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Not sure I know what you mean? The current is the CCS current. I'd set it anywhere from 12mA to 18mA to experiment. Voltage is the Vf of the LED. In an ideal world, you'd pick an LED with the lowers dynamic impedance you can. In the real world, this information is rarely clear from the datasheets, and LEDs don't really differ that much, so worrying about it is likely not that important. You know the old saying, that in theory, theory and practice are the same, but in practice, ...

While the thread has been bumped ... I was doing some research on something else the other night, and happened upon distortion spectra for tubes drawing grid current. To my eye, it looked a lot like what we've been seeing on this amp when run hard. That is, I think grid current starts well before 0 bias. I tried biasing the grid a little more negative, and this seemed to help a bit. My recommendation is that the grid be at least -2.5V, with perhaps -3V being even better. You can do this by swapping out the red cathode bias LED for something in the 2.5V to 3V range (you can search at Digikey by forward voltage.) Or, if you increased the CCS current, then just jumpering to the WE connection should get you to -2.5V with the stock Rk. The issue is that as bias is increased, plate voltage increases, too. The upside is that, even outside of the grid current issue, the plate curves look a lot more linear at -3V than at -1.8V and this moves heat dissipation from the CCS to the tube. The downside is less room to swing before clipping the CCS. Though, even at -3V and 18mA there is about 50Vp-p. We'll have to see if that's enough. And, increasing the power transformer from 200V to 225V is probably OK, though it will run little hotter. Anyhow, if anyone gets to do this, please let us know what you think.

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There is no need for the output cap to be 22u. Most preamps have a input impedance of 50K or higher for which 1u is enough. Even under the worst case scenario if your preamp has a 10K Zin, 4.7u is plenty high. One thing you might do, though, is increase R18 from 100K to 1M or so. It is in parallel with the input of the next stage, and does nothing at 100K but increase the size of cap you need. Second, the parallel 0.1u interstage couplers are unnecessary. There is some thought that paralleling identical caps is a good thing (it is the theory behind the multicap) but with most modern caps the benefits are less than they were in the past. But, you'll do fine with a single .22u cap there. As for recommendations for coupling, there are a number of "cap review" sites around. If you don't want to spend a lot of money, I think I'd look around for some Vitamin Q's, or maybe some of the Russian PIO or teflon stuff. If you want to spend more, I like the new Jupiters and the Mundorf SIO's the best of things I've tried. I also like Cardas caps, but even among capacitor marketing which is over the top and silly, Cardas seems especially over the top and silly. The best middle of the road option to my ear are the Dynamicaps which I think are a nice step up from Auricaps and their ilk. But, there are lots of things I've not tried. For the RIAA stuff, what you really need is precision. Unless they are too big, I like the Wima film and foil (FKP) caps available from Mouser. Otherwise, the Wima MKP's are OK, too. I've used sonicaps in this spot, too , which worked OK. MKP, by the way, just means metalized polypropylene, so Solens are MKP, too.

How about Dec. 4th?

That would be really disappointing, and would signal that Sean was no longer in charge. It's the sort of detail Slim Devices would not have left out.

Will the squeezebox bridge the HDD to the network so that files can be used on other computers?

Like Flynn

It's worth calling around, but most anodizers are slop shops used to doing 50000 pieces or more per run where per piece quality is not that important. Finding someone willing to take care of your stuff and not charge a ton is difficult. If you want anodized, you'll probably get the best results for the least money having CamExpert/FPE mill a new plate out of pre-anodized stock. The edges won't be anodized then, but you can embed the plate in a rabbet cut in the wood so it won't show. The quality of the anodizing will be a lot higher. And, if you want brushed, they can special order brushed/anodized material. If you don't want to measure, send the precut sheet to Heinz and I'm sure he can just replicate it for you.

That post from Benchmark is the perhaps the worst post on the subject I've ever seen. It is basically exactly wrong, and suggests that Benchmark has no idea how to drive headphones. There is a distinction to be made between balanced and differential. All differential amps are balanced, but not all balanced amps are differential. Differential amps (unlike the standard balanced amps that are really two SE amps back to back, and that offer little to no benefit for headphones) isolate AC from DC, and keep the signal current loops away from things like the power supply, and PS caps. There is no reference to either B+ or ground, and thus little to no noise. There is a blurb on it at Vacuum State (on page 3): http://www.vacuumstate.com/fileupload/RTP3D_brochure_b.pdf

Basically, any small signal NPN transistor should work. The higher the Hfe of that part, the better the CCS is at being a CCS (i.e., the higher the impedance). The 2N2222 is kind of crappy in this regard. You could use a higher Hfe part, but if you do that, the LED bias current will probably need to be increased, particularly if the CCS runs at high currents. As for it touching the board, so long as the case is not shorting anything, it should be fine. The part does not dissipate any real heat.

Hammonds just don't meet their specs. If you try to run them at specified currents, the voltage sags, they hum, and they run on the verge of burning up. The 300's are better than the 200's, but neither are great.

I was about to say that I'd trust technical transformer information from Hammond about as far as I could throw one of their power transformers. But, since they undersize the cores, I cold probably throw one pretty far Try these articles -- the bottom two are on PS design audioXpress - Track Listings - Audio Classroom Series

If you are doing a L'es style parafeed amp, those caps are really pretty incidental. The CCSes keep them completely isolated from the signal current loops and can provide up to something like 120dB of ripple rejection meaning that the ripple will be lower than anything you can probably measure. I'd use an electrolytic, and then put a small (0.1u to 1u) film bypass cap right at the CCS. The problem with the Solens is that the big ones are a pain to mount. Oh, and 5ar4 has a slower warm up than the 5u4 as the 5u4 is directly heated. Me, I prefer good sand rectifiers. Tube rectifier impedance is too high. In this amp, again because of the CCS isolation, it makes no difference.

I could be totally wrong, but I'd think a 24V relay would only really need something like 18 to 20V to actually work. That said, could you get away with an 18V relay? Probably easier to find than a 22V transformer

How important are those extra 4 volts? Could you get by with a VPP36-820 at Mouser?

I am certainly happy to help with schematics, ideas, or whatever else you need. The kit is basically a pared down version intended for beginners. But, if you are going to do it P2P, there are a number of things you might do differently -- better CCS, different tubes, a regulator before the CCS, tube rectifier, perhaps a SRPP or a mu follower instead of a solid state CCS (for the record, I don't think this is better, but it is different), etc. Oh, and I happen to know of someone selling some Electra-Print 5K:32 parafeed transformers: FS: Electra-Print 5K:32 2.5W Parafeed Headphone Output Transformer - diyAudio

Proper fuse size is 1.33x the VA rating of the power transformer. Most here use power transformers that are hugely overrated meaning that the fuse is often very large for the circuitry. If that was the case here, you should figure out why the fuse blew as this should not happen under normal operation. This is particularly the case since it happened at a meet so you have no idea what the cause was. Good luck

Just be sure they don't short to the case. And I'll be happy to look at your pre, but probably not till fall or winter.

Take it with a little grain of salt. If you look at the 45 data sheet, renowned as the lowest distortion tube around, at full "undistorted" output it's still up around 5%. Now, it's 5% has basically nothing past the 3rd, but the point is that distortion numbers are nt everything. At least the shape of the distortion is more or less appropriate. My guess is that RMAA is requiring you to run this at a pretty high level, probably higher than you might listen with most common headphones, which results in the 1% THD. If you run it at more like a few mW, the distortion looks a lot better with most upper level stuff buried in the noise. When I switched from using my M-Audio FastTrack Pro as the sound source to instead running it's spdif to my own DAC, a lot of the higher harmonics dropped off quite a bit, so I'd suspect the soundcard is having some influence. All that said, it is what it is -- the tube is not a super low distortion tube, we are running it at the maximum of what it can do, and the OPTs are not super high quality. If you want better tubes/transformers, it will cost a lot more.

There are a whole bunch of tube with different names that may or not be different tubes. I know Tomb likes one of the alternatives better, though I can't remember for the life of me which one it is. Some of them list the acceptabe plate dissipation as being much lower than how we are using them. However, I believe this is the case with Tomb's favorite and to date he hasn't had any issues that I know of. Worst case with this amp if you kill a tube is that the tube dies. Nothing else should be damaged as a consequence, with the possible exception that if the tube shorts internally the CCS transistor could overheat. So, please experiment and let us know what you find out.

"Kiwame" resistors are available from Mouser for $0.16 each. Try this link KOA Speer resistor

Hard to know. Higher might mean a better tube? b/t/w/, looking at the curves again, increasing the cathode bias with a 3 to 3.3V led might be a good thing. It looks like it is a little more linear there.

As the current in the CCSes is increased, the amount of voltage they have to drop decreases both because the plate's voltage increases, and because B+ sags a little. So, the heat will not necessarilly go up too much. I would not use a trimmer, at least not one that is adjusted by hand. These resistors are at high voltage, and the reason we didn't want a trimmer was to keep fingers out. Indeed, there are better CCS designs, but they rely on mosfet Vgs to set current which is a lot less predictable than BJT Vbe and would thus require a trimmer. We decided that it was better to be safe here. Plus, trimmer's can move when they heat up. In the big version of this amp, I use mosfets that are very closely matched. We are going to increase the HS size in the final (boh for BJTs and for the reg). But, I don't think an IC running at 55 or 60C is a big deal.

When you are dealing with tubes that might be +/- 20% and transistors that could be worse, matching resistors hardly matters. If you do want to match anything, tubes would be first followed by Q1L and Q1R, and really matching Q's is not important at all. However, we are recommending that R4L and R4R be closer to 56 ohms instead of 75 which will boost the current through the tubes quite a bit -- up to around 18mA. It measures better, and Tom says it sounds a lot better, particularly on high Z phones for some reason. If you don't want to pry the 75R resistors out, just bypass (parallel) them with something around 200R. To determine more specifically what resistor to use, just divide 1 by the current you want. 1 is approximate here, and depends on how close Vbe is to 0.7, and how close the drop across the CCS's biasing LED is to 1.7 (notice that 1.7-0.7 = 1). As for measurements, these transformers are not likely to be inductive enough that they will give very much bass when run into a high Z load like a sound card. Though, I don't have any idea with the primary inductance is. But, assuming it is not sure high, the high load will likely give you better than expected results at mid to high frequencies, and worse than expected results at low ones. @Colin - THD was definitely better with the LED. I didn't save any of the measurements I did, and it was a while ago, so I'll have to redo them at some point. Overall sound was flat and bass was really flabby and weak. Ryan listened to it for about 8 seconds before declaring the WE option inferior -- it was not a subtle difference. I can't say, though, if this is just this amp, or if this is a generalizable result. I have a DAC that runs an AD1865 into the grid of WE connected, trafo coupled, C3g's and it works fine there. Maybe it would work better with LED bias, though?