How Do I Get Better Results?
No, this won’t be a missive about how you can swap out the capacitors for audiophile-grade products the size of a beer can, or use specially formulated solder with silver and gold to increase performance. Boringly, but fittingly, this is about a few engineering-y, measure-y things you can do to improve the results of your build.
- Use a tube cushion. The original Vali used 30 durometer Sorbothane pads under the subminiature tubes, as well as another piece of foam under the board to keep the tiiiiinngggggg (microphonic) noise down.
- If you don’t have Sorbothane, use a soft rubber or foam pad under the tubes—you’re looking for something very soft. 30 duro Sorbothane feels a lot like bacon.
- Different tubes have different levels of microphonics. Some never settle down. Listen while you match the tubes (below) and get rid of any perma-ringers.
- Match the tubes.These tubes do vary. Actually, quite a bit. This matters because the variation changes the gain of the amplifier. If you use two unmatched tubes, you could have gain that’s off by a couple of dB on one side. Lower gain tubes also tend to have higher distortion, at least in our measurements.
- To match tubes, measure the gain of each side with a 250mV RMS sine wave input at 1K. The amp will put out something like about 1V RMS. However, one channel might be 1.08V RMS, and the other one 0.92V RMS. Swap the tubes until they’re closer (say, 1.08/1.02V RMS, or about 5%).
- Consider adding pin sockets to the build to speed up tube testing. There are small machined sockets you can use for this purpose. They’re not perfect, but they are better than soldering and desoldering a half-dozen tubes. You can solder in the tube when you find the right one.
- The 6418s seem worse than the 6088s in terms of variation, but the 6418s also have better measured performance in terms of THD.
- Adjust plate load for true midpoint operation.You’ll get maximum output from the amp when the output DC level (before the coupling capacitor) is 7.5V. This allows the amp to swing the rails (or pretty close).
- I’ve provided a 10K recommended plate load (R101 and R201) for both 6418 and 6088 tubes, but, depending on the tube (see above, they vary), you may want to change the plate loads to get the output DC closer to 7.5V.
- If the voltage is lower than 7.5V, increase the plate load
- If the voltage is higher than 7.5V, decrease the plate load.
- If you’re going to do this, change both loads to the same value, and don’t stress if one comes out at 7.7V and one comes out at 7.3V. That’s the tube variation again.
So, if you do all three of these steps, does it transform the amp into a super-high-performance product suitable for all headphones from CIEMs to HE-6s?
In short, nope. It’s still going to be a simple, limited, slightly noisy, high distortion, slightly microphonic little tube hybrid. But that doesn’t mean it sounds bad.
To go farther with the design, you’re moving into flying-parts-land. Adding feedback would lower the gain, noise, and THD. But that’s more parts, and it adds feedback, which some people like to avoid. Going to a Darlington output stage would improve performance as well, but that’s a huge change, and, again, more parts. Increasing voltage would be welcome, but then we’re getting well beyond what this was supposed to be—a coaster, which can also be built into a simple little hybrid amp.
Time To Build Your Own Coaster?
I hope you enjoyed this engineering odyssey, and learned a few things about the many decisions that go into making even the simplest product. Now, imagine the engineering decisions needed on something as complex as, say, an Yggdrasil (which has three different sets of firmware, a couple of dozen power supplies and voltage references, and both analog and digital sides to worry about. Or even a Freya, with its relay ladder, tube and buffer stages, and remote control.
Sounds like fun? Yep, it is.
Or at least we think so!
Here is part of Jason's last post from around July 4th, he talks here about tube matching and such.This is the last post on the Coaster amp and probably the most important one IMHO>
