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Re: Morrisons "Siren Song" preamp

Posted by L.D. Moore on November 28, 2000 at 14:15:48

In Reply to: Re: Morrisons "Siren Song" preamp posted by julioiglesias on November 28, 2000 at 09:00:11:

I'm looking at the schematic now. The 100k off the plate of the second half of the 6sn7 forms one of the poles of the time constants with the .033uF and 10K resistor. In fact, it is (100k +10k)*.033uF or the 3,630uS which when used in conjunction with the rest of what's there, namely the grid of the 6n7s in parallel and the coupling network comprised of .1uF coupling capacitor and the 249k grid resistor of the 6n7, yields the 3,180uS pole in the RIAA eq. In the same fashion, the 10k and the .033uF or the 330uS, used with the surrounding componets in like manner, yield the 318uS pole in the RIAA eq. So, as you can see the component values are a bit off in order to compensate for the actual in circuit result when used in conjunction with the other components.

Having said that....in order to place anything in the circuit in place of the parallel 6n7, it would have to approximate the grids of these two tubes in parallel, such that the two previously discussed poles are the same and the eq corresponding to such are equal. This is the only way the RIAA eq will be correct.

Now, the grids of a couple of 6n7s tied in parallel are I'd guess approximately the 500k to more likely mega ohm range. There is also some parasetic miller capacitance to be dealt with.

Now, you wanna put a 600 ohm 1:1 tramsformer there??? Not even close! The component values will have to all be changed and not by a small amount. In short, for-get it. I'm not trying to hard here...just trying to help.

You need the 6n7 there to act as a buffer to isolate the RIAA eq from the rest of the world, dude!

If the gain of the last stage is something like 75% of mu which is 35 or 26....How does one reduce this? Not too easily since you really don't have a lower voltage to work with and the max plate voltage of the 6n7 is 300. This rules out using a step down transformer in place of the plate resistor which is 25K. You could go to a lower mu tube in this application....but, you still effect the RIAA eq a bit.

What is you line stage like???

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