Wharfedale ID1 Isodynamic
The Wharfedale Isodynamic (sometimes referred to as ID1) was the first commercial orthodynamic headphone released. The large white circum-aural cups gave it a striking look. The design has a 1970s space age feel. The ID1 has square ceramic-rubber magnets. The headphone has low sensitivity, requiring more power and at a higher voltage than most others, and may not produce adequate volume when connected to a headphone socket, although this can be worked around.
Being the first of the many orthodynamic headphones produced over the next two decades, it is surprising to see that Wharfedale's designers got it right the first time round, where many other companies failed with their following efforts. The headphone is comfortable with its soft circumaural earpads. Backwave control has been considered in that the cups are reasonably opened, and just as crucially, effective acoustic backwave damping has been implemented, producing sound that is pleasurable when driven by adequate amplification.
One version of the headset has silver stickers with black text; the other has black stickers with silver text. There are also two kinds of drivers, one with more bent magnets, and another kind with less bent magnets. It is possible that they were all equally flat 40 years ago.
The ID1 has a tendency to develop a buzz in the drivers because of a problem with the membrane. Some units have this problem, others are fine.
Isodynamics require significantly higher drive than most headphones; this was true even when they were in production. Typical headphones give a sound pressure level (SPL) of around 90dB from 1mW of input power; a table shows very few below 90dB, and some up to 120dB (the higher the SPL for 1mW the more sensitive the headphones are). The ID1's stated SPL of 95dB for 30mW equates to 80dB at 1mW, very insensitive. To make matters worse, the ID1's relatively high impedance means that quite a high voltage is needed; some headphone amplifiers may deliver enough power, but only into a lower impedance. An SPL of 95dB is considered safe to listen to for 4 hours at a stretch; 110dB is safe for half an hour according to a table of safe SPL exposure. This is for constant volume; headphones should be capable of peaks of 110dB while listening at safe volume. To deliver 110dB the Isodynamic would need a power of 1 watt (1000mW). The signal voltage required for 30mW (SPL 95dB) is just under 2V, with 10V needed for 1W peaks. A typical headphone output will deliver less than 2 volts to a 32 ohm load (a widely-used computer sound chip, the Realtek ALC887, can deliver 1.2V into 32 ohms). To summarise, the sound source should be capable of delivering 1 watt and a voltage of about 10Vrms to listen at a loud but safe volume and cater for peaks.
It is clear that Isodynamics are unlikely to give sufficient volume when plugged into a headphone socket. The instructions with the Isodynamics recommended that if headphone sockets did not provide sufficient volume they should be connected to the speaker outputs; they can withstand voltages of up to 25Vrms. Obviously this is a nuisance when using amplifiers with a single set of speaker outputs. They were supplied with an adaptor to plug into DIN speaker sockets, no longer used. At the time 35W was a high power; with the higher power amplifiers available nowadays, a protective resistor in series will provide protection should the volume inadvertently be set very high. The resistor should be chosen to give reasonable volume at a normal setting of the volume control. Isodynamics came fitted with large 1/4" jack plugs, rather than the 3.5mm type almost universally used today; a safety resistor can easily be fitted in a line jack socket with a connector or bare wires to connect to a speaker output; the adaptor should be labelled to remind users. (To connect to a headphone output, 1/4" to 3.5mm stereo socket adaptors are widely available.) When used with portable devices and computers, an external headphone amplifier capable of driving the ID1 is needed.
General Description of the Sound
When amplification is not quite sufficient, there is a 'plasticky' resonance present, and the bass is not in full control. Exact effect will depend upon what the amplifier does with peaks it can't handle. Otherwise sound is quite good. When amplified adequately the sound is overall quite flat, with a slight hump in the mid-bass. Soundstage is above average for an orthodynamic. It feels flat and relaxed. It is not dynamic or aggressive like most modern headphones, but if you like smooth sound this is very enjoyable.--Kabeer
Pricing and Other Data of Interest
Wharfedale Isodynamic headphones were designed by the Rank Radio International Ltd design team with consultant designer Oliver Hill. On sale from 1972, price about £20, low for quality headphones. Won a Design Council Award in 1973. Rank Arena sold these in the German market in 1977.
As with all headphones of this age, the padded headband and earcups will have deteriorated, although the sound may be perfect.
Impedance: 120 ohm
Magnets: anisotropic synthetic ceramic rubber
4 micron copper conductor
Frequency response 20-22 000 hz
Maximal voltage: 25V
Sensitivity: 30mW for 95db
Wharfedale Isodynamic driver
The key features of the Wharfedale driver are :
1> Monolithic rubber magnets with round holes.
2> Pleated diaphragm, 100 mg
3> plastic frame for the magnets to slot into and glued down.
4> small plastic interlocking posts on one side on each of the magnet holders.
5> holes on the edges of the diaphragm which will slot into these plastic posts, stretching the diaphragm out.
--Sachu 01-19-2010 at 4:30 PM
These images show the ID1 headset in pieces.