balanced line output

[Milkmansound]

Does anyone have a schematic for adding a balanced line output?

I just want to double check what I have done on a bass amp I built for a friend.

My signal path is:

DC coupled cathode follower 12AU7

Edcor 600:600 transformer

pins 2-3 of the XLR

Pin 1 goes to ground via a lift switch

This is pretty much straight out of the Thunderfunk book. The pot will make some noise when turned but it should not need to be turned often and its much quieter than the signal coming from the output.

[Leo_Gnardo]

I'm guessin' the 'AU7's cathode is what, a 10K 2-watt wirewound pot? And the 'AU7 plate surely isn't connected to ground but some sturdy DC hi voltage, say 150V or more. That circuit looks like it'll handle the job, and as simple as it could get. I'm sure you'll use a quality film capacitor well over-rated for voltage.

If I knew I was going to a mic input, I'd swap out that transformer for say a 10K:600. If you're looking for a line out then stick with the 600:600. If you run that into a mic input be prepared to pad it some, maybe a lot.

[Milkmansound]

It's a DC coupled follower so the plate is grounded and the b+ and input signal go to the grid. The pot is 100k into a .47uF 600V cap.

That should be very low impedance - under 1k which is why I chose 600:600 instead of 10k:600

Is pretty hot but I tested it with a mixer into the mic preamp and it was usable with the output turned all the way up.

I may end up switching to an AC coupled follower and putting a 1M pot ahead of it. Should have the same result just more parts.

[EtherealWidow]

I don't understand how that's working if it is indeed wired the way the schematic implies. The cathode would be at a higher potential then the plate.

Typical DC CF's are as follows.

http://www.valvewizard.co.uk/dccf.html

AC CF's are like this.

http://www.valvewizard.co.uk/accf.html

Maybe you'd have better luck trying one of the schematics Merlin has.

[M Fowler]

Don't forget to add a ground lift it may be needed.

[EtherealWidow]

Also, I think you can omit the coupling cap before the transformer. Doesn't really serve any purpose.

[Milkmansound]

the ground lift is there Mark - I forgot to draw it

After closer reading of the Tunderfunk book, it says you *can* ground the plate but instead it goes directly to the power supply. Guess I had better change that!

I will try without the big cap too - its true, the transformer won't let the DC through. I am used to doing FX loop type non transformer outputs and the cap is a relic.

thanks for the tips guys! Here is an updated drawing:

[Milkmansound]

well, that didn't work too well.

The large capacitor needs to be in there in order to prevent voltage drop for the entire node.

I added a gain stage before the CF and I really like where its at now - that 100K pot is a terrible idea not sure what Thunderfunk was thinking with that design.

This circuit has a lot of range - can be at mic level or line level and then some.

[EtherealWidow]

Glad it worked out for you! I learned something myself. I think without the cap, current would flow "up" through the xformer and through the tube. I might use this schem myself as a cheaper alternative to the one on Jensen's website.

[Smitty]

Connect the top of the primary winding directly to the bottom of the 10K cathode load resister and the bottom of the primary winding to ground. A transformer is a current device. All of the current will flow through the primary.

You'll be able to drive a mile of low impedance cable this way.

You can still connect a cap to the top of the 10K load resistor and use that for a high impedance output. The high impedance output will present an AC parallel load. You can unload it a bunch with a series resistor above 20K after the cap.

[gingertube]

You needed that 0.47uF cap to stop the transformer primary shorting out your tube current. That is there is nothing to limit the current anymore.
With a 100K resistor it will establish its own operating current although a 100K is more typical for a 12AX7. You could try say 22K for a 12AU7.

I've used 10K:10K transformers for this with good success.

If you really need 600 Ohms out then use the 600:600 tranny. Even with that you probably don't need to use a 12AU7. The Zout at the tube cathode is approx 1/gm. Even a 12AX7 can manage 600'ish Ohms. (gm is 1.6 mA/V so 1/gm = 1/0.0016 = 625 Ohms)

It is likely that the signal voltage level at the cathode will be way more than you need so you could use a 10K:600 Ohm step down. That is only a divide by 4 in signal voltage.

Cheers,
Ian

[Milkmansound]

Wow great posts thanks!

I will either drop the resistor to 22k or try a 12ax7 in place of the 12au7

I feel like I too have learned a lot by posting this. I tried searching online and was unable to find a good schematic for this kind of output - I'm happy to re-draw once I'm done to share.

Now we can all make our own microphone preamps

[EtherealWidow]

A mic preamp is probably my next build. Behringer mixers kind of suck just a little.

This guy looks pretty cool. The actual mic pre schem is on the 4th page.

http://site.triodestore.com/st-70-mp-project.pdf

[Smitty]

You needed that 0.47uF cap to stop the transformer primary shorting out your tube current.

That's what the 10K load resistor will do. So to be clear:

Ground => transformer primary winding => cathode load resistor => cathode