Bought this little beast for some 100 euros to use it as my practice amp in home. However, it had some serious design mistakes, which made this amp a sleeper. Generally, you cannot expect much when a bass amp has a 8″ speaker. In addition, the stock speaker was a piece of crap. So, let’s start from it.

Speaker swap

I explored around the internet to find a suitable speaker for the use. The problem is that BA108’s cabinet’s volume is only around 18 litres. It limits the performance of most the speakers available. Finally I found one which would be suitable for the cab this small. Fane Sovereign 8-125 seemed to be good one for use, and I compared it with the stock speaker using WinISD:

Green is for Fane, purple is for stock speaker. I am considering -3db to be the lower limit of usable frequency range. As we can see, it is 74Hz for the stock speaker, and 61Hz for Fane. Huge difference it is. I wasn’t able to find Thiele/Small -parameters for stock speaker (suprise), so I needed to measure them myself. Resonant frequency is something we are interested about when we are designing a vented cabinet. It is good point to start. I measured it to be 71,5Hz for the stock speaker. I also measured the cabinet and found out that it was tuned to 71Hz. As the resonant frequency for Fane is 65Hz, I needed to modify the reflex tube to tune the cabinet down. After careful measurements I found out that optimal cabinet tuning would be 61Hz for Fane. That means modifying the reflex tube.

Modifying the cabinet

The workmanship of the cabinet was at most fair. It leaked from several points causing odd rattling. Glueing every single seam with wood glue fixed the problem. I also added some foam plastic tape used in drain piping. In stock there’s nothing but a small dampening cloth inside the cabinet, which I left there. It was attached very loosely, so I added some five million staples to it to attach it firmly. All the rattling is gone now.

Reflex tube is made of paper pulp and cutted carelessly. The inner end of the tube was uneven and partly collapsed. I bought some plastic drainpipe from the local dealer for 3 euros, and used it as reflex tube by sawing it to appropriate lenght. Used short part of the original tube as a gasket for the new one, because it happened to fit inside the original tube just perfectly.

Changing the Op-amp

This amp has an JRC4558 op-amp in it. Not bad, and used widely in Tube Screamer -type overdrive pedals.  However, I decided to swap it, as I had a Burr-Brown OPA2132 in stock. It’s considered to be more hi-fi and tube-like than 4558. For future tests I added a socket in the place of the op-amp. It’s easy to change op-amps now without a need to solder. Also, by acting like this you won’t fry your op-amp with soldering iron during installation.

Adding filter caps

By default, this amp has two 3300uF capacitors acting as filter caps. It’s not much, and adding some more should thicken the lower end and give more clean headroom. I also had a buzzing problem. It came from the speaker, but I wasn’t able to find any reason for it. I decided to add two 2200uF caps in parallel with existing ones. Because they were attached very close to the rectifier, I needed to solder the rectifier off, and move it to separate daughterboard along with new filter caps. So it goes like this:

Yes, you can connect poles 2 and 3 backwards. I tried it, and it only blows the fuse. Really, don’t try it. It was just a stupid mistake. After the smoke evaporated, the result is very neat:

As a result, this little devil has far more power and wider frequency range. I measured the -3db limit to be in 59Hz.  Think about it. It was 74Hz with stock parts. In addition, that odd buzzing is gone. It is widely reported over the internet, so it appears to be a real problem. Crossover distortion? However, adding some filter caps will fix it.