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In Reply to: RE: phono stage RIAA question. posted by lovetube on February 25, 2011 at 20:24:13
Your circuit has a slight error, causing the lack of bass boost. You have a 330K resistor across the 33K resistor. Change that 330K R to a 3300 pf cap for a quick fix. The resistors can be tweaked, as well, but, this 3300 pf cap will quickly yield the required bass...
Concerning the required RIAA treble deemphasis, if your circuit does not satisfy your ears and you want more high frequency cut, you can add another EQ filter pole to ground after this network.
Research the passive phono EQ published circuits beginning circa early 1950s. RIAA was standardized in 1954. Many, many passive EQ circuits have been implemented with 6SL7s since then. Many are available online. In my lower post to this thread, I gave the link to the JE Labs site. Joe's mono playback system has many virtues. His design topology can easily be implemented as dual mono stereo preamp stages. By using separate power supplies for each stage as well as each channel, ultimate detail and stereo separation with amazing soundstage effects can be achieved.
While I am a bigger fan of active feedback phono EQ, made popular in the early stereo era, I do know more than a few folks that have implemented the passive EQ to their satisfaction. Happy tunes await you; so, ENJOY !
Follow Ups:
isn't the 330k is the grid leak for the 6CG7 ?
It can take a long time for the light bulb to be fully lit concerning electronics. Sometimes, the technical theory simply requires memory to "move on" and understand further. As a great example, current flow theory is often taught as passing from positive to negative, or vice versa. In order to work with high current carrying voltages safely, understanding the flow is much less important than remembering not to get any part of your body in the way of any possible current flow, in order to reduce shocks...
You are very welcome !
With EQ circuits, only via studying and rereading, again and again, will the filtering/boosting be comprehended. Seeing a wide variety of circuits, they actually start looking similar. While many peers might believe tweaking cap and R values is confusing, I believe that if you can trust your ears, simple value changing is extremely enlightening. In fact, simply changing the type of EQ capacitor can be ear opening; say ceramic to silver mica or polystyrene.
You are correct that the second 330K to ground is the 6CG7 grid leak R. In many passive preamps, this grid leak value is even higher. For instant satisfaction, simply install a 3300 pf across that 33K R in each channel's EQ pole. Then, later you can upgrade or change at will, after a bit of research. I assure you that the many circuits really do start appearing similar. Remember that the type of EQ cap installed or reinstalled will probably startle your ears. After a bit of break-in time for new parts, your ears will easily tell you whether that circuit prefers silver mica or polystyrene caps. Enjoy the ride and happy listening...
Thanks.
will have a lot of trial that is for sure but you are right , i always truth my ears .
I will have to learn a bit more on this RIAA stuff.
look like it is going to be complicated to get right.
Cheers
LT
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