I-Pod to Gram input of vintage radio options

[Yorkie]

I’ve seen several option for connecting an i-pod or similar MP3 player to the gram input of a vintage valve radio.

Three of the options I've seen are shown in the attached pics entitled 'options 1, 2, 3'.

The simplest is option 1 - just a couple of resistors which could be squeezed into a 3.5mm plug.

The rationale of Option 2 is - to quote the circuit originator:

The headphone jack of your iPod is not ideal for connecting to your stereo. OK for headphones, but sub-par for connecting to a stereo.

There are two problems:

1. The impedance mismatch that makes your tunes sound mushy.
   
2. A lack of output voltage means that you have to crank up your stereo volume to get a decent listening level.
   

Solution: Use two audio transformers. The designer of the circuit states thus:

The end result? A great sounding boost. Not earth-shattering volume as would be realized in an active (opamp or transistor) circuit, but all of the mush was gone. Bass and treble were both significantly enhanced, and the sound improvement was pretty dramatic for a passive circuit.

This device is really simply, basically four parts. Here's what you'll need:

1. a pair of Radio Shack Audio Transformers (Catalog #273-1380)
   
2. a pair of 3.5mm stereo input jacks
   
3. an enclosure to hold the parts together.
   

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Option 3 has the merit of isolating the gram socket from the MP3 player, and the designer of that circuit explains it thus:

About a year ago I built a circuit similar to this using these same RS transformers. The significant difference on mine was that it combined the two audio channels from the Ipod into a single channel to run directly into the auxiliary input of a tube radio. I also used the secondary of the transformer winding on my project. If you were to use this adapter with a AC/DC set and had concerns about isolation, the transformer coupling provides DC isolation to the MP3 player.
 
The term passive amplifier is a little misleading. Although it is a passive circuit, it is more of an impedance matching network than anything else.

Since it matches the low impedance output of the Ipod to the relatively high impedance input of a tube radio audio stage, it might appear to provide gain, but what I think you are really seeing is more efficient transfer of the Ipods output power to the radio. The frequency response and fidelity was better with a transformer network than my other attempts with just a resistive network.

Clearly the first option is cheapest and simplest, but do the other two options provide worthwhile benefits?

Certainly option 3 has the merit of isolating the I-pod from the chassis of the radio into which the adaptor is fed, though I don't think I'd trust connecting it to an AC/DC 'live chassis' radio due to the dubious insulation qualities of the cheap audio transformers, the secondary of which isn't on a separate bobbin, but overwound on the primary. Maybe via two Y class caps? (A bit academic as I've no intention of doing that anyway).

Presumably the widely available cheap ‘Eagle’ LT700 transistor output transformers would be suitable for the above options?

Source of the above two transformer circuits, and a thread discussing the same is here:

http://antiqueradios.com/forums/viewtopic.php?f=6&t=180091

Any thoughts on any of these circuits please anyone?

[tiv48]

Or a pantry transmitter ?
Rob T

[Old Sparky]

A similar circuit could probably made, using an LT700 transformer, which has primary Z of 1k or 1k2, depending on whether you read Cricklewood Electronics or Maplin's descriptions!, and secondary Z 3.2 Ohms (Maplin0 or 2 x 3.2 Ohms (Cricklewood) I don't have one to hand to check which description is correct, but, IIRC it's a centre-tapped primary and a single secondary. Until now, I've always used method #1 when fittin 'aux-in' connectors, almost always to vintage car radios, but may go down the isolating transformer route in future

[Mark Hennessy]

The rationale of Option 2 is - to quote the circuit originator:

The headphone jack of your iPod is not ideal for connecting to your stereo. OK for headphones, but sub-par for connecting to a stereo. There are two problems:

1. The impedance mismatch that makes your tunes sound mushy.
   
2. A lack of output voltage means that you have to crank up your stereo volume to get a decent listening level.
   

I disagree with his first assertion (point 1). Apart from in a small handful of situations - not including this one - we don't aim to match impedance when we work with audio. Matching impedance is something that you have to do when considering transmission line effects - not an issue at audio frequencies unless you're travelling long distance

The headphone socket of a portable audio device will have a source impedance measured in Ohms, or tens of Ohms. In other words, it is a decently "stiff" voltage source. You can change the load impedance between, say, 100 Ohms and infinity with no effect on the audio.

Point 2 is correct, and that is the only problem here. If there is "mush", that's because of a lack of voltage, plain and simple.

Solution: Use two audio transformers. The designer of the circuit states thus:

The end result? A great sounding boost. Not earth-shattering volume as would be realized in an active (opamp or transistor) circuit, but all of the mush was gone. Bass and treble were both significantly enhanced, and the sound improvement was pretty dramatic for a passive circuit.

But for mono operation, this is a waste of transformers... Simply resistively sum to mono ahead of a single transformer...

Option 3 has the merit of isolating the gram socket from the MP3 player, and the designer of that circuit explains it thus:

About a year ago I built a circuit similar to this using these same RS transformers. The significant difference on mine was that it combined the two audio channels from the Ipod into a single channel to run directly into the auxiliary input of a tube radio. I also used the secondary of the transformer winding on my project. If you were to use this adapter with a AC/DC set and had concerns about isolation, the transformer coupling provides DC isolation to the MP3 player.

 
The term passive amplifier is a little misleading. Although it is a passive circuit, it is more of an impedance matching network than anything else.

Since it matches the low impedance output of the Ipod to the relatively high impedance input of a tube radio audio stage, it might appear to provide gain, but what I think you are really seeing is more efficient transfer of the Ipods output power to the radio. The frequency response and fidelity was better with a transformer network than my other attempts with just a resistive network.

Well, the term "passive pre-amplifier" is widespread in hi-fi circles, so although I agree that it seems illogical, it is "accepted usage"

However, it does provide voltage gain. That's what we need here - remember, audio is all about voltage, not power. The input impedance of a typical valve radio is 100s of kΩ, perhaps more.

For anyone building this, watch out for the phase of the transformer windings - get one wrong and you'll only the "S" signal - that is, Left minus Right. For a mono recording, that's nothing at all. For stereo programme, you'll probably lose the main vocal to some extent!

I've used circuits like this for professional audio in the past, using transformers that were staggeringly expensive to maintain the audio quality. It works well.

Clearly the first option is cheapest and simplest, but do the other two options provide worthwhile benefits?

Certainly option 3 has the merit of isolating the I-pod from the chassis of the radio into which the adaptor is fed, though I don't think I'd trust connecting it to an AC/DC 'live chassis' radio due to the dubious insulation qualities of the cheap audio transformers, the secondary of which isn't on a separate bobbin, but overwound on the primary. Maybe via two Y class caps? (A bit academic as I've no intention of doing that anyway).

Well, I definitely share your concerns regarding safety.

Option 1 is good if there is enough voltage available. If the device has a "Line" output, that will give you a much higher output voltage than the headphone output will. And because the source impedance is generally higher (perhaps 1kΩ or thereabouts), you can get away without the resistors - although I'd recommend them personally.

If not, you need voltage amplification. Your options are transformers or active circuitry. Cheap transformers are a bit "hit and miss" in terms of their audio performance, but then so are valve radios . And can you find one with a suitable turns ratio? It seems to me that he's using a CT primary to achieve a doubling in voltage (just +6dB), and has decided to not use the secondary to add to the output - seems like a waste to me.

Alternatively, a simple op-amp circuit could be built - just a handful of resistors and capacitors to add, plus a PP3 battery - and objectively it would be much better than any transformer. Current consumption could be just a milliamp or two. That's what I would do, but it's personal choice.

Cheers,

Mark

[Eddystonepete]

Hi David, I agree with Sparky, keep it simple. Provided your not using a live chassis radio and your MP3 player is not earthed there is no need for isolation i,e. a transformer. The players output impedance is low, (50 ohms or so) so it will easily drive the gram input of the radio. Impedance matching is only important in that the load, about 500k ohms (the radio) does not unduly load the source (the player).  Its unfortunate that these players rarely have line outs so your stuck with the phones out.

Peter

[Alistair D]

My IPOD headphone  output socket output is peaking at 1Vpp into the scope. That should be enough signal for all but the deafest of audio amplifiers.

If opting for option 1 it would be worth adding a DC blocking capacitor before the probe,. It might just save the output  stage  in the event the probe touching the wrong place.

Al

[Mark Hennessy]

My IPOD headphone  output socket output is peaking at 1Vpp into the scope. That should be enough signal for all but the deafest of audio amplifiers.

If opting for option 1 it would be worth adding a DC blocking capacitor before the probe,. It might just save the output  stage  in the event the probe touching the wrong place.

That's pretty good going - I get quite a bit less than that from my Sony MP3 player.

The addition of a capacitor is good advice. If especially worried, you could add back to back diodes as well.

Mark

[Aerodyne]

Mark is spot-on as usual. I've had great success using a portable DVD player, directly from the headphone output into the gram sockets of many valve radios. Volume well up, adjustable with the player's own VC, no problems with mush, whatever that is, though I did combine the two channels using - I believe, if memory serves aright - 1k resistors. Actually I doubt even these are essential.
I would NEVER use the pick-up sockets on any AC/DC receiver.
Tony

[Yorkie]

Thanks guys for reading and responding to my post with so much helpful information.

I have got some LT700 audio transformers, but was sceptical as to whether the circuits I outlined in option 2 & 3 above were strictly necessary,  and are likely to provide any discernible  improvement in performance over option 1. The general consensus is that option 1 would provide satisfactory results. Alistair D suggested that perhaps a blocking cap would be a worthwhile addition - presumably as I've shown in the updated diagram below? (I don't know how come the values of 4k7 for the two resistors arose).

It was my son who asked me about this - he shares my interest in vintage radios and has a small collection, but doesn't himself have any that have gram input sockets. (He has a couple of Ekco A22s, and uses a 'spitfire' with them). He'd asked whether direct connection of i-pods to radios that do have gram sockets was possible and I said yes, without considering the aspect of stereo to mono. It caused me to consider whether it's advisable to simply join the two channels of the I-pod output together in parallel to feed  this into a mono gram socket, or whether doing so would have an adverse effect on the I-pod (or whatever other gizmos people use today).

As to 'pantry transmitters', fine if a radio doesn't have a gram input socket, or is AC/DC, but no required if a gram socket or 'aux' input is present.

[Yorkie]

Just a little update on this thread. I've tried all the options with no success whatsoever. Tried it on a Fenman1 and a Bush AC41 with the same results - with the radio tuned to LW MW etc, ear splitting volume on tuned in stations, but switch to gram, and feed in an I-pod using any of the options I mentioned in my earlier post, and with the I-pod itself and the radio turned up to maximum volume, the sound is barely audible. Obviously there's a mismatch between an I-pod output, which is intended to feed low resistance headphones, and the gram input socket of a radio, meant to be fed from a gram pickup.  I've tried using an LT700 output transformer in reverse to see if it will match the ipod output to the gram input, as per the attached sketch, but it makes no difference. I'm thinking of poking the output of the ipod into a little LM386 amp, then using  the LT700 'back to front' as in the sketch with the output of the LM386 amp fed into the LT700,  to see if that will lift the output of the ipod.

If I poke a signal injector into the gram socket, I get a nice loud tone from the speaker, so the gram input socket connects fine via the 'switchery' through to the audio stage of the radio.

I'd naively assumed that all one had to do was to make up a lead to combine the LH/RH outputs of the I-pod and poke it into the gram socket. Not so, it seems.

Ho hum