UPDATE 5/1/22
Tonight I am listening to what will no doubt be considered the best sounding amplifier Decware ever built. If you can live with 4 clean watts and some docile very soft hum on 100dB speakers than fasten your seat belt because it's about to get really real. When I started Decware we had Drool towels for people who heard the first Zen Triode. With this amp I'm going to have to recommend Depends.
I took a giant leap this weekend. I have been evaluating it with and without a preamp and with and without a ZROCK2 set flat. Those of you who own a ZROCK2 have no doubt found the magic spot on the dial at about 1:15 where magic happens. This position actually measures flat. It occurs before there is any bass boost or treble cut. The midrange, texture and density of a ZROCK2 set here is so over the top no one who owns one can live without it.
It should be pretty obvious that if you're going to build an amplifier for a pair of output tubes that cost $700 a piece that it's going to have to sound as good or preferably better than our current reference the SE84UFO25. So that is the mind set, and it has taken me on a journey of realization about the sound I want. For example, I start with the question 'What is the best sound I can get in here' and then look at the gear used. Well, it has basically been the SE84UFO25 paired with the CSP325 matching preamp. The preamp adds so much beauty and density to the sound that when you hear it in front of most any amplifier, you will want it. So the reality is that for the best sound, you want both. And a good power cord for each, as well as interconnects.
But as good as that is, if I really want to wet myself, I'll get out a ZROCK2 and put it in front of the preamp. I'll set it to that magic spot which is flat, and just shit my pants at what the three components create! So add another power cord, and another pair of interconnects. As well, don't forget that you need full anniversary mods with the tube regulation inside. I'm pretty sure with the cables and shipping that's getting close to $10K.
So that is the reference, and I am directly A/B this amplifier design with that.
When we left off, I had mentioned that the amp already sounds like it has the preamp because of the 300B tubes. It's like free density. Well it's not free, but it actually costs half as much as the preamp.
I have been enjoying it all week with no preamp and then that what if moment happened and I stuck the ZROCK2 on it. This was a mind numbing moment because it offered the same gain I was getting from my favorite settings on the CSP325, and that insane midrange dimensionality that once you hear it you can't live without it. Only now it's magnified. It's like it is in sync with the 300B and everything just shifted to sounding lucid and completely real.
Haha, it's a good thing I'm experienced at blowing my own mind, because you can't just jump into stuff like this without a lot of seasoning.
THE PLAN
Ok here's the plan come late Friday night - measure the exact values of where I set a ZROCK2 and install one inside the amp set exactly that way. This will be the needed gain stage giving my around 5 volts (that I will later compress) for every volt of input and put the input sensitivity just ahead of the Zen Triode SE84 series and even the SE34I. So this is what I did. I recreated the exact circuit with the exact settings inside the amplifier and added an additional tube to the front of the chassis. The tube is a 12AU7. This compliments the 6922 signature so beautifully not to mention opens an entire world of tube rolling. A single tube will change the sound of your amp and there are millions of them on the planet.
Like a ZROCK2, the tube is at full blast all the time. The output of the tube feeds the level control which in this case is the input of the amplifier, or put another way, the volume control. This is what we refer to as riding the gain. The input to the amplifier is raised from 2 volts max to 10 volts and then squeezed back down to 2 volts. This creates dynamics and speed.
The EQ switch is deleted, and the adjustment control and the bypass switch all deleted. Everything is hard wired to the exact setting that is dead flat. Plus it is feed from it's own vacuum tube regulator just like the 6922 (or 6N5P/6N1P/6N6P) bringing the total of internal voltage regulator tubes to 3. These are SG5B tubes that have leads and are soldered directly into the circuit. In this case all three are directly in series with the plate resistors with no capacitor.
I guess that will make this 6 tubes on the outside and 3 tubes on the inside for a total of 9.
LOOKING BACK
I know when I started this afraid of the thickness associated with these tubes that I wanted a Zen amp style attack with only 3 parts to it doesn't get any thicker.
I learned quickly that the tube has precious little to do with it other than to make it apparent when it's there. Everything is the driver stage. It's 94% of the sound. I made it sound sharp, dull, and every other way possible until I realized that it too can be made to sound like anything you like dependent on the driver stage.
Quickly the design moved towards the Zen Triode transparency and kept the scale tilted towards neutral rather than thickness. Once there, that refreshing sense of air is back and it now gets better with a preamp or higher gain on the inputs without getting thicker.
So now that I have basically installed the ideal preamp inside the amplifier, we have two stages instead of one. That means 2 capacitors instead of 1 and 4 resistors instead of 2. OMG help! No worries, when each stage is feedback free and has this kind of transparency it is like water in a cup. Dirty water 1/2 inch deep in a cup will not obscure the bottom. Another 1/2 inch layer of water (2nd stage or component) begins to blur the bottom of the cup. The more you add the worse it gets. Clear water on the other hand can be added to reach the top of the cup without obscuring the bottom. And this is what has resulted. Everything just got WAY better on every level. Just proves no matter how good your source is, it could have been better.
Right now I have the HOLO audio May dac directly connected to the amplifier, and it sounds like it did last night when I tested the ZROCK2 with it, but twice as clean. All the extra crap gone, no connectors, pots, switches, cables, cords and so on... the sound is so clean and round.
Frankly this seems like the deal of the century to me right now. Simple RCA inputs, your favorite source and 93-96dB speakers if you're triggered by soft hum, 100dB if your not.
Just to be clear, if you don't like to tube roll, want the best, want to get it done with a single component you will have the sound of a ZROCK2 with anniversary mods feeding a CSP325 and the fully anniversary modded SE84UFO series amplifiers but with 4 watts instead of 2 and with 3 times less distortion at 2 watts and up to 8 watts at 10% distortion not to mention seldom if ever having to replace the output tubes since they are likely to outlast many of us.
So it was a long two days but raining the whole time, so time well spent. I will so put my name on this -- a 100% original circuit that sets a new benchmark. I think the Audio Gods even raised an eyebrow on this one.
Steve