K6JRF's Page
formerly W6FZC
ESSB Audio Techniques Page
(New: Nov 3, 2014 Update: Feb 12, 2020)

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

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 and LINE 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 + Line In Analysis
The picture here shows a section of the actual FTdx3000 Main Board schematic centering on the Mic Preamp ckt outlined in RED and the LINE In shown in Yellow. The schematic is courtesy of Frank, NR6AM from the User Manual. Tnx, Frank!
FT3000 Mic and Line Preamp Ckt from the User 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) and Line Input ("Line" = VS2) 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. I've recently found the Spice model for the 2SC4116 transistor which is now included.

Mic Preamp Ckt captured for 5Spice Simulation

The ckt "form" is identical to the Mic Preamp ckt in the FT2000 series. Check out the similarity 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 include high frequency rolloff capability but the cap wasn't employed. So they must have "thought" about the "ttbf" mode - 4Khz TX BW!

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

For the MIC IN, the -3dB point is 11Hz for C1368 = 0.22uF. Changing C1350 to 47pF increases the Hi Freq cutoff to 5.1Khz.

5Spice has shown that replacing three (3) parts will give the required ESSB audio BW. Please make sure that the radio is in the "ttbf" mode to take advantage of the broadband audio response.

FTdx3000 5Spice Line In Analysis
5Spice Results: Line Input

For the LINE IN, the Lo cut -3dB point is apx 15Hz; the Hi cut -3dB point is 8.1Khz. Changing C1368 = 0.22uF supports the Lo Freq cutoff to 15hz.

This is the lowest possible cap change that is acceptable since too much low frequency response MAY cause some VOX falsing b/c of the very low frequency response.
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 ---------- 47pf ----------------- T-B-D ------------------ T-B-D ------------- $??
C1399 ---------- 1uf ------------------- T-B-D ------------------ T-B-D ------------- $??
C1368 ---------- 0.22uf --------------- 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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