Not really vintage but an interesting little box nevertheless. I have little experience with headphone amplifiers, even headphones in general but this unit was very surprising indeed.
I picked this up at a boot fair a few years back and its just sat around in a cardboard box ever since. However the other day I was refurbishing my cherished Cambridge CD1 and felt the need to go hunting for op-amps. This lead me to the Pro-Ject Head Box but sadly it used dual op-amps, not what I was looking for. But upon opening the unit I was so impressed with its design I felt I should take a closer look.
I came across a thread where someone had also been impressed with it and modified theirs with great results. I could feel me figures twitching for the soldering iron, I wanted to get stuck in, it was Lampizator all over again!
I plugged the unit in for the first time and was shocked, it sounded better than my Sansui Ca-2000 on headphones! How can this be, my world has been crushed. Cheap and cheerful op-amps never sounded so good!
So lets gets started!
First off, new caps
470uf 35V x2 main power supply, after ac-dc conversion
330uf 16V x2 main power after regulator
10uf 63V x2 op-amps (no need for such high voltage surely?)
4.7uf 100v x4 decoupling (by volume pots)
Main power supply caps can be increased but I kept original values as space is at a premium and very little current drain in this amplifier. I used Sanyo oscon on op-amp and 1uf polyester for decoupling. The polyester are by no means the best thing to use but its all I had. Polypropylene should be used instead, wima ect.
Second, op-amp sockets for easy swapping!
Above we have the finished article, or so I thought. After plugging my phones in I had too much mid-range, very crisp and transparent but it was hurting my ears! Hmmm I felt the polyesters may have given this effect but I wasn't sure. So now I had to get those op-amps out and start swapping!
The 4580 came out and the LM833 went in, not much difference on paper but the 833 is used in a lot of the good old TDA1541 Philips players.
Headphones went in and BAM! there it was, perfect. The LM833 sounded lush, shimmery mid with very realistic vocals and as a bonus it had less distortion at higher levels. Anyone who says op-amps have no impact on sound is talking rubbish!
I had to play the whole London Grammer album, start to finish, I was helpless. So this is me signing off in a state of bliss, until the next time folks!
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