Friday, March 13, 2015

Making an RIAA/phono preamp

Before we start today's blog post, I think I really need to clarify what we’re discussing. I’ve recently got back into vinyl in a big way. Part of the reason for this is the fantastic “Vinyl Decision” in Taipei, run by a couple of very fine gentlemen by the names of Mark and Terry ( / If you're in Taipei, and like music, pay them a visit. A recent purchase from there brought back memories of trying again, and again (and again) to play guitar like my guitar heroes as a teenager in the 1980s.

In fact, vinyl is making quite a comeback worldwide. A lot of people are realizing that digital versions just lack some of the personality that vinyl can provide. Many of them are buying old record players/turntables and wiring them up to their modern amps. However, they are often disappointed to find that the output audio is very quiet and lacking bass. This is because records are/were recorded using a very specific frequency response. The output of a record player is much lower than “line level”, but it is also much heavier on the treble and lighter on the bass. This frequency curve is what is known as the RIAA (Recording Industry Association of America) equalization curve. This was necessary primarily because heavy bass would have the needle jumping around too much on the record, affecting sound quality, and, I’d imagine, increasing wear and tear. Additionally it allowed the grooves to be thinner, hence allowing more grooves (and therefore longer playing time) per record.

The RIAA Equalisation Curve:

Older amps had a specific input labelled “phono”, which specifically catered to record players. This input would be wired to a built-in preamp which would not only amplify the signal to line level, but would also correct the frequency curve, hence enabling the amp to output the sound as it was originally intended. Many newer amplifiers do not have a phono input and therefore an external preamp is required between the record player and the amp to enable the use of one of the line-level inputs.

And that’s where today’s project comes in. We’re going to make an RIAA/phono preamp.

There are many, many preamp schematics out there—some simple, and some insanely complex. I wanted to find one which was relatively simple, while still accurately correcting the RIAA recording frequency curve. I finally settled on this one: (“The Very Simple Phono Stage”). I highly recommend you go to that page and read through it if you decide to tackle this project. It’s very well explained and highly educational.

Rather than build a power supply, I decided to buy a 30V DC one ready-made, and then split it so that it would have +15V, OV and -15V. To do that, I made a simple voltage splitter like so:

And a filter where the power plugs in to remove any unwanted hum from the power supply, like so:

Additionally, I added a diode right where the 30V meets the board to protect the circuit in case anyone ever plugs the power supply in the wrong way around.

I also decided to add some small 100nF capacitors at the +15V and -15V legs of the chips to reduce any picked up noise to a minimum.

OK, let's get going. We're going to need an enclosure. Here’s the box I’m going to use with my markings for where to put the connectors:

From left to right, it’s going to have a grounding connector (which may or may not be needed depending on whether there is a humming noise – note that the humming noise can sometimes be removed by unplugging your record player turning the plug around and then plugging it back in [assuming it’s symmetrical]), left output RCA jack, right output RCA jack, left input RCA jack, right input RCA jack, and finally the power input (30V DC negative sleeve [it is imperative that the 0V from the power supply is electrically isolated from the enclosure, so don’t be using any metal connectors that might join them]).

Here it is with the holes drilled and the connectors, erm, connected:

It’s a good idea to test stuff like this out on breadboard before you switch your soldering iron on. Here’s an early version of mine:

Once the circuit design is more or less finalized, it’s time to lay it out for veroboard, like so. Yes, this is how I do it:

And just for you, here it is done properly on the computer. You may notice a few small changes between the initial pencil sketch and the final layout. As with all images, you can click to enlarge.
Here we have the components mounted on the board:

Now it’s time to think about mounting the board in the box. It’s pretty important to secure the board in some way so it doesn’t move around and touch something it shouldn’t. You can see here that I’ve drilled a couple of holes through the board:

Now we drill a couple of holes in the enclosure that match up with the hole positions on the board. I’ll eventually use plastic mounts to hold the board securely in place.

Before placing the board in the box, we add the wires (apologies for the overexposed photo):

Now that the board’s in place, we can connect the wires to the connectors, like so:

Once we're happy that the circuit’s done, we secure the connectors in place with thread lock:

Here's how it looks after that:

Finally, a little bit of the old in and out so we don’t mix up the cables:

And we're done. I'm happy to be able to verify that “The Very Simple Phono Stage” sounds fantastic.

Wednesday, January 28, 2015

Shielding and upgrading the electronics on a Stratocaster-style guitar

Today we have a guest post by my good friend Adam. I gave him some advice and tips, but he did all of the work himself.

I’ll let Adam take it from here:

“I found this guitar at the bottom of my in-laws’ stairwell. It was being thrown out. When I touched the gig bag it was in, my fingers came away black with dust and grime yet inside was this near perfectly clean guitar. I did a bit of a quick clean and initial set up but now it’s time to get serious and get her playing and looking right. She’s an Epiphone Strat copy. I’d expect Scarlet to be getting on for 18-20 years old too. Further search reveals it to be an Epiphone S310 Batwing Stratocaster.

This large cavity was what I was hoping for rather than the intricately routed ones on Fenders and Squires. It will make coating the cavity easier with copper sheeting. It also means I may be able to buy a Fender tortoiseshell guard. The pickups were set in different positions to a Fender and I was worried that if the cavity was shaped like a Fender I wouldn’t be able to fit pick ups in different positions.

Back of the pick guard. Just this little sliver of shielding behind the pots.

Taking the neck off to make the body easier to work with. Note the nice plate on the back of the neck – pretty cool.

Taking the bridge apart. It’s good to have a tin handy to chuck all those screws and bits and pieces into so they don’t get lost.

Voila! Some rust and gunk on the bridge. Surprised it’s not more after almost 20 years of use (or non-use). We’ll see what we can do about that.

All the hardware is off the body. Ready for the copper sheeting now. Maybe it wasn’t necessary to take everything off but I wanted to get a proper look at everything. When I put it all back together I’ll know it’s right. At this stage I tried to fit a Fender pick guard I had lying around to the body with the idea of buying a tortoiseshell one (I’ve seen one about town with an HSS layout) but unfortunately the shape of the pick guard was just out enough for it not to be possible.

Yeah, this job is going to be so much quicker than lining a Fender. This stretch is one strip of sheet.

Yes, a much quicker job than a Squier and many more big strips were laid down rather than little pieces.

I’ve left the electrics soldered together for now. It looks like the factory did a nice job on the solder and I’m not too deft with the stuff so I want to keep my clumsy work to a minimum. I did make sure to mark the parts I could confuse easily with some tape. Yellow tape on a pickup or pot means it’s nearest the neck. Black tape means it’s nearest the bridge. There is no confusing the humbucker!

I don’t think I will replace these pots. They look pretty good quality. I will try to clean them up a little though.

Again, although this is an import style switch I don’t think I will change it for a Fender. This one seems to be working OK for now and if it goes wrong in the future, it is an easy fix

The copper has been applied to the rear of the pickguard. I almost got it on in one piece!
Fitting the pickups to the back of the pickguard. I’ve found that one of the holes on the middle pickup has been shorn of its thread somehow. This means that the pickup cannot be fitted securely or have its height adjusted. (Note: I solved the problem by simply finding an old screw that was a slightly larger diameter thread than the original.)

Remounting the parts on the copper covered pickguard.

Now it’s time to resolder according to this diagram:

I’ve soldered the ground to the back of the pickguard. I will solder the ground to the tremolo unit to the copper lining in the cavity. Don’t worry, I didn’t leave the soldering looking as bad as this! I’ve also covered the selector switch with tape to reduce the chance of anything touching the copper lining and sending the pickup signal to ground.

The bridge cleans up pretty well with some polish:

When I’d put the whole thing back together again I found that the import type switch was really scratchy and horrible sounding so I decided to go out and get a good quality switch.

In the end I decided to go for black replacement knobs and tremolo end. I also decided to replace the pots.

I have some Fender pick up covers lying around. One of them is black and I think that will make a nice touch. Had to file down the pick guard because the Fender pick up covers are 1mm longer than the Epiphone ones! It’s not completely even but you’ll never notice when the pickups are mounted.

When I strung the whole thing up I found that the tremolo bridge was rising up far too much. The tremolo springs are clearly to blame so I got a new set. I didn’t notice it before because I had the tremolo blocked off. Because the tremolo unit isn’t great I decided to have it flush to the body rather than floating to help the guitar stay in tune better. I also needed to file the nut a little on D and G string grooves. I had to use all five strings in this pack to get the bridge flush!

The finished article. Took her for a test drive with the boys. Even though she isn’t a patch on my American Fender and isn’t as nice as the Squire I worked on recently, she did the job well enough. I think I’ll just play her at home to save wear and tear on my nicer guitars. Hey, but pretty nice for something found on the stairs! I’m happy she has a home. I have a nephew who seems interested in guitar. I might sort him out an acoustic and if he gets into that let him have Scarlet."

Wednesday, November 19, 2014

Wiring a Stratocaster for Two Volumes and One Tone

My friend Adam was recently upgrading an Epiphone Strat copy (which we’ll be covering in the next blog post) and asked if I could help him with a somewhat unorthodox wiring idea. So, continuing on somewhat from the previous post about the Stratocaster 5-way switch, today we’re going to look at how to wire up a Strat so that it has two volume controls and one tone control.

To quote Adam: "I want to rewire my Strat so that I have two volume controls and only one tone control. I want the knob nearest the strings to control the volume for the bridge and middle pickups, the middle knob to control the volume for the neck pick up and the knob furthest away from the strings to be a master tone control. "

I couldn’t find much on Google about anyone else doing this, at least not without using a super switch, but I was pretty sure it could be done.

So this is what I came up with. Click on the image to enlarge it.

Here’s how it works:

In the bridge position, pin 1 (bridge) is connected to pin 0 (common) on both sides. The yellow wire from the bridge pickup goes to pin 1 and the signal then continues to pin 0 on the left side of the switch. This then connects to the bridge/middle volume control via the blue wire. Additionally, the purple wire from that volume control comes back to pins 3 and 1 on the right side of the switch and this then passes to pin 0 on that side since it is currently connected to pin 1. Pin 0 then connects to the tone control through the red wire and continues to the output jack.

In the middle position, pin 3 (middle) is connected to pin 0 (common) on both sides. The green wire from the middle pickup goes to pin 3 and the signal then continues to pin 0 on the left side of the switch. This then connects to the bridge/middle volume control via the blue wire as before. Additionally, the purple wire from the volume control comes back to pins 3 and 1 on the right side of the switch and this then passes to pin 0 on that side since it is currently connected to pin 3. Pin 0 then connects to the tone control through the red wire and continues to the output jack.

In the neck position, pin 5 (neck) is connected to pin 0 (common) on both sides. However, the neck pickup wiring is quite different from the other two pickups. The grey wire from the neck pickup goes directly to the neck volume control. It connects there no matter what position the switch is in, so you may wonder how that doesn’t affect the output even in the other switch positions, well, that’s because the pink wire coming OUT of the volume control connects to pin 5 of the switch and that doesn’t connect to anything unless the switch is in the neck position, at which time the signal can now continue to pin 0 on the right side of the switch. This then connects to the tone control through the red wire and continues to the output jack.

Wednesday, October 29, 2014

The Stratocaster 5-way Switch

Today we’re going to talk about the standard 5-way switch used in Fender Strat-style guitars—how it works, how to wire it up and variations of the switch itself.

Back when Strats were first made, they came with a 3-way switch (one position for each pickup), but some savvy guitarists soon realized that if they moved the switch into a position midway between two of the “official” positions, they could get two of the pickups to turn on at the same time, resulting in new sound options. This happened because the switch was what is known as a “make before break” switch, which means it touches the next contact before disconnecting from the previous one. It wasn’t easy to keep the switches in these in-between positions and guitarists often resorted to sticking matchsticks, etc., into the switch slot to hold them in position.

Here’s how the switch looks as more of an electrical diagram. As with all images, click to enlarge.

Seeing how customers were doing this, Fender eventually started installing 5-position switches. Note that these were still really 3-way switches, just with a couple of in-between positions added on so that matches were no longer required to hold the switch in those positions.

Also of note is that the switch is “dual pole”, which means that it is basically two switches in parallel that move at exactly the same time, without being electrically connected to each other.  If you take a good look at the switch below, you will see that there is a second row of contacts on the other side.

Here’s how THAT looks as more of an electrical diagram:

This allows for more wiring possibilities, which we’ll discuss later.

Looking at the photo of the switch a couple of images back, you can see that there are four contacts on the side facing us (and four on the other side that aren’t so easy to see). Of the four on this side, the left-most one is the “common” contact, which is to say that it is always connected. The three contacts to the right correspond with the three main switch positions (with the additional two positions being in-between 1 & 2 and 2 & 3 respectively). The other side of the switch has a similar setup.

Now given that there are only (normally) three pickups, and the switch has three contacts on each side (in addition to the common contact), you may be wondering why we need two sets of contacts in the first place. Well, this is so that you can have a bit more freedom when it comes to choosing how to control the volume and tone of each pickup. For example, with standard Strat wiring, the volume pot controls all three pickups, whereas the two tone pots control one pickup each (with the bridge pickup having no tone control at all). Older Strat versions had one of the tone pots controlling two of the pickups at the same time (and there are many more options too!)

Here’s how the switch would look from below. Rather than numbering the pins--which I think can be confusing given that there are 3 pins (not counting the common pins), but five positions—I’ve named them B (for bridge), M (for middle), N (for neck) and C (for common).

Again, remember that you also have two additional positions, as mentioned previously, in-between bridge and middle (so pins B and M would both be connected to C at the same time), and in-between middle and neck (so pins M and N would both be connected to C at the same time).

As for the other side of the switch, despite the fact that the pins appear to be in a different order, they still work the same way, so when the switch is in the Bridge position, the left-hand pin B is connected to the left-hand pin C, and the right-hand pin B is connected to the right-hand pin C.

I hope you’re still with us here. The above can be a bit of a brain melter, but once it clicks it makes a lot of sense.

In an effort to make this as simple as possible, here’s what a standard Strat’s wiring looks like just around the switch itself (remember that there are many, many ways to wire up a Strat and this is just one of the more common ones):
Without going into too much detail about how the tone controls actually work (that’s for another day), I’ll try to walk through how the switch works as clearly as I can.

When the switch is in the neck position, left-hand pin N (neck) is connected to left-hand pin C (common). This allows the signal from the neck pickup to pass to the common pin on this side of the switch. Since left-hand pin C is shorted to right-hand pin C, the signal from the neck pickup continues to right-hand pin C, where it then goes to the volume control. Additionally, right-hand pin C is currently connected to right-hand pin N, allowing the signal from the neck pickup to ALSO travel to the neck tone control.

When the switch is in the middle position, left-hand pin M (middle) is connected to left-hand pin C (common). This allows the signal from the middle pickup to pass to the common pin on this side of the switch. Since left-hand pin C is shorted to right-hand pin C, the signal from the middle pickup continues to right-hand pin C, where it then goes to the volume control. Additionally, right-hand pin C is currently connected to right-hand pin M, allowing the signal from the middle pickup to ALSO travel to the middle tone control.

When the switch is in the bridge position, left-hand pin B (bridge) is connected to left-hand pin C (common). This allows the signal from the bridge pickup to pass to the common pin on this side of the switch. Since left-hand pin C is shorted to right-hand pin C, the signal from the bridge pickup continues to right-hand pin C, where it then goes to the volume control. Additionally, right-hand pin C is currently connected to right-hand pin N, but right-hand pin N is not connected to anything else, the sound is unaffected by any tone control.

Remember that we also have those two additional in-between positions, but all that happens there is that everything is true for both positions either side of it, so for example in the in-between position between the neck and the middle pickup, everything I wrote about the neck position AND everything I wrote about the middle position is true.

Finally, in addition to a standard Stratocaster 5-way switch, you can also get import types, which look more like this:
Note that these are just wired up in the same way as the switch that I’ve already been describing, except that the pins are laid out a bit differently. In the above image, the pins are numbered (left to right) 3,2,1,0,0,3,2,1. These represent the following positions:

    3 – Bridge
    2 – Middle
    1 – Neck
    0 – Common
    0 – Common
    3 – Bridge
    2 – Middle
    1 – Neck

Finally, if you want to get all fancy with your wiring, you can get “super switches”, which have many more contacts, allowing you a much greater range of wiring options. Here’s an example of one below: