Friday, 4 January 2019

Clean Jazz Guitar Amp Builder Notes — Part 1: DC Power Supply


— D I S C L A I M E R —

The text, diagrams and images on this blog are presented for information purposes only.  You are entirely responsible for any damages, harm, or injuries that arise from any equipment you build, or from any experiments you perform.  —  Experiment and build at your own risk —

Building a power supply & audio frequency equipment poses a serious risk of personal injury, death, destruction of personal property and for fire. Safety first !  Always keep children and pets away from your bench. 

Further, all home built equipment should never be connected to your household Mains AC electric system when not in use. Connect your AC power supply containing devices to a suitably rated, commercial switched power bar, or switch system that is certified/approved in your Country. For example by the CSA Group, ANSI, ASME, ASSE, ASTM, ASFE, UL, NSF etc..  Turn your device AC supply on and off with this power bar/ switch system —  or better yet, unplug it when it's not in use, or unattended.   

Work safely!  Prevent accidents and fires.  Keep an annually certified fire extinguisher in your home shop, or lab at all times in case a fire starts.

Above — The fire extinguisher that hangs in my lab. We also have 2 others: 1 next to the electric clothes dryer & 1 in the kitchen. According to local fire officials, these 3 places are the most likely areas for a fire to start in a non-smoking home.

A. Basic  Power Supply Goals

[1] Electrical safety.
[2] Low ripple.
[3] No amplifier hum, or noise caused by ground loops, or inductively and/or capacitively coupled rectifier pulsations.
[4] Manage heat and/or prevent expensive component damage from thermal runaway, or a catastrophic parts or design failure.

I'll show my pursuit of a split DC supply for a jazz guitar amplifier [ ~22-24 Watts into an 8  Ω load ].  I learned a ton making this power supply, and, of course, this knowledge will boost the success of obtaining the above 4 goals in future designs.

B. Transformers 

In 2018, I bought 2 suitable iron transformers: a toroidal + conventional frame type. Likely, the transformer is the most important plus costly object in your whole amplifier build.

This blog will lean towards low power clean jazz guitar amps, however, all the same principles apply for higher power guitar amps.
I show how I measure my amplifier power ratings near the bottom of  this blog page:  Click here



Above 2 photos — My 2 transformers.  I purchased the "Torel" toroidal unit on eBay from hansurmann from the Russian Federation.  I purchased the Hammond 'heavy weight' locally & mainly because I got a very good deal on it.  The heavy Hammond transformer will either go in a lab power supply, or perhaps a combo amp that never leaves the lab.  It's a well built transformer that offers separate bobbins for the 2 windings eliminating the need for an electrostatic shield between the primary and secondary coils.

Hammond Manufacturing also offers a good selection of Toroidal transformers to view and to learn more about them.  Click here.

Transformer power rating isn't usually expressed in Watts; rather VA (Volts [RMS] and Ampere units). So my Hammond center-tapped transformer as shown is 30 VAC * 5 Amps = 150VA. This would work very well for a 4 ohm load guitar amp.

Your transformer's secondary AC voltage and rectified DC voltage varies with the diode type + rectifier topology, how you filter the pulsating DC into low-ripple DC voltage  — and the load on the power supply .  If you seek to make a 'specific' wattage guitar amp, this makes choosing your transformer output AC voltages a little more difficult than you might like.

Hammond Manufacturing offers a file with some math and rectifier circuits to make this job a little easier:  Click here

For a ~22 Watt 8 Ω jazz guitar amp. You'll roughly need +/- 23 to 24 volts on your power amp stage DC rails (under heavy load) to get that maximum clean power.
For this project, I do not regulate the DC supply going to the final power amplifier.  Only the pre-amplifier stage op-amps get regulated DC.  I think this is typical for guitar amplifier power stages.  Unregulated DC on your PA rails offers a proven, reliable strategy, and of course, gives lower cost & complexity than when you put high power linear regulators on the power amp stage DC supply.

Unregulated, you won't achieve the somewhat theoretical doubling of maximum amplifier power when going from an 8 to 4 Ω load, since the current draw doubles and this results in a greater voltage drop in the DC supply system.  This also may occur because of AC signal factors: mainly increased PA transistor beta drop which I'll discuss in Part 2.  As aforementioned, you need to consider many factors when choosing what DC rail voltages you'll need to get a certain clean signal amplifier power.

When driving our PA stage, we suffer DC power losses from many factors. These include transformer winding resistance and leakage reactance, I squared R energy losses in your power supply system wires + any resistors including PA emitter degeneration resistors and so forth.

Above — A simple bench setup to measure AC + DC voltages on either of the DC supply rails. I'll use this to measure my positive DC rail voltages -- and later,  peak-peak voltages [ AC ripple voltages ] on the positive rail.

Above — DC measure of the positive rail of my power supply that's connected as shown in Figure 1.  The signal generator is switched off, or to 0 output.  The unloaded positive rail measures + 27.1 VDC.

Above — DC measure of the power supply when connected to a PA driven to maximum clean signal power [ 23W ] by the 1 KHz signal generator.
The unregulated positive rail has sagged to 23.1 VDC under this heavy load.  We'll use the above measure when we calculate % ripple in Section E. Reservoir Capacitors and Ripple.

You can't just casually order a transformer like you would some power resistors. Also, the cost may shock you; especially if you live in Canada.  Making a power supply takes some thought & planning.  Think about your transformer's VA, weight, mounting requirements, RMS AC output voltage, what maximum peak - peak signal voltage, you seek, plus other factors such as whether or not you plan to put the power supply right on the amp chassis, or sit it on the floor of a combo amp with patch wires connecting the power supply to DC power rails.

As you go up in amplifier power rating & apply bigger transformers that provide a larger split DC voltage with more current, your build increases in difficulty. Heat sinking requirements,   I squared R losses, the need for thermal protection/current limitation circuitry -- and the potential for unwanted noises all go up.  This is especially true if you wish to drive a 4 Ω load.

No one showed me how to make a solid state guitar amplifier, that's why my blog needs to be taken with a grain of salt.  Experts and critics may just lurk and smirk —  however, I welcome constructive criticism from any readers who know more than I.  We all learn that way.

That's why I chose a  roughly 23W into 8 Ω clean guitar amp for my first build. Keeping the power down hopefully will allow us to enjoy success in order build confidence, plus gain the skills we might apply to future higher power amps if so desired.
  
C.  Schematic

Above — Power supply schematic. For this project, I used the center-tapped Torel toroidal transformer that was listed as 50VA with + 18, 0,  and -18 VAC RMS output.  Mine measured 20 VAC peak -- and rectified + filtered under load, I get a nominal +/- 25 VDC output.  The panel mounted fuse is connected to the hot polarized AC Mains plug.  This power supply gets used in all experiments shown on this blog page.

Above — Measuring the positive DC rail with no load on a DVM.  I built my power supply enclosure from 2 - sided copper clad board.  The lid is off for testing and photography.  Although toroidal transformers leak less magnetic flux than conventional frame types, they still require electrostatic shielding to prevent capacitive coupling to nearby circuitry.  Sometimes moving or rotating them may reduce their hum field.  In general, keep you power supply away from your signal path circuitry; and especially your pre-amplifier.

If you consider the wires that lead to and from the transformer, the rectifier diodes and the reservoir capacitors as a loop — the smaller the loop area, the better for EMI prevention.  A loop potentially may inductively couple the transformer pulse currents into your nearby circuitry causing an annoying background AF buzzing noise that cannot be filtered off with bypass capacitors.

I'll go through the circuit starting with the Mains AC supply.

Years ago — for anything I make that uses AC mains electricity.  I switched to only using an IEC320 C14 receptacle — plus a cord containing an IEC320 C13 appliance plug on 1 end, and a grounded NEMA 5-15P polarized plug at the other  Since they're commonly used on personal computers in North America,  you seemingly can always find a cord if you forget or misplace one.


Above — Another view of the power supply with the cover off.  The IEC320 C14 receptacle contains a 10 amp DPDT switch.  The 2-sided copper board is well soldered on both sides so it's technically RF tight around the transformer and mechanically solid to strongly support the AC  receptacle and fuse.


Above — The IEC320 C14 receptacle with DPDT switch & the toroidal transformer mounting hardware.

D.  In Rush Current

When you turn on and energize a power transformer, an instantaneous surge of current gets drawn to charge up the reservoir caps and to magnetize the transformer.  The magnitude of the in rush current is not related to the amplifier load, rather it's dependent on the point in the Mains AC wave cycle when the transformer is switched on.

On the (low resistance) primary coil side this puts potential strain on your switch and fuse -- some larger toroidal transformers will even snap off a house circuit breaker the odd time.  Thus ensure you use a slow blow panel fuse, plus a well rated switch.  For example, I used a 10 amp @120 VAC rated switch.  Perhaps overkill for a small guitar amp, but you get the picture.

On the transformer secondary coil side, your rectifier diodes and reservoir caps get strained during any start up surge.  Any rectifier diodes must be tough.  I initially purchased a commercial 25 Amp rated diode ring that featured a heat sink and mounting hole. Sadly, this part tested defective. I looked in my rectifier parts drawer and saw about 40 Vishay brand 1N5822 3A Schottky barrier rectifier diodes and then studied their datasheet online. These should well handle my voltage, current needs and any brief inrush surge. They work well for my particular guitar amp. I have no idea when, or who I bought them from however.

I put a slow-blow fuse on each AC secondary to keep the transformer alive should the diode ring ever go shunt to ground.  Currently, I've got a 2 Amp slow blow fuse in each slot. The X capacitors are to bypass any HF noise created by the switching diodes from going into the house wiring.
I used hulky capacitors because in the presence of AC bias, some small size ceramic caps may not hold their capacitance value.  These 1 kV caps are cheap and easy to find. Carefully check the datasheets for any parts you place in your power supply and consider avoiding 'no - name' parts.  It's really sad when you fry a $50 transformer because you decided to save 50 cents by plying a, cheap no-name, part.


Above — The improvised diode ring.  I'll shorten the wires even more.  Sadly, I misplaced my 0.01 µF/ 1kV  X capacitors and had to order more. No doubt, I'll locate them the day the fresh capacitors arrive.  I'll shorten the diode ring wires when I install those X capacitors.

E. Reservoir Capacitors and Ripple

The main question we all ask — how big should my reservoir capacitors be?  Professor Ken Kuhn wrote a great pdf document for his students.  Click here

I consider this document essential reading.  Also check out his fantastic web site.

Some audiophiles get carried away with 'requiring" ultra low ripple at high power.  From Ken's article, consider aiming for medium to low ripple and use the smallest capacitance that will achieve that goal.  Higher capacitance increase surge current as Ken mentioned in his article. More capacitor = more dollars too!

Ripple is a nuanced number that's conditional and contextual. DC supply ripple increases in tandem with PA stage current draw. So ripple could be quoted with no power supply load, with the amp at 1/2 power, full power and so on.  For a clean jazz guitar amplifier, the best case seems to be with no applied test signal and the worst case occurs when you apply the maximum test signal before the amplifier sine wave distorts. This is often quoted as the maximum signal drive where amplifier THD is under 1%.

On the web, you'll find a number of write ups and videos how to calculate percent ripple, or, the reservoir capacitor value you need to get a certain percentage ripple. Sometimes they lack precision and seem a bit theoretical. We amp builders need a practical, measurement-based way to evaluate ripple.  I'll show some simple oscilloscope measurements and apply Professor Kuhn's formula below.  It seems to work OK.  It's also fun to actually view the ripple on your DC rails with various reservoir capacitor values and power supply loads. Visceral stuff.

Above —  The formula taken from Professor Kuhn's document.  To get data to calculate, you'll need a power supply, a signal generator, a PA stage, a dummy load and a oscilloscope or DSO. I've already shown how to measure rail DC with a 'scope DC coupled to your rail earlier in Figure 1.   Again, I'll only show the positive rail.

I place additional ripple filtration capacitors on both my pre-amplifier and PA boards. We'll only consider the PA board.  As shown in my power supply schematic, my main reservoir caps are 6800 µF.  On my PA board lie additional capacitors and on my test 23 W power amplifier,  I've got a 2200 µF /50 volt electrolytic capacitor on each of the DC supply rails.  So per rail, that's a total of 9000 µF.

Above — DSO screen capture with an AC coupled 10X probe on the positive rail with no applied signal drive. This would be the "best case ripple", but seems totally unrealistic as the PA is drawing only 16 - 20 mA quiescent PA bias current from the power supply.

Above — A DSO capture of the AC signal on my positive DC rail using an AC-coupled 10X probe. The V peak-peak = 660 mV.   This is 'worst case' ripple as my PA is drawing maximum current; a heavy load for the DC power supply.

Using Ken's formula, let's calculate the percent ripple from the data we've gleaned.

Ripple = 100 *  RMS Ripple Voltage / Average DC Voltage

Your DSO may calculate and display the RMS ripple voltage, however, let's assume you've got a 25 year old oscilloscope with no math functions.  We might simply just estimate RMS ripple voltage by taking the measured V peak-peak value and dividing it by 3.  We bring the 'worst case' DC voltage from the earlier DC measurement of the positive PA rail.

Ripple = 100 * (0.66 / 3) / 23.1 = 0.952%.

On a popular (but unnamed) jazz guitar amp I recently measured,  the AC ripple signal was around 2 V peak-peak with maximum clean drive signal into a 8 Ω resistive load. So I did OK.

This simple method allows you to measure and crudely calculate ripple percentage to make comparisons with different capacitors in real time.  So my 9000 µF of C in a low-power jazz guitar amp ranks as ultra-low ripple at 'worst case'.  Of course, if you go for a 4 Ω load, it could be a game changer.  More on that in Part 2 of this series.



Above — My 2 main reservoir capacitors.  Installing them proved a little time consuming. I built my power supply using Ugly Construction on 2-side copper clad board. Since you can only solder their short, thick leads on the bottom side of the main circuit board, I had to improvise.  I cut Cu islands for the DC rails and O volt pathways on the top side.

I also had to cut islands on the bottom side of the Cu board.  I connected the positive, negative and 0V top islands to their respective capacitor leads below by joining the top and bottom carved islands with via wires. I drilled 5 via holes for each lead and soldered 20 gauge solid copper wire to join each appropriate top and bottom island. I'm used to doing this to provide a low impedance ground in UHF circuits, so it's not a big deal for me. After final testing with an ohmmeter, the 6800 µF caps were installed. I did the soldering with my 80W iron.

I installed the filter capacitors by drilling 1 hole for each cap lead into each of appropriate carved islands and then soldered each capacitor lead to the bottom copper board.

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 builders 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.

Click for my Guitar-Related Index