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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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 Post subject: Altoid's Headphone Amp
PostPosted: 07 Oct 2011, 08:21 
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Joined: 28 Dec 2010, 22:07
Posts: 270
Hi all,

I mentioned in another thread a small headphone amp that fits in a candy tin. I am posting info here in case anyone wants to build one.

Image

It uses a 9v battery in a split configuration. The capacitor value was chosen by their size, nothing magical about 220 microfarads. There is a bit of hiss in the output that is only noticeable when there is no signal in. I'd like to remove this but I don't think it has to do with the power supply. The 4700 ohm resistors aren't critical either, but they don't need to be too low or too high.

The TLC272 chip is the only IC I have that will work. I tried TL072 and TL082 (according to TI's website, the TL072 is the same as the 82 but with lower noise characteristic) but each breaks up much sooner than the 272. It's a CMOS dual op amp that can source/sink up to 30 mA and I think that's why it's most capable of driving the output transistors. I haven't tried any other op amp types, mainly because I am not willing to drop $15 on one DIP-8.

The transistors are biased by the two diodes at about 10 mA. This value isn't critical but the transistors aren't supposed to be hot (either one). The diodes aren't critical, but better performance would be achieved by comparing the diode forward drop to the base-emitter forward drop of their corresponding transistors. The transistors themselves would benefit from matching, but at very least need to be complimentary types. Suggested types are 2222/2907, 3904/3906, and 4401/4403.

I didn't use metal film resistors and that may be contributing to the fuzz noise. Here is a list of improvements I could make, but most will not fit in a small Altoid's tin. The amp, as built, works pretty well. The resistors are all carbon-film, the caps are electrolytic and green mylar, and the IC socket is one of those solder-tail sockets. You can find better sockets out there although they aren't as cheap.

-Extra battery filtering/isolation or a DC jack. External DC would increase the power but defeat the purpose of portability that this scheme offers.

-Make the output stage a darlington pair stage by adding another NPN and PNP transistor. This will probably allow the use of lower-output chips such as TL072 and TL082.

-Add a volume control on the front end. There is not enough room in the can as-is but a larger enclosure would accommodate.

-Use metal-film resistors for lower internal noise.

-Replace each 220uF power cap with three 100uF caps in parallel. This will lower capacitor ESR and perhaps increase bass response.

-Expensive audiophile caps (or not)

If there are any suggestions, comments, angry rants, etc, I would like to hear them.
Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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PostPosted: 08 Oct 2011, 00:39 
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Joined: 28 Dec 2010, 22:07
Posts: 270
I have included pictures. There are bitmaps I drew up of the layout, but I think these will be more helpful for potential builders. It was a bit cramped but I've seen people do more amazing things with a soldering iron.

Image

The top shells of the cans were painted with acrylic paint. The white paint kind of sucked and had a tendency to brown. I coated it with clear gloss and it improved the appearance somewhat. The next time I paint one, I'll paint it with spray and give it several coats. Thicker paint chips less and a hard lacquer will further prevent undesired attrition.

Image

The browning of the white paint is more evident here. The amplifier on the right had the LED indicator put in the lid. I'd recommend not doing that...it made extra difficulty in closing the lid and had extra wires getting in the way. The hot glue eventually came undone and allowed the LED to float. I removed the plastic coating from the capacitors in the second amp. I might have gone so far as to polish the bare capacitors but I decided it was a waste of time. For the input caps I used the green mylar/film ones rated at 100v. I don't think I'll ever contact anything with 100v there but time may tell. Some audio nuts would argue that those caps are insufficient and I should be buying Swedish foil caps made from rare materials. I shake my fist at thee.

Image

A close-up view of the circuit. The resistors are stood on end to maximize component density. A lot of avionics gear is done this way for the same reason. Someday I will find an op amp that performs here as well as the TLC272CP. Someday.

Ed

_________________
Completed Projects:

6L6G / 12AX7 25W Guitar Amplifier
TLC272CP/2N3904/3906 Headphone Amp (Can Man)
MPF102/IRF630 Headphone Amp (FETZilla)
12AU7 / IRFX Headphone Amp (x2)
TIP31C/32C 5W Stereo
More to come...

Bow to Ed!!


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