Tube headphone amp kit for $25!? - Page 14

No, but I worried about it, as the lines to the switch and back are physically near the signal lines. I would reroute them if I needed to, but so far I don't hear a need.

Hi Barbarous, I'm so not familiar with setting up a balanced circuit, so kudos to you mate 

Nice work and I can relate to the various challenges with putting the kit into cases though, having just completed two of these delightful little amps, one as a preamp/amp and the other I use just with an iPod Classic sending signals from the dock port. 

Not to ressurect an old thread...

I have a question, Sennsay, I'm probably going to build a few of these, and was planning on just getting rid of the transpots and putting in a stereo pot. You mentioned an inbalance in channels, and the need for the transpots. Is that really going to be a large problem, in which case can I just put the pot behind the transpots? I was planning on using a 1M pot, if I put the pot behind the transpots, could I use a smaller pot?

Thanks!

Hello all!

So, I got a couple of these a few months back and have been playing a little bit here and there. At my bench, I keep a baggie of extra caps that I don't use for other projects. Many of these are from the RadioShack 20 cap bag (which, if you pick your bag well, can actually be quite the deal - I've seen some fairly decent caps in there). Anyway, through a variety of projects, I've accrued a few decent caps.

Among those I'm looking at:

1. Nichicon Muse ES 470uF 50V

2. Nichicon Ultra Low ESR 220uF 6.3V

3. Wima 0.22uF 100V

4. Wima 1.0uF 63V

This amp seems like the perfect tinkering amp, given the evident potential and the cost. As I see it, there are six caps in this amp that will make the most substantial impact on sound quality, the input decoupling caps, the caps that decouple the tubes from the opamp, and the output decoupling caps. These are the only caps directly in the signal path.

The plan:

I'm going to first ensure the two channels are perfectly balanced (trimpots). I rolled the opamp, I think I'm going to unroll it, I want to start close to the original build. I have replaced most of the resistors in the signal path with metal-films.

Then, I'm going to pull the three resistors mentioned above from the right channel and replace them with screw terminals, and do some testing. I am expecting that changing the output caps will make the largest impact. I'm thinking of using the 220uFs there (though I need to be careful about the power ratings...), also a possibility are the 470uFs, either alone or bypassed with the Wimas (which Wimas? not sure). I will try as many combos as possible (though 1 and 2 are clearly output only caps).

I'll post results and pictures as I go.

The best combo was, by far the 0.22uF film Wima caps as the input caps, the 1uF as the intermediate caps, and the low ESR 220uF as the output caps. I didn't actually have the 470uF Nichicon Muses at my disposal, but I do now and will probably substitute them in at the output side and put the 200uF low ESR caps back in my skeletonDAC.

Some pictures:

Here is how I tested it, I left one channel stock, the other, I replaced the caps with screw terminals (and one set of alligator clips)

Here are the capacitors I experimented with:

Edit, if L2 stays lit, I assume I'm not getting the 3v supply from the AA batteries, meaning the tubes ain't being heated?

Put it together but I'm getting no sound :(

should the LED labeled L1 say lit once the amp is turned on, or should it flash then stay off? Mine flashes, not too sure it should do that. L2 seems to always be on.

Looked at the joints and they all look OK (would post a pic but I don't have a camera atm.). What would be the best way to troubleshoot it?

Thanks in advanced.

L1 should flash and stay off. If the 9V battery drops below 6.8 V (working from memory, so I may be wrong on the exact voltage), it will flash on and off continuously, meaning that you need a new battery. It should only flash once.

The other LED, L2 should stay on, that is the heater circuit LED. If it doesn't light, you need new AA batteries.

Hmm, L1 flashes and L2 stays on. Circuit seems to be getting power at least :)

FIXED IT!!!

Turns out I had soldered the audio out to the wrong pins of the 1/4 inch socket :p

Might have to order another set to modify the heck out of, or finally get around to building a starving student.

I just wanted to post some changes that I've made. The output buffer opamp is super low current, somewhere in the area of 12mA, which with low impedance cans is not enough. So, I removed the voltage regulator, bridging pins 1 to 8, so that 9V goes to the opamp. I replaced the opamp with TS922, which can be had inexpensively and can push 80mA. Then, I changed the output caps to Nichicon Muse 470uF, increasing the bass response. The result? This thing is loud, and sounds pretty damn great. It is a tiny bit sibilant, but the TS922 can be sibilant, but you really can't beat its efficiency.