Bit By Bit Boneyard Breakdown

[shawnee]

Hello forum Brothers,
   I have been thinking for a while how cool it would be if we took a circuit and broke it down and gave a brief description of what each component does in relation to the rest of the circuit. This would be absolutely invaluable for us that build but don't totaly understand how everything fits together. To understand the design would get the newer guys (like myself) on the forum to another level and maybe move us toward the simple design stage and not just build or copy. I think the Boneyard would be a great one because it sounds amazing and does quite a bit for the amount of components it contains. I have become infatuated with it but still don't know enough to try some of the things that I would like to add/change or put into another circuit. So would anyone else find this valuable? Would you more advanced guys be willing to participate? There really is no time frame. If it takes months, that's OK too. Any takers?

[mjcyates]

I think that is a great idea. I would like to see it.

[irmcdermott]

Not a circuit like a Boneyard... but it's a start.

Here Dano at Beavis Audio breaks down a typical booster circuit.

[mgwhit]

It's not the Boneyard, but here's the most detailed analysis of the Big Muff Pi I've ever seen: http://www.kitrae.net/music/big_muff_guts.html#Circuit.  It covers a lot of ground: Transistor gain stages, feedback, soft clipping, RC filters, tone stacks, etc.

Other great full-circuit walk-throughs are R.G. Keen's Technology of the Tube Screamer (http://www.geofex.com/article_folders/tstech/tsxtech.htm) and Technology of the Fuzz Face (http://www.geofex.com/article_folders/fuzzface/fffram.htm).

With the exception of the voltage pump, there's very little going on in the Boneyard that isn't covered in one of those three (most of it in the Tube Screamer).

[shawnee]

Thanks guys. I will study these links and see if I can understand enough to eventually break down the Boneyard cicuit.

[jimmybjj]

I love walk throughs. thanks for the big muff link, that one is awesome! here is one for the phase90 not as comprehensive but still informative.

http://www.circuitbenders.co.uk/forum/index.php/topic,1532.15.html

[jkokura]

Thanks guys. I will study these links and see if I can understand enough to eventually break down the Boneyard cicuit.

Please do! And when you are done anyzing, I would love to see your efforts as a new post here - break the circuit down for us!

Jacob

[shawnee]

I really love the Boneyard and find it to be the best sounding most useful overdrive/distortion made to date. I would like to know more about it to maybe add more EQ options or other tweaks. The boost is so clean that I even thought that it may be worth isolating into a small circuit by itself. Then there is the power supply with +/- voltages. This circuit really has a lot of ground covered! Anyway, I'll do my best but I'm not sure if I can learn enough to be the authority on this circuit compared to some of the pros we have here.

[mgwhit]

I've got a few minutes to knock out a simple summary of the Boneyard circuit.  I hope you'll find this helpful.

The Boneyard uses all four op-amps in the quad-op-amp LF347 chip to amplify your input signal, regardless of which mode is being used or whether or not the Boost is engaged.  The first op-amp (IC1B) is a unity gain non-inverting amplifier.  Signal goes into the non-inverting input, and the lack of any resistors in the  negative feedback loop keeps the amplification factor as close to 1 as possible.  I assume this stage is being used as a buffer, but since impedance is a subject that gives me headaches I'll leave that for someone else to explain.

The second op-amp stage (IC1A) is where most of the action is.  It's another non-inverting amplifier, but this time it has a more developed negative feedback loop.  Capacitor C4 is a high-pass filter that tamps down some of the highs in the output.  D1-D3 clip the signal somewhat.  Diode clipping implemented like this in a negative feedback loop is frequently referred to as "soft clipping", as it is not as aggressive as diode clipping that is implemented using diodes that directly connect the signal path to ground (like in a Rat).  Note that the Boneyard uses LED's, which are known for a crunchier (i.e. Marshall) tone, for the clipping diodes, and that the two-on-one-side-one-on-the-other layout produces asymmetrical clipping, which many people feel produces a more amp-like distortion.

The Crunch/High Gain switch is implemented here in the second op-amp's negative feedback, and all it does is toggle between those two pots to determine the amount of resistance in the negative feedback loop.  The High Gain pot, at 1M, can produce less negative feedback than the 100K Crunch pot.  Less negative feedback results in more gain.  For the record, this stage is set up for massive gain: a median setting on the Crunch knob will produce a gain factor of 1000, and a similar setting on High Gain will reach 10,000.

The next op amp stage (IC1D) is an inverting amplifier set for fairly low gain and is used apparently for the sole purpose of implementing the tone control in the negative feedback loop.  The Tone pot appears to control the ratio of negative feedback that is affected by the C7 capacitor.  Also note that the gain level from the preceding stage might be causing this stage to distort.  Not 100% sure about that, but it seems reasonable.

The Volume pot is implemented in a very standard way as a voltage splitter shunting rolled-off signal to ground.  What's odd about it is that it is implemented before the end of the circuit, but that's probably just a way to reduce distortion in the Boost stage that follows.

The fourth and last stage (IC1C) is the Boost stage, another inverting amplifier.  When the Boost switch is not engaged it appears to be configured for pretty much unity gain (R8 = R9 @ 22k), but when the switch is engaged, the 500K pot enters the negative feedback loop, resulting in a gain factor of up to 25 at maximum setting.

Another interesting aspect of this pedal is the power supply.  Typical 9V op-amp overdrives provide the IC with 9V and 0V forcing you use a voltage divider in your power supply to bias the signal input of the op-amp at around 4.5V.  The Boneyard's design uses a charge pump to generate -9V for the other half of the IC's power requirement.  This frees you up from having to provide the secondary bias DC voltage on all of your op-amp inputs, which is nice.  And you get more headroom from the 18V peak-to-peak power.

I have not built a Boneyard yet, but a few fantastic sounding demos have showed up here in the past few weeks (you know who you are) and it is definitely on my short list.

[jimmybjj]

Awesome thanks for taking the time. I will study it later when I get home

[shawnee]

Matthew-  Thank you so much! I could tell that there was a buffer, clipping gain stage, tone stage, and a boost but that was about it. The information about resistance in the feedback loop increasing gain was a revelation for me.

This input buffer video was pretty informative at the 5:00 mark (and there is a short part 2).
http://www.youtube.com/watch?v=cdjXT4HdVmY

[oldhousescott]

Great job, Matthew! A few clarifications and extensions if I may:

Input impedance is roughly R1 || R2 (in parallel with), or about 500k. C2 knocks out the really low lows, and C1 rolls off the high highs.

C4 forms a low-pass filter (not hi-pass). A typo, I'm sure.

R7, C7 and the tone pot form a variable low-pass filter. Even at full CW rotation there is some cutting of the highs as the cap puts R7 and the tone pot in parallel, reducing the gain for high frequencies. This is not uncommon as, in screamer circuits, you'll often see fixed caps to ground after the clipping stage to dump some of the scratchy highs created by the clipping.

Bipolar supplies are nice, too, because it allows the designer to use fewer coupling caps between stages and to make direct connections to ground for biasing. As they say, "no cap is as good as no cap".

[gtr2]

oldhousescott or others

Is the opamp stage at the end of the circuit primarily for the boost gain stage or does it also function to maintaining the proper impedance regardless of the volume pot position?  Like the some of the others, impedance always "loads" me down.  My reading on the subject always leaves me scratching my head. 

Josh

[mgwhit]

C4 forms a low-pass filter (not hi-pass). A typo, I'm sure.

Actually, I intentionally wrote high-pass, but I may be mis-using the term for this application.  Or I might just be dead wrong.

What I meant is that C4 lets highs pass through the negative feedback loop without having to deal with the soft clipping and voltage divider.  Since these highs are still part of the negative feedback, I assume this tames the highs more in the output of that op-amp stage.  It actually functions as a traditional RC low-pass filter in that it passes the lows (in this case the majority of the signal) on to the remainder of the negative feedback loop.

Thanks for your input -- and please let me know if I'm still wrong...!

[Haberdasher]

Great thread guys.  As soon as I get a chance I'm going to sit down with the schematic and try to make sense of all this.  Much appreciated!