Document:

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                                                                    Exhibit 10.4

                                                                  EXECUTION COPY

                                 TRANCHE A NOTE

$5,000,000                                                          MAY 31, 2000

      FOR VALUE RECEIVED, the undersigned, CRL SYSTEMS, INC., a Nevada
corporation (the "Borrower"), hereby unconditionally promises to pay to the
order of ORBAN, INC., a Delaware corporation (the "Lender"), at the Lender's
Payment Office, in lawful money of the United States of America and in
immediately available funds, the principal amount of Five Million Dollars
($5,000,000). The principal amount of this Note shall be payable in the amounts
and on the dates specified in the Credit Agreement. The Borrower further agrees
to pay interest in like money at such office on the unpaid principal amount
hereof from time to time outstanding at the rates and on the dates specified in
the Credit Agreement.

      The holder of this Note is authorized to endorse on the schedule annexed
hereto and made a part hereof or on a continuation thereof which shall be
attached hereto and made a part hereof the date and amount of each payment or
prepayment of principal thereof. Each such endorsement shall constitute prima
facie evidence of the accuracy of the information endorsed. The failure to make
any such endorsement (or any error therein) shall not affect the obligations of
the Borrower in respect of this Note or the Credit Agreement.

      This Note (a) is the Tranche A Note referred to in the Credit Agreement
dated as of even date herewith (as amended, restated, supplemented or otherwise
modified from time to time, the "Credit Agreement"), between the Borrower and
the Lender, (b) is subject to the provisions of the Credit Agreement, and (c) is
subject to optional and mandatory prepayment in whole or in part as provided in
the Credit Agreement. This Note is secured and guaranteed as provided in the
Security Documents. Reference is hereby made to the Security Documents for a
description of the properties and assets in which a security interest has been
granted, the nature and extent of the security and the guarantees, the terms and
conditions upon which the security interests and each guarantee were granted and
the rights of the holder of this Note in respect thereof.

      Upon the occurrence of any one or more Events of Default, all amounts then
remaining unpaid on this Note shall become, or may be declared to be,
immediately due and payable, all as provided in the Credit Agreement.

      All parties now and hereafter liable with respect to this Note, whether
maker, principal, surety, guarantor, endorser or otherwise, hereby waive
presentment, demand, protest, notice of acceleration and intent to accelerate,
and all other notices of any kind.

      Unless otherwise defined herein, terms defined in the Credit Agreement and
used herein shall have the meanings given to them in the Credit Agreement.
<PAGE>   2
      THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED AND INTERPRETED IN
ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK, WITHOUT REGARD TO THE
PRINCIPLES OF CONFLICT OF LAWS THEREOF EXCEPT SECTIONS 5-1401 AND 5-1402 OF THE
NEW YORK GENERAL OBLIGATIONS LAW.

                              CRL SYSTEMS, INC.

                              ________________________________________________
                              C. Jayson Brentlinger, President and Chairman of
                              the Board
<PAGE>   3
                           Schedule to Tranche A Note

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<CAPTION>
<S>                   <C>               <C>                   <C>
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</TABLE><PAGE>   1
                                                                    Exhibit 10.5

                                 TRANCHE B NOTE

$3,500,000                                                          MAY 31, 2000

         FOR VALUE RECEIVED, the undersigned, CRL SYSTEMS, INC., a Nevada
corporation (the "Borrower"), hereby unconditionally promises to pay to the
order of ORBAN, INC., a Delaware corporation (the "Lender"), at the Lender's
Payment Office, in lawful money of the United States of America and in
immediately available funds, the principal amount of Three Million Five Hundred
Thousand Dollars ($3,500,000). The principal amount of this Note shall be
payable in the amounts and on the dates specified in the Credit Agreement. The
Borrower further agrees to pay interest in like money at such office on the
unpaid principal amount hereof from time to time outstanding at the rates and on
the dates specified in the Credit Agreement.

         The holder of this Note is authorized to endorse on the schedule
annexed hereto and made a part hereof or on a continuation thereof which shall
be attached hereto and made a part hereof the date and amount of each payment or
prepayment of principal thereof. Each such endorsement shall constitute prima
facie evidence of the accuracy of the information endorsed. The failure to make
any such endorsement (or any error therein) shall not affect the obligations of
the Borrower in respect of this Note or the Credit Agreement.

         This Note (a) is the Tranche B Note referred to in the Credit Agreement
dated as of even date herewith (as amended, restated, supplemented or otherwise
modified from time to time, the "Credit Agreement"), between the Borrower and
the Lender, (b) is subject to the provisions of the Credit Agreement, and (c) is
subject to optional and mandatory prepayment in whole or in part as provided in
the Credit Agreement. This Note is secured and guaranteed as provided in the
Security Documents. Reference is hereby made to the Security Documents for a
description of the properties and assets in which a security interest has been
granted, the nature and extent of the security and the guarantees, the terms and
conditions upon which the security interests and each guarantee were granted and
the rights of the holder of this Note in respect thereof.

         Upon the occurrence of any one or more Events of Default, all amounts
then remaining unpaid on this Note shall become, or may be declared to be,
immediately due and payable, all as provided in the Credit Agreement.

         All parties now and hereafter liable with respect to this Note, whether
maker, principal, surety, guarantor, endorser or otherwise, hereby waive
presentment, demand, protest, notice of acceleration and intent to accelerate,
and all other notices of any kind.

         Unless otherwise defined herein, terms defined in the Credit Agreement
and used herein shall have the meanings given to them in the Credit Agreement.
<PAGE>   2
         THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED AND INTERPRETED IN
ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK, WITHOUT REGARD TO THE
PRINCIPLES OF CONFLICT OF LAWS THEREOF EXCEPT SECTIONS 5-1401 AND 5-1402 OF THE
NEW YORK GENERAL OBLIGATIONS LAW.

                                   CRL SYSTEMS, INC.

                                   ---------------------------------------------
                                   C. Jayson Brentlinger, President and
                                     Chairman of the Board
<PAGE>   3
                           Schedule to Tranche B Note

<TABLE>
<CAPTION>
     Date of Principal              Amount of              Unpaid Principal
      Payment on Note             Principal Paid            Balance of Note            Notation Made By
<S>                               <C>                      <C>                         <C>

</TABLE><PAGE>   1
                                                                    Exhibit 10.6

                                    AGREEMENT

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

      1.   Lender shall extend the Tranche B Maturity Date to November 30,
           2000.

      2.   The Tranche A Note and the Tranche B Note shall bear interest
           beginning October 1, 2000 at the rate of 12% per annum. Upon receipt
           by Lender of full payment of the Tranche B Note, together with all
           accrued but unpaid interest thereon, the Tranche A Note shall, from
           and after the date of such payment in full, bear interest at the rate
           of 8% per annum.

      3.   Interest on the Tranche A Note and the Tranche B Note shall be
           payable bimonthly in arrears on the 15"' and the last day of each
           calendar month.

      4.   Borrower shall pay an Extension Fee (the "EXTENSION FEE") equal to
           $150,000 by deduction of funds due Borrower in the Chase Account
           or Bank of America Account, to the extent sufficient funds are
           available in such account(s), and thereafter, by wire transfer to
           such account(s) by Borrower or the Company until sufficient funds
           are available, payable as follows - $50,000 on October 1, 2000,
           $50,000 on October 30, 2000 and $50,000 on November 15, 2000. In
           the event Lender receives a principal payment of at least $2
           million on the Tranche B Note prior to November 15, 2000, any
           unpaid portion of the Extension Fee not due prior to or as of the
           date of such principal payment shall be waived.  The Extension Fee
           shall not be applied towards or reduce the Obligations.

      5.   Lender will have the right, exercisable by written notice, to
           require Circuit Research Labs, Inc, (the "Company") to register,
           in accordance with the provisions of the Securities Act of 1933,
           as amended, any or all of the shares of Common Stock, par value
           $.10 per share of the Company ("COMMON STOCK") underlying warrants
           for Common Stock held by Lender as of the date hereof, and the
           Company will bear all costs and expenses associated with such
           registration.  The Company will use its best efforts to file a
           registration statement as promptly as possible upon receipt of
           written notice from Lender to do so.  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.

      6.   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.

      7.   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.
<PAGE>   2
      8.   THIS 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.

      9.   Except as specifically set forth in this Agreement, the terms of the
           Loan Documents and the 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.

      10.  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.

      11.  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.

      12.  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 September 29, 2000.

                                    CRL SYSTEMS, 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

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