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Old 9th January 2013, 06:05 PM   #1541
Telstar is offline Telstar  Italy
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Happy new year to everyone!

I'm starting to prepare things to build a scaled-down version of the l'Amp with CCS for use with lowZ headphones.

The first question is about the input impedance of the K82. Since I would like to use a 10k pot, i would have to raise the input resistor to 100k - is this ok?

Second, if someone -possibly in EU- has a pair of IXTH6N50D2 that could sell me.
Otherwise I have to order from digikey.
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Old 10th January 2013, 09:34 AM   #1542
generg is offline generg  Germany
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hi telstar,

I suppose the input capacitance of the SITs is to high to allow 100k.....

you can see that Mike went down from 47k to 10k. Same did Nelson with his SITs, he offers now the choice, 10k direct input or 100k but with a J-Fet buffer. How this buffer looks, that could be the right one for you, I do not know. May be B1 like......

If understood right, one of the reasons to go down was also the gate leakage current. More resistance more voltage against or in cooperation with the Vgs...
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Last edited by generg; 10th January 2013 at 09:37 AM.
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Old 10th January 2013, 09:57 AM   #1543
protos is offline protos  Greece
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By chance I had a similar idea for my Senn 600 and I built a small version one channel prototype to see the feasibility. I just thought I could get away with a nice resistor loaded version about 35 R with about 250 ma current 24 v supply and a 2W source resistor wirewound pot for bias and tweaking the sweet spot. I did not feel it was necessary to get into the trouble of a separate neg bias circuit and since the current was low I could use the 2W pot for simplicity.
The circuit worked quite well and the sound as far as I could tell was quite punchy. I wired the phones as for mono to get a better idea of the quality of sound since it is hard to audition on one side only.
The problem is that this circuit has almost no gain at all if you are taking a normal line level out. Perhaps this is ok with lower impedance phones which do not need many volts but I felt I needed a few db more for satisfaction on the 300ohm Senns. Of course if you already are driving from a preamp there is no need. I guess the next step is a ccs which should add a few db.
The other problem is that there is a big turn on/off thump which is scary over the headphones. I was wondering of the probability of damage.
Who can help here?
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Old 10th January 2013, 10:10 AM   #1544
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I used 68k. I was a bit concerned, but it seems fine, didn't select any devices, just the first two out of the box. YMMV, of course.
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Old 10th January 2013, 10:47 AM   #1545
generg is offline generg  Germany
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@protos

"I guess the next step is a ccs which should add a few db.
The other problem is that there is a big turn on/off thump which is scary over the headphones. I was wondering of the probability of damage."

here L'Amp: A simple SIT Amp

in the picture with the CCS the 10k and the 220uF in the upper halve try to minimize the Plopp.

In SIT1 with no CCS I suppose Nelson is filling many caps with a higher resistor slowly....
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Old 10th January 2013, 01:33 PM   #1546
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by generg View Post
hi telstar,

I suppose the input capacitance of the SITs is to high to allow 100k.....

you can see that Mike went down from 47k to 10k. Same did Nelson with his SITs, he offers now the choice, 10k direct input or 100k but with a J-Fet buffer. How this buffer looks, that could be the right one for you, I do not know. May be B1 like......

If understood right, one of the reasons to go down was also the gate leakage current. More resistance more voltage against or in cooperation with the Vgs...
I was afraid of this. I think the best solution is to make a 1k stepped attenuator. A buffer would color the sound and I definitely dont want to go that route.
My HPs are efficient and lowZ. I'm going with the default +35V and about 0,5A bias.

Georg, I have to re-read all the articles, but can you remind me about how much gain has the CCS version?
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Old 10th January 2013, 01:53 PM   #1547
generg is offline generg  Germany
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Mike wrote in his lampCCS.pdF

"...So, I tried (D). It uses a self-biased IXYS depletion mode MOSFET.
I apologize for the simplicity.
But, lest we think this simple solution won’t deliver the goods, let’s take a look at how it actually performs.
Noise is typically less than 25uV, in a couple of samples much less.
Gain is now 9dB.
The frequency response is -.17dB at 10Hz and -.22dB at 100kHz...."
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Old 10th January 2013, 02:01 PM   #1548
Telstar is offline Telstar  Italy
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Thanks.
I did check all permutations and the one with CCS performs better at low power.
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Old 10th January 2013, 02:42 PM   #1549
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hi Teslar , i have a 27K load on the gate , and use a passive optocoupler as volume pot ( near 1K of impedance) and it works fine . the add of the B1 buffer in front of my sIT perhaps had some stronger bass but the passive with no Buffer gives me the more "natural" sound
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Old 10th January 2013, 02:55 PM   #1550
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by juanitox View Post
hi Teslar , i have a 27K load on the gate , and use a passive optocoupler as volume pot ( near 1K of impedance) and it works fine .
This would be a good option, yes
I just checked the cost of attenuators and I froze I think diy is the only option either way.
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