(c) 2002 JŲrg Hohensohn                              last change 05/24/2002

Preface

Iíve been too much of a perfectionist with this. Please take my possible criticism with a grain of salt. Plus, being from Germany, English is not my mother tongue. I can express myself, but possess no particular writing skills. So the reading my be dull to you, and often I need some wore words to come to the point.

In this article Iím describing my modifications on an Amphony 1000 headphone, particulary the DAC part I built. Although I give out this information for free, I still reserve my rights on it. Youíre welcome to use it for private purpose, but not for business.

Introduction

Since quite some years I have an unfulfilled product wish: An audiophile quality wireless headphone. A top of the line headphone where you donít notice the absence of the cable, sound-wise I mean. This would require a digital transmission.

As a teenager I happened to get a used Jecklin Float Electrostat, an old model. But I enjoyed listening to it very much, the precise highs were unsurpassed (and at that age I was most able to enjoy them). It lacked some bass, though. The Jecklin is now in bad shape, with the cushion foam worn out and rotten, plus is doesnít reach the volume any more, with one side even weaker than the other. So right now Iím without a true reference headphone. I had a Grado SR80 for mobile listening, now replaced by a Koss Porta Pro, but thatís a different league. Iím telling this for you to get an idea of my expectations.

There are a few upper class digital wireless headphones on the market, I tried some of them:

With these disappointments I was already thinking about a DIY project, well fearing that itís over my head. My favorite would have been a wireless electrostat. I started to gather some material, today there are interesting components like piezo transformers for the high voltage generation. But the efficiency is too bad for battery powering.

For the transmission, I started to look for those 2.4 GHz A/V links. The video part would have enough bandwidth to carry an S/P DIF signal. But the receivers are bulky and power-consuming.

About the end of the year 2001 I happened to find the Amphony1000 phones in the web. They use 2.4 GHz for uncompressed PCM transmission, exactly the technology I wanted. The price ($130) is too modest for them to be audiophile, unless these guys made miracles happen. You donít get serious corded headphones for that price! But I ordered them straight away because worst-case I was willing to scrap them for the embedded transmission or had the feeling that replacing the transducers can upgrade them.

Amphony 1000 Review

Main features:

Quality

The phones arrived shortly before Christmas, my gift to myself. Right out of the box they had some scratches on the outside, and on one of the earpads the thin material was torn about one cm. I called the dealership, angry about probably getting their worn out demo model, but they swore theyíve never touched it, offered to return it to the manufacturer. But in fact I saved them the hassle, kept it that way. For taking it apart it didnít matter, but they may have a quality problem. German engineering meets Chinese manufacturing, a culture clash?

Later I found that on one of transducers the excess glue it was fixed with got spilled into it, being dry it bridged the enclosure and the diaphragm. Thatís a good reason to sound bad.

The phones look rather cheap from a closer look, like cordless phones for $25. The plastic has burrs, the cans themselves are painted silver and appear imprecise. So no reason to worry about my out-of-the-box scratches, itíll get more anyway. The battery compartments (one on each side, for an AA cell) donít open too easily, tend to stick. The cushioned earpads are covered with a very thin leather imitation, rather sweaty. I guess Amphony took an existing El Cheapo phone and upgraded it with their digital transmission technology?

The good news is that they fit well. An elastic headband makes them weightless, and there is no need to adjust anything. They rotate and tilt into place. The outer diameter of the round cans is 9.5 cm, comparably modest.

Sound

Iím sorry to have given them a devastating sound review in an earlier www.headwize.com posting, my particular ones must have sounded crappy because of that glue bridge. Later I was able to check another pair, sounding way better. So hereís my revised review:

The sound is ďhonestĒ, pretty natural without coloration. I always find voices to be most challenging in terms of colorations, but they pass. (My favorite test material: Suzanne Vega Ė Tomís Diner, Tracy Chapman Ė Mountains oí Things)

The highs show decent resolution, but reveal the limitations of the transducers. Cymbals can sound a lot more detailed. (Favorite test material: Ana Caram Ė Viola Fora de Moda)

The bass goes down to about 30 Hz, below the transducers just canít radiate sound. For comparison, my Grado transducers are well audible at 20 Hz. So not too much in for bass lovers.

The phones may lack some volume, if you like to listen really loud (or have to, say as a DJ). For me it was OK.

Overall Iím surprised to find them sounding so well, considering the price tag and what part of that might be left for the transducers. Donít expect these to have true audiophile qualities, thatís a different league. I would rank them close to my Koss PortaPro, but without the Koss bass emphasis (which I never really liked, BTW).

For those interested: The transducers are 32 mm in diameter, the diaphragm may have 28 mm of that.

Reception range

Under ďaverageĒ conditions Iíd say it ranges about 10 m. The 2.4 GHz is in the microwave range. So the signal likes to travel straight paths and suffers if thatís blocked by metal. Microwaves are also absorbed by water and fat, which is what people are made of (no offence!). Amphony gave the phones 2 antennas, one in each can. I donít know how effective that in fact is, but it sounds like a good idea.

My flat is almost covered by the transmission, I have a few holes though. It works well into the next room, but may fail in the following. The failures are sometimes nasty. Usually the phones instantly mute when they detect errors, but sometimes can give you wild crackles. If you consider the digital transmission, you can imagine that it can sound really bad if the numbers are screwed up.

There is no channel switch for the transmission (maybe because they use up all available bandwidth of the ISM band already), so you have no chance of avoiding others. I have an analog 2.4 GHz A/V transmission which I have to shut off when using the phones. Fortunately it doesnít give audible interference, just reduces the range. Bluetooth and WLAN also operate at 2.4 GHz, but I donít know how they interfere. In the future that band will likely be crowded.

What you really have to avoid is operating a microwave oven nearby. Another reviewer described the audible effect like ďbeing hit by a Romulan disruptor beamĒ. Well, luckily I havenít been exposed to such yet. But well, it sounds loud and unpleasant.

There is a nice feature about the transmitter: It shuts itself down after a minute or so if there is no input signal. So you donít have to worry about switching it off to avoid RF pollution. (It has no switch, anyway.)

Battery life

The phones run on two AA cells. Mine came with a pair of half empty Chinese batteries. Iíve never used them, Iím afraid they could leak. Probably most people will run the phones from rechargeables. They take about 31 mA of current, volume doesnít really matter. Iíve put a pair of 1800 mAh NiMH cells in. You cannot exploit the cells, because the phones stop working below 2.25 Volts or so. I havenít done stamina tests, but Iíd guess theyíre good for about 20 hours at max. Amphony advertizes 100 hours, but you theoretically would need a 3 Ah cells that keeps the voltage up above threshold. There are high performance primary cells based on Lithium with 3Ah, but they cost a fortune.

Anyway, battery consumption is low enough not to be bothered.

Technical+Inside

The transmitter takes an analog stereo signal. The levelling is fixed, no manual adjustment possible. The signal is sampled by a Burr-Brown (nowadays Texas Instruments) PCM1801 ADC. Thatís a low cost model. It may be possible to upgrade this to a better one, but I rather went for an S/P DIF input later. Then you could connect a DAT recorder as an external ADC, for example. The ADC has 64 times oversampling and includes the nyquist lowpass filter, so practically it can be directly fed with the signal. This is done via a capacitor, the first in the signal path. In total itís three, two more in the receiver.

The digital stream is subject to an FEC (forward error correction), which doubles the data rate to a whopping 3 MBit/s. The added redundancy allows the receiver to correct errors, if there are not too many. The FEC signal is available on a cinch socket, it is meant to connect extra transmitters here to increase the range by having extra ďcellsĒ.

The receiver mixes the signal from its 2 antennas. The RF reception circuit is in the left can, the right one hosts a digital part for the error correction and the actual DAC and amp. Both PCBs are nicely shielded. The DAC being used is a Philips TDA1311. Itís not featuring any oversampling whatsoever. The signal coming out looks like a staircase. It has to be filtered externally, which is done by the output amp. I didnít analyze the circuit, but itís probably something like a 2nd order lowpass filter. There are still steps to be seen on the final output signal, but surprisingly the sound doesnít suffer from it. Probable the transducers themselves are doing the remaining filtering job.

The single-ended output signal is coupled out with 220uF capacitors, the smallest I ever saw. Together with the 24 Ohms impedance of the transducers this means a low-frequency rolloff of 30 Hz.

Modifications

By now, I spent a ridiculous amount of time into this project. It became sort of my model train.

As I mentioned, the obvious and most fruitful is to exchange the transducers. But since I thought that to be the trivial part (not quite right, see later), I took a different challenge first.

DAC+Amp Replacement

Because of no oversampling and the filtering all in the analog domain, the frequency response I measured is rather uneven, the phase response has a non-constant group delay. I felt that this can be improved and searched for oversampling DACs suitable for mobile equipment.

DAC Selection

A big obstacle was the unusual system clock within the receiver. An oversampling DAC needs an additional, higher frequency locked to the sampling frequency for its filter logic. Normally, this master clock is 256 or 512 times the sampling frequency (Fs), rarely 384 times. Thatís what most DACs can be configured for.

The receiver locks itself to the clock of the transmitter using a VCXO circuit. Thatís very good news, because a VCXO has way lower jitter than a PLL. But the frequency is very odd, 6.528 MHz which is 136 times the sampling frequency, not useful to standard DACs, bummer. This means I still would have to use a PLL to generate 256*Fs and canít benefit from the nice VCXO.

But I was lucky to find a DAC which can handle it, and that is the poop of this modification. The secret weapon is the Texas Instruments TLV320DAC23. It has an odd mode to feature 44.1 kHz sampling with an input clock of 12 MHz from a USB bus. Division tells this means master clock is 272*Fs. That number happened to strike me while browsing the datasheet, it is twice of what I need. It can also be programmed to use 6 MHz input, and then I get my 136*Fs, hooray!. A DAC with itís internal filtering doesnít care about absolute frequencies, just the ratio matters. So if I set it to above mode and feed a 6.528 MHz steam instead of 6 MHz, Iíd get my desired 48 kHz output instead of 44.1 kHz. This doesnít stress the DAC at all, it can do up to 18.43 MHz and 96 kHz Fs.

Itís not the best DAC in the world, but at least doesnít require a PLL and is targeted for mobile use. Power consumption is less than the original one. The high end ones are really thirsty and require symmetric, split supply. Another interesting feature of this DAC is that it includes electronic volume control and a headphone amp! The downside is that it requires programming, via I2C or SPI bus. An onboard controller is needed.

Iím a software developer by profession (hardware developer by passion), so that didnít scare me too much. On a prototype board attached to the receiver, I built my first test setup, see picture.

The two DACs are side-by-side here, the new being the chip with the red and green wires, the old is the DIL-8 chip on the original board. So I was able to compare. The small chip on the left is another headphone amp (also from TI to have it symmetric), which I used as inverting buffers to form a bridged output, the 4-pin header next to it. This has the benefit of not needing output capacitors and the doubles voltage swing gives quad power. I thought that might be a good idea for a design that runs on 3 Volt supply, or 2.4 if rechargeables used. The output below (3-pin header) is a classical one directly from the DAC, and you easily notice it needs capacitors. I picked 1000uF for the low frequencies.

The I2C bus was controlled externally here. The top header connected the whole to my PC, were some printer port hardware I built forms an I2C master. So I could test the programming.

The results were quite promising, the frequency response flat as Utah salt lake, the group delay constant. I decided to carry on with a bridged amp, since it gave a nice and powerful punch. The drawback of bridging is you get twice the distortions, both amps add their share, but with todayís amps thatís no big deal. The benefit is that you get almost immune to noise on the power supply. That later became very useful, but at that time I didnít know.

Microcontroller

For the volume control, I decided to try the builtin electronic one controlled by software. The idea was to use the existing pot (anything else would have required mechanical work which usually looks amateurish) for an analog input of the controller, which in turn can issue the proper I2C sequence to the DAC. I can use both halves of the stereo pot in parallel, so Iím pretty much immune to wiper failures which these cheap pots so often develop.

For a past project, I have used a small 8-pin controller with onboard ADC (for analog input) from the PIC family, a 12C671. But that is either UV-erased or OTP, which was a pain. This time I wanted a flash controller, which can be programmed in circuit. Microchip is really behind technology with their PIC line, only a few feature Flash memory. For my really small 8 pin model, there was nothing available.

This time I chose an Atmel controller, ATtiny15L. It has a 10 bit analog input (well enough for the possible 128 volume steps), Flash and EEPROM. The latter is nice for altering settings without the need to recompile the software. I had to buildup a toolchain for this controller, since Iíve never used Atmel before. On the Web I found ICC Tiny from ImageCraft as a C compiler in a 30 days evaluation version and PonyProg for actually programming the device, via a to be built parallel port cable. (Again the parallel part, Iím glad my printer is connected via USB.)

I was able to complete the programming before the compiler expired. The remaining fine tuning was possible via EEPROM settings, which PonyProg can do. A special thing about this controller is that it contains no RAM. Everything has to be kept in the 32 registers, which worked out for me.

Software operation

On power-up, the controller sets up the DAC for the required modes, with the initialization data being stored in EEPROM. Then it programs the watchdog timer to reset the controller periodically and goes into a very low power sleep mode. The average power consumption is neglible.

Within the periodic wakeup, the controller applies a voltage across the potentiometer and measures the voltage at the wiper. The pot only gets power during the measurements, to save energy. My great trick, ha! If the wiper position has changed, the volume is reprogrammed. As a very useful extra I use a floating hysteresis for this measurements, so that a ďnervousĒ ADC reading does not constantly change the volume.

The compiler had a catch for me: it clears all global variables (registers used as such) on reset. The watchdog loop also is a reset, and I loose my previous level to compare against. I had to patch this out of the compiled hex file, canít use the compiler output directly.

First real board

I made my first PCB, which contains the circuitry of the prototyping board with the BTL amp plus the controller. The whole is supposed to be a daughterboard ontop of the original and still fits within the shielding. It takes the power supply and digital audio pins from now vacant through-holes of the original board, from which I removed the DAC and analog part. So most signals come and go with straight pins from the original board, as you can see from the bottom view. Only 3 wires have to be laid: 2 for the potentiometer, one for the clock. (the empty holes below.)

The DAC is the one with the many pins, the controller is at the bottom (with the 5 ISP pins for programming above it), the inverted BTL amp half at the right top.

This got soldered into place and I hoped thatíll be it.

Second board

Unfortunately the first shot wasnít quite satisfactory. I noticed audible crossover distortions behind the amps when loaded with low impedance, that went unnoticed with the prototype. I did more measurements and found out that the TI headphone amps are no good when loaded. Finally I redesigned the board to have a fully external BTL amp. And I took the chance to improve the power layout. I still use the headphone output because only that can be volume-controlled in software, and I found it as good as the line output the DAC also has. Both outputs have a residual noise (hiss) which is about the same magnitude as an LSB, the faintest reproducible signal. My stationary CD player does better, has less noise in that discipline. Iím afraid I have to live with that for this DAC.

Below you see the 2nd board ontop of the original:

(click to get large view)

Digital Input

To fully exploit the digital transmission, I wanted an S/P DIF input. That task was not so hard, fortunately. The 3rd cinch socket of the transmitter (the FEC output which I like supposedly most people donít use) was crying to be my digital in.

The transmitter is the systemís clock master. It ďdictatesĒ the 48 kHz it uses to sample the input. But anything attached to the digital in also requires to be master. A sample rate converter (SRC) can bridge that gap. It would have been to difficult, or rather impossible, to change the transmitter to be a slave. Such SRCs are standard parts, and the CS8420 from Crystal Semiconductor also contains an S/P DIF receiver. So all I need comes in one chip.

This time I havenít done a PCB, because the chip is in a standard SO package which fits on available prototyping boards. Below is the hookup:

(click to get large view)

Well, there are two more chips on the board. The one that looks like a transistor is a reset generator, and a very small 6 pin mux (left of the SRC) selects between the data from the DAC or the S/P DIF data, based on whether there is a valid signal. I didnít like to drill a switch or so into the transmitter, therefore the electronic selection. The digital in has priority. When a valid signal is found there, it gets fed trough. Otherwise the analog input is used.

There was a side project to equip my home CD player with a digital out, because it didnít have one yet. But thatís a different story.

With the digital in I was able to really test the DAC part, because now I can be sure about what signal reaches there. Thatís how I did the LSB test. I have a homemade test CD which contains computer-generated test signals. In fact I did the digital in mod sometime during the DAC upgrade.

Transducers

That was the last part, which I considered easy. The whole time I have been watching Ebay for suitable broken or worn out phones with nice transducers. After the DAC part was done and I had a mobile assembly of those two modules (RF and DAC) again, I went to some local hifi stores to test headphones on it. The salespeople looked a bit puzzled. It turned out to be Sennheiser HD600 against Grado SRx25, with my old SR80s falling short behind those. My favorites would have been the Senns, while the transducers are available as replacement parts for a reasonable price ($40 apiece). But theyíre 300 Ohms impedance, which doesnít really match my amp even in BTL mode. The volume didnít have enough headroom. Grados are 32 Ohms, perfect for me. But they donít sell parts in Germany, and new theyíre quite expensive here, not very common. Thereís no 2nd hand market at Ebay Germany for those.

I decided to scrap my SR80s, the most reasonable solution. Now I donít have the upper-end transducers I dreamed about, but maybe one day I get an opportunity for a used SR125 or SR225.

The final soundcheck with everything put together again was at first disappointing. The Grado transducers suffer from not being in an open headphone any more. I was partially expecting this, since they sound crappy when you hold the outside closed with your hands while listening. A layer of damming wool (correct term?) partially got me rid of the effect. I suspect the Amphony cabinet to have some resonance, maybe I can improve that with a dampening coating on the inside. Voices still sound colored, which the original transducers didnít show. At least the improvements are that my bass is lower and precise, and the highs are as good as it can get with the SR80s. I consider this modification not yet completed. More to follow on this chapter.

If anybody has nice and low impedance transducers lying around, please get in contact with me.

Earpads

I sweat under the original ones, since they canít breathe. During my Ebay days, I cheaply acquired several replacement pads for other headphones of which I thought they could be fitted. But none in fact did, what a foolish attempt. Anybody in need of pads for Sony MDR-CD480/580/780/2000 Sennheiser HD560, Beyer DT220?

My very latest modification was to tailor new pads out of velvet. Recently Amphony sent me a free replacement set of earpads for my torn original, thanks! After replacing my bad one, I carefully cut open the seams, took them apart to get measures for a pattern. Two elliptic rings of fabric are needed to replace the visible part, the thicker inner pleather backside can be reused, as well as the foam.

The tailoring effort is ridiculous, at least for me usually being far from those tasks. It took me half a day per side. But likely the summed Ebay searching/ordering was more. A sewing machine may help a lot, if it can be precise enough on the elliptic shape. I have no experience with such, did the sewing job by hand.

With the second one I found a great trick to more accurately and cleanly process the velvet: Before cutting, circle your marking with a glue that stays elastic when dry. Soak about the bordering 2 mm meant to stay with it, the cut away side doesnít matter. Let it dry, then cut the shape. This way the sealed edges stay perfect and are a lot easier to handle. With my first one I always had to be very careful to avoid the fabric from falling apart. The glued edges will later be invisibly hidden within the seams. Just make sure you donít glue more than about 3 mm inside.

The result was better than I expected, I got the shape pretty well. Now I have velvet pads, no sweating. They are also more quiet than the original material.

Ideas for the future

Power Regulator

It would be possible to extend battery life with a step up switching regulator. That would keep the supply voltage up while the battery voltage already goes way below. Or it can give the option to use just one cell, saves same weight, or run from AAA cells instead of AA.