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In Reply to: RE: 5670 or 12ax7 phono posted by lugnut1 on April 17, 2015 at 11:01:33
How much gain are you looking for? You might be able to use both (12AX7 first stage, 5670 out).
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I would like to use a MM cartridge. Phono gain 35-40DB(5670) and line stage gain of 20-25DB(6dj8)
this is one I'm considering. Any thoughts?
There are some outright grievous errors in that schematic. It actually won't work as built. The EQ looks like something "borrowed" from a phono preamp with 12AX7's. It's not optimized for 5670's.
To be honest, I have built differential phono preamps before (What you're referring to as balanced), and they don't sound as good as something more traditional. Additionally, the parts count is higher, and noise performance is actually worse.
I would recommend trying something more like a 12AT7 up front with a CCS load, then let that drive the 5670. The 12AT7 with a CCS load still has a lot of gain, but is much more capable of driving a passive EQ network (which I would also recommend). You can also find spiral heater 12AT7's that are a bit quieter, and that may be helpful. Something like this would be far more technologically advanced than 99% of the tube phono preamps on the market.
Points taken CB. How about something like this.
Do you like SRPP? It was very popular in the late 90's, then fizzled. I'd recommend doing some research to learn why.
The SRPP setup in this schematic does not offer additional gain, but rather gain reduction. Again, there is gain traded here for lowered output impedance, then there's this extremely high impedance RIAA network (which could be driven by a single 12AX7 gain stage), but then a 15K resistor has been thrown across the whole thing, which will roll off the bass.
Built a guitar effects Hotbox using NOS TS 5670Ws, long time ago. Very quiet tube. Nice linear character. AFAIK, that box is still being used.
Not sure if mu is enough for your use, but these are good tubes.
Now that I think about it, the 5670 might be just fine in the differential gain block of a refurbished Heath W6M. I mentioned reworking the gain structure in the thread linked below.
I like high gm devices in circuitry containing a GNFB loop.
Eli D.
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