SWR owners: Aural Enhancer/tone stack mod?

[mellohawk]

Anybody tried this modification? I'm referring to this TB thread:

SWR Aural Enhancer Bypass

Supposedly, if you remove the .1 uF and .001 uF caps, then replace the .001 uF cap with the .1 uF cap, the preamp voicing will be flat instead of the scooped mids that SWR uses for their tone stack.

Just curious as to whether anyone has tried this mod. If so, please share your experience. Would it be worth my time/effort to perform this mod on my SM-500?

Thanks,
Curt

[JimmyM]

you could always boost the mids with your eq and accomplish the same thing.

[will33]

Originally Posted by JimmyM

you could always boost the mids with your eq and accomplish the same thing.

I boost around 250 or so with mine as well as taking out the lowest of lows 30-40hz stuff. It "un-SWR's" the sound but still isn't quite the same as the mod. The mod makes it more poweramp type flat power and would make it sound louder, fuller. The eq tweaking gets close but it sounds like there is still a little hump and dip stuff happening. I was going to do the mod on mine but the PITA part is you have to get at the bottom of the board to do the soldering, which means taking off about 20 or so knobs, buttons, sliders etc. just to be able to flip the card over and change 2 parts. I'll still do it someday when I have an afternoon with nothing else to do and post my thoughts. For now it sounds good by turning the lowest sweep down to 31hz and dumping it, then putting a little boost in around 200-300hz. If you look at the results graphs in that other thread you'll see what I mean.

Edit: As long as you have to take that much apart and not wanting to do it twice, I want to make the mod reversible like hooking it up to a "fat" switch or something so you can run it either way without having to disassemble the amp again.

[lowfreqgeek]

The *best* way to get a flat sound that I've found is to completely bypass the AE circuit. The AE circuit is what gives SWR that "SWR sound", so eliminating it from your signal path gives you a very close to "flat" amp in the true sense of the word. There are still some tone-altering elements in the path, but they are minimal.

You'll have to put in a load resistor on the bypass side to attenuate to approximately the same level as the AE attenuates, but that's easy. Also, a 2M to ground on the grid to reduce popping. Any higher and the pops become more prominent as the high impedance limits the rate at which the grid will pull to ground; any lower and the pull-down will start to effect the overall operation and sound of the AE circuit if you want to use it.

It's been a few years since I simulated the whole AE circuit, but replacing the 2 caps you mentioned won't completely make it flat as I recall, nor will simply adding "mids". If you want a "flat" response, you need to completely bypass it. I suggest you use shielded wire, too, as I've had problems with 60Hz getting through after doing the mod. The shielded wire (shield tied to ground on the board) fixed that issue.

Personally, I find that I actually use the AE some when I have the option to disengage it at any moment. It's more rare, but my G&L basses in parallel mode with the AE engaged sounds fantastic! Hate it with my Pbass, though, so I disengage and get happy again. I've done it to several SWR amps over the years and I've never been disappointed. In fact, I've come back around to using them once that mod was done.

[Planet Boulder]

Originally Posted by lowfreqgeek

The *best* way to get a flat sound that I've found is to completely bypass the AE circuit. The AE circuit is what gives SWR that "SWR sound", so eliminating it from your signal path gives you a very close to "flat" amp in the true sense of the word. There are still some tone-altering elements in the path, but they are minimal.

You'll have to put in a load resistor on the bypass side to attenuate to approximately the same level as the AE attenuates, but that's easy. Also, a 2M to ground on the grid to reduce popping. Any higher and the pops become more prominent as the high impedance limits the rate at which the grid will pull to ground; any lower and the pull-down will start to effect the overall operation and sound of the AE circuit if you want to use it.

It's been a few years since I simulated the whole AE circuit, but replacing the 2 caps you mentioned won't completely make it flat as I recall, nor will simply adding "mids". If you want a "flat" response, you need to completely bypass it. I suggest you use shielded wire, too, as I've had problems with 60Hz getting through after doing the mod. The shielded wire (shield tied to ground on the board) fixed that issue.

Personally, I find that I actually use the AE some when I have the option to disengage it at any moment. It's more rare, but my G&L basses in parallel mode with the AE engaged sounds fantastic! Hate it with my Pbass, though, so I disengage and get happy again. I've done it to several SWR amps over the years and I've never been disappointed. In fact, I've come back around to using them once that mod was done.

Can't you simply turn the AE knob all the way down? That's what I do with my WM15 and it seems to do the trick.

[pin_head_47]

Originally Posted by Planet Boulder

Can't you simply turn the AE knob all the way down? That's what I do with my WM15 and it seems to do the trick.

That's what I'm wondering, I usually run mine all the way down on my Bass 750. Does completely bypassing it sound that much different than having it turned down all the way?

[mellohawk]

Originally Posted by will33

I boost around 250 or so with mine as well as taking out the lowest of lows 30-40hz stuff. It "un-SWR's" the sound but still isn't quite the same as the mod. The mod makes it more poweramp type flat power and would make it sound louder, fuller. The eq tweaking gets close but it sounds like there is still a little hump and dip stuff happening.

This is exactly what I do on my SM-500. I usually boost ~200hz, add a bit of 500hz (for articulation), and I also set the first paragraphic slider to dump 31hz. It would seem that if you had a flat response to begin with, you'd have far more control with the paragraphic EQ.

From what I have gleaned from the original thread, the mod drastically changes the sound, un-SWR'ing the preamp, and allowing you to still use the AE to saturate the second stage of the 12ax7 for further gain and harmonic enhancement. I personally do not favor scooping the mids in a live setting as that's where I can 'hear' the bass. Installing a switch to choose either voicing would be way cool too.

I've been using an old Carvin Pro Bass 15 preamp to bypass the SWR preamp and it sounds very good set flat, but being solid state, it sounds a bit too 'clean' for my taste. I'm thinking that if I modified the SWR to get close to that flat response, and kept the tube character of the amp, and had more tonal control that it would be win/win/win situation.

Thoughts?

[lowfreqgeek]

Originally Posted by pin_head_47

That's what I'm wondering, I usually run mine all the way down on my Bass 750. Does completely bypassing it sound that much different than having it turned down all the way?

Turing the AE knob down does not give a flat response. It's still very heavily contoured. Bypassing it make a significant difference in the tone compared to turning the AE down, to my ears. It sounds fuller in the mids and not as razor-sharp-bright.

The AE circuit is an interactive filter that simultaneously moves the bass/mid/treble freqs while boosting lows and highs and cutting mids. The audible effects are also volume-dependent. IMO, the AE sounds better with the amp cranked than at more listenable levels - which is a function of our our ears work. Our hearing sensitivity is higher between 300 and 3kHz (more or less - the nominal range of speech and right in the mid range for bass). The louder something gets, the flatter our ears become and the more pronounced the midrange seems.

With the AE bypassed, it sounds full and clear at lower volumes. At higher volumes, it may seem too mid-heavy and aggressive, so it's nice to have the AE available when you may use it for a low-volume acoustic duo one night and a rock gig the next.

[mellohawk]

Originally Posted by pin_head_47

That's what I'm wondering, I usually run mine all the way down on my Bass 750. Does completely bypassing it sound that much different than having it turned down all the way?

As lowfreqgeek explained, no, it won't. The tone stack circuitry is still there 'contouring' what is represented as flat on the paragraphic EQ. Putting the mids back where they've been attenuated won't give you a truly flat EQ considering the Q of each paragraphic frequency sweep knob has a fairly narrow bandwidth. You can try to boost/cut the desired freqs back into the neighborhood to attempt a flat response, but you will likely see some bumps/ruts on the graph (like a bumpy road). Close maybe, but not truly flat.

It would be nice if you had the option to bypass (or flatten) the tone stack altogether so you could have total control - starting with a flat spectrum, then adding/subtracting freqs to find your desired tone.

I know I could just buy a good tube preamp and run it into the effects return to bypass SWR's tube preamp, but why not use what's onboard if it can be given this flexibility and have the best of both worlds -- flat response and 'SWR sound'?

[lowfreqgeek]

Originally Posted by mellohawk

I know I could just buy a good tube preamp and run it into the effects return to bypass SWR's tube preamp, but why not use what's onboard if it can be given this flexibility and have the best of both worlds -- flat response and 'SWR sound'?

That's the route I took, and I don't regret it AT ALL. In fact, I rather like that amp now, and my main amp is an Epifani UL502. Now I'm not saying the SWR is "the same" or "better" than the 502, and I still prefer the 502 for *most* things, but I'm really enjoying the SWR again. The best part is the *POP* that happens when I turn it on at home. The wife, kids, and pets all jump at the same time.

I've also found the DI on the Bass 350 to be exceptional - even compared to my Avalon U5. I'd even say the SWR DI sounds more natural with a good bit of warmth and detail than the U5. I prefer it in the direct mode, taken off the first-stage tube, before the gain control/AE/second-stage tube/EQ.

Speaking of the DI, the next mod I'd like to make is the inclusion of a Jensen transformer in place of the differential amp. Just haven't gotten to it. In the mean time, I may rig up an easy way to use my Radial JDI after the tube.

Another thing I'm experimenting with is running a separate preamp (VTbass, U5, etc) into the effect return and using the effects blend to blend channels. I really need to get an A-B-Y box to run the same bass into each pre for an even different flavor, but it's really nice to use a separate preamp into the return for a second bass. My Epifani has 2 channels, so I don't need to go that route with it.

My old Studio 220 was super-versatile with the AE bypass mod. The additional EQ and the crossover on the back added more flexibility. I really miss that amp! A little underpowered (ok, a LOT), but an amazing studio amp, for sure. Perhaps I need to find an SM-400s with the same original preamp as the ST220.