Buzz in decay - Blackstone Mosfet OD

Discussion in 'The Stomp Box' started by jondanger, Feb 23, 2011.

  1. jondanger

    jondanger Poster Extraordinaire

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    So I just got a Blackstone Mosfet Overdrive from someone on Craigslist. I got a good deal on it, and in general, am pretty happy with the pedal. I am having one issue with it though. On the brown channel, as the note decays, there is this weird buzz that hangs around, and then sharply stops. Any idea what could be causing this? I put a fresh battery in it right away, and I also took the board out and de-oxed the pots (kind of anyway, they are hard to get to). I thought I'd put it to you guys before I write Jon and ask him. Here's a sound sample of what's going on here.

    Blackstone Buzz
     
  2. Pinball_Wizard

    Pinball_Wizard Tele-Holic

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    How big is your room?

    Sounds like interference/ambience involved. I had the exact same sound with my cool cat fuzz. Stopped happening when I rolled the treble down or played in a larger room.

    I'm lazy so I adjusted the amp settings and pedal settings to eq it out.
     
  3. Bill

    Bill Poster Extraordinaire Ad Free Member

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    I had that happen when using an old battery. I put in a new alkaline and the problem went away. I think he recommends alkaline batteries.

    For what it's worth I do not get that sound on either of mine.
     
  4. jondanger

    jondanger Poster Extraordinaire

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    P.W. - It's small, just about 12xx15, but I reproduced it going direct into a digital 16 track, so I'm pretty sure it's in the box.

    Bill - I did use a cheap battery, so I may switch that out.

    I got a recommendation on The Gear Page to back the internal gain trimmer down, so I'm going to try that too. I had thought that it only affected the red channel, but apparently that's not so. We'll see what I can do tonight. Thanks for the help!
     
  5. Impulse

    Impulse Tele-Meister

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    That IS the distortion fading into the clean tone. Happens with most dirt pedals, especially audible playing into a clean amp.
     
  6. 11 Gauge

    11 Gauge Doctor of Teleocity

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    That pedal uses what is called a CMOS logic "inverter" chip. It's in the CD40XX family of chips. Since it was never intended to create distortion, it doesn't have the best power consumption parameters.

    IIRC, it uses a 120 ohm current limiting resistor. It helps, but it's far from perfect. The Tube Sound Fuzz uses the same chip, and it has a 100 ohm resistor. The Emma Reezafratzitz uses a similar chip that you can set to a minimum of 120 ohms, but it has a "starve" control (I think called "bias") that will let you completely destroy the amount of juice that the chip gets.

    ...The Red Llama, which is a Tube Sound Fuzz derivative, uses a whopping 1K resistor. It REALLY cuts the current, but you lose much of the clean headroom or ability to clean it up/have dynamics like the TSF.

    Either way, the BMOD is a beast on current, and those CMOS chips sound like doo doo (unless you like FUZZ) unless they get ample power.

    I use a 390 ohm resistor in my TSF variants, but it is WAY crunchy, and fuzzy at higher settings. But it is a really good compromise, IMO.
     
  7. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Those inverter chip pedals don't have the best output impedance unless they have an output buffer (and the BMOD does not). The loading can do funky things to your preamp (especially if your power source isn't up to snuff).

    ...If your preamp is D.I. or solid state, it's not that hard to load down it's input or simply make it clip from sufficient voltage gain. Put a Tube Sound Fuzz or an old EHX Hot Tubes (they used the same type of chip) into a D.I. setup and you'll get the same buzz in most instances.

    I can't stand testing TSF variants through my little Vox Valvetronix for those exact reasons.

    If you put a buffered bypass pedal after your BMOD, it might take the loading off of the input of whatever you plug it into. It's a cheap and easy workaround.
     
  8. Al Watsky

    Al Watsky Tele-Afflicted

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    OP
    I get the same sort of buzzing decay on mine when using a weak battery.
    With a power supply no problems.
     
  9. pedalo

    pedalo TDPRI Member

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    @.011 (and any other friend of the thread...)

    I saw a separate post of yours relating the lineage of the BAMO, tracing it all the way back to other CMOS inverter-based designs like that of Craig Anderton's TSF. Encyclopedic knowledge, this. I am not worthy. You truly go to eleven!

    Sheepishly, I will confess to being an Electrical Engineer of the digital persuasion. To me, an inverter is just that, a digital device that turns 1s into 0s, and vice versa. It don't do nuance :)

    Clearly, the BAMO and other TSF descendants have proven me to be in the dark. To quote Bruce (the Canadian one): you gotta kick at the darkness until it bleeds daylight! So here is moi kicking away: I wonder what the basic operating principle is when using CMOS inverters for audio applications? Is it merely that tying the output back to the input through a relatively low resistor gets the inverter to hover at mid-supply, and that from there, any signal injecting current into the input forces the system to react, driving the output to zero out the input signal? That makes it a crude inverting amp...?

    The other thing I wonder about is why CMOS-based inverters are particularly good at generating even harmonics? Is it intrinsic to the CMOS nature of the thing? is it circuit-topology related? Not obvious to me at all.

    Any and all help will be rewarded with 10,000 good karma points, double the normal amount.
     
  10. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Part of what makes inverters work well is that they have a lower gain factor than an op amp, so they won't give you the rail banging splat at higher voltage gains that a true differential will do. Just check out the Ubescreamer project over at runoffgroove.com - it takes two inverters to do what a singular op amp stage is capable of.

    The other part is the metal oxide construction that Mr. Blackstone refers to, as well as the field effect. It's what makes both mosFET and jFET devices seem to work so well at enhancing the even order harmonics. mosFET's in particular also seem to have good parasitic capacitance capabilities.

    The other issue with the inverters is the loading. What is interesting is that you can build something with multiple inverter stages only, and it has a tendency to behave more like a fuzz at higher gain settings. But if you smack the front end with an op amp or jFET or something (Blackstone uses a TLO72, the 3 legged dog at runoffgroove.com uses a J201 - both pedals use three inverters), you get tighter distortion.

    The inverters handle boosters (and OD's and all sorts of dirt pedals) cascaded into them very well. This is great for the DIY crowd, as they can build a TSF and whack the front end with a MXR Micro Amp or the boost of their choice (or a TS, or whatever).

    I actually like the capability of getting either raucous OD or fuzz out of a singular box, so I will just drive the inverters hard - in addition to even order harmonics, you can get some really sweet soft octave effects, too! I suspect that there is some crossover distortion at extreme gains that makes things a bit crispy, too. I've never put one on a scope to check for kinks where the sine crosses, but would be surprised if it wasn't there.

    I'm not an EE, so I only know what I have informally studied about inverters. I've been playing with the TSF circuit for a long time, and wondered when other pedal builders might catch on and adopt in in their stuff. It now seems to be popping up everywhere (just in time for Fairchild Semiconductors to discontinue the "good" lower noise ones that Texas Instruments cannot match).
     
  11. pedalo

    pedalo TDPRI Member

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    .011,
    Your good karma points are on their way. Many thx.
    So: the CD4049s, I need to grab me some Fairchilds? Meaning, that seems to YABAMO (or should it be YATSF?) circuits to be what the JRC4558D is to the YATS design?
     
  12. 11 Gauge

    11 Gauge Doctor of Teleocity

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    Either the CD4049 or 14 pin CD4069 (like in the Emma pedal) work fine. The more important thing is that it be UNBUFFERED.

    The "industry standard" seems to be the CD4049UBE, but I've had much better luck with the CD4049UBCN, which AFAIK only Fairchild makes. I've only been able to find the UBE in the Texas Instruments chip.

    The TI chip isn't bad - tonally equivalent to the Fairchild. But the latter has WAY LESS hiss. If you can find a TI chip that matches the Fairchild UBCN, I'd like to know about it!

    Places like Mouser have about 1,000 to 2,000 UBCN's left, but that's it! I'm sure you could source them from an Asian distributor, but that is a lot of work (plus having to buy in quantity) for a 60 cent chip!

    IOW, there's got to be a more reasonable solution. It might make sense to just go through the unbuffered inverter line completely (as possible) and audition them all. I'm sure that Emma, Blackstone, et al are working on it. Emma has a notch filter near the output of the circuit - that might be there to knock out some hiss (I haven't calculated the notch frequency).
     
  13. pedalo

    pedalo TDPRI Member

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    If the hiss is white/pink noise, then a notch may not prove that effective.
    But I get your point: finding un-buffered quiet CMOS-based inverters is a worthy hunt.

    thx again for the information: always appreciated.
     
  14. Pillarcat

    Pillarcat NEW MEMBER!

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    Pickup height

    Hello,

    I am new here but thought I would spread my recent findings with the Blackstone mosfet.

    I was having a similar problem with a splattery note decay buzz type thing. I had my les paul serviced recently and the luthier found the pickups to be far too high. He lowered them significantly and since then the buzz has gone. The tones are much more balanced. There seems to have been magnetic interference between the string and the pickup causing all sorts of problems.

    This may be the cause of the OP's problems.

    Pillarcat
     
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