Sunday, September 25, 2011

Aikido Pre-Amp Project - Part 2

In this post I'll go about the chassis layout.
This is going to be boring.... but its absolutely crucial for one to get this right before any physical work...

Again, I would lay down the requirement for this session
1) Safety
2) Hum free
3) Easy construction
4) Good ventilation

Start the crunch....

A well laid chassis will keep noise & hum from bothering our sanity, prolong equipment life & safeguard any user from electrocution.
Hum & noise, a part from destructing for our sanity, they will definitely corrupt the signal we wish to process. The most significant effect of corrupted signal is noticeable loss in micro details, in a bad case, a blur broadcast with a constant hummmmmmmm...
A good electronic circuit is one which will reject unwanted signal while going about their intended purpose...
Weak signals are easily influence/corrupted by unwanted signal. such as RFI, EMI...etc. hence we must keep them away from noisy stuff such as EMI (electromagnetic interference) which are mostly generated by electromagnetic stuff such as electric motors, transformers, chokes, magnets...etc. RFI (Radio frequency interference) are generated by high frequency switches, PWM devices, radio wave transmission, microwave oven, solar burst... etc etc..
We can just shield external noise with a Faraday's cage which is basically putting everything inside a grounded non-magnetic metal chasing such as aluminium casing. Steel casing is ok but it does conduct magnetic thingy (which can be a PITA to debug) within the steel sheet unlike aluminium.
That take cares of the external noise. How about there are those internally generated noises within an amplifier.
General good practices in setting out an amp's interior layout is simply to separate the noisy stuff from the quiet stuff with air space. From my experience, most EMI can be minimised by a clear space of  50~65mm. To do this it is a good practice to make sure all the irons are amply spaced up with their coils perpendicular from each other & away from any signal path. Another good practice is to keep all the dirty & noisy stuff in one end & all the signal path in the furthest corner.The noisy stuff being anything closer to the mains supply & the quiet stuff are those closest to the signal inlet socket.
High current AC stuff induce electromagnetic field around them, disregard of their voltage.  Filament wire is one of the know culprit. There are many solution to this, one of which is to employ DC heater supply as DC current  induce very little electromagnetic field. That is the main reason for phono amps. DC filament supply can be complex & it has great impact on the sonic, particularly the size of the filter capacitor, it won't filter properly when its too small, & dull/death sound when you overdose... Some of the reader will enjoy tweaking the DC filament circuit but for me it is a PITA. 

It is no secret that I personally prefer AC heated tubes; IMHO, it is easier to build a fantastic AC heater filament implementation then to do a lousy DC filament implementation. Most importantly the lively & unstrained sonic energy it radiates. 

The simple solution to this is to use twisted filament wire, tug them into chassis corner, & keep them away from signal path. If crossing signal path is unavoidable, cross them perpendicular to each other. Less for more :)

Ground loop
Ground loop is the main culprit to hum... The practical approach of ground loop management is a faculty by itself  hence  not going to talk much of it. As for this build, I'll just take thing easy & go with the flow....

The Aikido PCB has a good star-ground layout, hence I'll use star ground scheme to manage the ground loop.

Transformers layout
It is necessary to finalise the PSU component selection before going about the irons placement in the chassis.
We need the PSU specs. before we can start & these are the numbers needed:-
Current requirement :  7.2ma x4= 28.8ma call it 30ma for convenient sake.
Voltage requirement : Since Aikido is not picky with B+ value, I'll just use what ever 230Vac, 50Hz can give which should fall anywhere between 300 & 325Vdc
Components I have : 8H @ 164 DCR Choke,  10uf, 68uf, 100uf, 120uf, 150uf, 220uf capacitor all rated at 450Vdc
Get PSUII out & play around with different parameters... Finally satisfy with the  following concoction...

from here, supplementing  other good design considerations & good DIY practices. An exact scaled 2D dimensioned chassis layout appears.

Well.... that goes another good 10 hours on the project  :)

As you can see, when one take all good practices into design considerations, there is no way a tube amp will get anywhere close to the size of 20 iPod stack.

.....Part 3 Preview.......

Churn out a 3D chassis design base on the 2D plan we had just cooked up...

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