J. Rockett Tim Pierce signature OD (traced)

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mauman
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Post by mauman »

Here's a trace of a J. Rockett Tim Pierce signature OD that I had on hand. It's a dual pedal with a four-knob OD followed by a MOSFET "power amp" boost. This PCB is stenciled rev 5 and the pedal dates from 2015. This post is mostly for curiosity's sake since I couldn't find an existing trace. It's a decent but not spectacular pedal and there's nothing innovative inside.
Tim Pierce OD front
Tim Pierce OD front
Tim Pierce board
Tim Pierce board
Tim Pierce board with numbers matching schematic
Tim Pierce board with numbers matching schematic
Tim Pierce OD schematic.pdf
Tim Pierce OD schematic
(119.19 KiB) Downloaded 118 times
The most interesting thing about the OD side is that is uses just half of a dual op amp, and leaves the other half unterminated. Maximum OD gain measured on the bench is just over 16 dB, and it symmetrically peaks the waveform as the gain is increased. The Tone (treble) and Bass pots have a pleasant response.

The "power amp" side is advertised as emulating the power amp section of a Naylor Super-Drive 60. It's a stock BS170 MOSFET boost, much like the AMZ MOSFET or the EQD Black Eye but less flexible, with a fixed gain measuring about 12 dB on the bench. You can get either clean or distorted outputs by varying the input level to the boost. As you increase the input level, the boost clips off the top of the waveform and peaks the bottom. I don't have an SD60 so I can't say if it sounds similar.

Both sides seem to be designed for input signals at the level of a guitar pickup (< 500 mV) rather than from another OD or boost pedal in front. Which makes sense if Tim asked Rockett for a pedal to use in the studio with just a guitar in front of it. With higher input levels, the OD side just limits the output with the same waveform, while the boost side increases distortion. Maximum gain with both sides in series is ~25 dB on the bench.

Bypass switching is true bypass for each side, with the OD wired first ("On/Off") and boost wired second ("Power Amp") in an "either or both" configuration. Current draw @ 9V is 3.5 mA max with both sides active.

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Manfred
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Post by Manfred »

The most interesting thing about the OD side is that is uses just half of a dual op amp, and leaves the other half unterminated.
I think there is no single op-amp that has the same characteristics as the JRC4558DD type.

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mauman
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Post by mauman »

What surprised me was not terminating the unused half. What Rockett did is shown in figure A below, quoting from Analog Devices application note # 1957: A. This is the worst thing to do with an uncommitted op amp. Both inputs are floating and will pick up noise and the output may switch from rail to rail unpredictably. This configuration will draw varying amounts of supply current and will couple noise into the other op amps and may add noise to the power and ground traces.
Uncomitted op amp.gif
Uncomitted op amp.gif (8.72 KiB) Viewed 334 times

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ppluis0
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Post by ppluis0 »

mauman wrote: 22 Sep 2022, 03:01 What surprised me was not terminating the unused half.
The best use of that remaining opamp would have been to have a more stable reference voltage, IMHO

BTW, what is the point of making a chain with a 1N4001 and a 1N4148 in series ?

Is it something very different from having put the four diodes of the same type ?

Cheers,
Jose

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CheapPedalCollector
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Post by CheapPedalCollector »

Its possible the traces on the op amp are terminated underneath the socket as it is a double sided board. Did you check them with a multimeter?

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mauman
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Post by mauman »

CheapPedalCollector wrote: 23 Sep 2022, 00:07 Its possible the traces on the op amp are terminated underneath the socket as it is a double sided board. Did you check them with a multimeter?
Double sided board with traces visible on both sides, although under a black top layer. I was skeptical too, so I removed the IC from the socket and checked each pin on the socket with a meter. Pins 5, 6 & 7 were completely unterminated.

Jose, I don't have an insight into the use of 1N400x vs. 1N4148 in the feedback loop, they're almost identical. I modeled it in LTSpice and the only difference is a small increase (100 mV) in maximum gain with the two 1N400x compared with four 1N4148. The waveform is the same.

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