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Old 16th April 2012, 02:03 AM   #1
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Default Another RIAA Preamp

Attached is an RIAA preamp circuit that came out of the work I've been doing (documented in the analog line forum) with feedback connected mosfets used to simulate or fake a SIT, which has a triodish characteristic. That led to a "pseudo-Aikido" using mosfets and jfets that needed feedback to tame its gain. That circuit is used in the second stage of this preamp. It's the first feedback RIAA amp I've designed in years. It probably won't be quite optimal with respect to noise because of the high value feedback resistors. However, those resistors are in the second stage, which may somewhat mitigate their effect.

The second stage provides a pretty optimal load for the SRPP first stage. One could also use a simple cascoded common source 2SK170 at the input, but a follower would be needed. I've laid out my circuit board for both input circuit types. The one thing standing in the way of going ahead with this is putting together a suitable 2n8 cap for the RIAA filter. I already have the 10nF caps. I should be able to scrounge something fairly soon.

At any rate, this may not be the hottest circuit in town but it's food for thought.
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File Type: gif pseud_aikido_RIAA.gif (20.7 KB, 538 views)

Last edited by wrenchone; 16th April 2012 at 06:20 AM.
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Old 17th April 2012, 04:44 AM   #2
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I found the fixings for my 2n8 cap (high quality 2n7 + 100p polypropylene), so the board is laid out, and will get listened to sometime down the line.
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Old 17th April 2012, 04:51 AM   #3
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Quote:
Originally Posted by wrenchone View Post
I found the fixings for my 2n8 cap (high quality 2n7 + 100p polypropylene), so the board is laid out, and will get listened to sometime down the line.
Wrenchone,

Looks good, keep us posted.

DT
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Old 5th May 2012, 02:43 AM   #4
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I'm actually stuffing a board for one of these preamps. The power supply/regulator came up without any hitches today, and I may finish selecting and stuffing the rest of the jfets and mosfets this weekend.

Actual listening may not happen until I put together a special test chassis (no selector switch, just a pair of RCA jumpers to switch between RIAA preamp and line source). The first pairing of test modules in the box will be this preamp and a "MOSFET-Schade" line amp.
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Old 7th May 2012, 04:39 AM   #5
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I finished selecting the jfets and stuffing the proto board of this preamp. Things balance out sufficiently well that I will hook it up to the gain-phase analyzer at work when time permits to test gain and channel-channel balance. I will probably change R14 to ~120k to better center the second stage output.

This amp is still not optimal because of the high impedance RIAA feedback network, but it's an interesting topology and deserves at least a cursory listen. I suspect that the second stage will provide fairly optimal loading for the PN4393 SRPP input stage.
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Old 8th May 2012, 02:58 AM   #6
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Here's a pic of the little freak on the bench getting gain-phased.
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Old 8th May 2012, 03:04 AM   #7
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Attached are the gain-phase plots, done with an HP4194A with 0.5 mV input. The gain is a little on the high side, but both sides match to within 0.3 dB.

It occurs to me that the load on the first stage SRPP is still a little lighter than optimal, so I can reduce the impedance of the second stage feedback network, pulling down the overall 1 kHz gain closer to the desired 40 dB and reducing 2nd stage noise contribution. Before I make that change, I'll probably listen to the thing as-is.
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Old 8th May 2012, 03:47 AM   #8
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Nice. Did you run an FFT also we can maybe see?
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Old 8th May 2012, 05:28 AM   #9
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No FFTs on anything, yet. My sound card interface is not functional yet, and it would need a decent sound card to go with it.
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Old 8th May 2012, 09:56 AM   #10
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Similarity to Creek's SRPP version with BjT's
How convert Creek OBH-8 to MC cartridges?
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