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Old 7th May 2012, 12:26 AM   #1
Simon B is offline Simon B  United Kingdom
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Default Advice sought on thermionic headphone amp

I wonder if any forum members could offer practical advice on the construction of this circuit:

Class_A_amplifier_headphone_amplifier

I think it might be more demanding in some respects than the equipment I usually work on, which is musical instrument electronics.

I'd like to avoid any errors or oversights that might spoil my enjoyment of this headphone amp when I use it in its intended environment. Perhaps some members might have useful practical experience they could share? Serving and ex-military comms guys might have the most to offer.

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Old 7th May 2012, 01:45 AM   #2
Fenris is offline Fenris  United States
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Personally, I prefer solid state rectification and regulation, it's more efficient. Sourcing the transformers may present the biggest challenge, but otherwise this is a fairly textbook two stage single ended amp. I personally prefer using a triode rather than a pentode in the first stage and you might be able to replace the cathode resistor/capacitor with a LED string that drops the appropriate voltage, which many people prefer.
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Old 7th May 2012, 05:59 AM   #3
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If it will be for headphones only then that circuit is a bit over kill since it is a power amp with a headphone plug. Try looking at some of the projects here HeadWize Library - Projects
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Old 7th May 2012, 07:16 AM   #4
marce is offline marce  United Kingdom
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Hi Simon, Nice little headphone amp, with the right enclosure would look nice. Any idea of the parts costs.
For construction, I would reccomend a 2.4-3.2mm thick double sided PCB with quite heazy copper (2oz min, if you use 2oz on a PTH board, it will be about about 2.5 ounce the plating process has finished). Though the design will probably go on one layer with the odd jumper, going for a two layer solution will give more options. Then assembly is just like any PCB. This way avoids any wiring errors, if you go from (checked) schematic direct to PCB. You have to observe the creepage and clearance spacings on a PCB, but easily achived. Also you can pick your enclosure and match the PCB to it, making assembly easier. The thicker boards are much more rigid than standard 1.6mm boards, when you are putting valves in their sockets they feel solid and give a impression of quality, plus they make a rigid assembly less affected by vibration and shock, not to much of a problem in a houshold envoirenment, but in more taxing envoirenments it is critical.
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Old 7th May 2012, 07:31 AM   #5
marce is offline marce  United Kingdom
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Thinking a little more, depending on your tastes...
You could do a retro design with curvy tracks with teardrops on the pads, brown bakerlite sockets, iron cases for the transformers and a plain varnished PCB (no coloured solder resist). Or a modern sleek design with MU metal shields for the transformers (mounted on the PCB) with modern routing, nice sockets and a nice black resist to contrast the metalwork.
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Old 7th May 2012, 09:01 AM   #6
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I would search around for a better headphone amp to build up.

these "designs" are littered with bugs because they are very generic.

try to stay away from odd tubes. the more standard the tube types used, the ease of swapping and availability of replacement is a great advantage.

I take an "old school" approach to building - Point to point.
I know a lot of people don't have a friend that is a machinist.
precut chassis are available. There are electronics merchants on the web that cater to DIY audiophile and guitar amp building.

tube vs solid state rectification, I recommend using solid state but put the high voltage DC on a separate standby switch. Tube rectification is an added cost in tubes and power transformer requirements.

I posted on this fourm techniques for heater supply design as well as others. The simple approach seems to work for me.

When I build point to point, I keep all of my high voltage away from signal lines. My heater wires are assembled in twisted pair. and routed physically as much as possible in 90 degree angles from the signal path.

Placement of tubes in a staright line helps construction as different sections are identified easier. a good rule of spacing is 1 to 1 1/2 the socket distance as this will help the tubes and the parts ventilation.

try to place power transformer and choke (if used) as far away from the output transformer. in stereo output, place one in 90 degrees orientation from the other channel this will help preserved .

this is a picture of point to point design on a prototyping chassis. I use polypropylene caps where ever possible. and 1 w resistors that are temperature stable ( less than 50 ppm ).

electrolytic caps should be low leakage or low impedance (low esr) type.
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Old 7th May 2012, 10:35 AM   #7
DF96 is offline DF96  England
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I'm puzzled. What advantage would military comms experience give with a valve-based bog standard amplifier circuit?
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Old 7th May 2012, 12:14 PM   #8
Simon B is offline Simon B  United Kingdom
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Thank you for your replies so far.

I'm thinking of using a vibrator-type power supply for the HT,
possibly a synchronous rectifier type, fed from a rugged sealed lead acid battery, though I haven't worked with this rather old technology before.

I'm not necessarily averse to using a pcb, though I need to research which board materials are least prone to absorbing moisture as this could obviously be a problem. Even point-to-point on turrets or eyelets might be susceptible to this, though I should think the range of materials that these can be mounted on is basically anything I can drill suitable holes in. Perhaps with attention to creepage and clearance spacings, and the right board material, a pcb would be suitable.

I need to source suitable tar-potted transformers and the valves used will be metal can pentodes.
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Old 7th May 2012, 12:51 PM   #9
DF96 is offline DF96  England
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To achieve compatible documentation, please ensure that any notes you put on the web about this project use DjVu format.
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Old 7th May 2012, 01:55 PM   #10
marce is offline marce  United Kingdom
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Er wouldn't worry much about moisture content, best material is a decent Tg rated FR4. Unless you are going into outer space or somehwere, the moisture absorbancy of the PCB material wont be a problem, I know this for a fact as I have done boards with high voltages that work in -40 to +85 all around the world no problem. Even if you design to pollution class 3 (not likeley in your house) the creapage is 8mm for 600V peak.
All you can do with point to point wiring can be achieved on PCB's, thats why they came along, I have valve amps from both design types, so I have no view on the effect on sound, but a PCB based project is an easier build.
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