Blackstar ISF Control - What's going on there?

Blackstar Amps have quickly established themselves as one of the forerunners of the "new to the scene" amp  manufacturers. Some great amps twinned with some really interesting tech have really helped push Blackstar to the front of a pretty saturated market.

One of their most interesting innovations is their patented "ISF Control" ("ISF stands for Infinate Shape Feature") - a control which allows you to alter the position of the mid cut in a standard passive guitar amp tonestack:

Turn the ISF toward USA and you lower the frequency of the mids scoop, go towards UK and you raise it.

So, how on earth does that work? Firstly check out this video from Blackstar themselves:

A pretty impressive control in itself (not too sure about that demo at the end, not quite to my tastes!). So now we want to know how it works! Well, check out the actual patent application Blackstar made to the UK (Big shout to the UK based manufacturers :-) Patent Office:

 

So it's basically a dual ganged pot with each of it's individual pots added into a key location in the tonestack network:

Unfortunately we have no values presented in the Patent Application to play with. Luckily some clever fellow has pulled apart a Blackstar HT-5 which has an ISF control located in it's preamp section:

Part of the Blackstar HT-5 preamp - note the ISF control and the surprising amount of solid state semiconductors!

Granted not all the values are there but it does give a starting point at least. So if we start out with C1 = 470nF, C2 = 220nF, C3 = 220nF and C4 = 4.7nF, R4 = 1k what else do we need, well, quite a bit really. I have it on reliable information that the dual ganged ISF pot is a dual 10k linear pot. This is still enough to start playing with though - don't use it in any manner you're not supposed to though - remember that this is Patented technology and, as such, Blackstar has complete control over it, this article is just a quick look at one of the most interesting tone shaping ideas developed in recent years in the guitar amplification scene.