Impedance Matching
How to Impedance Match Sound Powered Headphones (and other headphones) to Crystal Radios (and just about any other radio)

Dedicated Circuits

Stanley Match
by Mike Tuggle (new)

Select to Match ("S-T-M")
by Steve Bringhurst

Universal Match-Box ("Uni-Match")
by Steve Bringhurst

UltiMatch and UltiMatch 2
by Steve Bringhurst (new)

Transformer Information Page
(Data Sheets and Schmatics and where to purchace them)

The Selec-To-Match, UniMatch, and UltiMatch are original concepts by Steven C. Bringhurst  The other devices are either 'common sense' or suggested by Ben Tongue, and other sources. With the exception of the UniMatch, the devices on this page are based on the research & ideas of Ben Tongue, without which they would not exist in the form presented. Our thanks and gratitude are extended to Ben Tongue for his work, inspiring this page's contents.  Steve Bringhurst

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Simple Dedicated Circuits
Now we will show a very simple circuits, dedicated to a particular set of headphones, but first,
you may want to go to Ben Tongue's web page and read article #05 (maybe all of the articles).
Ben's Web site is at Welcome to Ben Tongue's Technical Talk
 

This "fixed" setup is designed for standard 600 ohm impedance elements wired in series.
It is a standard matching transformer circuit.

 

The following is another dedicated circuit for two 600 ohm series wired elements.
The transformer is set up more as a autoformer.
I think it performs a little better than the above circuit.

Try both circuits. They use the same parts!

T1    Calrad 45-700  audio transformer (Ocean Electronic)
R1    250k ohm pot (a mini pot would work great!)
C1    .1uf capacitor
C2    .47uf capacitor

 
 
 

 
 
 

Circuit for a UTC A27 Transformer
An excellent very low loss transformer
(This circuit has been revised on 29 January 2004)
 
 

Dual Calrad 45-700 xformers
Excellent when using Strombergs Carlson and Dynalec Decktalkers.
Strombergs are lower AC Impedance than most other sound powered headsets and can
benefit greatly with this circuit. This will help them perform closer to the other "Big Can" decktalkers.
Also works great if using the "mic" elements out of decktalkers.
"Mic" elements are also lower impedance.

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Stanley Match
by Mike Tuggle

The StanleyMatch combines two of the bargain Fair Radio transformers with a 4-pole, 5-position switch to provide several high-to-low impedance matches especially suitable for low impedance sound powered phones. StanleyMatch by Mike Tuggle The StanleyMatch is nothing radically different from other phone matching units, just a way to get more flexibility from the Stanley/UTC TF-1A-10-YY input transformers still available at a bargain from Fair Radio.  By some nifty switching, one can combine two single 100 k-to-100 ohm units to get five different input-output impedance pairs.  The key component is the 4-pole, 5-position switch.  These come in the form of 4-deck rotary switches, as 2-deck switches with two independent 5-position sections on each deck, as shown here (actually, the sections are 6-position with one position blocked out) -- or possibly as single deck switches with two independent sections on each side!  On a 4-deck switch several positions will likely need to be blocked out.  That's what those tabs at the collar are for. The 2 x 3 x 5 in. mini-box is about as small as I would suggest going -- unless you are a micro-circuitry freak and/or don't mind the smell of insulation burning on your soldering iron.  Even so, I had to pre-wire several of the components before installing them in the box.  Details like exact pot and capacitor value and connector types are user's choice.  I just happened to have 'standardized' on 1/4-in. phone plugs and 1/8-in. miniature output jack on my sets. The low impedance (50 - 200 ohms) outputs make this unit especially suitable for low impedance sound powered phones.  I suppose you could Bogenize the low impedance end to match higher impedance magnetic phones.  I've been quite happy with SP's and haven't seriously tried other types. The test points (TP) mounted on the rear apron are for audio voltage output measurements if so desired.

 

Stanley Match built by Mike Tuggle

 
 
 

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S-T-M
"Select to Match"
by Steve Bringhurst

The "Select to Match" was designed to allow a person to experiment with different headphones on the same crystal radio.
If you are using only one set of headphones only, you might want to build the
dedicated circuit below which will save you space, time and money.

STM introduction

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S-T-M  Calrad/Bogen
"Select to Match" Impedance Matching Circuit

Circuit Design by Steve Bringhurst "baldy3823"

T1     Calrad 45-700  audio transformer  (Ocean Electronic)
T2     Bogen T725  4 watt  P/A transformer (Check the bottom of this page)
R1     250k ohm variable resistor
SW1  12 position single pole rotary switch  (Radio Shack)
C1     .1 uf capacitor
C2     Value of C2 depends on the set up of the headsets.
          (all non polar)
          1) one 600 ohm impedance element = 1 uf
          2) two 600 ohm impedance. elements wired in parallel = 2.2uf
          3) two 600 ohm impedance elements wired in series = .5 uf
          4) 10k Hi "Z" headphones = .068uf to .05uf
          5) two low "Z" elements  in series like Stromberg
              Carlsons can be run on 2.2 uf, but 4.7uf might be
              a better match up.

Notes:
1)    1 uF is a good fixed compromise value for C2 if several different impedance headsets
        will be frequently used.
2)    The range of C1 can be from .05uf to .22uf. It is not that critical.
3)    Only ten of the twelve positions  are used on SW1
4)    White lead on T1 goes to the rotary switch SW1 with yellow
5)    Experimentation is the key to getting the right values on this unit.
       What is right for you and your set up is what counts.
6)    See photo below for proper phase ID of the two "pink" wires on the Bogen 725
       transformer.

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My version of Steve's S-T-M  Calrad/Bogen

 
 

 

The S-T-M is excellent for comparison of headphones. It works very well for one or two sets of phones as well. It will run a range of headphones, from magnetic to sound powered.
More S-T-M circuits using different transformers are in the works to be placed here on this page in the near future.

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S-T-M     Stanley/Bogen
"Select to Match" Impedance Matching Circuit

Circuit Design by Steve Bringhurst "baldy3823"

T1     Stanley audio transformer Part Number 227-E-1-3-98
         Pri 100 ohm  Sec. 100M (100k) ohm
         (Fair Radio Sales number T3/AM-20)
T2     Bogen T725  4 watt  P/A transformer  (Check the bottom of this page)
R1     250k ohm variable resistor
SW1  12 position single pole rotary switch  (Radio Shack)
SW2  6 position single pole rotary switch (Radio Shack)
SW3  SPDT toggle switch (Radio Shack)
C1     .1 uf capacitor (Radio Shack)
C2     4.7 uf capacitor non polar (Radio Shack)
C3     2.2 uf  capacitor non polar (Radio Shack)
C4     1.0 uf  capacitor non polar (Radio Shack)
C5     .5 uf capacitor non polar (Radio Shack)
C6     .05 uf capacitor non polar - not shown in circuit can be
          put in for 10k Hi "Z" headphones (Radio Shack)

Notes:
1)    The range of C1 can be from .05uf to .22uf. It is not that critical.
2)    Only ten of the twelve positions  are used on SW1
4)    #1 lead on T1 goes to SW3. It is split and one lead goes to brown on SW1 and  the
       other goes to red on SW1. This allows you to shifts the ratio range a full
       step for better use of the Stromberg Carlson elements.
5)    Experimentation is the key to getting the right values on this unit.
       What is right for you and your set up is what counts.
6)    See photo below for proper phase ID of the two "pink" wires on the Bogen 725
       transformer.

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My version of Steve's S-T-M  Stanley/Bogen

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Universal Match-Box
"Uni-Match"
by Steve Bringhurst

Another idea from Steve Bringhurst is the "Uni-Match" or Universal Match-Box. This simple box will match just about any
type headphones to just about any type radio. Not designed just for crystal radios, but for just about any type radio.

"Say you want to use your S/Ps or Brush-Clevite piezo's, or your Telephonics TDH-39s or?? on a tube or xistor regenny,
MK-484 TRF, the output of your regular radio, ect, ect. You could do any and all with this."  Steve Bringhurst

Starting at the first position on the switches and going clockwise the impedance should be approximately
40k, 20K, 10k, 5k, 2.5k. 1.2k, 600, 300, 150,75, 8 (all in ohms)

T2     Bogen T725  4 watt  P/A transformer  (Check the bottom of this page)
sw1  12 position single pole rotary switch  (Radio Shack)
sw2  12 position single pole rotary switch  (Radio Shack)

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UltiMatch and UltiMatch 2(new)
by Steve Bringhurst

This is a "All in one box" unit
This incorporates the Select to Match ("S-T-M") and the Universal Match-Box ("Uni-Match")
all in one unit. Get the most out of your components!!

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Original UltiMatchCircuit
Original circuit has been left here so those who have already
built the circuit can see easily the differences in the two circuits.
This will help if you would like to "convert" to the new circuit.

 
 

UltiMatch 2(new)

 
 

Directions for use of Ulti Match 2

Input 1 General directions for using the UltiMatch with crystal detector receivers; Crystal detector input is made at input 1 with the diode to SW4 & the detector return to the other terminal. 1st position of SW-4 is for higher impedance detectors and the 2nd position is for lower impedance detector ckts & mineral detectors. SW5 selects an over-all ratio shift to better match some low impedance headsets at pos#2, pos#1 is 'normal'. SW5 doesn't function with SW4 in 'low'. R1 is used to adjust for lowest strong-signal distortion with lower impedance detectors, R2 is adjusted for lowest strong-signal distortion with very high impedance detectors like Schottky barrier diodes, otherwise, it's 'normal' setting is 'minimum' with the adjustments for 'low' SW4 setting and germanium and similar detectors on SW4 'high' being made with R1. SW2 allows selection of the best match, by ear, of the input device to the headphone being used. The position giving the loudest & clearest sound is usually the best setting. SW3 is used to select bass response balance for the headset and input device in use, the 1st position is 'direct'.  This adjustment will have a more subtle effect than a regular tone control. Input 2 Input 2 is used for powered devices like tube & transistor detectors & amplifier ckts with SW4 in the 'low' position. SW1 is used to select the proper input match for the powered device. Always start from the highest setting first when using this function, too low a setting may damage the powered device. Using SW2 selects the best match for the headset as before, or position 1 can drive a loudspeaker directly.  The loudest & clearest sound is usually obtained when SW1 & SW2 are optimum.  Output Any set of headphones or single earpiece, such as a telephone earpiece, may be connected to the 'output' terminals. It is usually preferable to have 2 earpieces connected in series, rather than parallel, & properly phased so that the sounds seem to occur 'inside' ones head between the ears and not 'at' either ear. Switching the connections to one earpiece will correct phasing. Some listeners can hear a definite difference in intelligibility when the headphone connections to the transformer device are switched around (which reverses 'absolute phase'), the clearer sound has correct 'absolute phase' and is of course preferable.  It is generally a one-time adjustment, but not always for every station, program, ect . This can change if the station or studio is careless about engineering practices and allows reversed absolute-phase signals to occur. If you cannot hear a difference when switching the absolute phasing, don't worry about it. Many cannot and it's one less thing for you to concern yourself. Position 1 on SW1 permits use of an efficient 4, 8, or 16^ speaker on very loud signals, or use of headsets with very low impedance ratings similar to those speakers. Output Using  Input2/Output Combo The 'High' side terminal of input 2 may be used as the 'return' for the headset for the xtal detector function if it is desired to select a winding ratio 'inside' the normal selections. Connect the headset to the 'high' side output terminal (from SW3) and it's other lead to the 'high' side of input 2. SW1 is then used to select a tap for the 'low' side of the headset connection, SW2 selects the 'high' side as before. This 'inside' option excludes selection/use of the position '1'  8^ speaker tap.Some headsets will match up better using this option.  Absolute  phasing may then be reversed/corrected by reversing the switching order of  SW1 and SW2 , thus reversing the 'high' and 'low' sides of the headphone conections without need to disconnect and swap them, once the best ratio selection is found by ear. Correct  "Absolute phase" refers to the the condition where a pressure 'push' into the studio mic gives a pressure 'push' from the speaker or headphone. Absolute  phase reversal results in an unnatural result of a 'push' giving a 'pull'. Some  listeners find clearer & more lifelike voice sounds when absolute phasing is  correct.  Steve Bringhurst

 

Parts List
(UltiMatch and UltiMatch 2)

T1     Stanley audio transformer Part Number 227-E-1-3-98
         Pri 100 ohm  Sec. 100M (100k) ohm
         (Fair Radio Sales number T3/AM-20)
T2     Bogen T725  4 watt  P/A transformer (Check the bottom of this page)
R1     250k ohm variable resistor #
R2     2 to 3 Meg ohm variable resistor
SW1  12 position single pole rotary switch  (Radio Shack)
SW2  12 position single pole rotary switch (Radio Shack)
SW3  6 position single pole rotary switch (Radio Shack)
SW4  DPST Toggle switch
SW5  SPST Toggle switch
C1     .1 uf capacitor
C2     4.7 uf capacitor non polar (Radio Shack)
C3     2.2 uf  capacitor non polar (Radio Shack)
C4     1.0 uf  capacitor non polar (Radio Shack)
C5     .5 uf capacitor non polar (Radio Shack)
C6     .05 uf capacitor non polar - (Radio Shack)
C7     .1 uF Capacitor #
 

Notes: # Not used in UltiMatch, only in UltiMatch 2

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Bogen T725 transformers can be purchaced at

Electronics Outfitter $4.06 ea. No min. order
http://www.electronicsoutfitter.com/eostore/items_html/BOG-T725.html

Lashen Electronics $4.69 ea. (must buy 16 at a time)
http://www.lashen.com/vendors/bogen/Speaker_Transformers.asp

Consolidated Electronics $5.61 ea. No min. order
http://www.ceitron.com/audio/bogen.html

Mar Vac $6.00 ea. No min. order
http://www.marvac.com/products/bogen/t725_t72510.htm

Modern Radio Labs $7.50 ea. No min. order
http://www.modernradiolabs.com/Parts%202.htm

Dave’s Homemade Crystal Radio Parts $8.00 ea.
http://www.schmarder.com/radios/sale/parts.htm

Paging Systems Equipment $10.59 ea. No min. order
http://www.pagingsystemequipment.com/Catalog/Model_T725.htm

A-I Consolidated, Inc. $11.99 ea. No min. order
http://shop.store.yahoo.com/aicons/bogt7t725vol.html

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