Audio transformer : digging into details

Audio input transformer is a step-up kind with ratios of 90 for treble and 270 for bass (quote from quadesl.com). Sheldon Stokes says 96/270. diyAudio/bolserst says 87/290. diyAudio/Imazed says 96/289 … But this is based on voltage or turns measurement ? Probably first choice: voltage measurement.

You can read 55/110 here. This ratio is for windings themselves, and overall this makes 2*55 and (110+2*55+110): “110 / 330”. It may be related to turns ratio this time.

Maximum voltage at treble side should remain far below 3kV, which can be forced by an optional clipping board that clamps at 2200V. This 3kV corresponds to about 33Vp at input side.

Components used inside audio transformer do have to support up to 6kV. But this happens only with high power amplifiers such as Quad 303 (2x45W). With 5W, voltage inside audio transformer will stay below 100V. But who wants to use as little power as 5W with a low sensitivity speaker? Therefore, use high voltage components only! quadesl.org specifies 10kV for resistors and 4 to 6 kV for capacitors. Considering schematic (3 resistors  inserted in serie in the bass line), VR37 are probably good enough. VR68 will be overkill but are still good choice since same size as original components. Besides, 1W vs 1/2W.

Model

An interesting discussion occured on diyAudio, here. See some extracts from contributor ‘bolserst‘ below:

For a complete understanding of the ESL-57 crossover as well as its acoustic design I would highly recommend reading the description by Peter Baxandall on pages 169 – 179 of “Loudspeaker and Headphone Handbook”, J. Borwick, Ed. See attached ESL-57 schematic to which I have added the transformer leakage inductance and winding capacitance since they are directly involved in how the crossover works. The values were taken from the Baxandall description…actual production values may vary slightly. Loading Choke The purpose of the loading choke was to reduce the magnitude of the impedance peak that occurs at the low-to-mid frequencies of all transformer driven ESLs. It is my understanding that most tube amplifier output stages do not like to be driven at high levels without some amount of loading…internal tube arcing can occur. Without the loading choke, the impedance in the mid-bass region might have been high enough to cause problems with some tube amps of the day. The KLH Nine had a similar loading choke. For the brief description below to make sense, you must understand that an ESL panel that is small relative to listening distance has a response that naturally rises 6dB/octave when driven from a voltage source; response is flat when driven from a current source. Bass panel Crossover Ignoring diaphragm resonance, the bass panels have a natural 6dB/oct rising response. The 6 x 180K resistors in series with the Bass Panel effectively provide current drive above about 300Hz and flatten the response of the bass panels. Looking at it another way, the RC filter puts a 6dB/oct LP filter on the voltage reaching the bass panels. Either way you look at it, the response remains flat until about 1kHz where there is a 2nd order LP network formed by the leakage inductance (46H) and the panel capacitance(400pF). Below 300Hz, the response would still fall at 6dB/octave. The addition of baffling around the panels and small side wings keeps the response from falling quite this quickly. The diaphragm resonance with Q of 2 or 3 provides equalization for the bottom octave or so. Tweeter panel Crossover The sum of the winding capacitance and tweeter panel capacitance in combination with the leakage inductance(145mH) forms a 2nd order LP network just above the audio band at about 24Khz. The resonance is damped by the 150K resistor(junction 9 & 14). This is not immediately obvious, but at 24Khz, the 560pF caps and the mid panel capacitance are essentially short circuits provided the electrical path for the damping. Most other ESLs damp this HF resonance by including a small series resistance of 1-2 ohm in the primary circuit. The two 560pF caps and 270K resistor were added for production models after serial number 16800. The purpose was to shelve down the drive voltage at lower frequencies by 6dB to avoid arcing the tweeter panel when used with amplifier power > 15W, which was the power level originally designed for. Mid panel Crossover The two 150K resistors provide current drive for the mid panel flattening the response as was described in the description of the bass panel crossover. The 560pF cap in parallel with one of the 150K resistors provides a bypass for this resistance so the right amount of damping is applied to the 24kHz tweeter panel crossover resonance. Without it, the response of the tweeter panel would peak up at the top end. Mid panel & Tweeter Panel Acoustic Crossover Both the mid panels and tweeter panel are rolled off on the top end (above about 2kHz) due to the curvature of the panels spreading out the radiated HF sound. So the LP for the mid panel is purely acoustic, not electrical. The tweeter panel response is flattened back out using the slightly under-damped resonance at 24kHz.

As you can see on pictures above:

• outer wires of the 2 small coils connect to outer terminals (1 and 5 for serie 2)
• inner wires of the 2 small coils connect to inner terminals (2 and 4 for serie 2)
• small coils are on the top side (near plots 1 .. 5) and are a priori identical

How many turns ? I got this from one source:

• Small coils are expected 7850 turns gauge #39 (0.0889mm) !! not what I measured, see below !!
• Audio inputs (B/R) make 74 turns, gauge #21 (0.7239mm)
• Last, big coils, 2 times 3926 turns gauge #39, turns ratio 53

On my side, I measured for small coils:

• for one transfo, 180 turns per layer (width = 1 inch, thus diameter<0.141mm), and computed 1715 Ω/km: gauge #37 (0.113mm)
• for another transfo, I got this on just one layer (slightly less accurate): 34.7m for layer #4 and 64.7Ohm and 3.1g -> 1.86Ohm/m, 89g/km: definitely gauge #37
• for one coil, 41 layers, first and last being partly filled, around 180 turns/layer, 180*40.5=7290 turns in total
• for another coil, 40 layers only, around 182.5 turns/layer, 128 only for first and last, 7211 turns in total, 113g of wire
• this makes turns ratio 97.5 .. 98.5, resistance close to 2.2 kΩ. Obviously only total number of turns was monitored

For big coils, and each instance of high voltage wiring:

• 8 layers, first and last being half filled
• 565 turns per complete layer: about 3955 turns in total (turns ratio 53)
• target is 1.93/1.73 kΩ (note that inner and outer wiring have same # of turns but not same radius thus not the same resistance)

For big coils, low voltage wiring:

• I confirm gauge 21, 74 turns, 0.58 Ohm, 13.25m, 1 layer only

Additional notes:

• 1st layer is half-filled, and starts towards left
• seen from connection wires, small and big coils are NOT wired in the same sense: anti-clockwise for small ones
• 3.5 turns of paper is used between layers for insulation of big coils, only 1.5 turns for small coils
• big coils have additional external insulation layers (4m30!), less for small coils (1m)
• not all coils measure the same !

My summary:

• apply turns ratio of 53/98 (this makes a voltage ratio of “106/302” before applying yield and loss, to obtain hopefully the famous “90/290” after)
• small coils: layers of 180 .. 185 turns, gauge 37, 40 .. 41 layers, 7250 turns (first and last layers filled at 75%), set winding tool with a spacing for a gauge 2 steps below (25.4mm/180). 1.5 turn of insulation between each layer (1.5 inch), 5 turns for final insulation
• big coils, HV: layers of 565 turns, gauge 39, 8 layers, 3950 turns (first and last half-filled)
• big coils, LV: 1 layer of 74 turns gauge 21
• once completed with external insulation, small coils should mesure less than 42x76mm (paper core is 21x56mm, height 36mm, external side). One turn will vary between 19.3cm and 15.4cm (avg 17.35cm thus roughly 1265m in total and 2.2kOhm)
• for big coils, height of paper core is 74mm (insulation can be 2×1.5 inches wide)
• the 2 small coils are identical. Take care to wire direction: see picture

Impedance computation:

• http://www.dicks-website.eu/coilcalculator/
• https://www.eeweb.com/tools/rectangle-loop-inductance/
• https://www.diyaudio.com/community/threads/explaining-the-quad-esl-57-crossover.224615/

• big coil, LV, 100mH (Al value = 18260nH/N^2)
• big coil, HV, 300H
• small coil, 1000H

Here is a code to reproduce measurements above, with figures from Baxandall:

Version 4
SHEET 1 1176 760
WIRE 144 -32 0 -32
WIRE 848 -32 224 -32
WIRE 880 -32 848 -32
WIRE 992 -32 960 -32
WIRE 1056 -32 992 -32
WIRE 848 32 848 -32
WIRE 992 32 992 -32
WIRE 720 48 288 48
WIRE -96 80 -320 80
WIRE 0 80 0 48
WIRE 720 96 720 48
WIRE 720 96 640 96
WIRE 992 128 992 96
WIRE 1056 128 992 128
WIRE -176 160 -320 160
WIRE -96 160 -176 160
WIRE 0 176 0 160
WIRE 144 176 0 176
WIRE 288 176 288 48
WIRE 288 176 224 176
WIRE 368 176 368 128
WIRE 368 176 288 176
WIRE 432 176 432 128
WIRE 480 176 432 176
WIRE 560 176 480 176
WIRE 640 176 640 160
WIRE 720 176 640 176
WIRE 0 192 0 176
WIRE 368 208 368 176
WIRE 432 208 432 176
WIRE 480 208 480 176
WIRE 288 224 288 176
WIRE 720 256 720 176
WIRE 768 256 720 256
WIRE 560 272 560 176
WIRE 672 272 560 272
WIRE -176 288 -176 160
WIRE 560 288 560 272
WIRE 720 288 720 256
WIRE 672 304 672 272
WIRE 0 320 0 272
WIRE 480 320 480 288
WIRE 480 320 0 320
WIRE 0 352 0 320
WIRE 816 352 720 352
WIRE 480 400 480 320
WIRE 144 432 0 432
WIRE 288 432 288 288
WIRE 288 432 224 432
WIRE 560 432 560 352
WIRE 560 432 288 432
WIRE 720 432 560 432
WIRE 560 480 560 432
WIRE 0 544 0 512
WIRE 144 624 0 624
WIRE 848 624 848 96
WIRE 848 624 224 624
WIRE 880 624 848 624
WIRE 992 624 992 128
WIRE 992 624 960 624
FLAG -176 288 0
FLAG 1056 -32 BASS+
IOPIN 1056 -32 Out
FLAG 480 400 0
FLAG 672 304 TREBLE+
IOPIN 672 304 Out
FLAG 768 256 MID+
IOPIN 768 256 Out
FLAG 1056 128 BASS-
IOPIN 1056 128 Out
FLAG 560 480 TREBLE-
IOPIN 560 480 Out
FLAG 816 352 MID-
IOPIN 816 352 Out
SYMBOL ind2 -112 64 R0
SYMATTR InstName L1
SYMATTR Value 0.1
SYMATTR Type ind
SYMATTR SpiceLine Rser=0.6
SYMBOL ind2 -16 176 R0
SYMATTR InstName L2
SYMATTR Value 189.225
SYMATTR Type ind
SYMATTR SpiceLine Rser=1500
SYMBOL ind2 -16 -48 R0
SYMATTR InstName L3
SYMATTR Value 1030.225
SYMATTR Type ind
SYMATTR SpiceLine Rser=2350
SYMBOL res -16 64 R0
SYMATTR InstName R1
SYMATTR Value 180k
SYMBOL ind 128 192 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L4
SYMATTR Value 72.5m
SYMBOL ind 128 -16 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L5
SYMATTR Value 23
SYMBOL cap 272 224 R0
SYMATTR InstName C1
SYMATTR Value 200p
SYMBOL cap 544 288 R0
SYMATTR InstName C2
SYMATTR Value 100p
SYMBOL cap 624 96 R0
SYMATTR InstName C3
SYMATTR Value 560p
SYMBOL cap 832 32 R0
SYMATTR InstName C5
SYMATTR Value 70p
SYMBOL cap 976 32 R0
SYMATTR InstName C6
SYMATTR Value 400p
SYMBOL res 704 80 R0
SYMATTR InstName R2
SYMATTR Value 150k
SYMBOL res 976 -48 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 360k
SYMBOL voltage -320 64 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 24 118 Left 2
SYMATTR InstName V1
SYMATTR Value SINE()
SYMATTR Value2 AC 10 0
SYMBOL ind2 -16 336 R0
SYMATTR InstName L6
SYMATTR Value 189.225
SYMATTR Type ind
SYMATTR SpiceLine Rser=1500
SYMBOL res -16 416 R0
SYMATTR InstName R4
SYMATTR Value 180k
SYMBOL ind2 -16 528 R0
SYMATTR InstName L7
SYMATTR Value 1030.225
SYMATTR Type ind
SYMATTR SpiceLine Rser=2350
SYMBOL ind 128 640 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L8
SYMATTR Value 23
SYMBOL cap 704 288 R0
SYMATTR InstName C4
SYMATTR Value 200p
SYMBOL ind 128 448 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L9
SYMATTR Value 72.5m
SYMBOL res 976 608 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 360k
SYMBOL res 704 336 R0
SYMATTR InstName R6
SYMATTR Value 150k
SYMBOL res 464 192 R0
SYMATTR InstName R7
SYMATTR Value 270k
SYMBOL cap 432 192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value 560p
SYMBOL cap 432 112 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 560p
TEXT -128 -120 Left 2 !K1 L1 L2 L3 L6 L7 1
TEXT -368 208 Left 2 !.ac oct 1000 20 100k

Pictures imported from other sites: