Early stompboxes were exclusively powered by batteries. Their low consumption made it possible. When most demanding stompboxes came around 1978, external power supplies, less or more stabilized, were necessary. Today, current switching power supplies are widespread because they are cheap for the manufacturer and reliable for the user. The problem is they are often noisy when used on analog equipment.
So I had to integrate the issue of power supply in my Guitar Poppa project : My products had to operate as silently as possible with any negative grounded DC power supply, and providing 7 to 12V in charge. As I moreover use both silicon NPN and germanium PNP transistors, the question of polarity was added to the problem.
From these objectives came a specific supply circuit that is in all my effects pedals. It uses an active noise filtering and decoupling cells.
Power supply circuit
in all my stompboxes
The schematic is to be read from left to right, from current sources to the active circuits.
The battery is switched standard way.
• Negative pole grounded when input jack connected.
• Positive pole connected to + Ua if nothing is inserted into the coaxial power plug.
• Otherwise, the coax plug takes over and transmits the external voltage to+ Ua.
+Ua is the positive input of the power supply.
• Decoupled by 220μF.
• Powers the LED indicator.
• Powers filtering transistor.
The AC181 transistor constitutes an active filtering device.
• Its base is controlled by the RC cell 2k2 × 47μF.
• This cell forms a filter whose efficiency is multiplied by the transistor gain.
• The filtered output current from the emitter is available on the point +Ub.
+Ub is the actual positive pole of the regulated power supply.
• Decoupled by 220μF.
• Powers the buffer stage (orange) through a decoupling cell 220R × 47μF
• Powers the active circuit of the pedal (blue).
The active germanium circuit is connected “upside down”
The PNP polarity being the reverse of the”normal” , thus its connections are pivoted :
• Vee pole is connected to + Ub (+ 8,7V) ; Vcc negative pole is connected to ground.
• PNP polarity is respected: Vcc stays negative with respect to Vee.
In functional terms, we have an electronic filter that feeds two circuits, each with its own polarity, and both separated by the decoupling cell 220R × 47μF.
This mounting may seem odd at a quick glance, and can be sometine discussed.
This is the perfect time to take stock.
Discussion
Power supply circuitry in LikeYourFace Fuzz.
8. 2.1mm socket.
9. AC181 Transistor with base capacitor.
10. +Ub decoupling
capacitor.
Decoupling = filtering + stabilization
This is to insert capacitors between each power supply node and the ground:
• +Ua ; +Ub ; +Uc, etc. …
• Anyone who has seen tube amps schematics knows this : it’s an indispensable habit.
These capacitors bypass any alternative signals on their terminals.
• They enforce an “AC zero level ” between each stage.
• This prevents audio oscillations due to feedbacks between stages.
• This contributes to the general filtering
That’s why you see lots of electrolytic capacitors on my cards!
And why they are less noisy than average.
Guitar Poppa
Go and see :
Stompbox Power Supply : Overview (article)
SStompbox Power Supply : Things to know (article)
