I opened the amp and saw the following: MIT coupling caps, and diodes with the word FR on them. Whoever built this amp already upgraded the parts. I also noticed a solen cap, along with nichicon power caps and high quality resistors. The quality of soldering was average, with many parts soldered on with not a lot of mechanical support. In fact one grounded end of a cap popped loose and I resoldered it.

Given this, as well as the small amount of room in the box, I will not be upgrading anything in this amp. The input tube is a GE 12ax7, and the outputs are sovtek. I may play around with a telefunken input tube. Overall, i am very satisfied with my purchase, given the money i paid. I would not pay $499 for this amp (the list price of the KIT alone), esp. since my speakers are fine with the 1.5 watts the zen has. However, if i has 91-92 db speakers, this amp would be much more preferable than the zen. The lack of negative feedback in the zen, along with tube rectification seems to make the music come together a little better, PROVIDED YOUR SPEAKERS CAN deal with 1.5 watts!

At the next Tulsa audio meet, i'll bring this amp for others to listen.
-akhilesh

At the risk of sounding too "know it all," I would like to offer a comment concerning one statement. You stated that the Zen hummed because it was tube rectified. A properly designed power supply using a tube rectifier will have zero hum. Hum can also be had from other sources like heater to cathode induced hum and a poor grounding scheme. Just my 2c.

Hope you enjoy the new amp! Have fun with it!

Thermionic

I agree completely that so much of our impressions are internally driven (or subject to sonic interference even) that it is more like appreciating different paintings, rather than admiring engineering artifacts.

-akhilesh

Overall, the sand diodes yield a "harder" sound, and will also generally produce tighter bass definition in Class AB operation amps. Class A operation amps do not exhibit enough plate current draw increase on loud/bassy transients for them to make much of a difference. With Class AB operation push-pull amps, the increase in current draw may be quite dramatic.

However, even modern high speed diodes do not produce DC that's as clean as a tube rectifier. Sand diodes produce a sharp high frequency spike with all sorts of harmonics as they switch. You can liken a FRED diode to a MOSFET. Better than the "Plain Jane" model, but it still ain't no tube. Vacuum tube diodes just switch softer, bar none.

Another problem is that the capacitor input topologies used on 99% of tube amps produce DC with a residual AC waveform component that is a sine wave on the top, and a triangle wave on the bottom. The triangle wave produces many harmonics at the sharp point of the waveform as it switches from descending to ascending voltage. SS diodes with a C-R-C filter not only can and may indeed may have very low residual ripple, but it's often as gunked up with hash as can be. Yes, the filter caps nuke a lot of it by forming a low pass filter, but aluminum electrolytic caps introduce trash of their own. Add an LED-based constant current source and you've got the electrical equivalent of a toxic waste dump.

The common C-R-C filter also suffers from voltage drop on spikes in plate current draw more than any other supply topology. The voltage drop across the resistor increases with current draw according to Ohm's Law. The tubes get starved the most right at the spot where they need the most juice!

A C-L-C filter also suffers from it but at a lesser degree, due to the often high DC resistance of the small chokes used as the L filter. When the power supply recovers after such a load passes, it "overshoots" the nominal intended voltage and "rings" with oscillations on the way back down to the nominal stable operating point. It creates all kinds of harmonics in addition to the voltage/current instability when this happens. It causes the sound to lose the intended sharp focus and definition.

A tube rectifier with a choke input power supply and metallized polypropylene, Cerafine, or Black Gate caps is the ultimate power supply for a tube amp, especially a Class AB push-pull. A well designed choke input power supply has excellent regulation, far in excess of any other PS topology. The residual AC component is a pure sine wave, with no harmonics, and the above caps don't add their own say so to the mix, besides being much faster, less resistive, and more efficient than aluminum electrolytics. When bypassed with small value film and foil caps, they're unbeatable, IMO.

The choke input supply also provides a constant current source for both your power and preamp tubes. It also reduces the necessary conduction angle of your power iron and rectifier tube, so both are stressed less and run cooler. Rectifier tube life is extended by a choke input. Also, a choke input supply greatly reduces overshoot and ringing.

A choke input is not without drawbacks. Electrically, it's pretty awesome. In practice, it has it's physical problems. It requires a quite higher than normal power transformer secondary voltage, a low DC resistance choke capable of handling the current draw of the entire amplifier, and dealing with the enormous size, weight, and expense of such a choke. Most often, a choke input really done right requires a choke larger than the power transformer!

Thermionic

I assume your comments about semiconductor diodes and constant current sources are in reference to the Bottlehead Paramour amplifier and maybe others like it. I have a couple of Paramour amplifiers, and while they aren't entirely quiet, it seems to me that the problem is with rectified line cycle hum and a little bit of high frequency switching artifacts. Stock Paramours have a touch of hum and buzz.

The Paramour is an entry level kit. As such, I suppose some cost-saving measures are in order. Personally, I would have liked to have an all-tube kit, but more for consistency than for performance reasons. It just seems cool to me to be all "old-school" running all tubes than to have a mix of tubes and solid state in the circuit. But the Paramour saves maybe twenty bucks or more by using a couple of ten cent 1N400x rectifier diodes and inexpensive electrolytic caps for the DC supply. It was probably done to keep costs down, and it seems to work just fine.

If anything, I think the biggest problem with this amp is that there is not enough power supply regulation. I'm not as concerned with what is used to regulate power supply voltage as the fact that it doesn't appear to have adequate regulation. But then again, it is an entry level kit.

I think it's really cool to have an amplifier with no semiconductors at all. So I think it's cool to use tube rectifiers in the circuit, and to build a noise-free amplifier using them. I consider such an amplifier to be an impressive piece of hardware. But to tell the truth, I'm not sure I would expect to find any performance penalty when using semiconductor rectifiers or regulators. In these places, I would have expected using semiconductors would be an easy way to get the desired goal. Some might see it as kind of like "cheating" but I would have expected it to work well.

Seems to me that shunt capacitance in the power supply should really knock down the ripple pretty well, and that caps across the diodes would help remove the switching noise. So I'm not sure I fault the diodes used as much as the lack of adequate filtering. Add a few more filter caps, and it probably would quiet down a lot.

thanx
-akhilesh