Eddystone 888A & High Impedance Headphones

[carnivalpete]

Dear Reader (!) - I believe this problem must be a common one but I haven't found much discussion on it. Apologies if it's already been done to death on some other thread.

I have a vintage comrad and the specification says the headphones need to be a high impedance between 2000 and 4000 ohms. Has anyone found any? The old SGBrowns and Ericssons seem to have completely disappeared.

I've searched long and strong and newer Hi-Imps don't seem to exist. The best I can find are used ex-aviation Telex 610 phones at 600 ohms from USA (from our favourite buying site).

Does anyone have any experience of using the ubiquitous low impedance phones with or without some sort of interface?

Thanks in advance. Pete

[Skywave]

No, I haven't. But may I make one remark that is relevant in practice?
Prior to connecting a pair of low-Z 'phones to an item that specifies high-Z ones, I would suggest that a look at the cct. diag. of the equipment first would be a good idea. Essentially, in doing that, what you are looking for is enough information to determine if the source of the signal to operate the 'phones comes from a low-ish impedance source. If it is low, then you should be O.K. to proceed. Otherwise an impedance matching device will be required, such as a transformer.

Al.

[Station X]

Circuit here:-

http://eddystoneusergroup.org.uk/Man...l%20888Av2.pdf

[G6Tanuki]

The "high impedance" phones appear to be being connected to the anode of the output-valve via a capacitor, and using the output transformer as an audio-frequency choke.

Given the general unavailability of *comfortable* high-impedance phones, it would be dead simple to rewire the phones-socket so you could use modern more-easily-obtainable phones fed from the output-transformer secondary.

That's what I've done in the past, so allowing me to use low-impedance phones [usually an "Airlite-62" headset as used by the wonderful Brian Trubshaw in his Concorde-test-pilot days].

[HamishBoxer]

If comfort is not too much of an issue then you can find high impedance old headphones on Ebay normally at a fair price.

[carnivalpete]

Thanks for comments guys. The phones are only likely to be in use occasionally, mainly for late night use when I'm still too awake to sleep. My study/shack is next to the master bedroom and I dont want great dx 5 and 9's waking my mrs. So resolving this is a convenience rather than a desperate need. What I have done for now is to buy the ex aviation 600 ohm phones and a 4000/600 ohm audio trf. I'll set that up and see how it goes. rgds, pete

[turretslug]

This deriving of headphone output technique seems to be common across a number of Eddystones, with the output valve anode isolation capacitor feeding a potential divider consisting of (often!) a 33k and something around 1k (varied a bit). In other words, a resistive output impedance of a bit less than a kilohm. So, it'll be pretty tolerant of loading- a low impedance headset may produce disappointingly low output but at least it won't result in harm or distortion. Whilst the potting-down may reduce DC potential effects of isolating capacitor leakage, there's always the chance of the lower resistor going O/C and a nasty tingle- it might be prudent to replace the original isolating capacitor with a reputable 1kV type.

[carnivalpete]

As a final footnote on this topic it may be of interest to note that the Telex 610 600 ohm phones turned up from the USA a few days ago. They have a standard quarter inch jack and they work well on a 888A without the need for any matching transformer. So, issue resolved. rgds, pete

[m0cemdave]

Some of the older Sennheiser hi-fi headphones were 2000 ohms (early versions of HD414 and HD424). Many of the older AKG pro-audio headphones (K240 etc) are 600 ohms. They sometimes appear on the secondhand market.

The industry standard Beyer DT100 and its derivatives have been available in various impedences - 2000, 800, and 600 ohm versions occasionally turn up, as well as the usual 400 ohm type.