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Hi there, welcome to my blog - La Revolution Deux. It's an odd name - but I like it! Here you will find all the info on my various DIY Guitar effects builds, amplifiers and guitars. Everything from a humble Ibanez tubescreamer to the holiest KLON Overdrive.

You may also find a few effects builds that I am looking to move on - usually in exchange for other effects/gear/cash. You can always check my ebay account to see what I've got up for grabs.

Have fun, enjoy the blog - Fred Briggs :-)


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Saturday 19 May 2012

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.


  1. Thanks for this!

    I've been wondering how this works.
    The HT series are hybrids in my book. Too much semiconductors in it.

    How do these achieve gain? Cascading tubes with IC's?


  2. LT spice is Your friend, experimenting this topic!

  3. This article is worthy of attention.

  4. Found another, higher res schematic online, the values are as follows:

    RV1a=10k Log
    RV1b=10k Log
    *RV3=100k A-Log
    RV4=47k Lin

    *Pretty sure this is the value.

    Modeled it in LTSpice and indeed the mid scoop Q is shifted from 380Hz to about 740Hz. However, I analysed the clip at the end of the YouTube video with Audacity's Frequency Spectrum and the range sounds more like 400Hz-1.9kHz.

    Although a very useful control, the circuit is overly complicated and the name bombastic, it's nothing but a parametric EQ.

    There's another much simpler way to introduce a similar control, and in a Marshall-style tonestack no less!!

    It is mentioned in the tonestack section of a valve design book I have.

    The mod is incredibly simple, just play around with the values of one component at a time in a regular Marshall tonestack to see what happens to the frequency curve and it becomes obvious ;)

  5. What about VR2 (Middle) value?

  6. https://www.partsisparts.net/category/catalog/blackstar-amp-pots

  7. Keep up continue sharing such well post guy . Thank you so much.
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  8. http://schems.com/bmampscom/blackstar/blackstar_ht5-schematic.pdf


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