Making a reverse/inverse RIAA attenuator

A couple of blog posts ago, we made an RIAA preamplifier (http://diystrat.blogspot.com/2015/03/making-riaaphono-preamp.html). Hopefully you understand what that is now, but if you don’t, maybe have a look at that post now.

As part of the process of making that preamp, I found it useful for testing purposes to have an inverse RIAA attenuator on-hand. Wait, a what? An inverse RIAA attenuator. It's basically the opposite of an RIAA pre-amp.

If we refer back to the RIAA Equalization Curve graph that we used on the RIAA preamp post, you'll see that it has two lines (see below). One is called the recording curve (dotted blue line) and one is the playback curve. The RIAA preamp follows the red curve. The Inverse RIAA attenuator follows the dotted blue one. If you were to put one after the other, you would end up with a flat horizontal line running right across the middle of the graph at 0dB.

So why would you want one of THOSE? Well in my case, it made it possible for me to take the line-out of any device, send the signal through the attenuator, and then feed its output into the RIAA preamp I was building at the time. If the output was back to line level, then I knew the preamp had no major problems (though I would still test it later with a record player for sound quality, etc.)

Now I understand that most people would not be in this situation, and therefore this attenuator would seem to be of little use. However, it does have one other fantastic use, and I wish I’d had one when I encountered this exact problem a few years ago. Imagine the situation—you have an amplifier with just a few inputs. One of those inputs is labelled “phono”, but you don’t have a record player, so it’s basically wasted. Well, with a reverse RIAA attenuator, you can plug just about any device with a line-out connection into the phono input using this attenuator as a go-between. In fact you’d probably get away with the headphone output of many devices, though you may have to play with the device’s volume levels to get a decent sound quality/frequency response—in fact I tested this with an iPod and it worked just fine.

Now, as I'm sure you know, an attenuator is the opposite of an amplifier. Since we’re not actually amplifying anything, we can do this circuit completely passively (no power supply required). In fact, it’s an incredibly simple circuit once you’ve worked out which resistor/capacitor values you need. This is no mean feat, but luckily there’s almost always someone else on the Internet that has already worked this kind of thing out. In my case I used this circuit, which I got from here: http://sound.westhost.com/project80.htm

I highly recommend visiting that site if you want a little more information about the circuit and how it does what it does.

The schematic is extremely simple, requiring only four (or even three--see below) capacitors and three resistors for each channel. So simple in fact, that you don’t require a circuit board.

These are the values I ended up using:

R1 = 909k
R2 = 75k
R3 = 1.5k
C1 = 3.3nF
C2 = 270pF
C3 = 1nF
No need for C4

I boxed them in a fairly small project box with a couple of RCA jacks at each end.

Then covered it up and marked which end was which (don’t mix this up).

And here’s an action shot of me using it to test out the RIAA preamp I made in the previous post:

This ended up being a simple, cheap and fun project and I ended up with a very handy little device.

Building a Bazz Fuss fuzz pedal

Hey, what should we do today? How about making a fuzz pedal?

Cool, I knew you were up for it.

As you may be aware, I’ve recently been helping my friend Adam with a few guitar projects and he was keen to get started on pedal making (specifically a fuzz pedal). I suggested the Bazz Fuss as an easy starter project. It’s only 5 components (and a pot) and it sounds good too. It's designed primarily for a bass guitar, but it sounds great with a normal guitar too.

Here it is (note that I laid mine out slightly differently, and I eventually swapped the 10k resistor for a 100k one):

[from http://diyyoguitare.hautetfort.com/archive/2010/07/13/bazz-fuss.html]

And here's my very professional layout:

Since I hadn’t already made a Bazz Fuss myself, I decided it might be fun to do so. Apart from having one for myself, it would be handy to have one to refer to if Adam ran into any issues while he made his.

I decided to challenge myself a bit and see if I could fit it into a Hammond 1590A enclosure. They’re pretty hard to find round these parts, but I did manage to find an enclosure that had almost the same dimensions. In fact it’s slightly tighter than a 1590A, but hey, that’s just part of the fun, amirite?

Just for reference, a Hammond 1590A enclosure measures 93 mm x 39 mm x 27 mm. Mine measures 90 mm x 36 mm x 30 mm. The walls are a bit thicker and there are some extra bits inside which will make it even more cramped.

Here’s the enclosure in question: 

And here’s the inside. You can see some additional metal in there that you would ordinarily screw a ground screw into, as well as some circuit board slots (which I won’t be using).

Here’s a bit of a test fitting just to see if I can even get the main mechanical parts in there:

The answer is “just about”. So we start drilling very carefully.

First the switch:

Then the first jack:

 

Second jack in place:

I’m testing the jacks with something plugged in, just to make sure I can still do that without any contacts banging up against anything they shouldn’t. In a normal-sized enclosure this would be less of an issue, but things are very close together in here.

Next to go in is the DC-in jack, since there’s really not much flexibility there. Once that’s done we’ll be able to see what space is left for the pot and the circuit.

OK, the pot can just about fit in here, with the legs bent up (in fact I ended up cutting off parts of the legs to help them fit).

Just one more item and that’s the LED. I decided to go with a smaller 3 mm LED. And I also decided to make use of one of the little grounding threaded holes I mentioned earlier. 

This (little) one, which I drill all the way through so the LED can sit in it:

Here’s how that looks from the front:

And we’re done (apart from the circuit and wiring of course):

Here’s how THAT looks from the outside:

When you’re trying to fit a circuit into something this small you really have to think about how big your circuit board can be and still physically fit. Not only that, but things like whether the capacitors can sit vertically without hitting the back of the enclosure (they couldn’t, which is why they’re lying flat).

Anyway here’s the board done, and wired to the pot:

I stick some insulating material between the pot and the board so there are no short-circuits:

Before we wire this up, I’m going to paint the enclosure. How about blue?

And here it is wired up. Not the easiest pedal wiring job I’ve ever done.

I’ve put some insulating tape anywhere where there was a danger that something might touch something it shouldn’t, and that included the inside of the back cover.

 

Finally it’s time to put it together.

As you can see above, my "Sharpie" decal didn't turn out as well as I'd hoped, so I made something up in Illustrator, printed it out on normal paper, stuck some Scotch tape on top to make it look laminated, and then put double-sided sticky tape on the bottom. Et voilà, a cheapskate decal.

The first time I tried it out, it sounded bad. It worked, but not well. I replaced the 10k resistor with a 100k one and it sounds great now. You may want to do the same.

Here's a quick and dirty (see what I did there?) demo of the pedal: