Some additions and improvements as promised last month. These improvements consist of a two-channel buffer amp which defines the input impedance at about 130k ohms and the output impedance at a little over 1k ohms. Also, a power supply filter which permits the buffer amp to run off the signal-light supply without picking up noticeable hum.
The relatively high input impedance makes the choice of guitar leads a little more critical, but may give your guitar a more impressive sound, depending on what sort of amplifier you use.
The low output impedance makes the operation of the "Link" switch more predictable and makes it easier to fit isolating output transformers, for additional safety and to prevent hum loops when driving two different amps.
I have gone to considerable trouble in the design of the buffer amps to minimise interference from mains, lighting cables, and unwanted radio transmissions. They provide less noise and hum than most guitar-amplifier combinations, and under most circumstances will handle a signal of three volts peak to peak. This is the highest level I have ever known to come from a guitar pick-up and at least 60 times the nominal input needs of most guitar amps.
Jermyn, (Contact Details)
can supply all transistors (including BD518) and the mains voltage capacitor for £1.49 inc. tax, if you telephone your order and give your Barclaycard no. (or for £2.24 including tax if you send written order and postal order.) These components are marked with an asterisk on the list.
Construction of circuit boards and new wiring.
Remove the mains lead. All wires should be insulated unless otherwise stated. Remove base, and place lid with circuits face down on the bench.
1. Remove the chassis lead from the lighting circuit, cover with an additional piece of heavy insulating sleeve and connect to the "sleeve" tag on the nearest guitar input socket. ("Sleeve" tags are nearest to the end panel, then "ring" tag if fitted, then "tip" tag at the back of each socket). This lead connects to the guitar strings and therefore to you! The extra sleeving is to counter the remote possibility of the earth lead fouling the mains socket and damaging its insulation.
2. Unscrew the power supply, remove its lid, and connect a wire to the metal clip which holds the mains transformer together, either by soldering top centre, or by a tag under the mounting screw at the opposite end to the wiring.
In either case, scrape off all varnish around the contact area. Take the lead out alongside the six volt output wires, cut it, leaving about two inches, and fit a 4BA tag. Remote the screw, holding earth tags to the chassis, add the third tag and replace screw and nut tightly.
If you are uncertain of mains wiring techniques, omit this stage and do not modify the power supply. The equipment will work just as well, but may be slightly more sensitive to mains-carried interference.
3. Build the hum filter board. Don't be surprised if your components are smaller than mine; I deliberately used the largest you are likely to find. Perhaps I should begin with some notes on soldering veroboard.
Both boards use a standard size of 0.1 pitch vero, as supplied to radio-parts shops by Norman Rose Ltd. If you cut your boards from a larger piece, use a junior hacksaw with a new blade, and cut along the holes, copper side up.
The outline of the filter board on the diagram shows the edge of the board; copper strips are indicated by numbers, and crossrows of holes by letters. It is relatively easy to blob solder across two adjacent strips. To prevent this, I suggest you hold the board horizontal while soldering and use a miniature iron between 10 and 20 watts with a tip of 2mm or 1/16 inch wide, or less. Standard "Multicore Savbit" solder is use-able but the finer gauges are more convenient. If you still have trouble, Desolder Braid will soak up any unwanted solder. The diagram shows the board with components facing you and copper behind. Significant parts of the copper strips are shown shaded, (as if the board was held up to a strong light). It is not necessary to remove unshaded parts. Begin by cutting through the copper strips at the points shown by an X. The easiest way of locating the correct hole on the back is to count from the front, push a pin through the hole and turn the board over.
Remove the pin slowly while putting the cutting tool in the same hole. Rotate the tool clockwise until it has cut the copper strip completely at both sides, but stop before it damages adjacent strips. The cutting tool can be purchased, or made from a new
1/8 inch drill in a small handle. There are breaks at: 4C, 6C, 5D, 4X, 6X, 5W and 2P. You will also need clearance holes for 6BA screws at 5B and 5Y. Check carefully around any cut edges or cut strips for copper shavings leaning towards adjacent strips. Cut these off neatly using a scalpel with a strong stubby blade, or a similar modelling knife. The next step is to insert (from the copper side,) single-ended, headed, terminal pins at 2F, 8F, 2Y, and 2Z. Insert small components, checking each one first if you have suitable equipment, and solder. Then add the two large capacitors, which may overlap the board if necessary, and solder these, and the terminal pins. (Yes, the two capacitors are
pointing in opposite directions). The highest part of each transistor should be about 15mm from the board.
This hum filter is an ingenious circuit attributed to Clive Sinclair many years ago, and could be considered as a form of gyrator. That is to say the transistor pair converts one of the capacitors to act as an inductor. Its value is proportional to R.re and at low currents, is very large indeed. Together with the second capacitor, it reduces the hum level carried to the output sockets, to considerably less than that coming from the guitar. When manufacturers start screening their guitars, I will sort out a better supply for you. Until then I think this hum filter and a cheap commercial power supply, represent the best value for time and money spent.
The filter has a "warm up time" of about one minute before supplying power to the rest of the circuit, and may need ½ to one minute after a temporary interruption of the mains supply, as may be caused by a dirty, or loose plug.
The transistors are cheap and convenient but could be substituted. The larger transistor should have a gain of 50 to 100 at about 10 m.A. and the smaller driver transistor needs a similar gain at 20 to 50 microamps
. Both are P.N.P. silicon types.
4. Build the buffer amp board. In this part of the circuit, transistor changes are entirely at your own risk. The recommended Motorola 2N 5087 and 2N5210 are sophisticated low noise transistors, which are necessary to give a reasonable performance with the wide range of source impedances likely to be connected to the inputs. If you wish to experiment, the nearest similar transistors are probably Texas BC 214 L.C. and B.C. 184 L, although the lead identification will be different.
Most of the components on this board are mounted, standing on end. This brings the solder joint very close to the component body and there should be a minimum of 2 mm. between component and board. Otherwise construction is similar to the filter board except that the largest capacitors should be fitted before the others. The cuts in the copper strips are easier to place, than on the filter board. The entire L and O rows are cut through, also the points 10 F and 10 U. You will need clearance holes for 6 B.A. screws at 3B and 3Y.
The terminal pins at inputs 1 and 2 have Ferrite beads glued over them and so must be longer than the others (about 8 mm.) As the input wires connected to them are very light, these pins can be made from thick copper wire.
If you get glue on the terminal pin, it can make soldering difficult. I suggest you solder the pins in place, then run a small ring of Clear Bostik or Clear Evostik around the BASE of the pin and press the bead into place. It should hold firmly within half an hour. You will find if you use the recommended transistors, that the lead identification of each one is marked on the body or brass tab, which should avoid confusion. Other types which look very similar may have different lead arrangements. Also, the two output capacitors C4, marked 4.7/uf are Tantalum Bead types. These have — and + leads and must be connected as shown. The method of lead identification varies with different makes and you should be quite sure of which lead is which before buying. (The + lead is sometimes called "Anode").
After all components and pins are connected, solder a tinned copper wire right across the board, joining all the M to N strips and the part-strips at top and bottom edges.
(Click image for higher resolution version)
5. Hold the two completed boards up to the aluminium screen and mark suitable points on it, for attaching the boards. Remove the screen and fit 1/2 inch by 6 B.A. threaded bushes (like very long nuts) with 1/4 inch by 6 B.A. screws from the back of the screen. Check that the bushes line up with the holes in the boards: if not, file out the holes in the screen — not the board.
6. Replace the screen and fit the filter board, also with 1/4 inch by 6 B.A. screws. Connect it to + and — lighting wires and link the connection points with a 0.2/uF plastic film or ceramic capacitor of 50 volts rating or over. Connect a 1k resistor across "- out" and "+ out" pins and attach a voltmeter across the resistor leads set to 10 volts full scale. Connect the mains lead, STAND WELL CLEAR AND PLUG IN. If the meter reads about 5 volts after about 1 minute, all is well: if not, check for errors. IN ANY CASE, DISCONNECT MAINS BEFORE TOUCHING ANYTHING. Now remove the 1k resistor.
7. Solder two screened wires to the IN and E pins of channels 1 and 2. The braidings go to the pins marked 'E'. Screw the board in place and connect it to the "+ out" and "- out" pins on the hum filter board.
8. Remove the two insulated wires from the ring and tip contacts of the stereo socket and solder them to the Out 1 and Out 2 pins on the amp board.
9. Clean up the loose ends of the input leads to the amp board. Connect both screens to the sleeve contact on the stereo socket, and the inner wires to the tip and ring contacts. You can also replace the links between the sockets with screened wire, but there is little improvement.
10. Solder a capacitor of 0.01/uF across the live and neutral terminals of the mains socket. This capacitor must
be rated at 275 volts A.C. and be suitable for continuous mains operation. A capacitor rated at 275 volts D.C. will probably work for some weeks and then blow itself, or the nearest fuse, or both, and may start a fire in the process.
11. Dress all the long leads between switches (and output leads from the amp board), close to the top panel, between the switches and the front edge. They should be as close together as possible to minimise induced hum from the magnetic field of the mains transformer.
12. Replace the cover, connect to the mains and wait one minute for the power supply. Connect the switch box to guitar and amplifier(s) and check for correct operation. The "link" switch will link the outputs and connect them to whatever input(s) are selected. If both channels are switched on, each is halved by the link circuit, which keeps the total output roughly constant. If you link in an input with nothing plugged into it, you are mixing in a "nothing" signal and the output level will drop. So if you have a "mono" guitar, connect it to both
mono input sockets with a 1 into 2 adaptor, perhaps via two different effects units.
Don't be surprised if your guitar seems to have a bit more punch when using a high quality, straight lead. The input impedance of the buffer amp is fairly tightly controlled at about 130k ohms for just this reason. Poor quality leads, particularly long "coiled" ones, may lose some treble and add some hum and crackles. A lower input impedance loses some of the hum and crackles of a poor lead; it also loses most of the guts from the guitar's sound. I would rather use a good lead, which in any case is likely to be more reliable.
If the box does not work correctly (or at all), suspect wrongly placed components, solder joining adjacent copper strips, wrongly placed transistor leads, overheated screened cables or small capacitors (in either case the result is a short-circuit), incorrect wiring runs, and in desperation, dud components.
This is fairly common, but by far the most difficult to trace. It is really worthwhile to check each component before fitting. If you still can't find the trouble, you will have to shout for help, and you will need a specialist audio engineer. You can't expect a general serviceman to want to rebuild a "one-off" piece of equipment.
Next month I will describe how to tit the output isolating transformers if they are required, and some reasons for the choice of resistors feeding the link switch.
|Resistors: (R.S. Components or DORAM 1/8 watt High Stab — 10%) |
|4 off ||270k |
|1 off ||330k |
|1 off ||27k |
|2 off ||1.5k |
|2 off ||1k |
|1 off ||100 ohms. |
|2 off ||470/uF. 10 or 16 volt Siemens B41283 Electrovalue |
|2 off ||4.7/uF. 16 volt Tantalum Bead R.S./DORAM, or Siemens B45134 Electrovalue |
|2 off ||0.047/uF. anything 50 volts or over such as Siemens MKH or R.S./Doram 400 volt polyester. |
|2 off ||100 pF. Polystyrene |
|2 off ||470 pF. Polystyrene. |
|1 off ||0.22/uF. Axial lead polyester — anything over 50 volt |
|Ferrite Beads: |
|2 off ||R.S. Components or similar. |
|Terminal Pins: size 0.040 about 12 off. |
|Veroboard 0.1 matrix see text |
|2 off ||2N 5087 Motorola Jermyn Industries |
|2 off ||2N 5210 Motorola do |
|1 off ||2N 3906 Motorola do |
|1 off ||BD 516.518* or 520 Motorola Jermyn. |
|4 off ||1/2 inch bushes (Pillars) 6 B.A. Electrovalue |
|4 off ||1/4 inch screw 6 B.A. Cheese Head Electrovalue. |
|Special Capacitor: |
|1 off ||0.02/uf. 275 volt A. C. Suitable for continuous Mains operation, such as Electrovalue 1,000 volt range, or preferably a specialised RIF A type from Jermyn. |
| ||RIFA type PME 271 0.022 (or 0.01) |