Vinyl hunting in NYC

Last Sunday had the great opportunity to spend a day out in New York prior to a hectic work week ahead. Obviously record hunting was on my list of things to do. It’s been a long time since I last went to NYC and can say that the record scene has changed a lot. I’d like to highlight this great place owned by Jonathan Sklute. A passionate guy that loves vinyl records so much that has this cool small shop located in 218 East 5th Street, right in the East Village. The record store is called “Good Records NYC” and the owner proudly curates a selection of Jazz, Rock, Blues and other genres. Each record is carefully clean using a nice RCM (which I didn’t manage to capture model as it was placed on the back).
I came out with four gems from the Rolling Stones, The Kinks, Eric Dolphy and Little Sonny.

Highly recommended if you are around New York 🙂
Ale

4P1L driver – LL7903+LL1671 tests

Last week I did some preliminar tests with the LL2746 in 1:2 step-up mode. Despite having measured good results with it, it will be a challenge to drive grid current given that the output impedance of the 4P1L will be multiplied by 4 so about 5KΩ.

Before looking at the LL1671/20mA which is suitable for multiple driver valves, let’s see how the LL2746 driver performs with the addition of the input step-up microphone transformer LL7903. I’m currently using the LL7903 in my 814 SE A2 amplifier and sounds really nice. The LL7903 was wired up in 1:4 setup so gain can get about 63:

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6e5p beam tetrode SPICE model

Thanks to the great work that Derk put into his great Extract Model tool, I helped him to refine and debug the application by tracing the challenging 6e5p beam tetrode. After 4 versions we managed to optimise the model:

The model fits really well including the kink section but given the saturation of the 6e5p tetrode at currents above 50mA there is a slight divergence as the valve cannot reach the same anode current at higher voltages.

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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:

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:

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:

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4П1Л / 4P1L Pentode Curves

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:

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:

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:

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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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