DIY Audio Projects Forum

So I heard some clipping and decided to check those pots again. At 100% resistance its only brought down to about 9v. I'm using the exact trim pots linked in the tutorial from mouser. Though the trim knob faces sideways for some reason. Any Ideas?

Thanks,
Max

Hi Max, can you take some readings?

Below describes how to set the bias. From: http://diyaudioprojects.com/Solid/IRF61 ... phone-Amp/

The pinout diagram for IRF510 and IRF610 MOSFETs which shows the Source and Drain pins is attached.

Let us know how you make out.
Cheers

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]

I have been wanting to build a tube headphone amp as a way to get started with tubes for some time. When I saw this one on Make Magazine I knew it was time to take the plunge. Although I have never built any audio projects before I am reasonably competent with electronics. Although I couldn't design my own (yet) I can build from a schematic. I haven't etched a board since high school so I figured I'd just proto board it for now. I bought everything except the tube at radio shack. I ordered an Electo-Harmonix and a JJ from Tube Depot. I also included a socket in the order. I figured I'd go inexpensive for now in case I screwed it up. The caps from Radio shack are probably not the best, but again, I figured I'd get it working before going exotic. Over the last week I put it all together. I have a 13.8 V bench supply that I haven't used in a while I figured would work. I know that's a little high on the voltage so I picked up a 12 volt regulator from Radio Shack as well. I figured if I included the regulator in the circuit I could use a variety of different DC inputs and it would still work without adjustment. Once I had it all proto-boarded up I remembered my 13.8V supply was DOA. It was about 3:00 AM and I really wanted to see if it would work so I scrounged up an 18V laptop supply. The laptop supply worked fine, but it really heated up the 12 reg. I ended up pulling a heat sink out of a dead computer supply and using it on the reg. With that on it would be very hot to the touch, but seemed stable at that temperature. If felt comparably hot to the MOSFET's that didn't have heat sinks. It fired up the first time without any smoke. I adjusted the bias, but I was only able to achieve ~ 6.8 volts. The pot still had several turns left, but the lowest the voltage ever went was ~ 6.8. The input side was 11.98. Not sure why, but it didn't seem to matter. I plugged in some crappy headphones and heard music. I fooled around with it for awhile before getting out a decent pair of headphone to try. It sounds good! My initial impression was slightly bright as a few people have mentioned. I don't have any other stand alone headphone amps to compare it to. It sounds obviously better than the amp in my iPod. I tried doing some comparisons to the amp built into my CD player but it was kind of tough. I don't have an A/B switch so I had to unplug from one, screw on the adapter and plug into the other. I think I'll build an A/B switch so I can really compare. The CD player I used isn't a new model, but it's always sounded good. I only have the pair of Sony headphones to try as well. I put a few pictures on Flickr if you care to look: http://www.flickr.com/photos/49896052@N04/
The amp is very quiet. I cannot hear any noise during quiet passages. if I pause the CD player and crank the volume I don't notice anything except the occasional pop. This was a very satisfying build, particularly since it worked right the first time.

Todd

I'm betting you're testing what I tested.

I *was* testing the voltage at the tube PLATE - (Tube Pins 6 or 1) - this will give you close to 9 volts, maybe a bit more, but I was confused that I wasn't getting 6 volts. You won't be able to get this down below around 9 volts.

The bias value he intends you to find is actually tested at the IRF510's 'Source' pin (or where it joins to the LM317 INPUT - same).
Here, you'll get a range such that you can (by adjusting the pots) set to 6 volts (or really, 1/2 of your input voltage; if you're providing 12.8 volts, then you're looking for around 6.4 volts at this point).

I haven't experienced clipping yet; just some excessively strong high frequencies (I've been calling it 'brightness').
I've changed from earbuds to some phones and it seems to be really superb now - great highs but very strong lows as well.
Surprising flatness - strong all 'round.

I had to change out my heat sink for a bigger one. It was just getting too hot for the little 'VGA stick-on sink.

Now that I've got it in a 'settled' state, I need to shoot some photos of it.

Mike Y
Dallas, Texas

Ya done good.

Yup - I'm also using an orphan 18V laptop supply - it'll get quite hot; as a 'linear' regulator, it has to burn off the approximately 6 volts at about 400ma (approximately what the amp consumes). Power = Volts x Amps, so it's emitting about 6.0 x 0.40 = 2.4 watts in pure heat. The more heat-sink you can put on it the better.

Tips: the tube will last longer the lower the filament voltage you can keep. Tubes are usually spec'ed to be within 0.4 volts + or minus of the listed heater voltage, but keeping it near 12.6 would be best. It won't last AS long at higher voltages, although it'll still probably last a long time. In my few years rebuilding old tube radios and tube transceivers, I've yet to find a completely 'bad' tube - i.e. bad filament, vacuum gone, internally broken. Sometimes they're note quite 100%, but they usually work. The oldest radio I have - a 1947 Zenith Transoceanic receiver - all but one of the tubes are original (the 117Z6 rectifier had been replaced) but all were good.

Sounds like you did all the right things, and the first time. Congrats.

It's a good little amp for what goes into it. I built mine entirely from what I had on-hand, including the tube and socket (left over from some ham radio projects). I finally bought a second enclosure for a regulator board, and now I'm going to have to 'embiggen' the heat sink on the regulator as well.

Mike Y
Dallas, Texas

Todd,
Great work, I have seen the article on Make and I am rather humble and excited.

This amp will give you the basics, and the satisfaction of creating an amp. You can springboard into larger, higher voltage and different circuits like SRPP and push pull. With the basic knowledge and understanding it makes troubleshooting higher end projects much easier.

The other reason I recommend a small project as this, is as you build higher voltage and more complex units, you will experience how the sound opens up and even more detail.

Thanks and post some pics when you are ready.

Rogers

Just one more thing about the pot, you did wire it as a variable resistor, the center pin should be shorted to either the #1 or #3 pin. Other things could be recheck the value of the resistor for the LM317 should be 10 ohms and verify the pinout of the LM317 as well. There are a couple of variation I have seen out there.

I went and got a DPDT relay at Radio Shack and made an A/B switch so I could instantaneously switch back and forth between headphone amps. Tonight I tested the built in CD player headphone amp and the project amp. My first impression was that I screwed up and it wasn't switching between sources. The sound was that close. I kept unplugging one or the other just to make sure it was actually switching. Once I satisfied myself that it was I settled in for a few hours of listening. I don't really have a criterion collection of reference material. I just went though my CD's, mostly stuff from the 80's and 90's. My initial impression was that the tube amp is slightly more open on the top end and the CD player has slightly more mid-bass. After considerable listening I still feel the same way, although I'd say the mid-bass is a close run thing, it may actually be gone now that it has been running for a few hours. (Nope, I'm listening to it as I type this and I just flipped the relay a few times, there is slightly more mid-bass on the CD players headphone amp.) I'm actually surprised at the quality of the CD player's headphone amp. So surprised I took it apart to see what it was using. The CD player has an XRA15218 amp chip. It is entirely possible that better headphones would reveal more from the tube. My best pair are Sony MDR-7506's. I bought them about a year ago when I wanted something better than my iPod earbuds. I didn't audition them, I just looked on Amazon and found highly rated headphones for around $100.00. I've never tried the Grado 80's that so many people talk about. At the time I had never heard of Grado so I bought the Sony's. Suggestions for my next pair of headphones are welcome.

Tube Rolling.
When I ordered my tube from Tube Depot I ordered an Electo-harmonix 12AU7 and a JJ 12AU7. I also ordered an Electro-Harmonix 12AT7. After a few hours tonight with the Electro-Harmonix I decided to try the JJ. I pulled the board out to check the bias with the new tube. The voltage was ~ 8.50 volts, it wouldn't go any lower. The lowest I could get with the Electo-Harmonix was ~6.5. I listened to the JJ for only a short time, it was horribly lopsided. One channel was much louder than the other. Nothing had changed, but I tried the Electro-Harmonix again just to be sure, perfectly balanced. I guess I won't be using the JJ anytime soon. I decided I would try the EH 12AT7. It biased like the JJ, about 8.5 volts. It was slightly lopsided as well, but this time it was the opposite channel that was louder. The lopsidedness was more subtle, but annoying when doing A/B tests. So, I switched back to the EH 12AU7. Maybe someday I'll get something more exotic to see how it sounds. My next step will probably be to make a PCB and get better caps.

Todd

I've listened a little more with the EH 12AU7 back in. The top end is definitely more open and clear than the CD player's built in amp. I'm listening to an Enya CD as I type this and flipping back and forth reveals a better top end on the Tube. A few more discs and I'm very satisfied that the tube is giving a cleaner and more open sound, particularly higher up. I'll have to admit, I really have to listen for it. It'll be interesting to see what changes will occur when I start tweaking the project.

Todd