“Upgrading” a Pimeta v1 Headphone Amplifier

As I’ve mentioned in the M³ post, there are a few headphone amplifiers that I was always fond of. They weren’t always expensive or exotic amplifiers, but they simply sounded right to my ears. One of these is the Pimeta from Tangent. I first came across the Pimeta quite a few years ago when one of my friends had a portable unit powered from a battery. A few years later, one of the amplifiers I’ve built was a Pimeta, that ran from a 24V regulated PS. Over the years that amplifier has been modified a few times to suit the needs of the time, including a reduction of gain and PS voltage to fit more sensitive headphones and a smaller case. Recently, after a few years of not using it, I’ve had a renewed need for, and decided it was as good of a reason as any to give it a little “upgrade”. This post is meant to share those modifications, as well as give the Pimeta some more attention, as I think its a great little amp that isn’t getting enough love on the forums.

First things first, the Pimeta I’m talking about is the v1. It has since been superseded by a modified v2, but I’m still using the original version. The new version has a few modifications in the PS section, as well as new output buffer IC’s. However, the topology is basically the same. It uses an “active ground” topology where the ground is generated by splitting the single rail supply using a TLE2426. The design uses an op-amp as the main amplifier, with a buffer IC that is capable of driving larger currents. It incorporates a multiloop topology for the feedback network, as was published by Walt-Jung. By now the Pimeta isn’t a new design, and in its earlier days it was quite well known on the Head-Fi forums and other headphones/audio forums online. Nowadays it seems there is a fairly limited number of people have ever heard of. That’s a shame in my opinion, it really is a compact, easy to build, low BOM cost, and good sounding little amplifier.

I’ve built mine a few years ago, and for some time it was my main headphone amplifier. Over time, larger and more expensive amplifiers took its place. Recently, I’ve decided it would be wise to bring a decent headphones setup to the office as I have very little time  to listen to music at home nowadays. Since I wanted something compact I’ve first started looking at an integrated device with a USB DAC and a decent headphone amplifier. After a bit of reading online, I’ve decided to go for the Topping DX3 Pro. Unfortunately, a short time after getting it, it died on me. I’ve read online that I was far from the only one experiencing this problem, and therefore decided to send it back for a refund and look for a different solution. Finally, I’ve remember I’ve had a Pimeta amplifier within reach, as well as an AMB Gamma2 USB DAC that I wasn’t using. That actually sounded like a fairly good combo, why not use that?

So I’ve hooked everything up, connected to my MrSpeakers Aeon Flow closed headphones, and it sounded just wonderful. The low impedance of the Aeon’s (15ohm) didn’t seem to be a problem for the Pimeta, and the gain I’ve had set inside the amplifier (x2.5, 8dB) seemed to be an excellent match to these headphones and the volume I listen at.
So overall this was a great match, but I’ve still had a couple of issues. I’ve built this amplifier quite a few years ago (over a decade ago probably), and was missing a few of the things I now consider as mandatory in all of my builds, such as output protection. Therefore I’ve decided to add an output protection based on the DC protection circuit I’ve designed for a different project. However, as you can see below, the original case wasn’t large enough for this additional circuitry, so that had to be changed as well.

Fig. 1. Front of the Original Case
Fig. 2. Inside the Original Case

The PCB already had AD8610/8620 op-amps biased into class A. The output buffers were a BUF634 with increased bias. The PS is a straight forward implementation using an LM317 (mounted to the bottom of the case). There is a 12VAC input from an external transformer, rectified and regulated on the small protoboard mounted to the LM317.

I have decide to look for a black color case that will match in width to the Gamma2 DAC so that the two could be stacked to save space on my desk. Luckily for us, the selection of aluminium enclosures on Aliexpress is excellent, and the prices are fairly low if you aren’t looking for something special. My plan was fairly straight forward, move it all to the new case, add the DC protection circuit, and that’s all.
However, the 12VAC input, is regulated to a 12VDC rail, which isn’t quite high enough for the original design of the DC protection circuit. Therefore, it had to be slightly modified. This is fairly straight forward, especially in this project where the power rail is already regulated. The power supply section of the circuit looks like this:

Fig. 3. Power-Supply Section of the DC Protection Circuit

This assumed a high voltage unregulated PS is used, and therefore created a low ripple local 12V (+/-) rail. For the Pimeta with a single 12V rail, R15/R16 were omitted. Q6/Q7 were shorted (all 3 legs of each device), and D4/D5 replaced by 1Kohm resistors. These 1Kohm resistors set the local “ground” voltage at approximately mid-rail. This is more that sufficient for this application as there is very little ground pin current in this design. Additionally, since the DC offset is measured relative to signal ground, the local “ground” of the circuit doesn’t have to be identical to it. For complete explanation and schematics of the circuit, please have a look at the post dedicated to it. After assembly I’ve verified that the circuit properly detects DC offset of either polarity on both inputs, and it was ready for assembly in its new case.

Fig. 4. Amplifier/PS/Standoffs in Place
Fig. 5. Adding DC Protection and Wiring
Fig. 6. Adding Connectors, Power Switch, Fuse, and Panels

The DC protection circuit was mounted upside down on tall standoffs to save space. The LM317 is still mounted to the bottom of the case with an insulating pad/washer to prevent a short.

As you can already see (partially) the new panels were engraved on the CNC machine to give it a better look. I went for an “old-school” look with a tinted green LED on the whole black panel, matching some of the older headphone amplifiers, in the spirit of the Pimeta 🙂

Fig. 7. Back-Panel with RCA Input, Power Input Jack, and Power Switch
Fig. 8. “Old-School” Front Panel with Whole Black Color, and Tinted Green LED
Fig. 9. Assembled Pimeta Amplifier

The amplifier has since returned to the office, where its once again connected to the Gamma2 DAC, and drives the Aeon C Flow headphones. Only difference is that its now also driving the Ether CX that have since joined the family. I’m still on the fence as to which of these 2 headphones I prefer (Ether is more fun sounding, but I still think the Aeon is more to my liking with a more natural sound). However, one thing’s for sure, this combo sounds excellent with both, and the Pimeta does a great job at driving these headphones. If you are looking for a DIY amplifier at a reasonable cost, do yourself a favor, give the Pimeta v2 a look.

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