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Passive RIAA Preamplifier(BC546)

2015-01-06 20:41  
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This article describes Passive RIAA Preamplifier (BC546). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: BC546.

There are two types of preamplifiers for magnetic phono cartridges. An example of the most common type is the one described in the March 2002 issue ofSILICONCHIP. It has theRIAAequalisation network in the feedback loop. The second type was previously used in valve circuits which typically had no feedback loop and used passive RC networks to provide the phono equalisation. This experimental preamp was put together using inexpensiveFETs to compare the performance of these two types of preamp. The first stage, consisting of Q1 and Q2, is a simpleFETaudio amplifier, where theFETs are connected in parallel to reduce noise. This is followed by a passiveRIAAnetwork consisting of 240kO and 15kO resistors and the associated 0.1OF .022OF and .0047OF capacitors.

Circuit diagram:

Figure:1 Passive RIAA Preamplifier Circuit Diagram

Figure 1 PassiveRIAAPreamplifier Circuit Diagram

Some of the gain loss in the passive network is then made up byFETQ3. It also has a 51kO drain resistor and is buffered by bipolar transistor Q4 which is connected as an emitter-follower stage. All resistors are 1% tolerance metal film type while the capacitors for equalisation areMKTpolyester types. Ideally, the Idss of allFETs should be matched for both channels. Resistors R3 and R8 should be adjusted so that the drain voltage in each stage is between 13V and 14V, to give symmetrical signal clipping. The power supply can be three 9V batteries connected in series. Current consumption is only 3mA for the stereo circuit.



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