Listening to the new RIAA phono stage

It’s been some time since I built my JFET shunt-cascode RIAA MC stage. I have listened to several well know records for some time with this phono stage in my system and others. I took it to the London Circle Audio meeting recently and it was very well received by most of the circle members. I even had some requests to build units!

Now I can say that it’s completed. Albeit I haven’t built the charger for the lead-acid battery pack, it sounds fantastic and has great autonomy. Long gone days were I was recharging my NiMH pack! Now I can relax and enjoy music without worrying about charging the batteries now and then. I haven’t measured the voltage across the battery cells so far.

More lasting than bronze – John Coltrane

Usually I play my John Coltrane records. Not just because I personally find Coltrane to be one of the best musicians ever existed on earth, I have some specific songs I use for testing. When I was younger, I used to listen for hours the same Coltrane records when learning saxophone. I was keen to listen to every single detail on the phrases, solos and arrangements of the Quartet, Quintet or the Sextet formations. As a sax player, you learn many solo parts, tunes and you know by memory every expression made when blowing the horn. If you don’t have any Trane’s record or even you haven’t heard any of his compositions, I urge you to seek his wide and bast repertoire. From “Love Supreme” to “Giant Steps” and many other gems. Several years ago when I started to rebuild my vinyl collection here in London, I found this excellent compilation of Coltrane: “More lasting than bronze“. It’s a great sample of Coltrane’s music. I use it to test my system. The horn bass notes, the pulsed bass, drums and pianos have a great level of dynamics and detail I constantly listen to. It grabs my mind and takes me to another planet. That is music, more lasting than bronze…

JFET RIAA Preamp – battery supply

I built a new pack of 12V+6V lead-acid batteries to provide +/- 18V for my JFET RIAA MC phono stage. Despite the bad reputation of these being noisy, Geoff tried them with great success. The Haze brand are the recommended and the low capacity ones (i.e. 1.3Ah) are very quiet.

I decided to build and test it. As the proof is in the pudding!

differential chokes and OSCON caps

Battery pack

Battery pack completed

Measuring noise

Here is a simple test on my workbench of the 18V battery set with a 20mA load to simulate the RIAA stage consumption. The LC is formed by a 33mH choke in differential mode + 100uF/20V OSCON capacitor.

No twisted cable pair, just banana alligator clips. The noise level is really low and is obvious that 3.3μV 50Hz hum is picked from the workbench. I also listened to it for a while and can hear the difference. I monitor the FFT with no average and lower FFT size and also couldn’t see any spikes due to chemical reactions. They may happen in future though 🙂

The performance of this battery pack is outstanding, so far so good!

RIAA phono stage completed

FET RIAA Phono Stage

Finally completed today. It seems impossible from me to get a project finally boxed properly. I’ve done it this time with the RIAA phono stage built recently thanks to the help and guidance from Rod Coleman. This MC stage has a gain of about 70dB for MC cartridges and currently running my DL103a with a 200Ω input load. The beauty of the folded cascode (or shunt cascode as Rod refers it to) is that Miller capacitance is not a problem thanks to the fixed voltage at the drain of the FET input stage. This helps us to have a low input capacitance stage. The second stage is op amp OPA637.

I need to take final measurements of this Phono stage but this is how it looked when I was initially testing it on the bench:

Nice RIAA compliance to +/- 0.1dB. LF noise and gain levels made it difficult to capture the LF response below 100Hz.

The distortion is also really nice with less than 0.025% @ 1kHz and nice harmonic distribution thanks to the CCS at the output which is forcing the operational amplifier to operate in class A:

This stage was well received at the recent London Audio Circle Meeting. It has a nice clean and detail sound. I tried it with multiple MC and even low MM cartridges and has a nice response across the whole bandwidth. Great bass and delicate treble.

I think it is a very quiet phono stage given its high gain. I really like the overall sound and response and I guess that the negative side of it is the 18V battery pack made up of PP3 rechargeable batteries. It needs charge every two weeks and may be a pain in the back. I will try a DC supply, but will have to be really quiet to substitute my battery pack.

IRIAA test box

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:

Extracted from "On Reference RIAA Networks" by Jim Hagerman — Extracted from “On Reference RIAA Networks” by Jim Hagerman

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:

Inside the IRIAA box

MC and MM outputs

IRIAA box

Input connector

Inside box 2

Inside box 3

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:

Continue reading “IRIAA test box”

DL103 cartridge review

This may not be any news to most of you, but after a long time I managed to get the upgrade needed on my turntable: the cartridge. The Denon DL103 was a perfect decision to step up into the MC territory at an affordable cost. Many will say out there that there are better cartridges, but that is not the point of my review.

I tried some MM ones and until now couldn’t get the Lenco + Audiomods arm to sound as good as it sounds now. The DL103 is challenging due to its low compliance and output levels.

Although I have a step-up transformer, I’m saving it for my valve phono stage project which I hopefully will get my hands on next year. Currently still have a solid stage RIAA stage made by Project which sounds really good in my opinion. Shortly will complete my JFET shunt cascode RIAA stage and listen to the difference.

Jeff from Audiomods sent me the copper shim for the headshell but my cartridge screws aren’t long enough to fit both. I was worried on the performance by not adding sufficent mass to the arm, but after testing it, I was surprised to see that it was not the case.

The sound performance is really good. No loss in bass or treble, with a flat response, the dynamics are brilliant. Noise level is very low. Tight drums and clearer sound. I like the detail and even playing loud sections it is great. I will continue to listen to this until I can find anything to dislike from it, so far it’s superb.

JFET RIAA preamp build

Last week I started with my JFET RIAA stage. This stage is designed for my current MM cartridge (Ortofon 3e) which has a relatively high output at about 4mV. The stage is a variation of the Werner Ogiers’ circuit, inspired by the John Curl Vendetta. Rod Coleman helped me out here to get this design optimised, so thanks Rod again for the contribution.

A two stage RIAA stage with only about 42dB gain is not ideal. I’m looking at replacing my cartridge soon and hopefully buying an DL-103 MC cartridge. I will then easily tweak the gain and RIAA network to use the Denon one instead.

I built a breadboard of one channel and tested it:

Here is a first test of the RIAA response: Continue reading “JFET RIAA preamp build”