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Pacific RIAA phono pre: failure!
Pacific RIAA phono pre: failure!
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Old 28th June 2018, 11:53 AM   #141
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Default improved bass response by moving coupling cap

Update: my soldering iron did not run cold after my last postings. In my newest, cleanest version i applied some helpful changes. I use 50 ohm trimmers as source resistors only for the second transistor stage. Cant hear the difference to fixed resistors, but it helps me searching for the sweet spot biasing point. Together with the variable VCC it gives me more different ways to set the voltages and biasing points on the first jfet (with fixed Rs of 12R7) and on the second jfet.

But the biggest improvement came from moving the coupling cap at the output of the first jfet stage to the input of the second stage. I saw a lot of people in the forums doing this with the pacific topology. But i did not understand why it would sound different.

Now i thought about it again and realised that right after the first jfet stage the frequency spectrum is still bended according to the RIAA curve.
But after the RIAA filter stage, right in front of the second jfet stage, the signal is low level, but already "flat". If you place a coupling cap there it will see a higher amount of bass (compared to the amount of highs) in the signal. In combination with the gate-to-ground resistor of the second jfet (220k in my case) a mild highpass with fc around 3 Hz is formed without too much attentuation of the audible band.
I guess there are more professional ways to describe this, sorry

Will come back with my actual schematic for everyone that might find this thread somewhen in the future :-)

Last edited by AddiDub; 28th June 2018 at 12:11 PM. Reason: typo
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Old 28th June 2018, 12:26 PM   #142
Salas is online now Salas  Greece
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Pacific RIAA phono pre: failure!
Not only that but its uF value is lowered due to the higher impedance point and you can use a better quality capacitor for the money or even Teflon
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Old 28th June 2018, 02:52 PM   #143
Malefoda is offline Malefoda  France
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Quote:
Originally Posted by Salas View Post
Comprehensive schematic.
That was about my hand drawing. But I wanted it cleaner for archive file.

So I've done my homework, nice little supply with top circuit in a remote case, bottom one in the phono's one.
The software is iCircuit on Mac OS, lacking parts but I guess they're working on it.
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File Type: png Capture décran 2018-06-28 à 16.51.48.png (47.7 KB, 137 views)
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Old 28th June 2018, 07:56 PM   #144
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Looks good, nice drawing :-) How does your power supply sound like? Mine is exactly the same lm317 circuit followed by the capacitance multiplier, but powered by a smps wallwart. I checked different models of wallwarts, some are noisy, but mine (the cheapest Voltcraft model) seems to be dead quiet..

I found some hints in the web, that it is good when all the capacitors of the capacitance multiplier share one star ground point. Changing the capacitor ground rail to a star ground totally killed the last amount of hum in my case..

Example: http://sound.whsites.net/p15_fig3.gif

Last edited by AddiDub; 28th June 2018 at 07:59 PM. Reason: inserted image link
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Old 28th June 2018, 08:20 PM   #145
Malefoda is offline Malefoda  France
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Hi, sadly the shape of my circuit is the only one that fits in my tiny phono box, it's only a quick build with mostly drawer spare parts (but the JFet). Anyway it's a very good sounding "waiting until better" phono pre, and quite silent!
Let's enjoy the LPs And thanks that guy named Salas!
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Old 11th July 2018, 06:32 AM   #146
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
finally my Hagtech inverse Riaa unit arrived :-) I did some basic square wave measurements on some the phono preamps using the inverse Riaa.

Phono Input Allen Heath xone 92: weak bass response under 50 Hz, bass boost at about 70 Hz, not-so-square looking waves at the output.

No-Name opamp phono preamp: way better looking square wave response.

Pacific: very good looking square wave output at 100Hz, 1khz and 10khz. At 10khz the square wave output looks like there is too much attentuation at high frequencies. But keep in mind that the Hagtech inverse Riaa comes with an additional zero at 50khz, which is not represented in the circuit i used. So the square wave output will always look a bit "rounded" as long as i do not implement the "missing Neuman constant" at 3,18us.

After my holidays i will continue with more comprehensive measurements including some integrated phono preamps. Very interesting topic!
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Old 11th July 2018, 11:58 AM   #147
andyr is offline andyr  Australia
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Quote:
Originally Posted by AddiDub View Post
finally my Hagtech inverse Riaa unit arrived :-)
Yes, a very useful device!

Quote:
Originally Posted by AddiDub View Post
But keep in mind that the Hagtech inverse Riaa comes with an additional zero at 50khz, which is not represented in the circuit i used.
I did not know that. Is it mentioned in the Hagtech documentation ... and I missed it?

Andy
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Old 11th July 2018, 01:20 PM   #148
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Quote:
Unlike traditional networks, the iRIAA Filter includes a correctly placed upper 3.18us (50kHz) corner in its transfer function.
Quote taken from the sales page of hagtech IRiaa2
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Old 11th July 2018, 02:56 PM   #149
Salas is online now Salas  Greece
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Pacific RIAA phono pre: failure!
It should have been a switchable feature IMHO
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Old 11th July 2018, 03:18 PM   #150
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Quote:
Originally Posted by Salas View Post
It should have been a switchable feature IMHO
I agree! That would make measuring accurate when it comes to old designs that did not use this fourth corner at 3.18us.

Maybe i will build myself a passive RIAA with the "classical curve" / without the corner at 3.18us.
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