Ale Moglia

SiC cathode bias

Playing this afternoon with the SiC C3D02060F, which can happily run +20mA with very low dynamic resistance. Ideal for the 6e5p/6e6p driver I had in mind for the 300B amp:

At 20mA of cathode current the forward voltage is 0.85V and dynamic resistance 1.5Ω. If cathode current is 40mA instead the resistance drops down to 1Ω:

The 6e5P/6e6P will run comfortably around 30-40mA and bias tends to be around 3.5-3.7V to swing nice volts as needed. Therefore, we will need 4 SiC in series. 4Ω resistance is good enough and not adding much when reflected to the anode…

Diode cathode bias

Playing with the semiconductor curve tracer I did a quick test of potential candidates for diode cathode bias:

The popular HLMP-6000 is a superb LED with its low impedance. The SiC diodes have proved to be a great match with an impedance lower than 2Ω. Bias voltages will be around 0.7-0.8V for low currents. The classic Schottky SB540 has a very low impedance, but its forward voltage is so low that is not practical for diode cathode bias. What surprised me was to see the 1N4007 to be a good match. The impedance is higher than the LEDs or SiCs, but good enough. The green LED on the opposite extent has a significant dynamic resistance over 10Ω.

Interesting to see that a minimum of 2mA should be run through with small signals to ensure the diode operates in the linear region. The higher the better. An arrangement with an extra source of diode current (e.g. LND150 or DN2540 CCS shunting current to the diode) can be used when dealing with lower cathode currents due to the valve being used.

Further tests are required….

Welcome baby Sofia

On Sunday 19th our lovely baby girl Sofia arrived in St George’s Hospital in London. Both mum and baby girl are doing fine and we are trying to adjust to the new lifestyle.

She’s a musical baby. In fact, back in the operations theatre room I sung to her “Rocky Racoon” to sooth her once the medical team gave it to me for the first time. We used to play the Beatles’ white album frequently during her pregnancy and am sure she recognised the tune.

She loves “Ella Fitzgerald” and it’s one of my favourite records to play on at 2 or 3 in the morning to get her back to sleep

I’m still enjoying playing the valve amps whilst it last. It will be probably for another 4 months so best to get the most out of them

I’m a happy dad…

Ale

Sound Card interface (revisited)

For measuring my system, I was struggling with not having a floating USB sound card to use with Pete Millett’s interface. I found a cheap alternative (Focusrite Scarlett 2i2) which can do ASIO 96kHZ and 24-bit.

Above is the response with internal loopback at 1Vrms on the 2Vrms scale of the SC interface.The distortion is really good below 0.01% just above 25Hz:

With the balanced input and outputs the noise level is really low. Below -120dB mostly across the entire 20Hz – 50kHz band. Inband noise level is below 100uV when the DAC volume is at highest level and the ASIO generator is muted:

For distortion testing this USB audio interface has a very good oscillator output performance. Check out the 100mV level below:

AVC finished

Today I soldered the remaining connections to get the second channel up and running. The result is impressive. The AVC project is now finished. And I’m a happy bunny.

The sound is stunning. As good as I was expecting to be. Very detailed and no loss at bass or treble. The combination of 4P1L driving the AVC and the 814 input step-up transformer seems to work much better this way. The 4P1L preamp has no challenges in driving the AVC, however the full signal swing from the 4P1L step-down via by the AVC seems to sound extremely well.

Thanks to Dave Slagle for such a wonderful AVC. I measured the AVC and matching with a FR sweep and the scope. I was pleased to see the matching between channels and also the nice response 10Hz up to 30-40kHz.

Hard work to wire but it’s worth every penny, trust me…

I have also tried a new set of interconnect cables I made using PTFE shielded RF coaxial. Very simple to build and with nice performance.

The AVC is placed very closed to the power supplies (as you can see on the picture below). The AVC is still very quiet and there is no hum picked up due to the system layout. Lucky me.

AVC work continues

Started yesterday and soldered so many enamelled cables between the AVCs and the rotary switch. Tested one channel last night and was amazed with the sound. Still have 10 cables to finish the remaining channel:

Slagle’s AVC project

It’s been a long time since I had the chance to do any work on audio. I’m working on my solid state amp albeit very slowly. I need a volume control before I get the opportunity to install the new solid state system.

The Slagle’s AVC have been waiting for more than 6 months. I’ve been put off by the milling and woodwork involved. I simply don’t like that work. I found time and energy today to get started and managed to make very good progress.

I used existing 4mm aluminium top plates I had at hand, hence the layout of this box. The hardwood frame has a first coat of wax. Need to add a second to reinforce the tone and make it slightly darker. The rotary switch is a Russian 28-step military unit. Way more cheaper than the expensive hi-fi ones. The AVCs are from Dave Slagle. They have a fantastic reputation and having heard them before there is no doubt this will be a great addition to my system.

I had to build a mounting frame for the AVCs. This support unit will hold the AVCs to the top plate. I have also added a mute switch which is quite handy.

Soldering the AVC is a daunting task. Something I will do another day.

Happy Easter!

Transistor Curve Tracer

Finally I’ve had the chance to test Locky’s Intelligent Curve Tracer. This is a great piece of testing gear if you’re looking to match transistors or derive some curves for solid state amps. Great for FET matching in RIAA stages as well.

Here are some quick tests I performed after installing the software:

More cartoons

A set of quick drawings whilst listening to good music…

Now my heart is full

Don’t be ashamed to have friends

I’m coming

Chelsea Girl (Nico)

Social drink

Walking

PL84 triode

The PL84 pentode is very well known out there. I’m not going to write about it as there is plenty of information about the use of this pentode in push-pull amps, etc as well as comparisons with the EL84, EL86 and Russian equivalents such as 6P14P, 6P15P and 6P43P. I recommend you to check Klau’s work here.

I have a nice set of Telefunken PL84 which I may be using in triode mode as part of a Spud project I’ve been working on the design for some time. I’m interested in triode-strapped curves but also will be looking at tracing the Schade-feedback curves when I get the time to do so.

I quickly found a sample valve that was close to the pentode specs at 90%. Good enough for my purpose so I proceed to trace the triode curves quickly in uTracer.

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