How to set the DPA 220

аSet the R13 toа theа middleа positionа andа theа R23а toа minimum

аresistance. Connect aа signalа generatorа toа theа inputа andа an

аoscilloscope to the output. Take out a fussа inа oneа brancheа of

power supply and connect a current meter instead. Slowly raise the

аsupply voltage above zero and watch the current. Since aboutа +/-

а3а Vа theа ampа shouldа startа toа workа andа thereа shouldааа be

symmetrically limited signal at the output. Ifа theа current аdoes

not grow too high (30 mA), raise the voltage to the full +/- 40а V

and raise the input signal to check the symmetry of limitation.

аIf everything's all right, connect 4/8ohms/100Wа loadа andа raise

the signalа levelа toа 3а dBа underа limitation.а Whenа theа power

transistors grow hot (5 minutes?) set the signal generatorа toа 20

kHz at the output voltage of the amp 1V. Aа distortionа shouldа be

visible on the sinusoid - eliminate it by raising the BIAS current

by R28. A correctly set amp should have a BIAS current about 50 mA

in each branch of power supply.

аThen connect a DC voltage meter to the output and by R13 setа as

small DC output voltage as possible. 10 mV is acceptable, 1 mVа is

possible with a bit of patience. You can check frequencyа response

if you want. In the end you can check the current securing circuit:

аShorten the output with a 0.1 ohm resistor Set theа inputа signal

to 1 kHz/500mV. Slowly raise the supplyа voltageа -а thereа should

appear a sharply limited signal on the output with smallа glitches

on the front edges. If the circuit works as described, you can try

complete short circuitа atа theа nominalа voltageа -а theа current

consumption should be about 3A in each branch of the supply.

My own advice: when you start and set the amp for the first time -

or any otherа circuitа -а beа absolutelyа careful.а Whenа changing

configuration of signal generator, oscilloscope, meters andа load,

ALWAYS switch off the power supply. I even unplug it ever sinceа i

burned a TDA2040. Another practiceа hasа provenа usefulа overа the

years: for the first power-up, replace fuses in both powerа supply

rails with lightbulbs - I'm using regular 100W / 240V bulbs.

аNaturally you mustn't apply load toа theа amp'sа outputа in аthis

setup. If something goes wrong, the lightbulbs start toа glowа and

basically limitа theа currentа flowingа throughа theа circuit.а If

everything's right, the lightbulbs remain cool and the amp behaves

as expected.

аThen you can try to make them glow byа turningа theа bias-current

trimpot. If you succed, you know you have another clueа everything

is working fine. Then you turn the bias current back to zero,а put

the fusesа backа andа youа canа proceedа withа theа aforementioned

procedure. In general, it cost me about a handfulа ofа transistors

to gain this knowledge. Note that the frequency response is a full

graph and measured 1 dB under limitation (=maximum sinus power)а -

when measuring voltage, value in decibels = 20 log (amplification)

- 6 dB means amplification 2x.

The DPA 220 schematic

The DPA 220 low detail schematic (click on the imageа toа seeа the

high detailed version (47 Kb)

Do not try to print this picture in Netscape because it's too big.

T1 to T6 create the input differential stage. The D7 and D8а zener

diodes stabilize at 5V. Theseа areа justа theа simplestа low-power

zeners,а onlyа theyа haveа toа beа coupledа inа toleranceа ofа 200

milivolts, which should not be a problem.а T1а toа T6а areа common

allpurpose low-power transistors withа highа Hfe.а Theseа sixа and

maybe the next four have to be coupled in tolerance of 25%.

The T7 and T8 are fast, switching application types.

T9 and T10 have to be fast and must hold a high voltage, thusа the

best are the "video" types - BF469/470. T15 and T16 areа theа same

types.

The C9, C10 and C15 should stand voltages higher than usual 50 V -

I don't know why.

D3 to D6 can be any silicon type, notа Scottky,а theа onesа listed

below are just allpurpose low-current ones for 150 V. These diodes

should be rather fast - "switching types".

The T11 and T12 stabilize the BIAS current forа theа powerа stage.

T11 also serves as a temperature sensor, andа isа mountedа toа the

cooler of power transistors.

T13 and T14 secure the output current - in cooperation with R38 and R39.

The output transistors used here are Teslaа typesа -а Teslaа isа a

former local devices manufacturer - the pair in each branch can be

replaced with a single power darlington,а likeа BD649/BD650.а They

should have Pc > 150W, Ic > 15A, Uceo > 100V.

In this case obviously the R38+R40 /а R39+R41а mustа beа connected

parallel. These resistors should be able to absorb high power - at

least 2 W, but I'd use 5W ones.

The output filter improves stability of the amp when workingа with

complex impedanceа of аspeakersа -а itа isа quiteа important.а The

resistors are high-power ones again, the coil is 13 turns of a 1.2

mm wire on a 8 mm thorn (diameter). R43 is placed coaxially in the

coil.

The schematic also includes power supply capacitors andа rectifier

- the capacitors' size is not crucial, generallyа theа biggerа the

better. The rectifier originallyа consistsа ofа fourа siliconа 10A

diodes, but you can use whatever you have - rectifier bridgeа etc.

The trafo should be a 2 * 30 V / 7 A type so that you have +/-а 40

V on the power supply capacitors.

In the scheme there's also a thermistor thatа isа supposedа toа be

connected to some additional circuits that secure temperatureа and

other things.

The complementary input stage of DPA amps is an unmistakableа heir

of earlier designs published by Mr. Borbely in severalа issuesа of

Volume 1984 of the Audio Amateur.