From: fdeck@bashful.helios.nd.edu (francis deck)
Subject: Amp Plans
Summary: Simple audio amplifier, 10 W into 8 Ohms 
Organization: OUC, University of Notre Dame
Date: Fri, 22 Jan 1993 17:22:27 GMT

                  PLANS FOR SINGLE-CHIP BASS PRACTICE AMP

                              Francis J. Deck
                         fdeck@grumpy.helios.nd.edu


INTRODUCTION

     I have built a little amplifier which is totally simple and bare-
     bones, but which has good sound quality and costs under $30 in parts,
     excluding speaker.  Power is rated at 10 W RMS into 8 Ohms.

     The amplifier is based on a Signetics TDA1515 automotive power
     amplifier chip, with enough gain to amplify the low-level signal from
     an instrument pickup.  There is an optional volume control, which
     should not be necessary if your instrument has a volume knob.

     I have designed a simple add-on compressor/limiter for this circuit,
     and will post the plans soon.  Use of the limiter prevents "clipping,"
     and thus eliminates the need for headroom in most situations.  The
     result is an increase in the useful power of the amplifier by as much
     as 10 dB.

     Beware that, with any homemade electronic equipment, you are
     responsible for making sure that what you build is safe.  I cannot
     guarantee the safety of this device because I cannot predict what
     construction techniques will be used.  For beginners, please stick
     with the wall-wart power supply.

SCHEMATIC DIAGRAM

                                TDA1515
                    VC        ------------
                    |        |            |1
                    R1       |            |-------------------------
                    |       2|            |6    +                   |
INPUT o-------C1-------------|            |------C3----             |
                             |            |5           |------R1----|
  GND o---G                  |            |------------|    |       |
                    VC       |            |            |    R3      R2
                    |        |            |            S    |       |+
                    R1       |            |9           |    C1      C4
                    |      12|            |------------|    |       |
      G-------C1-------------|            |8    +      |------R1----|
                        +   4|            |------C3----             |
                 G----C4-----|            |13                       |
                            3|            |-------------------------|
                 VC----------|            |7                        |
                       |+    |            |----G      VS            R2
                       C3    |            |10         |             |
                       |     |            |-----------|             C2
                       G     |            |11         |             |
                             |            |-----------|             G
                              ------------            |+
                                                      C3
                                                      |
                                                      G

SCHEMATIC--POWER SUPPLY (Omit for car battery power)

                Bridge Rect.
                   ------
12 VAC    o-------|~    +|--------------------VS
1 A               |      |           |+
WALL XFMR o-------|~    -|---G       C5
                   ------            |
                                     G

OPTIONAL VOLUME CONTROL

Input  o-----
             |
             R4<----o To input of above circuit
             |
             G 

LEGEND

     VC = midtap voltage--connect all VC's together
     S  = speaker, 8 Ohms--don't ground either terminal
     +  = positive terminal of electrolytic capacitor
     G  = ground

CONSTRUCTION TIPS

     You must bolt the TDA1515 to a heat sink.  Don't power the circuit up
     without the heat sink--it generates enough quiescent power to burn
     itself up.  A metal chassis box or panel provides sufficient heat
     sinking for this circuit.  Keep all leads very short.  Pin 1 of the
     TDA1515 is identified by a stripe on the chip.  The pins are numbered
     as if they are all straightened out in a single row.

     The gain of the amplifier is deterimined by the resistor which goes
     between pins 1 and 13 (in series with the capacitor).  Reducing the
     resistance increases the gain proportionately.  The recommended
     minimum value is 330 Ohms.  You will then need to replace the 10 uF
     capacitor with 100 uF.

PARTS LIST

     I have chosen a single supplier for these parts, but you may be able
     to scrounge around and get better prices.  If you build the limiter,
     you can get some of these parts from Hosfelt Electronics, and split
     the bill up.  No one, however, has better service than:

          Digi-Key Corporation
          701 Brooks Ave. South
          P.O. Box 677
          Thief River Falls, MN 56701
          1-800-DIGI-KEY

     Part Quan Description         Cat #     Price
     ---- ---- -----------         ------    ----
          1    TDA1515 IC          TDA1515AU  6.52
          1    2 A bridge rect.    2KBP005M   1.37
     C1   3    0.1 uF              EF1104      .25 ea.
     C2   1    330 pF              P4030       .71/10
     C3   4    100 uF              P6252       .21 ea.
     C4   2    10 uF               P6248       .12 ea.
     C5   1    15000 uF            P6510      5.19
     R1   4    100k                100KQ       .26/5
     R2   2    1.8k                1.8KQ       .26/5
     R3   1    2.7 Ohm             2.7Q        .26/5
          1    12 VAC 1 A XFMR     T608-ND    6.35
          1    1/4" phone jack     SC1085-ND  1.80
                                             -----
                                   TOTAL:    $24.55

REFERENCE

     The circuit design was taken from the Signetics _Linear_Data_Manual_,
     Vol. 1, with some minor component substitutions.
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