Archive for the ‘Build Your Own’ Category

So, I finally finished the channel switcher for my Hughes & Kettner Tubemeister 36!


And here it is all lit up…


Perhaps somewhat foolishly I have built this footswitch at a time when I don’t actually have my amp, due to it being off away somewhere in Germany being repaired, but I’ve tried it with other amps  that are set up the same way with RTS jacks, so I know it’s all good.

It’s pretty full on – it’s a buffer (the same circuit as the 072), killswitch, has dual switchable loops (one for clean, one for dirty channels) and also controls the amps FX and reverb. I very nearly put a starvable dc outlet on it, but when it came down to it I decided I would probably never use it, plus it was getting a bit cluttered without mounting components on the upward face of the pedal.

To say it is a squeeze getting all of that in there is an understatement – I used the jack sockets that are typically mounted on PCB’s so that I could file off certain bits of the plastic casing so they would fit into the corners of the pedal.

It uses 3 x 3PDT toggle switches, a momentary soft touch SPST footswitch switch and a 4DPT footswitch, which are not rare components, but certainly not as readily available as most pedal parts
In reality it is a bit too small for large feet stomping it on stage – there’s a real potential of stomping the wrong thing. I probably wouldn’t worry too much about adding the buffer again, although it may come in handy to have. But hey, it is what it is, something I wouldn’t build again, but it was a good thing to do an a lesson on how much you can really squeeze into an enclosure if you want to go OTT!

So if you want to build your own I have included the schematic I drew up for it.


A word of warning – arrangement of this into a Hammond 1590BB is pretty tricky, consider a bigger enclosure if you have the  room on your pedal board and want to make you life that bit easier! Plus, by going bigger you could probably use right-angled jacks for the send and return loops, which would save space on the board in the long run!


The past few weeks I’ve mainly been working on the op-amp based clean booster I mentioned in my last post. Since then I have made a few tweaks and built up a vero version ready to be boxed up as soon as the enclosure and the artwork arrive. The artwork is a transparent vinyl sticker with a white foreground, which should look great on the powdercoated hammond 1590b’s, I’ve gone for the red one for this version. I’ve also got one of these large aluminium knobs which should feel great to use and look cool. Anyway here’s the artwork:


Here’s the circuit:


And here’s the vero layout, verified by myself!


Pics and videos coming soon…

Over the Christmas break I’ve been pretty busy playing with 2 new pedals I have been working on. The first I have mentioned a few times before – the Square Earth Theory, which still isn’t working too well to be honest. Here’s the schematic I have so far, trying to filter out the popping is proving to be such an enormous pain in the ass you wouldn’t believe. This uses a FET to switch the signal, in the past I used a 4053 and 4066, with OK results. The 4053 worked the best of the 2 CMOS IC’s, set up so that the signal is shunted to the Vref rail, and is also broken, but the FET seems to be the quietest option when set up to allow a slower switch on time as in the schematic. If you have any luck getting this to work better than I have please let me know!

Square Earth Theory

I have also been playing with an op-amp clean boost circuit that uses a charge pump to internally double the +9v supply run the op-amp that does the boosting at +18v which allows a decent amount of headroom before the signal starts clipping when boosting. I’m pretty sure the Klon used a charge pump to get ~18v without at 18v supply, and a clean boost seems like a great application for using a charge pump.

When you talk about clean boosts everyone throws the term ‘transparent’ around as if it is the holy grail, and it kind of annoys me as what most people are really after when they think of a clean boost is an all around ‘tone fattener’, which is far from transparent! Op-amps get discarded as being slightly sterile when used as clean boosts, but I don’t necessarily agree. Mosfets do tend to be slightly ‘warmer’ than op-amps and do other things to the tone as they boost, but I actually really like the way the op-amps stay so clean (especially when you have a the increased headroom that is provided from a larger voltage than the standard 9v) even when the boost is many decibels.

Anyway, here’s the schematic.


The amount of boost the pedal can kick out is set by the trimmer, which sounds great around +24dB (trimmer set to 8k8). This worked well for the pick ups of my PRS Custom 22 as it allowed loads of boost without any clipping. I made adjustable with a trimmer so that it can be adjusted to suit your guitar, as some with really hot pickups, especially active, may clip earlier. The available range is between 9-35dB. This could have been a pot, but I would rather have it set internally and then just forget about it. With the setup I have here the signal is always boosted by x amount, and then attenuated with the volume control. I think it was Jack Orman (who incidentally designed the Mosfet Booster, which is the basis for many commercially available clean boost pedals) who said that this is like driving a car and always having your foot on the accelerator and jamming on the brakes to slow down, which is a great analogy of what is going on here. I opted to do it this way as in practice it felt more natural to me than adjusting the gain when using it. You could substitute the 100k trimmer (R7) with a 100k pot and do away with the volume control to control the gain directly, which, on paper at least, makes more sense (and actually my original plan). If you are thinking about doing this I would consider changing R9 to 510k, R3 to 10k and using a 500k pot, which will give you around +34dB of gain on tap, but not all of which will be clean.

I think I’m going to call this pedal the Scallywag, I’ll get it built up and boxed asap, and may make a small batch.

So, here is a simplified schematic for the Solar Lifeforce. Basically I have removed the expression/CV out stuff, removed the toggle for the buffer so the buffer is always on, and made it as simple as possible. It could maybe do with some small value caps between both R5 and ground and R9 and ground, but they aren’t essential. The LDR is a pretty unreliable thing for any consistent readings so maybe buy a bunch and try them out until you find one that suits you, I have tried the circuit with ones of varying specs and they all have behaved OK. The best I found was on doctor tweeks website which can be found here. It has a dark resistance of 20M which is plenty, 10 lux (‘room’ light) resistance is between 50k-100k and it has a rise time of 20ms and a decay time of 30ms.

Before I go into the circuit I will warn you – LDR’s aren’t perfect for cutting volume alone as they roll off treble, so your tone will be altered. This isn’t a problem so much when you are playing in really well-lit conditions but it does become noticeable when you are in stage type conditions. Of course this may be what you want, but if you are after a ‘transparent’ volume cut a LDR is not the way to go.

Anyway, here’s the circuit. I havent included the LED/switch wiring etc in this one, and I haven’t put in reverse polarity protection. It is up to you if you think it is needed. If you think it is you could just use a diode such as a 1N4001 in series with the 9v rail.

R1 is a pull down resistor to help prevent popping when the circuit is bypassed/engaged. R2, R3 and C1 form the network which establishes the reference voltage for the op-amp. The first side of the op-amp acts as a buffer, and the second side acts as a gain allowing you to increase the volume to compensate for poor lighting conditions. VR1 controls the range of the LDR between being covered and uncovered and R5 sets the minimum for this. R9 and R10 set the gain of the b side of the op-amp, which is around 5  (G = 1 + (R10/R9) = 5.545), which is then attenuated by VR2 just before the output. C4 stops any DC leaking from the circuit into your amp. You could use a lot of different op-amps for this circuit. I used a TL072 because I like them, have a few kicking around and is a dual op-amp so takes up less room than two signals. They are also pretty cheap and quiet so are good for this kind of work. On the Barefoot which was really the predecessor to the Solar Lifeforce I used DPDT toggle switch to change between if the signal kills when the LDR is covered or is restored there, you could probably mod this circuit to include it. You could also use a DPDT toggle to cut the buffer out of the circuit if you wanted too easily enough. I wired the original so the buffer was independent of the pedals bypass, so the signal hit the buffer, then the bypass, then the rest of the circuit, but you could wire it after the bypass so that you have a switchable buffer aswell. Just to clarify on this diagram the buffer is always in the circuit.

Here’s the part list:

R1         1M
R2         10k
R3         10k
R4         1M
R5         10k
R6         LDR
R7         10k
R8         1M
R9         2.2k
R10        10k

C1         10uF
C2         100nF
C3         10uF
C4         10uF
J1         Input
J2         Output
U1         TL072
VR1        500k Lin
VR2        10k Log

I found the best way of mounting those odd shaped LDR’s that are specified as 5mm diameter but aren’t actually round is to use a 5mm bezel mount. And be sure no light can get to it from inside the enclosure from the LED!

Update 12/07/12:

The circuit above is a little crude in places, it is a barebones version of the hand drawn circuit I drew when experimenting with it. Perhaps a nicer version would be to have a gain control directly controlling the gain of the b side of the op amp (ala the MXR Distortion+) and a separate volume control as the gain pot is wired in this circuit in this circuit. That way you are not boosting the signal only to attenuate it.  The volume control is then completely optional as in many ways the gain will act as a kind of volume control anyway. Here is my revised circuit with the aforementioned mod, a few additional caps and a resistor to help cut out the really high frequencies before the buffer. Probably still a fair bit of room for improvement, but here we go! I haven’t built this version yet, but it should be OK.

Update 11/12/12:

I’ve modded this circuit again (haven’t tested it but thought I would post anyway, so beware!) to have some basic filtering on the power supply, set the output impedance at 1k and also adjusted the values of a few components to make them a bit more sensible. If you build this and can verify it works well please let me know!

Solar Lifeforce Revised Schematic


Here’s a dead simple one, the circuit for the Morse Device. This uses just 4 components: an input jack, and output jack, a momentary DPDT footswitch and a latching DPDT footswitch. The latching footswitch could be a SPDT but as I have a fair few DPDT around I tend to use that.

What is going on here is pretty easy to explain. Basically the two footswitches control if the signal is connected to ground or not. If it is the signal is muted, else it reaches the output jack unaffected. The latching switch determines which throw on the momentary switch the signal connects to, which in effect reverses the NO/NC action of the momentary switch.

072 Buffer Circuit

Posted: May 10, 2012 in 072, Build Your Own, FX
Tags: , , , , ,

So, here’s the circuit I used to create the 072 with. It is pretty much as basic as you can get, especially as it is always on but it is very effective at its job.

D1 is for reverse polarity protection incase people use a positive center adaptor instead of negative center (or vice versa depending how you wire it). R1, R2 and C2 form the voltage divider which provides the reference voltage for the opamp. R1 and R2 don’t have to be 47k, they can be anything (I would stick between 10k and 100k though to ensure the divider is reasonably stable) as long as the values are equal. C1 helps prevent any noise while  C3 prevents and DC from the circuit leaking into the amp and R3 limits the current so the LED doesn’t burn out. You could get away with something like a 680r resistor for R3, but the I like the 2.2k because I have a load of them, and they are commonly used for this purpose! I would hook up pin 7 (the output for the second side of the opamp) of the TL072 to ground as it isn’t going to be used.

And that is really all there is too it. Fits nicely in the 1590A enclosures, providing your layout fits. On the only 072 I have built I had quite a big vero layout and it still fit, but I redesigned it after to be that bit smaller to allow slightly more room in the enclosure for everything else.


R1    47k

R2    47k

R3    2.2k

R4    1M

C1    100nF

C2    10uF

C3    10uF

D1    1N4001

D2    3mm LED

U1    TL072

And the all important circuit:

You can pretty much drop this into other builds, and if they require an opamp you will probably have a reference voltage you can tap into to get your Vr, which cuts out the whole voltage divider section. There are many, many mods you can do to this circuit, but this is meant as a simple low noise, low part count buffer in a small package.

Revision 04/02/2013: I have updated the circuit and added a vero:

Revised Circuit Vero

This circuit requires a digital voltmeter panel, you can get them on ebay for around £4-8, depending on where you get them from and what you want. Bear in mind you will want to mount it in an enclosure, so getting a panel with a bezel is a good idea as it will sit nicely. Otherwise you may have to mount it inside the enclosure, in which case you will probably want a window, which can be done using some thin transparent perspex, but is a right P.I.T.A.

The output is around 1.6v-9v depending on your input supply, the minimum voltage set by R2. On the pedals I’ve produced I used a 680r as R1, which is fine, but 2.2k cover you better. You could put caps on the input and output of the 7805, something like a 0.33uF tying the input to ground and a 0.1uF tying the output to ground should work OK, and just helps to stabilize the voltage, but it will work well without it.

Make sure you use a linear pot for the VR1, as an audio taper would be very strange here! For the output I used a jack rather than a socket, mainly because it stops people plugging into the wrong side. If you want reverse polarity protection then you could put a 1N4001 or similar in series with the input voltage, but this will reduce the output voltage by around 0.6v (off the top of my head), so bear this in mind. Depending what voltmeter panel you choose it may have reverse polarity protection built into it. You shouldn’t need a heat sink on your 7805, but if you find it getting too hot then you can connect it to you enclosure to help dissipate the heat, providing you’re using a Hammond style enclosure. I used a 1590A, and it is a tight fit, but you can get everything in there, even if you have to use a shoe horn!


D1      3mm LED
R1       2.2k 0.25w
R2      2.2k 1w
SW1     3PDT
U1      7805
U2      Digital Voltmeter Panel
VR1     10k Lin

As you can see, it’s a pretty basic circuit, even if you add the mods I have described above. The hard part is firstly sourcing a good panel, and then squeezing it into the enclosure. I’m probably not going to make any more of these, at least for the foreseeable future, as I don’t have the facilities to cut the rectangular hole required for the panel efficiently enough to be covered by the price I charge for each unit.