Modifying Preamp Frequency Response - Eden WT-800 to WT-400

[duff2]

I have recently measured the frequency response curves for my Eden WT-400 (400 watt mono) and Eden WT-800 (2 x 400 watt stereo) bass amps. My ears have always suspected their preamps were voiced a little differently, and my measurements have confirmed this (see graphs below). Specifically, the response of my WT-400 rolls off faster in the treble range than the WT-800. I actually prefer the way my WT-400 sounds.

Despite my best attempts, I cannot accomplish a WT-800 to WT-400 preamp voicing transformation using the EQ alone. So I am posting here looking for answers to the following questions:

1. How do the additional circuit features highlighted in Red in the WT-400 schematic below accomplish the altered high frequency voicing?

2. Since the rest of the circuitry detailed in the full preamp schematics are the same for both amps, can I have my WT-800 modified (via installation of the circuit features highlighted in Red) to have it sound like my WT-400?

Thanks,
Duff2

[Enzo]

HArd to say. it is four very inexpensive parts, so tack them on and find out.

Beyond that, are the 12AX7s in both amps identical? Did you move one of them back and forth to make thins set of measurements? I would expect a range of variation in the tube selected. Tubes are certainly not all alike.

And have you measured several of either model? Normal part variations may also be contributing to the differences you have.

[duff2]

HArd to say. it is four very inexpensive parts, so tack them on and find out.

Beyond that, are the 12AX7s in both amps identical? Did you move one of them back and forth to make thins set of measurements? I would expect a range of variation in the tube selected. Tubes are certainly not all alike.

And have you measured several of either model? Normal part variations may also be contributing to the differences you have.

I opened up my WT-800 today and found that those four parts are already there. I have also found another version of the WT-800 schematic that included these RC components on the input jack.

With regards to the 12AX7, I used the exact same tube for each measurement (swapped it between the amps).

On someone else's suggestion from another discussion forum (Eden), I measured the response pre-EQ and post-EQ for both amps. The pre-EQ signal is the identical for both amps (so none of the above components are contributing to the preamp voicing differences), but the amps differ post-EQ which is from where I took the measurements for the curves above.

I have not measured several models of each amp, but bass players (myself included) have been saying for years that the WT-400 sounds warmer than the WT-800. I indulged in this exploration to try to understand the reason for this with hopes of eventually modifying my WT-800 to make it sound like my WT-400.

I could post the entire schematic diagrams here if anyone wants to try figuring it it out. I have a lot of reading to do before I could figure this out on my own.

Thanks for your reply Enzo.

Cheers,
Duff2

[defaced]

Yea, post the full schems.

My thinking would be "what's different between the tone stacks?".

[duff2]

Here are the schematics for the WT-800. The WT-400 schematics are on my other computer. I'll upload them tomorrow morning.

Cheers,
Duff2

[duff2]

Didn't get a chance this morning. Will upload 'em tonight.

Cheers,
Duff2

[duff2]

Here's the schematic for the WT-400.

Have fun!

Cheers,
Duff2

[duff2]

For those of you following along, trying to figure this out from only the schematics will very likely end up being a lost cause.

I went through my WT-400 preamp tonight schematic in hand and found that there are extra resistors and capacitors on both sides of the board that are not accounted for in the schematics I have posted.

So it appears that solving this mystery will require removing the board and tracing out a schematic de novo. I am neither qualified nor willing to do this at this point. Sorry if I wasted your time or dashed any hopes.

Cheers,
Duff2

[bsco]

Hi Duff2. Have a question for you if I may....you say that your preamp board has extra components that are not shown on the schematic...Does your board have a bunch of jumper wires underneath on the solder side?? I have some there on mine and they run all over the board....the amp I have has a problem with the clip led light coming on when you first power it up....If you power it down and then back up again, the clip led gos out but no sound. If you remove IC4 you get sound..reinstall the IC and there is no sound.....did you have any issues with your 400 besides your tone difference??
Cheers,
Bernie

[doctor]

Hi Duff2. Have a question for you if I may....you say that your preamp board has extra components that are not shown on the schematic...Does your board have a bunch of jumper wires underneath on the solder side?? I have some there on mine and they run all over the board....the amp I have has a problem with the clip led light coming on when you first power it up....If you power it down and then back up again, the clip led gos out but no sound. If you remove IC4 you get sound..reinstall the IC and there is no sound.....did you have any issues with your 400 besides your tone difference??
Cheers,
Bernie

May be this file will add some info to this issue