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A Super-Simple 6DJ8 Headphone Amp - Page 8

post #106 of 471
2/23/09 at 3:24pm
Yeah, I've noticed that the sizes of the caps are off a bit. When I place my mouser order for the BantamDAC I'm building I'll order the correct output caps for this thing. I'm thinking the 470uF caps I put there for now will probably sound just fine tho.

Just a note, the output caps have 7.5mm spacing and the main power cap has 10mm. Finding values as small as the ones they want for the amp and getting those spacings isn't easy. One solution is using larger values, like the 4700uF 50V power cap kansei uses. I think I put a 35V 2200uF cap there.

I'm almost done with the power supply; I only have a couple of wires to solder in then, if I'm brave enough, I'll plug it in and test if my hand drawn, protoboard regulator actually works. Fingers crossed!
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post #107 of 471
2/23/09 at 4:34pm
Quote:
Originally Posted by DKJones96 View Post
One solution is using larger values, like the 4700uF 50V power cap kansei uses. I think I put a 35V 2200uF cap there.
That 4700uF cap dwarfs the tube, I'm thinking of mounting it horizontally.
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post #108 of 471
2/23/09 at 4:48pm
Thread Starter 
Quote:
Originally Posted by kansei View Post
Sound
Nothing short of amazing. Female vocals sound incredible. Drums and guitars sound great. I like this amp a lot better than the Starving Student. Well worth the price and the effort.
Congratulations on finishing the amp; I still haven't found time to add the last few bits to mine, yet. Soon, very soon.

"Better than the Starving Student"? That should pique a lot of people's interest, but are you sure you're allowed to say that kind of thing around here?

Have you measured your heater voltage, et cetera, and found them to be roughly correct, even with the 33V power supply? I've cobbled together a fairly nice, regulated, 24VDC, 1A supply for this, but if it works off of 32VDC or so as well, I may just use a different supply instead.

Also, did you run into any problems with input levels, as the guy who bought the prebuilt version did last year?
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post #109 of 471
2/23/09 at 5:48pm
Input level issues? Got a link?
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post #110 of 471
2/23/09 at 5:50pm
Quote:
Originally Posted by Nemo de Monet View Post
Congratulations on finishing the amp; I still haven't found time to add the last few bits to mine, yet. Soon, very soon.

"Better than the Starving Student"? That should pique a lot of people's interest, but are you sure you're allowed to say that kind of thing around here?

Have you measured your heater voltage, et cetera, and found them to be roughly correct, even with the 33V power supply? I've cobbled together a fairly nice, regulated, 24VDC, 1A supply for this, but if it works off of 32VDC or so as well, I may just use a different supply instead.

Also, did you run into any problems with input levels, as the guy who bought the prebuilt version did last year?
I like the "Super Simple" a lot. The "Starving Student" is almost as good with my coveted RCA "brown text", square getter tubes. I should note that the SS amp was my first project and was built with standard capacitors instead of audio-quality ones.

I didn't measure any voltages, just plugged in and went. The tube lit up and I was ready to go. Good idea to measure, though, will do some measuring tonight. I tried to use a 19v power supply as well but I got a lot of distortion. This may be the reason why I was having issues with the Philips Jan 6922 tube, it may need more heater voltage than the Tung Sol e88cc. More research for me...

No issues with input levels whatsoever, just smooth and quiet all around. My amp likes less input, I ran Foobar at 3/4 or 2/3 volume level and it worked very well. The link to the pre-built amp and issues with it is here: http://www.head-fi.org/forums/f5/95-...photos-347465/
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post #111 of 471
2/23/09 at 7:23pm
Hmm, that thread is making me wonder about grounding.

I'm guessing that it does but I'm kinda new to these kinds of amps. Educate me if I'm wrong, but this is how I understand this amp to be working. Unlike a solid state amp that runs a rail splitter and has a virtual ground that the phones can sit on at idle, this amp makes its 'virtual ground' by having a dc offset at all times from the fets/tube. Say you run 6 volts, at idle the fets supply 3v to the output caps. You get your +- output to drive the phones by varying from the 3v offset. moving to 4v gives you +1 on the other side of the cap and 2v gives you -1.

So, ground on the PCB and DC-in is at the same voltage potential as the inputs and outputs. Is that right?
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post #112 of 471
2/25/09 at 5:30pm
Quote:
Originally Posted by kansei View Post
Another observation is that the mosfets get really hot.
That is correct - I purchased the pre-built version of this amp from the eBay seller and have been using it now for several weeks. What I noticed when testing it right after it arrived is that the all four mosfets and their associated heat sinks (which are large, apparently for good reason) get very hot -almost to the point where they will burn a finger if contacted too long.

I haven't found that to hamper performance and the heat from the sinks doesn't appear to be "cooking" anything else on the board. However, it does mean I remove the top of the plastic Hammond enclosure enclosure I mounted it in when the unit is running. It also suggests that this circuit shouldn't be housed in an enclosure without adequate ventilation.

Regarding the "issues" another member found with the pre-built board, I did replace the input and output jacks with Neutrik 1/4 and RCAs, but kept the ALPS volume/on-off switch. I haven't had any of the noise or gain issues. The amp drives my Senn 600s easily and is dead quiet. No distortion or bass weakness/bloat, perhaps because the input is being fed directly by the tape out on my BEZ preamp (ergo, a pure low level line source, not the headphone output of a portable). I have noticed improved air when listening to tape off my Nakamichi deck (compared to running the phones directly off the Nakamichi headphone output). Things are also likely being helped by the tube I'm using - a 1963 Amperex Bugle boy. I kind of like the errie blue glow it has when it's running, a function of the blue LED the designed squeezed in under the tube socket.
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post #113 of 471
2/25/09 at 7:15pm
I got mine together but my power source has an issue(solid 24v at idle and 6v under load, no idea wth) and it doesn't give me anything more than a thump when turned on and an eerie low hum that doesn't change with a source or anything. I'm using a 19.8v switching power supply on it but I have a hard time believing that 4v means the difference between nothing at all and full sound.

I left it on and the 317s get warm but not the 310s. I've got some real troubleshooting to do since I noticed the LED doesn't light up either and I'm pretty sure I didn't install it backwards. Should the tube get warm to the touch? And if so, how long after it's on should it be?

**EDIT** I was looking at the schematic and the diagram together and am wondering something, the adjustable pots I used at 110/210 have 3 legs in a tripod formation. One of the legs sits on the same line as the wiper and the 3rd leg is on the other side. Could this be causing my issue? I'm thinking no because the path of least resistance is through the wiper at anything but turned full right on the pot.
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post #114 of 471
2/25/09 at 9:01pm
Your tube shouldn't get hot at all DK. I think if your dropping that much voltage you have a PS that doesn't put out enough juice or something in your amp has way lower resistance(possibly close to zero) than it should be.
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post #115 of 471
2/25/09 at 9:06pm
Thread Starter 
Huzzah, mine's alive and for the most part working as designed.

I bought, and then misplaced, trimpots for R110/R210, so I just put a resistor (470 ohm, I think) in its place. My LM317s get fairly hot, but the 2SK310s just get warm.

I tried powering it off a 30VDC 1A printer power supply, and this does work - you get the correct heater voltage, so yay. However, with my printer power supply, I also get a horrible hum, so I'm running it off a homebrew 24VDC, 1.5A supply. I can't imagine why yours wouldn't work off a 19V supply - you did get the polarity right, right? The tube heaters should start to visibly glow within, oh, thirty seconds or so, and the glass envelope should be warm to the touch a minute or two later.

With earbuds or other highly-efficient 'phones there's a slight hum on mine that doesn't increase with volume and is fairly ignorable; it isn't audible with my ATH-M30s. I have a few ideas to try to minimize this...

I, too, used the standard Alps pot, and it's probably just my pot, but the power switch doesn't do anything! If the amp's plugged in, it's on. (At first I thought the tube - an RCA ECC88 - just took a while to dim, but after about thirty seconds standing there, I started to get suspicious...) Doh! Eh, when I case it up I'll just have to put a switch on the power line, but it's still odd.

Oh, and also, my amplifier does something that I suspect none of yours does: picks up local 50KW AM radio station KSTP fairly clearly, if I accidentally touch any part of the input signal "upstream" of the tube. Hmmn...

Other than that, it sounds ridiculously good.
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post #116 of 471
2/25/09 at 10:16pm
Make sure your input ground is tied to pcb ground, and not just connected to HP ground. The schematic shows the pot ground not connected to pcb ground but I am sure it is supposed to be.
So is this your first hybrid? I thought you guys would love the sound.
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post #117 of 471
2/25/09 at 11:04pm
Quote:
Originally Posted by digger945 View Post
Make sure your input ground is tied to pcb ground, and not just connected to HP ground. The schematic shows the pot ground not connected to pcb ground but I am sure it is supposed to be.
I assume the board on the filled (pre-built and tested version that I have) is identical to the bare PCB others are building off of. The way mine came the adustable resistor (a.k.a. volume control) is connected the PCB ground trace.

On the subject of hum: for what it's worth, the input jacks on my setup are at the back of the enclosure, ergo at the rear of the board. They are connected to the input pads at the front via solid 24g wires inside a braided shield. I also put a small sheet of aluminum between the input jacks and the rear of the board. So the entire input line is shielded, which may be a factor in why I have absolutely no hum in my unit.
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post #118 of 471
2/25/09 at 11:09pm
^ x2, digger945. I had some hum at first, then connected (-) from the power plug to the ground on the RCA plugs to the ground on the headphone jack. This fixed the hum. At first I also connected these three to the ground on the pot but this made the volume control inconsistent.
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post #119 of 471
2/25/09 at 11:19pm
Quote:
Originally Posted by Nemo de Monet View Post
I can't imagine why yours wouldn't work off a 19V supply - you did get the polarity right, right?
Mine wouldn't work right on 19v, I was getting distortion.
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post #120 of 471
2/26/09 at 12:11am
I measured the heater (pin 4) it and it says 6.2v. I also measured pin 1 and 6 (anode #1 and #2) and made them the same voltage with the trim pots (25v). Can't hear the difference with the adjustments, though. I'm still new at this so I could be completely off on both the measuring and on the pin outs.

I got the Philips tube working as well, some contact cleaning and a thorough warm-up fixed it up.

On an unrelated note, I am way into this as evidenced by posting to the same thread 3 times in a row.
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