JB has decided to leave us. As a parting gift, he asked us to post his schematic of the Denneson amp, and I'm pleased to be able to do so.
This version of the JC-80 line stage looks pretty good. I have not looked at every detail yet, so I may find something in future.
This is an early version of this topology. It can be improved, because of later experience, but in some ways, it cannot be as good as the original, because the active parts that made it are not available at this time. Substitute parts, may or may not work as well.
The important thing to note is the dual servo. One servo is common mode, the other is differential mode. Since this preamp has balanced outputs, one servo works to keep the DC offset relative to each output, the same, and then the other servo puts the DC offset close to ground. It is interesting to watch what happens when you disable one servo or the other. Either the outputs will be near ground, BUT equidistant from it from both sides of zero, or the outputs will be the same, but offset from ground.
The servos must be slow, or they will effect the sound quality. An OPA 134,2134 is a good substitute for the 411, 412 series. That is what I use today.
This is an early version of this topology. It can be improved, because of later experience, but in some ways, it cannot be as good as the original, because the active parts that made it are not available at this time. Substitute parts, may or may not work as well.
The important thing to note is the dual servo. One servo is common mode, the other is differential mode. Since this preamp has balanced outputs, one servo works to keep the DC offset relative to each output, the same, and then the other servo puts the DC offset close to ground. It is interesting to watch what happens when you disable one servo or the other. Either the outputs will be near ground, BUT equidistant from it from both sides of zero, or the outputs will be the same, but offset from ground.
The servos must be slow, or they will effect the sound quality. An OPA 134,2134 is a good substitute for the 411, 412 series. That is what I use today.
Off topic posts were removed last night.
Interesting servo John. I would normally have used a servo to ground for each phase. Your trick is neat, I like it.
-Chris
Yes, clever circuit.
May I ask what the patched 10k "noise gain" resistor in the CM servo is for?
Regards, Klaus
May I ask what the patched 10k "noise gain" resistor in the CM servo is for?
Regards, Klaus
I don't know. It looks like a mistake to me. Actually, it is to reduce the maximum input level to the servo IC's to below their power supply voltage to avoid latch-up during turn on.
john curl said:
was wondering about this / thanks.
An externally hosted image should be here but it was not working when we last tested it.
Attachments
![surprise[1].jpg](/community/data/attachments/101/101351-38ea965849b1fd718fdff5fbe847de05.jpg?hash=OOqWWEmx_X)
On reflection, this was a 'BAND-AID' added on to fix a problem that developed after the circuit board was finished. The best way, and the way that I normally solve the same problem with Vendetta or CTC designs is to add 2 low power 12-15V zeners wired in series, opposite each other and put them at the servo inputs to ground. Then, if an input signal higher than the power supply appears, the zeners get it first.
John,
Not so. I am always interested in your thinking, if for no other reason than the fact that you are one of the few people around here who shares my predilection for wide bandwidth and low-to-no feedback.
I could, I suppose, make a nuisance of myself by demanding or asking or begging or pleading...but my notion is that you'll talk when you're good and ready and not an instant sooner.
So I'll wait until you're ready.
It's that simple.
Grey
P.S.: Incidentally, I etched a board this afternoon for that JC-3-ish circuit that I've been working on. If all goes well, I'll etch another tomorrow. I've got the front end bandwidth above 200kHz (yes, it'll drop when I attach the output...I'll retune the thing then and see where things stand) and things look promising for this version of the circuit. I've got two other variations I want to try before I commit to one particular design.
And after all, the Blowtorch circuit isn't so very different from the JC-3's front end...
Not so. I am always interested in your thinking, if for no other reason than the fact that you are one of the few people around here who shares my predilection for wide bandwidth and low-to-no feedback.
I could, I suppose, make a nuisance of myself by demanding or asking or begging or pleading...but my notion is that you'll talk when you're good and ready and not an instant sooner.
So I'll wait until you're ready.
It's that simple.
Grey
P.S.: Incidentally, I etched a board this afternoon for that JC-3-ish circuit that I've been working on. If all goes well, I'll etch another tomorrow. I've got the front end bandwidth above 200kHz (yes, it'll drop when I attach the output...I'll retune the thing then and see where things stand) and things look promising for this version of the circuit. I've got two other variations I want to try before I commit to one particular design.
And after all, the Blowtorch circuit isn't so very different from the JC-3's front end...
john curl said:Well, fellow designers, I guess that a complete line amp is of little interest around here.:
I am always watching, reading, learning. You make me stop, reasses and think... please don't stop now.
Fellow designers, I did not get my point across very well. The line amp in this thread IS complete, and only needs a power supply and input volume control.
Other functions are not what I wanted to talk about on this thread.
Other functions are not what I wanted to talk about on this thread.
Let's us begin to build
This is very interesting, I want one of those, I want to throw away my LK1.
I want some information where I can find good hardware like enclosure, knobs and switches.
Where can I find the subsitute for the semiconductors.
It is possible to build this line amp on traditional old type of soldering turrets on glassfiber board.
Kamskoma
This is very interesting, I want one of those, I want to throw away my LK1.
I want some information where I can find good hardware like enclosure, knobs and switches.
Where can I find the subsitute for the semiconductors.
It is possible to build this line amp on traditional old type of soldering turrets on glassfiber board.
Kamskoma
It may have been a mistake for me to put the JC-80 up on this website. Virtually everything about the Blowtorch design has been discussed here, except I will not publish the complete schematic. It is still a contemporay design, and I do not publish my working schematics.
If people with sufficient experience wanted to make a design similar to the Blowtorch, the physical details already included here could be incorporated, and the JC-80 topology could be developed into a working preamp. This JC-80 topology has the advantage of being easier to make work properly, than an no global feedback design with a greater selection of fets, than the Blowtorch.
For example, the JC-80 input fets could be 2sk170, 2sj74 gr, bl, or V. OR 2sk146,2sj73 gr, bl ,or V. OR 2sk389, 2sj109, gr, bl, or V.
The Blowtorch circuits are more critical and only V parts would work properly.
If people with sufficient experience wanted to make a design similar to the Blowtorch, the physical details already included here could be incorporated, and the JC-80 topology could be developed into a working preamp. This JC-80 topology has the advantage of being easier to make work properly, than an no global feedback design with a greater selection of fets, than the Blowtorch.
For example, the JC-80 input fets could be 2sk170, 2sj74 gr, bl, or V. OR 2sk146,2sj73 gr, bl ,or V. OR 2sk389, 2sj109, gr, bl, or V.
The Blowtorch circuits are more critical and only V parts would work properly.
Hi John,
Okay, there is an amplifier thread running. This thread has probably run it's course except for the odd question.
I guess there can be a new thread for any other topic you have an interest in discussing.
-Chris
Okay, there is an amplifier thread running. This thread has probably run it's course except for the odd question.
I guess there can be a new thread for any other topic you have an interest in discussing.
-Chris
Do not call the ghosts, I am just starting up further heavily biased class A and smell of the solder is the last thing I would like to meet 🙂.
john curl said:
Id everyone has learned all that is possible rom this BT thread, please feel free to start the JC-80 thread anyone.
RK
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