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.

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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.

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TDA8954TH Based Subwoofer Amplifier Board from AliExpress (25$) Any Good?

In this post I’d like to briefly share my thoughts on a subwoofer amplifier board I’ve purchased from AliExpress a few weeks ago. I have purchased it as a cheap solution to test an in-wall subwoofer I’m building (2x10inch woofers model R1S4-10 from RF in sealed enclosures). I looked for an amplifier that will have ~400W output power into an 8ohm load (bridge connection), although I doubt I will ever need this much power, or even half that. I’m actually more interested to see how much output power I can get before distortion gets out of hand, with the hope I could get ~200W before onset of clipping. I would obviously much prefer to have a plate amplifier for this, but I haven’t found one that seemed to meet all my needs for a reasonable cost, so I went with plan B. As all temporary things, if it works well it will probably be turned into a fixed solution, so I was interested in seeing how it will perform on the test bench.

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Agilent 54831B Oscilloscope Taming

Just like any other DIY electronics hobbyist, a scope is one tool I can’t live without. Over the past few years I’ve owned the renowned Rigol DS1052E (which was hacked up to the 100MHz model), and more recently a Siglent SDS1204X-E. Both were great for the price and their respective time to market. The DS1052E was probably among the first scopes that almost every hobbyist could afford, and the SDS1204X-E was much more capable with more channels and processing power and its MSO option (which at the time was quite buggy, but I understand it has improved). However, the thing I was missing is the ability to probe (relatively) high speed signals. I wanted something that could do 500MHz, and preferably even 1GHz. Getting such a high BW scope would cost quite a lot of money brand new, so I knew I had to look for some older used units. After reading a bit on different forums, I’ve decided that an Agilent 54831B would be a good match for my needs. In this post I’d like to describe some mods I did to the scope to make it better suited for my needs. Hopefully some other readers will find this useful to learn about this scope, or how to mod it if the need arise.

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Audio Measurement Pre-Amplifier – Part 4 – Casing the Pre-Amplifier

This is part 4 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 part 3, I have gone through measurement results of the assembled pre-amplifier board, as well as some circuit modifications to extend its performance. In this post, part 4, I will briefly show the assembled unit, along with slight discussion of external and power supply coupling into the signal.

As with many of my recent projects, I stuck to PCB’s for the front an rear panels of the pre-amplifier. The benefits are clear, its cheap, its very easy to design in the same software tools used for all of my circuit designs, and it offers electrical shielding due to the internal copper layers that are available to us. Unlike in my previous builds, this one is significantly larger, has very large holes, and even square cut-outs. Therefore, I wasn’t sure how well it will come out. To minimize the chance of an error I’ve printed the panels on a piece of paper and measured it in place before placing the orders. You don’t want to spend a few 10’s of $’s, and wait for a few weeks before you realize you’ve made a mistake 🙂 Continue reading “Audio Measurement Pre-Amplifier – Part 4 – Casing the Pre-Amplifier”

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.

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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.

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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.

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EMU 0404 USB – Op-Amps Replacement Worth the Effort?

As I’ve posted in the past, my audio measurement setup is built around an EMU 0404 USB sound card. Its a fairly old device, its driver is old too. On the other hand, you can get it for almost nothing on eBay, and it has excellent measurable performance for the price. It is good for 0.001% THD at 1KHz without any modifications. With some help, its front-end is good enough for even 0.0001% THD measurement, as I’ve showed in this post. However, as you increase the frequency, the distortion will grow, as you’d expect. Additionally, if you look inside the box, there are quite a few parts there that make you wonder “how good can it be if I put a few extra $ into it?”. That’s exactly what I wanted to find out. I didn’t want to spend much time, nor funds, as I was happy with the performance I was getting. This was mostly for fun, and the results are shown in this post.

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Quick and Simple Notch Filter for THD Measurements

One important tool that can help extend the capabilities of a distortion measurement setup is a notch filter. The logic behind it is fairly simple, if we are only interested in the distortion components, why should we even feed the fundamental frequency into the measurement setup? By eliminating it (or simply attenuating it sufficiently), we can reduce the harmonic distortion generated by the test equipment as a result of the large tone, effectively extending its capabilities for harmonic distortion measurement.  There is obviously more than one way of doing it, and in this post I will only describe one way  which was a good match for my needs.

I wanted to create a small box that would implement this function for my needs to allow me to extend further the THD measurement setup I have. In its simplest form, using the EMU 0404USB I’m able to measure THD of ~0.001% at 1KHz. By using an external low distortion 1KHz oscillator I was able to extend this down to ~0.0004%. However, I was looking for a way to get down to 0.0001% to allow measurement of high quality DAC’s. Since I know the external oscillator I use has sufficiently low distortion to support these figures, I needed a way to reduce the distortion caused by the input stage and ADC of the EMU. I have considered trying to hack the EMU and improve its input stage, but I expect the ADC will limit me before I can reach the target performance. Therefore I went with the option of removing the fundamental frequency from the signal before feeding it into the EMU, to reduce the distortion it generates.

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