Reverse Your RIAA Filter
“More Aair”
Need more Aair in your sound?
Last modified: 18-Nov-2006
As you know, I like to design and build Phono preamplifiers. And although these designs are quite good simulated and even calculated with my spreadsheets, there will always be small deviations from the theoretical best curve. Why? because tubes are just NOT lineair devices, and the specifications of most electronics components are not at all within 1% accuracy too. Environmental conditions and the load of cartridge and connecting pre-amplifiers is also of influence on the sound experience.
And testing my amplifiers seemed like a good idea, if only these phono amps would be just amps and not big filters with amps. Therefore I decided to build a reverse RIAA device that would invert the function of a normal RIAA amplifier. This way, by connecting this device to the input of the RIAA phono amp and connecting normal music of a wave generator to the input of the device, the output of the phono amp would have a lineair relationship with the input of the reverse RIAA device. Of course this would only work if the reverse RIAA filter would be of excellent quality. And with this I do not necessarily mean audiophile but measuring right. And needless to say the phono amp must be good.
But most important, it would be possible to test the filter in the phono amp in an easy way: Connect the input of the wave generator to one channel of the oscilloscope and the output of the phono to the other input and it would be easy to see whether the amplification/gain was constant over the total frequancy spectrum of 20-20k Hz.
This small project has been described on the RIAA background pages as well, only there the focus is on the design of this filter.
The following circuit was used for the reverse RIAA module (see for an in-depth discussion the RIAA bakground page 6):
The values used for the components were as follows:
| Component | Value | Type of component |
| C1 | 6.2nF | 1.5nF and 4.7nF parallel Both Polypropylene 1% |
| C2 | 1.8nF | Polypropylene 1% |
| R1 | 511k | Metalfilm 0.6Watt, 1% |
| R2 | 41k2 | Metalfilm 0.6Watt, 1% |
| R3 |
1740 |
Metalfilm 0.6Watt, 1% |
| R out | 604 | Metalfilm 0.6Watt, 1% |
As described on the background pages, I designed a filter and built it on a breadboard. Next I have to find an enclosure for the filter. For the moment I just used a standard Black interconnect, cut it in half and soldered it directly to the board.
I plan to use one of my standard enclosures for this project, but the "open air" version works well so I'm not really in a hurry.
Actually, it works quite well and I'm wondering why I did not build a reverse filter years ago. Since it does not make sense to have a turntable in my hobby room anyway this solution saves me lot's of space and I can use my iPod as a source of music and test the Aair.
Besides the listening test I need to perform an objective test with the scope as well. But since I'm in the process of moving my DIY stuff from one room to the other in the house, I did not power up all tools yet, so please be patient as I have to connect the scope and other stuff first ...
| Reverse RIAA filter background >> |