Tube Amp Progress

For the tube amp, my initial thought was to design and build it completely myself. However, the following factors contributed to my use of the existing kit solution (AES kit )

• I want as better audio quality as possible.
In my high school, I played with building tube amps a lot, but results were never succesful, with lots of noise and hum. With the fact that this particular circuit was designed a while ago by a famous George Fathauer, will accept it as it is, maybe trying to improve the quality, introduce proper grounding, etc.
• Ordering the components separately is not much cheaper than bying that kit.
Transformers, for instance, are quite pricy
• Designing high-quality analog circuitry is far outside the scope of this class.

I have ordered the kit and after a half a day of work, assembled everything (and spotted a few design errors, like an improper grounding of the pot)

Ordered tube amp kit

Assembled kit

Now, one might think that the tube is ready and it is over. Unfortunately, though it does amplify the signal, the amount of 60 Hz hum and AM-radio noise that this tool emits is almost unbearable. I need to fix that.

I have spotted a few general ideas circuling over the internet that might contribute to the hum in tube amps.

• The tube filaments are powered with an 12V AC voltage, with traces going very close to the tube pins responsible for the amplification. Well, I have taken a 12 Vdc balanced source and powered the filaments with DC. The hum and AM noise was still present, so that was an unlikely cause of the noise.
• The capacitors are chinese and made of low quality materials
I have ordered the good (american) ones from Digikey. Still failed to work.
• All the transformers shells should be connected to each other and connected to the signal ground.
The hum was slightly reduced, but was still significant.
• Transformers induce EM wave, which influences the amplification tract.
I tried everything from rotating the transformers (while they were off!!!) to relocating them to as far away from the circuit as possible. The noise was still there.

After investigating a topic, I almost gave up at some point. But then it occured to me that we all live in the areas with a huge amount of RF and 60 Hz radiation present pretty much everywhere, coming from our microwaves, fridges, outlets, cellular networks, etc. What if I shield the entire amp in a metal enclosure, i.e. introduce a chassis

DAC and pre-amp

Grounding

In addition to insulating my amp from the outer world, the grounding will serve a critical purpose: it will make this device safe to operate and prevent me or my beloved ones from any possible electrocutions. The current design of the kit is horrible in terms of safety: the high voltages are floating, the device is not grounded and all the parts are exposed.

Now the problem of grounding is that this represents a whole new world, in which one (properly trained) individual with some effort probably can make the device being save. However, getting rid of the newly introduced noise due a number of probable ground loops is the problem that is very far from easy to resolve. Hence, in a way, I need to not only shield the device from the outer world, but also shield it from itself.

A lot has been written on the topic of grounding, where the discussions of the proper relations between the power ground, safety ground and signal ground are described. In terms of the Tube amps, a very good read can be found here.

In short, I need to make sure that my signal inputs, signal outputs, volume pot, power ground and chassis (and the future DC ground for the DAC microcontroller) should be connected only in one point, ideally on the chassis, and ideally close either to the input signal ground or to the power ground.