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Old 4th June 2011, 04:44 PM   #1
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Join Date: Feb 2011
Default Tube RIAA(something easy)

So,I want to make something with tubes,and the case is that I need a RIAA preamp.I have:
2x ECC85
2x EL84
L7R (tube rectifier ?)
various old USSR tubes
some USA penthodes
a chasis with a transformer(~300V + ~6.3V I think)
Suggest a simple schematic.There's no big expectations.
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Old 4th June 2011, 08:46 PM   #2
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In the research I have done with an eye to building a phono preamp I have come to the conclusion that phono preamp and "something easy" are not compatible. It seems that phono preamps are the hardest thing to do well. What do you have in the line of those old USSR tubes and US pentodes?

I suppose one could do something simple with passive eq. using the ECC85s and ECC83 but there would have to be some significant compromise there. The EL84s are power tubes which one wouldn't normally use for phono preamp but I suppose they could be used for overkill cathode followers at the output.

One possible option is to do something like Steve's preamp...

Using your ECC85s for the cascodes (but using a single section of the ECC85 for the bottom instead of the two bottom triodes that he uses) and the ECC83 on the output (using appropriate resistor values for these tubes). Problem is the ECC83 needs very high impedance load so an added CF may be necessary if your line preamp is not of quite high Zin.

Another possibility is a cascode input like that with two step eq. (50Hz corner after the cascode and the 500Hz corner after the second gain stage) followed by a cathode follower. It is possible that some of your Russian and American tubes can be used for the CF. It is also possible that one of your Russian or American tubes could be better than the ECC83 for the second gain stage.

A simple but compromised option would be to implement the classic RCA topology but using the ECC85. Gain might be a bit less but might still be adequate.

You might want to check out this thread...

RCA Phono Stage Redux

The first post has a pic of the classic RCA and the further discussion afterward may be instructive as well.

Best of luck.
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Old 5th June 2011, 01:18 AM   #3
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Location: US
I've made Steve Bench's phono pre and it's a knockout. Try it with Salas' shunt regulated supply.
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Old 5th June 2011, 01:35 AM   #4
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Location: Monroe Township, NJ
I'm uploading a schematic for a fairly straight forward modernized RCA setup. That design calls for a 12AX7/ECC83 for each channel. However, 1 'X7 section for the 1st gain block and an ECC85 section for the 2nd gain block might work out well. If the ECC85 section is constant current source (CCS) loaded, stage gain working into the source follower will be approx. 55X. That's adequate. I leave selecting IB for a grid leak biased ECC85 section to you, but 3 mA. might work out well. The 6AQ8/ECC85 pins out differently than the 12AT7/ECC81, but the triodes in the bottles are quite similar.

BTW, CCS loading of triodes is GREAT for linearity, thanks to the horizontal load line.
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File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 479 views)
Eli D.
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Old 5th June 2011, 07:29 AM   #5
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Ok,looks good.
The USA tubes are 6D6,6A7 and "Hytron 75","Purotron 82"
Also I've got many of these tiny ТХ4Б-1
That transformer outputs 500V
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Old 5th June 2011, 06:53 PM   #6
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If that tranny is 500V center tapped (i.e. 250-0-250) you are fine, just use a full wave rectifier (two diode type not bridge) and you have a voltage a bit under 300.
Generic Signature
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Old 5th June 2011, 07:29 PM   #7
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More complicated but the valve itch is just about the best phonostage i've heard. Thread is over in the analog source forum.
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Old 6th June 2011, 01:01 AM   #8
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I have not built this however I think it looks interesting and easy:

RJM Audio - Tube passive phono preamplifier
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