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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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PostPosted: 13 Feb 2012, 17:34 
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Suncalc wrote:
mwhouston wrote:
I would image tube life should be around three years with normal to high use? Agree?
At 10.5W you should get more than that. Remember that the ratings are set such that given normal variability in mass manufacturing any particular tube will not be over stressed. I would think that for individual builds, +5% is as good as meeting the dissipation limit.

Matt: Good news and it will allow us to extract max wattage from the design. I increase the inter-stage cap to 0.033uf After a day or two of burn in I'll put the amp on the CRO to check power and frequency response. Paris had the cleanest FFT graphing I have ever seen.

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PostPosted: 14 Feb 2012, 02:14 
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All finished and sounding great. The amp reminds me very much of the original Paris, very clean and detailed mid-range. Bass rolls of, in room, at 40hz. 31Hz is very quiet but down to 40hz is OK.The Dragon goes to 31hz then loses it. But I'm pleased. Need lots of burn in time now.

I didn't use shielded wire from RCAs to tube. I used wire-wrap wire which I twisted heavily and ran flat on the chassis. The connection from the OPT to speaker terminals is also twisted wire-wrap wire. There is no hum or buzz. Another dead quiate amp. The power transformers are rubber mounted. The filter stage is: CLCRC\RC.
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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
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PostPosted: 14 Feb 2012, 13:27 
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Hey Mark, the amps looks great. Nice work! :up: Glad to hear they are dead quiet. I know you like the appearance of amps, so I'm going to challenge you for your next monoblock build. How about a mirrored layout! :idea:
Cheers

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PostPosted: 14 Feb 2012, 17:57 
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Gio wrote:
Hey Mark, the amps looks great. Nice work! :up: Glad to hear they are dead quiet. I know you like the appearance of amps, so I'm going to challenge you for your next monoblock build. How about a mirrored layout! :idea:
Cheers

Yes Gio I have to admit they may have looked better mirrored. But I was trying to make it easier for me to build. Building two of the same seem the way to go.

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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
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PostPosted: 14 Feb 2012, 21:46 
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Thought the amps only have a few hours up I thought I would do some basic testing:

Into 8.2ohms:

Max power 1.8W RMS
Gain (@1Khz) 2.84Vpp in and 7.7Vpp out on my calcs 8db gain
Frequ. response at -3db points 17Hz to 51KHz

Into 3.9ohms:

Max power 2.3W RMS with gain about 6db here.
Frequ response at -3db points 19Hz to 39KHz

My speakers are 6 ohms so I get the best of both world.

The FFT graph is amazingly flat. Again better than every other amp I have built?? Could this be why the amp sounds so good?

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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
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PostPosted: 15 Feb 2012, 00:37 
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mwhouston wrote:
Could this be why the amp sounds so good?
I'm going to go out on a limb here but I would theorize that the sound of the amp is a combination of a relatively low Rp at the bias point (≈1000Ω), a good selection of rolloff points, good sized grid stoppers to minimize overdrive effects, a nice low impedance and low sag power supply, good quality build and components, and the tube itself.

These deflection tubes were all specifically designed to drive fairly large inductive deflection coils with the 15.484kHz flyback waveform. This waveform has harmonic components from very low all the way up to over 100kHz. If the tubes had any problem driving the coil then the picture would be distorted. I suspect that for these little tubes, driving the harmonic content of music through an output transformer is thermionic child's play.

After hearing about how well this design has worked out I really can't wait to build one.

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PostPosted: 15 Feb 2012, 10:35 
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Oops. That should be 15.734kHz.

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PostPosted: 15 Feb 2012, 17:19 
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Only after a few hours listening, from my memory, I'm not wondering if the original Paris sounded even better. This could be due to the fact that it has the integrated tube preamp and currently I'm using my BoZ preamp. A few days from now I deliver Paris Duo and get back the original Paris. It will be time to compare.

On another note I was asked to give a demo. at the Melbourne Audio club. The night was about digital music from laptop to speakers. I used an external software based D2A which uses the XMOS chip as a DSP. For amplification I took the Silver Dragon, my BoZ preamp and Fostex FE206En back loaded horn speakers. The Dragon had no problems filling the large hall with sound and over 100 people. If fact It could play very loud with no strain at all.

I took Paris Duo along to promote more of my work and toward the end of the 3 hour demo. swapped Paris Duo for the Dragon. 1.8W of power had no problems filling the large hall with sound. The Fostex FE206En are 96db efficient, I not sure what that would translate too in db but there was plenty of volume. Some liked Duo over the Dragon saying the bass was far tighter and just as extend. The hall causes most speakers to sound "boomy" and with the Fostex being a little light on bass the hall brought up the bottom end to give a great balance. In fact many comment how the overall sound balance with Paris Duo was better than with the Dragon.

So a 1.8W compact tube amp can fill a large hall (with high ceilings) with good music at high volume levels and plenty of bass. The Dragon also gave a great performance at 5W!

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Projects: "Abstinence" - 6EM7 power amp | "Sanguine" - 6AS7 Headphone amp with Sophia Mesh Plate 274B rec. tube | "VoXDeuX65" - Inexpensive 2-Way Speakers ]
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PostPosted: 16 Feb 2012, 07:17 
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Matt would you please tell me about the grid stopper (10K) and what it does and how it works and what effect on sound and amp? Ta!

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PostPosted: 16 Feb 2012, 20:49 
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mwouston wrote:
Matt would you please tell me about the grid stopper (10K) and what it does and how it works and what effect on sound and amp?
The short answer is that the grid stopper limits low level bias excursions in the coupling capacitor.

Without a grid stopper even very short duration bias excursions can cause clipping and give rise to odd order harmonics in the output. This problem is then exacerbated when the grid leak resistor is large because this lengthens the recovery time making the odd order action more apparent. With the sizable grid stopper in place, bias excursion take place much more slowly. This converts the clipping action to more of a compressive action for small excursions therefore replacing the discordant odd order action with a more pleasing even order action. This means that with real music (with typical instantaneous dynamic range), the amp can be run closer to max without discordant effects. This is one of the reasons that when listening to real music, a well designed tube amp sounds louder than a transistor amp at the onset of distortion. The perceptible distortion onset takes place only well after small bias excursions are already taking place.

This topic is really worthy of a much longer discussion. Perhaps when I'm done working on my current design article for the web page, I'll do a short article on grid stoppers, bias excursions, and the effects on amplifier sound.

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