Bass Booster - for FX loop

[JohnH]

I’d like to show you my weekend project, just for interest. It’s probably not much use to others, unless you have an amp like mine. But for me, this is one of the most useful, and most minimal, effects boxes that I have built. It addresses an issue with my Marshall DSL401, which is a great amp, but has a tendency to thinness in the overdrive modes, when driving the stock speaker. For this reason, some owners replace it with a Vintage 30 for more bass, but they are not cheap. The OD modes are set up for screaming Marshall leads, with strong bass de-emphasis. This is fine at high gain, where there’s enough boost to bring the bass back, but at more moderate crunch, it wants more bass.

So my gadget is a bass booster which sits in the parallel FX loop. It has just one function, with no controls, which is to filter out the low bass register and boost the cr@p out of it. This gets fed back as a pure bass signal, and the FX knob on the amp then becomes a low bass control, with the ability to dial in as much low end as I could ever need.

It’s designed in a similar way to several other pedals that I’ve made, using JFETS for gain and buffering. Two inverting gain stages and a non-inverting buffer result in an output which is in phase with the input, and ready to be fed back to the mix. At each stage, RC networks cut down on signals above about 200 to 300Hz. It also cuts down on signals below guitar frequency, so spurious thumps and bumps do not get boosted.

It seems to be well behaved, and does not add noticeable extra distortion or noise. It is like a single slider on a graphic eq, but more purpose-designed. On the clean channel, with FX at about 3, the sound is deep and resonant like a 4x12. The OD channels now do a perfect classic crunch with FX mix at about 5.

Heres the schematic



Here it is, built into a diecast box, sitting like a cyborg implant on the DSL



cheers
John

[Runewalker]

JH (MasterBlaster):

In my nascent review of the world of tube amp mods, you can find many approaches to this same type of problem in various mfg products, where the mfg voices the overdrive channels as too dark, too bright, too diodey-distorted, too middy, too pink.... etc.

Many modders decide on an internal approach opposed to the input approach with a box or EQ. In that approach they shift the voicing with changing the values of various resistors or cap in the signal stream. More permanent perhaps, more experimental as different values are tried.

Your approach at least avoids trial and error within the chassis. I have a new/old amp that I am trying to fine tune to my ear, and while I executed a few smaller mods, I hesitate on other fine tuning with different cap/resistor values.

So I will experiment with a similar concept as yours, except with a effects looped 16 band EQ.

Your little device is a nice singular approach to fine tune your amp.

One other approach would be to take your 6 string input devices (guitars), replace the strings with telephone pole guy wires, and tune down an octive. Completely reversable and retains the purity of the Marshall. I would recommend the steel opposed to aluminum guy wires.

RW

[UnklMickey]

hello Runewalker,

Welcome to Guitarnuts2.


you know, once-upon-a-time, we had another member, who called himself Runewalker.

he graced these boards with a moderate number of in-depth. insightful posts, spiced with liberal dose of humor.

hopefully you'll aspire to be just like him.

LSH....

runewalker said:

....the mfg voices the overdrive channels as too dark, too bright, too diodey-distorted, too middy, too pink.... etc....

buzzy, fuzzy, muddy, fizzy, boomy, boxy, flabby, brittle, sterile, dark, bright, middy, and scooped -- i've heard those terms used before, and understand..........."diodey-distorted"? - okay, i get that one too.

pink?





John,

it looks like you've spent a bit of time and thought, tuning this.

i take it, you've "right-sized" C2, C3, and C4 to roll off the lower-than-lows.

it also looks like you have classic low-pass configurations comprised of:

R11 and C8, R5 and C7, R7 and C5.

but, my guess is, bypassing the drain resistors with C6 and C9 is the extra-thick icing, on this cake.



i'm particularly interested in the "partial" bypass of the source resistor for J1.

it's common to see completely un-bypassed or fully-bypassed source/(emitter)/(cathode) resistors.

and sometimes even a high-boost of sorts, by reducing the size of the cap, so the extra gain rolls off at lower frequencies.

but this is a bit different.

the cap is large enough to go to low enough frequencies, that we would consider this as fully-bypassed in terms of frequency response.

but then there's R12,

i've seen a pot used in that location as a "gain" control before.

so i would guess that might have been the kind of thinking that spawned this?

dial in the proper amount of gain, to get the overall gain where you want (determined either through modeling or testing), then use a fixed resistor?


cheers,

unk

[Channelman]

John,
Just out of interest I've simulated your bass booster.
It's rolling off at about 30dB/octave which is correct
for 5 Low Pass RC nerworks.
Regards,
Channelman

[sumgai]

John,

Nice work! Way too much possibility here for us to play with!

+1, just for the sheer potential. ;D


unk,

The partial bypass is fairly much as you expected, it's fine tuning for the particular JFET being used. Dialing up different values for R12 would probably let one experiment with many different JFET types.

BTW, for those interested, the 2N5457 has pretty much been supersceded by the new nomenclature, the J201. And you might recall that ccoleman has a thread going right now that references an auction on eBay wherein a seller has a whole passel of these little beauties for sale in small- or large-quantity lots. Along with with beaucoup other goodies for DIY experimenters.


sumgai

[JohnH]

Thanks for the comments.
Unk - R12 allowed me to adjust the overall gain - I first trialed it without this resistor, and it was too much. I was aiming to get the most useful range out of the sweep of the amps fx pot.

C2, C3 and C4 are the low passes, but I set them at different frequencies, to get a steep cut of ultra lows, but a more controlled response within musical range

Channelman- thanks for trying the sim - I cant see the image that you posted however

Sumgai - Ive tried some J201's, but they have a much lower Vgs voltage, leading to higher gain in this configuration. I find I get nicer results in general with the 2N5457, but I note that all the circuits on www.runoffgroove.com are based on J201

cheers

John

[Channelman]

johnh said:

Channelman- thanks for trying the sim - I cant see the image that you posted however
John

John, I've sorted that out now...I was just sorting some old files out. Try a 'refresh'....should be there now.
Channelman

[UnklMickey]

johnh said:

...C2, C3 and C4 are the low passes, but I set them at different frequencies, to get a steep cut of ultra lows, but a more controlled response within musical range...

hi John,

yes, i get that they are at different frequencies.

about 87Hz, 56Hz, and 23Hz.

but aren't they high pass (low-cut) filters?


wouldn't the low passes (high cuts) be C8, C7, and C5?

322Hz, 219Hz, and 219Hz

along with C6 and C9?

286Hz?, and 286Hz?


cheers,

unk

[JohnH]

Yes Unk - but you know what I mean - typing at speed pre-coffee!
CM - thanks for graph
J

[UnklMickey]

yeah, i figgered as much.

to calculate the frequency of C6, do you just use the resistance of R1, and the usual formula?

or does it require calculating the impedance at the drain of J1, and including that in some more complicated formula along with R1?

[Runewalker]

unklmickey said:

hello Runewalker,

Welcome to Guitarnuts2.


you know, once-upon-a-time, we had another member, who called himself Runewalker.

he graced these boards with a moderate number of in-depth. insightful posts, spiced with liberal dose of humor.

hopefully you'll aspire to be just like him.

He sounds like an uberhero. Oh, be still my heart, in the hopes of meeting him.

Not to divert from JH's thread.

Re-reading your initial post where the cure from other hobbyists is a darker Celestian driver, your box almost seems like a targeted cab and speaker emulation -- built for the specific deficiencies of the stock speaker.

[UnklMickey]

hey Rune, old buddy, not so fast.

ya still didn't answer my question:

unklmickey said:

runewalker said:

....the mfg voices the overdrive channels as too dark, too bright, too diodey-distorted, too middy, too pink.... etc....

buzzy, fuzzy, muddy, fizzy, boomy, boxy, flabby, brittle, sterile, dark, bright, middy, and scooped -- i've heard those terms used before, and understand..........."diodey-distorted"? - okay, i get that one too.

pink?

so what's pink? wait, let me re-phrase that. what does the term "pink" mean -- in regards to a sound quality?

[Channelman]

My guess is he means 'Pink' as in 'Pink Noise' which is
'White Noise' with an element off Bass Boost.
'Pink' beacause it's 'warm' like 'red', maybe?
Channelman

[JohnH]

runewalker said:

Re-reading your initial post where the cure from other hobbyists is a darker Celestian driver, your box almost seems like a targeted cab and speaker emulation -- built for the specific deficiencies of the stock speaker.

Thats basically what it does, plus ballancing the bass cut that is built into the OD channel. But I figured that with a 40W amp driving a 100W speaker, and me needing probably no more than 10W, then I have plenty of overhead available to use some on bass boosting.

I sized the caps initially just matching the relevent resistor to the cap impedance at a frequency I guessed. Then I looked at it as a sim, and then built it on a breadboard to listen to. That lead me to reduce the high-cut caps to raise the cutoff frequency compared to my first guess.

John

[Runewalker]

unklmickey said:

hey Rune, old buddy, not so fast.

ya still didn't answer my question:
......
so what's pink? wait, let me re-phrase that. what does the term "pink" mean -- in regards to a sound quality?

"pink" was just a troll to see if anyone is actually reading this stuff. Amazingly they apparently are. I hear RuneW was a smartA$$ so I am trying to aspire to that standard. Apparently needs work. Perseverance.

I think the term "too pink" was a riffing but vague reference to pink noise (as noted by Channelman), which is somehow different from white noise, but I don't really remember how (and fortunately C-Man does). But it was really just a toss away for fun (writer needs to get a life).

However, I sort of loved your first [implied][ interpretation .... and would expect nothing less from UM.

Going back to J's device. This little tool is intriguing. As some may remember I like to use a modeler a bit. I am finding as I move around between amp the patches are quite specific to the amp, cab and speakers. It is tiresome to have to keep re-programing or to set up respective banks to a target amp/cab/speaker combo. I like to switch up speaker destinations between amp, because I am fickle and like to switchhit in the harem, and frequently wish for a little device in the direction of Johns, but with more controls over the parameters.

A multiband EQ gets close, but with 16 bands or more, it almost becomes a programing device.

Something like John has here with maybe three parametric and level controls might be very nice. But that of course exceeds the objectives of JH's solution.

[UnklMickey]

runewalker said:

...However, I sort of loved your first [implied] interpretation ....

i have absolutely no idea what you are talking about. - *snicker*

Rune,

i think using a graphic would work okay, but they tend to sound a bit harsh when you have adjacent bands with one cutting heavily and it's neighbor boosting heavily.

even a single band parametric would be easier to use.


there's an awful lot to love about JohnH's design.

the most noteworthy is it's complement of controls!

so no matter where you place the box, the bass control is just as easy to reach.


i could imagine using this in a bi-amped situation.

maybe not so much for guitar, but perhaps in a bass version, with filtering scaled down an octave.


+1 John, and congratulations.



unk