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Old 20th November 2009, 09:46 AM   #1
Dxvideo is offline Dxvideo  Turkey
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Default My new baby: TBGC

Hi all,

After a long silence, last month I have decided to make another amplifier..
I was listening my LM4702+lateral mosfet amplifier for more than two years. It was a sweet (and very analytical) amplifier, however by the time it gets boring and sounded more bright to me..
So I thought it will be a good idea to make a tube buffered gainclone. Because a lot of times I've heard that the tube buffers (or amplifiers) sound more musical than the solid state amplifiers. And in older (BC) times I had a tube amplifier which was mono but really good sounding.
Anyway,
After a long decision period, I decided to make a triode buffer stage and a LM3875 in inverted mode. In my drawer I already had a 6N6P double triode and it was a good tube for this like applications. So an 3.5mA bias was enough for my input stage, I did that with a simple resistor and feeded my 6N6P with the same supply as LM3875. Just after the buffer I added a LPF to prevent possible oscillations. For some reasons I kept LM3875's gain around ~33dB wich is fairly high. And to have a low DC offset, added a 1M // 100n to LM3875's non inverted input to GND. All the signal capacitors are MKP and high voltage ones. And all the resistors Vishay MRS25 type 0.6W metal film. I feed the tubes heater with 6.5v DC regulated. And the main suppy is dual regulated with LT1083 at +/-35v. I also use 225VA 2x30v toroid, 25A metal type rectifiers and 10,000uF/63v BHC Low ESRs for the rectfying and 2,200u/63v Nichicon FW Low ESRs for onboard reservoirs plus 100n for each ICs and tubes power pin to GND also a 470n MKP is added between rails.
So, if we come to the listenning impressions;
First I need to remind that, I have listened a lot of buffered or nude gainclones and other type (have really low distortion ones) solid state amplifiers in my life.
First time I listened it sounded a bit dull to me. However after a while I realized that it was not dull but more soft. Especially the high mids were really smooth and not bright. Soundstage was very very wide. And no compensation from any kind of details as I felt. I will use it for a long time I guess because I love it sound too much! At least I never sleep while long listenings like the solid state ones.
May be its a bit "noisy" amplifier. I can hear a hum and hiss from ~10cm distance. I think its a natural result of high gain and SE input stage without a real CCS. But I can live with it, if you consider that I cannot hear it from a normal listenning position.
As a final conclusion;
Its really different amplifier from any SS one that I listened. Strongly recommended.
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Old 20th November 2009, 11:15 AM   #2
ratza is offline ratza  Romania
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Use twisted wires to minimize the EMI and keep them away from the grid. 6N6P has balls, so you can use a sepparate power supply and increase the plate current to 10...15mA without any problems.
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Old 20th November 2009, 11:29 AM   #3
Dxvideo is offline Dxvideo  Turkey
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Sure! But for now I am happy with common supply (its much easier ).
Regarding the twisted wires; is it about the heaters connections?
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Old 20th November 2009, 11:56 AM   #4
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Quote:
Originally Posted by Dxvideo View Post
First time I listened it sounded a bit dull to me.


Yup. The cathode follower in action. A CCS may help a bit but it's still a dog.
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Old 20th November 2009, 12:18 PM   #5
ratza is offline ratza  Romania
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Quote:
Originally Posted by Dxvideo View Post
Regarding the twisted wires; is it about the heaters connections?
Yes, for heater and also for power lines in general. If you really want to se how tubes sound, change the configuration for anode amplifier or follower. SRPP also does a good job, although you'll need higher voltage. The cathode followers tend to be quite neutral; nothing added, nothing lost.
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Old 20th November 2009, 01:36 PM   #6
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Originally Posted by analog_sa View Post
Yup. The cathode follower in action. A CCS may help a bit but it's still a dog.
Sure if you say so. But that was my first impression. But after a while my ears adapted to this sound.. Now I am happy (I think!)
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Old 20th November 2009, 01:38 PM   #7
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Quote:
Originally Posted by ratza View Post
Yes, for heater and also for power lines in general. If you really want to se how tubes sound, change the configuration for anode amplifier or follower. SRPP also does a good job, although you'll need higher voltage. The cathode followers tend to be quite neutral; nothing added, nothing lost.
Hmm.... So you say, my amplifier is not sounding real tube sound now!
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Old 20th November 2009, 02:10 PM   #8
ratza is offline ratza  Romania
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Absolutely not. For tube sound you should build a tube amplifier

Try the anode amplifier, it's easy and cheap to build. Class A really doesn't compare with anything, no matter if tubes, bipolar or mosfets are used. The circuit used doesn't have any gain, it's just an impedance adapter. As an analogy, it is something like an emitter or source repeater.
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Old 20th November 2009, 02:27 PM   #9
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Hmmmmm.... Thats really interesting! But I ve heard a lot of differences... What does that mean?
And concerning the class a matter, I have already built a single ended headphone amplifier three years ago.. In fact I loved her sound too much also. But it was still "bright"...
Actually I have a PCB for Pass F5 and I was thinking about to build it, but this amplifier stopped me about F5!
What do you think? Will F5 sound better than this?
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Old 20th November 2009, 06:45 PM   #10
ratza is offline ratza  Romania
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I have no idea because I didn't build the F5. I didnt't even look on it's schematic. The differences you heard are coming from better impedance match. Input of tube has a pretty big impedance and the output is for sure lower than 1k (I'm too lazy to calculate it properly). There's a big difference between headphone and speakers. It's very personal, but I like bright sound, with crisp details.
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