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Old 8th April 2014, 03:01 PM   #1501
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I spoke to Canterbury windings about the production of EI transformers and this was their response

Dear James,

Unfortunately I don’t make EI core transformers + I am reluctant to recommend anyone as last time that I did my customer told me that the quality was poor from the recommended manufacturer
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Old 8th April 2014, 03:10 PM   #1502
dwjames is offline dwjames  England
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Quote:
Originally Posted by stijn001 View Post
Ok, hereís one for you. Iíll probably be shot for suggesting this. But itís been bothering me that with only using single-ended output Iím in affect only using half the power/gain output of the DDDAC, by connecting either ENG or POS and common and I could do with a bit more gain anyway. I was also curious about the amount of DC de DDDAC produced. Well in my case it was 0.008Vdc&0.000Vdc R/L respectively. So I thought what the heck, it will be a nice experiment to do away with the output buffer caps and since I knew Rload resistance in my power amp (I put it there), I got rid of Rload in the DDDAC too. So now Iíve connected POS/NEG single-ended RCA out to my amp. Iím sitting here listening to it now and I can not put any fault to it, if anything it sound better with +/-15Db more gain (someone will be able to do the math on this). I also made change to my PSU so itís hard to tell whatís what but less is more in my book & no output caps..

Any comments?
Yup, if you measure the DC offset across and POS and NEG and it's real small like that (mine is 0.007v and 0.009v) then there shouldn't be any issue running direct from these without coupling caps, especially if you have an amp with inline coupling capacitors like many valve amps do.

I have mine running like this now since the weekend and it sounds really good.
The only downside I have found is that with the increased gain, I get a little clipping on my amp if I put the volume up very loud. I guess it's over the 380mv sensitivity on my amp's input, but as long as I keep it attenuated to a sensible volume, it sounds much better than with coupling caps, especially the cheap ones I had in there
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Old 8th April 2014, 03:55 PM   #1503
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Quote:
Originally Posted by dwjames View Post
..... it sounds much better than with coupling caps, especially the cheap ones I had in there
.. Glad to hear I'm not the only one.. My First watt F5 is coping fine , no clipping so far, and it's "cap"less all the way from DAC chip to driver..!

You could try a little attenuation to bring down the gain. Just now, I have Rload (at the amp end) 47K between pos/neg and 4k7 inline, to give you an idea.
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Old 8th April 2014, 04:26 PM   #1504
carlsor is offline carlsor  United States
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Default JFET Buffer Circuit

Attached are a schematic and photo of my BF862 JFET buffer. There is almost too much detail. I could hear the valves and mechanical linkage of a bassoon clacking on a Vivaldi recording through my Grado GS1000 headphones Ė something I have never heard before. The break-in period was UGLY at times and bad recordings are punishing. The best CDs sound amazing. I am going to test HF filter capacitors on the DDDAC outputs to try to soften the SQ a hair. I donít need to hear each 0 and 1 from the CD. This buffer can pass a signal greater than 100 MHz.

This Buffer is for the masochistic DIYer. It requires testing and matching of BF862 JFETs. A negative power supply is needed. Surface mount components require more care. Nevertheless, a buffer is the only alternative for the person who wants to totally tweak out a single board DDDAC and drive an output transformer without compromising the musical experience. On the positive side, all semiconductors and resistors are inexpensive and available as SOIC-23 surface mount. The BF862 is only available as a SOIC-23. Anyone want to design a surface mount PCB for this buffer? I could provide a set of components with matched BF862s. Maybe Doede could drop the buffer circuit into the main board foil trace for optional use in a future revision.

The circuit uses the ZTX450 which is used in the highly acclaimed Pearl 2 DIY phono preamp to provide a constant voltage source for the 1st JFET gain stage. The ZTX450 makes the lower BF862 constant current load - more constant, and allows a more standard -12VDC power supply to be used. My prototype uses a mixture of surface mount and standard size components. The TL431 regulates the voltage to the base of the ZTX450s. The 20 ohm pot is used to zero out the voltage offset between the + and Ė outputs to the transformers.

If the ZTX450s and the TL431 and its associated components are eliminated, you end up with the Nelson Pass B1 direct coupled buffer design which should sound a bit more euphonic. It would be much simpler and fit easier into Doedeís main board. My 2SK170 version of this sounded great, but Iím a masochistic DIYer.
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File Type: jpg BF862 Buffer.jpg (709.7 KB, 428 views)
File Type: jpg BF862 Buffer Photo.jpg (481.7 KB, 409 views)
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Old 8th April 2014, 04:39 PM   #1505
dwjames is offline dwjames  England
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Carlsor, do you have a basic explanation of what that is and what it does and how it's better than either an output transformer, a good coupling cap or simply using the pos and neg direct if the DC offset is low?
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Old 8th April 2014, 06:09 PM   #1506
carlsor is offline carlsor  United States
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Default Buffer Explaination

dwjames,

The JFET buffer does not replace coupling capacitors or transformers - it is used to interface the DDDAC output with these devices. The Buffer load (JFET gate) to the DDDAC output is over 1meg ohm - almost nothing. The DDDAC doesn't see the transformers or capacitors or the load that they terminate to. Meanwhile, the Buffer follows the DDDAC output signal but provides its own power to drive the signal through these devices.

An electrical load can affect a signal source. The Cinemags interfere with some of the musical detail and dynamics from a single board DDDAC. Multiple boards help overwhelm the Cinemag and its load with a lower combined output impedance (ie stronger signal)

Its like the difference between a bicycle and military tank both moving at 20kph. The bicycle may be affected by the terrain, but the tank will move ahead unaffected. The Buffer is like a tank following the pedal speed of a stationary bicycle - both will operate uninhibited.

The Buffer pot adjustment is an opportunity to bring the DC offset even lower, but this is not the primary reason for using a buffer.

I hope this helps.

Last edited by carlsor; 8th April 2014 at 06:19 PM.
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Old 8th April 2014, 07:47 PM   #1507
dwjames is offline dwjames  England
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Excellent explanation, thanks
Makes me wonder as I have a spare SRPP valve output stage lying around...
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Old 8th April 2014, 08:43 PM   #1508
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I hope someone has an explanation for this:

When I tried to use the balanced outputs directly without trannies or caps. The stereo image collapsed and I got a lot of extra bass. Totally not amused with the result.

Then I reconnected my output trannies and voila there it was back again. A nice wide stereo image with a lot of envelopment.

Do any of you know what could have been the problem? Did I do anything wrong?

I measured the DC offset. 2mV on the right channel and 0.3mV (very low) on the left channel. I connected the pin of the cinch plug to the pos en the sleeve to the neg terminal.

Any ideas?
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Old 9th April 2014, 09:23 AM   #1509
WalterW is offline WalterW  Netherlands
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Quote:
Originally Posted by dwjames View Post
Excellent explanation, thanks
Makes me wonder as I have a spare SRPP valve output stage lying around...
+1

Would a buffer like this, from the VFET article, be fine?
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File Type: png buffer-JFET.png (25.1 KB, 325 views)
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Old 9th April 2014, 09:51 AM   #1510
dddac is offline dddac  Germany
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Stijn, one question, you are NOT using the Rload resistors? the Ra and Rb ones, that should not be possible, or do you mean you run it without the 100k bleeders which are normally after the capacitor? that would be perfectly correct to remove them. and no cap is the best cap of course !
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