Decibel Dungeon


There is little doubt (IMHO) that the inverted Gainclone works better when used with either a buffer or a pre amp. I have written about my buffered Gainclones so I am adding this page on how to build a pre amp to go with a Gainclone.
Of course, the pre amp described below will work well with many other types of power amplifier but as many Gainclone builders are new to building hi-fi, I thought that a detailed description of adding a pre amp would not be out of place in this section of Decibel Dungeon!

A block diagram showing stages of the preamp.
A block diagram of the pre amp.

The pre amp is little more than a source selector, a means of controlling the volume, and a buffer. And of course, we will need a power supply for the buffer (with two stages of regulation) so I will describe how to build one with discrete regulators that should sound better than the chip types although you could use those if you want to save yourself some work.
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The housing.

Well, we may as well start with the case as it will dictate what goes where and if it will all fit inside. I'm going to be describing the pre amp that I built and you can see that the case is home-made (to match my GC mono blocks).

My GC preamp to match my GC monblocks.
My GC pre amp to match my GC mono blocks.

If you do not have the skills/time/tools to make your own case consider something like an aluminium box or an old hi-fi case. You don't need too much space inside; the volume control and selector switch don't take up much room and the buffer circuit and PSU are quite small too. As you can see, I fitted all this into a 4 inch (100 mm) diameter tube about 12 inches (300 mm) long, although the PSU is in another case.
There are pros and cons to putting the PSU in a separate case. Some like to keep all the AC power well away from the signal circuitry. It may also be an advantage to keep the transformer (with its magnetic field and mechanical vibrations) isolated from the signal circuitry. On the other hand, two cases are needed instead of one and there is the additional expense and work in providing the sockets/cables to go between the two boxes. So whether to use one case or two is a personal decision and often dictated more by the practicalities than anything else.
Planning is the key to success here so whatever case that you use, make sure that you sit down before you begin the project and work out on paper where everything will go and how it will connect to each other. Remember to leave room to get your fingers (or a pair of needle-nose pliers) and a soldering iron in where you have to solder connections.
Other things that you may need to complete your case include the connectors, phono for interconnects, and either a mains connector if you have the transformer in the same case, or a three pole connector if you bring in the supply for the buffer from another 'box' as I have done here. All these connectors need to be on a non-conductive panel (plastic or similar) unless the sockets that you use are insulated.
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The PSU (part 1).

OK, let's get the PSU out of the way first as it is quite important. Based on experience, I strongly recommend a dual regulated power supply for a pre amp. Recently I tried using the buffer circuit that I have chosen for this pre amp project with one and two stages of regulation and I could clearly hear the improvement that having a second stage makes.
The first part of the PSU is exactly the same as the supply for my active crossovers but I'll put it here for convenience.

Preamp psu design

The components that you will need to build it are: (codes shown are Farnell Electronics unless stated as Maplin). I believe that both these suppliers will ship worldwide but if you want to purchase locally, you should have enough details of the components to do so.

If you do not fit the components recommended below, make sure that the alternatives are safely rated. All capacitors must be rated at a minimum of 35 volts DC or above. The resistors between the transformer and rectifier must be rated at 5 watts or above. The other resistors must be rated at .125 watts or above.

Mains lead (and plug)

Use a length of flexible mains cable or make your own by plating together three lengths of insulated wire and enclosing them in some heat shrink tubing.

A - Fuse holder and fuse

Use an open type fuse holder with a cover (Maplin codes KU30H and VJ57M) and an anti-surge fuse rated at 1 amp (Maplin code GL91Y).

ANTI-SURGE FUSE - when electrical equipment is switched on, there is a temporary inrush of current (especially true when using toroidal transformers and large value capacitors). Although it only lasts very briefly, this current is higher than the level of current drawn during operation and can blow a standard fuse. The anti-surge fuse is designed to withstand this initial surge of current but will blow if there is a sustained rise in the current level caused by a fault.
B - Mains switch (DPST)

Depending on the type of case you use and your skills/tools, it may be easier to fit a switch requiring a circular aperture (140-621) rather than rectangular (157-788).

DPST - stands for Double Pole Single Throw. With a double pole switch, both live and neutral conductors are switched for extra safety.
C - Mains transformer - 50-80VA 18-0-18 (step down)

I prefer toroidal type transformers, others prefer laminated and someone I know who has a lot of experience upgrading hi-fi swears by C-core types. For this project I recommend the Antrim brand toroidal transformer supplied by Maplin, code number DH59P. Antrim transformers are one of the better quality brands of toroidal transformers.

D - Resistors

The rectifier transforms AC to DC. A silicon rectifier is a semi-conductor, only letting current go through it when the output voltage is more than 0,5 Volt below the input. At that very moment it switches "ON". Inside a semiconductor there is a capacitance as well. This capacitor, together with the inductance of the wiring, causes a very sharp peak voltage to occur. This peak is too fast to be dampened by the connected (electrolytic) capacitors.

The solution is simple, put a resistor between the transformer and the rectifier. The value is not absolutely critical but I suggest for a pre amp supply, a value of 10 ohms and rated at 5 watts (987-580)

A resistor rated at 1 watt would be high enough for this pre amp but I couldn't find a suitable example in the Farnell catalogue, so I have specified the 5 watt.
E - Rectifier diodes (4 off)

Four rectifier diodes form the bridge rectifier which converts the electrical supply from alternating current to direct current. Note, that at the same time the rectifying stage changes the voltage by a factor of 1.414. I am currently using ultra-fast soft recovery diodes (367-357) which perform better than the standard equivalents.

BRIDGE RECTIFIER - 2 full-wave rectifiers connected together so that, both a positive & a negative voltage source is available using only 1 transformer secondary winding.
  • F - Smoothing capacitors (2 off)
  • The smoothing (or reservoir) capacitors store the electrical power (when there is a lower demand by the equipment) and release it at times when there is a large demand. A good compromise between performance and cost is to use the types made by BHC Aerovox. 4700 micro farads (536-192) is sufficient for this pre amp supply.

    MICRO FARAD - which is abbreviated as uF, is the unit of measurement used for capacitors. It can be divided into pica farads (pdf) (1/1000,000ufd) or nano farads (nf) (1/1000ufd).
    G-K - Bypass capacitors (2 off each)

    The large value smoothing capacitors should be bypassed with smaller values. I have used 4.7ufd polypropylene's, part number JY82D from Maplin which are in turn bypassed by 1ufd (286-916), 0.1ufd (286-886) and 0.01ufd (106-780) polypropylene's.

    POLYPROPYLENE - a type of dielectric (insulating material) used in capacitors. Usually considered better than polyester and polycarbonate. Polystyrene is also considered to be good.

    0.01ufd=10nf or 10,000 pf.
    LM317 - Voltage Regulator (1 off for positive rail) LM337 - Voltage regulator (1 off for negative rail) RAIL - the name given to the conductors carrying a voltage, eg positive, negative or 0 (zero) volt.

    As their name suggests, these devices regulate the voltage supply. Start off with the standard LM317 (412-132) and LM337 type (412-223). There are more exotic and costly regulators but I have not yet tried them.

    Voltage regulator pins Make sure that you connect the regulators correctly as their pin arrangement differs. Looking at the LM337 from the front, the pins are (from left to right) Adj, vIn and vOut. The LM317 viewed from the front also has the Adj pin to the left but followed by vOut and vIn.

    N & P - Resistors to set regulator output

    The LM317/337 regulators are adjustable types which need to have their output set using a voltage divider consisting of two resistors. The output of this power supply is to be + or- 18 volts so, starting with a UAR value of 220 ohms (513-866) we can calculate from the formula

    Vout = 1.25*(UAR+LAR)/UAR

    that the LAR value should be 3K (514-135).

    VOLTAGE DIVIDER - a circuit, usually consisting of two resistors which splits the voltage into two. In the case of an attenuator, it enables part of the voltage to be shorted to ground thus 'controlling' the volume.

    LAR - lower arm resistor (P).
    UAR - upper arm resistor (N).
    Vout - output voltage.

    Please note, if you intend to use the discrete regulators described later on, you should set the voltage output of the first stage of regulation to +/-21 volts. That is achieved with a UAR of 220R and LAR of 3K6.

    Q - 100 nF capacitors (2 off)

    These capacitors are to filter off any unwanted AC components on the voltage rails by allowing them to pass to ground via the 0 volt rail.

    Output socket (and matching plug)

    This will provide the means of connecting the psu to the pre amp. The items I use (149-325 & 149-322) are designated 'for audio' in the Farnell catalogue but are rated high enough for this job.

    A PCB

    You could make up a PCB from a sheet of copper clad board but I suggest gluing (hot melt glue is ideal) the components to a suitable board (I use a 3mm thick piece of fibreglass) and then hard wiring them together.

    PCB - Abbreviation for Printed Circuit Board. The board on which electronic components are mounted to build a circuit.
    A suitable case

    I'll leave this up to you but I prefer to build my own. Remember, if you use a metal casing, it must be earthed (point Q in the diagram).

    Odds and ends

    You will also need some insulated terminals (to connect wiring to the fuse-holder and mains switch), a terminal block, a cable gland and some method of securing the mains lead where it enters the casing. As these items will depend on your type of casing I cannot make specific recommendations.

    The above design can be modified in many ways although it will work well enough as it is. Potential improvements may come from placing RC filters either before or after the regulators to remove ripple and other mains nasties. Diodes could be placed across the regulators to protect them from reverse voltages although I have used this design without them for six years without problems. I prefer to use a combined circuit-breaker/switch (248-472) in place of the fuse and mains switch. It costs a bit more but circuit-breakers are reputed to perform better than fuses in audio equipment.
    If you would like to add an LED to show when the power is on, I suggest the following method:
    1. (Before you install the transformer) Take some thin enamelled copper winding wire and wind about 25-28 turns around the (toroidal)transformer. Make sure that you leave enough free wire to reach your LED location.
    2. Add a bit of tape to secure the wire to the side of the transformer.
    3. Carefully scrape off the enamel from each end of the wire using a sharp craft knife or similar.
    4. When you have built and tested the PSU, connect your multimeter to each end of the enamelled wire and you should (hopefully) find that there is a reading of 1.5 to 2 volts. Remember that this is AC so set your meter accordingly.
    5. If the voltage is around 1.5 to 2 volts (most LED's operate around that voltage but not all!), remove power and connect the wire ends to each leg of the LED (polarity doesn't matter with AC).
    6. Power up again and you should see your LED light up.
    Please note that the number of windings that I have recommended is suitable when using an 18 volt 80VA transformer. If you use something different, you will have to experiment a little to achieve the correct voltage. Always check the voltage before connecting the LED. If it is too high remove some windings and vice-versa.
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    The PSU (part 2).

    The discrete method.

    I hope that you enjoyed the commercial break and got yourself a beer from the fridge! Don't worry, this part of the PSU is much less work so sit back and find out how you too can enjoy the benefits of a discrete regulator.
    OK, we now have a power supply with a regulated +/- 21-22 volts DC. The next stage of regulation will take the rails down to +/- 15 volts (and of course give us that second stage of regulation).

    Circuit diagram of the discrete reulators.
    The negative and positive discrete regulators courtesy of Andrew Rothwell.

    These are fairly simple circuits that can be constructed on stripboard, or if you like to try, they make good starter projects for a PCB. I made some PCB's for mine although I would be just as happy to use stripboard.
    Any old red LED will do, the large cap is an electrolytic (Panasonic FC for example) rated at 35 volts or higher. The smaller cap is a film type (I use polypropylene). Resistors are all 0.25 watt rating.
    And all that should give you a first-class power supply for your pre amp. Yes, there is quite a lot of work in it but you cannot skimp on power supplies, especially for small signal circuits like pre amps. One bonus to all this work, is that you can take a feed from the output of the first stage of regulation, add more regulators and then 'feed' a phono pre amp, CD output buffer, active crossovers or whatever else you use that requires a +/-15 volt supply.
    Again, I prefer to use separate regulation stages for each channel but you could use just one pair of negative/positive regulators to supply both channels. If you can afford the time and expense, try both methods and see if it makes a difference in your system!
    Now for those of you who want to know how the discrete regulators work, and how you can set different output voltages, here is an explanation of the circuit by Mick Feuerbacher.

    (for the positive regulator):

    R8 and C4 form a low-pass filter at the input with a pole at 12.5 Hz. After that filter, the regulator circuit begins. R7 sets the source current for Q4. Q3 is simply a pass transistor. The Voltage drop over R5 is given by the voltage drop over the LED (which is constant at 1.7 V and therefore acts as a voltage reference) and Vbe (which normally is 0.65 V). As a result, the voltage drop over R5 is also constant. The output voltage is then regulated via R6 and R5, forming a voltage divider. The output voltage can therefore easily be calculated as

    Vout = (Vd + Vbe)*(R6+R5)/R6 = 2.35 * (R6+R5)/R6.

    C3 together with R6 make up another low-pass filter with a pole at 145 Hz.

    The analysis seems to be ok, as I get an output voltage of 15.28 V according to the above equation using the values for R1 and R2 in the schematic above.

    Mick has also kindly redrawn the circuit diagrams for the discrete regulators which you can see here.

    The 'easy' method.

    The easy way to make the second stage of regulation is simply to copy the first stage using the LM317/337 regulators.
    The only difference will be the values of resistors, UAR and LAR to set the output voltage. To set it to +/-15 volts use 220R for the UAR but change LAR to 2K4.
    If the second stage is some way from the first stage, I like to add a small amount of capacitance between each rail and the 0v rail. Anything between 10 uF and 100 uF should do. I also add 0.47 uF between the adjust pin and 0v and place 0.1 uF polypropylene capacitors on the outputs.

    TIP - it is recommended that when using the regulators with opamp circuits, it is best to keep the regulators as close as is practical to the opamps voltage supply pins.

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    The buffer.

    After all your hard work building the PSU, I am going to make the buffer section very easy. And in this case, 'easy' doesn't mean compromising sound quality! You could put any buffer circuit in this position; I have shown a few options on the Buffered Gainclone page, but I have chosen the buffer that, to my ears sounds the best.
    This is a discrete buffer so we won't even be going near the 'what opamp is best' argument although we do pose another question, 'which transistor sounds best'! But, if you build the buffer circuit using transistor sockets, you can very easily swap transistors in and out as you find them. For now, take it from me, this buffer sounds extremely good with the BC547/ZTX 653 combination. Discrete - built from individual components as opposed to most of the components being supplied on one chip (as in the case of an opamp)

    Circuit diagram of the discrete buffer.
    The simple discrete buffer courtesy of Andrew Rothwell and based on an idea by Les Sage.

    This simple circuit can easily and quickly be constructed on stripboard, or once again, it would make an ideal beginners project if you want to try making your own PCBs. The circuit works in true class A. While you can bias an opamp circuit to work nearer class A operation, you cannot make it work in true class A.
    Here are some notes that should enable you to build this circuit:
    • All three transistors are NPN types and choice is not too critical. The originator of this circuit, the late Les Sage, suggested using BFQ232 transistors that are available from Cricklewood Electronics (in the UK).
      I have used BC547C transistors for Q1 and Q3 and ZTX653 for Q2. This choice was based on what I actually had in my parts box at the time.

      TIP - be careful when soldering in transistors not to overheat them with the soldering iron. It is good practice to use a heat sink which clips to the legs of the transistor while it is being soldered. The heat sink looks like a pair of tweezers. Of course, if you use sockets, this is not an issue.

      Transistor heatsink.
      Transistor heat sink (click on image for larger picture)

      As stated above, if you use transistor sockets (Farnell part number 177-128), auditioning different types of transistor will be very easy.

    • Note, it is important to put the transistors in the correct way or the circuit will not work and you will probably damage the transistors. Transistors of this type have an emitter (e), a base (b) and a collector (c). Which ever transistors you use, you will need to know which leadouts are which. You can usually find this information somewhere on the internet or in the electronics suppliers' catalogues. In the case of the BC547 and ZTX653, the wiring is e b c as you look at the transistor while it is lying face down.

      TIP - If you are not sure about the pins on a transistor that you wish to use, test the transistor in your multimeter. Most DMM's have a test facility and will only give a readout when the transistor is inserted the correct way.

      Transistor pinout details.
      Transistor pinout details for BC547 and ZTX 653.

    • The resistors are all 1% metal film types. The 100K values can be anything from 91K-270K. The 68R can be anything from 68R-82R. But stick to the 27K and 120R values as they are critical to the proper operation of the circuit.
    • The output (DC blocking) cap can be anything you like that is between 2 uF and 4.7 uF. A polypropylene film cap is probably the best option.
      This discrete circuit out-performs the opamp circuits that I have tried to date. The trade-off is that there is an unavoidably high DC offset (around 670mV in my case) so the DC blocking cap is a must, not an option!
    • The supply voltage can range from +/-15 volts to +/-21 volts.

    The discrete BJT buffer.
    The buffer, so compact it will hardly take any more room than an opamp buffer. The transistors simply plug in to the sockets so you can try different types without the use of a soldering iron.

    Update 3rd June 2005

    It may be necessary in some cases to add a 1uF (film) cap together with a 1K resistor (in series) between the output of the pot/attenuator and the input of the buffer. If you have problems with noise when you use the volume control, try this mod.
    I have described the discrete buffer as I prefer the sound over that of the other buffers I have tried but this pre amp can use virtually any other buffer that you like to try. Most buffer circuits use a similar voltage for the power supply so they should fit into this pre amp without problem. Another very good discrete buffer, using Jfets instead of the BJT transistors can be found on Pedja Rogic's excellent web site.
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    Source selector (for beginners).

    Some people only ever use one source, usually CD but if you use more than one source then you will want to be able to select between each of those sources. This is most often done using some sort of switch. There is a much more complicated method using relays that I will leave for another time.
    The rotary switch is useful for switching more than two sources. If you have just two sources, then an ordinary DPDT toggle switch will take up much less room. I used one of these in my 'pod pre amp' to save on space but usually I use the rotary type.

    DPDT selector switch.
    DPDT selector switch (click on image for larger picture)
    Wiring A="Left channel source 1, B right channel source 1, C left channel out, D right channel out, E left channel source 2, F right channel source 2.

    Rotary types come in different configurations that allow you to switch a different number of sources. If you have, say three sources to switch, a 4 pole 3-way switch will do the job and enable you to switch both 'hot' and 'cold' lines at the same time (for two channels). Hot and cold lines - the 'hot' line is the wire carrying the signal. The one that you connect to the centre pin of the phono plug/socket. The 'cold' wire carries the return signal to ground.

    Rotary selector switch.
    Rotary selector switch (click on image for larger picture)
    Wiring depends on switch type

    If you need to switch between more than three sources, use a 2 pole 6-way rotary switch. This would allow you to switch up to six sources for two channels but only on the 'hot' line and not the 'cold'.
    Another option is to make your own rotary switch from a switch mechanisms and rotary wafers (use the break-before-make type also called 'non-shorting'). However, ready-made switches are cheaper and sealed!
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    Volume control.


    The volume control can be a simple potentiometer (better known as a pot) or a stepped attenuator. We will take a look at pots first.
    You can buy an almost infinite number of types and make of pot, both mono and stereo. These range in cost from about 1 UKP upwards but you don't always get a better pot for a higher price. In addition there are two types of pot called 'log' (logarithmic) and 'lin' (linear). The linear type simply increases or decreases resistance in a linear fashion. If you looked at a graph of the resistance for a given position of the knob, it would be a straight line. But our hearing doesn't work in a 'straight line' so we have log pots that more closely follow the curve of our hearing.
    Pots also come with different value impedances, just to make things more complicated. You need a pot with a high enough input impedance that the source will work into it without problems, but not too high an output impedance to go into the next section. Fortunately, when using a buffer with a highish input impedance, pot selection is not too critical but I still like to keep the pot to around the 10K value in this GC pre amp.
    One essential quality for any volume control is that it controls both channels equally. This is to ensure that the sounds stay in the same place in the sound stage and don't wander left to right or vise versa. Unfortunately many cheaper pots don't track both channels accurately but there is a way around this problem.
    The solution is to use a linear pot which has better tracking than it's equivalent log type. Then, we modify it to fake the curve that the log pot produces. The modification involves soldering a resistor between the out and ground pins of the pot, and this is called 'Law faking'. The value of the resistor used is between one tenth and one twentieth of the value of the pot. So, for a 100K pot the law faking resistor value is 5K and 10K, for a 50K pot 2K4 and 5K1, and for a 200K pot, 20K and 39K.
    Adding the Law-fake resistor effectively reduces the impedance of the pot so I recommend you sticking with pot values of 50K and above unless you know what you are doing.

    A pot showing the pin connections.
    A typical stereo pot showing the connections. The law-fake resistor is soldered between the 'out' and '0v' pins.

    Of course, there are always the more expensive pots that won't need modifying. The best thing to do if you want to buy a more expensive pot is to look on audio forums and find out what other people recommend. Or you can go for the best sound quality and use a stepped attenuator!

    Stepped attenuators

    The main argument against stepped attenuators has always been their higher cost when compared to a pot. However, this needn't be a problem if you make your own attenuator, and this is not a particularly difficult job. The first problem is to find a suitable switch and this is probably the most difficult part of a DIY attenuator project!
    Ideally, we need a switch that will cope with both channels (mono volume controls are a pain in the backside!), that can offer around a minimum of 20 steps of attenuation. Switches of that specification tend to be fairly expensive. But what if we can get away with a smaller range of attenuation? When we make our own hi-fi, we can do this because we can tailor the attenuator to our system. A commercial attenuator could go in any system and therefore has to have a wide range of attenuation to work in any system.
    I have found from experience that I can happily use a 12 way attenuator without problem. And fortunately, it is possible to build a 12 way (stereo) switch using a switch mechanism and wafers (the make-before-break type also called shorting).

    Home-brew attenuator.
    Home-brew attenuator (click on image for larger picture)

    A make-before-break switch means that for an instant while you move the wiper from one switch position to the next, both the position that you are moving from, and the one that you are moving to, are connected. This is because if you have nothing connected while the wiper is 'between positions' you break the circuit before re-making it and that causes the 'clicks and pops' when you change the volume setting.
    In the above picture you can see the black plastic switch mechanism and the two blue switch wafers. The attenuator is a shunt type which means that there are only two resistors in the signal path at any setting. Other designs of attenuator can have up to thirteen resistors in the signal path. The trade-off with the shunt design is that impedance changes at each setting but in practice, this is not a problem if you use the correct value resistors.
    I had found that my passive pre (which is just a pot in a pod) worked well with my buffered GC's so I decided to more-or-less use the same value resistors for the attenuator used in the active pre amp. Here is a list of those values.
    Series resistor = 10K
    Step#ResistorDb cut
    Note that these values and this range of attenuation works well if you use a unity gain buffer (no gain) and a gain in your power amp of around 20, as is the case with the basic Gainclone designs. If your system has more gain, in the pre amp and/or power amp sections, you may find that you need more attenuation than the 34 decibels of the above design. If you need more attenuation, contact me and I will send you a different set of values.
    The construction of the attenuator is as follows:
    • Assemble the wafers onto the switch mechanism.
      The switch mechanism has an adjustable stop so make sure that it is set so that you have a full 12 positions available.
      Make sure that you place the switch wafers the same way around and also ensure that the same position is selected on both wafers.
    • Find the 'starting' position of the wafers by turning the shaft anti-clockwise until it can not go any further, ie it comes up against the adjustable stop. Now, using your multimeter, check which tab on the wafer is connected to the input tab on the opposite face of the wafer. Mark this tab with a pen.
    • I suggest cutting one leadout of all the resistors to about a quarter of an inch (6mm). Leave the other end intact for now.
    • Solder the short end of the lowest value resistor (100R in this case) to the tab that you have just marked. The resistor should form a straight line with the centre of the wafer and the solder tag.
    • Continue soldering all the resistors to the tabs of the first wafer in a clockwise direction (when looking from the shaft side of the wafer).
    • Take a piece of copper wire (0.8mm to 1mm diameter) and remove any insulation. The wire should be about six inches (300 mm) in length.
    • Find something circular about 2 inches (50 mm) in diameter that you can use as a former. Form the copper wire into a neat circle, trim the ends and solder them together.
    • Place the attenuator into something like a vice (or a hole drilled into a piece of wood) so that the wafer is horizontal. Place the copper wire circle over the resistor ends so that it is centrally positioned in relation to the wafer. Carefully fold the leadout of one resistor over the copper wire making sure that you don't reposition the copper ring. Do the same with the resistor on the opposite side of the circle and then the two resistors between these two. And so on until the wire ring is securely held in place. Then trim the excess leadouts and solder all connections.
    • Repeat the above steps for the second wafer.
    • Solder the input resistors to the single tabs on the wafers together with the wires that will connect to the buffer input.
    • Solder a wire to each wire ring. These wires will go to the ground connection of the buffer.
    The attenuator is now completed. A slightly fiddly job but nothing that requires special tools or skills! Depending on your choice of case, you will need to either mount the switch in one side of the case, or make up a suitable bracket to hold it in position inside the case as I have done (made from acrylic sheet) in the above picture.
    You can 'tune' the sound of your stepped attenuator by your choice of resistors. I prefer to use some carbon film resistors for the series (input) resistors and 1% metal films for the shunt resistors. I have also used Welwyn RC55 types for the shunt positions.

    TIP - If you use anything with a greater tolerance than 1%, I suggest that you sit down with a meter and measure the resistors to get the closest match as possible. Remember, keeping both channels as close as possible is very important!

    Another version of the DIY attenuator.
    Another version of the DIY attenuator.

    The attenuator shown above uses a metal switch mechanism, using the same wafers but the resistors parallel to the shaft. This results in a longer but narrower attenuator. The resistors rest on some home-brew dummy wafers but you could use another set of wafers if you don't mind the added cost. Make the attenuator style that will best fit into the space in your pre amp.
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    Wiring it all together.

    When you have built your PSU and extra regulator section, the buffer and attenuator, install all the necessary sockets into your case. You can then wire it all together to make a working active pre amp.
    The following diagram shows how everything is connected.

    Preamp wiring diagram.
    Pre amp wiring diagram shown for just one channel. This also assumes that you do not switch the 'cold' line from the input.

    Note that you need two buffers to make a stereo pre amp. You can use either one set (neg and pos) of regulators for both channels, or one set for each channel.

    I don't know if I can add much more but if you are very new to hi-fi building, and there is something that you cannot understand, please use the contact form to ask me.
    As I said previously, you could easily alter this pre amp to make it do exactly what you want. You could change the buffer design, or you could add a phono pre amp. If I can find the time, I will draw up a circuit diagram for my phono pre amp which you can see here.
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    I recommend the following tests:
    • Measure the output of the transformer secondaries before you connect it to the rectifier bridge.
    • Measure the voltage after the rectifier bridge. You measure the voltage between each DC output of the rectifier bridge and the zero volt rail.
    • Measure the voltage at the output of the first stage of the PSU, ie before the regulator circuits.
    • Measure the voltages on the outputs of the regulators. They should read +15 VDC for the positive regulator and -15VDC for the negative regulator.
    • With the buffer powered up, measure each rail voltage (should be +/-15VDC.)
    • Check the DC offset at the output of the buffer. It should be very close to 0mV.
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