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PostPosted: Tue Aug 11, 2009 10:56 am 
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Below is an image of the v.2.0 Large Beaver PCB with the 5 major stages of the circuit color coded. As shown on the schematic below (correspondingly color-coded), the signal flow through the LB circuit is as follows:

Input --> Transistor Stage 1 --> Transistor Stage 2 --> Transistor Stage 3 --> Tone Stack --> Transistor Stage 4 --> Output

Components left gray are either the LED loop, the "Pickled Beaver" mod components, or the ground and +9V connection points.

Image

Color-coded schematic of the v.2.0 Large Beaver, Triangle v.1 configuration:

Image

For descriptions of the function of these stages, refer to CallMeRog's excellent Large Beaver v.1.0 Walkthrough. Though the walkthrough was written for the v.1.0 Beaver and the schematic and PCB graphics there are no longer valid, the descriptions of what happens in each of the five stages is still largely accurate. The biggest change for the v.2.0 kit is the new tone stack with its four switchable EQ settings. The rotary switch allows selection of different combinations of capacitors in the low pass and high pass filters that determine the frequencies where the treble and bass are rolled off. At a "flat" midpoint setting of the tone pot, the first "stock" switch setting has the typical Muff mid-range response "notch" that gives it its characteristic scooped mids sound. The second setting gives a flat response through the mid-range, while the third produces a "hump" in the mids, analogous to that found in a Tube Screamer (though obviously sounding completely different!). The 4th switch setting bypasses the tone stack completely, so an unaltered signal goes straight through to the final transistor stage. Consequently, there's no volume loss through the tone stack and that switch setting is noticeably louder than the other three.

It should also be noted that the "missing" 82K bias resistor for the 2nd transistor stage in the v.1.0 layout is present in the v.2.0 kit, so that the transistor is properly biased to ~4.5V.

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PostPosted: Thu Aug 13, 2009 7:19 am 
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Occasionally the question comes up about how hard it would be to build a Large Beaver that could be "switched" between the Triangle v.1 and Ram's Head configurations. Answer: REALLY HARD.

The two versions use different resistor and capacitor ratings in many places in the circuit....20 of them, to be exact, over half of the total number on the board. A total of 16 resistors and 4 caps are different between the two. So you'd either need a BUNCH of switches or a lot of sockets on the PCB (and a lot of patience changing 20 components manually!). Bottom line: if you want both Triangle and a Ram's Head versions of the Beaver, buy two kits.

Here's a summary of the differences; wherever you see red, that component is different between the two versions:

Image

Here are the component positions labeled on the PCB, courtesy of Moderator Stephen:

Image

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2


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PostPosted: Thu Aug 13, 2009 1:36 pm 
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A question occasionally comes up about the way the rotary "EQ voicing" switch works in the LB v.2.0. So here's a rundown on the subject:

As described in the Beaver instructions, this 4-position rotary switch allows you to change the voicing imparted by the tone stack, which was a common modification that builders used to perform on the original v.1.0 kit. In a nutshell, the switch allows you to select different combinations of capacitors in the tone stack stage of the circuit, changing how mid-range frequencies are processed.

The Big Muff style tone stack is different than what you find in most dirt pedals, which is most commonly a simple treble cut control, i.e. as the tone pot is turned down, more and more of the high frequencies are shunted out of the signal path to ground. Instead, the Muff style tone stack uses two RC filters, one a high pass filter (HPF) that removes lower frequencies and the other a low pass filter (LPF) that removes highs. Both are wired into the same control pot, so that turning it has the dual effect of reducing one side of the frequency range while increasing the other. The choice of the ratings of the resistor-capacitor pair that makes up each filter determines its rolloff frequency, beyond which a significant portion of the signal is scrubbed off. The farther beyond this point the signal frequency is, the greater the portion of it that is removed. For the HPF, frequencies below the rolloff point are removed, and for the LPF, frequencies above the rolloff point are scrubbed off. Consequently, the values of those two rolloff frequencies become critically important to how the tone stack functions.

For the Triangle version of the Large Beaver at the "stock" Muff EQ setting, the HPF rolloff point is 1046Hz, and for the LPF, it calculates as 408Hz. (You can see how this is calculated HERE, or plug in the R and C values HERE to have it done for you.) This means that, assuming that the tone pot is set at the midpoint, the tone stack is removing an increasing portion of the frequencies below 1046Hz, while also attenuating an increasing amount of the frequencies above 408Hz. Take note that the two attenuation ranges OVERLAP between 408Hz and 1046Hz, which means that within that interval, there will be an additive loss effect. This creates a "notch" in the mid-range response and gives the Muff-based pedals their characteristic "scooped mids" sound. (For the Ram's Head version, the overlap is even greater and the scooping somewhat more pronounced.)

The v.2.0 Beaver gives exactly this behavior at position 1 of the rotary switch, which is the "stock Muff" position. At position 2, the HPF cap is changed, and the new rolloff value is 408Hz--the same value as the LPF rolloff. With the two rolloff points equal, the additive loss notch is eliminated, so this switch position gives a flat mid-range profile. In position 3, the LPF cap changes, and its rolloff frequency jumps up to 1046Hz. Now, the 408 - 1046Hz interval is getting little to no attenuation from either filter, resulting in a mid-range "hump" in the signal. Think of it as "Big Muff meets the Tube Screamer"! :mrgreen:

In switch position 4, the signal is bypassed around the tone stack completely, so an unfiltered signal goes directly into the final transistor stage, making for a very full and open sounding (and noticeably louder) output. Refer to the green-shaded "Tone Stack" portion of the schematic shown in the first post of this thread to see how this portion of the LB circuit is laid out, and how the switching between components is configured.

Here are a diagram and a couple of tables that summarize what's going on in the switch sequence, and the impact on the circuit and tonal output:

Image

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My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2


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