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I just finished a Lite Audio #37 12AX7 tube phono section. I built a seperate power supply in a seperate chassis, using a 6 Ft. cable to connect the power supply to the phono section. With a Thorens TD-125, Sumiko Premier tone arm, and a Shure V15 MM cartridge, the phono section has way to much hiss for my liking.How do I eliminate hiss from this schematic? Any thoughts?
Follow Ups:
You mentioned a long umbilical. Install local bypass caps to reduce RF on the connections. The best places are at the plate load resistors themselves to ground. 0.1uf ought to do, assuming that the power supply is reasonably stable (regulation is nice, BTW...).Also, the grids need grid stop resistors. The body of the resistor needs to be as close to the tube socket as you can get it- no lead length in between. For phono, lower grid stops resistances of 300-600 ohms work fine.
BTW, the most important resistors in the circuit are the cathode resistors. Low noise- Resista, Draloric or the like if you are on a budget, Vishay or Caddock if the sky is the limit...
Ralph,The 12AX7 is a tube that grid stoppers can be left off of. Low gm and high Rp, as found in the 'X7, are not likely to be associated with parasitic oscillation.
I LIKE the idea of noise suppression caps. from the B+ side of the load resistors to ground.
Eli D.
We build a couple of guitar amplifiers and the grid stops for the 12AX7s are essential. Without them RF interference is a problem as well as excess noise (both hiss and hum).
Elwood
Everyone of these guys gave you excellent ideas especially the WW non inductive resistors and getting selected low noise tubes..
As I looked at your phono module I had notice dip switches which Im assuming is for impeadance matching and you may have to experiemnt to get the best for your cartridge..If you have to the volume control up super high to get nominal volume,you have a mismatch and that can easily cause excessive hiss.
low signal stages --such as phono preamps-- NEED TO BE LOW NOISE
TYPES. Did you use non-inductive wirewound resitors for the plate
loads? If not-- then order some up from Parts Connexion or
Handmade Electronics, wherever. These will make a BIG difference
of much less noise than if you are presently using carbon or metal
film types for the plate loads. Also: your tube/s might indeed
also noisier than normal (12AX7 not a particularly low-noise tube
design to begin with --regardless of manufacturer) but, FIX THE PLATE LOADS FIRST --unless you already are using wirewounds here.
Good Luck,-T.M.
What I would check:1. Is the first stage (MC pre pre) disconnected in the MM mode?
2. Regime of the input tubes. Lower plate voltage and current produce less noise. In a typical phono section, the input 12AX7 is run at less than 100 V and 0.1 mA.
3. Noisy tubes.
4. Grid resistor of the second stage more than 47 K, or noisy resistor at this position.
5. Too high heater voltageThere are other things, but these will make most inpact.
Anything that attempts to use 12AX7s with LOMC carts. is (IMO) suspect.You're using a good MM cart. PHEW! You need tubes that have been carefully selected for LOW noise. AA sponsor Jim McShane will sell you a set of QUIET Sovtek 12AX7LPS tubes at a fair price.
Eli D.
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