RIAA pre amp oscilates. - diyAudio
Go Back   Home > Forums > Source & Line > Analogue Source

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

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 22nd April 2012, 05:52 PM   #1
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Default RIAA pre amp oscilates.

I have just had a pcb made for an RIAA pre amp.
The circuit has 100nf and 47uf decoupling but oscilates at 250KHz. The circuit doesnt seem much different to other examples so I dont know why it oscillates. The op-amp is a NE5532.

Click the image to open in full size.

Last edited by nigelwright7557; 22nd April 2012 at 06:03 PM.
  Reply With Quote
Old 22nd April 2012, 06:29 PM   #2
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
If it oscillates, you've got positive feedback via some path. Without seeing the layout its hard to tell, but I'd start with the physical locations of the ground of R6 and C9. If C9 can "wiggle" the voltage at R6 slightly due to trace resistance and daisy-chaining, that'll do it. I also have had odd things happen when grounding inputs like pin 3 of the second opamp. I like to put some series resistance there.
__________________
May the root sum of the squares of the Forces be with you.
  Reply With Quote
Old 22nd April 2012, 07:40 PM   #3
diyAudio Member
 
Join Date: Mar 2012
There always is internal capacitance between output and input.
If you noticed in original construction there is suppressing input capacitor.
http://www.paia.com/riaa.asp
  Reply With Quote
Old 22nd April 2012, 09:47 PM   #4
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by Moon Track View Post
There always is internal capacitance between output and input.
If you noticed in original construction there is suppressing input capacitor.
http://www.paia.com/riaa.asp
I added the 100pf capacitors but they made no difference to the oscillation.
If I short the input to ground it stop oscillating.
Looks like there is some feedback between output getting capacitively coupled to the input.
I dont have a cartridge connected so dont know if that would stop the oscillation.
  Reply With Quote
Old 22nd April 2012, 10:01 PM   #5
kaos is offline kaos  United States
diyAudio Member
 
Join Date: Dec 2005
Location: Los Angeles
Should have a series resistor on pin 3 of the input (100 – 510 ohms). A parallel cap across R6 is sometimes needed too (or sometimes from pin 3 to ground). What’s the point of the second stage unity gain inverter?
  Reply With Quote
Old 22nd April 2012, 10:09 PM   #6
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by kaos View Post
Should have a series resistor on pin 3 of the input (100 – 510 ohms). A parallel cap across R6 is sometimes needed too (or sometimes from pin 3 to ground). What’s the point of the second stage unity gain inverter?
It could be coz the input is floating, I dont have a cartridge to load it.
On the scope there is 400mV on the input of what looks like oscillation feedback through capacitive tracks. Its certainly the same frequency as the oscillation. Pin 2 and 3 have tracks in parallel so this could be part of the problem.

The unity stage is for adjusting the gain if required.
  Reply With Quote
Old 22nd April 2012, 10:33 PM   #7
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by kaos View Post
Should have a series resistor on pin 3 of the input (100 – 510 ohms). A parallel cap across R6 is sometimes needed too (or sometimes from pin 3 to ground). What’s the point of the second stage unity gain inverter?
I tried a 1k on input and it still oscillates.

Does anyone know what the loading of the cartridge is ?
It could that would load the input and stop the feedback breaking through.

If I connect a signal generator the circuit settles down but it is being loaded by 600ohms.

Last edited by nigelwright7557; 22nd April 2012 at 10:40 PM.
  Reply With Quote
Old 22nd April 2012, 11:13 PM   #8
grufti is offline grufti  United States
wombat
diyAudio Member
 
grufti's Avatar
 
Join Date: Jun 2004
Location: Hillside
MM cartridges are often about 1k. You can check specs on the ortofon website for example.
  Reply With Quote
Old 22nd April 2012, 11:14 PM   #9
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I'd start by seeing if the amplifier is stable without the EQ caps installed, if it isn't you need to do some work to figure out what the mechanism is.

IIRC The 5532 is unity gain stable so the high levels of HF feedback should not cause a problem..

Note that if you have a lot of capacitance at the output of the first amplifier stage a small series resistor right at the output (usually after the feedback connection) may help.. If the feedback network is part of the problem a small (say 100 ohms) resistor between the op-amp output and the network and connection to the next stage can help.

You can also scale the feedback network, lower resistor values and larger caps also help wrt to current noise performance with 5532. This also would reduce the effect of stray capacitances on EQ accuracy and phase margin. IMO Your resistors should be 1/10 the current values and the caps 10X. They are way too high for a 5532 wrt to input current noise. I suspect the amplitudes involved would not cause significant problems despite the effective 150 ohm load at high frequencies. (If you are concerned scale for 600 ohms) I bet you also have significant offset at the output that would go away.

Not unlikely that layout design is part of the issue - can you take a picture of your implementation?
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 22nd April 2012, 11:21 PM   #10
kaos is offline kaos  United States
diyAudio Member
 
Join Date: Dec 2005
Location: Los Angeles
Quote:
Originally Posted by nigelwright7557 View Post
I tried a 1k on input and it still oscillates.
Sounding like a ground or layout issue. The 5532 is usually pretty easy to apply. Are you using a star ground? Are the 5532s prime units, or junk box?
  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
Morgan Jones' RIAA Pre-Amp skajam66 Tubes / Valves 2 4th January 2011 04:04 PM
Precision op-amp RIAA pre.. Dxvideo Analogue Source 8 27th November 2010 08:19 PM
Needs simple RIAA pre-amp parmar664 Analogue Source 2 30th March 2007 10:29 PM
RIAA pre amp jethari Solid State 1 30th April 2002 04:21 AM


New To Site? Need Help?

All times are GMT. The time now is 03:53 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