The AKG K1000’s have a somewhat of a legendary status as a unique pair of headphones. They are more like “floating” speakers than typical headphones. These are a fairly old model which was produced for a fairly long time, but it was discontinued some years ago. Many people still own these, but as all things, they do need some TLC over the years. In this post I’d like to briefly share my comments on these headphones along with some pictures to describe the work that was needed keep my pair of K1000’s in proper working condition.

Audio Measurement Pre-Amplifier – Part 5 – Updated Ver1.1

Over the past year and a half since posting the series of pages about the measurement pre-amplifier I’ve designed and built, I’ve received emails from multiple people who were interested in building the pre-amp. I’ve happily shared with them the remaining boards I’ve had from that first batch I ordered at the time. These few boards were all given out, and I’ve therefore ordered a few extra boards to be able to keep offering these boards to people who would like to build such an instrument for themselves. Unlike the first batch, this time I’ve printed boards that fixed the issues I’ve reported when building my own unit and were fixed by a “bodge” over the original board. These modifications to the board design (and the writing of this post) were all done well over a year a go when I’ve originally assembled my unit, but I didn’t see a need to post them until this point in time.

This post is meant to share the updated schematic, as well as to offer additional information that can be of help to people who would like to assemble such an instrument.  I didn’t make any functional changes to the pre-amp at this revision, therefore I will offer no additional measurements in this post.

Hafler 9505/9303 Power Amplifiers Repair and Measurements

Over the past few months a few different Hafler amplifiers passed through my bench. First it was a combo of a 915 pre-amplifier with a 9303 power-amplifier to accompany it, and some time later I got a 9505 power-amplifier. If you’ll have a look at the schematic you will see that while the pre-amp isn’t too exciting, the power amplifier has a few interesting points in its schematic. So I’ve decided these amplifiers are interesting enough to warrant a short post about them with some pictures and measurement results.

Kenwood KA-7100 Vintage Amplifier Revisited

One of the amplifiers I own is a Kenwood KA-7100 stereo amplifier. It is a fairly old amplifier (late 70’s), with a modest 60WPC into 8ohm specification. I was a fan of the KA-XXXX amplifier series and its siblings from the moment I first heard its smaller brother (KA-601). While that KA-601 was in fairly bad shape, I was amazed at the time by how much better it sounded than my (back then) modern Denon AVR HT receiver. I’ve since had an opportunity to listen to quite a few amplifiers from this series including the KA-7300, KA-8100, KA-9100, and others. Therefore, when I’ve had the chance (more than a decade ago) to get my hands on a KA-7100 I grabbed it right away. It wasn’t in bad shape, but it has seen better days, no doubt. As I learned a bit more about electronics, that amplifier became one of my first projects.
I have recently had the opportunity to revisit this amplifier by doing another small modification, which gave me an opportunity to write something about it, and add some measurement results while at it.

Audio Measurement Pre-Amplifier – Part 3 – Initial Setup and Measurement Results

This is part 3 in the series of posts discussing the (audio) measurement pre-amplifier project. In part 1 I’ve covered the motivation for this project along with the circuit schematic and detailed circuit description. In part 2, I have gone through the board layout consideration and showed the assembled boards.  In this post, part 3, I will show some of the measurement results of the assembled boards. I will start with describing what it is I would like to measure, and how I plan on measuring it, including the limitations of the measurements I can make with the gear available to me. Then I will show the relevant result and discuss them.

The measurements I plan on performing can be split into 3 different groups. The first has to do with linearity of the pre-amplifier, to measure how much distortion it will have. Next are the noise measurements, as I want to verify the input referred voltage noise of the pre-amplifier to make sure it meets my target figures to allow measurement of low noise voltage regulators (and other devices). Finally are the “other” tests such as the accuracy of the True-RMS reading, the voltage limits of the output protection circuit, and so on.

Audio Measurement Pre-Amplifier – Part 2 – Board Layout and Assembly

This is part 2 in the series of posts describing the audio measurement pre-amplifier project. In part 1 I’ve covered the motivation for this project along with the circuit schematic and detailed circuit description. In this post, part 2, I will discuss the next steps related to the board design and assembly. This part won’t be as long and the first (I hope), but I would like to share some of the consideration I’ve made when laying out the board design.

The first step was deciding on a case size and layout for the front panel, as this will set some constraints on board dimensions and placement of connectors/switches/LED’s. I wanted to use a case that will be made of aluminium to use it as a shield, as at the highest gain setting the pre-amp has 60dB (X1000) of gain which makes it very sensitive to coupling from external signals. I also plan on placing the completed pre-amplifier on my work bench, so I wanted something that is relatively compact, but isn’t too cramped so that it isn’t comfortable to use. Something similar (or slightly smaller) than a bench DMM seemed like a good size for this as I would be able to stack it on top of my other instruments. The plan was to have all the relevant connectors and switches at the front, along with some LED’s for visual representation of the selected range, and a panel mounted voltmeter. Placing it all in a single row seemed impossible, or at least very uncomfortable to use. Therefore I’ve decide to split this into 2 different rows (heights). This put a constraint on the minimum height of the case, and meant I will have to split the design into 2 boards to support this since I don’t want to solder any wires. The schematics posted in part 1 of this series already represented this split board solution, with the second board used mostly for range selection.

Audio Measurement Pre-Amplifier – Part 1 – Motivation and Circuit Design

As some my other posts show, I have been spending a significant amount of my spare time over the past few months on audio measurements related stuff. This included a low distortion oscillator, a notch filter to go with it, as well as modifying the EMU 0404 USB to extend its performance. One other item that has been in the works for quite a long time, is an audio measurement pre-amplifier. The motivation for this work is quite straight forward, I needed to find some way of turning the sound-card I’m using into a versatile measurement tool to do general audio measurements. The most significant limitation with sound-cards is their limited input voltage range, as most audio amplifiers put out voltages that are significantly higher than what you can safely feed into a sound-card. Indeed, this is what most people would use such instruments for. However, this is actually just a portion of what such a pre-amplifier could be used for.

This post will the first part of a series of posts that will describe my take on a measurement pre-amplifier. I will describe the motivation (requirements), the circuit design and implementation, measurement results, and more. I  will try to make this as informative as I can, and share some of the reasoning behind design decision. I think this can be of value for both people who would like to understand the circuit better, and people who would like to modify the circuit to better suit their needs.