Help With adding balanced Output to stomp box pedal

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sonolink

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I have 2 tube based preamp stompboxes I built recently. First one is JTM45 based, the other is a Fender AB763 single channel.





I would like to be able to place a speaker simulator at the output and balance it to plug it into a mixer or a soundcard.
I found this speaker simulator:



And this very simple balancing circuit on the Elliott Sound Website:



Can someone please tell me if I feed it 9v can I connect it to the output of the stompbox after the cabsim to have a balanced out?

Thanks a lot
Cheers
Sono
 

abbey road d enfer

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sonolink said:
And this very simple balancing circuit on the Elliott Sound Website:



Can someone please tell me if I feed it 9v can I connect it to the output of the stompbox after the cabsim to have a balanced out?
As John already mentioned, you'll have to modify the circuit for single rail operation. Many examples are available on the net; however, if you're not comfortable with it, we can provide you assistance.
You may have to increase R1 to about 100k, in consideration for the 4.7k resistance at the output of the cab simulator.

For a more complete approach, check attached pic
 

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  • cab sim+bal out.jpg
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john12ax7

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Actually what Abbey posted is better, as the output of Q3 is already close to +4.5V, you can tie it right to the non-inverting input as shown.  Don't forget about DC blocking caps on the output of the opamp. I would wire the output impedance balance as recommended by Jensen, you can see some good info here:

https://www.jhbrandt.net/wp-content/uploads/2014/11/Interconnection_of_Balanced_and-Unbalanced-Equipment.pdf
 

sonolink

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Hi guys,

First of all, thanks A LOT for your feedback to both of you!!
Sorry if it took me a few days to reply but this is still a bit dense to me.
I'm trying to understand and digest all this info and do my homework so please bear with me :)

The original Cabsim circuit was designed with J201 and BC550/BC560s. I can't find these at Mouser, they're way obsolete. I can't find cheap J201 TO92 subs so I'll probably get some MMBFJ201  instead and use a small adapter board for them. Can I sub the BC550/BC560s with BC546/BC556s? I found some with extensions BC546CTA/BC556ATA but I don't know if they have the same gain and/or are compatible to be used in the circuit.

Now on to the balanced output circuit. I read the article by Bill Whitlock, that I found very interesting (thanks for the link). I understand (well I try) the concept looking at the typical unbalanced and balanced outputs schematics he shows. But then I get lost at some things you mention :(
Sorry for the language barrier...

john12ax7 said:
For an opamp you need to split the rail, meaning bias the non-inverting to 4.5V with a 9V supply. It's a common technique in pedals.
You mean like this?


abbey road d enfer said:
You may have to increase R1 to about 100k, in consideration for the 4.7k resistance at the output of the cab simulator.
You mean the resistor between the opamp +input (pin 3) and GND, right?

abbey road d enfer said:
For a more complete approach, check attached pic

Sorry if I'm slow, I'm trying my best to follow...
The pic says "output of the cabsim goes into opamp pin3 after a 10k resistor between the output and GND, and 9v to pin 4 and GND to pin 8"?

So the Elliott Sound Website circuit is ok? I'm asking because he says it's cheating and after reading the Jensen paper I don't know...wouldn't this be the proper way to go?



Thanks again for your time and help
Cheers
Sono
 

abbey road d enfer

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sonolink said:
Can I sub the BC550/BC560s with BC546/BC556s?
You can probably replace them with any NPN that you have lying in your dirtbox. Not critical.

Forget about this; I showed you how.

You mean the resistor between the opamp +input (pin 3) and GND, right?
Correct.

So the Elliott Sound Website circuit is ok?
No it's not. It is known as the "Tascam problem". Impedance balanced is preferrable, particularly if the unit is connected to various equipment.
 

sonolink

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abbey road d enfer said:
No it's not. It is known as the "Tascam problem". Impedance balanced is preferrable, particularly if the unit is connected to various equipment.

I'm sorry about the language barrier but I'm not sure if you mean I should use a different circuit altogether and are inviting me to look for it on the net (which I'll be glad to do) or if I should use the Elliott Sound schematic I posted before with the changes you made.

I'm doing a search to read and try to understand "impedance balanced" vs "opposite polarity"...

Cheers
Sono
 

abbey road d enfer

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sonolink said:
Abbey, thanks a lot for your reply. I'm sorry about the language barrier but I'm not sure if you mean I should use a different circuit altogether and are inviting me to look for it on the net (which I'll be glad to do) or if I should use the Elliott Sound schematic I posted before with the changes you made.
Use the circuit from Ellior Sound that you posted at the bottom of your first post.

I'm doing a search to read and try to understand "impedance balanced" vs "opposite polarity"...
Many people think balancing is about having symmetrical voltages, i.e. one leg has a signal and the other leg has the same signal but inverted in polarity. This is of limited interest. It allows 6dB more output than unbalanced, that's all, and doesn't really matter in practice.
The benefits of "balancing" signals is hum and interference rejection, which relies on the two legs having the same impedance (as far as outputs are concerned). This condition is utterly independant of the way signals are presented.
Impedance balance is cheap, works well and is never unstable.
 

sonolink

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abbey road d enfer said:
Use the circuit from Ellior Sound that you posted at the bottom of your first post.

Excellent. Thanks!

abbey road d enfer said:
The benefits of "balancing" signals is hum and interference rejection, which relies on the two legs having the same impedance (as far as outputs are concerned). This condition is utterly independant of the way signals are presented.

That's what I'm trying to grasp now. For years to me it seemed obvious and easy to understand that balancing was using the cancelling property of 2 inverted polarity exact copies to "preserve" signal integrity. From what I read so far I get the impression that impedance balancing is almost the opposite, meaning that instead of "encoding" the signal, the signal is the "difference".
Not sure this makes much sense actually :)

A simple technical question please: is it ok to sub the J201 with any JFet N-channel like a BF256 (marked as N-Channel transistor on Mouser) or a J111 (marked as N-Channel SWITCH)? If not I'll go the SMD way :)

Thanks again for your time and help :)
Cheers
Sono
 

abbey road d enfer

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sonolink said:
A simple technical question please: is it ok to sub the J201 with any JFet N-channel like a BF256 (marked as N-Channel transistor on Mouser) or a J111 (marked as N-Channel SWITCH)? If not I'll go the SMD way :)
I'd probably favour the BF256, but both could work, since the source follower is very tolerant.
 

sonolink

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I bumped into this schematic for the same Speaker Simulator I intend to use, only this version says it includes a DI out. Is that really a balanced out? If so, maybe this option would be simpler and avoid having to use another opamp?



Cheers
Sono
 

abbey road d enfer

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sonolink said:
I bumped into this schematic for the same Speaker Simulator I intend to use, only this version says it includes a DI out. Is that really a balanced out? If so, maybe this option would be simpler and avoid having to use another opamp?



Cheers
Sono
The image is too small. I can't see any detail, however I don't see anything that would make it balanced.
 

sonolink

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I think you have to click on it Abbey....  ;)

Here is again as an attachment just in case

 

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  • Simple-Cab-Sim-rev2-With-JFET-DI-SCH.jpg
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PRR

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It is terrible. The 10K resistors will not pull large line capacitance. No matter, because the lack of proper bias clips any not-small signal.

You can do better. (MUCH better.)
 

sonolink

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Thanks for chiming in PRR  :)
I wish I could design and understand electronics TBH. I'm still a self taught novice in that field, still reading "Basic Electronics" by Valkenburgh trying to grasp things.

I found that schematic in another forum.
I'll stick to the Rod Elliott circuit as advised by Abbey  ;)
 
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