Zen Headphone Amp Lives (Sort of!) - diyAudio
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Old 17th March 2004, 12:37 AM   #1
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Default Zen Headphone Amp Lives (Sort of!)

Ladies and Gents, I present to you a stuffed perfboard for a zen headphone amplifier based closely upon tortello's design (Thank you very much Tortello for sharing your excellently put together project!). My aim is to run my amp off a regulated 30V supply at a bais of about 300mA. I based my approach not off the schematic posted at Headwize, but off the first schematic posted in this forum by Tortello.

Today, unable to wait for my transformer to arrive to complete the project, I hooked it up to a variable DC power supply, and slowly fed it voltage until it was running off 30V. The good news, no pops, sparks or smoke! The bad news, the bias is only ~250mA and the drain of Q2 is at 10.4V referenced to ground instead of ~15. I am posting the schematic with the measured voltage at various points labelled in blue.

My thought at this point is that I need to replace R3 and R13 with pots (200K?) to allow me to adjust the bias and the voltage of Q2's drain. I'd appreciate and feedback of help that can be offered.
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Old 17th March 2004, 12:39 AM   #2
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A quick shot of the assembled board for the record.
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Old 17th March 2004, 12:40 AM   #3
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Lastly a schematic of the power supply I am building.
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Old 17th March 2004, 12:42 AM   #4
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I forgot to mention that both channels are operating exactly the same (voltage readings at each point are within a few mV for both channels).
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Old 17th March 2004, 02:14 AM   #5
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Shooting from the hip having never built a zen or aleph, but the Vbe of Q3 looks low - if it was the standard .66 volts (as shown in Zen V4) you'd end up with more current - perhaps 300 mA. What current were you expecting? Odd that both channels ended up this way, but maybe new ZTX450s would help. Lowering the current source resistor will increase the bias.

A pot for R13 probably would be a good idea - Q2 seems turned on too much. Z V4 has the ratios of your R10 and R13 at least 2:1. You could try 75K-100K for R13, too, which should not turn on Q2 so much.

Odd that R4 shows DC current - you'd expect no DC current with a good mosfet.

Hope that this helps a little.
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Old 17th March 2004, 02:46 AM   #6
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Default Recommended change

I haven't built the exact circuit you've got there. However in very similar circuits, the power supply rejection and current source output impedance was much better if C1 was attached to the source of Q1 instead of the emitter of Q3.
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Old 17th March 2004, 09:15 AM   #7
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I like vary much Zen design, and have buid some samples of ZenV4 and hybrid - ZenV3 with tube (6N23P) SRPP input stage - all work very nice!

But for the headamp - I'm not clearly understand why You use so complicate schematics?
Idle current 250-300mA - the low impedance (40 Ohm) headphones works very well, and with the rail of 30v - it is quite enough to drive any 600 Ohm headphones, without controlable current source!

So, I use more simple schematic - LM317 as a current source at the top, and IRF510 at the bottom, powered from "electronic capacitor" - i.e. mosfet with capacitor from the gate to ground. Of course, it possible to put also a zenner in parallel, to obtain not only filtered but alse stabilozed voltage, but I havn't do this, think it is not nessesary.

Click the image to open in full size.

http://altor.sytes.net/headamp.jpg
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Old 17th March 2004, 01:45 PM   #8
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Did my homework last night and figured out at least part of the problem. I was on the right track, and Bob confirmed it for me. R13 is too large, and a correct value should be around 130K, so I will replace R13 with a 100K resistor plus a 100K trimpot in series later today and see what effect that has on my current source problem.

As to why I am using the Aleph current source? I built the original Zen a couple of years ago, and I want to build another power amp, maybe a Zen4 or an Aleph 5 or an Aleph-X - haven't decided yet, and since I need a headphone amp I thought this would be a good exercise in playing with a more complicated topology on a smaller scale before I sale up to a larger project.

Thanks everyone for the suggestions and insight, and I will update how the toubleshooting goes as I make some progress.

Metalman
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Old 17th March 2004, 08:58 PM   #9
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Default Progress with Trimpots

Over my lunch break I replaced R13 on both channels with 500K trimpots, powered them up and Voila! After a little adjusting the drain of Q2 is sitting happily at 15.1V. One problem down, one to go!

I am posting my revised schematic again with the new measured voltages reference to ground listed again in blue. Of particular concern to me is tha fact that the base of Q3 is at a lower voltage than the emitter. My suspicion is that the value of R3 is too high creating too much of a voltage drop at Q3's gate. My plan is to remove R3 altogether, check the voltages again, and depending on what happens, I may replace it with a 100K trimpot.

Once again, any comments or suggestions will be greatly appreciated. Fun stuff this DIY, and so far haven't hit a brick wall!
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Old 18th March 2004, 12:33 AM   #10
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Default AAARGHHHHH!

OK! Now I'm really confused! I replaced R3 with a 100K trimpot on both channels, connected it to a 30VDC supply, set R3 to max resistance (100K) and took some measurements. The schematic below shows what I found, which is not what I expected.
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