Fully 21st Century Tube Preamplifier

[Charles G]

@Jaytor

Relay attenuators are my default preference, but the fact they occupy they large amount of space on circuit boards, are expensive, call for hard to obtain resistor values, and elevate the design risk for degraded audio signal integrity, make them not qualify for my designs. Also, some could be annoyed by the typewriter-like sound relay bank switching makes. Furthermore, relay attenuators can have issues with zipper noise.

The Muses chip is essentially switchable resistor ladders on a chip (or the equivalent of a relay attenuator). Per channel, it has a network of two ladders, one for a negative feedback loop to set gain and the other for volume. The chip was designed for op-amp use but I adapted it for vacuum tubes. I cannot distinguish the sound between a regular volume control and the chip. This could be because the chip has no positive gain, or op-amps on it, probably just JFETs to work in place of the relays (hence the negative rail).

What also is nice is the zero crossing detection feature. This prevents the attenuator from changing the volume until the signal passing through it is zero, which helps mitigate zipper noise. Good luck doing that with mechanical relays!

Thanks,
@TorroidDoughnut

Out of curiosity, what are you using the 10k gain pot in the muses chip for? Some other gain setting function in the preamp separate from the volume control?

 

[TorroidDoughnut]

@Charles G

The gain pot is used for the line stage's negative feedback loop, which makes the gain programmable. I have the unit set so puts out a gain of +12dbV.

@tomchr

The Bluetooth uses an off the shelf module designed for audio use. For a future revision, I will explore the possibility of using I2S-based data converters, and replacing the module entirely with a Bluetooth low energy chip. This is in case the manufacturer of the module closes or a supply chain issue prevents me from buying it.

As for the display, it is an off the shelf RGB LED dot matrix. It is not ideal for the application because the frame rate and resolution are lower than i'd like. Furthermore the unit's firmware is required to block when the display is written to, which causes unnecessary slow downs. I am currently designing a custom LED dot matrix to address these issues. I will need to write a driver and character sets, which is its own project.

 

[tomchr]

Yeah. I'm looking at a dot-matrix LED display for a client and am going in the same direction as you. Thankfully I don't need that many different letters, so the 'driver' should be manageable, but still. Just managing the I/Os is a bit of an undertaking.

Tom

 

[TorroidDoughnut]

Below is the unit on my rack with its companion power amplifiers on the shelf below.

These units I completed a year or two ago but are a topic for another day.