RIAA Active EQ - 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 5th July 2013, 07:14 PM   #1
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
Default RIAA Active EQ

What is the best way to compute the values for the feedback network for this active RIAA circuit?

I have been attempting trial and error with a few different configurations but am getting nowhere.

I already made a passive stage which was much easier to manage.

Attached is a Tina file along with a schematic screen.
Attached Images
File Type: png RIAA.png (27.5 KB, 171 views)
Attached Files
File Type: zip RIAA Phono.zip (49.0 KB, 8 views)
  Reply With Quote
Old 5th July 2013, 07:17 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
That cathode follower is going to be very unhappy trying to drive the RIAA network at high frequencies!

The basic equations for calculating feedback EQ were published by Lipshitz back in the '70s and that article is likely floating around the Web somewhere. The 12AX7 is a very poor CF anyway, so you ought to consider a better choice (high gm, higher current). Loop gain for an SP6/SP3/Marantz type circuit is marginal, overload characteristics are somewhat dreadful.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 5th July 2013, 08:27 PM   #3
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
If you really want to use 12AX7's in that configuration, or you simply must diy, why not avoid the mental masturbation of all that math and just copy the Marantz 7, McIntosh C22, Audio Research SP3 or several others that did that long ago with fair results. And put the feedback point right to the cathode instead of trying to drive a 39 ohm load. Also, does your inverse RIAA network input match the output impedance of your signal generator? If not you'll get response errors.
__________________
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking
  Reply With Quote
Old 5th July 2013, 09:10 PM   #4
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 7
Quote:
Originally Posted by kingneb View Post
What is the best way to compute the values for the feedback network for this active RIAA circuit?
Not computing the values at all. That design is so horrible it's not worth pursuing.

A) The 12AX7A makes an absolutely horrible cathode follower: the current supply capability is severely lacking due to the ENNNNNNORMOUS plate resistance of the type (90K nominal). No way will it be able to drive a 39R load.

B) As if the plate resistance weren't high enough already, the unbypassed cathode resistors will drive it up even higher. That will likely make for even worse distortion. 12AX7s need very light plate loading if they are to sound good, and without either active plate loads, or enough DC rail for really big passive loads, they don't sound so good. There are way too many designs out there with 12AX7s that load the plate excessively. This is why there are so many complaints about how bad it sounds. (Small signal pentodes would often be a better choice and would sound better.)

If you're gonna pursue a design that involves active RIAA compensation, best to implement it as a glass op-amp: differential front end -> VAS -> Lo-Z buffer. Then connect the RIAA network between the output and the inverting input side of the differential.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 6th July 2013, 03:46 AM   #5
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
I cleaned the circuit up.

The Audio Research and other designs look like they have drive impedance issues too.

I raised the impedance to 1MEG on a diff amp. I am messing with values again and nothing seems to respond well in this configuration. Do you have to scale the components based upon that 1MEG resistor?
Attached Images
File Type: png RIAA.png (26.8 KB, 126 views)
  Reply With Quote
Old 6th July 2013, 03:51 AM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Noisy and won't even vaguely conform to RIAA. I won't bother to mention the use of 12AX7 as a cathode follower again... If you scale the components for the 1M resistor, you'll end up with bizarrely high values which can't actually be built and would be hideously noisy if they were.

Have you worked your way through the phono stage design portion of "Valve Amplifiers"? If not, I think it would be worth your while. You're trying to pull off a difficult design and you need a better grasp of the basics to make your life easier. And read the Lipshitz paper!
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 6th July 2013, 03:23 PM   #7
roline is offline roline  United States
diyAudio Member
 
roline's Avatar
 
Join Date: Sep 2010
Location: East Tennessee
Check out the groovewatt, a proven design.....
Attached Images
File Type: jpg phonoRIAA.jpg (161.0 KB, 94 views)
__________________
SO many tubes, SO little time!!!
  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
Variability of RIAA eq with tube characteristics? leadbelly Tubes / Valves 21 15th April 2008 07:29 PM
OK to use .wav files with RIAA EQ reversed to test phonostage? LowRedMoon Analog Line Level 8 20th November 2006 06:13 PM
Active RIAA Eq for tubes- Brian Clark's article jackinnj Tubes / Valves 0 2nd November 2005 11:18 PM
Passive EQ to active EQ Nuuk Multi-Way 13 24th May 2004 08:59 AM
New web page for active RIAA EQ design andy_c Analogue Source 1 15th February 2004 03:01 AM


New To Site? Need Help?

All times are GMT. The time now is 04:02 PM.


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