Hi,
I just joined diyAudio.com. I have spend my entire life designing, repairing and buildingjavascript:smilie('
') electronic equipment, electronics is my passion. I have designed a lot of High-End audio equipment over the last decades. The following projects are the "newest":
-Fully symmetrical cascode MOSFET amplifier with high resolution microcontroller based VU meters.
-Modular 6 channel control amplifier with remote control
-Modular input switchbox
-Twin TDA1541A DAC in differential mode with 8th order active hybrid filter
-Sonic resonators (passive semi 4-way 360 degree omnidirectional radiators
with real-time error correction). How do they sound? hearing is believing!
-Non inductive copper wire resistors for High-End passive filters, using stranded wire and a spiderweb winding technique. These resestors have extremely low noise and improve sound clarity.
javascript:smilie(':att\'n:')I am now developing a NON oversampling DAC using 4 X TDA1541A. The 44.1 kHz problem (ultrasonic interference in the audio band) is solved by using a trick I named Direct Interpolation (well this system had to have a namejavascript:smilie('
')). This results in a virtual sample-rate of 176.4 KHz, 18 bits resolution, quadruple output voltage, improved signal to noise ratio and lower distortionjavascript:smilie('
'). I used a second order bessel filter at 40KHz for linear group delay just in case. The sound and dynamics are breathtakingjavascript:smilie('
') since both phase-shift and ultrasonic interference are greatly reduced. Now finally I could hear music the way it was originally recorded. At the moment I am optimizing the Direct Interpolation system. Tips or suggestions from you all are highly appreciated
- Why the TDA1541A? it uses current sources based on dynamic element matching instead of resistor networks. The current sources are externally decoupled so they produce clean signals. Furthermore a smart design elliminates signal transients all together. In my humble opinion this is the purest form of D/A conversion, and the sound quality of these DAC's prove this. It is really a shame this state of the art DAC is discontinued, but I guess that is called "progress". Modern DAC's have the digital brickwall filter and decimation noise generators built in. I cannot follow the logic of spoiling a fine signal by decimating and then try to filter it out the mess you've createdjavascript:smilie('
'). The high oversampling frequencies will make sure there is lots of addittional noise and interference to listen tojavascript:smilie('
'), it's as if you put your High-End amplifier inside a PCjavascript:smilie('
').
- Why more than 1 DAC? With 2 ore more DAC's linearity errors are reduced, and normal lower cost TDA1541A's can be used. Signal to noise ratio improves, differential output can be used, elliminating residual interference and... the DC component can be cancelled out, enabeling a fully DC coupled DAC. In the new design Multiple DAC's are a fundamental part of Direct Interpolation. Many listening sessions confirm that a well designed multiple DAC system can sound significantly better than a single one.
- Jitter, I solved this by designing a custom made differential interface for the SPDIF signal, so no coax and no TOSlink. My opinion is to avoid jitter at the source. (asynchronous reclocking just seems to add jitter instead of removing it caused by the coincidental D-flipflop trigger). The higher the reclock frequency, the lower the jitter.
- I/V conversion, I used the classical OP-AMP approach, an OPA627, 470 Ohm and 220pF for this (shame on mejavascript:smilie('
')). Passive I/V conversion using 33 OHms already creates a voltage drop at 4mA full scale of 132mV. Philips datasheets indicate that more than 25mV already causes distortion. Of course I also tried a tube output stage, it sounded nicejavascript:smilie('
') but it doesn't solve the distortion problem caused by the I/V resistor and/or inductors (inductors seem to produce addittional noise). It also requires capacitors or transformers in the signal path. Personally I don't like non-linear components in the signal pathjavascript:smilie('
').
- Clock syncing, I used separate 1% polystyrene 470pF capacitors on all TDA1541 (the close tolerance is important). It is used for internal timing of the current source switching. If you want to sync them use a 100pF between both pin's 16 instead of a direct connection, the oscillators run in phase but oscillator output amplitude may vary.
- Decoupling, The decoupling capacitors are just used for that, decoupling, not sampling. However, leakage currents cause massive distortion. High quality polyesterfilm or polypropylene will do fine. Keep wiring as short as possible.
- Differential mode, I inverted the data signal on one DAC. The first dac outputs L and R, the second one L- and R-. So basically I used 4 I/V converters that feed into a differential amplifier. Interference is significantly reduced this way and the output is DC-coupled since the offset voltage of both TDA1541A's is cancelled out.
- Filtering, I experimented for over a year with the most exotic filter setups. I finally used 2 closely tuned Allen key 3rd order butterworth-isch filters followed by a 2nd order MBF butterworth-isch stage. Filter was optimized for both flatness and linear group delay that's why I stated butterworth-isch. The sound is about the best you can expect from a 8th order filter. By the way, there is a nice filter calculation program available from TI.
- Printed circuit board, Over the many years I designed hundred's of circuit boards (without autorouter), so this is a piece of cake. The twin DAC was placed on a single circuit board, but the new quad Direct Interpolation DAC will be a modularar design to achieve optimal results and future updates.
Well as you can see I can get quite enthousiastic about electronics.....Well that was it for now, I am pleased to join diyAudio. If you want more information about my D.I.Y. projects, please feel free to ask, your comments, tips and reactions are very welcome.
FifoPiQ7_0 by Ian, on Flickr
