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Old 25th May 2013, 05:11 PM   #1
chirs is offline chirs  United States
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Default Newbie advice - Phono Preamp/1st electronics build

I'm looking to build a very simple phono preamp for use on my low-end "garage" stereo (10 year old Sony AV receiver which has no phono hookup).
Consider me a complete newbie (apart from knowing how to solder)
I'm posting this in the tube section because I'd like to have this little project live on past the crap it's hooked up to. But if any of you think I should start with a chipamp or solid state, I'm happy to go there.
I just want to make something, have it work, and get the confidence to make more!

thx!
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Old 25th May 2013, 06:06 PM   #2
lexx21 is offline lexx21  United States
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Welcome to the forum...

Here is a relatively simply phono preamp that you can try:

RJM Audio - Tube passive phono preamplifier

I am building one out as well among other projects that I have going on. You can either use the 6DJ8 tube, which can be somewhat expensive or you can use the 6n23p which are FAR more reasonable. Normally those go for around $25 or so on ebay for new old stock (NOS).

There are other projects on that site as well. This is something that you can build out on a perf board or just go point to point. Get a couple of 9 pin sockets for your tubes, some good caps (I suggest the K40 Russian paper in oil, or PIO as they are called), and get metal film resistors for the riaa section. Those are the odd resistor values and metal film 1% resistors will work well for that. The rest can be carbon film.

Once you build it out, if you have any issues with it we can help you work through them so don't be discouraged if something doesn't turn out quite right. Working with tubes can be a lot of fun.

Hope this helps.
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Old 26th May 2013, 10:22 AM   #3
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That 6DJ8 design could get into trouble driving a low I/P impedance "sand" unit.

Scan the archives for my posts about the tweaked RCA phono preamp. That setup can drive the IHF "standard" 10 KOhm downstream load.
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Old 30th May 2013, 02:56 PM   #4
rongon is offline rongon  United States
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The only RCA Phono Redux schematic I could find was here :
RE: Need Recommendation For Phono Preamp Build - Eli Duttman - Tube DIY Asylum

Click the image to open in full size.

Is that the right one?
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Old 30th May 2013, 04:18 PM   #5
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I built the phono preamp at this link:
Looking for 12AT7/ECC81 phonostage schematic

Same circuit as the RJM but uses 12AT7.

Worked out great for me.

Scott
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File Type: jpg 12AT7phono.jpg (69.2 KB, 110 views)
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Old 30th May 2013, 07:13 PM   #6
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Quote:
Is that the right one?
Yes.
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Last edited by Eli Duttman; 30th May 2013 at 07:14 PM. Reason: typo
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Old 30th May 2013, 08:04 PM   #7
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I built the schematic Rongon found and added the tweaks in blue after further discussions with Eli and others. These balance some changes made in adding the follower stage and grounding the second section cathode.

Either the 6n2p-ev or a quiet 12AX7 will work well. I have tried both the Sovtek 12AX7LPS and the Mullard re-issued 12AX7 long plate, and they sound great.

After blowing the follower MOSFET from (1) a static discharge and (2) detaching and reattaching interconnects while everything was on, I'd suggest adding back to back 15V 0.5W Zeners between source and gate.

The Panasonic 0.47 muF ECP-Q(U) bypass cap on the output is getting hard to find. The SBE 716P film and foil cap would be a good replacement (not the 715P, which is metallized film). The GE 5 muF cap is metallized film, and there are similar types that would work.

Eli recommended to me that the 30 muF decoupling caps should be motor run oil caps or at least metallized film caps, not electrolytics. I am using Solens at the moment, because of space available. They work fine.

There is an associated power supply for this build, attached. It provides 12VDC regulated to the 12AX7s and 250VDC regulated for B+. Change out the heater doubler and the regulator for 6V tubes.

Overall, this is a great sounding phonostage, and with a Shure 97xe MM cartridge with ~4mV output, the gain matches the output of a typical CD player, so no additional pre-amp is needed if you normally don't need one for a CD input.
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Old 30th May 2013, 11:30 PM   #8
chirs is offline chirs  United States
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Really helpful!
I'm going to start exploring some of these schematics and familiarize myself with the parts (and, I must admit...what they do).
I have a few 12ax7's from a guitar amp build a friend did for me...what's an ideal candidate there? Once it's built, is it safe to swap tubes and see how they perform?
Also, just curious...do any of you have pictures of these builds?
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