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Old 13th November 2010, 09:39 PM   #111
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I've updated the schematic a bit. I replaced master current source Q9 with an LM317 current source, polluting the precious bodily fluids of the circuit with a nasty IC. However, it was more a accurate current source and had a higher output impedance than the fet I was using. When I find my stash or buy a new bunch at Mouser, I plan to replace the LM317L with a Supertex TO-92 depletion mode mosfet, which should have decent output impedance.

Anyway, U1/Q9 is the master current source for the circuit. Q5-7 are current mirrors that multiply the output of U1/Q9 and direct it to the drain of the gain fet for each stage, Q1 and Q11. Resistors R7, 9, and 22 scale the current going to each stage. I may revamp the circuit to use two separate depletion mode mosfets as current sources, one for each stage. It's fewer (but more expensive) parts, and I would appreciate a little less clutter. It also modularizes the design, making it easier to separate and bring up each stage.

I played with the idea (in simulation) of using an LED at the source of Q1 instead of a resistor, modelling it roughly with a voltage source and series resistor. Gain and distortion are higher, as the local feedback is much lower due to the lower impedance of the LED. I may try this approach later on, as the distortion, though higher, is almost entirely 2nd harmonic, and is similar in magnitude to that of a simple 2SK170 input stage. Using the resistor instead of an LED trades off less distortion in turn for a bit more noise.
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Last edited by wrenchone; 13th November 2010 at 09:41 PM. Reason: more explanation
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Old 13th November 2010, 09:55 PM   #112
Salas is offline Salas  Greece
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Quote:
Originally Posted by wrenchone View Post
I replaced master current source Q9 with an LM317 current source, polluting the precious bodily fluids of the circuit with a nasty IC.
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Old 13th November 2010, 10:03 PM   #113
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The attached schematic shows the circuit with two separate mosfet current sources instead of a master with mirrors - less cluttered.
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Old 13th November 2010, 10:06 PM   #114
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When pumping the cascode's lower floor common source current gain part with a CCS, is there any gain in PSRR for the whole cascode?
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Old 14th November 2010, 09:02 PM   #115
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I don't know if the added current does anything to the PSRR. I could try wiggling the VCC voltage in simulation, but that may not be terribly conclusive. Best to assume that the cascode pretty much trashes the PSRR due to the current divider effect between the high impedance cascode fet drain and the load attached to it, and try to arrange for as quiet a VCC supply as possible. This is a good approach anyway - if you don't track dirt into the house, you don't have to sweep it out...

I'm going to look around tonight in the basement and try to find an appropriately sized die-cast aluminum box so that I can make a permanent well-shielded test jig to use with the AP or whatever other sort of analyzer I gin up. Distortion measurements with the RIAA preamp will be pretty much meaningless until I come up with some decent shielding, and I might as well take the trouble to make a proper jig so that I can easily pop boards in and out, attach test points, etc. I have a box put together with copper tape and copper-clad board, but it's a pain to deal with, and I don't quite trust it.

This afternoon will hopefully be spent in the final stages of bringing up another tube amp, another vacuum/jfet hybrid.

I was thinking of using an outboard usb interface for audio measurements with my new netbook, but it looks like there very few makers of those rigs that have Linux drivers, and my netbook runs Ubuntu Linux. Anyone have any suggestions for an outboard sound card that will run Linux and is of sufficient quality to use in audio analysis?
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Old 15th November 2010, 06:47 AM   #116
Calvin is offline Calvin  Germany
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Hi,

have You by any chance simmed the noise? Active sources -as the current sources in this example- may introduce considerable noise to a circuit.

jauu
Calvin
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Old 15th November 2010, 12:09 PM   #117
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Quote:
Originally Posted by wrenchone View Post
I'm going to look around tonight in the basement and try to find an appropriately sized die-cast aluminum box so that I can make a permanent well-shielded test jig to use with the AP or whatever other sort of analyzer I gin up. Distortion measurements with the RIAA preamp will be pretty much meaningless until I come up with some decent shielding, and I might as well take the trouble to make a proper jig so that I can easily pop boards in and out, attach test points, etc. I have a box put together with copper tape and copper-clad board, but it's a pain to deal with, and I don't quite trust it.
Cookie tins are great for this application -- you can get the big ones at Costco -- and place a smaller one inside. A big cookie tin by itself, however, is a big capacitor, and it's got to be grounded to the test instrument.

I think that Scott mentioned that he had double shielded for his JFET measurements AND placed the rig in the center of the room.
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Old 15th November 2010, 09:23 PM   #118
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I found one of my die-cast aluminum boxes and fitted it with isolated RCA in and out jacks, as well as a dedicated ground screw bonded securely to the case via a kep net and lock washer. I added ez-hooks to the connectors inside so I can quickly swap boads in and out. I'll eventually have two sets of hole patterns to fit my standard board sizes for RIAA preamps and line amps.

Last edited by wrenchone; 15th November 2010 at 09:36 PM. Reason: sp
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Old 15th November 2010, 10:26 PM   #119
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Before I forget, here's the gain-phase plots for the current-augmented common source RIAA. The two plots are offset in gain by ~0.5dB. Listening? That'll happen later.
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Old 21st January 2011, 05:50 PM   #120
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Default Audio Research PH3 Hybrid phono pre amp

Hello Wrenchone & others,
First time writer here.
I have seen discussions about phono pre amp using JFET.
I am a tube DIY (wanabe, at least) and have very limited knowledge about transistors. I am planning to build a phono pre amp using the Audio Research PH3 topology. However, I can't identify the Q1 & Q2 (same) JFET used in the first stage. I can't cross reference to Fairchild, Toshiba, Motorola or Vishay part numbers. Can you help me out in this ?
The ARC PH3 schematic can be viewed here :
ARCDB - PH3

Thank you,
Ken
Vancouver, BC Canada
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