Testing RIAA stages is a real challenge. However, with current testing gear becoming affordable to us thanks to the development of PCs and sound card interfaces, now you can test your RIAA stage without further excuses. There are software-based solutions to implement IRIAA transfer curve for looking at a flat frequency response of our RIAA stage, but they are not free or easy to implement unless you are skilled on data manipulation on your PC. You can also build your own analogue IRIAA stage. In fact, Morgan Jones suggests a great way of implementing this in his great “Building Valve Amplifiers” (Second Edition) which was recently released. I recommend the book highly, albeit I will not fall in the trap again of buying the e-book version. I was highly disappointed with the e-book release of 4th edition of “Valve Amplifiers”. I guess I will just use my Kindle for reading plain books instead.
The circuit proposed by Morgan Jones is inspired on Hagerman’s paper (thanks Gary B for providing the link) which includes an IRIAA stage for same purposes:
Morgan Jones changed Hagerman’s design component values to fit to the 3.18uS Neuman pole instead of 3.5uS and combined 1% and 0.1% resistor tolerances to improve the accuracy of the IRIAA response. You can get the essence of the circuit from the above diagram.
I modified the circuit proposed by Morgan Jones slightly to fit my requirements and in particular I added the additional resistors to simulate DL103’s output resistance of 40Ω.
I had a nice strong mints sweet metal box from Marks & Spencer and used it ast the housing for the IRIAA stage. A BNC adapter also was added to the input to ensure I could hang the IRIAA box from either the waveform generator or the SC interface:
Building this box takes just a couple of hours and you will be amazed with the results you can achieve. I wish I could post the diagram but I don’t have Morgan Jones’ permission so I hope this post encourages you to buy the book if you weren’t still convinced to do so.
This IRIAA stage has two outputs. One for MC and the other one for MM. You can see the ideal frequency response below:
At low frequencies, the MM output will go down as low as 70dB, whereas the MC will attenuate as much as nearly 100dB.
I did a quick sweep MLS frequency test on my workbench and here are the results achieved:
You can see from the MM output the accuracy is really good. You can test your RIAA stages with acceptable accuracy, which is what we need at the DIYAudio level.
Just as an example, the below test shows my work in progress MM RIAA stage which is actually being converted to MC given the recent acquisition of the DL103 cartridge:
You can see from the IRIAA-RIAA response that the curve remains flat between +/- 0.1dB. You can get a more accurate measurement with individual frequency measurements but this was good enough to demonstrate the value of the IRIAA box.
An invaluable piece of testing gear for measuring our RIAA phono stages 🙂
Nice implementation and overall great website. Thanks for the great job documenting your results and progress.
Until you get permission from Morgan Jones to publish the iriaa schematic, you can point people to Jim Hagerman’s website where he’s posted a nice article on RIAA networks and includes an iriaa design that is probably similar to the one from Morgan.
Take a look here: http://hagtech.com/pdf/riaa.pdf
—Gary
Thanks for your kind words and also for providing the link to the Hagerman’s paper. I had it around but completely missed out the web source. I added and hopefully this will give everyone and idea of the simplicity of the circuit. Morgan Jones has changed the component values to fit to the 3.18uS Neuman pole and mixed resistor 1% and 0.1% tolerances to improve the accuracy of the IRIAA response.
By the way, there is no reason why you cannot publish the exact circuit you used, even if it is the same as Jones’s (which is isn’t), as long as you draw the image yourself. It is quite normal and acceptable to re-draw and re-publish public domain circuits.