The Four Pole RIAA Network

 

I have been a DIY builder of phono-stages for quite some time now but it took me an article by Jim Hagerman titled On Reference RIAA Networks to realize that I have been doing it all wrong. The RIAA equalization is the ‘tool’ that enables records to store information across the auditable spectrum on LP records. It was created with the single purpose: to increase playing time and quality. Information is stored on the record grooves by an amplitude modulation from a zero plain. Lower frequencies thus require more space and it is impractical to maintain a constant signal level because you diminish playing time. This did not meet the goal of long playing time while maintaining the full audio spectrum. In order to achieve this you need to lower the signal level at the low end during recording so that the low level information is stored in smaller space along the vinyl track. The inverse is done at the higher frequencies while bumping up the level at the high end. RIAA equalization is therefore a form of preemphasis on a recording, and deemphasis in playback mode. To achieve playback Phono stages are equipped with what is called a RIAA Network. Below you can see one in use from a pre-amp I have built. The Network is comprised of Capacitors and Resistors that form the filters necessary for equalization.

 

 

The Process:

 

The original recording comes in with a level frequency spectrum.

Then comes the first RIAA network which pre-emphasizes the signal

The signal is not ready to the printed onto the vinyl record.

The Pickup head now translates that signal and brings it to the phono stage.

Here the RIAANetwork within the preamp de-emphasizes the RIAA curve.

With the 4th time constant you have the original signal in its entirety.

 

 

                   

After reading the article I came to the conclusion that I had been building the RIAA networks with a missing factor. That component was the mythical 3.18ms corner in the RIAA response curve. Most of the amplifiers I had built till this point did not utilize this final component because there was so little information available about it. It is almost like a dirty little secret that has its roots in the recording industry. Records are cut using cutting heads (the inverse of a pickup cartridge) and those heads can burn out their cutter heads if the frequency gets to be too high. What the companies did to alleviate this problem was to create a 4th time constant that would counter the effects of burned cutting heads. This element was not communicated to the builders of phono preamplifiers and for years this constant was overlooked. It might have been overlooked for financial reasons or because the engineers did not consider it a factor in sound quality. Well how can it not be if you are trying to replicate the exact original recording? Below is a diagram from Jim Hagerman’s article that perfectly illustrates practical application of the 4^th cutoff.

 

 

 

The 4^th time constant is at 50,048Hz.

 

 

The next step was to go back to all of my old Spice models and apply the new constant. The most accurate method to simulation of a RIAA stage in simulation is to create the inverse RIAA circuit within Spice. I set the frequency range as wide as possible from 0 Hz to 500 kHz 20 to 20,000Hz “audio band”.

 

 

 

 

Graphical representation for generating reference inverse RIAA curve.

 

New Network Design

•                      50 ohm source impedance (many generators do not use the 600 ohm audio standard)

•                      600 ohm output impedance

•                      Dual output gains of –40dB and –60dB @1kHz

•                      Standard capacitance values. Figure 6 shows my new modified RIAA network.