Tonestack load

[javacody]

I'm going to give adding a Fender type tonestack to the Little Smoky another go. The problem is that when I use the Tone Stack Calculator, and I change the load from 1M to 50k (from the lm386), I lose lots of db. If I add a buffer after the tone stack with an input impedance of 1M, would this be the same as the load?

[Peter Snowberg]

Yes.

JFETs have a VERY high gate impedance by themselves. 1,000,000,000,000 ohms (yes, 1 teraohm) is what you'll see specified for an opamp like the TL072 with JFET inputs. As this value is huge, you can just ignore it and look at the resistance on the input as the value you seek.

Best of luck with your next experiment!

Take care,
-Peter

[javacody]

Thanks Peter! That is the answer I was hoping for. Also, I took your tip on your favorite JFET and bought a lot of 30 on ebay. I also got a lot of 30 mosfets (BS170) and a lot of 100 2n3904 transistors.

I'm also bidding on a diode assortment and a capacitor assortment. I want to never have to rely on Radio Shack again.

Also, I wanted to ask you if you had any sources for metal film resistor assortments? I've looked on ebay, and there are a couple of good lots, but I was wondering if you knew of any?

[Peter Snowberg]

Excellent! It always helps to buy a bunch and then us a meter (for BJTs) or a test jig & meter (for MOSFETs & JFETs) to grade your stock. It's also great to have a selection of different types.

I wish I knew of a good source for a metal film assortment. That is unfortunately a sorely missing item out there. The best deals I've seen so far have been at http://www.circuitspecialists.com/  but their 200 price is still $0.06 ea which mans $12 for a pack of 200. If you got a pack of 10K, one of 100K, and one of 1M, you could just use a few in series to breadboard and then replace them with a single value later. Metal films make the largest difference when the values get big. The same outfit I just mentioned sells 1000 carbon films for $3 and you can always use those until you find a place where noise really matters.

Take care,
-Peter

[Doug H]

Another idea you may want to consider is scaling the impedance of the tone stack you want to use.  Then you won't need the extra parts for the buffer, it's just a matter of using different values for the existing components in the tone stack.  Here is a good article on it from Randall Aiken's amp site: http://www.aikenamps.com/ToneControlScaling.html

You can use the tone stack calculator with the new values to see how well it would perform.

Doug