<PAGE>
                                                                   Exhibit 10.20

                    AMENDMENT NO. 3 dated as of December 12, 2001 (this
               "Amendment"), to the Credit Agreement dated as of June 28, 1999,
               as amended and restated as of July 21, 1999 and as amended on
               December 23, 1999 and on April 10, 2000 (the "Credit Agreement"),
               among Knowles Electronics Inc., a Delaware corporation (the
               "Parent Borrower"); the financial institutions party thereto as
               Lenders (the "Lenders"); The Chase Manhattan Bank, as
               administrative agent (in such capacity, the "Administrative
               Agent") and Morgan Stanley Senior Funding, Inc., as Syndication
               Agent.

     A.   The Lenders and the Issuing Bank have extended credit to the Borrower,
and have agreed to extend credit to the Parent Borrower, in each case pursuant
to the terms and subject to the conditions set forth in the Credit Agreement.

     B.   The Parent Borrower has requested that the Credit Agreement be amended
as set forth herein. The Required Lenders are willing to amend the Credit
Agreement pursuant to the terms and subject to the conditions set forth herein.

     C.   Capitalized terms used but not defined herein have the meanings
assigned to them in the Credit Agreement as amended hereby.

     Accordingly, in consideration of the mutual agreements herein contained and
other good and valuable consideration, the sufficiency and receipt of which are
hereby acknowledged, the parties hereto agree as follows:

     SECTION 1. Amendment. The Credit Agreement is hereby amended as follows:

     (a)  The table in the definition of the term "Applicable Rate" in Section
1.01 of the Credit Agreement is replaced with the following table:

<Table>
<Caption>
-----------------------------------------------------------------------------------------------------------------
                                                   Eurodollar                          Eurodollar
                              ABR Revolving/       Revolving/       ABR Tranche B      Tranche B       Commitment
Leverage Ratios:             Tranche A Spread   Tranche A Spread       Spread           Spread          Fee Rate
-----------------------------------------------------------------------------------------------------------------
<S>                          <C>               <C>                 <C>               <C>              <C>
Category 1
Greater than or equal to
  4:50 to 1:00                   2.00%              3.00%              3.00%            4.00%            0.50%
-----------------------------------------------------------------------------------------------------------------
Category 2
Greater than or equal to
  3.75 to 1.00 and less than
  4.50 to 1.00                   1.75%              2.75%              3.00%            4.00%           0.375%
-----------------------------------------------------------------------------------------------------------------
Category 3
Greater than or equal to
  3.00 to 1.00 and less than
  3.75 to 1.00                   1.50%              2.50%              3.00%            4.00%           0.375%
-----------------------------------------------------------------------------------------------------------------
Category 4
Greater than or equal to
  2.25 to 1.00 and less than
  3.00 to 1.00                   1.25%              2.25%              3.00%            4.00%           0.375%
-----------------------------------------------------------------------------------------------------------------
Category 5
Less than 2.25 to 1.00           1.00%              2.00%              3.00%            4.00%           0.375%
-----------------------------------------------------------------------------------------------------------------
</Table>

<PAGE>
                                                                               2

          (b)  The definition of the term "Consolidated EBITDA" in Section 1.01
of the Credit Agreement is hereby amended by (i) deleting the word "and" at the
end of clause (a)(iii) thereof and substituting in lieu thereof a comma and
(ii) adding, following the words "for such period" appearing at the end of
clause (a)(iv) thereof, the following: "and (v) non-cash charges relating to the
sale of RufElectronics Gmbh,".

          (c)  The amount "$25,000,000" appearing in clause (i) of the proviso
of the definition of the term "Permitted Acquisition" in Section 1.01 of the
Credit Agreement is replaced with the following: "(x) on or prior to March 30,
2003, $5,000,000 or (y) on or after March 31, 2003, $25,000,000".

          (d)  Section 2.01 of the Credit Agreement is amended by (i) deleting
the comma at the end of clause (b) of the first sentence thereof and
substituting in lieu thereof the word "and" and (ii) deleting from the end of
the first sentence thereof the following:

     and (d) to make Multicurrency Revolving Loans to any Borrower from time to
     time during the Multicurrency Revolving Availability Period in an aggregate
     principal amount that will not result in such Lender's Multicurrency
     Revolving Exposure (determined base on Assigned Dollar Value, in the case
     of Alternative Currency Loans) exceeding such Lender's Multicurrency
     Revolving Commitment

          (e)  Clause (a) of Section 2.04 of the Credit Agreement is amended by
(i) deleting the number "(i)", (ii) deleting the letter "(A)" and substituting
in lieu thereof the number "(i)", (iii) deleting the letter "(B)" and
substituting in lieu thereof the number "(ii)" and (iv) deleting the following:

     and (ii) Multicurrency Swingline Loans to any Borrower from time to time
     during the Multicurrency Revolving Availability Period, in an aggregate
     principal amount at any time outstanding that will not result in (A) the
     aggregate principal amount of outstanding Multicurrency Swingline Loans
     exceeding $5,000,000 or (B) the sum of the total Multicurrency Revolving
     Exposures (determined based on Assigned Dollar Values, in the case of
     Alternative Currency Loans, Alternative Currency Letters of Credit and
     Alternative Currency LC Disbursements) exceeding the total Multicurrency
     Revolving Commitments

          (f)  Clause (a) of Section 2.05 of the Credit Agreement is amended by
(i) deleting the number "(i)", (ii) deleting the following:

     and (ii) each Borrower may request the issuance of Multicurrency Letters of
     Credit for its own account, in a form reasonably acceptable to the
     Administrative Agent and the Issuing Bank, at any time and from time to
     time during the Multicurrency Revolving Availability Period and prior to
     the date that is five Business Days prior to the Revolving Maturity Date

and (iii) deleting the following:

     Multicurrency Letters of Credit shall be denominated only in Dollars or an
     Alternative Currency.

          (g)  Clause (b) of Section 2.05 of the Credit Agreement is amended by
(i) deleting the following:




<PAGE>
     whether such Letter of Credit is a Domestic Letter of Credit or a
     Multicurrency Letter of Credit,

(ii), deleting the following:

     in the case of a Multicurrency Letter of Credit, whether such Multicurrency
     Letter of Credit is to be denominated in Dollars, Euro or Sterling,

and (iii) deleting the following:

     and (ii) in the case of the issuance, amendment, renewal or extension of a
     Multicurrency Letter of Credit, (A) the Multicurrency LC Exposure
     (determined based on Assigned Dollar Values, in the case of Alternative
     Currency Letters of Credit and Alternative Currency LC Disbursements) shall
     not exceed $5,000,000 and (B) the total Multicurrency Revolving Exposures
     (determined based on Assigned Dollar Values, in the case of Alternative
     Currency Loans, Alternative Currency Letters of Credit and Alternative
     Currency LC Disbursements) shall not exceed the total Multicurrency
     Revolving Commitments. In the case of an Alternative Currency Letter of
     Credit, (i) on the Denomination Date for such Letter of Credit, the Issuing
     Bank shall determine the Alternative Currency Equivalent of the Dollar
     amount of such Letter of Credit specified in the applicable notice
     requesting such Letter of Credit, (ii) the Issuing Bank shall notify the
     applicable Borrower and the Administrative Agent of the Alternative
     Currency Equivalent so determined and the Administrative Agent shall notify
     each Multicurrency Revolving Lender of its participation therein as
     denominated in the applicable Alternative Currency in accordance with
     paragraph (d) of this Section and (iii) such Letter of Credit shall be
     issued, amended, renewed or extended in the applicable Alternative Currency
     in a principal amount equal to the Alternative Currency Equivalent so
     determined.

          (h) Section 6.05 of the Credit Agreement is hereby amended by deleting
the words "this clause (c) shall not exceed $25,000,000 during any fiscal year
of the Parent Borrower or $50,000,000 in the aggregate during the term of this
Agreement;" appearing in clause (d) thereof and substituting in lieu thereof the
following:

     this clause (d) shall not exceed $10,000,000 during any fiscal year of the
     Parent Borrower or $35,000,000 in the aggregate during the term of this
     Agreement;

          (i) The table in Section 6.12 of the Credit Agreement is replaced
with the following table:

<Table>
<Caption>
          Period                                          Ratio
          ------                                          -----
<S>                                                    <C>
June 30, 2001 through December 30, 2001                1.60 to 1.00
December 31, 2001 through March 30, 2002               1.55 to 1.00
March 31, 2002 through June 29, 2002                   1.60 to 1.00
June 30, 2002 through September 29, 2002               1.65 to 1.00
September 30, 2002 through December 30, 2002           1.75 to 1.00
December 31, 2002 through March 30, 2003               1.85 to 1.00
</Table>
<PAGE>
                                                                               4

March 31, 2003 through September 29, 2003              2.25 to 1.00
September 30, 2003 through March 30, 2004              2.25 to 1.00
March 31, 2004 through March 30, 2005                  2.50 to 1.00
March 31, 2005 through March 30, 2006                  2.75 to 1.00
March 31, 2006 through March 30, 2007                  3.00 to 1.00
March 31, 2007 and thereafter                          3.25 to 1.00

           (j) The table in Section 6.13 of the Credit Agreement is replaced
with the following table:

             Period                                       Ratio
             ------                                       -----

June 30, 2001 through December 30, 2001                5.50 to 1.00
December 31, 2001 through March 30, 2002               6.20 to 1.00
March 31, 2002 through June 29, 2002                   6.10 to 1.00
June 30, 2002 through September 29, 2002               5.95 to 1.00
September 30, 2002 through December 30, 2002           5.75 to 1.00
December 31, 2002 through March 30, 2003               5.30 to 1.00
March 31, 2003 through September 29, 2003              4.00 to 1.00
September 30, 2003 through March 30, 2004              4.00 to 1.00
March 31, 2004 through March 30, 2006                  3.75 to 1.00
March 31, 2006 and thereafter                          3.50 to 1.00

           (k) Section 6.14 of the Credit Agreement is hereby amended by
(i) deleting the words "in any fiscal year of the Parent Borrower to exceed
$25,000,000" appearing in clause (a) thereof and substituting in lieu thereof
the following:

     (i) in any fiscal year of the Parent Borrower other than 2002 to exceed
     $25,000,000 and (ii) in 2002 to exceed $20,000,000

,(ii) adding, following the words "pursuant to paragraph (a) in any fiscal year"
appearing in paragraph (b) thereof, the following: "other than 2001", (iii)
deleting the word "and" appearing at the end of clause (i) appearing in clause
(b) thereof and substituting in lieu thereof a comma and (iv) adding before the
close parenthesis at the end of clause (b)(ii) thereof the following:

     and (iii) no amount may be carried forward from 2001 to 2002

           SECTION 2. Representations and Warranties.  The Parent Borrower
represents and warrants to the Administrative Agent and to each of the Lenders
that:

           (a) This Amendment has been duly authorized, executed and delivered
     by it and constitutes the legal, valid and binding obligation of the Parent
     Borrower, enforceable against the Parent Borrower in accordance with its
     terms subject to applicable bankruptcy, insolvency, reorganization,
     moratorium or other laws affecting creditors' rights generally and subject
     to general principles of equity, regardless of whether considered in a
     proceeding in equity or at law.

<PAGE>
                                                                               5

          (b)  On and as of the date of this Amendment, the representations and
     warranties set forth in Article III of the Credit Agreement are true and
     correct in all material respects with the same effect as if made on the
     date hereof, except to the extent such representations and warranties
     expressly relate to an earlier date.

          (c)  At the time of and immediately after giving effect to this
     Agreement, no Event of Default or Default has occurred and is continuing.

          SECTION 3. Conditions to Effectiveness. This Amendment shall become
effective as of the date (the "Effective Date") first above written when:

          (a)  The Administrative Agent shall have received counterparts of this
     Amendment that, when taken together, bear the signatures of the Parent
     Borrower and the Required Lenders.

          (b)  The representations and warranties set forth in Section 2 of this
     Amendment shall be true and correct.

          (c)  Each Lender that executes this Amendment on or before December
     21, 2001 shall have received an Amendment Fee equal to 0.25% of its Total
     Commitment under the Credit Agreement.

          (d)  The Borrowers shall have prepaid all Multicurrency Revolving
     Loans and Multicurrency Swingline Loans, if any, and all Multicurrency
     Letters of Credit, if any, shall have terminated or been cancelled.

          SECTION 4. Effect of Amendment. Except as expressly set forth herein,
this Amendment shall not by implication or otherwise limit, impair, constitute a
waiver of, or otherwise affect the rights or remedies of the Lenders under the
Credit Agreement or any other Loan Document, and shall not alter, modify, amend
or in any way affect any of the terms, conditions, obligations, covenants or
agreements contained in the Credit Agreement or any other Loan Document, all of
which are ratified and affirmed in all respects and shall continue in full force
and effect. Nothing herein shall be deemed to entitle the Parent Borrower to a
consent to, or a waiver, amendment, modification or other change of, any of the
terms, conditions, obligations, covenants or agreements contained in the Credit
Agreement or any other Loan Document in similar or different circumstances. This
Amendment shall apply and be effective only with respect to the provisions of
the Credit Agreement set forth herein. This Amendment shall constitute a Loan
Document for all purposes under the Credit Agreement.

          SECTION 5. APPLICABLE LAW. THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

          SECTION 6. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall constitute an original but all of which when
taken together shall constitute but one agreement. Delivery of an executed
signature page to this Amendment by facsimile shall be effective as delivery of
a manually executed counterpart hereof.

<PAGE>

                                                                               6


     SECTION 7. Expenses. The Parent Borrower agrees to reimburse the
Administrative Agent for its out-of-pocket expenses in connection with this
Amendment, including the reasonable fees, charges and disbursements of Cravath,
Swaine & Moore, counsel for the Administrative Agent.
<PAGE>
                                                                               7

     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed by their respective authorized officers as of the day and year
first written above.

                                   KNOWLES ELECTRONICS, INC.,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title:

                                   JPMORGAN CHASE BANK, individually and
                                   as Administrative Agent,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title:

                                   MORGAN STANLEY SENIOR FUNDING,
                                   INC., individually and as Syndication Agent,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title: