Headphones for Boatanchor Receivers

[Dave KA2J]

I began thinking about the headphones I use occasionally with my boatanchor receivers after reading a recent reply by Mark WQ9E on the “First Boatanchor semi restoration” posting on this forum.

Mark said…
“If your headphones are of the modern low impedance variety you will hear significant hum on most of the older tube type receivers.  As I recall you stated you have hum before the tubes warm up so the hum in your sensitive headphones may be due to coupling from the power transformer to the audio transformer; this happens in a number of receivers but it is generally not noticeable with "communications quality" headsets.”

I do not use headphones often, but I’m aware that some of my receivers have considerable hum when using a couple of the headphones I have in the shack.  I opened my drawer and pulled out some old (pre-war?) headphones my dad gave me when I was about 7 or 8 years old.  I plugged them into my 75A2 and no hum!  I should have given this a little thought in the past, but never did!  The headphones are “Trimm Featherweight”, made by the Trimm Inc., Chicago, USA.

You’ve probable all seen similar headphones before.  They have the two stiff wires with a black woven cloth strap on each that go over your head.  The ear pieces have black plastic like cover with a hole in the middle, that screws to the housing.  Inside is a circular metal plate that is held in place by two little magnets that are each surrounded by a coil of fine wire. As a young boy, I had taken these headphones apart many times.  I would even listen to static around the house by touching metal objects.  Eventually I used them with my NC-300 receiver when I was a Johnny novice.  I remember my ears getting sore after about a half hour of that black plastic (bakelite?) material flat up against your ear.  I've seen pictures of telephone switch board operators using them.  How could they where those all day?

The more modern low impedance headphones I've been using recently resulted in significant hum with my 75A2, 74A4 and HQ-170.  These same headphones had no hum when used with either of my SP-600’s or R-390A’s.

The good old Trimm Featherweight phones were "hum free" when listening on the 75A2’s, 75A4 and HQ-170 and audio wise, didn't sound too bad.

[W2XR]

Dave,

I think the issue here with the Trimm headset you described is the fact that the low-frequency response of this type of headset is virtually nonexistent. I don't know what the actual response of these are, but I would be surprised if there was much usable acoustic output below 300 to 400 hz. They are the epitome of the "tinny-sounding" headphone.

I also have a pair of the Trimm Featherweights that I use really for period-decorative purposes in my vintage SSB station. Due to the limited audio response, they are also great for CW.

And yes, I also had a pair that I received as a birthday present when I was about 5 years old! I vividly recall that they had a pair of Mueller copper-plated alligator clips at the end of the woven cable.

In the AM station here at W2XR, I use a pair of AKG K-240M hi-fidelity headphones. I use them for off-air monitoring and for listening through my R-390A or SP-600 receivers; the audio from both receivers is taken off of the diode load and drives an outboard 15 watt triode power amplifier that in turn drives either a speaker or the 'phones.

I also have two 75A-4s here, and when using the AKG headphones, I don't really notice that much hum at all. Yes, there is some residual hum, but it is so far down as to be not objectionable, and at normal listening levels, the receiver and atmospheric noise essentially mask the hum anyway.

73,

Bruce

[AF9J]

Hi Dave,

Bruce is right in what he's saying.  Like Bruce, I prefer the more modern headphones.   As for the hum when using modern headphones - the reason is due to the fact that the headphone outputs in old receivers (at least into the mid 50s or early 60s - my Drake R-4A doesn't have this issue, nor did my Halli SX-96) were set up for high impedance headphones of at least several thousand ohms.  A lot of the hum you hear while using modern headphones with say an HQ-129X, is due to the impedance mismatch.  Modern headphones typically don't have an impedance much higher than say 45 or 60 ohms (my nice, expensive, over the ear $130 Sonys have a 46 Ohm impedance).   Me, I prefer to use modern headphones to the old timers.  They're more comfortable when you're wearing them for long stretches on your ears.  I also think they have better audio (especially with regards to bass response). You can do 2 things:

1. Put up with the hum (typically it's not a large amount).

2. Use an audio transformer - you can either wire it internally next to the headphone jack, or build it into a box with a short cable you can plug into the receiver.  You would then plug your headphones into the audio transformer box.  Because it's external, the box can be used with multiple receivers.  With regards to the audio transformer - even a cheapo Radio Shack-type 10 to 1 will usually do a good job of giving you a match your modern headphones can live with.

73,
Ellen - AF9J

[Dave KA2J]

Hi Bruce and Ellen,
Thanks for the comments.  Yes, I’m sure the low end frequency response is pretty impaired on the old time headphones.  But for causal listening for communications quality, it works well with the old BA receivers.  The audio transformer is a good idea Ellen.  My 75A4 hum isn’t that bad with the newer headphones either Bruce.  The 75A2 is another story!  The high impedance phones make a huge difference with the A2.

[W2XR]

Hi Dave,

You are very welcome!

73,

Bruce

[AF9J]

No Problem Dave! 

73,
Ellen - AF9J

[K3ZS]

I have taken stereo headphones, that you don't want to use for stereo, and connected them to a mono headphone
plug.   I put a resistor in series with the hot wire going to each side of the headphone.    That makes them less sensitive, so that you have to run the audio level about the same as when using the speaker.     That usually lowers the hum so you can't hear it.

[w4bfs]

be carefull of too large a resistor value ... if the radio has a switching headphone jack and you plug in the headphones and things are wired to remove the speaker load the the audio pa MAY only be loaded by what is plugged in .... I would check schizmatic and verify this ... for a 8 Ohm output, I would not want to go above 25 or so Ohm load ... same deal as zorched modulation transformer ... a safer way is to wire a 22 Ohm 1 W resistor across the jack switched contacts so that the load is there ... I may be a bit too conservative but I'm sure some OEM audio output transformers are unobtanium.....beefus

[K3ZS]

If you hear hum in the headphones, and not in the speaker, the headphones are probably higher impedance and more sensitive to the hum.    The stereo headphones I had must have been pretty high impedance, it took a couple hundred ohms to lower the lever down to the same listening level as with the speaker.    This was using them with an HQ-140 and NC-183.   

[w1vtp]

Dave,

I think the issue here with the Trimm headset you described is the fact that the low-frequency response of this type of headset is virtually nonexistent. I don't know what the actual response of these are, but I would be surprised if there was much usable acoustic output below 300 to 400 hz. They are the epitome of the "tinny-sounding" headphone.

Yup -- agree.  I use Bose QC2's here on a Hallicrafters SX-146 - also a Ft-957 (a non-boatanchor rig)  There is no hum on either rig.  The low end audio is real bad on the SX-146 but not bad on the FT957.  No hum on either RX.  Dunno what the Z is on the QC2

Al VTP

[W2JBL]

    Hi Dave! for non HI-FI work i like the HS-53 and HS-54 style (B-17 aviator) phones. about 600 ohms with leather headband covering and nice rubber ear cushions. reasonably flat from 100-5000 cycles. just right for trying to copy a guy with 50% modulation and all his audio power below 500 cycles. they make copying those kinds of signals livable on a noisy band. if i'm in a HI-FI mood i use a good pair of Koss or Sony cans and matching transformer to get to 600 ohms. the phone line output on the R390 is just right to drive phones, and i normally use that. it's fun to watch that line level meter bounce too!

[Dave KA2J]

Hi Chris!  Nice to hear from you.  I enjoyed your strong signal from the Adirondack Mts a couple of weeks back.

Thanks for the tips on the headphones.  They sound like just the ticket.

All the best, Dave

[Rob K2CU]

a 12 VAC filament transformer will probably give you about the impedance ratio you need to make a low impedance headset at say 8 Ohms look like around 600 Ohms. give or take a few. There was an article in QST a few years ago called "Tin Ears" where the metal diaphram was cut to make a 600 Hz resonant tuning fork for improved CW use. BTW, I have a couple of NOS lead sets for the Trimm headphones should there be a need out there.

[w4bfs]

hi Rob ... used that trick (12 V fil xfmr) to drive a spkr from 600 Ohm spurce ... worked like a charm ... 73 ...beefus