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
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
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.