Pearl RIAA modifications? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 14th June 2015, 09:56 AM   #1
Skorpio is offline Skorpio  Denmark
diyAudio Member
 
Join Date: Mar 2004
Location: Denmark
Default Pearl RIAA modifications?

Hi,

I am considering a new RIAA based on the Pearl 1. It's for MM only, so I want to use only two parallel 2SK170 in the input stage.

As I want to keep the same gain level as the original, I will increase R19 from 499 to 1000...

But I think I then also will have to change the value of the RIAA resistor R23 (6k8), as the output impedance from the first stage will be increased...

Is that correct?

Thinking Ro=R19, then RIAA resistance is 499+6800 = 7299 in the original.

In the modded: 7299 - 1000 = 6299, this must be the new R19 value?

BR
  Reply With Quote
Old 14th June 2015, 01:07 PM   #2
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
Tough choice -- the paralleled JFET's reduce the noise in the front end and increase the gain, but this stage will overload with a MM cartridge at frequencies over 10K with peak in put.

Bob Cordell showed the way in which passively loading the JFET input (from 47k to, I believe 18k) rolls off the high end, but you need to know the inductance of the cartridge, and make some other adjustments down the line.
__________________
A preferential option for the poor, need not imply a preferential option for the state.
  Reply With Quote
Old 14th June 2015, 07:25 PM   #3
Skorpio is offline Skorpio  Denmark
diyAudio Member
 
Join Date: Mar 2004
Location: Denmark
Ok, so this overload problem will not be there with 4 parallel FETs?
  Reply With Quote
Old 15th June 2015, 05:30 PM   #4
Skorpio is offline Skorpio  Denmark
diyAudio Member
 
Join Date: Mar 2004
Location: Denmark
Well, I have found some related topics about the overload issue, and it seems to be basically unclear (information wise).

So far I have concluded:

1. The overload issue has nothing to do with parallel FETs (2 or 4), if gain is equal. The problem is the high gain linear first stage at high frequency combined with high-output MM...


2. A solution would be to reduce first stage gain, and by keeping R19 at 499R resistance will half the gain....(with 2 FETs)


3. As many users are reporting no problems with MM on Pearl (and a few with some problems), it is likely not all, but high output MM above a certain level...

It could be nice if someone could state a guideline regarding this, like: "With MM putting out above 3mV/1kHz, the first stage gain can be maximum 23dB"

Anybody?
  Reply With Quote
Old 17th June 2015, 06:21 PM   #5
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
Hi,

the pearl 2 has 53dB of gain which is a lot for an MM cartridge.

Normally a gain of around 40dB is used. So you must reduce the resistor value even further (by aprox. 10 dB.)

William
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  Reply With Quote
Old 18th June 2015, 04:28 PM   #6
Skorpio is offline Skorpio  Denmark
diyAudio Member
 
Join Date: Mar 2004
Location: Denmark
It is Pearl 1, so gain will be just right.....
  Reply With Quote
Old 18th June 2015, 04:53 PM   #7
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
Hi,

sorry, I overlooked the pearl 1 in the first post. Pearl one has almost 60dB of gain which is even more than the pearl 2 so also a bit much for a normal MM cartridge.
Reducing R to 120R would give around 10dB of gain for the first stage and around 40 in total.

William
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  Reply With Quote
Old 18th June 2015, 05:16 PM   #8
Skorpio is offline Skorpio  Denmark
diyAudio Member
 
Join Date: Mar 2004
Location: Denmark
You are right about the 60dB gain, but you forget the -20dB loss in the passive RIAA...

The Pearl 2 has +10dB more gain than Pearl 1, made in the second stage...
  Reply With Quote
Old 18th June 2015, 08:15 PM   #9
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
right.......

william
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  Reply With Quote
Old 19th June 2015, 05:26 PM   #10
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
Quote:
Originally Posted by Skorpio View Post
Well, I have found some related topics about the overload issue, and it seems to be basically unclear (information wise).

So far I have concluded:

1. The overload issue has nothing to do with parallel FETs (2 or 4), if gain is equal. The problem is the high gain linear first stage at high frequency combined with high-output MM...


2. A solution would be to reduce first stage gain, and by keeping R19 at 499R resistance will half the gain....(with 2 FETs)


3. As many users are reporting no problems with MM on Pearl (and a few with some problems), it is likely not all, but high output MM above a certain level...

It could be nice if someone could state a guideline regarding this, like: "With MM putting out above 3mV/1kHz, the first stage gain can be maximum 23dB"

Anybody?
The first stage gain will be reduced 3dB by halving the number of JFETs, but the problem isn't in the first stage. (On its own, with no feedback the first stage measures under 0.02% and gain is ~27dB.) So you don't want to cripple the first stage.

Take a look at the "Building a Pearl 2" thread for some suggestions. Building a Pearl 2
__________________
A preferential option for the poor, need not imply a preferential option for the state.
  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
RIAA caps for Pearl and Pearl 2 AudioLapDance Pass Labs 7 22nd May 2015 07:19 PM
Riaa Caps for Pearl 2 Renton76 Pass Labs 5 12th April 2013 05:18 PM
modifications Professor smith Subwoofers 7 20th July 2011 11:36 PM
Question about the pearl RIAA stage lordvader Pass Labs 25 20th December 2006 12:02 AM
Results of Modifications. pixie Solid State 0 2nd February 2001 04:31 PM


New To Site? Need Help?

All times are GMT. The time now is 05:32 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2