Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

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:

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Happy Christmas

20121220-202553.jpgTo many festivities give us an opportunity to work on belated projects. This year, I couldn’t take time off between Christmas and New Year, but can’t complain as will be taking a week off in January.  Either way, I look back and is amazing everything achieved over the year. And I’m grateful to have such an opportunity to realise many of the dreams and ideas. What makes me even happier is the bigger list of projects that I continuously dream about. That is what I call passion and I know many of you feel the same way.

Not only I wish you all a very merry Christmas with your loved ones and a fantastic 2014, but also I hope you can also find time to bring to life all those great audio projects piling up!

Cheers,

Ale

HT ripple measurement

 Measuring HT supply ripple with your Sound Card

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Despite having used the Pete Millett’s SC interface to measure HT supply ripple, it does have a limitation as the input capacitor is rated 400V. Morgan Jones recommends on his latest great “Building Valve Amplifiers” (second edition) three methods for doing this. I opted the one shown above as it does help rejecting common mode noise and also protecting the SC interface. I’m careful enough to not revert the input by accident. If that should happen, the transformer used in this case provides a 500VAC (700VDC) isolation. Normally I play around 400-600V, so should survive in case of misuse. I played with LTSpice to tweak the secondary resistor down to 200Ω when input capacitance is raised to 10μF. Other transformers may allow reducing size of capacitor and increasing resistor up to 10KΩ. I have an OEP E187F at hand so this is what I will use.  f-3dB is about 30Hz and provides flat response at 50Hz and 100Hz to measure accurately input ripple.

Will build and report 🙂

Input capacitance

Drive Mr. Miller, drive…

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Input capacitance test rig

Whilst designing triode driver stages for a 300B SE amplifier, I looked into the input capacitance of high-mu triode (or triode-strapped pentodes) that are suitable candidates here. We know Miller capacitance is the big drawback of the triode driver stage, so I kind of liked the idea of testing how much this capacitance could be for the lovely Russian triode-strapped pentodes like 6e5p, 6e6p, 6j11p, 6j5p, 6j52p, etc.

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DL103 cartridge review

IMG_1416This 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.
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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.

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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.

 

New Lundahl distributor in UK

I’m pleased to see Big Bear Audio to start the distribution of the Lundahl transformers in the UK. I’ve got my first order and everything went really smooth. A fantastic service from Big Bear Audio.

Highly recommended, Big Bear Audio is now my new iron supplier!

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814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

IMG_1401After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s.  I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

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