Hi everyone,
Just wanted to share the start of a project I’ve recently finished, a passive inverse RIAA filter that ended up being surprisingly useful (and a bit more involved than I expected). It’s designed to help with testing both MM and MC phono preamps, staying accurate across the full audio band: 20 Hz to 20 kHz.
Laser-engraved dual-channel filter enclosure:
The motivation was pretty straightforward. Most IRN filters I came across, whether commercial or DIY, either lacked the precision I needed, didn’t handle MC levels well, or relied on active stages that I wanted to avoid. If you’re building or measuring phono stages, having a clean, analog-domain IRN filter is super helpful, especially when you want to check frequency response or verify your RIAA curve.
The original “The Audio Amateur” article, 1980:
This one’s based on the well-known Lipshitz & Jung design from Audio Amateur (1980), with a few ideas borrowed from more modern builds like the Hifisonix Accurate IRN. But it’s been adapted and rebuilt from scratch with the following goals in mind:
•Dual outputs: –40 dB (MM) and –63 dB (MC)
•Fully passive RC topology, no active gain stages
• ±0.11 dB deviation from ideal RIAA (simulated)
•Separate shielded PCBs for left and right channels
•Premium hand-matched components for better tolerance
Resistors and capacitors were carefully hand-picked to ensure high precision and matched performance between boards:
I’ll go into the schematic and how I structured the attenuation stages for the next post. Even if IRN filters aren’t part of your regular toolkit, I’d love to hear what other folks here are using when testing phono preamps.
Cheers,
Alan
Just wanted to share the start of a project I’ve recently finished, a passive inverse RIAA filter that ended up being surprisingly useful (and a bit more involved than I expected). It’s designed to help with testing both MM and MC phono preamps, staying accurate across the full audio band: 20 Hz to 20 kHz.
Laser-engraved dual-channel filter enclosure:
The motivation was pretty straightforward. Most IRN filters I came across, whether commercial or DIY, either lacked the precision I needed, didn’t handle MC levels well, or relied on active stages that I wanted to avoid. If you’re building or measuring phono stages, having a clean, analog-domain IRN filter is super helpful, especially when you want to check frequency response or verify your RIAA curve.
The original “The Audio Amateur” article, 1980:
This one’s based on the well-known Lipshitz & Jung design from Audio Amateur (1980), with a few ideas borrowed from more modern builds like the Hifisonix Accurate IRN. But it’s been adapted and rebuilt from scratch with the following goals in mind:
•Dual outputs: –40 dB (MM) and –63 dB (MC)
•Fully passive RC topology, no active gain stages
• ±0.11 dB deviation from ideal RIAA (simulated)
•Separate shielded PCBs for left and right channels
•Premium hand-matched components for better tolerance
Resistors and capacitors were carefully hand-picked to ensure high precision and matched performance between boards:
I’ll go into the schematic and how I structured the attenuation stages for the next post. Even if IRN filters aren’t part of your regular toolkit, I’d love to hear what other folks here are using when testing phono preamps.
Cheers,
Alan
Following up from the first post, here’s a closer look at how the circuit was actually built. The filter uses a fully passive RC topology, simple in principle, but getting the precision right took a bit of effort.
The idea was to create two outputs, one at –40 dB for MM phono stages and another at –63 dB for MC inputs
Both paths follow the same curve, just scaled in level. Getting them to track each other closely required careful impedance planning, and quite a bit of part-matching, to be honest.
Many of the resistor and capacitor values didn’t exist as single parts, so I ended up building combinations (series/parallel) to hit the targets more accurately. All components are 1% metal film resistors and polypropylene caps from WIMA and Nissei. A bit old-school, maybe, but reliable and consistent.
The layout is dual mono: each channel has its own board and its own enclosure. That decision wasn’t just aesthetic, having the channels completely isolated actually helped with crosstalk performance when testing stereo phono stages.
Below is the schematic and a close-up from the build process. Would love to know what others here think, especially if you’ve designed something similar, or have tips on improving passive IRNs.
If resistors R13–R16 are changed to 562 Ω, and R17–R20 are changed to 40.2 Ω, the attenuation levels become:
1) –44 dB at 1 kHz for MM output
2) approximately –68 dB at 1 kHz for MC output
This modification may be useful if your phono stage has higher input sensitivity or if you want more headroom in measurements.
The idea was to create two outputs, one at –40 dB for MM phono stages and another at –63 dB for MC inputs
Both paths follow the same curve, just scaled in level. Getting them to track each other closely required careful impedance planning, and quite a bit of part-matching, to be honest.
Many of the resistor and capacitor values didn’t exist as single parts, so I ended up building combinations (series/parallel) to hit the targets more accurately. All components are 1% metal film resistors and polypropylene caps from WIMA and Nissei. A bit old-school, maybe, but reliable and consistent.
The layout is dual mono: each channel has its own board and its own enclosure. That decision wasn’t just aesthetic, having the channels completely isolated actually helped with crosstalk performance when testing stereo phono stages.
Below is the schematic and a close-up from the build process. Would love to know what others here think, especially if you’ve designed something similar, or have tips on improving passive IRNs.
If resistors R13–R16 are changed to 562 Ω, and R17–R20 are changed to 40.2 Ω, the attenuation levels become:
1) –44 dB at 1 kHz for MM output
2) approximately –68 dB at 1 kHz for MC output
This modification may be useful if your phono stage has higher input sensitivity or if you want more headroom in measurements.
I built mine from the Hagerman article here- https://www.hagtech.com/pdf/riaa.pdf
I admit to being lazy and only building one channel. Never really felt a need for two. I do have a construction advantage- the ability to measure small capacitance values with high accuracy and possession of a GR 1493 decade transformer. It turns out that measuring RIAA accuracy is harder to do than most people realize. If using a DVM, read the accuracy specs carefully.
I admit to being lazy and only building one channel. Never really felt a need for two. I do have a construction advantage- the ability to measure small capacitance values with high accuracy and possession of a GR 1493 decade transformer. It turns out that measuring RIAA accuracy is harder to do than most people realize. If using a DVM, read the accuracy specs carefully.
Good job!
I use a different approach: I use a close but lower value component and patch it up with "shims", i.e. the 3.6nF can be a 3.3nF // 270pF // 33pF.
I also use a bridge type LCR meter to sort at least the main components; a 0.5% bridge cost only about $60.
Finally, polystyrene caps have a better tempco than MKP, but you have to hunt them down. I buy the Philips 1% type from eBay.
I use a different approach: I use a close but lower value component and patch it up with "shims", i.e. the 3.6nF can be a 3.3nF // 270pF // 33pF.
I also use a bridge type LCR meter to sort at least the main components; a 0.5% bridge cost only about $60.
Finally, polystyrene caps have a better tempco than MKP, but you have to hunt them down. I buy the Philips 1% type from eBay.