[standarderror]

Hi,
first poster here.
 
Me and my wife are planning to get a decent turntable for headphone listening, and ideally we'd like a headphone amp that can drive to pairs of headphones with separate volume controls, so that we can listen together. Now, the only amp I've found that meets these requirements is the Pro-Ject Head Box SE II: http://box-designs.com/main.php?prod=headboxse&cat=headphones&lang=en, which goes for just below 400 USD in Sweden. I haven't found too many reviews of this, but my impression is that it's an ok but not excellent amp.
 
Initially, we'll probably be playing a Rega RP-1 or possibly a Pro-Ject Debut III through a cheap phono preamp, and listening on a pair of AKG K702 and one of our crappier pairs. We're thinking about adding a pair of Sennheiser HD650 further down the road.
 
Now, my question is this: are there any other options besides the Head Box for dual headphone listening with individual volume controls? Let me also clarify that we will probably upgrade to two separate and better headphone amps later, so for now we don't want anything more expensive than the Head Box SE II. Alternatively, is there a single amp that's half the price but of better sound quality thant the Head Box SE II?
 
Finally, I assume it would be catastrophic to get a good single headphone amp and use a splitter (and attenuator before one of the headphones for separate volume control) to drive to sets of headphones?

 

[caol ila]

You can build your own headphone volume control box and install as many good quality potentiometers as you need.
 
You should use a minimum of 1 watt power handling capacity, and the left and right channel pots must track each other if using a ganged pot. This eliminates cheap pots. A standard resustive track pot will change its impedance from minimum to maximum volume. Maximum impedance it at the ELECTRICAL halfway point (not halfway rotational point; you need an audio taper pot), and will be 1/4 of the impedance measured across the entire pot resistance. Impedance at full volume will be zero, but impedance to the amp will be headphone in parallel with the pot. Look up the Thevenin Equivalent formula, it is a simple mathematical formula, and you can figure impedance at various settings.
 
A 100 ohm pot at full volume in parallel with a 250 ohm headphone will show 71.4 ohms to your amp. Your headphone will see the impedance of the amplifier. At half volume the amp will see 91.7 ohms of impedance. The headphones will see 25 ohms + the amp impedance. This is the worst case (highest impedance) the headphones will be driven by, about a 10:1 damping factor, fully adequate. At zero volume the amp will see 100 ohms, the actual impedance of the pot alone, and the headphones will see 0 ohms as it is at electrical circuit ground.
 
You can keep the impedances the headphone sees at all volume levels by using a stepped attenuator with properly chosen resistance values in both legs. This is quite expensive to do it right and keep sound quality very high, so it might be less expensive to just use separate amplifiers instead. My Conrad-Johnson preamp volume control uses such a setup but takes it a step further by using computer control to pick 2 resistors from the resistor bank and then connect them properly through hermetically sealed gold plated contacts. That is probably beyond your ability to construct such a device, but it accomplishes a 100-step pot with far less number of expensive components and eliminates dirty pot contacts over time.
 
You can use a rather high quality pot of high impedance then connect the output to a buffer amp chip, which will require a DC power supply, which can be supplied to a Radio Shack battery holder and several D-cells, or get a high quality DC wall wart power supply.
 
If making your own gear, you can switch to high quality XLR connectors and wire your phones in balanced mode, etc. FWIW, Conrad-Johnson says that floating single ended circuits do much the same as balanced circuits, but use half the components and also eliminates matching of components on both sides of the circuit. That is why an isolated battery operated amp can do you just as well as a much more expensive balanced amp. Also, wire your interconnects in the configuration Conrad-Johnson suggests in their owners manuals. That is to connect the shield at ONLY one end and connect that end to a common point, normally the preamp in a home system. I was wiring my interconnects that way a long time before I owned my first C-J piece of gear, and it works, and common Mogami or Canare studio interconnect cable may just outperform very expensive audiophile interconnects if you give it a try.
 
Give simple pots in a box a try first, and use decent quality 1/4" phone jacks. You can use switched jacks, and when a headphone is pulled out of circuit it will connect a resistor of the same impedance as your headphones across the circuits which will eliminate volume changes.
 
The load your amp will see will be the impedance of the headphone + pot combo, divided by the number of circuits. That is 91.7/2 for the above example, or 45.85 ohms, at half volume for both cirrcuits. Much higher than a single 32-ohm headphone. Unless using very low impedance pots and headphones, any good headamp will drive a couple of headphones and a couple of pots.

 

[standarderror]

Wow, thank you! That's very detailed and helpful. I don't completely follow, but that is certainly due to my own lacking electronics skills. I've been thinking of learning some of this stuff. Actually, if you could recommend me a good resource for learning some basic audio electronics (or just general electronics), that would be great! I know very little about the subject, but I have a fairly good mathematics background.
 
Meanwhile, as I don't see myself finding the time to learn the necessary skills and build a volume control box in the nearest future, do you know if these kinds of boxes are available for sale anywhere?

 

[nikongod]

Just buy (or build) a second amp.
 
For building an amp, look at the Cmoy tutorial on tangent's site:
http://tangentsoft.net/audio/cmoy-tutorial/

 

[MalVeauX]

Heya,
 
My Matrix Cube DAC allows me to monitor with its internal amplifier on one headphone, while the external amp that I feed has headphones too. So I can listen on two separate headphones to the same source. I'm using the Matrix Cube DAC (with one headphone attached, powered by the Matrix's internal amp which is quite good and solid state), and it also feeds out to my Little Dot MK III (tube amp) which has another pair of headphones attached. Great for dual listening. Also, for me, great for comparing different headphones with the same source playing.
 
Very best,

 

[standarderror]

That's a good idea too. I do have a NAD C715 compact stereo, with an aux in, a headphone out and a tape out, so I guess I could use that until we can afford a second headphone amp. Tape out is always line level and independent of the volume control, right?
 
Also, it seems like noone thinks getting the Head Box is a good idea. Is this because it's known to be a bad amp, or has nobody heard it?
 
Thanks for all the replies, this is certainly a very helpful forum! I hope I'll be able to gather enough experience to contribute back in due time.

 

[caol ila]

standarderror,
 
Go search the back issues of Stereophile for an article by Corey Greenberg about the headphone amp design he gave to everybody for free. I believe it was a buffer amp, which means your source needs enough voltage, but it gives all the current drive you need for any headphone. In other words, if you have a tube preamp of at least 5 volts RMS output, it would lower your output impedance from 10K ohms to about 1 ohm.
 
An old tube preamp with a buffer amp (or several) and phone jacks added would make a killer phone driver. The volume control is already there. Simply do not turn on the power amp if listening to cans instead of loudspeakers.
 
Something like a old C-J PV10 with a tube phono preamp has a DC voltage source to drive 6-volt or 12-volt tube heaters. Make sure a tube preamp has DC heater supply and not AC if buying for a headphone amp project. BUT, you could add a bridge rectifier, capacitor, and voltage regulator and improve the sound while making it suitable as a buffer amp power supply. You probably have enough current drive left over to power a buffer amp chip. These C-J preamps were made in various versions over the years and outsold all other  C-J preamps, so they are easily found for reasonable prices. C-J has excellent listing of all their classic gear on their website. C-J sound is truly gorgeous, as they do power suppies right, but they never got into headphone amps.
 
Audio Amateur magazine was one I once supscribed to. Also, Glass Audio if you are into tubes. Haven't subscribed in years, but somebody has back issues available.
 
A buffer amp that runs on batteries may offer the best sound at the lowest cost if you have a good source. It TOTALLY gets your headamp off the noisy electrical grid, and if you team it with a good battery powered source component, you are essentially balanced with only single ended components. All phono cartridges are balanced, so if you buy a battery operated phono stage from Needle Doctor or similar dealer, you can have a very high end sound for chump change.
 
My HEARO 999 has 2 wireless cans that are battery powered and does not suffer from the problems that cheap consumer wireless cans suffer from. I believe the delay is only 5 milliseconds, so dialog and video image are still in sync. Digital radio transmission instead of infrared allows movement anywhere on my city lot or in the house and still listen to music. The HEARO 999 never got the respect it deserves, so I got my setup for far less than what it's worth on closeout back in 2007. Good thing I listened to a German recording engineer instead of audiophile forums as to the merits of the 999. It takes many charge and discharge cycles to get the wireless cans broken in, so I guess a lot of people made decisions on initial listening tests. He uses his every day in his mastering studio. The mode and surround settings are much more obvious after a proper break-in, and not so obvious fresh out of the box. Bass will not be full power for at least 100 hours. That is a month or more of full discharge and recharge if listening every night. IVA-Phones mode really is better than direct mode for opening up the sound of headphones, even if you have K601 or K701 phones that are known for open sound.
 
The HEARO 999 seems to be voiced for AKG pro cans if using the wired headamp. K501, K601, K701, K702, K271, K272HD, etc., I would recommend those.  Beyer DT880 not so good, and AKG K301 XTRA even worse. Grado might work, but Sennheiser probably not. Not for anything already considered really strong in the bass, as the bass gets stronger when driven by the HEARO 999 and then it goes over the edge.