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Old 9th September 2013, 02:05 AM   #21
tomchr is online now tomchr  United States
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Quote:
Originally Posted by e_fortier View Post
The 6LR8 does make nice music .
That it does! Especially when triode strapped.

Quote:
Originally Posted by e_fortier View Post
To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.
Hmmm... Hadn't thought of that. I'll give that a whirl.

D1 is actually not populated by default. That's why it says "DNP" = Do Not Populate on the schematic. I included it as an option. I actually get quite good performance with a green LED in that spot. Then you'll need R9 = 0 Ω and R10 = DNP.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Old 9th September 2013, 02:39 AM   #22
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I thought that this was the proper design.
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Old 9th September 2013, 02:43 AM   #23
tomchr is online now tomchr  United States
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Ya know... I might build my headphone amp like that. I want to use it at work...
One of those Heiniken mini-kegs would work as well.
Click the image to open in full size.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 9th September 2013, 12:22 PM   #24
roline is offline roline  United States
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6LU8's makes a killer PP amp..... I did a PCB concept for Triode, UL or Pentode mode, I did Pentode mode for the first one... Great with 6.6K Iron!. I used a 6CG3 diode for slow B+ start in the power supply filter chain.
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Old 12th September 2013, 02:34 PM   #25
nikon f is offline nikon f  United States
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When will the board be available?
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Old 13th September 2013, 12:03 AM   #26
tomchr is online now tomchr  United States
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I have opened up for preorders if you want to be among the early adopters. I expect the board to be available by the end of this month.

You can preorder on my website: Neurochrome :: Audio - Novar Spud.

I'm nearly done with the evaluation of my prototype. So far the board looks solid. I expect to submit it to the board fab on Monday or Tuesday. The turnaround time is two weeks (including shipping to my door). Once I have the boards in hand, expect it to take three days for shipping within the US and 1~3 weeks for international shipping.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 13th September 2013, 12:59 AM   #27
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Quote:
Originally Posted by tomchr View Post
One of those Heiniken mini-kegs would work as well.
As long as you don't have to drink it first.

jeff
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Old 13th September 2013, 02:57 AM   #28
tomchr is online now tomchr  United States
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Oh... I can think of worse beer than Heineken. Of course, five liters in one sitting may be a bit much...

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 13th September 2013, 11:39 PM   #29
tomchr is online now tomchr  United States
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Quote:
Originally Posted by e_fortier View Post
To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.
Tried that. Wouldn't recommend it.

So... I played around with the feedback resistor. I performed a sanity check with values of R3 both above and below 150 kOhm. R3 = 150 kOhm continues to provide the best performance.

I did find that the THD at higher frequencies was quite a bit higher than at midband, so I decided to investigate this further. It turned out that the transient response at the anode of the driver stage was less than stellar as the feedback from the output stage added significant overshoot. It made me wonder if the output stage was actually operating at the edge of stability.

To tame the transient response, I added a small (22~33 pF) cap in parallel with R3. That worked like a charm. This cap does limit the bandwidth of the amp to about 30 kHz (-3 dB), but its transient response is nice and symmetric without tendency to overshoot.

I need to run the circuit through my measurement suite, but I think this is it folks! Ready for prime time!

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 14th September 2013, 01:13 AM   #30
tomchr is online now tomchr  United States
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Thumbs up Primetime has come...

Wow. What a difference a sliver of mica makes. Adding a 'speedup' cap to the feedback really tightened up the transient response. Metallic instruments are now rendered with much more detail. Smooth...

The amp sounds good and measures well. The board will be submitted for fabrication by Monday morning. It should arrive on my doorstep by the 29th or 30th of September.

I will probably order a fairly small batch of boards as there doesn't seem to be as much excitement about it as I had anticipated. If you want to get your paws on one of these boards, I suggest getting your order in early.
The board will come with a full set of schematics, bill-of-materials, circuit description, getting started guide, and include the latest tweaks and optimizations.

Order from my website: Neurochrome :: Audio - Novar Spud.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

Last edited by tomchr; 14th September 2013 at 01:15 AM.
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