Hi-Fi PC Speaker System

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Hi-Fi PC Speaker System

The Audio Pag

lliott Sound Products Projec

Hi-Fi PC Speaker SystemRod Elliott (ESP)

Updated 31 Aug 2004

All required PCBs are available for this project. Click the image for the price list.

troduction

you like listening to music while using your PC, or enjoy games with all their sound effects, you willnerally be disappointed with most of the PC speaker systems available. Few systems use anythin

her than moulded plastic boxes, ratty little tweeters that don't really do very much at all, and commoing only a tiny bipolar electro as the "crossover". Zero acoustic damping and a 5W amplifiermpletes the picture.

for the sub-woofer, if it manages to wheeze its way down to 50Hz you are indeed fortunate, and wmuch as 20W in a tiny ported plastic enclosure you can think yourself lucky to get that far. I know

m experience, and eventually got so sick of the grossly inferior sound that I just had to do somethiout it.

is is not so much a project in its own right as a collection of existing projects, and using some of thards that I sell (I bet that came as a real surprise!). With a bit of work and not too many dollars,unds, shekels or whatever, you can make a PC speaker system that will outperform many small htups.

escription

ere are two parts to the project, small satellite speakers that sit on the desk next to the monitor, analed, equalised sub-woofer that also contains all the amplifiers, power supplies and other electronie sub extends happily to 30Hz, and makes quite a good account of itself considering its size. It als

akes a fine foot rest :-)

this an audiophile project - no. It is head and shoulders above any PC speaker system I have eveard, regardless of price, and also makes a great little hi-fi. The sound is very clean, and it hastended lows that put a lot of "proper" systems to shame.

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e selection of speaker drivers is up to you, since I have no idea what you will be able to get from ycal suppliers. Don't be too cheap - the better the drivers, the better the overall sound, and the happu will be with the end result. At the other end of the scale, using the best drivers you can find isobably not worth the effort either, since there is already a limitation in sound quality imposed by yound card.

ease note that all dimensions shown are in millimetres. For those who insist on those inch thingies

vide by 25.4. Litres can be converted to cubic feet by dividing by 28.

e parameters of the speaker drivers I used are shown below. These cost me a little under AU$15m local Australian suppliers. The only speaker likely to be recognised is the Peerless tweeter.

oofer Re/sponse 200mm Midrange Unknown 100mm Tweeter Peerless 81297

mpedance 8 Ohms Impedance 8 Ohms Impedance 8 Ohms

ower 120 W Power 30 W Power 100 W **

ensitivity 90 dB/W/m Sensitivity 91 dB/W/m Sensitivity 91 dB/W/mesonance 40 Hz Resonance 91 Hz Resonance 907 Hz

otal Q (Qts) 0.47 Total Q (Qts) 0.72 Total Q (Qts) 0.8944

as (litres) 32.8 Vas (litres) 2.11 Vas (litres) n/a

one Area (m) 0.0214 Cone Area (m) n/a Cone Area (m) n/a

Parameters For Prototype System Drivers

This is the nominal system power - the tweeter would self destruct with this power within its normal operating rang

a - Not Available / Not Applicable

e above information is mainly for the sake of interest. The drivers you use will have to be those yon obtain, but the above figures give you a goal to aim for. If you can get drivers with similar specsen you will have a system as good (or better) than mine.

atellite Speakers

rstly, you need to obtain some magnetically screened drivers. Ordinary drivers must not be used, aey will distort the colour and even the picture shape on your monitor. The ones I used are 100mmameter (outside) and are rated at 30W RMS or 50W peak. They are both 91dB/W/m and are 8 ohme crossover is a simple passive 6dB network at 4kHz. This is not ideal, but is a reasonablempromise between complexity and performance. With an internal volume of about 3 litres, the boxe sealed, well damped with fibreglass, and have a resonance of 110Hz.

s important that the low-midrange driver and tweeter have the same efficiency. If not, make sure te tweeter has the higher efficiency so it can be padded back to match the midrange. If not, you wilve to pad the midrange driver, and this will soak up much more power (as heat), and reduces dameither is acceptable. A simple resistive pad can be used to reduce the tweeter level, or you can us



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roper" wirewound attenuator pot so it can be adjusted.

e boxes are covered with carpet (the thin stuff for covering speaker boxes), which helps to eliminafraction, and adds extra damping to the boxes. The boxes themselves are made with 10mm medinsity fibreboard (MDF) and because of their small size, are extremely rigid.

Figure 1 - Satellite Speaker Dimensions

e boxes measure 125(w) x 270(h) x 145(d) outside. I rounded off all the edges with a router, but tnot really essential if you don't have access to one. If you do, use a rounding bit with a 10mm or

maller radius, or you will remove too much of the material next to the joins.

e crossover consists of a single 470uH inductor and a 3.3uF polypropylene capacitor, wired as shoFigure 2. This is a 6dB network, so it is important to make the crossover frequency high enough tosure that there is sufficient attenuation at the tweeter's resonant frequency. I measured the woofe

pedance at 4kHz, and found that it was only marginally higher than at the midband frequency (abo0Hz), so decided against using an impedance correction network. Although I'm sure that this wouprove matters, I suspect that it would be marginal, and not really worth the effort.



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Figure 2 - Crossover Network

nce I have already established (some time ago, actually) that the series connection for a 1st order f

to be preferred, I have changed the diagram to suit. While it is unlikely that the difference will bemediately audible in many cases, it is no more difficult to make the series version, and it does worktter.

e connectors that I used were some I happened to have in my junk box, and are actually the 2-pincrophone connectors that are commonly used on CB radios and the like. They are small, airtight, eap (even if you have to buy them), but are a pain to mount, as they are intended for mounting ineet metal. I recessed the mounting hole using a SpeedBor flat wood drill, and secured the finalsembly with hot-melt glue. Feel free to use any connector and mounting that suits you.

ub-Woofer

is is where all the action is, and I used the following project boards to make the final unit ...

1 x P3A dual 60W power amp - connected in bridge-tied-load (BTL) to obtain about 150Wcontinuous

1 x P05 +/-15V power supply 1 x P09 stereo Linkwitz-Riley crossover, configured for a 200Hz xover frequency 1 x P19 dual 50W IC power amp - one amp for each satellite speaker 1 x P71 Linkwitz transform circuit

e entire system is powered from a 25-0-25 toroidal transformer. The one I should have used woulted at 300VA, and I suggest that you don't use less than this or power will be severely restricted. Mstem is using a 160VA transformer, and this does reduce power considerably. Even so, total systewer is around 200W, but can exceed this on transients. This is very loud (as in VERY loud), and mconsidered overkill. Feel free to use a lower supply voltage, but the system as described can also

ed as a small hi-fi and will give a good account of itself in this role. Reducing the power will limit yo



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tions, and I will be replacing the transformer in my system as soon as I can.

e sub-woofer cabinet needs to be big enough to house the speaker, amplifiers, and the otherectronics. For the sub driver, I used a 120W 200mm poly-cone, because it was reasonably close tohat I knew would be needed for the job. Selection criteria for the sub are basically ...

Low Vas - this means that a small box can be used, without raising the resonance too much Good efficiency - since the sub is a sealed box, equalisation is needed to get a good low frequ

response. A low efficiency means that too much power is needed Low Fo - The lower the free air resonance, the better. The small sealed box will raise resonan

and if it's too high, excessive equalisation is needed (and an excessive power requirement res High Xmax - the maximum excursion will be quite high, so the cone needs to be able to move

freely 8 Ohms impedance - do not use a 4 ohm driver in this project!

e above may sound like a tall order, but such drivers exist at reasonable prices - I know, 'cos I boue. As with the satellite speakers, there is no point getting the best driver you can - it needs to be ado the job without distorting (too much). Remember that this is a PC speaker system, and makes

etensions at being audiophile hi-fi.

y sub box has an internal volume of about 22 litres - not accounting for speaker displacement and tlume taken up by the electronics, which is quite small. I used 16mm MDF, which is again very stud shows little or no signs of resonating because of the size. The box measures 480(w) x 280(h) x ), and again I rounded the edges - not because of diffraction, but I knew I would use the sub box asotrest, so I wanted comfortable :-)

e box is made with a removable back, secured with lots of screws and sealed with foam tape toevent air leaks. The back carries all the electronics, with the exception of the power transformer - tter is mounted in the main case, as far from the electronics as I could get it. This is important (ifmewhat inconvenient), because the system has high low frequency gain, and hum and buzz from tnsformer is all too easily picked up.



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Figure 3 - Sub-Woofer Enclosure

e enclosure is not critical, but you will need to do something before you start hacking holes in the bemovable!) panel. Bolt the speaker in position. You need to do this anyway to make sure thaterything fits, but in addition, include the fibreglass or other acoustic damping material, and solder auple of thin wires to the speaker. Bring them out through a screw hole and screw the back into thex using half the screw holes. Now you can measure the speaker's resonant frequency in the box.

tach an audio oscillator to the speaker leads. If your oscillator has a low output impedance (such aund card sinewave generator), then you will need a resistor in series - about 560 ohms will do fine

arefully adjust the frequency around the expected resonance frequency (in the vicinity of 50Hz if yoose your woofer wisely). The frequency where the voltage across the speaker terminals is higheste resonant frequency, and you will need this to double check the figures from the Linkwitz transformreadsheet. This represents the unequalised -3dB frequency.

sing the spreadsheet, insert your speaker parameters in the "Box" page, and include the internallume of your cabinet. The spreadsheet will calculate the values you need to properly equalise you

x. In the main page of the spreadsheet, select 30Hz as the -3dB frequency, and select a Q of 0.8 st results. The resistor and capacitor values obtained will be used in the Linkwitz transform circuitard. Have a look at the frequency response graph, and verify that the unequalised -3dB point is wew hertz of the resonant frequency you measured. If it is not, you may need to "fiddle" the numbee until the graph and your loudspeaker measurement agree. Small errors are insignificant, as roooustics will have a much greater effect than an error of a few hertz.

e Electronics



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the low level circuitry is powered from a P05 preamp supply. Note that I included 220 ohm resistoseries with the AC input - these must be 5W wirewound types. Since 35V is the absolute maximum

C input voltage for the 7815 and 7915 regulators, direct connection to the transformer would lead totential over-voltage, and the resistors tame that very nicely. The DC input to the regulators will be

pically about +/-22V with this arrangement.

OTE:- Do not operate the supply without a load from a 25-0-25 volt transformer..

gure 5 shows the power amplifier connections. The P3A (60-100W) amp is wired up using the intedging resistor. An insulated wire link is needed between the output of the Right channel and thedging resistor (R17) on the other. The input of the Left channel should be grounded with a wire line board. Note the 10 Ohm resistor and 100nF cap in the ground lead of the P3A power amp. Thisovides the high frequency ground, but the main earth line is back to the preamp (actually the crossd equaliser). Likewise, a 1 ohm (1W) resistor is in series with the power ground to the preamppply. These resistors were used to ensure continuity, but not allow any ground loops which will caassive hum.

e crossover inputs are the ground reference for the whole amplifier, and the ground is physically

nnected to this point, where the input connectors are in contact with the input panel. This is showngure 4.

Figure 5 - Power Amplifier Connections

efore you complain, I know that the diagram is a jumble, but it actually does make sense :-) All audads should be shielded at the amplifier end only. Do not connect the shields at both ends, or you weate an earth loop, and hum will surely result.



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e P19 50W IC power amp board provides the left and right channel feeds for the satellite speakersese require a connector for the satellite boxes, and although fixed leads can be used, I don't

commend it. All leads should be detachable, as dangling leads are a safety hazard when you arerrying the box - be warned, it is quite heavy when everything is installed.

eatsink selection is very important. Do not skimp on the heatsinks, as this is a high power system, eds as much heatsink as you can afford. Use a separate heatsink for each amp module, and eachould be no smaller than 1C/W. Bigger heatsinks mean cooler running and longer life. The amplif

odules must be tested as described in the construction details before you apply power to the finisheit.

e earthing arrangement is very important, and may need some experimentation to get it right. Asown in these diagrams, it should be OK, but your layout will be different from mine, and this can hainfluence on the final configuration. Note that the mains earth and signal earth must be joined, buly at one place. This is the classic "star" earthing, and must be used to avoid hum loops and otherzzes and noises. I made the star earth point the common connection between all four filter caps, as has proved quite satisfactory. This is also the speaker return point for the satellite speakers - theb-woofer driver is connected to the amp in bridge, so it has no earth return.

shielded wiring should have the shield earthed at one end only. Do not rely on the shield to compe earth connection, as noise will almost certainly be injected into the signal lead.

WARNING

Mains wiring should be carried out by qualified persons only. Do not attempt construction unless suitabqualified. Death or serious injury to yourself or others may result from a seemingly insignificant mistake

nally, Figure 6 shows the power supply in detail. As you can see, it is a dual supply, with only the

nsformer as a common point. This is not absolutely necessary, but does improve power output a d also prevents any possible interaction between the amps. I used an aluminium bracket fabricatem scrap sheet to secure the filter caps and act as a heatsink for the bridge rectifiers (I used 35Adges - you don't have to, but they run cooler and are more reliable than smaller ones). The capace mounted to the bracket using double-sided tape, hot-melt glue and a cable tie - I didn't want themttle, and you won't either.



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Figure 6 - Power Supply

e power supply mains wiring must be performed by a suitably qualified person, and all connectionsust be shrouded with heatshrink tubing to prevent accidental contact. The use of a fused IEC mainnnector is highly recommended for the mains input. The designators "A", "N" and "E" mean Activeot), Neutral (Cold) and Earth (Ground) respectively. If you don't understand this - don't even try to

previously discussed, I suggest a 25-0-25 volt transformer, preferably rated at 300VA. You will neuse an inline plug and socket (mains rated!) for the incoming AC, since the IEC mains input connese and power switch will be on the back panel, and the transformer is mounted in the case. Thecondary leads should be connected using heavy duty cables, and a terminal block to join the

nsformer output to the bridge rectifiers and a separate light duty feed to the preamp supply.

nal Assembly

give you a better idea of how everything fitted together in my system, Figure 7 shows the mechanyout on the rear panel. Yours will probably be a variation on that shown, since the heatsinks must commodated, and yours will be different from mine. Make sure that signal and power leads are weparated, and keep speaker leads well clear of all signal leads, boards and inputs in particular - you

n't need an RF oscillator !



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Figure 7 - Rear Panel Assembly Suggestion

e heatsink for the power amplifiers is shown dotted. This mounts on the outside of the rear panel,ust be securely fastened and sealed. The amplifiers are mounted directly to the heatsink through tles shown (you may only need one hole, depending on your heatsink). The sheet aluminium bracused as a convenient mounting point for the power supply filter caps, and is also the heatsink for thdge rectifiers.

nce everything is tested and operational, you can complete the final assembly. Make sure that notthe rear panel rattles before you screw it into position, and ensure that the box is completely seale

en a small hole will make noises at low frequencies, because of the large cone excursions andernal pressure. When powered on, the system should be silent after the small initial transient from

mps. Careful listening will reveal a slight hiss from the tweeter - this is quite normal.

onnect the system to your PC and hear your music and games as you have never heard them befoe prepared to make more than one unit, as your friends will be green with envy after they hear your

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opyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliottnd is Copyright 2000-2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanicectro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader tght to use this information for personal use only, and further allows that one (1) copy may be made for reference whileonstructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.

ge Created and Copyright Rod Elliott 18 Jan 2001. Updated 17 Jun 2002 (added info about heatsinking), 31 Aug 2004 (changed passier to series).
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