BAJA Orange AD200B MkII bass amplifier in a 9v pedal 270318

bajaman · Post by **bajaman** » 28 Mar 2018, 00:56

Hi folks
here is something a little different - a bass amplifier 9v effects pedal

not built yet, but will post pictures when it is

cheers
bajaman

: vero cuts.png (7.39 KiB) Viewed 7915 times

bajaman · Post by **bajaman** » 29 Mar 2018, 09:14

ok folks - I built it and it is now verified

HOWEVER - one small mistake on the vero layout (4k7 resistor connected to wrong side of driver stage clipping diodes).
I did feel that the gain was a little too low as well - i had to have all the controls on max to get any output stage clipping, so i increased the two preamp gain stages by 15dB (changed the 2k2 feedback resistors to 6k8). To maintain the same high frequency rolloff characteristic and phase shift I had to decrease the paralleled ceramic capacitors by the inverse of the gain (x .324 times).
Anyway here is the rev 1 vero layout and schematic - build pictures will follow tomorrow when it is daylight.
Happy Easter to all - try not to eat too much chocolate - lol

cheers
bajaman

deltafred · Post by **deltafred** » 29 Mar 2018, 13:57

Mmmmm, bass amps.

Many thanks Bajaman.

bajaman · Post by **bajaman** » 30 Mar 2018, 00:43

here is my prototype build - I ran out of TL072 opamps so used some OPA2134 devices I had lying around - a bit of overkill but sounds really nice.
I really like the sound and feel of the Orange AD200B bass amp - has a really fat sound with a natural growl - very simple but effective sounding design in my humble opinion

cheers
bajaman

BlueMeany · Post by **BlueMeany** » 30 Mar 2018, 16:06

Hi Baja, cool stuff! What is going on with the tl062 stage? Looks interesting.

Groovenut · Post by **Groovenut** » 30 Mar 2018, 16:18

BlueMeany wrote:Hi Baja, cool stuff! What is going on with the tl062 stage? Looks interesting.

I believe it is modeling the output stage behavior

BlueMeany · Post by **BlueMeany** » 01 Apr 2018, 16:37

Groovenut wrote:
BlueMeany wrote:Hi Baja, cool stuff! What is going on with the tl062 stage? Looks interesting.
I believe it is modeling the output stage behavior

If Bajaman would be so kind I would love to read a few details on the tricks and techniques which are being employed to create these emulations. They certainly seem a lot more involved than the usual "amp sim" with at least one or two more layers of thought applied to them.

bajaman · Post by **bajaman** » 01 Apr 2018, 22:29

If Bajaman would be so kind I would love to read a few details on the tricks and techniques which are being employed to create these emulations.

Hello BlueMeany.
Happy to oblige

All of my designs start off by analysing the actual tube amplifier schematic in a spice program to determine the transfer characteristics including it's phase responses (most important) at all stages. Vacuum tubes have a relatively high output impedance and some amplifier designs use cathode followers in an attempt to lower this so that proceeding stages do not load the gain stage too much. They also work on typically 300 volts which allows a lot of signal headroom, especially as most tube gain stages used in tube amplifiers have typically 30dB of gain. A 1 volt signal applied to the input of such a tube amplifier section would potentially (ignoring load effects) give a 40 volt signal at the output - leaving a massive 260 volts of clean headroom

However, apply this 40 volt signal to the next tube stage and the tube will attempt to deliver 1600 volts - impossible from a 300 volt supply so the tube will be driven into heavy clipping. Without going into the exact nature of how the tube clips - there are plenty of articles on line that explain grid current construction, blocking distortion etc. I would recommend Merlin Blencowes excellent "Designing Tube Preamps for Guitar and Bass" for a more in depth study

Back to my pedal designs - 9 volts does not allow a lot of headroom and consequently gain stages have to be trimmed considerably to give the same clean signal response as the tube amplifier individual gain stages. In my attempts to mimic the response characteristics of tube amplifiers but at 9v, I have tried using jfets, small signal low noise transistors, cmos, cmos op amps and have discovered the most realistic and practical devices to be the humble Bifet input op amps such as the TL07* and the low current draw TL06* series devices - occasionally I like to use the high spec lower noise OPA213* devices as input stage devices to keep the hiss level down, but i have found that these devices do not sound as good to my ears as output stage devices. Of course these are only my views and you may hear completely differently

The Bifet op amp has a high input impedance like a tube when used as a non inverting gain stage and a low output impedance (unlike the tube) - I like to think it has it's very own cathode follower stage built in

The vacuum tube has a greater input capacitance compared to the bifet op amp and some designs exploit this with a small resistor in series with the grid - this in effect acts as a low pass filter and rolls off some high frequency harmonics BEFORE they enter the tube. the bifet op amp sometimes needs an additional capacitor fitted at the non inverting input to duplicate this tube response characteristic - some times the high frequency rolloff is achieved by placing a small capacitor in parallel with the feedback resistor in the op amp equivalent - sometimes a combination is used - whatever method gives a similar frequency and phase shift characteristic to the tube design

Before any attempt is made to recreate the tube circuit with op amps, I examine which stage clips first in the tube amplifier simulation and whether it is symmetrical or not. Then i continue increasing the signal at the input until the preceding stage clips and note whether it is symmetrical or not - i make a note of the input signal levels

In the meantime - hope this helps - check out the latest design Baja Ampeg SVT bass amplifier

cheers
bajaman

johnk · Post by **johnk** » 02 Apr 2018, 08:51

being a bass player, i just drew up and etched a pcb for this one. it works and sounds good clean but unfortunately, i find the distortion to be less than desirable having a funky decay.

peterc · Post by **peterc** » 02 Apr 2018, 09:02

Great description Baja, thanks for that.

Peter

bajaman · Post by **bajaman** » 02 Apr 2018, 10:58

being a bass player, i just drew up and etched a pcb for this one. it works and sounds good clean but unfortunately, i find the distortion to be less than desirable having a funky decay.

try this then

cheers
bajaman

johnk · Post by **johnk** » 02 Apr 2018, 16:25

just tried rev2. it still has a really ugly decay on the distortion (it sputters out as it fades- unlike an actual tube amp).

johnk · Post by **johnk** » 02 Apr 2018, 19:38

i went back to Rev1 and omitted the volume control (since the actual amp doesn't have one) and it sounds A LOT better that way (it cured the funky distortion decay). i just connected treble 2 to where it used to connect to volume 3. I also changed the bass pot's taper to an audio pot for a better sweep of the control (it previously came almost fully on right off off fully counterclockwise). the pedal sounds excellent now. thanks bajaman!

johnk · Post by **johnk** » 03 Apr 2018, 17:36

BTW, this pedal sounds awesome running at 18 volts too. at 18V it has more clean headroom and a bit more output.

chili.dog · Post by **chili.dog** » 12 Sep 2018, 05:07

This may be a dumb question, but if designing the model while limited to 9V is making it difficult, why couldn't you design around 18V and use a charge pump? Would that make things better or would it just be more headaches?

johnk · Post by **johnk** » 12 Sep 2018, 07:01

that's exactly what i did on mine.

chili.dog · Post by **chili.dog** » 12 Sep 2018, 19:10

Yes, I see that. But the pedal wasn't specifically designed around 18V. I'm asking if baja would be able to get closer to the real amp if he designed it around 18V or even higher.

AAntonio · Post by **AAntonio** » 27 Sep 2019, 20:03

can it be connected to a cabinet?

thetragichero · Post by **thetragichero** » 28 Sep 2019, 02:54

you are welcome to hook it up to a cabinet, but if you want it to produce sound I'd suggest putting some sort of power amplifier in between

AAntonio · Post by **AAntonio** » 28 Sep 2019, 05:52

thetragichero wrote:you are welcome to hook it up to a cabinet, but if you want it to produce sound I'd suggest putting some sort of power amplifier in between

Is there a complete project to connect to a cabinet?

BAJA Orange AD200B MkII bass amplifier in a 9v pedal 270318 [documentation]