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Old 13th August 2003, 04:03 AM   #1
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Default ashok's tube/ss hybrid headphone amp

Hello all-

I was looking into a low voltage tube amp, but the more I learn about tubes, the more the low voltage option seemed to diminish. Odds are, I could not get good enough output with my headphones to warrant building. I did a little searching of the archives and came upon ashok's design (I had searched before, but had somehow missed this one), which I think looks fairly nice. Attached is the amp and regulated power supply:

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I would use this with a 6DJ8 variant, instead of the ECC83. Does anyone have any opinions as to how this would sound? What about the regulated PS? What voltage rating should my transformer be? Current rating?

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Old 13th August 2003, 04:15 AM   #2
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Skip the MOSFETs. Just use a nice high-current triode (circa 12B4, there's many with that range of specs). 150V supply is no problem for tubes.

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Old 13th August 2003, 04:20 AM   #3
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I am by no means a wiz at biasing or working with tubes apparently, and would prefer to do a tried design instead of work on my own as of now. I worked with the low voltage tubes, and now my power supply tranny is messed up. (don't ask) I also nearly fried a few things...

The need for tubes is killing me... I just want something up and running. If I had the money, I'd buy a kit. But, I cannot afford it. I doubt I could get anything running with another tube and experimental values (I probably wouldn't even get this running).
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Old 13th August 2003, 04:52 AM   #4
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Default 6DJ8

Hi Needtubes,
I am working on a 6DJ8 circuit with less components. It still uses an ss output with a 40 volt dc supply. It has a class A output . Will post the circuit and performance in a day or two.
Am trying to make it so that there is no pcb required.
Cheers.
Ashok.
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Old 13th August 2003, 05:32 AM   #5
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40V supply is a little more in my comfortable range. I will definitely take a look at this.
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Old 13th August 2003, 02:12 PM   #6
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Default Basic scheme

Hi Needtubes,
Tubes and low voltage I think is generally frowned upon. But if we can squeeze out decent performance it might be worth the trouble designing one.
My intention was to use a power supply that used capacitors rated at 50 Volts or lower. This keeps costs down. The only low priced low voltage tube that I can get is the 6DJ8 and the 6922.
So when I saw your post I decided to complete my design quickly.

The components have exceeded my target number but are essential for good performance.
Frequency response into a 300 ohm load is -0.5 db at 20Hz and about -0.1db at 20kHz. Distortion is less than 0.1% at nearly 4 volts peak ( 2.8 V rms). That will be about 26.7 milli watts into 300 ohms. With a 96db per mW headphone you will get 110 db!!
The output impedance is also very low - about 1 ohm over most of the bandwidth and rising to about 8 ohms at 20Hz. That I think is quite good.

The power supply is a simple capacitance multiplier. The 5.6 k ohm resistor may have to be chosen to get about 3 volts drop across the collector emitter. This circuit has not been tested yet.
I have collected the parts and should be able to do this in a couple of days. The bootstrap capacitor is essential to improve open loop gain and reduce distortion.
The output is a class A stage with an idle current of about 0.06 Amp. Q2 is a current source with a 9.1 volt zener at the gate.
All resistors are 1/4 watt and only the 3.9K resistor is a 1/2 watt type. Q1 and Q2 will have to be mounted on a simple heat sink. They don't dissipate equal amounts of heat.
Cheers.
Ashok.
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File Type: gif 6dk8 phones amp.gif (7.2 KB, 1126 views)
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Old 13th August 2003, 05:16 PM   #7
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I like this circuit. Shouldn't be all that expensive to build, and if it sounds good, should be well worth it.

R8... the 300-ohm resistor- I am assuming this is the load, correct?

If I want a volume control, can I just add a 50k pot in front without changing anything?

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Old 14th August 2003, 03:50 AM   #8
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Default R8

Yes R8 is the load. Actually it should have been a large value resistor say 100K . This would permit the output capacitor to charge even if the phones are not connected. Otherwise you would get a bang when you plugged the phones in .
So if R8 is 100k the phones load is not shown ! It will be in parallel with R8.

You can use a 50 K pot for volume. That's what I intend using.
Cheers.
Ashok.
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Old 14th August 2003, 04:21 AM   #9
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Is Q3 what's known as a capacitor multiplier? Uh I think...

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Old 14th August 2003, 05:04 AM   #10
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Default Capacitance multiplier doubt.

Hi Tim,
Check out http://sound.westhost.com/project15.htm

This describes the capacitance multiplier.

Q3 is the capacitance multiplier device. The collector base
resistor should ensure that you have enough voltage across the transistor collector emitter to keep it in the active zone - above 1.5 volts under all supply voltage conditions.
Cheers,
Ashok.
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