The Upgraded Pro-Ject Head Box Mk II

This morning the Pro-Ject underwent a major upgrade. All the electrolytic caps were replaced with Panasonic FM Caps, and the op-amp was socketed and replaced with an NE5532.

First impressions are WOW! :) The treble response is a lot better, the midrange has opened up and sounds absolutely spacious with deep bass, and a very extended treble. The Head Box Mk II plus this upgrade is highly recommended.

There were a few changes from the standard values which were made, here is what you need to do to replicate:

• 1000uF caps were upgraded from 16V to 25V. This is more of a longevity issue than sound.
  
• Input caps C13, 14, 16 and 17 were upgraded from 4.7uF to 22uF. For two reasons. First, they are almost the smallest FM caps I could find. Second, I wanted to beef up the bass response.
  
• Output caps C8 and C10 were upgraded from 470uF to 1,000uF. Again, to improve the bass response at the headphones.
  
• Op-Amp replaced with an NE5532. I know I dissed it as old, but it was a good old op-amp and I have them lying around.
  

If you wanted to do this yourself, you could probably do it for around $20 or less, including shipping from Digikey, making the total amount of time, and money well worth the quality of the head phone amp you are left with. This little amp already sounded better than the Rega EAR when I got it, it now sounds absolutely outstanding. The one minor issue that I'm having is that the case is getting a lot warmer than it did with the previous Op-Amp. I may have to put in some vent holes in the case to ventilate the voltage regulator. Especially since I'm still waiting on the pair of OPA617's from Futurelec. They will most likely warm up the regulator a little more.

The one problem is that it's picking up all the noise coming from my PC, which it masked before. I guess the next step is going to be an outboard DAC for it. :)

One tip, make sure you put the 1,000uF caps as close to the board as you possibly can. It's going to be a tight squeeze.

Upgrading the Suzuki

Well, in this case I'm actually downgrading it. At some point around 1989 I acquired a 1978 Yoshimura modified Suzuki GS 1000. It was actually nearly impossible to ride on the street. So I'm having Bikeworx rebuild the engine, repaint it and the frame, and add all the silly things that the DMV requires such as turn signals, horns, that sort of safety nonsense. So for those of you who care, here is a pic of the bottom half of the engine and transmission. With a little luck, when I'm not soldering uber expensive op-amps into inexpensive gear I'll be riding this.  

Inside the Pro-Ject Headphone Amplifier

It's only fair that if I show you the insides of the Rega EAR I also show you the insides of the Pro-Ject Mk II. Keep in mind that while the Pro-Ject has more parts in it than the Rega, it's case is half the size. Most of the Rega's ingides is air.

Something I forgot to mention in my last post is that another way in which the build of the Head Box is superior to the EAR is that it uses a double sided board with massive ground and power planes. This means less noise, lower power supply impedance. It also allows the board designer to run direct, short traces from one section to another.

It's clear from looking at the design that the designer worked very hard to keep the audio signal paths optimized, and any radiated EMI from the power supply away from them as much as possible. This is why the power supply caps are crammed in the way they are, and also why there's a mix of 470uF 25V caps and 1,000 uF 16V in there as well. The latter I'm a little worried about, because they are filtering the 15V supply, which gives the caps only about 1V margin of error before they are rated to break down rapidly. A pair of 25 Volt caps would make me feel better about the durability of them. I'll see if I can find some miniatures that will fit. 

The Pro-Ject Headphone Amplifier

I'm still looking for good values in headphone amplifiers. As you might recall, I previosly tried the Rega EAR, which was just too expensive and offered too little, and was noisy, so I started looking for something cheap enough that I could leave at work, and maybe give me equal or better value, and maybe be worth modding a little hear and there.

I think I've found it in the Pro-Ject Head Box MKII. The Pro-Ject headphone amplifier has a similar design to the Rega. Meaning, single ended supply, one op-amp driving a discrete amplifier stage. Externally there are two major differences. The price, $99 for the Pro-Ject vs. $299 for the EAR as well as the complete lack of switches on the Head Box. Which I guess makes sense if you are going to put in some BlackGate capacitors that means you'll have to leave the box on all the time anyway. :)

Listening Tests
Right out of the box, the Head Box is significantly quieter than the Rega EAR. I can hear no noise at all except at the loudest setting. It seems to be a little weak on the bass however, and perhaps a little on the treble as well.

Build Quality
The Pro-Ject headphone amp is less expensive than the EAR, but better made, in my opinion. This major difference probably has to do with the power supply filtering. The EAR had about 1,500 uF of filtration, the Head Box around 4,000, with a significant amount of filtration occuring after the voltage regulator.

Just like the EAR, the Pro-Ject amp has to have capacitors at the outputs. They are about half the size of the EAR's so this may account for why I feel the bass just isn't making it all the way through to the headphones.

Future Mods
There are two major areas where you can improve this little box. First is in the capacitors. Replace them all with Panasonic FM's. For the output caps, replace them with 1,000uFs if possible. This should get you some more oomf.

The other area is in the op-amp. There's nothing wrong with the original, but there is nothing great about it either. Socket it and experiment. If you can find an NE5535 to hear what vintage class-A op amps sounded like, I'd recommend it. :)

I'll post as I apply the mods.