Modifying Preamp Frequency Response - Eden WT-800 to WT-400 - diyAudio
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Old 1st February 2009, 03:50 AM   #1
duff2 is offline duff2  Canada
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Default Modifying Preamp Frequency Response - Eden WT-800 to WT-400

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

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Old 1st February 2009, 06:24 PM   #2
teemuk is offline teemuk  Finland
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Location: Suomi, Finland
1. They donīt. It's a mute switch that introduces a little "startup delay" in signal amplification. When the power is turned on it takes a while for the preamp DC voltages to settle. At the point when the supply rail capacitors charge (and supply rail voltages rise towards their normal operating levels) these voltages will resemble AC and hence will get coupled to the signal path. This often creates a nasty "pop" noise. The mute switch will prevent it, since it mutes the preamp signal for a brief moment.

Edit: Sorry, looks like I was too busy looking at the FET switch and missed the circuit in the input. The capacitor C2 shunt is the reason for high frequency roll off, as the circuit in conjuction with R4 forms a RC low-pass filter. The additional 1Meg resistor in parallel with another 1Meg resistors will cause some treble loss too, since 500K input impedance loads the guitar/bass more than 1Meg input Z. I'd still say the capacitor is a bigger "issue" in comparison to this effect.
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Old 2nd February 2009, 03:46 AM   #3
duff2 is offline duff2  Canada
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Join Date: Apr 2008
Words cannot express the feeling of gratification I am experiencing right now.

Thanks SO much for your reply Teemuk!!!

So just to be certain, do I simply need to incorporate R2 and C2 into the input of my WT-800 to accomplish the tone change I am seeking?

Cheers,
Duff2
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Old 2nd February 2009, 03:46 AM   #4
duff2 is offline duff2  Canada
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Well, I just popped open my WT-800 to have a look around. Turns out the 1M resistor and 470 pF capacitors are already present in my amp!

I have found another WT-800 schematic that indicates how these components have been implemented. Note how the 470 pF capacitor comes after the 1 microfarad capacitor in the WT-800 while it is wired in differently in the WT-400 (boxed in red). Is this what's responsible for the different preamp voicing? Do the other boxed in red changes contribute to the altered high frequency response?

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Cheers,
Duff2
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