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Old 8th January 2013, 07:58 PM   #1
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Default Pacific RIAA phono pre: failure!

Hi there!
happy new year to all

here I go with my freshly soldered RIAA Pacific, this very one but 47k input load.
The PSU is a linear one nicely smoothed, then a clean lm317 based supply, 21V (tx limts vs voltage drops
The thing plays records, RIAAses it, preamplifies it.
But it's unlistenable! On "S" and on some highs it's saturated, eachtime, each records, Shure M91ED, AT95E, Toshiba TD, Denon TD...
And these were playing fine with an integrated with a simple opamp design.
Something strange... now clue where too look at, need your help gents!
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File Type: png Capture décran 2013-01-08 à 21.32.42.png (103.7 KB, 579 views)
File Type: jpg 100_1791.JPG (367.5 KB, 567 views)
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Old 8th January 2013, 08:06 PM   #2
scafas is offline scafas  Italy
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Default similar

i build a similar phono pre and it sound very good!
Click the image to open in full size.
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Old 8th January 2013, 08:23 PM   #3
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Quote:
Originally Posted by Malefoda View Post
Hi there!
happy new year to all

here I go with my freshly soldered RIAA Pacific, this very one but 47k input load.
The PSU is a linear one nicely smoothed, then a clean lm317 based supply, 21V (tx limts vs voltage drops
The thing plays records, RIAAses it, preamplifies it.
But it's unlistenable! On "S" and on some highs it's saturated, eachtime, each records, Shure M91ED, AT95E, Toshiba TD, Denon TD...
And these were playing fine with an integrated with a simple opamp design.
Something strange... now clue where too look at, need your help gents!
Could be the RIAA capacitors.
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Old 8th January 2013, 08:28 PM   #4
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RIAA calculator

http://www.kabusa.com/riaa.htm

Important to use 1% tolerance capacitors & resistors for RIAA.
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Old 9th January 2013, 12:00 AM   #5
kevinkr is offline kevinkr  United States
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Try 100 ohm gate stopper resistors on each of the fets and add 47 ohm resistor in series with the source of the first fet for bias.
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Old 9th January 2013, 03:22 AM   #6
grufti is offline grufti  United States
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scafas' circuit looks better. it already has the modifications that Kevin [kevinkr] proposes plus a few other useful changes. don't change your RIAA capacitor values at this point. they are so similar to the values in scafas' circuit that you can worry about that later. same for the output cap: not much difference between 1.0 and 1.5uF.
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Old 9th January 2013, 06:39 PM   #7
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Quote:
Originally Posted by scafas View Post
i build a similar phono pre and it sound very good!
Quite close indeed! Hope mine can sing good soon...

Quote:
Originally Posted by merlin el mago View Post
Could be the RIAA capacitors.
...
http://www.kabusa.com/riaa.htm
Important to use 1% tolerance capacitors & resistors for RIAA.
I've matched caps to tightest specs and by pairs, maybe not 1% for the 6n8 under the 22n, my dmm is not that good.
Today with a fellow at work, apprentice engineer, we've had a quick look at the pre freq response. I only can show you that pic of 1k up to 20kHz, but later tests has shown no peak or such in audio band. The RIAA seems innocent.

Quote:
Originally Posted by kevinkr View Post
Try 100 ohm gate stopper resistors on each of the fets and add 47 ohm resistor in series with the source of the first fet for bias.
Quote:
Originally Posted by grufti View Post
scafas' circuit looks better. it already has the modifications that Kevin [kevinkr] proposes plus a few other useful changes. don't change your RIAA capacitor values at this point. they are so similar to the values in scafas' circuit that you can worry about that later. same for the output cap: not much difference between 1.0 and 1.5uF.
I've tried with my headamp, in case the 10K attenuator in my amp may trouble the phono, but the same terrible sound. So now I can only attempt theses mods, 2x 100Ω and that 47Ω and for the output caps I have 2.2F film caps, no problem here as far as I understand.

Let's warm the iron...
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Old 9th January 2013, 07:29 PM   #8
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Too late to edit...
No 100Ω at home, that will wait or maybe I'll find those tiny melf 110Ω that should be somewhere... value critical here?
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Old 10th January 2013, 12:06 PM   #9
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100Ω mini-melf ok and nice for such hack, got also 40 and 51Ω but not 47, wich one I use if that may change anything?
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Old 10th January 2013, 04:30 PM   #10
kevinkr is offline kevinkr  United States
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Use what you've got, 100/110 and 51 should be close enough. Place the 100/110 in series right at the gate lead.
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