4P1L – LL2746 driver test

After a recent discussion in the DYI Audio forum about the 4P1L drivers, I decided to do some quick tests on an idea I had around to use a step up transformer (1:4) – 4P1L and step up interstage transformer (1:2) to drive a 300B or similar using the 4P1L in filament bias.

First suspicion is on whether the 4P1L has the grunt to drive a capacitive load which would be a real challenge in a 1:2 step up as load capacitance is multiplied by 4 when impedance is reduced by a factor of N^2=4.

I built a test rig with the 4P1L in filament bias using a 15Ω wire-wound filament resistor and connected the filaments in parallel to obtain easily a nice bias voltage with 650mA of filament current. Also lower Rf will improve the low frequency response as helps keeping low the output impedance:

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4P1L rat nest

The valve was biased at Ia=30mA / Va=160V and grid bias is about -10.2V. A 10KΩ resistor was added as a primary Zobel as per recommendation of the datasheet. Then it was replaced by a 25kΩ potentiometer (P1) and the right value was found by looking at the frequency response.

Initial tests showed a very good response at 1kHz with only 0.24% THD @200Vpp output. The gain is approximately 16. The mu of the 4P1L with paralleled filaments is around 8 and lower than when used in series which is approximately 9-10.  Albeit the results were promising initially, the real test of this stage is by looking at high frequency response where the capacitance will makes it real pain.

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Measuring choke inductance

Recently, I built the “Noise Inspector” with the objective to help me measuring inductance and transformers. After working with it for a little while I realised that the proposed testing circuit wasn’t correct. The bias current of the inductor under test won’t allow the use of the noise inspector unless the loop back to the variable HT supply is done before the sensing resistor:
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Yet, there is a limitation with the above circuit. Unless we add an amplifier between the generator and the test circuit, the DUT will only see a couple of volts (rms) and measured inductance will be lower than the expected. Continue reading “Measuring choke inductance”

The Noise Inspector

Detecting those little creatures from the bottom

When recently measured the performance of my CCS designs, I found that I wasn’t able to measure below -114dBV (2uV) and therefore limited my ability to measure CCS impedance. I was also keen to measure inductors, transformers and other reactive components to derive 2 or 3 component models for more accurate simulations during the design process. A preamplifier was in order, so I looked at options with some of the ICs I had at hand. A very nice suggestion from Burr Brown/Texas Instruments is shown below. The INA106 is a precision 10x differential amplifier. Not a cheap device, but quality does comes with it unfortunately. When coupled with a pair of OPA37, a very accurate differential preamplifier can be built with extremely low noise and distortion:20140118-184418.jpg

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More on CCS

As I continue with my design of CCS to be used on my next designs as part of the supply filtering stage, I looked at testing the performance of my latest CCS using the following circuit:

CCS test The limitations I have currently is that my waveform generator can do 7Vrms maximum and in low frequency the existing noise level will be the limitation clearly.  As suggested by Gary Pimm, adding a battery operated differential preamp at the point of test will be a great way of raising the low level signal from the sound card interface noise floor. That, will be for a future day. I just want to see how well the CCS performs.

I set the CCS to 30mA and measured attenuation from 50Hz to 30kHz. The results are quite encouraging despite the lack of pre-amplification:

ccs NOISE TEST

The real life CCS is not that great as in the simulation. There is about 10dB difference with the Spice simulation. You can see that I can measure below -130dB attenuation without a pre-amplifier. Still is quite good, more than what we need for.

The CCS is operating to the level of what I need, so test passed 🙂

 

HT Power Supply Noise Measurement

I posted recently a great idea to measure noise levels from our power supplies. Yesterday I managed to put together this small interface circuit. I used a remainder piece of double layer PCB big enough to fit the bulky capacitor, the transformer and the output BNC connector. The input is a just a simple set of copper turrets. Special care is taken in laying out the ground planes to avoid ground loops. Also the transformer is grounded at one side only of the case. A finished interface looks like this:

noise test interface 1

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A versatile CCS load

I’m a heavy user of CCS loads. I generally use them to test my valves regardless of using my curve tracer or not. I tried multiple CCS types in the past with good results until I ended up burning one FET or protection zener or whatever due to the abuse of it.

Testing high current loads is not easy at high voltages. The DN2540 is rated at 400V. Not enough. You can use an expensive 01N100D which is another depletion 1KV MOSFET that has a lower Ciss (54pF against 200pF) or you can look at the cheaper enhancement FETs which require a different bias arrangement. If we are looking at modifying the classic cascode self-bias pair, it is a convenient opportunity to improve the VDS bias of the lower FET to improve the frequency response by lowering the Ciss. Remember that in a FET the Ciss is proportional to the VDS. The classic cascode pair has a disadvantage as the lower FET is biased with VDS lower than 1-2V to ensure the upper FET is biased correctly.

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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