Not all EE25 trafos out of Aliexpress (or Taobao) are alike.
I recently ran into difficulties trying to get the frequency response of a 10k+10k trafo approximately flat up to 20kHz. Normally a trafo will need either a resistor or RC network as a load in order to cancel out the resonance effect of its leakage inductance. However the trafo I had bought had such high leakage inductance that I found it impossible to get flat and have a bandwidth up to 20kHz. In the end I have about 2dB roll-off at 20kHz and no bumps or peaks below that from using a resistive load around 5kohm. The 'high leakage' trafos are these : https://www.aliexpress.com/item/1005004830376695.html. Wifey asked one of the sellers on Taobao about the lack of flatness and they responded that these trafos aren't optimized for FR, rather for 'balance'. And indeed the two half-windings on each side are very close to each other in DCR. So if you need 'balance' these might well be the trafos for you, if you're willing to trade that for insufficient bandwidth capability to handle audio.
As a result of less-than-satisfactory results with these trafos, wifey ordered me some similar looking ones to try out, to see if they're better. The ones I have on my bench now look like these - https://www.aliexpress.com/item/1005003353513635.html. However they're not identical to these as the Ali ones are Z11 steel cored whereas mine (from Taobao) are permalloy. From a quick measurement of inductance with and without a short on another winding its clear that the leakage is about 10X lower than the first ones. So initially promising.
(I'll update when I've evaluated these new trafos further)
I recently ran into difficulties trying to get the frequency response of a 10k+10k trafo approximately flat up to 20kHz. Normally a trafo will need either a resistor or RC network as a load in order to cancel out the resonance effect of its leakage inductance. However the trafo I had bought had such high leakage inductance that I found it impossible to get flat and have a bandwidth up to 20kHz. In the end I have about 2dB roll-off at 20kHz and no bumps or peaks below that from using a resistive load around 5kohm. The 'high leakage' trafos are these : https://www.aliexpress.com/item/1005004830376695.html. Wifey asked one of the sellers on Taobao about the lack of flatness and they responded that these trafos aren't optimized for FR, rather for 'balance'. And indeed the two half-windings on each side are very close to each other in DCR. So if you need 'balance' these might well be the trafos for you, if you're willing to trade that for insufficient bandwidth capability to handle audio.
As a result of less-than-satisfactory results with these trafos, wifey ordered me some similar looking ones to try out, to see if they're better. The ones I have on my bench now look like these - https://www.aliexpress.com/item/1005003353513635.html. However they're not identical to these as the Ali ones are Z11 steel cored whereas mine (from Taobao) are permalloy. From a quick measurement of inductance with and without a short on another winding its clear that the leakage is about 10X lower than the first ones. So initially promising.
(I'll update when I've evaluated these new trafos further)