Firestone Audio Bobby Balanced Headphone Amplifier
Firestone Audio Bobby Balanced Headphone Amplifier (MSRP $799)
I have to confess that Firestone Audio was never really on my radar. I knew the name, sure, and I even knew some of the products like Spitfire and Fubar. They seemed well liked and a good value but nothing ever really struck me as being a "must see" attraction. That changed recently when they moved upscale and launched the Tobby DAC and matching Bobby headphone amp. I got a hold of both and already covered the Tobby at HeadFiit's a very impressive DAC at $1100, especially over USB. The matching Bobby amp has an MSRP of $799 but I've seen it go for as low as $599 some places.
Bobby is deceptively large. It's pretty normal in width at around 9 inches, and height is also pretty standard at just under 3 inches. But depth is 13 inches, which may not sound like much but is far deeper than most other headphone amps. Not so deep that it won't fit on a rack, but after plugging in interconnects and AC cable you might not have much room to work with. Weight is on the heavy side too and the whole thing seems pretty substantial. There's a bit of venting on the sides and the device runs somewhat warm but not overly so, probably due to the large enclosure soaking up heat.
Firestone did a great job on the front panel which I find attractive and highly functional. The LCD display shows volume, input, and operation mode. The volume knob spins freely which reflects the digital volume control scheme. Push the knob to mute. There are two headphone jacks which both accept a 1/4" plug for simultaneous headphone listening. One side is for low impedance, the other for high, with a twist that I'll discuss shortly. Balanced mode is achieved by connecting dual 3-pin XLR plugs. Over on the far left is a power on/off button which is actually more like standby. A nearby LED blazes brightly indicating power status, as if the large LCD display being on or off wasn't enough. Around the back things are quite straight forward: XLR and RCA inputs, a IEC power receptacle, and a hardware on/off switch. Pretty simple.
Sliding off the top cover reveals the Bobby as a fully balanced design just as advertised. A rather large Ring Core brand toroidal transformer mates with dedicated voltage regulation and stiffening caps for each channel, and the separation continues from there on out. Quad OPA604 mono opamps biased into Class A drive a buffer of eight output transistors, with volume being handled digitally by a pair of Texas Instruments PGA2311 stereo volume controllers. Opamps are socketed but Firestone provided no info about swapping so I decided to leave them alone. Maximum output is a healthy 1500mW into 32 Ohms, down to 80mW at 600 Ohms.
Remember the twist I mentioned earlier? Here we go: the two headphone jacks are labeled as "low" and "high" impedance, or use both for balanced operation. Sounds simple enough, right? However, the "low impedance" jack has an output impedance of 227 Ohms, while the "high impedance" output measures 10.5 Ohms. Output impedance in balanced mode is 20.2 Ohms. The Firestone rep explained it like so: They feel low impedance cans like Grados and such sound best from the "low" jack (which has higher output impedance). And they feel higher impedance cans, like the 250 Ohm beyerdynamic models, sound best from the "high" jack (which has the lower output impedance). This puts us in a somewhat counter-intuitive position where the "low" and "high" markings actually refer to the impedance of the recommended headphone, and not the output impedance of that particular jack.
In UseOdds and Ends
The LCD display gets flanked by three small buttons. The top button labelled "source" is the most obvious, toggling between XLR and RCA inputs. The LCD will tell you which mode is selected and also reflects balanced mode or single ended output.
The middle button is labelled "stage" which remains somewhat confusing despite my best investigative efforts. Default mode is "normal" which means on board voltage stabilization remains active. Pressing the button switches to "high" mode, which apparently deactivates stabilization in order to swing more voltage. I chatted back and forth with Firestone but the difference between modes seemed to get lost in translation. Ultimately I couldn't discern any audible difference between the two modes anyway so I stopped trying to figure it out.
The last button is "gain" which is somewhat misleading but actually kind of cool. It doesn't technically change the gain at all; what it does is adjust the way volume gets cycled. In default "low" mode, turning the knob (which has very slight notches) results in one volume step up or down per "click". Each step equals a .5dB adjustment. Further pressing of the gain button cycles through the other modes, where the steps jump by increasingly large intervals1dB for medium, 2dB for high.
I really like this feature! The volume range goes from -60dB to +45dB, which is a lot of ground to cover in half decibel steps. The gain settings mean I can dial in sensitive IEMs just right, but am not stuck with those small increments when using larger cans. And since the volume knob has a great feel to it, and volume tracks perfectly even at low volumes, the overall user experience is very satisfying. True balanced amp designs require 4-gang balanced potentiometers which can get real pricey, so it appears the choice of dual PGA2311 volume controllers was a good one.
Lastly, a quick note about balanced operation. This amp uses the dual 3-pin XLR style which is not my favoriteit makes for a rather large, unwieldy connection. On the other hand, it allows the use of Neutrik hybrid jacks that double as 1/4" single ended outputs. I prefer balanced connections to use the single 4-pin XLR style, but that would require a separate dedicated jack for 1/4" plugs. So neither way is ideal. All my balanced headphones use the 4-pin XLR style so I contacted Ted over at CablePro for some help - he sent over a sweet Freedom Series 4-pin to dual 3-pin converter to allow my cables to work with the Bobby. Thanks Ted! This makes far more sense than getting a whole new cable.
So, it looks nice and works well.... but how does it sound?