15-09-2024, 08:14 AM
Colin
Start with a 680 ohm (or 820 ohm) resistor across the secondary of the 600 ohm transformer, then a series resistor to your 1K pot upper contact. The 680R resistor is there so that the impedance reflected back to the primary/source is in the vicinity of 600 ohms rather than some completely unknown value, and stays relatively constant regardless of the pot wiper position - and thus loading.
22dBu is about 60 millivolts so might already be close to the level required for the FT857 but that is the job the pot has. The series resistor plus the 1K value (ie the pot value) form a simple voltage divider so that is easy to work out. For example, a series 1K gives a 2:1 voltage division as a maximum depending on the pot rotation. And then you can always turn it down !
The FT857 manual does not quote a mic output level, just a nominal 600 ohm TX input impedance, so it is guesswork anyway.
Doug
Start with a 680 ohm (or 820 ohm) resistor across the secondary of the 600 ohm transformer, then a series resistor to your 1K pot upper contact. The 680R resistor is there so that the impedance reflected back to the primary/source is in the vicinity of 600 ohms rather than some completely unknown value, and stays relatively constant regardless of the pot wiper position - and thus loading.
22dBu is about 60 millivolts so might already be close to the level required for the FT857 but that is the job the pot has. The series resistor plus the 1K value (ie the pot value) form a simple voltage divider so that is easy to work out. For example, a series 1K gives a 2:1 voltage division as a maximum depending on the pot rotation. And then you can always turn it down !
The FT857 manual does not quote a mic output level, just a nominal 600 ohm TX input impedance, so it is guesswork anyway.
Doug
Doug VK4ADC @ QG62LG51
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http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.