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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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31st Mar 2018, 1:13 pm | #1 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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AR88D headphones once again
Hi again guys
I like to use my AR88D with headphones and I am a bit concerned before plugging my Koss stereo headphones into the AR88. The manual states that the AR88D require headphones with 20 Kohm impedance. However, such high Z headphones are hard to find so how have you guys solved this matter for your own "D" radios? Have you made some kind of adapter from High Z to 32 Ohm (usual value for stereo headphones) or do you just plug the lowZ phones into the radio and let go at that? Have looked for a high Z/lowZ adapter but not found any so far. Back in the 60s I had a very simple 20K headphone with my Ar88 but both that radio and the phones are long gone. Pls comment. Jan |
31st Mar 2018, 2:58 pm | #2 |
Pentode
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
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Re: AR88D headphones once again
A look at the manual shows that that set is designed for ... get this ... down to about 5 ohm headphones! Its on p.7 of the AR88-D manual under "output tube". Look at the circuit.
Note that it just says that it gives a certain power into 20K phones, not that it expects them. Read the manual carefully. If you don't have either phones or a suitable (3.2 or 4 ohm) speaker attached, I'd attach either a 5 ohm resistor to the 2.5 ohm terminals or a 1200 ohm one to the 600 ohm ones, to make a load for the tube. If you use 30 ohm phones either make sure they are plugged all the way in or, safer, permanently install a resistor. If the phones are too loud just plugged in, make a simple pad. If you are worried ... make an adapter so you can measure the voltage across the phones from a known safe source and the radio with a dummy load (as above) before trying them. Last edited by dtvmcdonald; 31st Mar 2018 at 3:08 pm. |
4th Apr 2018, 4:59 pm | #3 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: AR88D headphones once again
Thanks for your explanation, it is most helpful. I have the original RCA speaker attached at all times so no need for any resistor at the speaker terminals, but I'll keep it in mind if I have to remove the speaker for some reason.
Jan |
4th Apr 2018, 5:07 pm | #4 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,289
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Re: AR88D headphones once again
When phones are plugged in a 5R resistor (R56) is connected across tags 1 and 6 of the O/P transformer.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
4th Apr 2018, 9:19 pm | #5 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: AR88D headphones once again
Graham, thanks for that clarification
Jan |