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Old 21st April 2013, 11:26 PM   #1
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Default All-In-One Preamp Circuit

Attached is the schematic for the current implementation of an All-In One solution for MM phono and various inputs on one PCB with common shunt regulator for both channels. The line amp section has gain, so it might serve for folks who have lower gain power amp solutions similar to the SIT amp "L'Amp" or simulated SIT solutions such as "L'Fake Lite". This is a complete preamp on a board, no joke. Later on will be a setup for solutions more comfortable with a unity gain line amp - not a really big stretch.

This the the preface to a series of RIAA/line amp solutions that will be checked out in a no-frills test box.
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Old 23rd April 2013, 09:21 PM   #2
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Attached is a pic of the all-in-one board inside a bare-bones test chassis. Input coupling capa are at the bottom (Sprague 735P Polypropylene film). The pot at the left is a Vishay dual 10k CP unit with coencentric shafts.
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Old 23rd April 2013, 09:26 PM   #3
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Here is a rear view of the test chassis. The phono input and line outputs get the standard RCA jacks. The source selector has been omitted, and source switching is accomplished via the pigtail RCA jacks at the rear of the chassis. Keep in mind that this is a bare-bones test setup that can be easilly and rapidly swapped in to test a given preamp circuit. I only anticipate switching between a turnatble and a high quality CD player for listening tests, hence the ultimate bare-bones approach to source switching.
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Old 23rd April 2013, 09:30 PM   #4
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Here is a front view of the chassis, showing the on-off switch (which actually switches one the rest of the components in my system), and the coencentic volume control knob.
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Old 26th April 2013, 01:46 AM   #5
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Attached are gain-phase results for the two RIAA channels - the gains are pretty closely matched for open loop circuits.
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Old 26th April 2013, 08:14 PM   #6
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Here is the G-P response of the line amp section . Gain is ~4.8X, and is flat out past 1MHz. Some compensation will be needed to fix the peaking at ~15MHz. It appears that a simple pole will not suffice. More on that later when I can get the pole-zero tweaked to satisfaction.

BTW, on both sets of graphs, the top trace is gain, and the bottom is phase.
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Old 27th April 2013, 09:20 PM   #7
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I'm reading if nobody else is. In the stock box I have j111 for n channel jfets and 2n7000 for n channel mosfets, but that p channel mosfet in the output would cost me $8 minimum shipping plus whatever it costs. As all my power amps have a lot of gain to cope with a PAS2 output (12AX7), I might just delete that stage. The only problem might be the capacitance of the 6' coax RCA cable.
What C1 & R1 are you using? For what cartridge? My op amp disco mixer uses .033 uf on the input op amp feedback, if that means anything (hums a bit is why I'm thinking of an upgrade). Is C2 your RIAA roll-off capacitor?
Flat past 1 mhz may be a bit of a problem. I do have CB'er driving by. I managed to silence his continuously beeping "Dixie" with a steel box, chokes on the DC power supply inputs to the box, and 100 pf caps to ground on the mag phono inputs.
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Last edited by indianajo; 27th April 2013 at 09:36 PM.
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Old 1st May 2013, 08:58 AM   #8
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I am reading too.
Keep going wrenchone.I can't wait for the final version,since I'm looking for a complete preamp to build. Please include substitute fets.
Great thread.
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Old 1st May 2013, 10:42 AM   #9
AndrewT is offline AndrewT  Scotland
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Looks good.
What about adding in a switchable MC pre-pre stage?
What about switchable gain for the line stage, +0dB, +6dB, +13dB (4.8x)?
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Old 9th May 2013, 05:56 PM   #10
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I am not happy with the line amp section of the preamp, for what turns out to be well-founded (and fundamental) reasons. The problem got uncovered when I started trying to unsnarl the high frequency peaking in the gain-phase plots - more on this when tiime permits. It looks like there's also a solution, though with more parts, I'm afraid...
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