low impedance headphones for R1556A (AR88) - UK Vintage Radio Repair and Restoration Discussion Forum

cendoubleu · 15th Jan 2009, 8:44 pm

Does anyone know what headphones are suitable for the low-impedance output for the R1556A (AR88)? I'm assuming the high impedance o/p would be >2K ohm.

Charlie

XTC · 17th Jan 2009, 2:39 am

See the RCA manual and REME document E772, paras 46 to 48, available from the VMARS site as mentioned in a recent thread.

The headphone OP is 20K, the speaker output is 2.5K and the jack is dual position connecting a 5R load when the plug is pushed fully home. In the halfway position the headphones are connected, but the advice in E772 is not to use this without a speaker or 600R load.

The other O/P is 600R, presumably intended for PAs. You could connect 600R headphones to that.

The O/P transformers are notoriously delicate and don't cope with insufficient load on the secondaries, and replacements hard to come by. Check out some of the threads on this site. One of the articles mentioned recently suggests that transformer failures can be caused by excessive use of headphones.

Also, it's a reasonable view that using headphones with 60 year old mains powered equipment is asking for trouble and I've heard that electrocutions have resulted.

I'd say, connect a 600R resistor across the 600R secondary and use a 2.5R speaker connected securely to the 2.5R output. The issued RCA speaker is quite pricey and hard to find but 2.5R speakers are quite common in scrap domestic sets from the 50s.

Others may have a different view.

Pete.

Aub · 17th Jan 2009, 3:45 pm

Hi,

When I bought my AR88 it came with a pair of Eagle Products 8ohm headphones that the previous owner used to use. I've used them, I would guess, about a couple of dozen times, over the last 10 years I've had the receiver. Never really thought about it much.

Best regards

Aub

dave walsh · 17th Jan 2009, 6:34 pm

The advice from Pete is as I have understood it [although without much putting into practise]. Maybe some people are lucky [like Aub] or there is already a safety resistor in place. There have certainly been plenty of anguished individuals around when their OT has blown. Connecting LS or
Phone items seems to be like balancing a ming vase. As my set is marked "Slave 36", I suspect that many may have been continuously hard driven in wartime use without much plugging in or out to disturb things. On the other hand, the rare OT spares made available by a forum member recently suggest that OT replacement might have been a matter of course.

Somehow I doubt that as I have never heard this problem referred to by anyone who worked on them in service but of course, I don't know.
There are 2.5 ohm spkrs around but I would have thought that 4 or 8 might be ok [perhaps less output] but maybe I'm seriously wrong there?
The main maxim appears to be to have something firmly connected across the secondary while fiddling elsewhere [ as Pete advises].

As for headphones and HT, tapping in between anode and earth [via a suitably rated cap] used to be advocated. I think I tried it once in my youth

but probaly without actually wearing them. Still got some hair! Dave W

cendoubleu · 17th Jan 2009, 8:15 pm

Thanks for all the replies. The best advice seems to be steer clear of headphones. The receiver at the moment is connected to a 4 ohm speaker (not in a cabinet) which seems to be OK. I don't have the original transformer in use, well not the primary anyway. I have paralleled it with another transformer out of an old tape recorder. It gets a little warm but all seems well.

Charlie

15th Jan 2009, 8:44 pm	#1
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	low impedance headphones for R1556A (AR88) Does anyone know what headphones are suitable for the low-impedance output for the R1556A (AR88)? I'm assuming the high impedance o/p would be >2K ohm. Charlie

17th Jan 2009, 2:39 am	#2
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: low impedance headphones for R1556A (AR88) See the RCA manual and REME document E772, paras 46 to 48, available from the VMARS site as mentioned in a recent thread. The headphone OP is 20K, the speaker output is 2.5K and the jack is dual position connecting a 5R load when the plug is pushed fully home. In the halfway position the headphones are connected, but the advice in E772 is not to use this without a speaker or 600R load. The other O/P is 600R, presumably intended for PAs. You could connect 600R headphones to that. The O/P transformers are notoriously delicate and don't cope with insufficient load on the secondaries, and replacements hard to come by. Check out some of the threads on this site. One of the articles mentioned recently suggests that transformer failures can be caused by excessive use of headphones. Also, it's a reasonable view that using headphones with 60 year old mains powered equipment is asking for trouble and I've heard that electrocutions have resulted. I'd say, connect a 600R resistor across the 600R secondary and use a 2.5R speaker connected securely to the 2.5R output. The issued RCA speaker is quite pricey and hard to find but 2.5R speakers are quite common in scrap domestic sets from the 50s. Others may have a different view. Pete.

17th Jan 2009, 3:45 pm	#3
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,038	Re: low impedance headphones for R1556A (AR88) Hi, When I bought my AR88 it came with a pair of Eagle Products 8ohm headphones that the previous owner used to use. I've used them, I would guess, about a couple of dozen times, over the last 10 years I've had the receiver. Never really thought about it much. Best regards Aub

17th Jan 2009, 6:34 pm	#4
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: low impedance headphones for R1556A (AR88) The advice from Pete is as I have understood it [although without much putting into practise]. Maybe some people are lucky [like Aub] or there is already a safety resistor in place. There have certainly been plenty of anguished individuals around when their OT has blown. Connecting LS or Phone items seems to be like balancing a ming vase. As my set is marked "Slave 36", I suspect that many may have been continuously hard driven in wartime use without much plugging in or out to disturb things. On the other hand, the rare OT spares made available by a forum member recently suggest that OT replacement might have been a matter of course. Somehow I doubt that as I have never heard this problem referred to by anyone who worked on them in service but of course, I don't know. There are 2.5 ohm spkrs around but I would have thought that 4 or 8 might be ok [perhaps less output] but maybe I'm seriously wrong there? The main maxim appears to be to have something firmly connected across the secondary while fiddling elsewhere [ as Pete advises]. As for headphones and HT, tapping in between anode and earth [via a suitably rated cap] used to be advocated. I think I tried it once in my youth but probaly without actually wearing them. Still got some hair! Dave W

17th Jan 2009, 8:15 pm	#5
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: low impedance headphones for R1556A (AR88) Thanks for all the replies. The best advice seems to be steer clear of headphones. The receiver at the moment is connected to a 4 ohm speaker (not in a cabinet) which seems to be OK. I don't have the original transformer in use, well not the primary anyway. I have paralleled it with another transformer out of an old tape recorder. It gets a little warm but all seems well. Charlie