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Eli Duttmann's phono preamp voltages
Eli Duttmann's phono preamp voltages
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Old 27th January 2019, 07:26 PM   #11
aardvarkash10 is offline aardvarkash10  New Zealand
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The design is a progression from a very common RCA circuit. The original circuit lacked the ability to drive into pretty much any load. Other than that it was a staightforward workmanlike circuit.

The MOSFET output in the design Eli often boosts picks up the weight of the load and frees up the rest of the circuit to amplify and signal shape.

I haven't built it, but the original RCA design is probably out there in near millions of household record players - or would be if they were all still in use. Unless you are terribly picky about your RIAA curve (many are for better or worse) it is a relatively simple no frills build with all the usual caveats around amplifying low signals.
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Old 28th January 2019, 03:08 PM   #12
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by gregas View Post
I'd like to build a phono preamp and and I'm looking at building it. I was hoping someone had built it. I'm a newbie to building electronics and tubes, and was hoping to get some advice.
PCBs are available if you think it will help:
The Valve Wizard
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Old 29th January 2019, 10:57 PM   #13
aardvarkash10 is offline aardvarkash10  New Zealand
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This is a great option as it overcomes a lot of the issues with low signal circuit layout.

Merlin - the shop page doesn't seem to have this pcb on it - just the 3-valve pcb.
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Old 30th January 2019, 12:53 AM   #14
VictoriaGuy is offline VictoriaGuy  Canada
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Eli Duttmann's phono preamp voltages
Quote:
Originally Posted by aardvarkash10 View Post
Merlin - the shop page doesn't seem to have this pcb on it - just the 3-valve pcb.
Isn't this:
Compact Phono PCB the RCA phonostage?
Details at:
The Valve Wizard

I got directed to a 'Featured Items' page first, so I had to pull down the menu to get to the HiFi PCBs.
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Old 30th January 2019, 04:13 AM   #15
aardvarkash10 is offline aardvarkash10  New Zealand
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Doh! That's it! I had a bloke's look for it...

I think I shall order some - its incredibly good value.
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Old 31st January 2019, 02:40 AM   #16
gregas is offline gregas  Canada
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Looks like that pcb board schematic isn't the same circuit.
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Old 31st January 2019, 05:49 AM   #17
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally Posted by gregas View Post
Looks like that pcb board schematic isn't the same circuit.
It would be very easy to adapt that PCB to the tweaked RCA schematic. A jumper here, a value change there ... Very noteworthy is protective Zener D2. Field experience shows that is an essential item. FWIW, my preference is a 12 V. part.

Stop fretting about voltages. A regulated 250 VDC B+ rail is the key item. Please notice that the RCA original only specifies the B+ rail value too. 12AX7/ECC83 section plate current is in the 900 μA. to 1 mA. range.

The 2nd gain block in the Tweaked RCA Setup is grid leak, AKA contact, biased. Something on the order of a 2 V. contact potential is associated with the 20 megohm grid leak resistor.

Between the buffering FET and grid leak biasing, 2 of the 3 "weak" spots found in the RCA original are shored up. The tweaked setup has better drive capability and improved bass extension. The 3rd weakness is the high CMiller of the 'X7 triode, which is intrinsic to the type. Select a cartridge or SUT that doesn't adversely interact with that capacitance.
Attached Images
File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 459 views)
File Type: gif Original RCA Phono Preamp Schematic.gif (116.5 KB, 458 views)
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Old 31st January 2019, 06:39 PM   #18
Soonerorlater is offline Soonerorlater  United Kingdom
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Been looking at Eli's circuit for years. I think I should get a pair of these boards to gee me up. My 'spaghetti', point to point wiring just would't do.

Boards received, very nice.
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Old 21st February 2019, 04:04 PM   #19
gregas is offline gregas  Canada
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Quote:
Originally Posted by Eli Duttman View Post
The 3rd weakness is the high CMiller of the 'X7 triode, which is intrinsic to the type. Select a cartridge or SUT that doesn't adversely interact with that capacitance.
How would you know when selecting a cartridge If there is an issue?

Hey guys. Any idea if you need separate B+ rails for the 2 channels or whether you can use one rail with the 250v, 15k, 30uf, 39k, 30 uf and then feed the two channels from that. I'm getting very close to building this preamp. I've got all the components and thinking about layout now.

Greg
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Old 17th May 2019, 10:12 PM   #20
Eli Duttman is offline Eli Duttman  United States
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Each channel has its own set of decoupling parts. A single power trafo, rectifier, and filter is fine. Use individual LR8 regulators in each channel.

A schematic for a modest cost/high performance PSU is provided.
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File Type: gif RCA_Hybrid_phonostage_Power_Supply.gif (49.4 KB, 268 views)
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