Adding Output Transformer to existing circuit

[kimothebeatmaker]

Wanted to see if I could add an output transformer to this circuit to impart some color and mojo. Would I add it directly before the XLR Out right after the Parametric EQ at C29 / C31?

Ive attached the schematics.

 

[abbey road d enfer]

33079 will have a hard time driving a xfmr. The output impedance there is too high due to the impedance build-up resistors R37/50. And the 47uF capacitors will produce an undesirable LF hump.
The optimum solution would be to use a dedicated output stage, but you can have respectable performance using the output of a 5534, to drive the primary via an RC circuit of 50 ohms/470 uF in parallels.
BTW, what are RV11/C79 supposed to do? Weird tone control? Actually more a distortion control.

 

[kimothebeatmaker]

What if i swapped the 33079s for OPA1604's...and the 47uF to 470uF caps? RV11/C79 - adds saturation by adjusting the drain current on the input FET amplifier altering the even harmonic levels of the signal being passed

 

[Bo Deadly]

If your objective is to add "mojo" then you hardly need a fancy op amp. But it will need more drive than an MC33079 because the distortion of a transformer ("mojo?") is proportional to the voltage across it. So driving it differentially like in that circuit is good. Are you committed to a quad op amp? Is this from a board that's already built or are you going to build this? If you are still designing, then maybe just design for NE5532. Otherwise, you'll have to find a quad with some kick. What is the transformer? If the transformer has a split secondary to configure for a 2:1 step down (or has a 2:1 arrangement such as by reversing it), do that so that you have to put more voltage across the primary to get the same level out. Then add 2x gain in the output stage by making R44 10K to make up for the step down. Again this is to maximize "mojo". But bear in mind that you're not going to really hear any difference unless the transformer is relatively small and you really torture it. Unless maybe you're one of those folks that can hear that sort of thing anyway.

 

[abbey road d enfer]

RV11/C79 - adds saturation by adjusting the drain current on the input FET amplifier altering the even harmonic levels of the signal being passed

That's not what I see. RV12 does that, but RV11/C79 is a crude treble leak.

 

[kimothebeatmaker]

That's not what I see. RV12 does that, but RV11/C79 is a crude treble leak.

RV11 is what I said it is. RV12 isn't on the schematic I posted.

 

[kimothebeatmaker]

If your objective is to add "mojo" then you hardly need a fancy op amp. But it will need more drive than an MC33079 because the distortion of a transformer ("mojo?") is proportional to the voltage across it. So driving it differentially like in that circuit is good. Are you committed to a quad op amp? Is this from a board that's already built or are you going to build this? If you are still designing, then maybe just design for NE5532. Otherwise, you'll have to find a quad with some kick. What is the transformer? If the transformer has a split secondary to configure for a 2:1 step down (or has a 2:1 arrangement such as by reversing it), do that so that you have to put more voltage across the primary to get the same level out. Then add 2x gain in the output stage by making R44 10K to make up for the step down. Again this is to maximize "mojo". But bear in mind that you're not going to really hear any difference unless the transformer is relatively small and you really torture it. Unless maybe you're one of those folks that can hear that sort of thing anyway.

It's a pre-existing circuit, I was thinking of taking out the caps 47uF, and putting the transformer in its place.

 

[abbey road d enfer]

RV11 is what I said it is. RV12 isn't on the schematic I posted.

It's RV10 that does the bias trick. RV11 is some kind of tone control.

 

[kimothebeatmaker]

It's RV10 that does the bias trick. RV11 is some kind of tone control.

Rv10 is the pot resistor next to the jfet to bias the dual n channel jfet. Rv11 is the jfet saturation control like I said. I’m looking at the board right now. The BV10K is a potentiometer. One with an actual knob. Rv10 is a resistor with a turn screw pot on it to bias the jfet. Has nothing to do with saturation or tone control.

 

[abbey road d enfer]

Rv10 is the pot resistor next to the jfet to bias the dual n channel jfet.

Agreed.

Rv11 is the jfet saturation control like I said.

It's a weird treble bleed, that indeed creates distortion because it overlaods the voltage-follower.

Rv10 is a resistor with a turn screw pot on it to bias the jfet. Has nothing to do with saturation or tone control.

Did I say different? Oh yes, it bias the FET, which has undeniable effect on saturation and harmonic structure. And I never said it was a tone control.

 

[kimothebeatmaker]

Agreed.

It's a weird treble bleed, that indeed creates distortion because it overlaods the voltage-follower.

Did I say different? Oh yes, it bias the FET, which has undeniable effect on saturation and harmonic structure. And I never said it was a tone control.

Rv11 is a knob on the front that adjust the drain current on the jfet. Rv10 is a voltage bias for the dual channel jfet. One is calibrated to spec on the inside of the box for the unit to work as intended and left alone, the other is a feature on the outside of the box that is adjustable without opening up the box. You said that is not what Rv11 does, as Rv12 does it. I stated there is no Rv12 in the diagram. You said Rv10 does it. I said no, Rv10 biases the jfet. Nothing to do with saturation. Again Rv11 is the pot that controls the drain current on the jfet. Not Rv12, or Rv10. But we're getting off topic.

What got me thinking about adding a transformer inplace of the capacitors on the output was this picture of the chameleon labs 7720 modified with a jensen 123 output transformer. I know it's a compressor and a different beast, but was wandering how simple would it be to just do something similar to my unit. In the picture, he removed the electro alum caps on the output, ran the prim / sec leads to those in palce.

 

[abbey road d enfer]

Rv11 is a knob on the front that adjust the drain current on the jfet.

Do you agree with the fact there is no DC current into RV11, because there is capacitor C79 in series? As a result it has no effect on the FET bias.

Rv10 is a voltage bias for the dual channel jfet.

Agreed. It could be argued that it sets the FET's current, though.

One is calibrated to spec on the inside of the box for the unit to work as intended and left alone, the other is a feature on the outside of the box that is adjustable without opening up the box.

You said that is not what Rv11 does, as Rv12 does it.

I still claim that RV11 is a treble bleed, and I said in my first post that it's a crude source of distortion. And RV12, as a bias control, also changes the operating point of the FET, which in turn changes the distortion profile.

I stated there is no Rv12 in the diagram.

Correct, I had misread that for RV10.

You said Rv10 does it. I said no, Rv10 biases the jfet. Nothing to do with saturation.

Again Rv11 is the pot that controls the drain current on the jfet.

No. You can say that as much as you wish, but it's simply not true.

 

[Winston OBoogie]

If you go by the original marketing blurb for the unit (a Presonus) then kimothebeatmaker is correct.
If you go by what is actually happening in the circuit then Abbey is, of course, correct.

Kimo, if you want to put an output transformer on the unit for colour and mojo then, in what units are we going to measure things, in "mojos" ?
Maybe use beefier op-amps and up the values of your coupling caps as Abbey suggested and try it.

 

[lassoharp]

Why not pick a trafo, clip it in circuit, record some tracks showing an A/B comparison (a "mojo test") then post samples? That would be about a couple hours effort or so to get to the bottom line and seems vastly shorter than the time already spent pulling weeds.

 

[Newmarket]

I don't mean to negate anyone's input on this website/forum - but it does often seem worth noting that while some members seek high fidelity / low noise solutions, others are more focussed on deliberately introducing 'colour' / distortion / non-linearities etc through overdriving transformers / valves / jFets etc.
I'm not taking sides - but it's worth considering that whilst the the 'Colour' approach may be useful wrt 'Rock' music - and other broad genres (Jazz/Country maybe) - it may not be the preferred approach in others eg Spoken Word/Classical/Orchestral/Documentary.

 

[abbey road d enfer]

Why not pick a trafo, clip it in circuit, record some tracks showing an A/B comparison (a "mojo test") then post samples? That would be about a couple hours effort or so to get to the bottom line and seems vastly shorter than the time already spent pulling weeds.

That wouldn't be conclusive, because no one has the OP's ears and auditory preferences.
The search of mojo is a solitary endeavour.

 

[Winston OBoogie]

We're probably overdue some kind of international fixed standard as far as referencing 'Mojos"

 

[abbey road d enfer]

Let's start with a proper unit. I suggest the Morganfield.

 

[musipol]

Morganfield ???? I give up. What is the reference here?

 

[abbey road d enfer]

McKinley Morganfield, aka Muddy Waters, "Got my mojo working".