NELSON PASS 5 Schematic

voltage gain and a real damping factor.

Ampli ers that have low distortion and noise, and will drive a 4 ohm load.

you will  nd a pair of RCA inputs, speaker outputs, a fuse holder, an AC power receptacle, and on/off switch.

A caveat is in order here – this is a very wide band ampli er with a high input impedance. In order to prevent the output

voltage from bleeding back to the input at very high frequencies (thus making a  ne power oscillator), keep the input and

With everything connected up and the source equipment powered up  rst, you can proceed to turn on the power switch

At this point you should be able to listen to music. This ampli er has less gain than most (15 dB), but at 25 watts, it’s not

AC line voltage to ground until the fuse blows in case of transformer or other such failure.

Again, the heat sinks on this ampli er run fairly hot, and you want to make sure that they get adequate ventilation. They will

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Damping Factor

Voltage Gain

Warranty: Parts and labor for 3 years, not covering shipping costs or any sort of consequential damages. Warranty

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As many of you may know, First Watt is dedicated to exploring the performance quality of small simple power ampli ers.

You can follow the progression at

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A Quickie Tutorial on FET Ampli ers

will be helpful to repeat bits of it. All 31 years worth can be found at

and related links.

you can skip ahead.

function as a valve, a little bit like a water faucet. In this picture, the Drain (D) of the FET is attached to an electrical power

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Yes, I know some of you are thinking that maybe the Source should be on top and the Drain on the bottom, but they’re not.

You might not want to call a quantum mechanic if your plumbing stops up.

A FET is a three pin device, and there are three ways to amplify with them. Figure 2 illustrates these with an N channel

why it’s called Common-Source. Note that the output voltage is inverted in phase from the input voltage.

value, the AC voltage is ideally zero, and so it is called Common-Drain.

with respect to ampli ers.

voltage at 0 or slightly negative with respect to the Source, and for N channel MOSFETs the gate voltage is a couple volts

We put 47 ohms in series with the Source, raising its voltage to +.25V and this conveniently allows us to bias the Gate at 0V

voltage appearing from the Gate to Source of these devices is about 3.6 volts, and this biases the MOSFETs to about 1.3

voltage rail to the V- voltage rail, and none is going through the loudspeaker.

When a positive voltage appears at the Gates of Q1 and Q2, it makes the current through Q1 increase and the current

Actual Working Circuit

You never know what the ampli er will be attached to, so the input of the ampli er now sports R9 to avoid parasitic

We can stop here, and the ampli er will be fully functional. The remaining additions will enhance temperature tracking and

when their Base to Emitter voltage exceeds .4 volts or so. At .4 volts Q5 and Q6 draw off enough drive current from Q1 and

Where do we want to limit? The ampli er with 24 supply rails should be good for a 50 watt peak into 8 ohms, or about 2.5

We will calculate the values as follows, starting with arbitrary values of 1 Kohm for R17 and R19. At 10 amps peak, the

voltage across R11 or R12 will be about 4.7 volts. R19 and R21 will be chosen to divide that down to .6 volts, and we solve

And we get R19 = 150 ohms

Audiophiles are often concerned about the effects of capacitors in audio circuits, but a quick examination of the schematic

will put your concerns to rest.

At 1.3 amps per channel, you will see idle heat dissipation of 62 watts. To keep the temperature rise of the heat sink to

with a 1A fast blow fuse. Turn the Variac up slowly, and if you haven’t popped the fuse, then go ahead and con rm the rail

voltages to the channels.

would advise against using it during adjustment. For each channel you will be adjusting P1 and P2 alternately in order to

values across R11 and R12 will be equal.

You should be able to get the output DC offset down to 10 mV or so, and I would consider 50 mV the highest acceptable

 gure for this ampli er when warmed up. After the ampli er has been operated for a few weeks, it is a good idea to check

harmonic, as well as 2

th

and

th

You can additionally limit frequency response with an input capacitor across R10, and also in the feedback loop with

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