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 33V 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.
An interesting discussion occured on diyAudio, here. See some extracts from contributor ‘bolserst‘ below:
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
- 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 !
- 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). 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
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