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Author Topic: Quiet 6 volt power source for Grundig radio  (Read 239 times)

KK4GMU

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Quiet 6 volt power source for Grundig radio
« on: August 07, 2022, 07:15:23 AM »

Here is my inelegant solution for powering my Grundig Satallit 750 receiver. 

Background:  This radio is notorious for being susceptable to wall wart noise.  It can be powered by batteries or a wall wart.  The D cell batteries it uses are said to last a long time, but not rechargeable in the radio.  Regular switching wall wart power creates hum or whiney noises throughout the bands.  I'd prefer not to run off of the internal Ds because sooner or later they will leak or run down.  I just don't trust them. 

I tried a number of "better" $15 and $20 power supplies - they all produce noise.  I wanted to power the Grundig from my Jetstream 12 volt supply - which is very quiet on my ham rig - but my Grundig is 6 volts, not 12.  Marketplace (12 volt to 6 volt) converters either won't be quiet, or will be pricey. Homebrew is out of my skill set.

My inelegant solution: Use a 6 volt AGM battery ($10 to $20 for one that is 4.5 amp hours or a bit bigger). Get a cheap trickle charger ($10). Yes, they produce noise, too, but stand by.

I already have a plug in count down timer (1 to 6 hours). I connect the radio to the battery (I can't forget that the Grundig center pin is negative) so I need a polarity reversing adapter. I plug the battery to the charger, plug the charger into the countdown timer. The charge on that battery would probably last several weeks. Once every several weeks I hit the button on the countdown timer to charge up the battery for a few hours.

No more D cells, no more wall warts. No more noise.

Because trickle chargers themselves can produce noise in the radio, the battery would be charging only when I am NOT using the radio - thus the reason for the countdown timer for the charger.

My only concern is if I DO happen to power up the charger to the battery while the radio is "on", aside from likely noise from the charger, will it do anything adverse to the radio?   I don't like magic smoke.
« Last Edit: August 07, 2022, 07:19:50 AM by KK4GMU »
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WA3SKN

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Re: Quiet 6 volt power source for Grundig radio
« Reply #1 on: August 07, 2022, 10:21:01 AM »

The wall wart for that radio appears to be 6v 500ma, so your AGM 6v should work OK.  And the old "lantern battery" is also 6v and should work... not sure of the going price though.  Have you measured the actual current draw of the radio?

-Mike.
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KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #2 on: August 07, 2022, 11:51:48 AM »

The wall wart for that radio appears to be 6v 500ma, so your AGM 6v should work OK.  And the old "lantern battery" is also 6v and should work... not sure of the going price though.  Have you measured the actual current draw of the radio?

-Mike.

0.027 A
119.3 volts
1.88 watts
The volume settings makes no difference on current draw when sending audio into headphones "out" into a powered speaker.

When using the radio's audio amp and speaker, I can spike Amps to 0.046.

From the wart, if I increase the radios volume control and reduce the powered speaker's control, the hum is reduced to near inaudible - nice, but I know the hum is there. Inversely, the hum increases with the radio volume down and the powered speaker turned up.

Without the wart (powered by D cells only), I can turn the radio volume down and the powered speaker way up and NO hum.  So I expect the AGM battery to do the same.
« Last Edit: August 07, 2022, 12:05:48 PM by KK4GMU »
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KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #3 on: August 07, 2022, 12:22:28 PM »

ONE ODDITY I just noticed:  Using the D cells, no wart attached, when I turn OFF the radio, the same hum occurs as when I was using the wart.  I have snap on filter chokes on both ends of the cable leading from the radio to the audio mixer going to the powered speaker.  No other audio device that is plugged into the little mixer generates ANY hum, whether that device is on or off.  Those include my IC-7100, Alexa, and my computer.

The mixer has a ground lug.  There is no difference with or without a ground going to my ground buss to the ground rod a foot away.

I guess the simplest thing to do is just to turn down the radio input volume pot on the mixer when the radio is "off."  But it still bugs me.
« Last Edit: August 07, 2022, 12:43:44 PM by KK4GMU »
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W9IQ

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Re: Quiet 6 volt power source for Grundig radio
« Reply #5 on: August 07, 2022, 04:18:48 PM »

If the radio in question has no other electrical connections, it cannot be a ground loop.

What is the input impedance of the mixer, the output impedance of the radio and what are you using for an interconnect cable?

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #6 on: August 07, 2022, 04:48:41 PM »

If the radio in question has no other electrical connections, it cannot be a ground loop.

What is the input impedance of the mixer, the output impedance of the radio and what are you using for an interconnect cable?

- Glenn W9IQ

Good questions. I do not know either of the impedances.  The cable is a standard 12' long audio cable with mini phone plugs on either end, along with clip on ferrites on each end.

Why would the I get the 60 cycle hum with the radio OFF and it goes totally away with the radio ON?
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AA4PB

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Re: Quiet 6 volt power source for Grundig radio
« Reply #7 on: August 07, 2022, 05:15:17 PM »

Are you sure there is nothing else connected to the radio? An external antenna, ground, other cables, a metal table or bench, etc. You get 60Hz hum with the radio off and nothing connected to it except the audio output cable?
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Bob  AA4PB
Garrisonville, VA

LA9XNA

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Re: Quiet 6 volt power source for Grundig radio
« Reply #8 on: August 08, 2022, 12:31:31 AM »

If the hum comes from the wallwart you should get a linear powersupply. Get a supply with linear regulator and plenty of filtering.

If you want a challenge!
The supply can also be homebrewed with a 78L06 voltage regulator. The simplest way to build it is probably to reuse a 12 to 9 volt transformer and possible case from an old supply. In some cases where a 78L?? regulator is used you can just replace the regulator. You might find a local HAM to help you.




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KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #9 on: August 08, 2022, 04:17:24 AM »

Yes.  There is an FM and Shortwave antenna hooked up.  Neither of those are the culprit ...with radio off or on.

I found the "radio OFF" culprit.  It is the damned 5 volt wall wart that powers the mini mixer.
Snap-on ferrites on that power cord don't help.  The only thing that does reduce hearing the hum when the radio is OFF is turning mixer volume down.  When the radio is ON, its volume control needs to be turned up higher.

So now I'm on a hunt for a cleaner 5 volt power supply.  I'll start by seeing if my mixer can stand 6 volts instead of 5.
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AA4PB

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Re: Quiet 6 volt power source for Grundig radio
« Reply #10 on: August 08, 2022, 09:37:37 AM »

If it's 60Hz hum I'd suspect some sort of a ground loop (AC flowing in shield of an audio cable) rather than noise from a power supply. Any AC current flowing through the shield of a mixer input cable can cause hum when that channel is turned up. That's why I asked about other connections to the radio like antenna, ground, etc. when it's turned off.

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Bob  AA4PB
Garrisonville, VA

KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #11 on: August 08, 2022, 09:56:01 AM »

If it's 60Hz hum I'd suspect some sort of a ground loop (AC flowing in shield of an audio cable) rather than noise from a power supply. Any AC current flowing through the shield of a mixer input cable can cause hum when that channel is turned up. That's why I asked about other connections to the radio like antenna, ground, etc. when it's turned off.
And that is why I will be trying one of these cheapo gizmos:
https://www.amazon.com/gp/product/B019FC6ZQQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Can't hurt to try.  Can always use it on something.
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KK4GMU

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Re: Quiet 6 volt power source for Grundig radio
« Reply #12 on: August 10, 2022, 10:21:10 AM »

This: https://www.amazon.com/gp/product/B019FC6ZQQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

...resolved the 60Hz hum.  The only artifact left over is a slight hash a couple octaves higher that is only heard when the mixer volume is turned all the way up.

Problem solved!  I will get another for other similar potential issues.

I still get wall wart noise which I will resolve with an external AGM 6-volt battery/charger combo.

I tested the output voltage of the battery/charger combo, and it does go up to a bit more than 7 volts.  While I will avoid charging the battery while the radio is on, I want to make sure that voltage much above the rated 6 volts of the radio won't be detrimental.
« Last Edit: August 10, 2022, 10:32:10 AM by KK4GMU »
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