K6JRF's Page
formerly W6FZC
ESSB Audio Techniques Page
(New: Nov 3, 2014)

This new section shows the results of Spice analysis on the FTdx3000 Mic Preamp Ckt showing the "stock" repsonse.

Just is in the FT2000, the mic preamp ckt has limited audio response and needs to be modified in order to get LOW Freqs (below 100Hz) and HI Fregs (above 3Khz) that many ESSBers require.

I continue to use an excellent Spice program called "5Spice" written by Andresen Software. It features almost intuitive operation, has many powerful features and allows the import of various Spice models so that the ckt can be simulated using the actual real world parts. Click here to see more information on this fine circuit analysis product.

Frequency Response from MIC Input

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Feed the audio into the back AF input (LINE) to get 50hz Tx response If you take the time to checkout the schematic tracing the connection from the front panel "mic" input and rear panel "LINE" input, you will find that the mic input goes through a "preamp" stage while the "LINE" input is directly summed into the 2nd stage opamp in the TX mode. The 5Spice results are shown in the sections below. Checkout the charts the Tx sweep simulation showing each ckt's low frequency response.

FTdx3000 5Spice Mic Preamp Analysis
The picture here shows a section of the actual FTdx3000 Main Board schematic centering on the Mic Preamp input ckt. The schematic is courtesy of Frank, W6APO from the User Manual. Tnx, Frank!
FT3000 Mic Preamp Ckt from the Users Manual

The picture below shows the "capture" of the ckt values in a format that is acceptable for 5Spice. The analysis will accurately show the ckt's response as a function of 'frequency'. Here the front panel input ("Mic" = Vs1) as the ckt exists in the current FTdx3000. The reference designators (Ref Des) have been preserved and are as shown in the schematic if you wish to verify.

Mic Preamp Ckt captured for 5Spice Simulation

The ckt "form" is identical to the Mic Preamp ckt in the FT2000 series. Check out the simularity here. Note that the ckt is identical except for some R-C time constant changes. Also on the 2nd stage amp (NJM2737M), the rolloff cap (C1350) was omitted. This shows that there was a definite attempt to rolloff the high frequencies. Not the best design for a 4Khz TX BW radio!

FTdx3000 5Spice Mic Preamp Analysis
5Spice Results: Mic Input

For the MIC IN, the -3dB point is below 15Hz. Changing C1350 increases the Hi Freq cutoff to 5Khz.
5Spice has shown that replacing two (2) parts will give the required ESSB audio BW. The following chart shows what's needed.
Detailed Chip Cap Info: [FT2000 Part Designation]
The part numbers for the chip-caps has been taken from the FT2000 Service Manual [SM] since I don't have a FTdx3000 SM.
All of the chip-caps are mounted on the top side of the PCB so you can access them without having to remove the Main Unit Board. The following parts are available from Yaesu/Vertex Parts Department at 714-827-7600.

Ref Des ----- Value --------- Part Number -------- Yaesu PN ------ Price/Ea
C1350 ---------- 1uf ----------------- T-B-D ------------------ T-B-D ------------- $??
C1399 ---------- 47pf --------------- T-B-D ------------------ T-B-D ------------- $??

The Mic Preamp Ckt changes dramatically open the Mic Input TX audio path which will give you more room for low-bass experimentation. My personal comment is that there's really NO AUDIO information below 40hz other than "rumble" but you are now not limited by the radio's hardware. Audio at 50hz and above will come through w/o loss.

No other mods need be done prior to this modification.

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