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6DJ8 (ECC88) Tube Headphone Amp Project
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4161
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Author:  slotube [ 19 Jun 2020, 16:20 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bruce,

Thank you for the Headphone Build. This was my first tube amplifier build
and I learned a lot. Especially proper grounding techniques and the use
of a current regulator. Great job explaining those topics.

Observations:
Pros: The sound is clean and full.

Cons: Under powered. At .63dbv voltage gain, it can barely drive my Beyer Dynamic
DT-770 80 ohm headphones.

Suggestion: Please avoid hyperbole like "This is much more output than is needed for really
excessive sound levels with most headphones." It is misleading!

Author:  Bradley [ 19 Jun 2020, 16:30 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

@slotube Which output transformers are you using in your build? If they have additional taps on the output side (e.g., a center tap), I would try one of those to see whether it's a better match.

Author:  slotube [ 19 Jun 2020, 16:52 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bradley,
I am using the Edcor xsm10k/150 and using
the 75 ohm center tap. I measured an impedance
of 63 at that tap. So it's in the ballpark. The was done
measuring the output in Vpp with two different loads and
calculating for Z

Author:  gofar99 [ 19 Jun 2020, 20:31 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, I'm glad you liked the project but you are the first person who indicated it didn't have enough output. I did say for most phones. The large majority of which have sensitivities in the 100db per milli watt range. For them the 50-100 milli watts is quite excessive. Yours are the exception clearly. Yes I did say the gain was low and that some sources might be insufficient. Since a basic goal of that project was simplicity I don't consider any of that hyperbole. There is nothing saying you can't make a better or more suitable project. That is what diy is all about and indeed how I got started about 12 years ago.

Good listening
Bruce

Author:  Bradley [ 19 Jun 2020, 20:42 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

I'm using a modified circuit to run 6CG7 tubes with different outputs. Drives speakers. As Bruce mentions, some clever DIY might be the answer.

If the 6DJ8s can handle it, maybe a small step-up transformer on the front end would be enough? I'm using a 10K-90K step up on one of my inputs since the source is a bit lower level, which helps. Anything with an appropriate turns ratio would probably work well enough to test the idea.

Author:  slotube [ 19 Jun 2020, 22:41 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bruce,
Thanks for your reply. Yes, you were clear on the output.
But I like tweaking, building and improving things, so
further refining will be fun with this elegant design.

I am also in the process of building your 12AU7 Preamp Design and
looking ahead at possibly the Push-Pull EL84 Poddwatt. I am a fan of
the current regulating design you use.

Thanks again!

Author:  lolaschwimmt [ 21 Jun 2020, 04:02 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hello
Since a few weeks I am building the ecc88 headphone amp. Today I could listen to one channel of the amp, the first time.
So far it sounds very good without any distortion or hum.
But there are a few things I am wondering about.
Maybe you can help me to find the answers to my questions.

1. The input signal on the pin 7 is higher than the input signal on pin 2. I thought that the input signal on both inputs should be the same?
2. I am wondering about the amplification factor. The output signal in volts on the output of the output transformer is higher but only a bit higher than the input signal ( pin7 ) at all. It is even not as twice as high. That is the reason why I think, there is something wrong with the amp.

Thank you all in advance.

Author:  NickCool [ 12 Jul 2020, 14:16 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hello, I just finished the prototype. It is a very straightforward and elegant design.

I am using a 10k to 35ohm transformer to match my Audio-technica earphones, and NOS GE tubes.
I mounted everything on boards. The PS is oversized.

Frequency response is within 1db from 30 to 20khz (I couldn't go higher). No hum and no audible distortion.

The only thing is the output voltage. It is the same value than the input voltage.
The article claims "Approximately 100 millivolts (mV) of signal input will result in about 50 milliwatts (mW) of output power", i.e. 30db gain over a 300 ohm load.
I wonder what happen?

Anyway nice experience.

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