I-Pod to Gram input of vintage radio options

[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