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So you need a preamp - here are your options

Started by joecool85, April 25, 2006, 08:47:57 PM

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I have a few different setups myself , however , here is the real deal a power amp requires 1 to 2.6 volts to operate correctly. most of the new stompbox multi effect monsters(Roland , Digitech) can put out up to 2 volts , plenty to drive any good power amp.

                                     Rock On


From the NatSemi datasheet;

Low Voltage Audio Power Amplifier

General Description
The LM386 is a power amplifier designed for use in low voltage
consumer applications. The gain is internally set to 20 to
keep external part count low, but the addition of an external
resistor and capacitor between pins 1 and 8 will increase the
gain to any value from 20 to 200.

The inputs are ground referenced while the output automatically
biases to one-half the supply voltage. The quiescent
power drain is only 24 milliwatts when operating from a 6 volt
supply, making the LM386 ideal for battery operation.

n Battery operation
n Minimum external parts
n Wide supply voltage range: 4V–12V or 5V–18V
n Low quiescent current drain: 4mA
n Voltage gains from 20 to 200
n Ground referenced input
n Self-centering output quiescent voltage
n Low distortion: 0.2% (AV = 20, VS = 6V, RL = 8W, PO =
125mW, f = 1kHz)

note: comes in two versions;
low LM386N, M-1 12Vsup/0.75Wout, and
high LM386M-4 18Vsup/1.5Wout.

Vsup = 12V
Voutpk = 6V

Av = +26dB (from datasheet)
dB = 20 * log10(Av)
10^(26/20) = 19.95262315 times

Gain = x20

6/20 = 0.3V or 300mVpk in for full output.
If you say theory and practice don't agree you haven't applied enough theory.


Could I get some guidance on how to select 'equivalent' transistors, for example where a J201 or MPF102 is specified but not available. Apart from just googling 'J201 equivalent', is there way to select based on specs in the data sheet? At present I am planning a preamp to go in front of my TDA2003 amp.

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The first thing you need to know about JFETs is that the specs can vary quite allot. So much so, that it is hard to design a circuit that will operate for any JFET that meets the specs you find on a data sheet. The usual fix for this is one or two trim pots.

On the vast majority of JFETs, you can reverse connections to the Drain and Source with no change in performance.

The first thing to look at is Gate breakdown Voltage. BV(GSS) is a term you might see, it means Breakdown Voltage, Gate to Source with Drain shorted to Source. For a J201, the number is 40V. For the MPF102 the number is 25V. Most circuits don't provide any input protection and a big negative input Voltage will damage the JFET. Sorry, that's the way it is.

The next thing to look at is called I(DSS). It means Drain Current with Gate and Source shorted (to ground). It's usually specified at some Drain Voltage like 5 or 10 Volts. For the J201 the spec is 0.2 to 1.0 mA. For the MPF102 the spec is 2.0 to 20 mA. You have to look at the circuit the part will operate in, but generally try to keep the minimum I(DSS) of the replacement at or above the minimum of the part to be replaced. Try to keep the max I(DSS) less than twice the max value of the part to be replaced.

The orientation of the pins may be different for the replacement part. You will have to check this. J201 and MPF102 have the Gate at one end of the package. Same place the Collector would be on a 2N3904. Most Euro parts have the Gate in the middle. Keep in mind that you can swap Drain and Source. This can be a real PITA if the layout uses inline leads instead of a triangular pattern.

There are lots of other specs like gain and leakage, don't lose any sleep over them, just get some parts to try and be done with it. Beware of fake parts from China, but they might actually work.


Thanks for that. I think this one may fall into the correct range.

I note that the datasheet says , it used for '... low level switching...chopper-stablilised amplifiers...' etc. DOes that have a bearing on my intended use?

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these are the ones I've tried without success but maybe I need to mess around a bit more


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The J109 would make on OK low level amplifier, but is a challenge to bias and has almost zero chance of working in any J201 or MPF102 circuit without substantial changes. Notice that there is no maximum given for Idss. The parts will melt or explode if you try to measure Idss with DC. This is typical on switching type JFETs. They depend more on R(on), channel resistance.

The J175-77 are P channel. I've used them where I needed a P channel, but they are weird.


Thanks for that. Think I'll just get some MPF102 and J201's as recommended.

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OK I built up a buffer with MPF102. Definitely brightens the sound compared to going directly into my Kemo 40w power amp but less so in front of the TDA2003 or the LM386. This is using a 6" speaker, and fairly short guitar cables. Now to try it via my 1x12 and longer cables.

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