Hey Everyone,
I've had an Eden WT-500 since 1994, and it has been my main amp since that time. However, I found some aspects of its stock performance lacking. So, around 2007 I embarked on a lengthy modification process, hoping to make improvements in the amp's sound and performance.
A few of the issues I wanted to address:
- reduce hiss at idle with master off
- open up high frequency response
- clean up audio path
- increase dynamic headroom
In order to reduce the idle hiss, I increased the preamp gain slightly after the first gain stage and reduced the gain in the final section which drives the power amp.
High frequency response was improved by reducing/removing some low-pass filtering. Instead of rolling off around 12 kHz (stock), it now goes up to 20 kHz. Of course, some might say that there's not a whole lot of useful signal up there with bass, but I play a 12-string, and the increase in clarity is easily audible.
To clean up the audio, all electrolytic capacitors in the signal path were either replaced with film capacitors, removed, or bypassed with polypropylene film. I also replaced all of the original JRC072D op amps, most with high performance OPA827s (note that these op amps have negligible DC-offset, which allows some of the coupling capacitors to be removed). A pair of OPA211s drives the power amps. A TLO71 was used for the EQ clip circuit.
Note that before settling on these particular op amps, I auditioned many others, including OP249, OP275, OPA2134, OPA2604, OPA1641/2, LM4562, LME49720, LME49860, AD825, TLE2072, TLE2142, AD823, AD797, AD8599, ADA4898-1, LT1358, NE5532, THS4032, THS4052, and perhaps more that I can't recall at the moment. After considerable trial and error, and lots of listening, I settled on the op amps noted above.
After the original Eden 7025 became microphonic, I replaced it with a new Tung-Sol 12AX7. It's a pretty nice/clear-sounding tube, IMO.
Dynamic headroom was improved somewhat by adding four 10,000 μF capacitors to the power supply. The extra transient capability from the large caps is especially noticeable when running in bridged mode. I also replaced the original PSU-to-amp and amp-to-speaker wire with more direct, lower gauge wire.
The graph below shows the frequency response of Dingwall Guitar's stock WT-300 in white (thanks Sheldon!) and my modified WT-500 in green. Both were set flat on the EQ. Not sure why the readings are not centered at 0 dB, though the relative difference is what is worth noticing here. Also, note that this particular WT-300 has a bit different frequency response than my WT-500 had before it was modified.
These short audio clips should help to illustrate the difference in sound. My Pedulla Rapture 5 with Q-tuner pickup was sent straight into both amps, and run passive for this comparison, IIRC. The first clip is taken from the output of the stock WT-300 preamp, while the second is from my modded WT-500. As this recording was done some time ago, I'm not sure of all of the details. However, I think that the output was taken from either the headphone out, or the post-EQ send.
Please save these to disk instead of playing them directly.
WT-300 (stock):
http://members.shaw.ca/webplace/WT-300(stock).wav
WT-500 (modded)
http://members.shaw.ca/webplace/WT-500(modded).wav
