Sct controller described a below MVA event when issuing the correct descent clearance; but then fail...

Narrative:

I was working sder. A lear jet was inbound to nzy; checked on and I told him 'roger.' I later issued him a descent to 6;000. I handed him off to sdzr who would be working him into nzy. The sdzr radar controller later called my attention to the fact that the lear jet was below the 5;000 ft MVA to the east of pgy. I issued a low altitude alert to the lear jet and climbed him from approximately 3;800 back to 5;000. I told him that he had been assigned 6;000. He said he was cleared to 3;000. I later reviewed the audio of the event and although I did issue 6;000; I missed his read back of 3;000. Recommendation: listen for read back. When I first sat down at this sector; a few of the transmissions I made were not acknowledged. I realized that I was transmitting on the primary frequency. I have experienced this issue with this frequency on several occasions. I immediately switched to the backup frequency which seems to work more reliably.

Original NASA ASRS Text

Title: SCT Controller described a below MVA event when issuing the correct descent clearance; but then failed to hear the incorrect read back by the pilot. The reporter listed radio equipment as a possible contributing factor.

Narrative: I was working SDER. A Lear Jet was inbound to NZY; checked on and I told him 'Roger.' I later issued him a descent to 6;000. I handed him off to SDZR who would be working him into NZY. The SDZR RADAR Controller later called my attention to the fact that the Lear Jet was below the 5;000 FT MVA to the East of PGY. I issued a Low Altitude Alert to the Lear Jet and climbed him from approximately 3;800 back to 5;000. I told him that he had been assigned 6;000. He said he was cleared to 3;000. I later reviewed the audio of the event and although I did issue 6;000; I missed his read back of 3;000. Recommendation: listen for read back. When I first sat down at this sector; a few of the transmissions I made were not acknowledged. I realized that I was transmitting on the primary frequency. I have experienced this issue with this frequency on several occasions. I immediately switched to the backup frequency which seems to work more reliably.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1006832
Time
Date	201204
Local Time Of Day	0001-0600
Place
Locale Reference	SCT.TRACON
State Reference	CA
Aircraft 1
Make Model Name	Learjet 35
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Sct controller described a below MVA event when issuing the correct descent clearance; but then failed to hear the incorrect read back by the pilot. The reporter listed radio equipment as a possible contributing factor.