Eden WT-500 circuit modifications/improvements

[12bass]

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

 

[tink9975]

Thats very cool.

I've had my WT-500 since '96 and have had the same issue of hiss, but I don't have your skills to fix it. I have heard that you can minimize the hiss by rotating the toroid.

 

[12bass]

I've had my WT-500 since '96 and have had the same issue of hiss, but I don't have your skills to fix it. I have heard that you can minimize the hiss by rotating the toroid.

Rotating the transformer can reduce the hum, as does repositioning the signal cables; this is fairly easy to do. But from what I've determined, that doesn't impact idle hiss, as the hiss is the result of suboptimal circuit design, in part due to too much gain at the back end of the preamp.

 

[jjk2007]

Cool man. I bought my WT-500 in about 95 myself, but never recall the hiss being that bad for me. Since you play a 12 string it sounds like opening up the top end was a good move too. Always cool to see some of the sweet mods here on TB!

 

[bassgod76]

You guys have any issues with your WT-500, regarding an uncontroled howl-type feedback? My Highwayman has been giving me fits, and I'm not sure if it is a bad pot or the tube is old (circa 2000).

 

[12bass]

You guys have any issues with your WT-500, regarding an uncontroled howl-type feedback? My Highwayman has been giving me fits, and I'm not sure if it is a bad pot or the tube is old (circa 2000).

Not sure why you bumped this thread instead of starting a new one....

Anyway, under what specific conditions does this occur? And what happens exactly?

 

[bassgod76]

Not sure why you bumped this thread instead of starting a new one....

Anyway, under what specific conditions does this occur? And what happens exactly?

I did create a thread, but I was hoping to get more info, here.

It happens at random. Here's the thread that explains it all.
http://www.talkbass.com/forum/f15/eden-wt-500-feedback-problem-880047/

 

[vondran]

I have a WT-300 and am bumping this thread with some additional questions on these mod's mentioned by "12bass".

First, anyone try LM833 opamps in place of the TL072's? The TL072 has noise rating of 18nV/sqrt(Hz), while the LM833 has rating 4.5nV/sqrt(Hz), and the OPA827 mentioned at the top of this thread has 4.0nV/sqrt(Hz). The reason I ask is the LM833 is relatively cheap and easy to get (~$0.60), whereas the OPA827 seems more expensive per part ($9 on Mouser). Also the LM833 is specifically designed for low noise audio.

Second, anyone try shielding the wires going to the upper knobs that select the frequency in the parametric EQ section. I notice they make a nice loop and when I move my hand near them (when top cover off) the noise increases. Was thinking of wrapping them in shielding foil and ground where their pots connect to the chassis (being carefully not to make other ground connections, to prevent creating a ground loop).

Lastly, I do notice a 60Hz hum when I crank the gain. It's clearly in the input stage as I hear it both when I listen through the headphones out, as well as if I listen at the tuner out (which is immediately after the input stage tube on the schematics). One guess is that adding 220uf or 470uf capacitors between the +15V and -15V to ground on the preamp board near the tube would help as the schematic doesn't show any significant caps on these power lines on the preamp board. This should hopefully eliminate any remaining 60Hz from the power supply. "12bass" above mentions adding 4 10000uf caps to the power supply. Any thoughts on this may be addressing the same issue, or is it addressing something in the power amp section?

 

[12bass]

The OPA827 has a JFET input. LM833 is bipolar. I have not tried the LM833. Most of the parts that I have tried have been high performance devices which have been auditioned for best sound quality. At the moment, I'm using ADA4627-1 at the input, which sounds a bit better than OPA827, IMO.

How much noise an op amp will contribute in a given application depends upon the impedance in that part of the circuit. As a general rule, if there are large resistance values going to the op amp's inputs (>10 k), then a FET input is likely to be quieter in that section. I've found that FET devices are somewhat quieter in most parts of the Eden preamp. The LM833 is cheap enough that it might be worth trying to see if it makes any audible improvement. OPA1642 is a less expensive dual JFET that might be worth looking into.

The bundle of wires between the EQ board and preamp board can contribute some hum. However, I've not found it to be that significant in my amp, at least if I'm not deliberately moving the wires around. Might be worth trying to shield this area, as long as you're careful not to short anything out. Could also be worth looking into shielding the cabling between preamp and power amp.

Regarding the preamp supply filtering, I think I used two 2200 μF/25 V for the +-15V rails and another 2200 μF/ 25 V for the tube supply line. This should help reduce hum in the preamp section, as well as provide a more stable supply voltage when the amp is pushed. I also used 0.1 μF capacitors underneath the op amps for local supply bypassing. The four 10,000 μF capacitors were added to the main power supply in order to have a larger reserve on hand for increased headroom at higher volumes. Another way to address the hum issue is to try rotating the toroidal power transformer. You may find that you're able to find a null at one point as you rotate it, and it is free to try!

 

[Bassmec]

The OPA827 has a JFET input. LM833 is bipolar. I have not tried the LM833. Most of the parts that I have tried have been high performance devices which have been auditioned for best sound quality. At the moment, I'm using ADA4627-1 at the input, which sounds a bit better than OPA827, IMO.

How much noise an op amp will contribute in a given application depends upon the impedance in that part of the circuit. As a general rule, if there are large resistance values going to the op amp's inputs (>10 k), then a FET input is likely to be quieter in that section. I've found that FET devices are somewhat quieter in most parts of the Eden preamp. The LM833 is cheap enough that it might be worth trying to see if it makes any audible improvement. OPA1642 is a less expensive dual JFET that might be worth looking into.

The bundle of wires between the EQ board and preamp board can contribute some hum. However, I've not found it to be that significant in my amp, at least if I'm not deliberately moving the wires around. Might be worth trying to shield this area, as long as you're careful not to short anything out. Could also be worth looking into shielding the cabling between preamp and power amp.

Regarding the preamp supply filtering, I think I used two 2200 μF/25 V for the +-15V rails and another 2200 μF/ 25 V for the tube supply line. This should help reduce hum in the preamp section, as well as provide a more stable supply voltage when the amp is pushed. I also used 0.1 μF capacitors underneath the op amps for local supply bypassing. The four 10,000 μF capacitors were added to the main power supply in order to have a larger reserve on hand for increased headroom at higher volumes. Another way to address the hum issue is to try rotating the toroidal power transformer. You may find that you're able to find a null at one point as you rotate it, and it is free to try!

Chip rolling is such good fun. I tend to do it in fits and starts on some of my 80's studio mic pre's, lets face it there is not much that can be said in defence of the fidelity of the TLO chips in comparison to some of the burr brown/linear devices signal to noise ratio's, love the local bypassing idea as well
as building more capacitance into the supply. I am certain that your WT 500 must be a very superior sounding beast indeed.

 

[Cney]

Not to stir up an old thread. But i could use some help on my chip replacement on my Eden WTB-400. It is a good sounding solid state amp but because it's the amp of an Nemesis RS320 i i'm going to make it more worth of the Eden badge by using better capacitors and chips.
Capacitors i'm going to use are probably film capacitors. These will replac most of the electrolythic in the preamstage similar to 12bass.I have some questions regarding replacing of the JRC72's I was going to remove these, add IC sockets. My candidates are TL072, TLE2072, NE5532, OPA2132. Now i see that 12bass used more. Because i am not an electronic engineer (i am however a mechanical engineer) i am wondering if these are some good ones to try. Any others i need to try?
I do not know if there is anything more i need to do like changing out resistor values for the supply voltage. I have yet to find a schematic for this amp.
I am going to mail Eden about a schematic.

 

[12bass]

Unfortunately, Eden recently stopped providing schematics to anyone but service shops.

Polystyrene and polypropylene film capacitors will likely sound the most transparent. For parts of the circuit where only an electrolytic will work, you can add a small film bypass capacitor in parallel. It may be somewhat difficult without the schematic, however, try to upgrade only capacitors which are directly in the signal path (i.e. do not bother with supply caps). It may also be a good idea to add 0.1 μF across the op amp supply rails.

As for the op amps, ADA4627-1 is my current favorite, not inexpensive, and would require two single SOICs on a DIP adapter. OPA1642 is more economical, still nice sounding, but still requires a SOIC to DIP adapter, or something like this:

Finally, if you are going to perform modifications, please be sure to make everything as electrically and mechanically sound as possible in order to ensure reliability.

 

[Cney]

Unfortunately, Eden recently stopped providing schematics to anyone but service shops.

Polystyrene and polypropylene film capacitors will likely sound the most transparent. For parts of the circuit where only an electrolytic will work, you can add a small film bypass capacitor in parallel. It may be somewhat difficult without the schematic, however, try to upgrade only capacitors which are directly in the signal path (i.e. do not bother with supply caps). It may also be a good idea to add 0.1 μF across the op amp supply rails.

As for the op amps, ADA4627-1 is my current favorite, not inexpensive, and would require two single SOICs on a DIP adapter. OPA1642 is more economical, still nice sounding, but still requires a SOIC to DIP adapter, or something like this:

Finally, if you are going to perform modifications, please be sure to make everything as electrically and mechanically sound as possible in order to ensure reliability.

So it seems you have changed your opamp over the last couple of years to the ADA4627-1. Have you put this one in every place of your WT-500?
I have ordered TL072, AD712 and OPA2134 allready. But i wil try these as well. Are the ADA4627 and OPA1642 not available in dip8 format?
And could you send me a photo of your internals for reference and overall nerd porniness?

 

[Cney]

Oh... And whats with the 2 ADA4627-1 per dip socket? I see that it is a SOIC8 format. how does one put 2 on there?

 

[12bass]

Most new op amps are not available in DIP. ADA4627-1 is single, not dual, op amp.

Invalid Link Removed

 

[Cney]

Most new op amps are not available in DIP. ADA4627-1 is single, not dual, op amp.

Invalid Link Removed

Makes sense. I am going to try to buy these adapters. Although i have ordered standard dip sockets and not the precisions i am unable to see if that will fit. or do i have to order the precision ones and solder those on instead?

I am going to try to find a european source for these adapters and opamps.
Just to keep cost and time to deliver low.

Thanks for your help

 

[MuthaFunk]

I don't have any schematics for the WTB-400 but I do have a bunch of schematics for the other more popular Eden amps if anyone needs one. Just PM me.

 

[12bass]

Makes sense. I am going to try to buy these adapters. Although i have ordered standard dip sockets and not the precisions i am unable to see if that will fit. or do i have to order the precision ones and solder those on instead?

I am going to try to find a european source for these adapters and opamps.
Just to keep cost and time to deliver low.

Thanks for your help

You're welcome!

FWIW, I prefer milled DIP sockets, as they seem likely to be more reliable.

I think that using some OPA1642s soldered on to DIP sockets would make for a nice upgrade. I've tried OPA2132/34 and found the OPA1642 superior sounding, with OPA2132 being slightly better than OPA2134. Not sure about AD712. TL072 is very similar to JRC072.

Presently, my WT-500 has a combination of op amps, a pair of ADA4627-1s in the input section, OPA827s in the EQ, and a pair of LME49990s in the preamp output stage. Because the early WT amps use a resistor dropping method to get the preamp supply, you must be mindful of current consumption in the preamp section. Otherwise the voltage will drop below +-15 V. I've had to modify the resistor values to compensate for the high performance op amps. Hopefully the WTB-400 uses regulators. OPA1642 is a nice choice because it sounds decent plus it does not use much more current.

 

[Cney]

Unfortunately, Eden recently stopped providing schematics to anyone but service shops.

Polystyrene and polypropylene film capacitors will likely sound the most transparent. For parts of the circuit where only an electrolytic will work, you can add a small film bypass capacitor in parallel. It may be somewhat difficult without the schematic, however, try to upgrade only capacitors which are directly in the signal path (i.e. do not bother with supply caps). It may also be a good idea to add 0.1 μF across the op amp supply rails.

I guess i'm lucky because all the Q's in the preamp stage look like small regulators. 5 on the input side and 2 in the headphone/output side.
I'm going to use WIMA FKP caps for the signal path. From what i have read and seen on various fora is plethora of different caps for decoupling. A lot of people have different opinions. I've seen tantalum, elco's, film and ceramics in pictures. Is there anything difference of sound? what do you use? I read someone advised on using Ceramic over film at a HiFi forum.

I'm going to start modding after i have my caps. Feel stoked. I might not hear a lot of difference but i am a man that likes the process.

 

[12bass]

I am process-oriented as well. Because of that, I have been modifying my WT-500 for years, and have had a difficult time keeping the lid screwed on. Too much messing around inside can negatively impact reliability.

Generally speaking, I would suggest good quality film capacitors (like polypropylene) for the signal path, and ceramics and electrolytics for supply decoupling. For smaller values in the signal path, I prefer silver mica, as they sounds smoother (less "gritty" than ceramics).