In a message dated 00-09-14 02:33:43 EDT, you wrote:
<< Subj: Design flaws in the TT 1201 Desk Mic.....
To: <tentec@contesting.com>
Date: 00-09-14 02:33:43 EDT
From: jwl@telus.net (JWL)
To: KF6ZZ@aol.com
Phil: Here is what I found. Mic Bias
(Measured voltages at various points):
(Battery voltage used = 9.52V)
Before R11 = 7.43V
Between R11 and R14 = 1.38V
At Mic Element = 0.77V
(more than you got, but seems low still)
OP AMP (mine is an LF441):
VCC (Pin 7) = 6.00V
Pin 2 = 3.44V
Pin 3 = 3.44V
Pin 6 = 3.44V
I was then curious about the battery supply voltage and its effect on the
bias voltage, so I repeated all of the above with a supply battery voltage of
13.10V:
Mic Bias ( Measured voltages at various points ):
(Battery voltage used = 13.10V)
Before R11 = 10.80V
Between R11 and R14 = 4.23V
At Mic Element = 3.58V ( A ha ! )
OP AMP ( mine is an LF441):
VCC (Pin 7) = 9.28V
Pin 2 = 5.14V
Pin 3 = 5.14V
Pin 6 = 5.14V
ALL THE BIAS VOLTAGES ON THE OP AMP LOOK GOOD,
SO WE CAN ASSUME THE OP AMP IS WORKING PROPERLY.
It appears from the voltage drops that the mic element
consumes about 300uA with the correct bias
(i.e. voltage drop/R) correct?
YES, WHAT YOU ARE LOOKING AT IS THE CONSTANT CURRENT THRU THE INTERNAL FET
THAT IS BUILT INTO 99.99% OF THE ELECTRET ELEMENTS OUT THERE.
THE FET IS NECESSARY TO CONVERT THE HIGH IMPEDANCE OF THE ELECTRET
BOUNDARY TO A USEABLE LEVEL. USUALLY, A VOLTAGE GREATER THAN ~ 1.5 VOLTS IS
REQUIRED TO BRING THE V-I CHARACTERISTIC OF THE FET OUT OF THE LINEAR REGION
INTO THE CONSTANT CURRENT REGION, WHERE THE FET HAS ENOUGH TRANSCONDUCTANCE
TO DO IT'S JOB OF AMPLIFYING.....
If we knew the maximum bias voltage that the mic element can withstand,
we could work backwards and determine if your suggested new value for R11 is
safe for all supply voltages.
If the element can take 10V,
ALL THE SPEC SHEETS I HAVE SEEN ON ELECTRET MIC CAPSULES SPEC OPERATING
VOLTAGE RANGE 2~10 VDC.
THEY ALSO WARN OF A 3dB DEGRADATION OF OUTPUT
AT A BIAS VOLTAGE OF 1.5 VDC --- AND PROBABLY IT IS EVEN WORSE FOR VOLTAGES
LESS THAN 1.5 VDC .
then I think it's probably safe - would have to test it.
I guess the trick is to find R values
that allow a proper bias over
a wide range of supply voltages.
ALL YOU WOULD NEED TO DO IS: CALCULATE THE VALUE R11 AND R14 TO
PROVIDE THE MAXIMUM EXPECTED BIAS CURRENT AT THE MINIMUM SUPPLY VOLTAGE.
AND THEN ALLOW THE ELECTRET BIAS VOLTAGE TO RISE FOR MAX SUPPLY VOLTAGE
--- PROBABLY WOULDN'T EVEN GET TO 10 VOLTS ACROSS THE ELECTRET....
FOR YOUR ELEMENT THE NUMBERS WOULD BE:
MIN VOLTAGE AT TOP OF R11 ~ 6 V
MIN VOLTAGE ACROSS ELECTRET ~ 2 V
CURRENT CUNSUMPTION ~300 uA
THAT LEAVES 4 VOLTS DROP ACROSS RESISTIVE STRING.
4 VOLTS DIVIDED BY 300 microamps EQUALS 13333 OHMS.
SINCE R14 IS ALREADY 2200, R11 BECOMES ~11K.
NOW THEN, AT MAX SUPPLY VOLTAGE( 14- .7=13.3 V ), THE CURRENT CONSUMPTION
REMAINS THE SAME ( ~ 300 uA );
SO THEN THE CURRENT THRU THE R11 & R14 REMAINS THE SAME; AND THE VOLTAGE DROP
ACROSS THE RESISTIVE STRING REMAINS THE SAME, THE ONLY THING THAT CHANGES IS
THE VOLTAGE ACROSS THE MIC ELEMENT.
WHICH NOW WOULD BE 13.7- 4 = 9.7 VOLTS.
WHICH IS OKAY ...
I wonder if Ten Tec designed the voltage divider that biases the Mic
element for a 14V supply, then set the minimum supply voltage by what is
required by the ICs, but forgot that the lower supply voltage would not
adequately bias the Mic ?
Make any sense to you ?
YES, THIS WOULD BE AN EASY MISTAKE TO MAKE AT A SMALL COMPANY WITH LIMITED
RESOURCES
Your "typo" theory on the change in values also is a good
possibility and perhaps more likely.
James, VE7JLZ >>>
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