SWR Aural Enhancer Bypass Update - Level Normalization

[40Hz]

might be better to call it the RCA circui

Implementations vary. But the theory remains the same. So what’s in a name that doesn’t accurately describe its function really? It’s just marketing beyond that. I’ll just call it a tonestack -and call it a day.

 

[40Hz]

Consider the normalization mod renamed!!! Thanks Agedhorse!! LMAO

View attachment 3936889

LOL!

What comes next? The Martini tonestack with knobs for Gin, Vermouth, and Olive and a Dry/Bone Dry switch?

 

[agedhorse]

The Aural Enhancer circuit in SWR does something that the Fender implementation doesn't - in addition to the applying Fender-style scooped voicing (see * below), the AE circuit adds gain and harmonics as you turn it up and then exaggerates the scoop once you go past around 2 o'clock on the knob.

* the Fender implementation of the tone stack has a dip around 500Hz while SWR shifted it to around 240Hz.

Someone else will know the history better than I but I recall reading on here that the Fender tone stack was something Leo Fender got out of an RCA tube implementation guide? So (I'll stir up the pot here...) it might be better to call it the RCA circuit?

The AE circuit doesn't itself add either gain or harmonics, it's purely a set of "nested" passive filters.

Depending on how the circuit MAY load the circuit, that circuit MAY generate additional harmonics (or may not), and any apparent gain can't come from the passive filter either, that has to come from either the active circuit driving the filter or from the active circuit(s) following the filter.

 

[ihixulu]

Here's the post by duff2 that I'm referring to (sorry in advance for amateur quoting fu):

SWR Aural Enhancer Bypass

"...the mid-scoop is present even with the aural enhancer set to zero (unlike the Enhance control on the Eden and Thunderfunk amps). Up to a 2 O'clock setting the SWR aural enhancer controls how much additional signal from your bass passes through the second half of the 12AX7 tube. This adds gain and extra high end harmonic coloration to your sound but does not change the preset EQ curve present when the control is in the off position. At settings beyond 2 O'clock there's no extra gain, but rather an additional mid scoop added around 200 Hz.

Graphically, here's what the aural enhancer control does on an SWR amp:

"

I'm no EE so I am not going to get in the weeds of the argument as to whether it is adding gain or removing attenuation or something. As an end user, it behaves as described above.

 

[frankie5string]

I played around with the AE control on those amps for years and I don't recall any gain or boost from them. To my ears, it was more of a complicated scoop. It didn't seem as "interesting" to me as the Eden control of similar design (but I think had a different electronic design/intention) but it could actually get you out of trouble very quickly if you had to play a gig in a Gym/Cafetorium with a lot of hard walls.

I also never really understood why people wanted to go to the preamp and "solder out" the circuit. The amp sounded fine with the control set to zero if you didn't really like what it did.

 

[ihixulu]

I've noticed a change in the gain, and I can see it reflected in the metering on the GP. Granted we're not talking get-to-overdrive levels of gain (which isn't the design's point anyway) but it can easily push you into clipping territory.

I also never really understood why people wanted to go to the preamp and "solder out" the circuit. The amp sounded fine with the control set to zero if you didn't really like what it did.

I like mine stock but I can see why people would want a different tonality. I would use it as an excuse to buy a new amp! At the very least it's a fun customization project for an amp that isn't really vintage valued.

 

[Gianni "Orlandez" Orlati]

No matter what the called it, the circuit was based on the Fender tonestack. They could have called it "bourbon on the rocks" but that doesn't change the circuit.

Ah, correct from an electrical engineering perspective, but, commercially-wise, if we talk about “aural enhancer”, we talk about SWR @agedhorse

 

[wraub]

On my Grand Prix, I usually have the AE at around 9 o'clock. It's been that way for a while, I may have to mess with it some more, with the recent changes to my main bass...
I honestly don't think I've ever had it above noon for any length of time.

 

[SNCpre]

The old SWR amps definitely need some hi-pass filtering. While you're at it.

Very good advice.

No HPF:

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HPF@35Hz:

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[Ellery]

SWR Aural Enhancer Bypass

SWR Aural Enhancer Bypass Mod revisited and updated

I recently incorporated an Aural Enhancer Bypass mod into my SM-400. This time around, I didn't simply move the 100n cap in the fixed value Fender tone stack as described in the first thread above. Instead I went for a more elaborate version that was used in the Headlite so that the output level of the AE on and AE off confgurations were normalized. This is one drawback of the original AE bypass mod. The added gain in bypass mode renders the limiter circuit unusable in its stock configuration.

While incorporating the Headlite AE bypass circuit into my SM-400, I realized there was a much simpler way to do this. To normalize the output level of the stock and bypasssed configurations, all you need to do is change the value of a single resistor in addition to relocating the 100nF cap. After doing this, the limiter works equally well in bypass mode.

Here's how it's done:

Important: bring your amp to an experienced tech if you are not qualified to perform this mod yourself. There is 250V on the supply rails for the 12AX7 tube. If you do not discharge the caps properly, you will fry all the op amps if you ground the 250V supply rail during disassembly....or worse, you could receive a nasty shock if you aren't careful.

Stock Configuration

View attachment 3935819

Bypass Mod with Gain Normalization:

View attachment 3935820

3PDT Switch Version:

View attachment 3935821

Is the Aural Enhancer that undesirable? When my 750x was my regular live rig I usually had it turned up a little bit.

 

[agedhorse]

Here's the post by duff2 that I'm referring to (sorry in advance for amateur quoting fu):

SWR Aural Enhancer Bypass

"...the mid-scoop is present even with the aural enhancer set to zero (unlike the Enhance control on the Eden and Thunderfunk amps). Up to a 2 O'clock setting the SWR aural enhancer controls how much additional signal from your bass passes through the second half of the 12AX7 tube. This adds gain and extra high end harmonic coloration to your sound but does not change the preset EQ curve present when the control is in the off position. At settings beyond 2 O'clock there's no extra gain, but rather an additional mid scoop added around 200 Hz.

Graphically, here's what the aural enhancer control does on an SWR amp:

View attachment 3936983 "

I'm no EE so I am not going to get in the weeds of the argument as to whether it is adding gain or removing attenuation or something. As an end user, it behaves as described above.

His analysis about the gain of the circuit is simply wrong.

It's a totally passive network, there is no gain in the circuit only attenuation.

You can reduce the attenuation but that is not the same as increasing the gain (it will increase the level, which is different).

If you (or others) are going to claim something, at least be accurate for the benefit of other forum members. You don't need to be an EE to be accurate.

 

[agedhorse]

Ah, correct from an electrical engineering perspective, but, commercially-wise, if we talk about “aural enhancer”, we talk about SWR @agedhorse

ESPECIALLY commercially, it's still based on the Fender tone stack circuit. Why is this a problem to accept?

 

[Gianni "Orlandez" Orlati]

ESPECIALLY commercially, it's still based on the Fender tone stack circuit. Why is this a problem to accept?

Don’t think Fender called it like that @agedhorse but I may be wrong .. in that case the world would not collapse for this anyway .. I know experts tend to be quite proud of their specific knowledge, I’m more of a laid-back bass-player, you know

 

[SNCpre]

Is the Aural Enhancer that undesirable? When my 750x was my regular live rig I usually had it turned up a little bit.

I never intended it to be interpreted this way. Turning down the treble to 9 o'clock and boost @200Hz using the graphic EQ will accomplish pretty much the same thing. Bypassing the Aural Enhancer circuit just defeats the 'baked in' preamp voicing and frees up a little more flexibility in the EQ section. I just find this stuff interesting and posted it here for those who feel the same way. I never intended to offend anyone, start arguments or criticize SWR or Fender.

It seems that everytime I post something here, Talkbass is turning more into this:



Anyhoo, I hope someone finds my thread useful or enlightening in some way.

 

[agedhorse]

Don’t think Fender called it like that @agedhorse but I may be wrong .. in that case the world would not collapse for this anyway .. I know experts tend to be quite proud of their specific knowledge, I’m more of a laid-back bass-player, you know

Yes, in this case you would be wrong.

I'm just accuractly representing the statement and the circuit, it has nothing to do with being proud. That's an insulting comment.

 

[frankie5string]

Also, I have to ask: What is "Gain Normalization" in this context?

 

[agedhorse]

Also, I have to ask: What is "Gain Normalization" in this context?

How's about "level compensation"

 

[frankie5string]

How's about "level compensation"

I recall an old passive Langevin EQ unit that required a separate external line amplifier to do that. As you adjusted the EQ, it automatically controlled the line-amp. I think it covered insertion loss, too, once you calibrated it to the circuit... It certainly was not the lovely, "no-thinking" sort of thing like we have today.

 

[agedhorse]

I recall an old passive Langevin EQ unit that required a separate external line amplifier to do that. As you adjusted the EQ, it automatically controlled the line-amp. I think it covered insertion loss, too, once you calibrated it to the circuit... It certainly was not the lovely, "no-thinking" sort of thing like we have today.

This sort of circuit has been used within a lot of audio applications. There is Mackie's "AutoMix" circuit used in early Tapco mixers that spread the mic stage gain over 2 auto-tracking circuits which really improved both bandwidth and overload margins, there were the common graphic eq make-up gain circuits to normalize the output level post eq (which helped maintain similar levels in bypass mode versus active mode) and there were symmetrical gain-attenuation circuits used to match line level to instrument level on inputs and outputs simultaneously (used on many Yamaha effects processors, and on some effects loops where the gain and attenuation trached each other exactly).

This is a commonly used circuit type, most are manually adjusted, on some a single manual adjustment allocates gain and attenuation of the different stages simultaneously, and with DSP, there are more automatic ways to handle this within the micro-controller or DSP.

 

[Ellery]

I never intended it to be interpreted this way. Turning down the treble to 9 o'clock and boost @200Hz using the graphic EQ will accomplish pretty much the same thing. Bypassing the Aural Enhancer circuit just defeats the 'baked in' preamp voicing and frees up a little more flexibility in the EQ section. I just find this stuff interesting and posted it here for those who feel the same way. I never intended to offend anyone, start arguments or criticize SWR or Fender.

It seems that everytime I post something here, Talkbass is turning more into this:



Anyhoo, I hope someone finds my thread useful or enlightening in some way.

Nooo bro I didn't mean that I was offended! This stuff is really interesting to me too. I recall a thread here years ago, folks were talking about how to step around the AE with eq settings like you mentioned. Some did have disapproving words for the AE, but not for people who use it! I believe the discussion was about getting an SWR's tone as close to SVT territory as possible. THAT thread is why I stopped cranking my AE!! These kinds of discussions on TB helped me get started dialing in tones I liked. I totally get it if someone wants a different sound than I do. I also benefit when someone points out a perspective I hadn't considered. Keep on truckin!