ImPasse Preamplifier

[DaveM]

Ha!! yes thanks for the correction. Subscription. Nice. I love it when the fingers type things different than are in your mind.

[SY]

Originally posted by DaveM
Any chance of a scan of the article for those of us without prescriptions and local newsstands that carry it?

Thanks

AX would have a problem with copyright. I can ask them for reprint rights after the next issue comes out- the worst they can do is say no.

But really, you ought to subscribe.

[SY]

Kurt, the MN taper is what you want.

[tms0425]

hmmm... my Feb AX hasn't arrived yet, so it sounds like something nice to look forward to.

[Nelson Pass]

Originally posted by SY
AX would have a problem with copyright. I can ask them for reprint rights after the next issue comes out- the worst they can do is say no.

For future reference, I make a point of retaining copyright, and I never
accept money for submissions.

That said, people need to buy a subscription to AX. It's the least they
can do for the efforts made for their benefit.

[DaveM]

I agree that it is important to support things like AX if they provide a source of good information. Unfortunately, that is not usually the case. I had a subscription for about 5 years, and found that I wasn't interested in many of the articles they were writing and the ones that were interesting were things I already knew or were so far over my head without sufficient explanation that I found that the reading was not of much use to me. It is not that I don't want to support AX, but if I find that this board is a better source for information, inspiration and support in seeing projects through than AX is.

I just figured I would ask. I do have a friend who lives in Virginia who will buy this issue and scan me the article. I just wanted to see what SY's design looked like before having him go through the hassle.

[SY]

OK, let's look at that step network in some detail. We have 170-180V on the plate of the first tube, we want 118V on the grid of the second tube for reasons I explained in the article. So we know that the ratio R9/(R8 + R9) = 118/180 = 0.65. Now, this is one equation in two unknowns, so there's an infinite number of possible combinations. We would like to pick the resistors to be high enough that the plate of the first tube isn't loaded down very much, but not so large that stray capacitances and any grid current from the second tube will upset things.

Assume we're at midband where the capacitor has effectively bypassed R8. The first tube's plate load, then, is R9. The plate resistance of the first tube is something like 7k. So if we pick a value for R9 that's at least a hundred times greater, the load on the tube is negligible, and we will still get nearly the full mu as gain, i.e., we'll still be operating the first tube in nearly constant current loading.

1M is a nice, standard value that is known-good for the second tube (ECC88). It's a very common value for a grid leak, and since there's effectively 100% feedback in the second tube, any odd grid current is taken care of. So... we arbitrarily set R9 at 1M, which is more than 100x the plate resistance of the first tube. Plug 1M into the voltage divider equation and solve for R8. 1/(R8 + R9) = 0.65, so R8 = (1/0.65) - R9 = (1/0.65) - 1 = 0.52M. The closest standard value is 510k (0.510M).

Now, that value would work just fine. But in the 45 year tradition of my coffee-can engineering, I didn't have any good quality 510ks, but I did have some 680k. That would put the grid of the second tube at 108V, which is not very far from our target and barely changes the operating point of the second tube. So that's what I used.

In retrospect, I should have been a little clearer... In any case, the resistor values aren't very critical, and 510, 560, 620, or 680k will all work fine.

[hifimaker]

Originally posted by Nelson Pass


For future reference, I make a point of retaining copyright, and I never
accept money for submissions.

That said, people need to buy a subscription to AX. It's the least they
can do for the efforts made for their benefit.

I became a subscriber over a year ago based on Nelson's recommendation. I've learned a lot, and the 2 part tutorial in 2008 on speaker measurements was worth it alone for the year.

-David

[kurtbauer]

For the for the explanation of the step network. That's what just what I needed.

K

[jackinnj]

The problem w dropping the ImPasse into a PAS chassis is that the transformer probably isn't up to the task as the 6SN7 demands twice the heater current. The wimpy PAS transformer is a known issue.