Behringer HA4700 Headphone Amplifier – Mains Hum
Typically available for about Ł100 in the UK the Behringer HA4700 promises to deliver a professional amplifier with the highest sonic quality. Essentially it would do so if it wasn’t for the mains hum! Along with some other Behringer models the HA4700 features a built-in mains power supply. This is the norm if designed well and although Behringer uses toroid transformers, capacitor smoothing, DC regulators and decoupling, the mains hum problem persists.
Any owner can test their unit quite easily by disconnecting everything except the mains and a pair of headphones. Connect the headphones to output 4 and with the input turned fully down and CH4 volume set to maximum you should the characteristic mains hum.
The PSU circuit is standard stuff. It’s a split 44V AC supply (2 x 22V) feeding a bridge rectifier and 1000µF smoothing capacitors. The 15V split regulated supply is derived from standard 7815 and 7915 regulators, decoupled with 10µF capacitors. These supply a ripple free DC to all the audio circuitry. Earthing is well controlled and the unit is fully screened.
The problem is that the toroidal transformer emits a magnetic field well beyond itself and this interferes with any nearby circuitry. The transformer is directly behind channel 4 hence why the mains hum is worst on that channel. Shielding might solve the problem but there’s not much space.
To prove the theory I disconnected and removed the mains transformer. I then reconnected it with some long wires back to the PSU and connected the transformer to a safe block about 400mm away. This completely solved the mains hum now the channels are just outputting the expected very quiet circuitry hiss at max volume setting. The hiss would be extremely low at normal listening levels. I moved the transformer close to circuitry and the hum appeared again.
So the solution is to install the transformer in a separate housing and connect the low AC supply via removable connectors.
Hence this project…
The design fully insulated the transformer & components so that an earth connection to the PSU is not necessary.
This eliminated any further complications with earth hum loops.
An MB3 black plastic box houses the Behringer transformer. The mains is connected via a 2 way figure 8 input plug, then is connected to a 20mm fuse holder with a T315mA fuse. The low AC side is connected directly to a 3 pin Binder locking DC socket. A 3 way cable then connected to a 3 pin plug at the headphone amplifier.
The Behringer mains input IEC socket was removed and an aluminium plate was made to fit in its place to accommodate the low voltage AC input socket.
My Behringer RX1602 mixer suffered the same problem with an identical PSU arrangement so this was also modified in the same way. Total mains hum free listening now achieved.
Note that I also tried an ART Pro 6 headphone amplifier and that also suffered the same issue.
The ART Pro 6 is a really good quality unit and it’s a shame the manufacturers cannot get to grips with the mains hum issue.