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09-14-2026, 12:55 AM
Hi Guys
For the output transformer, the manufacturer just has to have experience designing/winding audio transformers. Specify the power rating, bandwidth (full audio 20-20kHz), the primary impedance (end-to-end) and the secondary impedances. If you want ultralinear taps, specify the percentage, even though it is almost always 40%.
You should try to keep the secondary simpler with not too many nominal matches; 4r and 8r are usually enough.
Specify the colours and ask for long leads, give a length.
So, an example spec might look like this:
25W 20-20kHz; all leads 24" stranded with 1/4" stripped ends)
PRI: 8k end-to-end; BRN (in-phase) - VIO (40% tap) - RED (centertap) - GRY (40% tap) - BLU (invert-phase)
SEC: GRY (8r in-phase) - YLW (4r) - BLK (0)
Leave out the taps for a more usual OT.
The natural center of an secondary is 1/4 of the end-to-end impedance, which is inconvenient most of the time.
Or, best to make a doodle with the colours specified on it and the impedances listed as appropriate, and lead length.
Best not to try to pin down dimensions as toroids have a "natural" aspect ratio for diameter to height, but if there is one dimension or the other that is limited, the manufacturer can make adjustments.
If the OT is to be surface mounted, you can specify an epoxy center and foam pad. This works if the bolt head can be proud of the bottom surface that the OT sits on. otherwise, the usual mounting with rubber pads and a disc is standard. if the bolt head cannot be proud of the mounting surface, then a dimple has to be made in the surface to the transformer sits on. Your chassis maker needs to do this, but you can do it by hand if the amp is a one-off.
The above are the standard colours London Power uses and Hammond follows this to some extent.
PTs have standard colours, as specified in the Power Supply Design thread.
For the design of the PT, you need to work it out for yourself as far as voltages and currents go. Use AC values instead of a mix of AC and DC. Use non-CTed windings to keep it smaller and simpler, in this way a standard PT will have one or two primaries and three secondaries:
PRI-1: BLK (120) - WHT (0)
PRI-2: BRN (120) - ORG (0)
Sec-1: REDs 300V 500mA, or could be 300V 150VA
Sec-2: VIOs 70V 100mA
Sec-3: GRNs 6V 6A
GRN - electrostatic screen
For me, Sec-3: YLW (6) - WHT (0) - YLW (6) 3A
If you need relay power BUT it does not have to be controlled by foot switches, then the heater winding can supply this. The 12Vct winding works well for this. If you need ground-referenced switching it is best to have a separate aux winding for that. I use BLU for aux.
For the output transformer, the manufacturer just has to have experience designing/winding audio transformers. Specify the power rating, bandwidth (full audio 20-20kHz), the primary impedance (end-to-end) and the secondary impedances. If you want ultralinear taps, specify the percentage, even though it is almost always 40%.
You should try to keep the secondary simpler with not too many nominal matches; 4r and 8r are usually enough.
Specify the colours and ask for long leads, give a length.
So, an example spec might look like this:
25W 20-20kHz; all leads 24" stranded with 1/4" stripped ends)
PRI: 8k end-to-end; BRN (in-phase) - VIO (40% tap) - RED (centertap) - GRY (40% tap) - BLU (invert-phase)
SEC: GRY (8r in-phase) - YLW (4r) - BLK (0)
Leave out the taps for a more usual OT.
The natural center of an secondary is 1/4 of the end-to-end impedance, which is inconvenient most of the time.
Or, best to make a doodle with the colours specified on it and the impedances listed as appropriate, and lead length.
Best not to try to pin down dimensions as toroids have a "natural" aspect ratio for diameter to height, but if there is one dimension or the other that is limited, the manufacturer can make adjustments.
If the OT is to be surface mounted, you can specify an epoxy center and foam pad. This works if the bolt head can be proud of the bottom surface that the OT sits on. otherwise, the usual mounting with rubber pads and a disc is standard. if the bolt head cannot be proud of the mounting surface, then a dimple has to be made in the surface to the transformer sits on. Your chassis maker needs to do this, but you can do it by hand if the amp is a one-off.
The above are the standard colours London Power uses and Hammond follows this to some extent.
PTs have standard colours, as specified in the Power Supply Design thread.
For the design of the PT, you need to work it out for yourself as far as voltages and currents go. Use AC values instead of a mix of AC and DC. Use non-CTed windings to keep it smaller and simpler, in this way a standard PT will have one or two primaries and three secondaries:
PRI-1: BLK (120) - WHT (0)
PRI-2: BRN (120) - ORG (0)
Sec-1: REDs 300V 500mA, or could be 300V 150VA
Sec-2: VIOs 70V 100mA
Sec-3: GRNs 6V 6A
GRN - electrostatic screen
For me, Sec-3: YLW (6) - WHT (0) - YLW (6) 3A
If you need relay power BUT it does not have to be controlled by foot switches, then the heater winding can supply this. The 12Vct winding works well for this. If you need ground-referenced switching it is best to have a separate aux winding for that. I use BLU for aux.
