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Old 3rd July 2004, 02:20 AM   #1
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Smile simple phono preamp circuit needed

Hi all-

I recently thought I'd give vinyl a try, so I just bought a Dual 1214 TT off of eBay and now need a phono preamp. I will be running this through my newer Onkyo receiver, which does not have phono inputs (only has CD and tape). From what I understand, the output from the TT will be too weak to work effectively... so I am in need of a preamp.

I could just use an opamp based circuit, but I'd really rather not, because my receiver doesn't have any opamps in the signal path (that I know of), and I'd rather not put any in there. I have a few 12xx7 tubes laying around, as well as two 6GM8's. Is there anything useful I can come up with using any of these? I would prefer it be as simple as possible, and not use very high voltage (I don't have any high-V transformers laying around unused... but I could use two transformers and get up to about 300V if I had to...). The TT will be here in a week, probably, so hopefully I could have the preamp done by then as well. Thanks all!
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Old 3rd July 2004, 02:56 PM   #2
sreten is offline sreten  United Kingdom
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IMO valve is total overkill for this application, and pointless.

If your Onkyo is fully discrete, then make a discrete transistor
op-amp and use it in an op-amp circuit or build a discrete
transistor (or FET if so inclined) phono stage.

sreten.
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Old 3rd July 2004, 03:31 PM   #3
BHD is offline BHD  United States
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Tube DIY is a hobby, and by it's very nature kind of pointless. And that's the... uh... point, I guess.

Besides, once he builds himself a tube phono stage, he'll be building himself a line stage and amp to go with it in no time.
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Old 3rd July 2004, 05:06 PM   #4
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The "classic" RCA circuit is about as simple as it gets. It uses a pair of 12AX7s and passive (no NFB) RIAA EQ. You will have to add a voltage follower to the RIAA circuit, as it can't drive a substantial load, like your receiver's line stage.

You can get an adequate B+ rail from a 120 VAC winding by using a full wave voltage doubler to drive a voltage regulator circuit. SS will have to be used for rectification and regulation as Volts are at a premium. I suggest you use 1 A./600 PIV Cree Silicon Carbide (SiC) Schottky diodes for rectification. The SiC parts have a low forward voltage drop and zero switching noise. Ignoring the unavoidable real world losses, 120 VAC into a doubler yields a 339 V. B+ rail. That seems to be enough to drive a regulator. The 120 VAC winding powering the doubler will have to be rated for a minimum of 4X the DC current draw.
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Old 3rd July 2004, 09:27 PM   #5
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Default Here is a modified RCA

This guy has an interesting variation that you might want to look at.

Bob's Phono Amp
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