PASSIVE AMPLIFER

While I was surfing around reading about iPod audio output levels I found out a ton of interesting stuff.
One problem most guys have is that the output level seems too low for feeding into other amps like your radio.

So one guy suggested using a passive amplifier that should give you a 2:1 boost.

It uses ONLY two tiny Radio-Shack output transformers for a couple of bucks each.
That's it... nothing else.... and it works great too.
You simply hook them together and that's it. No power or anything. You only use one side of the transformer... the CT side.
That's it.
I tried it tonight and it works great!


Here's the site where I saw the original schematic: (may be a dead link now - so I posted the red-border image above)
http://beavishifi.com/projects/Passive_ ... amplifier/

The guy who came up with this idea calls himself "PRR" on another form:
( http://gilmore2.chem.northwestern.edu/u ... 3&tid=6568 )

He says this:
" [A transformer] For DC, IS a short, or really a very low resistance. But at audio it has some impedance.

The iPod has plenty of Power. But the ratio of voltage to current is low, while line-in needs low current at higher voltage. Since we have enough audio Power, we don't need an external power supply, if we have a way to trade current for voltage.

A transformer is a 2-winding choke. Choke impedance rises from zero at DC to infinity at infinite frequency (ideally). When we want an audio choke we pick one with a "high" impedance at our lowest bass frequency.

The RS 1K:8 transformer, end-to-end on the 1K side, is probably 100R below 20Hz and over 1K above 200Hz.

It has a center-tap. This is equivalent to a 2-winding transformer with one end of each winding connected together. Transformer action is possible. Being a center-tap, the voltage ratio is 1:2, the current ratio is 2:1, and the impedance ratio is 1:4.

So if we put 0.1V from CT to one end, then the end-to-end voltage should be 0.2V. It doubles the audio voltage.

It also halves the audio current. Whatever current the Hi-Fi stereo's Line Input needs, we must supply twice that. However line inputs need very small current, and headphone output can supply large current.

The impedance at the CT of the winding is nominally 1K/4= 250 ohms. It is a cheap-radio transformer, so this is probably true only above 200Hz. At 100Hz it may be 125 ohms, at 50Hz it may be 62 ohms of choke impedance. Since the iPod is made to drive anything over 32 ohms, this is not a strain.

I agree that a 1:2 voltage step-up is not "Wow!" louder. I offer it just as a quick cheap hack. A good 37:600 transformer would give a 1:4 voltage step-up, still easily driven from iPod to any line in.... but such iron is rare, and generally costly. "
----------------
The author says, "In [auto-transformer mode] the low-gain (6 dB) configuration, the thing is just plain flat from 100 Hz to 20 kHz, dropping about a half dB at 20 Hz; distortion is negligible except at the low end, where it rises to about 0.25% at 40 Hz, and 0.5% at 20 Hz, for reasonably high player output levels."
Here's the Graph:

The author says:
""Perfromance?

Pretty darn good, considering the dimunitive transformers. In this
graph "Mark I" refers to the high-gain configuration (drive the 8 ohm
winding, headphones across the 1k winding), whereas "Mark II" is the
autotransformer configuration (drive CT of 1k secondary, ignore the
8 ohm winding altogether).

Obviously, gain comes at the expense of distortion and flatness: In
the low-gain (6 dB) configuration the thing is just plain flat from
100 Hz to 20 kHz, dropping about a half dB at 20 Hz; distortion is
negligible except at the low end, where it rises to about 0.25% at
40 Hz, and 0.5% at 20 Hz, for reasonably high player output levels.
In the high-gain (17 dB) configuration the falloff at low frequencies
is more serious, dropping 1 dB at 100 Hz, 2 dB at 50 Hz, and 4 dB at
20 Hz; distortion is much higher, reaching 1% around 60-80 Hz
(depending on level), and as much as 10% at 25 Hz at high output
levels.

Why is this? Well, an audio transformer is rated for some nominal
frequency range and impedance; they are designed so the magnetizing
inductance presents an impedance, at the low freq end of its intended
range, comparable to its nominal impedance (of each winding).
Thus, the "8 ohm" winding loads an 8 ohm source, reducing the output
(and, at high levels, risking core saturation) at the "low" freqency
end of its intended range. For these cheap tiny xfmrs, I found that
happens at about 100 Hz! That is why there is a dropoff below 100 Hz
in the high-gain configuration, and also distortion at low frequencies
that rises rapidly with signal level.

On the other hand, using the 1k winding alone, as an autotransformer,
improves the low end dramatically, because the magnetizing inductance
is higher by a factor of 60; it also reduces distortion to negligible
levels, because the magnetizing currents, even at low frequency, don't
take the core into saturation.

In actual use on airplanes, with the Sony NetMD as source, I find that
the low-gain setting is usually fine; only on pianissimo sections of
subtle classical music have I found it necessary to kick in the
afterburner. Even in the high-gain mode I haven't noticed distortion,
probably because the signal level at low frequencies is not terribly high,
and also maybe because it is so darned loud that one does not particularly
notice distortion."

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 auxilary 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.

I posted that circuit here at that time with some pictures of the whole project.

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.

If I could figure out how to do a search on the forum topics I might be able to find that old thread.


Pictures added:

Paul: I'd like to see that too.
I just searched your posts but only found where you asked a question about how to do it.

If you're trying to search the forum:
You can go to Google, then select the gear icon in upper right corner for advanced search. Then use the key words in the top box and enter http://www.antiqueradios.com in the bottom box.

One way to do it is to use a couple Edcor 15K balanced/unbalanced to 15K balanced/unbalanced transformers. You would connect the input to tne center tap with one end going to ground. You could then take the output from the full secondary or from the whole primary winding or some in phase combo of the primary and secondary windings.

I recommend the 15K transformers as they are flat in their frequency response to a bit above 20 KHz. The other impedances rolled off the high frequencies some.

http://www.edcorusa.com/products/152-wsm15k-15k.aspx

Added a couple pictures of my ipod adaptor.

Peter would probably approve of the re-purposed 35mm film container.

DONE
I went out for dinner tonight at about 8:30 and stopped at RadioShack to get a couple of those little transformers and a matchbox-sized project box.
Boy those transformers ARE small. Thumb nail sized. .. But only $2.95 each.

Continued on to dinner and an hour or so later got home and tested this stuff and decided to toss it all together.
Only took a few minuets.
Here it is already!
And it sounds great!
(Transformers are held in place with double sided tape.):


They provide two styles of box covers. A plastic and a metal recessed one.
I'll probably use the metal one so that my " IN & OUT " labels will be easier to read.

EDIT:
As you'll notice from the lower image I just added, that I had to use the plastic cover instead of the metal one to avoid touching one on the jack contacts because I positioned it a bit too high and vertically instead of horizontal.
As it turns out I think the plastic cover is nicer as it seems to look a little more professional.

Hi tube:
Those DO look like great transformers.
Why did you particularly chose the 15k impedance ones?

They are pretty cheap at $11 each.
However deliver is 16 bucks!! though.... lol

The RadioShack ones are only $2.95 each ...lol... and you get tiny too!

Anyway.. yes sure ... better transformer will provide a better frequency response.. sure!
But look how large they are. The ones in the article are only thumbnail-sized.
Remember this is for a tiny MP3 device too. And after all... would the MP3 signal quality and bandwidth be able to appreciate the audiophile quality?... or would the greater freq response be wasted?

Hi Paul:

Looks cute:
As far as amplification:
Did you read the authors detailed explanation that shows what it does and how it amplifies voltage?
He concludes with this line:
"So if we put 0.1V from CT to one end, then the end-to-end voltage should be 0.2V. It doubles the audio voltage."

You said something in your post about how you did something to combine channels? What was that?

BTW:
I sent you instructions of how to search the forum via Google.
Did you read that?

Interesting. Maybe I'll try one to boost the audio when feeding iPhone video to the agile modulator that serves as my in-house broadcaster for vintage TVs.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Here's the frequency response graph of those transformers. http://www.edcorusa.com/Plots/WSM15K-15K.pdf

Nearly flat frequency response from 10 Hz - 50 KHz. The other impedances available for those trasnsformers all roll off the highs somewhat which would not be good for HI-FI use.

Thing is you already have a poor quality signal using MP3 files with the tiny MP3 player and you do not want to further degrade the signal by using cheap transformers. Besides this can be used for anything that needs a level boost.

I might buy a couple transformers and try it.

I posted a schematic in my first post. It shows how L and R channels are combined in the two transformer secondaries to make a composite mono signal to feed the radio.

Did some site searches to try to find some of my earlier posts, but was only able to go back a few months. Seems like I might not be using the search terms correctly or something?

Phil, I love that old set on the left. Now we know where the PC makers got the tower concept.

That's a Philco Predicta (below).

Processhead, would you mind if I used your schematic and photos for a brief build-it article on my website? I frequently get the "how do I hook up iPod" question from people who visit my website. It's simpler to point them to an article than to make them scrounge around for forum discussions, especially as this forum's Search function seems "challenged." I'll give you a credit, if you like.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html



For more about the Philco Predicta, see http://antiqueradio.org/philc12.htm .

Yes. No problem.

Hi Paul:
I hadn't seen your schematic because each subsequent email alert placed me further down the thread and so I failed to scroll back.

That's a very good idea for combining channels permanently.
Why are the 10 ohm resistors needed on the mp3 input? Won't the impedance of those windings alone do the job?

My unit will almost always be using both channels so I built it as I showed in my original post.

While occasionally I need to combine channels for bench testing single channel amps I decided for that to use an output test-cable that ties the two channels together with 4.7uf non polar caps.
Any and all better ideas are welcome

You would just use the secondaries of the transformers connected in parallel. Series connection may work, but IDK.

An audio transformer, I was thinking the same thing when I read the subject line. I'm reading that chapter now in the NRI courses that I downloaded from the Classifieds section.
Philsoldradios: That Philco Predicta is one nice looking TV.

The very first schematic uses an unusual transformer connection for which a nit could be
picked unless there was a reason why they were so connected. Apart from crosstalk,
the left and the right channels see slightly different resistances for most transformers.


The thing to do for an experiment is to check phase shift between between input and
output jacks across the frequency range, especially for the one with the 2 10 ohm resistors.

Its simple to do, just overlay the two traces after adjusting them to be same size, and see
if they stay in step over the generator range. Use a load resistor on the output side.

"Why are the 10 ohm resistors needed on the mp3 input? Won't the impedance of those windings alone do the job?

I read that the output impedance of an Ipod was a 32 ohms. I added the 10 ohm resisters ins series with the 8 ohm transformer winding so they would more closely match the output impedance of the Ipod.

In reality, they probably are not really needed.

It does add gain. You're basically taking the transformer outputs and adding them in series for a mono signal which is larger than the input signal provided the transformers are 1:1 ratio.