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Old 4th March 2003, 04:53 AM   #1
Morse is offline Morse  United States
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Default 6922/6DJ8 headphone amp impressions....

Whew!

Here's an update on the Optimised Morgan Jones 6DJ8 headphone amp I recently 'completed' (is any project ever REALLY done? Here a tweak, there a tweak,....) for anyone considering building one.

First off: tried my own mods to the power supply and was less than impressed with the results - voltages were all lower than my initial calcs claimed they should be . Okay, so I backed off the size of the decoupling resistors for the separate stages and now B+ is only about 3VDC shy of spec. That added some real 'guts' to the sound right there. But, that was with my beat old Sony's (always use those for testing new headphone amps rather than my newer, pricier Sennheisers....).

Next: tried it with Senn 600's, Sony MDR V600's and Senn 570's in a 'shootout' and found that it was lean on bass with the Sony's, but overall they did okay. The Senn 600's sounded terrific (as always), but the 570's were a real disappointment. Bass was flabby but the amp seemed to lack the 'oomph' required for any real impact with the 570's. Worse, distortion was audible at moderate listening levels with these phones on music with a wide dynamic range. Overall the effect with the 570's was not acceptable for any orchestral music with an extended dynamic range. Folk music at low levels - yes. Mahler - NO!

Hmm, back to the beginning. This morning I added the NFB circuit and it's WAY better with the Senn 570's now. Also, the reduction in gain is making it easier to run off line sources. With NFB, the 570's are now an acceptable phone for this amp . WHEW!!

So, for anyone considering building one, the optimised circuit is definitely the way to go PARTICULARLY if you're using headphones with less impedence than the Senn 580/600 family. The Sony MDR V600's seem to be the low impedence exception - but this may be a result of their very high efficiency in spite of their 32 ohm impedence.

As always, YMMV, etc.

All the best,
Morse
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Old 4th March 2003, 07:32 AM   #2
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Ah.. figure out what was popping and flashing then?

Tim
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Old 4th March 2003, 03:37 PM   #3
Joel is offline Joel  United States
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Default Re: 6922/6DJ8 headphone amp impressions....

Quote:
Originally posted by Morse
...but the 570's were a real disappointment. Bass was flabby but the amp seemed to lack the 'oomph' required for any real impact with the 570's. Worse, distortion was audible at moderate listening levels with these phones on music with a wide dynamic range.
What is their impedance? It sounds like the amp is just running out of gas. You are exactly right that the Sony's are terribly efficient - so even with a low impedance, the amp can still get enough power to them. You need a lot of horsepower for low-impedance, low efficiency headphones.
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Old 4th March 2003, 08:58 PM   #4
Morse is offline Morse  United States
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Hi Tim;

The popping and flashing was an arcover from the cold plate at startup. A little experimentation showed that the flashover occurred at the peak voltage (about 370VDC). Also, it's source was clearly visible inside the open plate structure of those "made in England" pulls. Anyway, I switched to JAN Philips 6922's and they seem to handle the startup peak without issue (RCA claims 550VDC for Vpeak on a cold plate for either the 6DJ8 or the 6922). Maybe I'll put those oldies back in there after I work out some sort of time delay for the application of the B+; frankly they sound better than the JAN's that are in there now.

Hi Joel;

The impedence on the Senn 570's is 64 ohms. Yes, full agreement over here on the power issue - even with NFB applied this little amp is marginal with these cans. However, with the NFB the Senn 570's are acceptable if unexceptional paired with this amp, rather than being exceptionally unacceptable!

Anyway, just a hint for those out there contemplating building one of these amps - at least put the NFB on a switch so that you have the option with some headphones.

All the best,
Morse
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Old 5th March 2003, 09:48 PM   #5
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Hi,

Quote:
Maybe I'll put those oldies back in there after I work out some sort of time delay for the application of the B+; frankly they sound better than the JAN's that are in there now
A simple sollution is to switch on the heaters first, wait ten secs and than switch the B+.

Another is to use an NTC in the B+, after all you're not drawing much current so that should easy to do.

Cheers,
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Old 6th March 2003, 12:50 AM   #6
Morse is offline Morse  United States
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Hi Frank;

NTC? Not sure I follow you - do you mean something like a CL50 current inrush limiter? If so, can you recommend one for this (28-30mA at 217VDC)?

Thanks for the suggestion,
Morse
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Old 6th March 2003, 01:11 AM   #7
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Default NTC.

Hi,

Quote:
NTC? Not sure I follow you - do you mean something like a CL50 current inrush limiter?
Yes, that is what I mean.
Exactly which one would be appropriate...dunno, if you know how much current the circuit requires it shouldn't be that hard to find
out.

I think that in this case you can even do it with a ballast tube if you're a purist.

To be honnest with you, I'd rather browse tube manuals than sand docs, but I'm sure someone will trawl the net for you on this and tell you what is needed...approaching 2.30 in the morning and all that you see.

Come to think of it...I don't find it normal that your inrush current reaches 370VDC peak for such a small circuit, do you use bleeder resistors across the PSU caps?

Cheers,
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Old 6th March 2003, 01:55 AM   #8
Morse is offline Morse  United States
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Thanks Frank;

>>Come to think of it...I don't find it normal that your inrush current reaches 370VDC peak for such a small circuit, do you use bleeder resistors across the PSU caps?<<

I bleed out the PS caps with a 220 kOhm in series with a 12 kOhm resistor (I use the 12 kOhm to float the heaters by about 11 VDC). My suspicion is that it's a result of the overbuilt PS - it's rated at 250VDC @100mA draw.

>> I think that in this case you can even do it with a ballast tube if you're a purist.<<

Actually this one ended up very far from a purist approach - I actually broke down and used SS rectification (decided to use the 6X4's on another project....), as well as whatever resistors I had on hand in the right values - it's a motley assortment of metal films and carbon films.

Thanks again - I'll check around and see if I can find something appropriate as an inrush limiter. The reason I mentioned the CL50 is that they're the only ones I have on hand at the moment.

All the best,
Morse
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Old 6th March 2003, 02:04 AM   #9
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Default MY PLEASURE.

Hi,

O.K.I see what is going on now...try bleeding your caps with a 100k R and I think you'll be halfway home.

If that doesn't help, even a silly, say 10R 5W resistor in series with the B+ prior to rectification should calm it down.

Ciao,
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Old 6th March 2003, 02:18 AM   #10
Morse is offline Morse  United States
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>>O.K.I see what is going on now...try bleeding your caps with a 100k R and I think you'll be halfway home.<<

100k r on the way...

Thanks again Frank!!
Morse
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