<PAGE>   1
                                                                  Exhibit 10.4.3

                            THIRD EXTENSION AGREEMENT

         Reference is made to that certain Credit Agreement by and between CRL
Systems, Inc. ("Borrower") and Harman Acquisition Corp. (f/k/a Orban, Inc.)
dated May 31, 2000 (the "CREDIT AGREEMENT").

         1.       Lender shall extend the Tranche B Maturity Date to February
                  20, 2001, and, provided Lender has received payments reducing
                  the aggregate principal amount owing under the Tranche B Note
                  by at least $2.2 million by such date, shall thereafter extend
                  the Tranche B Maturity Date to such time as Lender determines
                  is reasonably necessary for Circuit Research Labs, Inc. ("the
                  Company") to satisfy its obligations under paragraph 2 below.

         2.       The Company will, as promptly as possible after the date
                  hereof, register, in accordance with the provisions of the
                  Securities Act of 1933, as amended, at least 714,158 shares of
                  Common Stock, par value $.10 per share of the Company ("Common
                  Stock") underlying warrants for Common Stock held by
                  accredited investors who purchased such warrants pursuant to
                  Subscription Agreements entered into on or prior to May 31,
                  2000. The Company will use its best efforts to cause such
                  registration to become effective and keep such registration
                  statement effective for a period of not less than 180 days.

         3.       Failure by Borrower or the Company of any of its obligations
                  under this Agreement shall be deemed an Event of Default under
                  the Credit Agreement.

         4.       Borrower agrees that the Obligations are valid and enforceable
                  obligations of Borrower and hereby confirms, acknowledges and
                  ratifies the existence of the Obligations and Borrower's
                  obligations to Lender with respect thereto as set forth in the
                  Credit Agreement, and all other obligations of Borrower and
                  the Company to Lender under the Agreement among the parties
                  hereto dated as of September 29, 2000 (the "FIRST EXTENSION
                  AGREEMENT"), the agreement among the parties hereto dated as
                  of November 28, 2000 (the "SECOND EXTENSION AGREEMENT"), and
                  the Loan Documents.

         5.       THIS AGREEMENT, THE FIRST EXTENSION AGREEMENT, THE SECOND
                  EXTENSION AGREEMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE
                  FINAL AGREEMENT OF THE BORROWER AND THE LENDER WITH RESPECT TO
                  THE SUBJECT MATTER HEREOF, AND THERE ARE NO PROMISES,
                  UNDERTAKINGS, REPRESENTATIONS OR WARRANTIES BY THE LENDER
                  RELATIVE TO THE SUBJECT MATTER HEREOF NOT EXPRESSLY SET FORTH
                  OR REFERRED TO HEREIN OR IN THE OTHER LOAN DOCUMENTS.

         6.       Except as specifically set forth in this Agreement, the terms
                  of the First Extension Agreement, the Second Extension
                  Agreement, the Loan Documents and the
<PAGE>   2
                  Warrant of the Company dated May 31, 2000 held by the Lender,
                  will remain in full force and effect and are hereby ratified
                  and confirmed.

         7.       This Agreement will be deemed to be a contract made under the
                  Laws of the State of New York and for all purposes will be
                  governed by and interpreted in accordance with the laws
                  prevailing in the State of New York, without regard to
                  principles of conflict of laws.

         8.       This Agreement may be executed in several counterparts each of
                  which when so executed will be deemed to be an original and
                  all of which will together constitute one and the same
                  agreement.

         9.       Capitalized terms used but not defined in this Agreement have
                  the meanings ascribed to them in the Credit Agreement.

         IN WITNESS WHEREOF Borrower and Harman Acquisition Corp. execute this
Agreement as of January 18, 2001.

                                       CRL SYSTEM, INC



                                       By: /s/ Charles Jayson Brentlinger
                                          --------------------------------------
                                       Name: Charles Jayson Brentlinger
                                            -----------------------------------
                                       Title: President, CEO, Chairman
                                             ----------------------------------


                                       HARMAN ACQUISITION CORP.


                                       By: /s/ Frank Meredith
                                          ------------------------------------
                                       Name: Frank Meredith
                                            --------------------------------
                                       Title: CFO and Secretary
                                             ---------------------------------


Agreed to and accepted by

CIRCUIT RESEARCH LABS, INC.


By: /s/ Charles Jayson Brentlinger
   ---------------------------------------
Name: Charles Jayson Brentlinger
     -----------------------------------
Title: President, CEO, Chairman
      ----------------------------------




                                        2