29 December 2012

Building a high gain amplifier part 1: The preamp

This has been a long time coming and will be the last ever Soldano influenced amplifier that I build.

All I'm doing here is rebuilding my original amp with a few tweaks that I have found that I prefer over the stock circuit.

First up we will start with populating the preamp board with the smaller components as it is always best to do it in height order to stop things slipping out and looking messy later.

So here we have the bridge rectifier for the channel switching circuit. It is the smallest component of the preamp therefore it is done first. Please note the polarity markings as this is critical for correct operation. The PCB also has a silkscreen that matches the markings so it's pretty hard to go wrong here.

 
And now it's time to go onto the bulk of the preamp which is the resistors. I am not going to go through these one by one as it will just be lots of repetition and there are no polarities to be concerned with so just solder away! One thing to keep in mind is that some resistors have very similar colour bandings so take your time and make sure you match them correct values up with the silkscreen.
 
 
With resistors I usually use a lead forming tool to help keep things nice and tidy.
 
And here is a nicely formed resistor ready to be soldered in. 
 
I've added a few extra shots of the preamp just for reference as to what it looks like as you go along.




In this last shot you will notice that I have left R14 blank. This resistor is the Soldano signature move, the cold clipper. It determines when the lead channel saturates and the amount of gain on tap. My reasoning for leaving this blank is that I am going to add a front panel toggle with the stock value and a lower value so that I can switch between two gain voicings as and when needed.

 
 

 
From here we are going to go onto the small value capacitors. For these I choose silver mica caps just because I like the way they sound over ceramics. However a cheaper ceramic cap will do a good enough job also it just depends if you want to spend the extra 30p or so.
 
These capacitors are unpolarised so again orientation isn't important here.
 


 
 
The last step that I am going to cover today are the three electrolytic capacitors in the preamp for the B+ & channel switching parts of the circuit.
 
These three capacitors are polarised and you really want to make sure that you get these in the right way as it isn't pretty if you power up with them the wrong way. Electrolytic capacitors failing can be very messy and sometimes dangerous. See this video for a demonstration.
 


 
 

 
As you can see the polarities are clearly marked on these capacitors making the chances of an error during installation very minimal as the PCB silkscreen also marks the polarity.

 
That is all for today but in the very near future I will be finishing the preamp in a bit more depth and also the power amp in the next post.
 
After that things will get a lot more in depth and also move into a more step by step manner as things get a little more complex.
 
If you have any questions just leave a comment and i'll do my best to answer you.