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Author Topic: headphones for a SP-600 ?  (Read 647 times)
W1JTO
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Posts: 118




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« on: January 10, 2019, 08:48:37 PM »

I am sure one of you knows the exact-best setup.  I have a SP-600 that is hooked up to a 600ohm->8ohm transformer and it works perfectly... But at night I want to use headphones to keep the family happy.

I have a Heil Proset elite 6 on my modern setup... assume it cant be utilized on the SP-600 ...Is there a good, comfortable alternative ?

Thank you and 73 

John W1JTO
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WA4JQS
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Posts: 277




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« Reply #1 on: January 10, 2019, 09:25:41 PM »

why not get another 600 to 8 ohm transformer and mount in a small box with a couple of phone jacks and use the Heil headset. same as with the speaker.. 73 WA4JQS
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W1JTO
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Posts: 118




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« Reply #2 on: January 11, 2019, 04:22:39 AM »

Are the headphone jack's 600 ohm ?  - My memory was that they were much higher ?

I could try that for sure.  Just a bump (big one) in the cable.
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K7MEM
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Posts: 667


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« Reply #3 on: January 11, 2019, 04:56:12 AM »

Are the headphone jack's 600 ohm ?  - My memory was that they were much higher ?

The following is an excerpt from the SP-600 Service Manual. That should tell you everything you need to know.

Quote
Audio Output - The audio output of V17 is transformer-coupled through a split balanced winding to deliver 2.0 watts undistorted output to a 600-Ohm load. The split balanced winding permits balancing of the direct kind in the output circuit, as used for teletype or similar service. A separate secondary winding provides attenuated audio signal output for headphone operation. This winding will deliver an output of 15 milliwatts into an 8000-Ohm resistive load when the 600-Ohm power secondary is delivering 500 milliwatts to a 600-Ohm resistive load.

Speaker - The speaker should be of the permanent-magnet dynamic type and should include a speaker voice coil to 600-Ohm line-matching transformer for connection to the 600-Ohm audio output terminals of the receiver. Caution: When the 600-Ohm output is not used connect a 600-Ohm, 2-watt resistor to these terminals to avoid component damage from high transient peak voltages. For applications requiring the insertion of direct current control or indicating voltages, the jumper connecting the two balanced sections of the 600-Ohm output may be removed and the insertion circuit, such as a low resistance balancing potentiometer, connected in its place.

Headphones - Either high or low impedance headphones may be used by plug connection to the phone jack located at the lower left side of the front panel. The high impedance type is recommended.
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Martin - K7MEM
http://www.k7mem.com
W8RXL
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Posts: 70




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« Reply #4 on: January 11, 2019, 12:14:35 PM »

I thought the headphone output was at 8 ohms. I never had the issue using an 8 ohm headphone with mine.

I never heard the headphone jack having anything to do with impedance.
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K7MEM
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« Reply #5 on: January 11, 2019, 01:58:56 PM »

I thought the headphone output was at 8 ohms. I never had the issue using an 8 ohm headphone with mine.

The manual does not say you can't use 8 Ohm headphones. High or low impedance headphones can be used.

The manual also does not say there will be any issues. They only recommend high impedance type.

You can, pretty much, use whatever you have without any issues.

I never heard the headphone jack having anything to do with impedance.

I'm not sure I understand this statement.
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Martin - K7MEM
http://www.k7mem.com
AA4HA
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« Reply #6 on: January 11, 2019, 02:35:33 PM »

I can say this with regret that I toasted the audio transformer on an SP-600 when I wasn't paying attention and used a 4 ohm speaker.

It was an expensive mistake and took me a few months to find a replacement original-equipment transformer from Fair Radio back in the 1980's.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AA4PB
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Posts: 14962




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« Reply #7 on: January 11, 2019, 04:15:12 PM »

"Caution: When the 600-Ohm output is not used connect a 600-Ohm, 2-watt resistor to these terminals to avoid component damage from high transient peak voltages."

Note the above warning. Even if you are using headphones connected to the headphone jack, you should NEVER run the receiver without a load (600 Ohm transformer + speaker OR a 600 Ohm resistor) connected to the speaker output. This was a common issue with older tube type audio power amplifiers.

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Bob  AA4PB
Garrisonville, VA
W1JTO
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Posts: 118




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« Reply #8 on: January 12, 2019, 05:26:14 PM »

Thank you for the help.

One question:   If the impedance's don't match I have heard it said that speaker volume would be "way off" or hard to control. Huh?

But, to summarize eham feedback:

Keep  my 600 ohm -> 8 ohm hammond transformer connected at all times to speaker terminals (this works perfectly for me, BTW) or use a 600 ohm 2w resistor in it's place.

And

Any headphone "should" work ok ?...

Even so, would an impedance match transformer for headphones be optimal ?

Thanks and 73   John W1JTO
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AA4PB
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Posts: 14962




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« Reply #9 on: January 12, 2019, 06:01:42 PM »

If you keep the transformer connected then the transformer MUST also be connected to either a speaker OR an 8 Ohm resistor. Alternately you can disconnect the transformer and connect a 600 Ohm resistor to the receiver speaker connection. You must NOT turn the receiver volume up with no load on the speaker output. A transformer with nothing connected to it's secondary winding is NOT a load on the receiver output.

You may find the volume a little low (i.e. you need to turn the volume control up higher) when using an 8 Ohm headset. The headset output is really designed for the older high impedance (2000 Ohm) headset that was common during the period when that receiver was made. Another alternative might be to connect the 8 Ohm headset to the secondary of the speaker transformer in lieu of the speaker, provided that you can turn the volume down enough to be a comfortable level. You are correct that when the impedance match is way off then the volume can be difficult to control.

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Bob  AA4PB
Garrisonville, VA
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