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Old 25th Oct 2020, 4:26 am   #1
hayerjoe
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Default Splice aux-in to old radiogram - diode issue

hello everyone

I have a 1960's valve radiogram with a BSR UA25 record changer. I'm trying to add an aux-in.

I wish to add the aux-in to the connecting strip under the BSR (so I can't use a switched 3.5mm socket as it will have 3.5mm plug permanently connected). This arrangement means that the signal from the record player cartridge will be live as well as the signal from anything connected to the aux-in. Whilst I won't be using both simultaneously I'd still like to avoid any potential damage to the cartridge whilst using the aux-in and vice versa, so I added diodes to each signal wire. The diodes are causing my problem. If I remove the diodes everything works great, when I add them to the signal wires I get low volume and much interference.

Can anybody help? Am I using the wrong spec diodes? or even the wrong component? Do I even need to protect the cartridge?

See attached for my handrawn schematic, it's pretty much a diagram I got from the net with my addition of the diodes (yep the bit that makes it fail!)

Any help would be welcome, also I am not an electrics expert

thanks
Jo

diodes = 1000v 1amp (too much resistance perhaps?)
resisters = 0.5w 1k
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Old 25th Oct 2020, 10:21 am   #2
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Default Re: Splice aux-in to old radiogram - diode issue

Diodes will not block AC of any sort, they will just distort it.

There is no way to do what you want without switching of some sort, for a number of reasons.
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Old 25th Oct 2020, 11:40 am   #3
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Default Re: Splice aux-in to old radiogram - diode issue

I think the chances of damage are small. Use resistors instead if you want, experiment to find values which work best.
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Old 25th Oct 2020, 12:12 pm   #4
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Default Re: Splice aux-in to old radiogram - diode issue

Additionally, some radiograms have live chassis connected to the mains. Connecting them to anything outside their case is very dangerous.

Before going any further, you need to find out whether the power supply section of that radiogram is isolated from the mains

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Old 25th Oct 2020, 10:23 pm   #5
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Default Re: Splice aux-in to old radiogram - diode issue

Many thanks for all your replies.

Firstly and importantly safety - I can confirm that it's not a hot chassis radiogram and I am taking all precautions relating to valve lethal voltages

Secondly I didn't even consider that signals would have been AC! I presumed (incorrectly) that a record groove and cartridge would not generate AC and that my phone (aux-in) would just use DC, but as originally stated works fine without diodes so I was wrong to presume. thanks

And finally for nickthedentist, should I go with the highest resistor I can that doesn't distort sound? I tried 1k, was fine, I tried 10k was also fine, do I just keep trying bigger resistors? and therefore 'bigger' provides better protection to the cartridge?

thanks
Jo
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Old 26th Oct 2020, 10:49 am   #6
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Default Re: Splice aux-in to old radiogram - diode issue

I think you are going about this in the wrong way.
You are attempting to put a line level low impedance signal into a medium/high high gain high impedance input.
Something you need to consider is the pickup input will probably have some RIAA equalisation suitable for record player cartridges, you won't want that on the line level signal from the iPod.
The result is likely to cause interference due to hum pick up on the record player.
My suggestion would be to insert the line level into the audio preamp stage at, for instance the volume control, depending on where the tone controls are in the circuit..
See similar ideas and threads about connecting an aux input into a car radio.
Alternatively is there a tape socket? What is the make and model of the radiogram, do you have the schematic.


Mike

Last edited by crackle; 26th Oct 2020 at 10:59 am.
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Old 26th Oct 2020, 11:36 am   #7
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Default Re: Splice aux-in to old radiogram - diode issue

Thanks, I have seen discussions about adding to the volume pot, but the signal wires go direct from BSR changer to the radio/gram selector switch and then direct to volume, so tapping in right under the changer seems easiest and effectively the same. The radiogram is a Kreisler Stereosonic from 1968, the schematic is stuck to the back, so hard to scan! but I've taken a photo, hope it's readable. And no tape or even headphone jack, which is weird for late 60's.

Also I should state that now I've removed the diodes the sound is just perfect from BSR and from my phone attached to my new aux-in, I'm just concerned that the aux-in signal could feed back into the cartridge and cause some damage
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Old 26th Oct 2020, 1:45 pm   #8
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Default Re: Splice aux-in to old radiogram - diode issue

If the selector switch is a conventional rotary one, you could replace it with one with one more ways, then it will act as a selector for your new input... just turn the knob one place beyond those marked and there you are. The radio will be muted in the gram position, so just duplicate however this is done for the new position.

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Old 26th Oct 2020, 2:45 pm   #9
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Default Re: Splice aux-in to old radiogram - diode issue

that's a clever idea David!
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Old 26th Oct 2020, 6:50 pm   #10
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Default Re: Splice aux-in to old radiogram - diode issue

FWIW a '60s radiogram is unlikely to have anything other than a ceramic or crystal cartridge feeding the gram input, so there's no particular equalization or signal level issues to consider. A low impedance external signal input would also be unaffected (at any rate audibly) by the 600pF or so capacitance of the cartridge loading it. It's also unlikely that any harm would come to the cartridge from an applied signal of similar magnitude to that which it develops itself when playing a record. It might "tizz" a bit as the piezoelectric effect works both ways mechanical to electrical (record playing) or electrical to mechanical (external applied signal voltage).
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Old 27th Oct 2020, 8:16 am   #11
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Default Re: Splice aux-in to old radiogram - diode issue

thanks herald1360. Glad to have it confirmed that it may need no protection. As you say it is a ceramic cartridge and probably not a very sophisticated one at that, but I didn't want it to survive 52 years to be ruined by me in a week!

Like I said in the thread now I've removed the diodes it's working really well and has a very nice sound. But I'll wait for any more replies to this thread before I make a decision on how to finalise it all

thanks again
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Old 27th Oct 2020, 12:45 pm   #12
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Default Re: Splice aux-in to old radiogram - diode issue

If you just connect a low impedance line level source across the cartridge it will work, but the cartridge won't because it will be excessively damped. You will need to disconnect the source if you want to play records.
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Old 28th Oct 2020, 11:38 am   #13
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Default Re: Splice aux-in to old radiogram - diode issue

Quote:
Originally Posted by paulsherwin View Post
If you just connect a low impedance line level source across the cartridge it will work, but the cartridge won't because it will be excessively damped. You will need to disconnect the source if you want to play records.
@paulsherwin actually I haven't tried the combination of record player playing with phone still attached to aux-in, it would be good experiment. If I try this is there any risk to the cartridge if it's "excessively damped"? Please let me know your thoughts

thanks Jo
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Old 28th Oct 2020, 11:45 am   #14
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Default Re: Splice aux-in to old radiogram - diode issue

No, it won't do any harm, it just won't work properly.

A ceramic cartridge needs to see an impedance of around 1M ohms, but a phone headphone socket will shunt this bringing it down to a few thousand ohms if that. Simply unplugging the phone will restore the impedance to its original value.
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Old 28th Oct 2020, 12:00 pm   #15
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Default Re: Splice aux-in to old radiogram - diode issue

Quote:
Originally Posted by paulsherwin View Post
No, it won't do any harm, it just won't work properly.

A ceramic cartridge needs to see an impedance of around 1M ohms, but a phone headphone socket will shunt this bringing it down to a few thousand ohms if that. Simply unplugging the phone will restore the impedance to its original value.
ok I'll give it go tomorrow and let you know how it goes
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Old 28th Oct 2020, 3:51 pm   #16
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Default Re: Splice aux-in to old radiogram - diode issue

With blocking capacitors and highish voltages you can get charge-up transients on plugging in big enough to dispatch phones to that great big WEEE pile in the sky.

Take care, or try it out with something sacrificial.

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Old 29th Oct 2020, 9:26 am   #17
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Default Re: Splice aux-in to old radiogram - diode issue

Quote:
Originally Posted by Radio Wrangler View Post
With blocking capacitors and highish voltages you can get charge-up transients on plugging in big enough to dispatch phones to that great big WEEE pile in the sky.

Take care, or try it out with something sacrificial.

David
Yep decided against this, didn't even want to brick my least useful phone! In fact I'm coming down on the side of using a switched 3.5mm socket which will totally disconnect the cartridge when something is inserted into the jack. It's a lot more soldering and involves more shielded wire, but it's a safer (and the more correct) way of doing what I wanted

thanks everyone for your help
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Old 29th Oct 2020, 10:38 am   #18
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Default Re: Splice aux-in to old radiogram - diode issue

For what its worth, a few years ago I fitted a 3.5 socket to a Bush SRP31 with a ceramic cartridge, with some resistors to sum the stereo input to mono. Worked fine and caused no harm to the cartridge at all.
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Old 29th Oct 2020, 11:17 am   #19
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Default Re: Splice aux-in to old radiogram - diode issue

Quote:
Originally Posted by PsychMan View Post
For what its worth, a few years ago I fitted a 3.5 socket to a Bush SRP31 with a ceramic cartridge, with some resistors to sum the stereo input to mono. Worked fine and caused no harm to the cartridge at all.

good to know!
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Old 29th Oct 2020, 11:40 am   #20
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Default Re: Splice aux-in to old radiogram - diode issue

Thanks for this thread, because I'm thinking of adding an aux input to my Pye G63, and I was concerned about the effect on the cartridge. Just for cassette and CD, no computer headphone socket stuff.
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