EVH 5150 Micro Stack modification !

Here's a new micro-amp project I've recently finished. I bought the new EVH 5150 III Micro Stack amplifier to test it out as it was affordable and I wanted to use it for practice and for quickly testing out a few guitars. The amp came in a nice box with EVH stripes on and a simple manual. It had a brand new GP 9V battery in it!



First impressions:
At first, I tried to use the back-stand of the amp and it simply fell off. Turned out the metallic bar was not bend properly and it was an easy fix. Then, I grabbed a simple Les Paul copy with ceramic humbuckers to test the amp. It sounds boxy of course but less boxier than the Marshall MS-2. There is a single channel and a gain pot. The cleanest setting is with the gain pot set to zero. Once you turn it up a bit, it sound crunchy and with the gain set to max, it sounds muddy with plenty of fuzzy distortion. The distortion is not that good as the tone is really muddy. Single-coils and low-output pickups sound better but the "EVH" amp should be used with a hot bridge-position humbuckers, right?... The amp is loud though, louder than most cheap 1-2W micro-amps available. The more powerful >3W models are louder. It looks good as well. There is attention to detail such as the grill-cloth, the evh stripes and logo, fake handle and corners, chicken head knobs etc.
Once I opened the amp I noticed there was a piece of damping material placed inside the "cabinet" at the back of the speaker which is cool. Many manufacturers use no such material and their cab sounds even boxier.

The speaker is big enough for this application. It's a 3", 8ohms/2Watts paper-cone speaker. The "power amp" section is based on the classic LM386/Jrc386 chip. Its 8 pins are in-line though. The preamp section is based on a dual RC4558 smd opamp. There are also a few smd and through-hole components on both sides of the pcb.


Mods
Ok, so let's get to the modifications! This amp is really easy to modify with a soldering iron and a pair of tweezers as its smd components are not that tiny. These mods will VOID the WARRANTY of your amplifier! Proceed with caution.

External Cabinet output
The amp has a headphones output jack on the side. That's something I wasn't planning to use as it sounds terrible (to my ears at least) and there's no point in using this little amp as recording amp. I am planning to use this tiny little amp with bigger 12" external speakers though! That's why I modified the headphones output. I wanted to use the existing female trs jack but it's a stereo jack and the mono-cable of the external cabs was shorting the power amp which is totally wrong. So, I desoldered the female trs jack using a solder suction pump and some solder-wick.



 Once the output jack was removed, I placed a jumper across two pcb traces to enable the internal speaker. Check the following pics to place the jumper wire:





I had a new plastic trs female jack. The new jack has a total of 6 pins but we are only going to use 4 of them. Remember, a 2 or 3 pin jack will not work in this case. That's because a simple female jack does not act as a switch once the male trs jack is unplugged. We need this switch to enable the internal 3" speaker once the external cabinet cable is unplugged.
I desoldered the two wires going to the 3" speaker and soldered them on the tip and ground/sleeve long pins of the new output jack. I also placed a 2.7ohms/5W resistor in series with the tip/hot wire to reduce the power output of the amp a little bit and let the LM386 chip run a bit cooler.




Caution! In order to plug this amp to an external speaker cab with a total impedance of less than 8 ohms (e.g 4ohms), you HAVE to place a 3.9 ohms/5W resistor in series instead of a 2.7 ohm one. DON'T omit this resistor as a 4 ohm speaker could overheat/BURN the 386 chip... 
If you cab is a 8 or 16ohms one, you can omit the series resistor if you want to hear the full output of this little amp.

We need a total of 4 wires connected to the female jack. The two remaining wires will be connected to the opposing pins of the female jack and the internal speaker terminals. Check out the following diagrams/pics:

The amp sounds really cool through a 2X12" cab with celestion speakers. It sounds huge compared to its tiny speaker. The tone is fuller, louder with added low end. It can even power a 4X12" cabinet!


Tone-mods
The amp sounds Ok but not great. The overdrive sound is muffled and fuzzy and this is because the C1 capacitor lets too much bass pass through. C1 is a 563 (56nF) capacitor. I replaced C1 with a 4.7nF (472) capacitor. The sound became much clearer but a bit of gain was lost in the process.




To make up for the gain reduction, I replaced the R1 1kOhm resistor with a short circuit (jumper) and the R12 4.3kOhm resistor with a 1.5kOhm one.



 If you're not familiar with smd components, you can simply solder a wire across the pins of R1 and solder a classic 1/8W 2.2kOhm resistor with short leads across the R12.




To reduce bass and muddy-tones even more I replaced the C12 electrolytic capacitor with a 1uF/25V one.

Then, I placed a jumper across the R8 1kOhm resistor to increase the output level of the preamp a little bit.

The mods made the amp sound a lot like a proper high-gain amp and got rid of the muddy/fuzzy tones. I wouldn't turn all pots to max. though. There is plenty of gain available now! You could do these mods one by one and listen to the result. It's a step by step procedure. You don't have to do all these mods at once. You can omit a few steps according to your taste/needs.


It's working!

Update! The battery connector came off at some point... The problem was that the soldering joints of the 9V connector were too thin. I pried the connector open with a small screwdriver and resoldered the battery leads to its pads. Be careful of the correct battery polarity or you will damage your amp...



So, how does it sound now?
Here's a short video clip of the modified EVH micro stack!

Cheers,
Thanos