JLM AUDIO

I'm looking to build something like this: A headphone amp with two inputs, one line-level output and one headphone jack with independent volume controls for the output and headphones. I'd like to be able to run it off of the psu I already have for my Baby Animals, if possible. Would two Dingo kits be suitable? I'm thinking of using either Hybrids or OPAs as the opamps.

...Adding, can the balanced and unbalanced inputs on the Dingo boards be used simultaneously? The outputs can be used simultaneously, so that would take care of my output requirements. If both inputs can be used at the same time then that's a big plus, I don't mind if one of the inputs is unbalanced since the source it will be connected to most of the time is also unbalanced.

I suspect there are much simpler ways of achieving what you want, but it depends a little on exactly what you are trying to do.

For example, given a line level input there are plenty of schematics on the internet for low component count headphone amps based on dual opamp ICs, including independent volume control, and quite capable of providing good fidelity into most headphones.

Are the two inputs you describe L and R, or are they independent? What is the one output going to go to? With a bit more info I suspect a fairly simple solution will exist.

Right, I've seen the CMoy and a couple of other things, but most of those are made to run off of batteries or at a lower voltage than that put out by the power supply used for the Baby Animal (the smaller one, +/- 24v). I'm not quite ready yet to futz around with redoing one of those schematics to deal with the higher voltage.

Anyways, the box I'd like to build would have the following topology. Two stereo inputs (one balanced, one unbalanced. I'd be fine with both balanced if it makes the circuit easier to implement), a balanced +4 line-level stereo output (or two) and a headphone output. Three knobs for volume; one for each input and one for the headphones. I'd like to have some gain in hand (say up to 40 db) for the inputs. It's o.k. if the two input controls affect the level going to the headphones. It'd be nice to have mutes on the inputs, or some way to switch a/b/both. Another cool feature would be inserts on the output/s, but that's not a necessity.

OK, that's all a bit more than just a headphone unit.

If you really need some gain (as distinct from a volume knob, which is attenuation), then my simple ideas wouldn't work. Even still, I would be thinking more a cut down BA kit (ie no front end switches or transformer) with a modded DI for the unbalanced side rather than a DINGO. Joe may have better ideas.

Do you mean something like this?



If you want the XLR outs to be fully balanced add 2 x JLM111DC output transformers.

Cool, thanks! That pretty much nails it. I reviewed the BAD pcbs and I can see how everything would fit together. Is there a way to wire up the BADs so they don't use input iron? I like iron, don't get me wrong, but there's a fair amount in the signal path as-is (mic'ed signals go through a JLM 1:4 (in the mic pre) and two Edcors (in the mixer)). I would expect that gain would suffer, though, if it were possible and I went that route so maybe it's best to use it.

Just to make sure, would the smaller of the two Baby Animal power supplies have enough juice for this and two Baby Animals running as mic pres? All of them would use OPA2604s, although I might upgrade the Baby Animals w/Hybrids (and a better box) in the near-ish future.

Balanced output isn't a huge issue, my cable runs are small. Thankfully I'm pretty close to getting a new job that pays more money -- it looks like I'm gonna need it!

Is there a way to wire up the BADs so they don't use input iron?

Yes you just need to add 2 resistor and 2 capacitors instead of the transformer but you cannot have a gain control but can set a fix gain or unity.

Just to make sure, would the smaller of the two Baby Animal power supplies have enough juice for this and two Baby Animals running as mic pres? All of them would use OPA2604s, although I might upgrade the Baby Animals w/Hybrids (and a better box) in the near-ish future.

If should be ok. Depends on what headphones and there impedance. If you are going to use OPA2604 to drive the headphones you will have to have 600ohm headphones. If you use Hybrid A/B or 99v on the output you could run 8 to 600 ohm headphones.

I'm a programmer by profession and thus somewhat succeptible to the "more widgets" mentality; I'll probably use input iron so that I can vary the gain. It's not very iPod of me to think that way, but I like options when it comes to stuff I'm building for myself. In that vein I'll probably order the larger power supply as well when I get around to building this so that I can be prepared for future upgrades/additions to my small arsenal of JLM-based stuff. Thanks again for the schematic!

Joe Malone wrote:

poserp wrote: Is there a way to wire up the BADs so they don't use input iron?

Yes you just need to add 2 resistor and 2 capacitors instead of the transformer but you cannot have a gain control but can set a fix gain or unity.

Hmmm - I thought that the BA gain control came about by adjusting the negative feedback loop around the opamp, and that if you made up a balanced line receiver (sort of like a cutdown DinGo, just the bit from the balanced in to the unbalanced out) you could just plug it into the opamp non-inverting (+) input and away you'd go - BA without the switches and input tranny, including gain control.

Now that's a bit more than just a couple of resistors and caps, but would it do the trick, or is there something I've missed?

Hmmm - I thought that the BA gain control came about by adjusting the negative feedback loop around the opamp, and that if you made up a balanced line receiver (sort of like a cutdown DinGo, just the bit from the balanced in to the unbalanced out) you could just plug it into the opamp non-inverting (+) input and away you'd go - BA without the switches and input tranny, including gain control.

Now that's a bit more than just a couple of resistors and caps, but would it do the trick, or is there something I've missed?

You can convert the input opamp on the BAD to a balanced electronic line receiver easy without an input transformer with just 2 caps and resistors. So the XLR gain trim would be gone but you would still have gain trim on the headphone output stage.

OK - so rather than building an external receiver, you configure the first gain stage of the BAD as a line receiver, at the cost of that stage's gain control (but with the proviso that you can setup that gain stage with something more than unity gain should you wish - like 11.8db to bring a -10dbv signal to +4dbu). Meaning you still then have the second stage as your amp, and that would be more than enough for line level input signals anyway.

I can see that.

OK - so rather than building an external receiver, you configure the first gain stage of the BAD as a line receiver, at the cost of that stage's gain control (but with the proviso that you can setup that gain stage with something more than unity gain should you wish - like 11.8db to bring a -10dbv signal to +4dbu). Meaning you still then have the second stage as your amp, and that would be more than enough for line level input signals anyway.

I can see that.

Yep that's right The 1k resistors shown going into the BAD +/-IN can be scaled to set the gain. So only 2 caps need to be fitted to the BAD to jump where the transformer used to be. RL value should = FB resistor which is 10k standard but could be also scaled up to get more gain on the line receiver.

i have nothing of use to add, except....

i believe the plural of dingo is dingoes, not dingos...

see:

http://en.wikipedia.org/wiki/Azaria_Cha ... appearance

As in "I'll go if Din goes".