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It is currently 04 Dec 2020, 21:45

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PostPosted: 22 Oct 2020, 05:20 
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Joined: 15 Nov 2018, 19:43
Posts: 13
Location: Belgium
Currently it's just one of those generic 8-32V to 5-390V ones, but it should be easy enough to swap that out to try a few different ones.

If it ends up being a bit noisy, it should be simple enough to try some others ones out. Worst case, I'll probably end up looking for a more suited power transformer/get one wound and just go back to the original design. My main goal was getting the resistors out and not have basically 20W of heat be sat there in summer

EDIT: I'll just 3D-print a bare-bones "test stand" to try out how noisy it actually is before finalising an enclosure design. Shouldn't be too hard to do.
If all else fails, I'd reckon an XPWR223 (ideally a custom wound 150V one) would be a good solution?


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PostPosted: 22 Oct 2020, 07:45 
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As an aside: I've never had power issue with any of my headphones, be it the 80ohm DT 770s or my 250 Ohm DT880s. The volume knob hardly ever goes over 30%

It's even fairly quiet with the 25ohm Emu-Teaks (which is probably pushing the 75-150 output transformer a bit, but I doubt I can easily find a ~25 to ~300 one)


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PostPosted: 23 Oct 2020, 15:08 
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Joined: 15 Nov 2018, 19:43
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Location: Belgium
Got the test stand printed, still need to wire it all up to test if there's any obvious noise left in the power stage.
Attachment:
Test_fixture_small.jpg

Attachment:
amp_test_layout_small.jpg


Having it printed out like this also gives me a better sense of how I want the overall amp enclosure to look. (AKA not as it looks on the test fixture)
I'll probably take some cues from how Suncalc built their purpleheart one


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PostPosted: 07 Nov 2020, 15:15 
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Joined: 28 Oct 2020, 18:26
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Location: Austin, TX
Just joined this forum specifically to build this project. This will be my first tube build. The plan is to use this for my final project in one of my classes. I bought the two output transistors from Edcor a couple days ago (Edcor XSM10K/600 for use with my HD650's). I called them first and was told that since they were making a batch currently I'd more than likely have mine in about 2 weeks. I'm starting to make my BOM now.
I tried to read every page of this topic but I might have missed some things.
2 concerns:
- I have a wooden box that I'd like to use that would be sufficient in size for the build. So if my "chassis" is all wood, what acceptable methods could I use to ensure proper grounding? A metal plate somewhere?
- I decided not to order the Edcor XPWR083A power transformer because my class will be over if I wait 6-8 weeks. I've been looking around on Mouser but my head is starting to spin. I know the primary should be 120V or 240V, 50/60Hz and the secondary should be 200V at 40mA and 8V at 1.5A, but am I missing anything else? If I match those specs do any other variables matter? Also, are there better places to be looking for this?
Thanks in advance for any help offered. I've been wanting a headphone amp for quite a while now, and I'm very excited to start this project.


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PostPosted: 08 Nov 2020, 20:58 
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Joined: 04 Jun 2008, 20:59
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Hi Welcome to the forums. I will try not to confuse you with the following info. This is a reasonable project for a start in vacuum tube audio. Since this is a low gain project and not subject to picking up hum and noise pretty much any case will do. I use metal ones because I have them around all the time. For grounding I would use a large diameter bare copper wire. Something like a 12 gauge one that is often used in house wire. If you can't find a bare one then just strip the insulation off of an insulated one. Component layout can be a bit tricky so plan ahead. Have the main ground wire attach directly to the ground connections on the input jacks and from there split off as needed to the power supply side and the active circuitry side. In those areas you can make connections with lighter gauge wire. I like to use 20 gauge bare "buss" wire. It is cheap. You can use insulated wire of the same size if it suits your build better. The reason for the two sides of the ground system is to avoid having significant power supply currents running through the same ground as the signal ones. If you don't separate them then you can introduce power supply hum and noise into the audio. The audio circuitry considers the signals to include both the "hot" lead and the the ground return lead as a pair and has no way to separate trash on the ground side. It would consider it part of the signal and amplify it. In this amplifier it is not critical, but in high gain ones it can ruin the build. Be sure to watch for "ground loops". They occur any time a circuit or part of it has two separate ways to get to ground. That will also introduce hum or noise. About now you are thinking "what about the power flowing in from the cathode circuit" ? Yes it is power, but it can mix with the signal ground as it is really part of the signal.

Any power transformer that can supply the correct voltages is fine. You may find it easier to get one that will have the correct 120 or 240 volt primary for use in your area, both are not needed. Also it should supply at least 150 volts on the secondary side and up to 200. It can have any heater voltage from 8 to 12 (at one amp) for the heater power. If you go for the higher voltages the LM7806 may get a bit warmer but ought to be fine with a heat sink of about 2 inches square by 1/2 inch thick. It should be in an area that has some ventilation otherwise it can cook some of the other components. There are a number of alternatives to this issue. If you actually have a 12 volt AC winding on the transformer you can put the heaters of the two tubes in series and attach them directly across the 12 VAC winding. This would eliminate the bridge rectifier, filter capacitors and regulator. With the low gain of the circuit and being a first project I would probably do this or the next choice with a 6 Volt AC winding. With the 12 VAC winding it ought to have a center tap and I would ground that. Another alternative is to use a transformer with a 6 VAC winding and use it to power the heaters in parallel. If it has a center tap it should be grounded, if not then one side of the winding should be grounded.

When you get it figured out how you would like to proceed let us know and we can guide you with the project.

BTW The reason that the original project uses DC voltage on the heaters is because I am a fanatic about hum and noise. I go to extremes to minimize it, even when it can't possibly be audible. This fits into that category. Also I have left over defective PCBs from prototypes of my commercial products and they use that transformer and filter arrangement. It makes for a sort of modular build.

EDIT: As a first project....Please be careful. It uses voltages that are potentially lethal. None of us want your first project to be the only one.

Good listening
Bruce

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