Orange Terror Bass amplifier - Attenuate DI Output

[Whoops]

Hello,
a friend of mine has a Terror Bass amplifier head.
The first models had the DI output at Line Level and not not at Mic Level.
It seems so many people complained the output was too hot and it was changed in later revisions.

My friend has the same problem when he uses this amp at concerts since most of the time the DI output will be connected to a Mic Input on the FOH console.  (or its more convenient that way in Live Sound scenario)

He asked me to modify his amp to have a MIC Livel DI output (he prffers to have the amp modified than using and attenuator barrel)

Here is the schematic for the DI output:

I want to implement a 40dbs pad, as most Terror Bass users seems to happy with this level of attenuation

Can I implement the mod for the PAD this way?

thank you so much

 

[Khron]

Looks good to me. Although i'd rather go with 10k resistors rather than the very oddball 8.5k, and maybe something like 220 ohm for the third resistor.

Or if you wanted to get really fancy, and have room on the panel, you could chuck in a 1k or 10k pot instead of the 150/220 ohm, and have adjustable output level ;D Although that's probably overkill...

 

[Whoops]

Or if you wanted to get really fancy, and have room on the panel, you could chuck in a 1k or 10k pot instead of the 150/220 ohm, and have adjustable output level ;D Although that's probably overkill...

thank you so much.
Yes I also thought about a Pot, but the owner doesn't really need it and this way there's no need to modify the chassis.
He just needs the attenuation.

What I think would be nice in this Direct Out is the option to have it Pre EQ and VOL, Orange did the Direct Out Post EQ and Post Vol, as a Sound Engineer I prefer my DI feed to the mixer to not change in volume when the musician wishes in a middle of a gig to turn his volume up, or increase a lot the Bass knob to suit his taste on stage.
But thats also an extra hole for a switch in the chassis and he didn't ask for it

Thank you

 

[Bo Deadly]

I would go lower impedance. If you want to drive long cables you might as well go as low as the op amps will allow. If an OA can handle 1.5K with no problem, since the OAs are driving each other anti-phase, we should shoot for ~3K series resistors. So maybe 3.3K and 68R.

But personally I would just use the barrel plug attenuator at the mixer end.

Although since mixers usually have a line setting I would just use that. Not sure why people would be complaining. I personally would rather the DI out be line level.

 

[Gus]

If you care about CMRR I would do this a different way

Attenuate the signal before the first part of the IC driver pair
and reduce R24 and R26 resistor values (if the opamp stays stable) and match better than .1%

Better cable drive (as squarewave posted) and better CMRR
However being a bass amp and playing live CMRR might not count as much live.

 

[Whoops]

I would go lower impedance.  we should shoot for ~3K series resistors. So maybe 3.3K and 68R.

thanks, how much attenuation will that combo provide?

 

[Whoops]

If you care about CMRR I would do this a different way
Attenuate the signal before the first part of the IC driver pair
and reduce R24 and R26 resistor values (if the opamp stays stable) and match better than .1%

Thanks for that idea, seems to make sense.
How would you proceed?

this is the schematic of the DI circuit:

this is the full schematic:

 

[Bo Deadly]

thanks, how much attenuation will that combo provide?

40db

 

[CJ]

strap a 22 K resistor across R41 for 10:1 reduction.

if it was my amp of course i would use a transformer because the sound is in the iron. 

and maybe a pot on the iron output, with a chicken head knob of course, because the sound is in the chicken heads.   

if you want to get fancy you can add a PIC chip microcontroller programmed to crowbar the main rectifier stack whenever the bass player plays Stairway, or Barracuda by Heart.

 

[Bo Deadly]

strap a 22 K resistor across R41 for 10:1 reduction.

This is why it's important to post the whole schematic. Now we can see the voltage divider in front of the op amps (which is pretty high Z for a bi-polar op amp incidentally).

But for an additional 40dB you would need to strap a 1.5K resistor across R41.

Or replace R41 with a 10K trimmer.

 

[Whoops]

This is why it's important to post the whole schematic. Now we can see the voltage divider in front of the op amps (which is pretty high Z for a bi-polar op amp incidentally).
But for an additional 40dB you would need to strap a 1.5K resistor across R41.
Or replace R41 with a 10K trimmer.

I should have posted the full schematic in the beginning.
Thanks Squarewave.

So overall,  what do you think it's the best solution? The 40dbs U Pad at the the end or attenuate before the opamp?

Most Professional (or a lot of them) live sound StageBoxes have MIC Inputs only, no line inputs at all, thats why it's more practical in a Live Sound scenario to have the DI output at mic level. Orange also changed that to later revisions of these amps (although I only have the schematic for the first versions).
These are 3 examples of Live Sound StageBoxes, all inputs are MIC Preamps:

Thank you so much for all your help

 

[PRR]

> If you want to drive long cables you might as well go as low as the op amps will allow.

Yes, but 150 Ohms is known to work great on very long lines.

And it's only a guitar.

 

[Bo Deadly]

So overall,  what do you think it's the best solution? The 40dbs U Pad at the the end or attenuate before the opamp?

Attenuate before like CJ described.

Most Professional (or a lot of them) live sound StageBoxes have MIC Inputs only, no line inputs at all, thats why it's more practical in a Live Sound scenario to have the DI output at mic level. Orange also changed that to later revisions of these amps (although I only have the schematic for the first versions).
These are 3 examples of Live Sound StageBoxes, all inputs are MIC Preamps:

I'm not familiar with those units specifically but I'm pretty sure they handle line-level signals no problem.

 

[Whoops]

Attenuate before like CJ described.
I'm not familiar with those units specifically but I'm pretty sure they handle line-level signals no problem.

No they don't Squarewave.

I'm quite familiar with these stageboxes and the consoles that work with them. 

Thank you so much for your help

 

[Gus]

what does the cathode of VC2a look like on a scope?

It is not bypassed so maybe you can change where you pickup the signal from the plate to the cathode(lower P to P voltage) and scale it to  the output you want. It is also lower resistance for possible lower noise with a bipolar opamp input

The opamp circuit gives about 6dB gain

 

[Whoops]

Hi,
I performed the mod today to the Di output of the Terror Bass.

I did the mod like CJ and Squarewave suggested, it works perfectly and it's really simple to perform as it's just 1 resistor that you can solder on top of the 220K already in the R41 place.

Thank you all for the help, specially CJ and Squarewave for the simple and efficient solution.

here is the mod:

Also did a video before and after the mod:
https://www.youtube.com/watch?v=kTHUAQFREoM

 

[Gus]

Do the math on how you changed the loading on VC2A.

Note all the loading on VC2A. You changed one part of the parallel loading from 690K (470K +220K)to about 470K.

It is probably OK but note the high pass formed with C12 and the output resistance of a 12ax7 and the loading after the tube.

It does not matter much being guitar and bass frequency ranges and amp distortion

 

[Bo Deadly]

Do the math on how you changed the loading on VC2A.

The load difference is negligible (note there are multiple paths of resistors and pots defining load) and effect on RC would be difficult to measure.

I agree the signal probably should just be taken from the cathode but it's hard to say if the last tube stage is contributing to the sound.

 

[Whoops]

what does the cathode of VC2a look like on a scope?
It is not bypassed so maybe you can change where you pickup the signal from the plate to the cathode(lower P to P voltage) and scale it to  the output you want. It is also lower resistance for possible lower noise with a bipolar opamp input

I agree the signal probably should just be taken from the cathode but it's hard to say if the last tube stage is contributing to the sound.

Hi Gus, thats not interesting for a live DI as the Di feed would be is Post Master Volume (at least if I understood correctly your suggestion),
I don't want the input bass level on the Front of House Console and the Monitor console to change when the bass player decides to bring the amp's master volume in the  full blast in the middle of the gig.
The feed for the DI out needs to be Pre Master Volume.

Thank you so much,
I loved the result of the mod

 

[radardoug]

You have seriously compromised the noise level of your d.i. output by padding before the opamp. You would have been better to go with your resistor across pins 2 and 3 on the XLR. Same result, but better signal to noise.