F: Power Supply, Signal and Filter Capacitor Grounding Scheme
We ought to consider ground loop hum and reduce it by carefully placing our parts, rails and various types of ground wires to avoid contaminating our signal ground with charging currents from filter capacitors and other unwanted AC currents. You’ll also need a scheme to connect your signal ground to chassis ground plus Earth.
A stellar reference for this topic and also my entire project =
Audio Power Amplifier Design by Douglas Self . I can’t recommend this book enough. Click for Doug Self’s web site
Doug Self’s practical knowledge, writing skills and published experiment results provide game-changing insight into the world of audio design from small signal to PA stages.
Above — Grounding scheme. We’ve got multiple ground terms which gets confusing. On the power supply circuit board lies Reservoir Ground. Reservoir Ground is the 0 volt rail on the power supply and it’s only connected to the transformer center tap and 1 node of each reservoir capacitor. Lots of pulsating charge current flowing here.
Signal, and Electrolytic Capacitor 0 Volt Grounds on any circuit boards are connected to the Reservoir Ground by wires. I made a tee off the Reservoir Ground rail and put an Electrolytic Capacitor Ground point at 1 end — and the Signal Ground point at the other end of this tee.
This separates pulsing capacitor charging currents from Signal Ground — and by way of the tee, keeps these 2 points confined away from the 2 main reservoir capacitors. Connecting Signal Grounds to 1 point on the Reservoir Ground rail by wires is commonly called star grounding.
Apart from the power supply board, most circuit boards will have only Signal Ground, or Electrolytic Capacitor Ground.
Signal Ground on any circuit board gets connected to the Star Point on the power supply board by a wire as shown.
Grind away any copper on the power supply, pre-amplifier, PA or any auxiliary circuit boards that your circuit board anchoring hardware passes through. This prevents you from connecting any type of circuit board ground to the chassis through any bolts, spacers, washers and nuts used to mount your various boards to the chassis.
In essence, the circuit boards are “floating” from the chassis. They won’t truly be floating since we’ll connect these boards to the chassis by a special means that’s described later.
Consider the DC rails on the PA and pre-amplifier boards. At the beginning of each positive and negative DC rail lies electrolytic capacitor(s) that connect to a small 0 volt rail (Electrolytic Capacitor Ground) that is separate from the Signal Ground on each board. On my boards, I carve out a small island for the Electrolytic Capacitor Ground rail. Run a copper wire from each Electrolytic Capacitor Ground island back to it’s proper grounding point on the power supply board as shown.
Auxiliary board 0 Volt ground returns such as those that contain relays, thermal, or non-signal support circuitry get connected to the their own special tee to Reservoir Ground on the power supply circuit board by a copper wire. See the diagram.
Mains ground from now on is called EARTH. EARTH is the AC ground lug of the IEC320 C14 receptacle. Connect a thick, shielded copper wire from this EARTH lug to the guitar amplifier input jack ground lug. Thus, the amplifier input is EARTHED by this wire.
The 2 – sided copper clad board surrounding my transformer is also connected to EARTH lug of the IEC320 C14 receptacle. Since my power supply shielding box is “floating” from the chassis it will only be connected to the chassis by way of the single wire mentioned above.
Signal Ground is connected to Chassis Ground at 1 point. Some designers do this with a 10 Ω resistor. For example, you’ll see this in the Fender Jazzmaster Ultralight guitar amplifier schematic.
Another way, I learned about from Doug Self, is through a shielded input cable from the guitar amplifier input jack to the first pre-amplifier stage.
The 1/4 inch guitar amp input jack’s grounded sleeve body is usually automatically connected to the metal chassis because it’s a conductor. Check it with an ohmmeter. If you use a plastic 1/4 inch amp input jack, you’ll have to connect its ground lug with a short wire to a bolt on the chassis. Whatever you do, ensure you’ve got a solid, low impedance connection to the chassis from the amplifier input jack’s ground node.
The ground braid or shield of the proximal input coaxial cable gets connected to the guitar input jack ground lug. However, recall that the jack ground lug ( and chassis ) is also EARTHED through a wire at exactly this point — providing strong ground loop immunity.
The distal end of the coaxial cable goes to your pre-amplifier input stage. The braid at this end goes to the star grounding point on the power supply circuit board as shown. So you now have connected your EARTH, Chassis and Signal Grounds to the Reservoir Ground in a manner which helps reduce grounds loops.
G. Regulated DC for Your Pre-Amplifier Board
To garner maximum headroom, it’s desirable to run op-amps like the NE5532 and/or TL072 with more DC voltage than the doldrum standard of +/- 15 VDC. You might go as high as 17 volts on the rails. Since, I’ve got an abundance of 16 volt zener diodes on hand, I opted to build my rail voltage regulators around these. The 17V 1N5247B zener diode might also prove a good choice.
Many builder just run linear regulators and adjust the rail voltages to somewhere between 15 and 17 VDC.
I chose the field-tested emitter-follower configured DC regulator with a gain of 1. The base-emitter voltage drop of this topology lays in series with the load, so load current changes will alter the regulator output voltage. Vout = V Zener – VBE: so the regulated voltage will be under 16 volts if you apply a 16 volt zener diode. Barring catastrophe, once the pre-amplifier board is connected, the DC voltages on each rail will hold steady.
The BJT base bypass capacitor value gets roughly multiplied by the transistor’s current gain which boosts low-pass filtration of the stage to further scrub down rail ripple. The electrolytic capacitor and zener diodes 0V leads go to Electrolytic Capacitor Ground on the power supply board as discussed earlier.
Above — Pre-amplifier voltage regulators: one for each rail. I added diodes to limit the stage current to a maximum value somewhere above 200 mA. One diode compensates for the BE junction of the transistor, while the second diode limits the voltage across the emitter resistor to the diode ON voltage ( around 0.6 V ).
I’ll show the actual capacitor values + my DC measures on the pre-amplifier boards later in this series of blog posts.
H. Output Rail Notes
Above — Power Amp Output Rail diagram. This isn’t really about the power supply circuitry, however, I don’t know where else to put it. Your PA output rail receives all of the amplifier’s power and some serious current is flowing here. Authors such as Doug Self recommend that we don’t connect our 3 PA output rail networks at the emitter connection points. Make a tee as shown and then choose your 3 takeoff points. This confines the PA transistor emitter energy away from these 3 nodes.
In Part 2, I’ll present some of my notes about making and measuring a good, clean PA stage. I’ll show a small, very ugly PA I built to learn from. I also listening tested many pre-amplifier designs through this smaller PA stage while hooked to various speakers. I learned a lot & hope you’ll find my notes informative.
In Part 3, I’ll present my final PA stage mounted in a chassis, but that’s will be later this year.