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Old 29th May 2012, 07:28 AM   #11
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Default Pushing the limits of the E801S version

After this succesfull experiment with the 6SN7 I reverted to the version in the living room to adapt what I just learned with the 6SN7 experiment: get a higher current for the CF. 3 mA plus looked rather good to me as it was, but I added a dummy resistor also here to earth, that consumes 2,5 mA.

This gave more air definitely. The - 3 dB bandwidth is now an astonishing 687 kHz. This limit is clearly slew-rate limited (downward slope), so the buffer can be optimised further. The bandwidth could then reach 1 MHz easily ... No overshoot on the square wave, a slight nervousness can be seen on the settling edge (not an oscillation). With the 6SN6 you can see a relaxed settling, here with the 801S it bam locked. It is the higher feedback.

The 801S listening is ultra dynamic. It highlights all nuances of the music, small changes of timbre in the highs and lows, in pitches and in placement. The low level details are astonishing: even in the bass frequencies.

I did adapt the resistances a bit to improve the operating points. The first anode load went up to 150 K, the second was reduced to 39 K.
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Old 29th May 2012, 12:55 PM   #12
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Default Distortion measurements, 6SN7 version

This is the picture of the distortion, at 10V RMS out:

SP6- 801S line 10V RMS 0,1%.jpg

Almost pure 2nd harmonics. My meter says 0,1%. But that figure itself is distorted: the distortion shows the bad regulation of my bench top power supply as the thickness of the trace.

The square wave of 100 kHz: pure music.

SP6- 801S line L1016745.jpg

This is nice behavior, no RC in the feedback, no HF shunt in the first stage . . .

Because of the circuit adaptions, I should not call it an SP-6 clone anymore.
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Old 3rd June 2012, 08:25 AM   #13
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I have implemented a shunt DC for the heater of the 6SL7 tubes. It has a slow start with an idle current of 120 mA for about 5 seconds, before the voltage is slowly increased. This way, there is no surprise for the tubes.

Power Supply RIAA V3.pdf

The result is excellent. After about 15 minutes of warming up, the amplifier is full and warm blooded.

I have the 6SL6 in series because the winding I have is 12.3 V nominal (actually more than 13), so I could have cut the winding into two parts (the winding of the center tap is available), but that would double my components...
The schema works at this lower voltage too, but resistances have to be adjusted for 6.25 out.
The drop over the slow start FET is < 0,1 V. So it is negligible. This FET does not have a heatsink.
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Old 1st February 2013, 09:21 PM   #14
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Very Interesting Albert, where are you now in this preamp ? May we get the definitive circuit and pictures ?
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Old 3rd February 2013, 06:55 PM   #15
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Quote:
Originally Posted by JLOP View Post
Very Interesting Albert, where are you now in this preamp ? May we get the definitive circuit and pictures ?
I am very in to this amplifier!
- not physically: I finished it quite some time ago and have not found reason to adapt anything. It is just marvelous.
I might though implement a small kind of bandwidth correction of + 2 db from 200 Hz down just because I have these Quad ELS that are a bit lean in the bass and on some sources this is noticeable at low sound levels; it is a light kind of physio you might call it.. But is is also the difference between the 300B amp (higher output gives a little boost in the low end) - and my F5 that can pump 15 amps into 1 ohm without a hickup.

I see I left some things to the imagination:
http://www.diyaudio.com/forums/attac...p6-final-2.jpg


But I omitted C4 and C5 altogether. Also I did not use the positive feedback of R14. At the end on the distortion analysis I think I did not like it; but audibly I heard no high extra dynamics. So conclusively I decided for keep it simple.
The value of R14 also has impact on the setting of the whole amplifier and would have required too much experiments to get a good value where its effect is audibly positive. I think with all the feedback its impact is also negligible and thus might result in higher order distortion, invisible for me.
I must say I have an old fashioned distortion meter and can not see more than about third harmonics.

I might have a look again at the final resistor values (R10, R11) what I finally did use.

hope this helps you
albert
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Old 4th February 2013, 10:06 PM   #16
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Thanks Albert, it's much infomative. I of course did not intend to put you in the box, sorry for my approximative command of English (:-(

I understand that there should not be too much tuning after power on ? I can copy slavishly but not go into too skilled practice unfortunately.If so, I may give it a try in a couple of months when finished optimizing my Dac.

Did you have opportunities to compare the SP6 clone to other preamp ?

BR
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Old 5th February 2013, 07:22 AM   #17
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Quote:
Originally Posted by JLOP View Post
I understand that there should not be too much tuning after power on ? I can copy slavishly but not go into too skilled practice unfortunately.If so, I may give it a try in a couple of months when finished optimizing my Dac.

Did you have opportunities to compare the SP6 clone to other preamp ?

BR
Oops I forgot something.
The point I found out experimentally is just that there needs to be enough current in the cathode follower, that was the biggest find. That is why there are two resistors to ground from the cathode: one to the input divider and one directly to ground.
I have described it in a later post:
Quote:
I added a dummy resistor also here to earth, that consumes 2,5 mA. At Vk 100-120V this is 39-43K.
The first anode load went up to 150 K, the second was reduced to 39 K.
It has a nice slow start, just a few seconds and the sound starts. Even power off is without a pop. And that with a hefty triode stage pre into a very heavy solid stage power amp.

I compared it with the Aikido 6SN7 and it is just a bit more dynamic, in the sense that the sound stage at any level is constant and has a very big depth. This characteristic is what I like most in an (pre)amp, some call it laid back. The Aikido is not bad though. Just a little less coloring, and it can't handle long capacitative interlinks as good as this preamp (>500pF).

I also had a nice balanced preamp, but I had that with 7 pin dual triodes, that shared the cathode, and these were microphonic, even so that when I turned the stepped pot I heard ringing and with heavy crescendo's this also happened. In my memory that was a very nice design too (I had it for 15-20 years).

Since I finished it, I never touched it again.
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Old 13th August 2013, 02:29 PM   #18
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Default The new Current Feedback preamp with ECC88.

I found I had some rectifier sound in the output. When I connected a Lowther to my power amps (PP 300B) there was a big 'buzz'. And there was a also some hiss. Without the preamp the power amp was completely silent. Even with the 5V filament in AC...

So I went ahead in fixing the preamp, and in the mean while - - - I redesign the whole lot.
  • A first design decision was, that if I wanted to get a preamp with the lowest noise floor, I should revert to E88CC or even E188CC in the first stage.
  • I would need a very silent HV, so I installed a Salas HV at 200V.
  • Noisy ECC88?? Well a subsequent find was that the heater influenced the cathodes. I had the heater at plus 50 volts (I thought) but measuring, it was lower (20V), so something was wrong. I connected a mA meter and found some 3 mA were dumping in the cathodes. The heaters were at a divider of 68K/20K. It just disappeared when I connected the heater center to earth. Then also the noise in the tubes disappeared (for a large part). I found the tubes to be most silent when I connected it with 20K, bypassed to earth; the current was then 0,0145 mA.
  • I took all care to get a star ground. Both halves and the output stage are connected separately to the ground and plus.
  • I also decided to omit the feedback from the cathode follower and revert to a proper line stage feedback, directly from the first anode to the input cathode. I had seen before this gives no overshoot, is perfectly stable and is wide band: from < 1 Hz - > 350 KHz. So all problems I had of loading the output before, from slew rate limiting are gone all at once.

I tested the input with a LED as current source at the cathode. But this was very noisy (the LED's zener noise is amplified...); even connecting a cap across it did not help. So I selected a resistor to give the same 1,88 V drop (1,8-2 k).

So I have simplified version here that I like much again.
Note I have reverted to ECC88's and I must mention the two halves are not similar! The first triode half (1,2,3) has a Vfk of 150V, the second one (6,7,8) of 50V. So I must use the first one in the entrance because it floats at +55V ; the second one is used for the second stage. In the CF output I have (1,2,3) on top of the cascode-connected CF, and the section (6,7,8) as the constant current source. Because of the CCS I do not need a diode safeguard between the top grid (2) and the cathode/anode (3;6). I always suspect these diodes of some sonic effect.
CurrentFeedbackPreamp.png
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File Type: pdf Preamp CurFb.pdf (37.7 KB, 13 views)
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