BBD Circuit Explanation

Hello All,

Here is a breakdown of the various stages of an analog BBD style delay circuit. This is overly simplified and is more concerned with the various stages and the components needed as explained in the “Delay” section of The Stompbox Cookbook. I have provided a visual reference with color coding to make the circuit a little more clear.

Input Buffer/A1-Salmon

This is the initial stage of the pedal and is run by a single op-amp. This section is very straightforward and can be used to increase gain, provide some filtering if desired and is the first section encountered by the input signal. It is biased by Bias Buffer/A3.

Pre-Filter/A2-Green

This is the pre-BBD filter section to remove clock noise. This section can get very extensive and complicated depending on how much filtering is desired and/or necessary. With with very long delays, such as those in excess of 400ms, the clock rate ends up in the audible spectrum and must be removed with a very steep filter or the resultant delays will be “noisy”. This section is also biased by the Bias Buffer/A3.

Bias Buffer/A3-Sepla

This section is used to provide a biasing voltage to the input and pre-filter op amp stages. This section also feeds the post filter with the same voltage at its non-inverting input.

Post-Filter/A4-Light Blue

This section, much like A2, is for post filtering of the clock signal from the audio signal. This section is very crucial and be quite extensive, as in the case of the Electro Harmonix Memory Man, with multiple filtering segments running through this section of the output stage. This section also provides the output buffer for feeding the next pedal or amp in your signal chain.

BBD IC Voltage Feeds-Olive-Sepla

The Bucket Brigade Delay is used to provide the delay in milliseconds for the operation of the chip. Here the chip can consist of up to 4096 capaciators that are alternately triggered to release voltage in an on/off fashion. The rate of this action is determined by the BBD Driver which is a separate clock source IC that feeds Clock 1 and Clock 2 inputs on the BBD IC. The BBD IC has many input and output paths that are similar in function on all BBD ICs and it is biased by its own trimpot.

BBD Driver-Light Orange

The BBD Driver is used to supply the clock 1 and 2 voltages to the BBD IC and to control the rate of delay. A trim pot is often used here to alter the delay time and this circuit is fairly straightforward in its design.

There is definitely more to any actual pedal design such as the addition of a compander that compresses the input signal to the BBD IC and then expands it upon output to provide consistency in the delay repeats and to provide optimal input/output signal. This section is not necessary to design a functioning pedal but in pedals that provide longer delay times, it may be desirable if signal fidelity is desired.