Phono pre amp RF problem - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th September 2007, 02:40 AM   #21
diyAudio Member
 
Join Date: Jun 2007
well that sounds like some strong advice. I do have a few sets of the gold plated insulated RCAs around - I can sub those in.

only one other question - so I should put a 10nf cap b/w the RCA input jack ground and the ground buss. Is this the only ground path for the ground of the input jack?

I have never understood the reason that phonographs have a dedicated ground wire in addition to the 'grounds' of the two phono jacks. Perhaps this is the basis of my confusion.

anyhow, thanks again for the advice. I am really looking forward to getting this thing cookin 100%. c/
  Reply With Quote
Old 30th September 2007, 02:40 AM   #22
diyAudio Member
 
Join Date: Jun 2007
oh actually one more thing... can you suggest an appropriate supplier of a modest "RFI suppressing ferrites." This is a type of product that I am not familiar with.

basically i am stuck in the land of 1950's technology/technique, as is prolly clear... c/
  Reply With Quote
Old 30th September 2007, 12:16 PM   #23
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Originally posted by eddievanjealous
well that sounds like some strong advice. I do have a few sets of the gold plated insulated RCAs around - I can sub those in.

only one other question - so I should put a 10nf cap b/w the RCA input jack ground and the ground buss. Is this the only ground path for the ground of the input jack?

I have never understood the reason that phonographs have a dedicated ground wire in addition to the 'grounds' of the two phono jacks. Perhaps this is the basis of my confusion.

anyhow, thanks again for the advice. I am really looking forward to getting this thing cookin 100%. c/

You are confusing signal ground, AKA common return, with chassis ground. The 5th wire from the TT is a chassis ground that's tied to the preamp's chassis. The 10 nF. caps. are noise suppressors. They connect to the chassis, not the signal ground bus. The shielded "twinax" wire I previously mentioned provides the connection between the I/P jacks and the signal ground bus.

McMaster-Carr is an industrial supply house that stocks RFI suppressing ferrites and all sorts of "GOODIES" a DIYer can use.
__________________
Eli D.
  Reply With Quote
Old 2nd October 2007, 08:10 PM   #24
diyAudio Member
 
Join Date: Jun 2007
Default RF problem - update II

hi again all,

so i tried eli's suggestions, thank you very much eli... i added the insulated gold-plated input jacks and the 10nf ceramic caps from input signal ground to chassis. Input signal ground also directly connects to copper ground buss which serves entire unit.

this seems to have solved the RF problem when i use my stanton DJ cartridge, but made the RF problem WORSE when i use the grado!

on the plus side, the sound quality has improved (more treble extension - this thing really sounds fantastic now) and the hum has dropped a few DBs - to the point where it is well, well below the surface noise of an LP.

anyhow, there are clearly a lot of factors at play here which i do not really understand. WOW this a lot tricker than building a guitar amp or power amp.

anyhow, i want to hold-off on using the ferrites because my decks/carts sound 100% fine when i use my solid-state pres, so i assume that this problem is correctable fully within the world of the pre amp itself. ???

I was going to try this suggestion next: " put a 47pF from grid to cathode."

Right now i am using a 820ohm grid stopper, I was also thinking of increasing this to 1500 or so.

If anyone has any other ideas, i would love to hear 'em! you have all been a tremendous help, thanks so much.
  Reply With Quote
Old 2nd October 2007, 11:32 PM   #25
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
WOW this a lot tricker than building a guitar amp or power amp.
Is it ever. Ironing the kinks out of a phono preamp may very well be the toughest of DIY jobs.

Quote:
Right now i am using a 820ohm grid stopper, I was also thinking of increasing this to 1500 or so.
100 Ohms is plenty for use with 12AX7 sections. Are the stoppers Carbon comp.? If not, replace them. Also, the bodies of the stoppers should be a close to the tube socket lugs as you can get.



Quote:
this seems to have solved the RF problem when i use my stanton DJ cartridge, but made the RF problem WORSE when i use the grado!
Get the ferrites! Grado carts. are intentionally unshielded, while Stanton carts. are shielded. IMO, there's sufficient proof that the cart. and cables are acting as an antenna.

BTW, did you put shields on the 12AX7s?
__________________
Eli D.
  Reply With Quote
Old 2nd October 2007, 11:48 PM   #26
diyAudio Member
 
Join Date: Jun 2007
Default reply to eli

hi eli,

yes i am using carbon comp 1/2w. the bodies are against the socket. i AM using tube shields, but they have no bearing on anything whether they are on or off.

this has been my general experience with tube shields - i have never experienced them making a difference - EXCEPT for the one time that i used a 12at7 paralleled as a current amp - i put a shield on it and the tube (a NOS GE or something similarilry valuable) DIED, and i mean, died, in 5 minutes flat.

Eventually I WILL get to the ferrites - i really need to get this thing working, it just sounds so great that i really want to use it (i transcribe a lot of vinyl to CD).

I guess i just assumed that if the same TT output cables cause no problem with the SS pre-amp, IE, they do not need the ferrites to function properly in that scenario, I can;t understand why they are necessary in with this pre-amp.

granted, i trust your advice that they will solve the problem, but perhaps there is a more elegant solution that is eluding me?
c/
  Reply With Quote
Old 21st January 2008, 01:33 PM   #27
diyAudio Member
 
Join Date: Jun 2007
Thumbs up Success! Phono pre RF is cured

Alright. After many attempts at removing the dreaded WEBE 108FM from my RCA receiveing-tube-maunal dereived phono pre, I can finally report that the problem is gone, and with no noticeable loss of high end when a/b-ing the signal against the phono pre-amp built into my $250 yamaha receiver. FYI i am using a GRADO green cartridge and listening on AKG 240's and Yamaha NS30s (NS30s driven by my homemade 6SL7/ 2A3 SE ended amp)

to refresh, i used the front end circuit described here (http://www.bonavolta.ch/hobby/en/audio/rca.htm) with a two stage line driver (12au7 plate coupled into cathode driver) after it.

The circuit above indicates no grid resistor, but after hearing the advice of the members of this list, i tried various combinations of resistors and grid-plate capacitors.

I first tried 1K grid resistor. It helped, but no enough. Then i tried 330 with a 10pf cap. Again, it worked, but not enough. Then, following the circuit shown here (http://www.bonavolta.ch/hobby/en/audio/dyn_upas.htm) , I tried a 10k resistor with a 10pf cap to plate, and it sure did get rid of the RF, but it also seemed to cut off everything above 12k. So then i backed down the resistor to a CC 3.3k and there it is! worked great.

Overall, my preamp has approx. 1/2 the white noise of the yamaha pre, the channel separation is much better, and the center image is much more focused. There is a slight 60 cycle hum present when the cartridge is lifted off the record, but it is totally subsumed by the surface noise of the LP, so i do not think it will bother me. My lead dress is pretty good, so i imagine that the only way i am going to improve this situation is by using DC filaments, which i do not know how to achieve using the 6.3v AC secondary that I have available.

So, that you all SO much for your help.

Next question: any suggestions for a better phono pre to build?

c/
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
TEAC A717 Phono Op Amp Problem Lectran Solid State 6 29th November 2008 07:07 PM
El Cheapo Phono Pre problem bremen nacht Analogue Source 7 17th August 2005 01:58 PM
Pearl Phono Problem -- Need Help!!! Mikelo Pass Labs 2 4th April 2004 09:38 PM


New To Site? Need Help?

All times are GMT. The time now is 05:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2