The loudness of your headphones will vary depending on their design and their input impedance. The output impedance of the HP4 is 51 (ohms), and it will work fine with professional and consumer headphones but the volume will differ depending on which type of headphone you use.

Headphones with lower input impedances will generally be louder than higher-impedance headphones. (In this case, louder does not mean better!) Most professional headphones have impedances between 150 and 600. Consumer headphones for CD players and the like often have impedances between 8 and 32.

Broadcast headphones are specialized, and you are unlikely to use them with an HP4; their impedances are typically between 1.5 k (kilohms) and 2 k.

Key Features

• 4 screaming-loud (150 mW per channel) stereo ¼†headphone outputs (front panel)
• 2 balanced ¼†line outputs (monitor outputs)
• 2 balanced ¼†line inputs
• Headphone level control for each channel
• Mono summing switch for each channel
• Monitor level control
• Monitor mute switch
• Daisy-chain multiple units via monitor outputs
• Compact 1/3U rack-mount chassis
• Sealed, detented, metal rotary controls
• External 16 VAC power supply

Technical Specs

Inputs
Connector Type
XLR- ¼†TS, Combo, balanced
THD+N     
0.0025%, 1 kHz, unity gain
Max Input Level        
+18 dBu, <0.5% THD+N, unity gain
Input Impedance      
10 k
Frequency Response
20 Hz - 20 kHz, ±0.5dB
Monitor Outputs
Connector Type  
¼†TRS, asymmetrical, impedance-balanced
THD+N     
0.0025%, 1 kHz, unity gain
Max Output Level      
+18 dBu, <0.5% THD+N
Output Impedance    
51
Frequency Response
20 Hz - 20 kHz, ±0.5dB
Headphone Outputs
Connector Type  
¼†TRS, stereo
Max Output Power    
130 mW/Ch., <0.5% THD+N, 60 load
Frequency Response
20 Hz - 20 kHz, ±0.5 dB, 60 load
Headphone Impedance Working Range     
32 - 600
Power
External Adapter Type       
+12 VDC, 1000 mA, center pin positive
Physical
Dimensions       
1/3U rack-mount (with standard rack tray, not included)
Height      
1.75†(44.45 mm)
Depth        
5.5" (139.7 mm)
Width        
5.5†(139.7 mm)
Weight     
5 lbs (2.27 kg)