Bartola® Valves

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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