Aikido Pre-Amp Project - Part 4

Works continue on chassis design...

We will take up where we left in part 3 where we gone as far as determining the required chassis dimension of such,
PCB chassis dimension :141 x 293 x 100mm (H)
                                         ~5-1/2" x 11-1/2" x 4" (H)
PSU chassis dimension : 121 x 293 x 100mm (H)
                                         ~4-3/4" x 11-1/2" x 4" (H)
Aluminium is the material of choice.

Determine Type of Chassis & its Construction Method
Nothing in my junk pile match the sizes required, not even aluminium metal containers or kitchen utensil which I can improvise.... :(

Instead, I found some 300 x 500 x 2mm thk aluminium sheets which sits on a shelve all by themselves.. These will be suitable for folding a chassis per sheet with some tweaks in the chassis dimension.
Before any further a do, I need details of the press brake that will be folding these chassis for further number twitching in the previous development drawings. A disappointing trip to my friendly neighbourhood sheet metal fabricator reviews that the smallest bending radius is 9mm which is IMHO too coarse for folding chassis with a million-dollar-look

 "Desperation is the mother of innovation"


More stock check on what are at my disposal, with a pinch of copy cat & a whole load of cheating... The following drawings appear progressively...

 

Don't know how you will coin it. Borrowed from wood joinery technique; Its basically a 3D jigsaw puzzle with tabs & tab holes.  Tabs & tab holes are employed for locating the panels in relation to each other accurately. They will be spot welded at the tab joints to ensure good electrical & rigid structural bonding.

While I was at it, conveniently added some perforation on the PCB chassis top panel to facilitate ventilation path while preserving the rigidity of the penal. No opening on the side panel as I'm too tired to do so...

Keen eye readers will notice by now that both PSU & PCB chassis are identical in size.

So, I cheated... .Uniformity is the current excuse of choice for my laziness... I am sticking to it.. :)

After burning more man hours on touching up the design with further detailing such as optimising material utilisation; The nested penal layouts were born.

 

I reckon this will be the eventual construction drawing... but I do expect "unforeseen" circumstances.

The next on the list is to put the design aside to simmer for a couple of days (maybe weeks) while I can go on with my day job. We will know if there are any need for further tweaking by then...

Before I switch off the PC, here are the summery,

~Dimensions~
External : 140(W) x 290(L) x 100mm(H)
                  ~5-1/2" x 11-3/8" x 3-15/16"
Internal  : 124(w) x 274(l) x 98mm(h)
                  ~4-7/8" x 10-25/32" x 3-7/8"

~Panel material~
2mm thick Alum Plate which is approximately 14Gauge.

~Construction method~
CNC machined panels with tabs & tab hole, held together by TIG spot weld.

~Finishing~
Wire brushed, with natural colour Anodised.


Another 10 man hour burnt....


Now I can stop staring at the monitor & rest....

...........PART 5 PREVIEW...........

It will be physical! 

Aikido Pre-Amp Project - Part 3

In the previous article, I've came to the following layout.

Commercial amplifiers does has similar form dimension, I will use Pioneer A400  foot print which is 16 1/2" x 12" (420x305mm) as a rough guide.
Layout is not strictly a 2D affair, in the real world, we have 3-dimensions to play with. We can stack irons on each other if I have a magnetic shield between them hence, I'll place the filament transformer under the aluminium plate & have the choke directly above of it without adverse effects.

That leads to Ver.2

We get a huge space at the bottom left corner, plenty of real estate for future additions.

It struck me that if we put the PCB & all the iron is 2 different casing & we get separate PSU arrangement!

 But I don't fell comfortable with that huge void in the PCB casing so one thing lead to another, another layout appears...

Here are the other views

  

I go for a 100mm height chassis for now but 80mm will be the bare minimum for any reasonable working space within the chassis.

All these work just to  obtain the dimensions require for chassis preparation, be it off the shelf or custom made. Do you feel why I always end up with chassis-less amp???  LOL!!!


I started with all the intention to produce a 3D model of the chassis, unfortunately, getting to this stage has eaten up over 24 man hours. Since I've got all the dimension required to start construction,  I'll have to take a short cut & skip the whole 3D modeling exercise... :(


A description of the 3D chassis instead... :(

PSU Chassis
The mains tranny and the filament tranny will be on top of the PSU chassis & the choke under the filament tranny separated by the chassis top panel. They are space up lavishly.

The void between the two are filled by the filtering circuit component within the casing.

The IEC socket will be on the back panel  since all commercial amps has the
On/Off switch on the front panel, I'll follow suit even though I feel like an idiot routing the wire loop from back to front & back again... also... I can use relay & implement remote activation but I don't have one in my junk pile... Lets just keep it simple...

PCB Chassis
The PCB mounted on the bottom of the top panel to keep them away from dust & most importantly exposure to any form of interference.
The tubes protrude through the 4 holes on the top panel for best possible ventilation. I choose to have the tube orientated as such for it is farthest possible from the PSU exposed irons.
Filament wire layout can make or break any tube circuit... they are very high on my priority list. Tug into the chassis corners & away from any signal wire, even if you have to detour. The twisted wire will be travelling along the bottom of the chassis as shown & go vertically into the +H & -H terminals on the PCB.
The signal in & out terminals are on the back panel as shown... Detailing the hole dimension will have to wait... 
The selector & volume pot will be located just beside the signal input terminals at the back panel. they are linked to the front panel via separate connecting rods. This is important as the raw input signal is weak, susceptible to noise corruption. The extra wire length will act as antenna to invite noise. For extra protection, tug the wire into the top corner (top & side panel).

Finally! We come to the point where there are enough numbers to proceed with chassis preparation.
Go find ready made chassis of in close proximity of 142x293x100mm & 120x293x100mm & start drilling.
OR

Fabicate from these developement drawings.

WATCH THIS SPACE!!!

Aikido Pre-Amp Project - Part 1

Looking back my DIY audio journey, my objective in every project undertaking is to get to the point where a safe & functional good sounding amp soonest. My effort in aesthetic are limited to point-to-point circuit construction which is  beauty to my eye... There is just not enough energy left for aesthetic works....

This time round I'll will emphasise on the aesthetic work...But within reasonable degree of form-follow-function.

Like any projects in life, embarking on a project without laying down the goals is a sin...
Hence, a list of objectives in the order of priority
1) Quality finishing & Aesthetically appealing. Good enough to go on a display shelve.
2) All components must be from my junk pile.
3) Class II electrical safety compliance (double insulated form user)
4) Complete the project within 80 man hours
5) A project even a 12year old can complete.

Now the path is laid, one can start to eat the elephant a bite at a time...

DECISIONS, DECISIONS, DECISIONS...
Designing from scratch is fun but you are committed to a total unknown out come... Hence, this time, one gotta let go for one's ego for the better & use a trial & proven circuit.

Amplifier Design
A quick summary on Aikido. It is created by John Broskie. Requires 4 triode per channel to work, & the common practice is to use dual triode envelope valve, such as 12AX7, ECC88, 6N1P, ECC81, 6SN7... etc etc.
Aikido is such that it is almost immune to whatever tube selection or operating point (within common sense limits of course), it will produce fantastic audio result disregard. The only decision you will have to make is what sort of gain you need & what sort of output impedance you require... Spooky... :) To cut all the theory, I'll just say distortion cancellation is at work in this design...
Also, this amp is death quiet! It also design for power supply ripple cancellation!

I had built a myriad of Aikido pre-amps even using it as the driving stage of my 2A3 SE DHT with various tubes combinations & operation conditions; Construct using point-to-point construction technique, some uses bus bar ground & some using ground plane.

Sonic quality is fantastic, the image actually defy physical boundary & literally extends outside of the room! Neutral, dynamic & great micro-details are really high on the top echelon equipments.


Form of Construction
This is a solid design. Unfortunately, collect all the parts & construction from scratch will eat up the 80 man hour budget... even cutting my own PCB is out of the question...

I bought a few kits in return for John Broskie's generously sharing his work freely. This will be the best time to put them to their intended purpose.

This will be the second kit I build since I took the DIY path, the other kit was my inaugurate tube project which is a 6EW7 Kit from audiohobbyist many winters ago... And you guess right, the 6EW7 didn't survive my relentless hacking & eventually fell into pieces... Hope this Aikido will have better luck...

Tube Choice
6SN7 is the tube of choice, it can yield a gain near 10 &amp & when running this circuit at 300V & 7.2mA per tube we can have a super low Zo of 685ohm; Its perfect for a line stage & even also an great headphone amp!

Power  Supply Considerations
The kit has all the bells & whistles but you gotta build your own power supply.
6SN7 will work safely with 250V on the anode & when stacking 2 6SN7 it can safely use 500V B+ but since I have a few isolation transformers lying around, it will by default using B+ of 300~325V. Aikido is designed to have superb PSRR by design, but it helps with a solid & well filtered power supply.
I'll use the following power supply topology which I got repeated good results in the past. It sounds really close to a tube rectifier power supply.

That concludes the first 2 hour of work on this project.

Next 2 hours is to is to get things organised.

Dig up the irons required. Huge 100VA @ 230V isolation transformer for main B+, 8H Choke & a 30VA @ 2x6V toroidal filament transformer.

A quick stock check of the kit.

Never proceed without this very crucial piece stow box!
One must have a large enough stow box for stowing you project between play time...

This is how its used... 

This IMHO is the bare minimum size10"x13"x8"deep, & it comes with lid & rollers too :)

No blog is not complete without eye candy

 

.........PART 2 preview .........

In my next post, I'll spend time on the chassis.