I built one to use with my TV broadcasting setup, following processhead's schematic and using the same Radio Shack transformers & box as pbpix. Seems to work well.

Thanks!

Phil Nelson

Hey... GOOD NEWS!
I'm glad to report that someone else ( a Harvard Engineer) who also built one of these passive amplifiers has done all the leg and research work relating to Frequency Response
... and the good news is that if you use it in the auto-transformer mode the response is beautifully flat from 100hz to 20khz.
Only if you use both sides of the transformer for higher gain will you get some roll off and distortion.
And ..Here's the whole article:
http://frank.harvard.edu/~paulh/misc/horobooster.htm

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."

I was assembling another one of these adapters today.

As an experiment, I wired the secondaries in parallel instead of putting them in series as I show in the schematic .

The result was that the audio output is lower when the two secondaries are in parallel.

For maximum output level, they should be wired in series with attention paid to the phasing. (blue connected to green at the tie point).

I figured series would be more signal, but wanted to be sure.

The circuit is mixing rather than adding. The only conclusion that overall program
power level has increased has to consider the sum of all the components of L+R that
are in additive polarity.

Since all the energy in a phonograph needle, stereo or mono, flows through one
shank, it is difficult to understand how a stereo signal can contain more energy than
a mono one considering all the energy comes from the turntable motor which only
has one shaft.

I had not thought of it that way.

In all actuality series connection would be the proper way to do it, because in parallel if both transformers do not have the same signal level you have a varying ability to supply current and are more limited as to the loads you can drive.

I suspect the higher apparent signal level has more to do the two secondary windings in series and the higher output impedance that this presents to the input of the radio.

With any network, maximum power transfer results when the impedance of the source matches the impedance of the load.

I wonder what the dielectric rating of the RS transformers is? I did some looking on Mouser and found the majority of those small trans are 100 volts. I'm afraid that rating might not be very safe when used with an AA5 chassis.

Mouser had a few rated at 1000 V for about $10 each.

Agree that the extra gain is needed. I just tried an iPod jack w/o the trans and it was a low on volume.

Wouldn't a 1000 / 8 ohm trans (used backwards) give a voltage boost as well as the isolation?

Tim

With any network, maximum power transfer results when the (conjugate of the) impedance of the source matches the impedance of the load.

The answer is YES... of course.
And the gain is much higher.
If you look at the freq response graph I posted above you'll see where THAT author shows how his arrangement uses a switch that allows him to use it both ways. He shows MARK-I and MARK-II.
(Mark-I) is 6db (auto-transformer mode) or (Mark-II) is the 17db step up isolation transformer mode.

I made one of those last year from Ron's schematic...
I must have done something wrong, as My jobber, sounds terrible. I just put it in a drawer
and forgot about it until now. I was dissapointed. I too used a scandisk clip mp3.
I'll pull it out & review.
Would better audio xformers "sound better" ?

I

I would suspect that better audio transformers will sound much better.

Better transformers ?.. Will they sound better? Maybe ...but unless it goes below 100hz.. why bother?

It's hard to beat this transformer (as it is clearly shown by the graph above ) that it has a beautiful flat response from 100hz to 20khz with no distortion.

I think many of us are experiencing an intuitive skepticism about the claimed freq. response in such a tiny transformer. It's probably because we're used to the big honkin' audio interstage and output transformers we find in tube gear, many of which have specs not as good as this.

But if anyone has taken apart a dynamic microphone with a built-in impedance matching transformer will probably marvel (as I did) at it's tiny size. Yet it is able to give an excellent flat response over a wide audio spectrum.

So what gives? How are these tiny transformers able to do so much better than their larger tube gear brethren? The only think I can think of is that the tube gear transformers usually have to carry large signal currents + DC currents, requiring heavier windings and more massive iron.

Those transformers will work if the device the transformers drive cannot reproduce frequencies below 100 Hz, but for HI-FI use one would need better transformers. Also consider this. The smaller transformers can have a lower allowable signal voltage before the core gets saturated.



You are right. The tube gear transformers have to handle larger voltages and currents so they need a larger core in order to allow the voltages and currents without saturating the core.

I have a small driver transformer (drives the output transistors) in a Sylvania RM-300X radio and the frequency response of the amp is nearly flat from 20-20,000 Hz.

Hi Terry:
Did you follow the link and read the author's whole article and his very clear wording on this?

"using the 1k winding alone, as an auto-transformer,
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."

Hi Tube Radio:
You are missing the author's point. He clearly states why these cheap transformers work well by explaining that we are ONLY using the 1k winding... and NOT the 8 ohm side...
So the core never gets saturated.
Go back and re-read the author's article and explanation.
He says:
"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."

Hi Pbpix,

Could you please post a picture of the circuit that you are describing in your first post of this thread?

The original link from the guy named PRR is too draft for me (not sure what's what) The other link (http://beavishifi.com/projects/Passive_ ... amplifier/) is no longer available.

I would like to build it too.

Thank you.

Thank you so much!!!