Splitting and attenuating a balanced signal

[Fredde79]

Hi!

I'm building a tube preamp for bass, and I'm planning to build in a line trafo to get a balanced output for my power amp. However, I would also like to be able to send the signal at mic level to the stage mixer.

So my question is, will this work? My thinking is, the first resistors of the H-pad should isolate the attenuating resistor from the higher level signal and keep it intact.

(The resistor values in the H-pad are missing because I still don't know how much attenuation I'll need.)

 

[turk 182]

fredde79
you're not listing anything about the transformer which in most cases would achieve the signal reduction your looking for but if you feel further padding is needed the values of resistors is easy to determine.by the way your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.

 

[Fredde79]

fredde79
you're not listing anything about the transformer which in most cases would achieve the signal reduction your looking for but if you feel further padding is needed the values of resistors is easy to determine.by the way your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.

Thanks for your reply!

The transformer is a Hammond 108N 10K/600R. It produces a suitable mic-level when fed with a passive guitar signal (I've tested this), but the preamp will have a higher output so I'm thinking a -20dB attenuation (or thereabouts) would be a good thing.

The pad should be ok for a balanced signal, but I may be wrong of course. You could probably pulldown to ground as well, but as the signals in a balanced line have opposite phases, a resistor between them should have the same effect.

But my main question (whether the pad is correct or not) remains, will the splitting of the signal have some negative effects? DI-boxes usually have a switch for attenuation, but I've never seen a schematic for one that produces two balanced signals at the same time (which is what I'm trying to achieve).

One way would be to buy another transformer, but they are expensive...

/Fredde

 

[JMFahey]

your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.

No, it's not wrong, it's a *real* transformer balanced circuit.
Any reference to ground will defeat its purpose.

To fredde79.
You chose the wrong Jensen transformer.
If you want to drive a power amp straight from a preamp, you need a 1:1 line out transformer, or signal will be **way** too weak.
As is, it will supply a mic level signal.
Any further attenuation is counter productive.

 

[Fredde79]

No, it's not wrong, it's a *real* transformer balanced circuit.
Any reference to ground will defeat its purpose.

To fredde79.
You chose the wrong Jensen transformer.
If you want to drive a power amp straight from a preamp, you need a 1:1 line out transformer, or signal will be **way** too weak.
As is, it will supply a mic level signal.
Any further attenuation is counter productive.

Thanks! Aah ok I see. So I'll have to buy another one then... Will my schematic (with the split mic level attenuation) work in case I have a 1:1 trafo, or should I use this one for the mic signal?

(If it works, I could save the original trafo for a DI box or something else later.)

 

[JMFahey]

Thanks! Aah ok I see. So I'll have to buy another one then... Will my schematic (with the split mic level attenuation) work in case I have a 1:1 trafo, or should I use this one for the mic signal?

(If it works, I could save the original trafo for a DI box or something else later.)

Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

 

[Fredde79]

Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

Yes, I might just do that, my only concern is the rather low input impedance of the unbalanced input of my class D power amp. That's why I wanted to use the balanced input instead. (But a FET-buffer is a lot cheaper than a trafo, should this become a problem.)

Cheers!

 

[cbdb]

If you are getting another transformer, use both, one for the power amp, one for the mic send.

 

[cbdb]

Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

If this is for gigs you will be happy you have the transformers, as this will almost gaurantee no ground loops.

 

[JMFahey]

my only concern is the rather low input impedance of the unbalanced input of my class D power amp. That's why I wanted to use the balanced input instead.

Are you sure?
*Normal* is
Unbalanced=higher impedance
Balanced=lower impedance.
Please post specs of your amplifier.

EDIT: and what preamp are you using which can't drive a relatively low impedance?

 

[Fredde79]

Are you sure?
*Normal* is
Unbalanced=higher impedance
Balanced=lower impedance.
Please post specs of your amplifier.

EDIT: and what preamp are you using which can't drive a relatively low impedance?

Yes that's correct, and my power amp (Peavey IPR 1600) has a 7.5k unbalanced input impedance, which is not IMO optimal. This was why I wanted to use the balanced input (15k) instead.

My preamp currently exists only on paper, and the idea is, of course, that it should be able to drive a lo-Z input, and that, in turn, was the reason for the question about using a transformer. The last stage of the preamp was intended to be a 12AX7-triode stage, and that might not have an outout impedance that's low enough. But this is no longer a problem because:

I've since changed the design, so now it will have a small 6L6GC-power stage with an output transformer (to get a better "tube head sound"), whose output I'll connect to a 8ohm false load and attenuate to line level. This will leave the output impedance low.

This is a bit hard to explain verbally, but I'll post the schematic in the tube section soon for further discussion.

 

[JMFahey]

Don't worry, I have a reasonable picture of what you want in my mind.
Well, 7500 ohms input Z can be driven by practically *any* preamp ... except the plate of a 12AX7 because it's internal impedance ends up being around 40K to 50K so only *really* suitable (by itself) to drive the typical 500K to 1 M impedance of another tube stage.
Both 7K5 and 15K are way too low for it, so a buffer is needed anyway.
As of the uncommon Peavey input impedance, I bet they really offer a balanced impedance, 7K5 per arm (15K balanced) and for unbalanced they just drive one of them and so you meet 7K5 .
I find no other explanation for those unusual values.
But it's not worth finding a special transformer and, by the way, the 1:1 I suggested would not work, I was thinking an SS (or SS buffered) preamp before.

But of course, you are most welcome to post your project

 

[Fredde79]

My project is here: http://www.diyaudio.com/forums/instruments-amps/230118-bass-guitar-tube-preamp-small-power-stage.html

 

[cyclecamper]

After your dropping resistors you still might want a switchable H-pad or two.

for driving the power amp balanced, consider also the "BM" series of tranformers from Jensen (the balanced transformer company, not the speaker company) for driving power amps. It handles lots of power and has a huge dynamic range so you don't have to constantly adjust that pad every time you change your amp volume. Expensive, and worth every penny. It's what they use on the main outputs of the very best live-performance mixing boards, where they really need the noise-cancelling balanced lines for the long runs from the board back to the crossover and power amp stacks. They run those lines with plenty of votage and enough current to drive a stack of power amp channels in parallel when necessary.

 

[cyclecamper]

If you call the guys at Jensen and tell them your application, they'll tell you exactly how to do it...using their transformers of course.

 

[Fredde79]

Thanks, but I've pretty much abandoned the idea, the new solution is here: http://www.diyaudio.com/forums/instruments-amps/230118-bass-guitar-tube-preamp-small-power-stage.html

My new solution with an attenuated speaker signal will have sufficiently low output impedance even as unbalanced. I'll use the line trafo for mic level output only.

Balanced line level out would still be nice, but not worth the price of a Jensen transformer...