Friday, September 30, 2011

Aikido Pre-amp Project - Part 6

We have bulldozed this far where we had enough numbers & design plans to start building.

Arfter simmering the whole battle plan for a few day; Lets give it a trim.

PSU Sanity Check

Since I decided on a PCB, I'll just let it free from hacking for now, that leasves only the PSU to comfort my bloated ego...

For the choke, a snubber is added for taming stray capacitance of the choke, also a diode for safety.
This bridge ractifier is build from discrete parts, & the actual parts connections are as shown in the following drawing.
All is rosy until I realised this is too much work for a 12 year old....
So I drop the idea & use a package bridge ratifier instead. BUT there are no free lunch....
We gotta do something about the SS diode switching spike which will back feed into the main transformer secondary coil, induce noise in the primary & this will pass on to the other tranny connected to the same main.

The CLC filter which will not do anything to better the situation, it has to handle at the AC state.

An easier cocktail concoction fix.... Use fast recovery bridge for least switching spikes & connect a small value capacitor across the AC terminals on the Bridge rectifier  to tame the naughty spikes, not ideal, but that should get the job done...

No PCB require, the circuit is simple enough for wiring the component point to point.

Wait-up !!!

Tube will not conduct instantly from start up, they will only conduct when heated sufficiently by the filament. Unlike a tube ractifier, our solid state ractifier will not wait for that to happen. This instant high voltage will stress the tube & components. The risk is very especially pronounce with direct coupling circuit, which Aikido is. The consequence is premature death of the tubes concerned or fire..... which is a bad bad thing...

Capacitor Multiplier, A.K.A Gyrator 

The conventional wisdom is to throw in a timer delay relay circuit to facilitate time for the heater to raise the tube temperature. This calls for extra components, relays, another power supply for the circuit, breadboarding on perforated board or even making another PCB... Hack, I'm loosing hair just to come up with excuses for not doing it!!!

I'll go without any relays by implement a capacitor multiplier instead. As the name implies, its will mimic  huge physical capacitor of your choice value with 1 transistor, 2 resistor & 1 capacitor.

Like a real capacitor, it is very good at reducing ripples (High PSRR). It is not a voltage regulator as it will track the raw voltage & floats on the B+. AND it can reduce the power supply impedance to a single digit which is desirable

All are rosy..... BUT not what I'm after. I am after its 6dB/octave voltage built-up feature. Others are just mare bonuses... 

 MOSFET is the Chosen One!

BJT requires to pass at least 1ma to the gate, that will limit the size of the R value which make up the RC network ( the 500K & 68uF ) without eating up voltage; MOSFET on the other hand just need to see voltage at the gate without drawing any current, this gives a free hand to the resistor size selection.

Even though MOSFET will need to drop 4V to work, but really, loosing 4V on B+ really isn't an issue here.

Component sellection isn't critical.
Even though the potential across the MOSFET is only a few volts since it is actually floating on the B+. Being SS, they will let out the magic smove if hit by high voltage. Using one with at least the B+ voltage rating is a safe bet. Hence a IRF820 with 500V, 1.2A rating. which I happened to have in my hoard pile.
The 1K resistor acts as the grid stopper of IRF820.
The product of 500k resistor & the 68uF RC network gives time constant of 34 seconds. Since it is a 1st order network,  B+ will ramp up to 63%(~189V) in 34seconds. 102 seconds to get to 95% (285V), & nearly 3 minute to get to full B+. Plenty of lead time for the tube to heat up.

That should get the job done...

 Let Go Upstream...

Parasitic noise in the mains can stir up a havoc if not kept in checked... Not only we must stop these nasty noise from getting into our equipment. It is even more important to prevent the noise generated in our equipments from getting into the mains & contaminate other equipment sharing the same mains. They usually comes as common mode noise. What it means in layman terms is it is indescrimal & will superimpose themselves on any form of signals.

To reject common mode noise, we can make ourselves some(obviously) common mode filter.
You can make one with ease.

Bi-filler wire means 2 wires side by side.
Drum Roll, Please....
Throw in the gyrator, snubbers, filament tranny, 0.1A slow blow fuse, a rocker switch, the IEC socket...  & he we have a family portrait.

This ought to do it....

>>>>Part 7 Preview<<<<

Your guess is as good as mine....


  1. Very nice documentation.
    I will refer to it when I start on this project in future.