Quick Summary:  This Article discusses the use of audio transformers with crystal sets and gives the results of loss measurements on several of them.  A method for measuring insertion power loss is also described.

Many crystal set designs provide impedance step down taps on the final RF tuned circuit.  If the diode is connected to one of these taps, its loading on the tuned circuit is reduced and selectivity is improved. Too much of a step down also reduces sensitivity.  RF tuned circuit loading by the diode is affected by the diodes' Saturation Current, the headphone impedance and the signal level. One can reduce the loading effect of headphone impedance and of high signal level by transforming the headphone impedance up to a value that matches the audio output resistance of the diode detector itself.  This approach can keep the selectivity high and also increase the sensitivity of the crystal set.  For info on measuring headphone impedance see article # 2.

It is important that the diode sees a DC load equal to its AC audio load. This will permit connecting the diode to a higher tap or maybe to the top of the tuned circuit.  The result will be to maintain selectivity and reduce audio distortion for medium and especially for strong signals.  Diodes of lower Saturation Current can be tapped up higher on the tuned circuit than those of higher Saturation Current and, all else being equal, will give higher receiver sensitivity. See articles #0, #1, #4 and #15.

Note that the switched transformation ratios shown below vary by a factor of about four from one to another.  Note also that an impedance mismatch of 2:1 gives an insertion loss of only 0.5 dB.  This means that all values of diode output resistance from 12k Ohms up to 750k can be utilized, with a mismatch insertion loss of no more than a maximum value of 0.5 dB, plus the transformer loss.  Measured transformer loss is about 1.0 +/- 0.5 dB from 300- 3300 Hz at the 63 times ratio and about 0.5 +/- 0.2 dB at the 16 and 4 ratios.  Note: The transformation ratio on the H switch position is shown as 63 instead of 72 because of shunt resistive losses in the transformers.  On this switch position the diode sees the 12k headphones transformed to 750k, not 860k.  T1 and T2 are preferably Antique Electronic Supply P-T157 transformers.  Alternatively, one can use A.E.S. P-T156 or Stancor A53-C units. One will get a small amount more loss with the alternatives, mainly at 300 Hz and when the signal is weak.  C2 can be used to peak up response at 300 Hz if Stancor or P-T156 transformers are used.  C2 can be omitted if the P-T157 transformers are used.  Experiment with values around 0.02 uF.  Sw1 and Sw2 are DPDT slide or toggle switches.  R can be a 1 Meg pot.  It is used to set the diode DC load resistance to be equal the transformed AC load impedance.  A log taper is preferred.  Set R for the lowest audio distortion and best selectivity on large signals.  The diode load at DC must be the same as for AC audio signals for best results. This setting has little effect with weak signals, however.  C1 should be about 0.05 uF.  See the later part of Article #1 for info on determining transformer winding polarity and how to reduce the effect of inter-winding capacitance.
 

There is no need to transform headphone impedance up to as high as 750k unless the RF tuned circuit, when loaded with the antenna, has a resonant resistance of around 750k Ohms.  It is very hard to attain an impedance this high.  The diode also would have to have an appropriate Saturation Current of about 38 nA. For experimental purposes, if a transformed impedance of no higher than 380k is desired, a one transformer circuit should be used as shown below. This will prove more practical in real world applications.  R may be a 250k or 500k Ohm pot, preferably with a log taper. The transformer insertion loss remains below 1.0 dB from 0.3-3.3 kHz with output loads between from 6k to 24k Ohms when using the A.E.S. P-T157.  Keep in mind that the saturation Current (Is) of the diode should be such that the diode's (Low signal) RF input resistance is about equal to the (Antenna loaded) RF tuned circuit resonant resistance and also to the transformed headphone impedance. This diode resistance is equal to (0.0257*n)/Is.  Is is in Amps. For more information on this, see article #4 listed on the home page.

First, some help. For best performance at the high end of the audio band, it is important to minimize the effect if transformer inter winding capacitance.  To do this, the start and finish leads of the transformer coils must be properly connected.  In the schematics shown above, the start and finish of the transformer windings are indicated by "s" and "f".  The start of the low impedance winding of the P-T157 transformer is the blue lead. The finish is the red lead. The green lead next to the red lead is the start of the center tapped high impedance winding. The green lead next to the blue lead is the finish.  If Stancor A53-C transformers are (is) being used, the color coding is different. The start of the low impedance winding is the red lead, the finish is the blue lead. The green lead of the center tapped winding next to the blue lead is the start, and the green lead next to the red lead is the finish.

The insertion loss values shown below are were measured using A.E.S. P-T157 transformers.  Stancor A53-C units will perform somewhat worse, as mentioned above.  If you are going to use a low cost Stancor A53-C or similar transformer, keep in mind that all or most of the extra insertion loss at 0.3 kHz can be eliminated by using the correct capacitor in series between the transformer and the headphones.
 

Here are general specifications of the A.E.S. P-T157 and Stancor A-53C  Interstage Transformer:  Single Plate (10,000 Ohms) to push-pull grids (90,000 Ohms).  Overall turns ratio: 1 to 3 Primary to Secondary.  Max. Primary D.C.: 10 mA.  These transformers are still relatively cheap and usually available at Hamfests, personal junk boxes and Used Component Vendors. 

Terminal Connections for UTC, AMERTRAN and MOUSER TM-117 Transformers

G. Join 1 & 3, 4 & 6, 8 & 9.  Input is 7. Output is 1. Ground is 4 and 10. H. Join 2 & 3, 4 & 5. 8 & 9.  Input is 7. Output is 2. Ground is 4 and 10.  I. Join 3 & 4.  8 & 9. Input is 7.  Output is 2.  Ground is 5 and 10.  J. Input is 3.  Output is 2.  Ground is 1 and 4.  K. Join 1 & 3, 2 & 4, 6 & 7.  Input is 8.  Output is 1.  Ground is 4 and 5. L. Join 2 & 3,  6 & 7.  Input is 8.  Output is 1.  Ground is 4 and 5.  M. Input is 4. Output is 1.  Ground is 3 and 6  N. Take four TM-117s and label them W, X, Y and Z.  They will be connected in an      autotransformer configuration.  Join W6 to X4, X6 to Y4, Y6 to Z4.  Join W1 to       Y1. Join X3 to Z3,  Join W3 to X1. Join Y3 to Z1.  Connect a parallel  RC from       Z6 to W1.  Input is W4.  Output is Y1.  Ground for input and output  is Z3.  For an      explanation of why to use the RC, see the second paragraph after the first graph in       article  #1. O. Take four TM-117s and label them W, X, Y and Z.  They will be connected in an       autotransformer configuration.  Join W6 to X4, X6 to Y4, Y6 to Z4.  Join W3 to       X1, X3 to Y1, Y3 to Z1,  Connect a parallel RC from Z6 to W1.  Input is W4.        Output is W1. Ground for input and output is Z3.  For an explanation of why to use the        RC, see the second paragraph after the first graph in article #1.  Desirable but optional:       Connect  X1 to the center of the 1,200 Ohm load (junction of two 600 Ohm        sound-powered elements connected in series).  This eliminates a narrow spurious 1 dB        dip   in the frequency response at about 1.2 kHz.

The transformer loss figures for the UTC and AMERTRAN transformers were measured at an output power of about -60 dBm.  Performance is retained at output power levels much less than -60 dBm.  A voice signal at this power level will be quite soft, but understandable through most sound powered headphones.

The MOUSER transformer deserves special discussion since it is so low in cost: 
Frequency response and distortion:  The loss figures at two different power levels for a TM117 purchased in March of '00 are as follows: Output power level of +15 dBm:  2.8 dB @ 0.3 kHz,  1.9 dB @ 1.0 kHz and 6.4 dB @ 3.3 kHz.  Output power level of  -60 dBm:  11.1 dB @ 0.3 kHz,  1.8 dB @ 1.0 kHz,  5.7 dB @ 3.3 kHz and 5.4 dB @ 0.6 kHz.  The 0.3 kHz loss is greater at a power level of -60 dBm than at +15 dBm.  Why?  The core laminations of the TM-117 (and most other very small transformers) have low permeability at the low magnetic flux levels generated by the -60 dBm signal.  This low permeability is called initial permeability. The initial permeability, in combination with other factors, results in the transformer having a specific shunt inductance (at low signal levels).  This shunt inductance controls the low frequency roll-off of the transformer.  At higher flux levels (signal levels), but before saturation occurs, the permeability increases to an "effective permeability" value which can be several times greater than the initial permeability.  This means that the transformer shunt inductance is higher at the higher signal level and the low frequency roll-off is much reduced.  There may be some production unit-to-unit variation in the low frequency response of the TM117.  One that I bought about a year ago showed 2.5 dB less loss at 0.3 kHz than the one tested above.  Some low frequency harmonic distortion is generated in the changeover region from initial to effective permeability.  This can easily be seen on a 'scope, especially at 300 Hz sine wave.  I doubt that it would be very noticeable in actual crystal set use.

One can see from lines three, four and five of data in the TM-117 Insertion Loss Chart above that  the loss at 0.3 kHz, relative to that at 1.0 kHz, gets less as the output power level is increased.  The loss at 1.0 kHz is minimum at the -42 dbm output power level.  The greater 1.0 kHz loss at the -72 dBm power level is caused by the reduced shunt inductance as explained above.  The increase in 1.0 kHz loss at -42 dBm occurs because the core is getting closer to saturation.  The loss at 3.3 kHz in the four-transformer configuration is greater than that for one transformer shown on line 2 because the primary-to-secondary capacitance of transformers A and B is effectively connected from high impedance points and ground, thus rolling off the high end response.  The single transformer in line 2 is wired so that the primary-to-secondary capacitance is not in shunt across the primary to ground.
 

The CALRAD line of small transformers offers two types that are suitable for use in transforming a high diode detector output resistance down to 300 or 1200 Ohms to drive SP phones.  Their insertion loss is quite low and within a fraction of a dB of that of the UTC LS-10.  One distributor of CALRAD transformers is Ocean State Electronics, 6 Industrial Drive, P.O. Box 1458, Westerly, RI.   The two transformers are #45-700, spec'd to transform 100k to 1000 Ohms and #45-703, spec'd to transform 200k to 1000 Ohms.  They sell for $5.95 ea.  The following chart shows the measured performance of a single transformer and of a combination of two of the same type, with the high impedance windings in series and the low impedance windings in parallel.  The performance is very good, especially so, considering the price.
 

Note: To preserve good 3.3 kHz performance, the start and finish ends of the high impedance windings must be properly connected.  When using one transformer, the finish lead of the high impedance winding should be connected to the diode, and the start lead to the parallel RC, the other end of which goes to ground.  When using two transformers, the finish lead of transformer #1 goes to the diode, its start lead goes to the finish of transformer #2, and its start goes to the parallel RC, the other end of which goes to ground.  The two low impedance windings are paralleled with the leads of like color connected together.

Connect the generator to the transformer high impedance primary with a resistor of value equal to Rs (the expected output resistance of the diode detector) placed in series with the hot lead.  Connect a load resistor Rl of value Zh (average impedance of the headphones) to the secondary.  Tune the generator to the first frequency of measurement, say 1000 Hz.  Connect the scope or meter to the low impedance secondary.  Adjust the audio generator to as low a level as possible while still being able to get an accurate reading of the voltage without much error from hum and noise. Call this voltage E3.  Now connect the scope probe to the hot end of the primary.  Read this voltage and call it E2.  Connect the scope probe or meter to the actual generator output (not the actual transformer hot lead).  Read this voltage and call it E1.  Calculate insertion loss:  Loss = 10*log (4*Rs*((E3/E1)^2)/Rl) dB.  Also take measurements at 300 and 3300 Hz.  If the 300 Hz loss is much higher than the 1000 Hz loss, a transformer with a higher primary inductance is needed.  If the 3300 Hz loss is much higher than the 1000 Hz loss, the transformer has too high a winding capacitance.  Hopefully all readings will be better than -2 dB.

If the transformer is doing a good job of impedance matching Rs to Rl, E2 will be about 1/2 the value of E1 and the transformer insertion loss will be at a minimum.  If E2 is lower than 1/2 of E1, a greater impedance transformation (turns ratio squared) is needed.  If the transformer has taps on the secondary, using a lower impedance tap might improve results.  If E2 is higher than 1/2 of E1, the impedance transformation ratio is too large and a higher impedance secondary tap should be tried (if available).

One can find the least possible loss the transformer can deliver when using the selected value of Rl.  To do this change the value of Rs to one that causes E2 equal to 1/2 E1.  This minimum loss can be calculated using the new value of Rs.  The experiment results in the optimum source resistance, Rs, with which to drive the transformer when it is loaded with the selected load, Rl.

An easy way to compare the performance (loss) of a particular transformer with that of an ideal no-loss transformer of just the right transformation ratio is to build and try out the 'Unilateral Ideal Transformer Simulator' described in Article #14.

• Wide frequency response specification such as +/- 1 dB, 20-20,000 Hz:  A transformer specified from, say, 200-5,000 Hz will probably have several dB more loss than a wide band unit when used over the 0.3-3.3 kHz range.  The reason for this is that we want to use the transformer with primary and secondary load resistances several times higher than that which the manufacturer specifies.  This always narrows up the transformer pass-band.  We should shoot for a final pass-band of at least 0.3 - 3.3 kHz. 
• High impedance winding specification:  Single grid, preferably push pull grids, single plate or preferably push pull plates. The impedance level, if specified, will usually be between 20,000 and 80,000 Ohms.  The high impedance winding is the one to connect to the diode detector output.
• Low impedance winding specification:  Low impedance mike, pickup, multiple-line or simply a number between 100 and 1000 (Ohms).  Several taps may be supplied to enable various  impedance levels.  The specification might be: 50, 125/150, 200/250, and 333, 500/600 Ohms.  This winding is the one to which the sound powered phones are to be connected.
• The correct low impedance tap to use for connecting the sound powered phones may be calculated as follows:
1. Decide the audio load resistance to be presented to the detector.  Let's select 200,000 Ohms.  (See Articles #1 and #4 for info on how to determine this value.)  Assume that the sound powered elements are connected in series. This will typically result a headphone average impedance of 1,200 Ohms.  Calculate the needed impedance transformation ratio as:  200,000/1,200 = 167.  Note the Manufacturer's specification for the high impedance winding of the transformer. (If you don't know what it is, estimate 80,000 Ohms.) and divide it by 167.  Select the Manufacturer's low impedance winding tap specification (if you have that info) that most closely equals the value calculated above. If you are using the 80,000 Ohm estimate, the desired tap impedance would be 80,000/167 = 479 Ohms.  Call this number A.  Now check what the result would be if the sound powered elements were connected in parallel.  They will now present an average impedance of 300 Ohms and require an impedance transformation ratio of 200,000/300=667.  The desired Manufacturer's tap marking will now be 80,000/667 = 120 Ohms.  Call this number B.  Pick the number A or B, whichever is closest to an available transformer tap marking.  Connect the phone elements appropriately.  Note that we are using the transformer at a higher impedance level than that for which it was designed.  What we lose is by doing this is audio bandwidth and a small increase of insertion loss.  We don't need the 20-20,000 Hz range anyway, do we?  What we gain is an ability to transform headphone impedance to a higher value than if we used the manufacturer's ratings.
2. If you have a transformer on which you have no specs. except that it is designed to couple from a low impedance to push-pull grids, a grid, push-pull plates of a plate or just "high impedance",  connect that winding to the crystal diode and experiment with connecting the headphones to the various taps provided on the low impedance winding.  Do this experimentation using a weak signal and pick the connection that gives the greatest volume.

Published: 10/22/99;  Last revision: 06/18/01