[coinmaster]

PREAMBLE - SAFETY PRECAUTIONS
 
Before building/modding your amp, you are supposed to be aware of all the safety rules that go with this activity. Voltages and currents may turn you into this

and in some cases this

 
 
even when switched off or unplugged.
 
- Always unplug your amp before touching any exposed metal..
- Wait for some minutes after switching-off before touching said metal (discharge of the remaining energy in the "discharge resistors") 
- Always check the voltage of your power rails.
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I've been talking about this mods thread since too long, so here it is.
 
 I will be feeding it in the next few weeks, filling the "reserved" posts and editing them as I progress in my writing. 
 
 
 
Introduction / Context
 
Little Dot has done a fine job with the MK6 and MK8 amps and we fully understand that the "price tag" leads to some compromises ...
A group of people have decided to learn from the existing design of the amazingly good sounding Little Dot flagship amplifiers and improve on their stock design with upgraded components and such.
The stock MK6/8's sound is agreed to be on par with the WA22 which is more than twice it's price. (no really, there's plenty of direct comparisons on the web)
 
I was a part of these initial modifications of the amplifiers, mine being the MK6.  However, the sound quality increase from these modifications has risen the sound quality so high above the stock sound that it is sort of mind blowing.
 
I've tried many flagship amps from many manufacturers, Hifiman, Mcintosh, Moonaudio, etc and at around $5k-$10k each they sound no where near as good as my MK6 did after modifications.
 
I was quite disappointed in the prices of the flagship amps when they didn't hold a candle to my cheapo one.
 
So I decided to stop at nothing to make this amp perfect in every way.
 
Here is the thread for the tried and tested modifications of the MK6/MK8 amplifiers.
http://www.head-fi.org/t/782183/little-dot-mk8se-mk6-super-mods
 
That thread is mostly dedicated to improvements on the current design of the amp and my plans involve potential alterations to the design which is beyond the scope of that thread.
 
This thread is dedicated any possible improvements to the design even if it means changing the original one.
 
Making it way cooler and better.
 
I tore my amp apart completely and spent months mapping out a schematic from the PCB traces and getting help from all over the web on maximizing the potential and design of the amplifier.
 
I have literally spent every waking hour over the last month and a half doing nothing but that.
 
The time is nigh.
 
 
What we want to do in this thread
 
1. It was (and still is) an adventure and I want to share what I have found and done.
I claim this to be the final truth of ultimate sound, and if you disagree with me then, well no one is perfect. You will forever live in my shadow and be driven mad by it.
2. Show you how to build a modded MK6 on the cheap from scratch.
 
What we DO NOT want to do in this thread
 
1. Disagree with my ultimate opinions which are not really opinions at all but are a fact of life that all other beings must live under.
2. Post selfies of yourself as fried chicken or a pile of dust.
 
 
Table of Contents:

1. My progress so far
 
2. Schematics
 
3. Upcoming mods
 
4. Cheaply build an MK6 from scratch.

 

[coinmaster]

1. Progress so far
 
I originally applied the modifications listed on the other modification thread, again listed here
http://www.head-fi.org/t/782183/little-dot-mk8se-mk6-super-mods
 
I highly recommend taking a read of that thread, it explains much of the techno-babble that comes out of my mouth and contains step by step beginner explanations for the many of the mods that I have done.
 
This was the preliminary result
 

 
Random guy-"Why coinmaster are those the fabled Duelund cast capacitors?"
 
Me- "Yes good sir, plus industrial MKP capacitors with the best specs I have ever seen anywhere for the output tube decoupling/power supply bypass, maybe one day you'll be as cool as me

"
 

 
The sound changed from really good to wait is this real life?
 
Then I decided to replace the cathode bypass capacitors with some of those industrial MKP caps which would simply not fit with the chassis in the way, so I did this to it.
 

 
It was not only extremely dangerous to leave hanging around but I also fried some components when I accidentally put it together wrong and it was pretty flimsy overall.
 
At this time I realized the original design is simply not compatible with the new components I was introducing.
 
I decided I would rebuild it using point to point construction which is basically soldering the components directly to each other rather than using a board to mount them.
 
After spending months mapping a schematic from the PCB traces and doing countless hours of research on the potential upgrades I have found that I will be using a prototyping system to experiment before doing an official build.

This is just an example, not my actual setup.
It's a drag and drop, cold-swap modding center for tube amps(or anything else).
Meaning I can easily switch out mods or entire sections of the amp whenever I please.
If you are interested in making this baby, give this a read http://tubelab.com/articles/tubelab-prototyping-system/

 

[coinmaster]

2. Schematics
 
These are the schematics I have made based off all my hard work and effort.
They do not include the protection circuit which is a rather important safety feature but not needed as of now in the schematic.
These schematics are preliminary until I have 100% confirmed them by not burning my house down in a fireball explosion when I turn it on.
Do not use these to build anything yet.

 
 
 

 

[coinmaster]

3. Upcoming mods
 
1: Constant current source biasing
The first addition I would like to test is the use of a constant current source (CCS) as a biasing method on all of the tubes.
Biasing is a way to manage the flow of electrons within the tube so your tubes don't melt themselves. There's a "sweet spot" for biasing that will provide the best sonics through them.
 
Normally, the amplifier uses an auto-biasing method using a cathode resistor with a capacitor bypassing it, referred to as a cathode bypass. This method is effective and there is no need to manually adjust the bias every 30 days or so like on amps that use fixed bias.
The downside to this is the signal passes through these bypass caps messing with the signal. Capacitors are never a good thing for signal purity.
 
A constant current source is simply a power source that provides a current rating that never changes, even when the voltage does.
This method would allow the tubes to bias themselves automatically without sending the signal through a (in my case, very expensive) capacitor.
 
 
 
 
2: Gyrator plate load
I will also be experimenting with a gyrator (a synthetic induction circuit) for the plate load of the input (driver) tube.
 
It simulates the use of an inductor but with much better performance than a real inductor for this purpose.
 
In theory, this will result in much better performance and sonics than using the resistor plate load (the two 220k resistors) we have right now.
 
This is an example of the CCS and gyrator implemented into the input stage. The power supply is included on the bottom.
 

 
3: Increase driver tube voltage
There is also the matter that the driver tube currently is running on fumes with the voltage is it receiving.
I have been told this is often done for small signal stages, however I have heard from a few amp builders that this may not always be the best choice, especially if you want to get some good current out of them.
 
The input voltage on the driver tube plates as they are now should be around 90-100v ish, which is more or less the bare minimum required for the tube to function.
 
I plan to increase this to about 250v which based on the load line curves of the 6SN7 tube would be the sweet spot to keeping the tube happy while using a CCS.
 
 
4: Regulate driver tube power supply
I would also like to regulate the driver tube power supply, currently there's almost no regulation going on at all. A regulated power supply reduces the AC ripple (the left over voltage variation when converting AC to DC power).
We have this in our power/output tube section but not in our driver/input tube section.
 
5: Convert power supply to LC filter from RC filter.
I will be using an LC filter on both the input and output stage power supplies fed into a salas shunt regulator which is a type of regulator that also functions as a constant current and constant voltage source. http://www.southampton.ac.uk/~apm3/diyaudio/GuideSSHV2.pdf
An LC filter is similar to the capacitor-resistor-capacitor(RC filter) design we have now in the output stage power supply but instead it uses a capacitor-inductor-capacitor which is basically superior in every way to an RC filter except for the fact inductors are big and heavy.
 
In theory the power supply upgrades in the output stage including the ones from the other mod thread may not impart any sonic benefits at all after using a CCS on the tubes but the driver supply needs some kind of regulation going on.
 
 
 
6: Remove NFB from the amp.
 
Notice how the input of the amp is connected directly to the output of the amp through the gain switches and how the negative input leads to the positive output.
This is what is referred to as an NFB switch.
 
 
NFB is negative feedback. It involves exchanging gain for linearity. It has serious drawbacks in certain situations.

Linking the output of the amp to the very input of the amp via a resistor, is the definition no global NFB, gNFB short.

The reason that gain switch controls 'Gain' is because changing the resistor in the feedback loop changes the equation of how much gain is exchanged to linearity.
Which explains why people think the low gain switch sounds better. The lower the gain, the lower the distortion with NFB.

NFB is often used to linearise sloppy design. In old times it was used because of necessity, these days often (not always) because the designer is lazy, and/or wants to uphold certain "purity rules" of philosophical, not technical or even auditory kind.
 
More information here http://www.aikenamps.com/index.php/designing-for-global-negative-feedback
 
 
 
In other words It's a method of controlling gain and reducing distortion, which this amp would have a whole lot of without the use of NFB.
You know solid state amps? The reason they sound sterile and soulless compared to tubes is because they use copious amounts of NFB.
 
The problem is the NFB is simply covering up the problems in the driver stage. 
 
The solution is to get rid of the driver stage distortion and balance the driver stage before it needs the NFB to begin with. Which is done by implementing the gyrator and the CCS.
 
But
The NFB does give lower output impedance to the amp which is a very good thing.
So in order to compensate for the higher output impedance due to the loss of the NFB the output stage will have an improved design which leads to my next mod.
 
7: Change the output stage to a source follower
With the removal of the NFB, the output impedance of the amp is raised. This is not a good thing.
In order to fix this the output stage must be improved to have a low output impedance.
So I will be testing the change to a source follower which is a solid state replacement to the output stage which would solve the issue and potentially sound better.
Yes I said solid state, this means no output tubes. Yes I realize how horrible and lame that sounds, but it may not be as horrible as you think (still lame though).
Based on the testing and research of many others that have experimented with this, an amplifier with pronounced sonic differences when tube rolling is simply changing the operating characteristics when changing between tubes which is a result of a less than optimal design that can't handle it's job properly. In fact the reason you get such varying results with the output tubes is the fact that it is the one place where triodes do worst - providing current to a low impedance load.
 
 
A mosfet based follower should not sound any different than a tube in the output stage and it has been proven as such. In fact it should sound more like the best tube because it performs the same job but with near absolute perfection. Don't mistake this for "solid state" sound because it's not the same thing.
Not only that, but the output impedance of the amp will be in the 10s or 20s rather than hundreds.
 
The plate curves of a mosfet are nearly identical to a pentode, what we are doing here is making a "fake" pentode that is actually better than a real pentode and by the way, pentodes are much better for output stages than a triode. You can even make it imitate a triode if you wanted. Luckily, the type of amp we have (circlotron) is one of the only designs this mosfet replacement would actually work well.
 
Don't quote me on this but I actually think you could make this amp powerful enough to drive speakers with the mosfet approach.
 
These statements about the source follower are based off the trials and research of others and has been a proven topic for years.
However until I personally test this within the MK6 I consider it theoretical because, you know, the whole "if it's too good to be true" thing.

 

[coinmaster]

4. BUILD A  COMPLETE MODDED MK6 AMPLIFIER FROM SCRATCH ON THE CHEAP!
 
 
Always dreamed of having the sound quality of the legendary WA22 amplifier without spending $2000?
Well good news! Time and again the stock Little Dot MK6/MK8 amplifiers have been reviewed to sound as good or better than the WA22 amplifier.
 
So what if you don't want to spend $800 on an MK6/MK8 and then spend more cash to implement these mods and get sound in a league above even that?
 
 
Well even more good news! The stock MK6/MK8 can be made for around $350 
The WA22 can also be built for a similar amount.
 
What no way?!
Yes way!
 
Then you'd have lots of money left over to implement all the mods you want and unlike a PCB board, a terminal board is easily changed and modified.
It's kind of like a mix between PCB and point to point.
 
You could actually have a complete modded MK6 and still spend less than retail.
 
 
Never done a DIY project before? Have no experience with electronics? Neither did I before I did all of that stuff above. It's really not that hard at all if you can follow instructions.
 
I will provide more information down the road assuming my house doesn't blow up in a fireball explosion.

 

[coinmaster]

I should have the parts within a week or so and a working prototype soon after.
 
I will post pics of either the working prototype or the remains of my burnt down house.

 

[coinmaster]

reserved

 

[coinmaster]

reserved

 

[coinmaster]

Reserved cuz I'm cool.

 

[Maxx134]

Before:



After:

I am only posting this because it irks me to see a diagram upside down and crazy looking...

... Notice how my sloppy mess is much more understandable.

 

[coinmaster]

Did you ever consider that I might live in the southern hemisphere where everything is backwards?

 

[Maxx134]

Did you ever consider that I might live in the southern hemisphere where everything is backwards?

Anyways, I like to say goodluck with the breadboard phase of testing,
Just be carefull if you eat while testing:

Remember,
Obstacles are expected, so dont be diacouraged,
Just wear goggles..

 

[coinmaster]

Oh god, were living in the trueman show! I never knew 

 

[coinmaster]

I just realized how crappy the name of this thread is 

 

 

[adpo]

Looking forward to the build-from-scratch instructions! I'm in the never-done-DIY-before camp, so I figure I might as well go big for my first build