BAJA Celestion Blue driven by tube amplifier filter circuit  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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bajaman
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Post by bajaman »

One of the major failings when using a speaker cabinet simulator with transistor power amplifiers is the complete absence of a varying impedance load which interacts with the output of a vacuum tube amplifier and gives far from a flat output level response characteristic. This is because a tube amplifier has a very high output impedance when compared to the very low output impedance of the transistor amplifier and consequently does not damp the speaker as much. Examination of a typical 12 inch guitar loudspeaker's impedance curve reveals that it is far from the nominal 8 ohms - at resonance it can rise to well over 30 ohms and the voice coil inductance causes a gradual rise in impedance above this resonant peak. The tube amplifier sees this varying reactive load impedance and accordingly puts out subjectively more musical power output at resonance and at the extreme high end - there is a gradual droop in the output response in the lower midrange. A transistor amplifier with it's very low output impedance does not see this varying speaker impedance anywhere near the same, in fact it damps the impedance curve and therefore will put equal power output levels into the speaker across it's complete frequency range. This is one of the reasons why transistor amplifiers all tend to sound the same regardless of what brand of loudspeaker is connected. Also because of the increased midrange response and lack of any resonance they tend to sound flat and lifeless when compared to the thump and sparkle characteristics of a tube amp driven speaker.
The following filter design is an attempt to model this low end resonant peak and gradually rising treble response characteristic inherent when connecting a speaker to a tube amplifier. It is intended to be added after any cabinet simulator to enhance not replace it. This particular response characteristic is based on the Celestion Blue 12 inch alnico speaker.
Comments and questions most welcome.
Cheers
bajaman
baja celestion blue IR tube amp filter.jpg
baja celestion blue IR tube amp filter response curves.jpg
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Post by bajaman »

for a Celestion G12T75 type response change 330n to 390n, and change 2k7 to 3k3 in the gyrator section (bottom op amp) - this lowers both the height of the resonant peak and the resonance frequency slightly. :wink:
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Post by DrNomis »

Cool, I might have a go at building this, my Zoom G2 has speaker simulation built into it but it's digital-modelling rather than analogue..... :thumbsup
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Post by bajaman »

this is NOT a speaker simulator :wink:
It is a response simulator for a tube (valve) driving a speaker - it is an attempt to simulate the effect of the loudspeaker's resonance and rising voice coil inductance effects on the overall frequency response when driven with a typical high output impedance tube amplifier. You may need to use one of the many cabsims available before this :wink:
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Post by DrNomis »

Cheers Bajaman, I'll have a good google for a cab simulator on the internet..... :thumbsup


Actually, have you heard of PAIA's Stack-In-A-Box Tube Preamp?, just wondering if the cab simulator circuit from it will work with this circuit, I'll see if I can find a schematic and post it here..... :thumbsup


Okay ,here's the schematic, the cab simulator sections are where IC 1 B and IC 2 A are :
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siabsch.pdf
PAIA SIAB Schematic
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Post by bajaman »

okay - small point of clarification :!:
If you are paying through a solid state transistor chip amp or similar this little circuit should get you closer to the speaker response characteristics of a tube amplifier.
If you are using a tube amplifier already this circuit is not for you :wink:
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Post by DrNomis »

Ah, I get it now, thanks for that Bajaman..... :thumbsup
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Post by bajaman »

I built one today - took me about 30 minutes on vero board :wink:
I connected flying lead input and output sockets and powered it with a 9 volt battery, then I plugged it into the input of my test bench 60 watt LM3886 chip amp (nothing fancy :wink: ) and plugged my guitar into the input - no tone controls just guitar, filter and transistor power amp connected to a small 12 inch boxed speaker under my workbench.
I was actually quite amazed with how it transformed a dull wooly middly power amp and speaker combination into a bright and much more lively sounding system. I was expecting a boomy bottom end due to the resonant peak but was presently surprised that this was not the case. :D
In my opinion it was well worth the design and build and definitely an improvement to the sound reproduction when placed at the the input of a transistor power amplifier.
Give it a try and let me know what YOU think :)
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Post by DrNomis »

bajaman wrote:I built one today - took me about 30 minutes on vero board :wink:
I connected flying lead input and output sockets and powered it with a 9 volt battery, then I plugged it into the input of my test bench 60 watt LM3886 chip amp (nothing fancy :wink: ) and plugged my guitar into the input - no tone controls just guitar, filter and transistor power amp connected to a small 12 inch boxed speaker under my workbench.
I was actually quite amazed with how it transformed a dull wooly middly power amp and speaker combination into a bright and much more lively sounding system. I was expecting a boomy bottom end due to the resonant peak but was presently surprised that this was not the case. :D
In my opinion it was well worth the design and build and definitely an improvement to the sound reproduction when placed at the the input of a transistor power amplifier.
Give it a try and let me know what YOU think :)
cheers
bajaman


I think I've got some spare pieces of vero laying about so I might build the circuit up this weekend, cheers Bajaman.... :thumbsup
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Post by bajaman »

also check the amptweaker tight metal thread page 18 :secret:
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Post by atreidesheir »

Is there no high frequency rolloff above 10k? The chart looks like the highs are on a shelf from 7k and higher.
I know this is not a speaker emulator, but is that correct? I ask because I am programming your curves in my ReaQ vst plugin in Reaper daw to enhance my cab sims. I will share the band law for each band when I hear from someone.

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Post by atreidesheir »

This is the curve as I see it for ReaEQ. If you have not tried Reaper's vst set, it is awesome and free, and that makes it more awesome. They are all excellent tools for anyone recording music, and if you have not tried Reaper and you record music, its demo is generous and it is $60 for a lifetime license. But you can download the effects set rfor free to use in any daw.

Image
Here are the specifics.

----------------------frequency ----------------gain ------------------bandwidth (octave)
band 1: ---------------59.8-------------------------+4.2 --------------------------------------2.00

band 2 ---------------85. -------------------------+12. ----------------------------------- 2.00

band 3 -----------------287. -----------------------------7.2 ------------------------------1.73

band 4 ------------------- 401.3 -------------------------+12. --------------------------------4.
HIGH SHELF
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Post by bajaman »

please be advised that this is how a tube amplifier reacts with an UNMOUNTED Celestion Blue 12 inch loudspeaker - it will of course respond quite differently when the speaker is mounted in a sealed cabinet such as a Marshall 1960A cabinet :wink:
the resonant frequency is raised from 75Hz to approximately 130Hz when enclosed :wink:
Check my post in this sub forum section regarding the Celestion G12M Greenback response characteristics :secret:
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Post by atreidesheir »

thanks
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Post by bajaman »

That curve is representative of an open backed cabinet response though :wink:
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Post by J0K3RX »

atreidesheir wrote:This is the curve as I see it for ReaEQ. If you have not tried Reaper's vst set, it is awesome and free, and that makes it more awesome. They are all excellent tools for anyone recording music, and if you have not tried Reaper and you record music, its demo is generous and it is $60 for a lifetime license. But you can download the effects set rfor free to use in any daw.

[ Image ]
Here are the specifics.

----------------------frequency ----------------gain ------------------bandwidth (octave)
band 1: ---------------59.8-------------------------+4.2 --------------------------------------2.00

band 2 ---------------85. -------------------------+12. ----------------------------------- 2.00

band 3 -----------------287. -----------------------------7.2 ------------------------------1.73

band 4 ------------------- 401.3 -------------------------+12. --------------------------------4.
HIGH SHELF
I tried this with my Hotline 2 cab sim, I use Reaper... HOLY SHIT!!! This sounds as KING KONG'S BALLS!!![/b] You got anymore tips???

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Post by atreidesheir »

That is the open back cab sound. If you want to experiment with closed back cab sound, roll off treble @7khz.
No more tips really. Just following bajaman's lead.
"Contemplate it - on the tree of woe." :Thulsa Doom

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