Phono pre amp RF problem - Page 2 - 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 26th September 2007, 01:34 AM   #11
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
With that passive RCA pre-amp...
Yes, changing that 22K to a 24K improves the RIAA acuracy...
As the circuit stands with the 22K resistor you get a 67.5uS Time Constant....
Using 24K moves this time constant to 73uS....
Technically it should be 75uS ......
Also the 3180uS time constant is actually at 3126uS as the circuit stands...changing over to 24K moves this to 3146uS .....
The part that I don't get is where the 318uS Time Constant comes from????
The second stage with it's 180pF produces a 6.27uS and this moves around a wee bit depending on output loading.... I figure if you drop the 180pF to 100pF you will get closer in the ball park like about 3.3uS ....

I am "assuming" that RCA "bean-counters" intended this circuit for low budget mass produced units and were selecting these resistor values based on price and standard chart values, also that it is "close enough" for consumer applications...

I see that the 1KHz mid Band is at +10dB Gain from the first stage...
The second stage contributes +35dB Gain low to mid band...
This makes about +45dB Gain at 1kHz for the whole bloody thing.....unless I goofed somewere in the math

Totally agree on the " Johnson Noise" ..... I don't usually include this in the phono-pre analysis, since the valves thermal noise swamps this out... I do however consider that when designing LNA's in RF applications with noise floors about -137dBm....

Chris
  Reply With Quote
Old 26th September 2007, 01:40 AM   #12
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
What you want is a zero at 3.18us. And it's not there.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 26th September 2007, 05:12 AM   #13
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally posted by SY
What you want is a zero at 3.18us. And it's not there.
Ouch... Take a look at the work by Stanley Lipshitz for clue about this issue or take a look at my sloppy, but reasonably accurate RIAA networks based on his work. (Come to think of it that zero might not be there either, if it's the one I am thinking of it is often omitted. SY correct me.. LOL)

An inverse RIAA network you can tweak response against is pretty useful.. I'd get me one of those. There is an old AA article on building on of these. Bear in mind source and terminating impedances for these networks can be critical to accuracy. (Typically 600 ohms source and termination.)
Kevin
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 26th September 2007, 06:26 AM   #14
diyAudio Member
 
Join Date: Sep 2006
First, if your RF problem occurs with the inputs shorted then it is probably within your RIAA circuit.

Years ago I talked with someone at Grado on the phone about a hum problem with one of their budget carts. I learned that they did not believe in shielding their carts. Maybe things have changed at Grado.

I ended up buying an 8MZ Signature Series cart. It has no hum problem.

There may be something about the Grado cart in conjunction with your preamp that is creating an RF antenna.
Mark
  Reply With Quote
Old 26th September 2007, 10:07 AM   #15
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Kevin, the 3.18us zero wasn't included in Lipshitz's paper (at least the one I have, from about 1980). In your two tube RIAA, it's implemented by putting a resistor in series with C3. I think about 900-1000 ohms would be right.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 27th September 2007, 07:56 PM   #16
diyAudio Member
 
Join Date: Jun 2007
Default Thank you all so much!

wow this is some amazing advice. I will make that sub in the RIAA section and change the load to 47k and try the various grid-stopped recommendations.

The problem is only present when a cable is inserted in the input jacks... so no I do not think it is in the RIAA section.

This is some great stuff to get started on, thank you all so much. c/
  Reply With Quote
Old 27th September 2007, 09:33 PM   #17
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Glad to be of help, remember also that the internal ground wiring is not a ground at 100MHz and you really want to use some caps from the input rca jack grounds directly to the chassis - use small ceramic caps with leads as short as you can reasonably make them. This alone has saved me more than once.

I have tried beads, but found them audible, and IIRC I think they did have a measurable effect on the thd above a couple of kHz. Anyway I usually remove them and replace with small resistors or air core (diy) inductors.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 29th September 2007, 10:07 PM   #18
diyAudio Member
 
Join Date: Jun 2007
Default UPDATE: RCA phono pre RF problem

Hi all

so - i implemented a number of the suggestions that the group proposed:

*Changed input load resistor to 47k (was 56k)
*added 820 ohm 1/2w cc resistor between input jack and 1st screen (body of resistor located at socket).
*added 75v .01 ceramic cap b/w heater pins and ground

The level of hum (which was never bad at all) did not change. the snubber caps did not seem to have any effect.

The RF (108 FM) dropped dramatically, but it is still audible... more than i think it should be. it is only present when cables are inserted.

I am using switchcraft-style steel non-insulated input jacks (IE the body of the jack has continuity with the chassis). I am using a ground buss (8 ga solid copper wire) which meets the chassis at the input jacks. Because of this, i did not try the suggestion to connect inout jack ground to chassis thru a .01uf cap... cos the jacks are essentially equivalent to ground in this case, no?

So... any other suggestions? my only other move left is to bump the input grid stopper value up to 2200 ohms.

c/
  Reply With Quote
Old 29th September 2007, 10:56 PM   #19
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
I am using switchcraft-style steel non-insulated input jacks (IE the body of the jack has continuity with the chassis).

YUCK!! Put Gold plated jacks with insulating washers in. Add the 10 nF. ceramic capacitors. Use shielded "twinax" wire between the I/P jacks and the signal circuitry. The shields should be grounded only at the same point the 10 nF. caps. are grounded. The shields do not carry signal. Connect the signal ground bus up near the tube sockets, not at the I/P jacks.

Finally, don't forget the RFI suppressing ferrites on the cables from the TT as close to the preamp as possible. The cables are acting as antennas.
__________________
Eli D.
  Reply With Quote
Old 29th September 2007, 11:55 PM   #20
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally posted by Eli Duttman



YUCK!! Put Gold plated jacks with insulating washers in. Add the 10 nF. ceramic capacitors. Use shielded "twinax" wire between the I/P jacks and the signal circuitry. The shields should be grounded only at the same point the 10 nF. caps. are grounded. The shields do not carry signal. Connect the signal ground bus up near the tube sockets, not at the I/P jacks.

Finally, don't forget the RFI suppressing ferrites on the cables from the TT as close to the preamp as possible. The cables are acting as antennas.

Eli, I couldn't have said it better myself!

What he says man...
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  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 11:33 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