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Old 14th April 2004, 03:36 AM   #1
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Default New mini Aleph headhpone amp

I've been thinking about a new headphone amp fro a while now. Most of my listening is after everyone is in bed, so the cans have been giving me much pleasure lately, but I want to raise the bar a bit over the crappy pot and built in the player HP amp in my crappy old Denon.

Even though headphones ar sensitive to offset voltage, I don't want caps in the signal chain, so the zen HP amp I've seen around doesn't excite me. Since dissapation isn't a problem for fractional watt amps, class A is a must.

My thought is to use Aleph front end and second stage, but a simple ccs for the + side. I'm adding a voltage divider to the feedback network with a pot to zero out offset and source follower voltage regulators to the rails. I'll use one toriod around 300 VA with a large bank of caps, and two regs (one per channel) feeding a second bank of caps. This should fix crosstalk concerns and give dead quiet rails.

The pic is a general topology. There are a few mistakes in this early pic. (I'm in the middle of a move to a new house and computer, so I don't have hardware and software on one computer to do the art)

Is there any other interest in this endeavor?
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Old 14th April 2004, 03:45 AM   #2
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The output source resistor are scaled to set bias at around .2A and neg swing current limit at .6A. I think that works out to 3 ohm for the ccs and 1 ohm for the active output. Final rails are 12 to 15 volts and gain 8. I also added resistors to both sides of the pot to make it more sensitive. I know that ifrp 244s are overkill, I have them on hand and plan to use them.
I already made some quick throwdown boards freehand with paint pen and hope to build in the next week. Later this week, I'll scan and post artwork and closer to final scematic.

PS the comp caps are also missing in the above scem
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Old 14th April 2004, 06:29 AM   #3
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Hi Brian,

Concept looks good to me. I agree with you that DC-coupled is the way to go. That's why I passed on the Zen designs as well. In my Aleph headamp I was able to zero the DC offset by merely adjusting the current in the differential pair.

If you want to read my experience with the Aleph headamp it is here: jwb's ultimate aleph headphone amp
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Old 24th April 2004, 03:27 AM   #4
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OK. It's up and running and all of my calculated voltages are spot on. It's playing through som POS disposable $.30 Emachine computer speakers now. I'm waiting a hour or so with the offset below 10 mv before I plug in the Sennheisers. The offset seems to be a little drifty, but it may be the old linear PSU that I'm using. I tied a pot between the rails for a DC null, but I'm not sure if the rails are stable enough to do what I want. Maybe a servo (aahhh no don't say it) is the cure. If not, maybe a very sensitive DC detection circuit that shorts an output cap if offset is below 15 mv or so. I don't want a output cap in the chain, but I also dont want to distort my HP diaphrams.... What are your thoughts Hopson.
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Old 24th April 2004, 04:28 AM   #5
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It works, but could use a little more gain. Dire Straits Latest Trick sounded great. Right there. But Kurt Elling sounds a little muddy. The bass seems to be a little heavy. I've noticed that many HP amps have a series resistor between the amp and phones. Why is that. I may buy a 20 ohm pot and see what that does to the sound. There is also a little 120 hz buzz, but i'll address that when I build a propper PSU and ainput attenuator. But it is very promising for A first go.
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Old 24th April 2004, 04:30 AM   #6
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Here's the artwork and layout for the boards
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Old 24th April 2004, 04:35 AM   #7
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The above pic is foil side. I printed it and traced it onto the copper with carbon paper, (the stuff people used to use to type a 2 copys) then overwrite with a paint pen for etchant resist. Etch and drill and you have an instant board. If anyone is interested, I can post component values.
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Old 24th April 2004, 04:46 AM   #8
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A better pic. I plan to epoxy the diff pair together and insulate the plastic part of the cases. Notice the multi turn pot. I found a neat little trick to do with the little peices of lead that go flying across the room when you trim. Make a loop and solder it into the board connections. They hold aligator clips very well and you can solder wires to them easily.
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Old 24th April 2004, 04:51 AM   #9
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Here's the not so glamourous bottom side. For the final installation, the board will be cut un half and be dual mono. But for now, cheesy wires to feed both sides. Sorry, my camera started sucking after it got washed cleaning the boat after a fishing trip.


I've been listening with the cans all this time. now that the fets are good and hot, It's sounding sweeter. The bass is still a little heavy, but all of the detail in the highs is there. And the offset has been hanging around 0v +- 6 mv.
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Old 25th April 2004, 03:25 AM   #10
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Updates. I cut the NFB by 40% or so, but my cans rattle befor I reach painful levels. It must be 20 yrs in the construction industry toughening my ears. The sound has really sweetened up. I think I was just used to the weak base of the built in amp. I now call it rich and controlled. Very injoyable. The only real problem is the offset. I fiddled with it for the first hour. After it cooled an I restarted it tonight, it was at 400 mv. I'll need to figure out a solution for that. It may be my PSU, but more likely not.


Edit.
I touched an ice cube to the output fet heat sink (neg one) and the offset raced right up to .4v. Maybe larger heatsink is the answer. And servo? I'll listen to output caps first.
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