You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
The Feb 2025 Radioddity Giveaway Results are In! Click Here to see who won!

Reply to thread

Message

[ATTACH=full]30001[/ATTACH]
The needed pre-amp parts are all on the left side of this schematic. They include everything from C1301 to C1308. VR1301 is not used. Amplified audio output comes from the ungrounded side of C1308. That cap is too large at .15uf and attenuates lots of the high end content. As I recall, a .047 worked much better here. C1305 controls how much bass passes through the circuit. Adding a variable resistor in series with C1305 allows adjustment of gain if needed. Q1302 is just a general purpose NPN. The old 2N2222A or equivalent will work fine.

The right hand side of the schematic contains a transmit switching transistor and "Private Line" switch for receive as used in a Motorola radio. They are not needed in other radios. This pre-amp can be built "dead bug" style on the backside of the stock mic board. It can fit if you remove the metal weight in the back of the stock mic and Dremel out a little of the extra plastic ribbing inside the mic. I did have to add a small copper shield made of roof flashing, soldered to the ground on the PC board and covering over the pre-amp to prevent RF feedback.

The stock mic element can be reused in place too. Just cut the circuit trace on the back of the board to insert the pre-amp in between.

<blockquote data-quote="Shockwave" data-source="post: 676030" data-attributes="member: 10882">[ATTACH=full]30001[/ATTACH]The needed pre-amp parts are all on the left side of this schematic. They include everything from C1301 to C1308. VR1301 is not used. Amplified audio output comes from the ungrounded side of C1308. That cap is too large at .15uf and attenuates lots of the high end content. As I recall, a .047 worked much better here. C1305 controls how much bass passes through the circuit. Adding a variable resistor in series with C1305 allows adjustment of gain if needed. Q1302 is just a general purpose NPN. The old 2N2222A or equivalent will work fine.The right hand side of the schematic contains a transmit switching transistor and &quot;Private Line&quot; switch for receive as used in a Motorola radio. They are not needed in other radios. This pre-amp can be built &quot;dead bug&quot; style on the backside of the stock mic board. It can fit if you remove the metal weight in the back of the stock mic and Dremel out a little of the extra plastic ribbing inside the mic. I did have to add a small copper shield made of roof flashing, soldered to the ground on the PC board and covering over the pre-amp to prevent RF feedback.The stock mic element can be reused in place too. Just cut the circuit trace on the back of the board to insert the pre-amp in between.</blockquote>

[QUOTE="Shockwave, post: 676030, member: 10882"] [ATTACH=full]30001[/ATTACH] The needed pre-amp parts are all on the left side of this schematic. They include everything from C1301 to C1308. VR1301 is not used. Amplified audio output comes from the ungrounded side of C1308. That cap is too large at .15uf and attenuates lots of the high end content. As I recall, a .047 worked much better here. C1305 controls how much bass passes through the circuit. Adding a variable resistor in series with C1305 allows adjustment of gain if needed. Q1302 is just a general purpose NPN. The old 2N2222A or equivalent will work fine. The right hand side of the schematic contains a transmit switching transistor and "Private Line" switch for receive as used in a Motorola radio. They are not needed in other radios. This pre-amp can be built "dead bug" style on the backside of the stock mic board. It can fit if you remove the metal weight in the back of the stock mic and Dremel out a little of the extra plastic ribbing inside the mic. I did have to add a small copper shield made of roof flashing, soldered to the ground on the PC board and covering over the pre-amp to prevent RF feedback. The stock mic element can be reused in place too. Just cut the circuit trace on the back of the board to insert the pre-amp in between. [/QUOTE]

Top Bottom