A low-cost optoelectronic coupler for PC-Soundcards

Circuit | Components | Details | Characteristics | Installation | Layout | Links | Index

Admittedly, the most simple and elegant way to perform ground-free analog sound transmission was a real good transformer... Unfortunately, those special audio transformers do not accommodate some peoples financial margin at all - or else they do not suffice CD or even HiFi demands; broadcast is more or less altered by magnetic hysteresis, mismatching, inductivity and resonance effects. 

Optocouplers virtually bypass all these unwanted effects - yet requiring additional electronic components to operate them properly. In my circuit concept, this workaround has been minimized and standard couplers were used to gain comfortable and affordable protection for the analog output drivers of a PC's soundcard.

prototype

  • HiFi Stereo broadcast

  • Groundloop protection

  • High voltage separation up to 2kV

  • Based on inexpensive standard components

  • Project cost: ~ 10 €




Picture: Prototype 50 x 70 x 20 mm

    Circuit

    circuit plan on the optocoupled PC soundcard output


    Components

    R1, R2
    		330
    R3, R4
    		2k2
    R5, R6
    		4k7
    R7, R8
    		100
    R9, R10
    		680
    C1, C2
    		100µF / 10V
    C3
    		470µF / 10V
    C4, C5
    		100nF ceramic
    C6
    		470µF / 10V
    C7, C8
    		220µF / 10V
    D1, D2, D3, D4
    		1N4148
    T1, T2
    		BC547C
    OK1
    		ILD74 / CNY74-2 (twin optocoupler)
    Tr1
    		ferrite choke (2 x 1mH, testing voltage 2kV)
    e.g. Type RADIOHM 42H22-3000 or similar
    Other Materials
    		cables; PCB or veroboard;
    gameport-connector D-SUB 15 pin (male);
    3.5mm stereo jacks (male and hole)
    fuse ~100mA/T (see adds)
    enclosure 70x50x20 mm (e.g. KS21 from Reichelt) 


    Details

    Even standard optocouplers like CNY74-2 resp. ILD74 are able to transmit analog waveforms up to 100kHz, which is far above the audio range. Because these couplers' CTR is about 100% and because the detector is a plain phototransistor (no darlington), and because the range of almost linear operation characteristics is compareably wide, an audio broadcast with good linearity over the whole audio spectrum of 20-20000 Hz is possible with few external components.

    Being active components, optocouplers need separate voltages for LEDs and phototransistors; second voltage, of course, should be groundfree and well isolated from the other side. This is what most parts of this circuit concept deal with: A standard mains choke utilized here as a highly-isolating transformer for VLF. The 'primary' side (PC's soundcard) is driven by a symmetric oscillator (T1/T2) to generate a magnetic flow of about 40...60 kHz and thus transferring several milliwatts of energy to the 'secondary' side. Voltage induced over there gets rectified and finally buffered by a capacitor to gain about 3.2 Volts stable DC for the output transistors. 

    The whole machinery matches a 55 x 65 mm piece of punched paper... A nice Layout is available, too.
    If you got comments or questions about that fine project - Contact me!

    Audio Characteristics

    Channel separation
    (one input disconnected)
    		better >65 dB
    Channel disparity< 5 %
    (depends on coupler series and other component's tolerance)
    Level ratio
    (input vs. output voltage)
    		1 : 1.3
    S/N-R(dynamic range)
    		>75 dB
    Linearity
    referred to 1 kHz as 0dB
    sine waveforms, 0.775V input,
    matching with 1k input and output impedance    		10 / 100 / 1000 / 5000 Hz = 0 dB
    10.000 Hz = -0.2 dB
    15.000 Hz = -0.5 dB
    20.000 Hz = -2.8 dB
    Distortions
    		less than 1% at the whole audio spectrum
    (more precise results not attainable at the time :-(


    Note: This design has been conscientiously tested with my modest equipment; and fortunately a few other people recently confirmed my measuring values. These are pretty astonishing and satisfactory results on a low-cost concept with standard components. But most of all, I use to say: "May be too bad for CD; but it's always better than HiFi..."

    In case of...

    Unbelievable Sounds? Playback volume correctly adjusted ? -> Check the audio menu

    Sounds unbelievable... Sad but true; nowadays there are still excellent soundcards on the market, but some of them no longer provide robust output drivers anymore. Without these drivers, one could not even plug some headphones directly to the card, and the optoelectronic sound coupler will not work properly. Yet there is still a chance that your soundcard is featured with some jumpers that allow to re-configure for "speaker-mode" (which provides low impedance - RTFM!)


    NOTE 10/2003
    Incredible! Now that I tested the sound coupler at several brandnew PCs with AC97-compliant onboard sound, I was positively surprised, as there was no level nor impedance problems at all! (Hope this was no 'exceptional case.)


    Adds

    HOW TO GET 5 VDC OUT OF THE GAMEPORT SAFELY
    Every Gameport provides some powerful 5V-terminals (1,8,9,15) which are internally connected to the 5V supply inside the PC and three pins (5,4,12) featuring GND potential. That's convenient to feed some external logical or even analog circuits as well, see picture left.
    Just consider: Accidental short-circuit on these terminals might severely damage parts on your soundcard or mainboard!
    It's a reasonably good idea to     insert some kind of fuse or current limitation into these 5V-lines!
    With small loads (<100mA), the fuse may be substitute by a cheap 0.25W resistor of about 10 Ohms, which is tiny enought for being soldered directly on the corresponding pins of the D-SUB connector.    		Schematics on how to get 5 volts DC from the PC's Gameport without severe trouble by accidental short circuit


    PCB (200 dpi GIF)

    layout design and site plan

    Links

    last revision on this document: 11/2002; additional notes and redesign 11/2003;