Schematic diagram:

Pre-Amp + Tone Control Components List:

Resistors:
R1, R2__________________ 220R
R3, R4__________________ 4K7
R5______________________ 2K2
R6______________________ 1K

Capacitors:
C1, 2, 7, 8, 17_________ 10 uF ecap
C3, 4___________________ 47 nF
C5, 6___________________ 15 nF
C9______________________ 220 nF poly
C10_____________________ 100 uF 25V
C11, 12, 13, 14, 16_____ 100 nF
C15_____________________ 1000 uF 35V
C18, 19_________________ 10 nF

Misc.:
IC1_____________________ TDA 1524A
IC2_____________________ LM 7812
P1______________________ 50k linear switch pot
P2, 3, 4________________ 50k linear pot
X1, 2, 3, 4_____________ RCA jack
D1______________________ Diode 1N4004
L1______________________ Red LED

Circuit Works:

All signal processing is done within the TDA1524A by voltage controlled amplifiers and voltage controlled filters. The IC provides a fixed voltage (~ 3.8V DC) at pin 17, and this is used by all the variable resistors to provide an adjustable DC voltage to the appropriate control pins.

Current sensing is used to provide a flat response when R5 is connected to pin 17, and a loudness contour when disconnected. 100 nF capacitors are used on each pot to decouple any AC signals from the control inputs. 10 uF capacitors are used to couple both input and output audio signals whilst blocking DC. R1 and R2, are to ensure stability with capacitive loads. R3 and R4 make sure there are no DC spikes at the output sockets if the load is switched. C3 and C4 control the loudness contour. C5 and C6 control the treble turn-over frequency. C18 and C19 have been added to roll off the gain above 70 kHz. Low volume settings coupled with treble boost was causing HF instability in some instances. This should no longer be a problem.

C15, 16, 17 provide power supply filtering. D1 provides protection in case of incorrect supply polarity. The LED is a power on indicator and may be omitted if not required, or preferably mounted on the enclosure. If you are not using a switch pot, you can connect an external switch across the P1 switch pins, or connect a wire link there and switch the power supply.

The power supply is critical to the noise? performance of the pre-amp. An on board regulator is provided to reduce mains hum. If you wish to use it with a car or other 12V battery, then you should omit the 7812 regulator, and place a wire link between the regulator input and output pin positions on the PC board. Do not short to earth! This will be necessary because the regulator must have an input voltage at least 2-3V greater than it”s output, for it to maintain regulation. However the regulator will not be necessary with a battery supply.

If using a plug pack, it”s output voltage should be 15 to 18V DC. Because most plug packs have poor regulation, one rated at 12V DC will often be around 15V when lightly loaded. The current drain of the pre-amp is less than 50 mA, so many 12 V unregulated supplies may be adequate if you have one. Replace D1 with a wire link if necessary, making sure you have the supply polarity correct!

If you are using a 15-20V supply for your power amplifier, you can use that as your pre-amp supply as well. Make sure you test the voltage first in all cases.

Download the Pre-Amp + Tone Control Circuit Manual in PDF:

Preamp and Tone Control with TDA1524A Manual

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Download TDA1524A Stereo-tone/volume control circuit:

TDA1524A Datasheet

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Download Datasheet

Here the response of the preamp’s frequency and phase:

Here the diagram of total noise at output, measured with 600R load:

circuit by Graham Maynard

schematic diagram:

The leftmost 10k resistor supplies plug-in-power to the electret, forming part of the FET amplifier in the electret capsule. This could be anything from 2k to 10k, the higher the better the stereo separation (another mic derives bias from the same rail). Apparently higher values also lower distortion, and the best bias power circuits involve actually breaking a trace on the electret capsule to allow the use of both a drain & source resistor, but I”m not going that far.

The leftmost 2.2uF cap blocks the bias voltage from the input. In conjunction with the following 27k resistor it forms a high pass filter, but cutoff is essentially near DC.

The input impedance is set by the two 27k resistors and the 10k resistor. The +ve rail is also connected to ground as far as the AC signal is concerned because of the power supply cap. So there are two 27k resistors in parallel, making 13.5k, in parallel with the 10k, making about 6k or so for the input impedance. But if you’re making it proper dual supply, you don’t need the upper 27k resistor, as the input doesn’t have to be biased mid rail anymore.

The feedback loop has two resistors 27k & 1k5 from the inverting input to ground. When they are both in circuit, the gain is a bit under 2 ((28.5/33)+1). The 27k resistor can be bypassed with a switch, then only the 1k5 sets the gain, to 23 ((33/1.5)+1).

The 10uF cap in the bottom half of the feedback loop reduces DC gain to ~1. The value isn’t very important. If any DC input offset were amplified it would create a larger output offset, pushing the output toward one of the rails and reducing headroom. (At a gain of 23 with the expected input levels it probably doesn’t matter.)

The optional 2pF cap in relation to the 33k resistor sets the high frequency rolloff. The cutoff frequency is in the 100″s of kHz. It has to go further than 20kHz to keep the phase shift at audio frequencies small, and also because output starts falling long before cutoff. The op-amps cannot maintain enough gain at these frequencies anyway and their output will already be falling, but the cap makes the circuit more stable, though it will probably work without it. There will probably be 2pF of capacitance just from the PCB traces, and op-amps tend to be fairly well compensated these days so it”s really not needed. I think in retrospect this cutoff frequency should be much lower, say 30kHz-50kHz.

The 100ohm resistors are there partly to limit current to protect the op-amp if the output is shorted, but the op-amps have internal protection anyway. They mainly allow the op-amp to drive capacitive loads (long/cheap cables) without oscillation.

The 2.2uF cap on the output blocks DC and the value is not specially important. It forms a highpass filter with the 10k pot, the cutoff is virtually at DC.

If you think you might accidentally start connecting the battery the wrong way round, you’d better put a diode in series with the battery clip, or you’ll smoke your ic. Put your IC in a socket too just in case you do want/need to change it. You could try several dual op-amps against each other, they’re all direct plug in replacements.

PCB layout:

Low Impedance Microphone Amplifier Component list:

PCB layout (bottom):

PCB layout (top-component placement):

Good luck with this low impedance microphone amplifier circuit project