed The power supply is a variation on the following.. I supply a link so as not to infringe copywrite issues. With all credit to the original authors for the work.
http://eshop.diyclub.biz/article_info.php?articles_id=3
Thanks
Nick

 

[SY]

Top view?

IIRC, this is the schematic:

http://www.hagtech.com/images/cornet2schem.gif

Is that correct?

And the red and white wires that are bundled together, are those power distribution? Where are the input signal leads for the phono?

 

[duderduderini]

Wires

Hi
Yes the schematic is the one you have listed.
The red and white wires are input signals. the selector switch is inbetween the middle valves.
Power distribution is either side.. in the larger picture you will see red and grey wires coming form the back panel.
I have given you a top shot
Thanks for your help

 

[duderduderini]

And the PSU

heres the psu

 

[duderduderini]

I could have cleaned the dust off them

They were clean 2 days ago!!
Heres both together

 

[duderduderini]

input into phono

You can see a short white wire near the 47k resistor on the right side of the close up photo --- to the left of the bundle of red/grey wires in top right corner of photo
Thanks
Nick

 

[dvisves]

Hai duderduderini

With such a lot of wiring it is really impossible to control feedback and hum.You are much better getting a pc board made.I can help if you send me the schematic and design a simple pc board
DeeVee

 

[duderduderini]

Thanks

Hi.
I really appreciate that very kind offer. I would consider it but that would be contingent to Jim (the owner of the circuit being cool with it).
I am thinking maybee i can reduce it by trying a few things and failing that maybee some form of noise cancelling circuit, the likes of which i have seen on tubecad.com.
So I will take a raincheck on your very generous offer but if i cant reduce the hum and more importantly Jim Hagermann says its ok then I will contact you
Thanks
Nick

 

[duderduderini]

To Sy its the comet or cornet OCTAL not the image you posted

HI
I just realised its not the schematic you posted.. its the OCTAL version
Nick

 

[SY]

If you can find a link to the octal version, post the link here. In any case, I'd look at the order of the grounds. From your photo, it appears that you have a star ground in the middle of the row of input jacks, with the phono grounds separated from the star point and a few other grounds tacked in between. The grounds from the phono input and the first stage grounds (i.e., the cathode and grid leak resistors) should return directly to the star point without any other grounds in that path. Return any other grounds to that point using separate wires.