Transformer combo comparison in modular tube pre amp

[Holger]

This post is related to my modular tube pre amp system
https://groupdiy.com/index.php?topic=67518.0
I post it here because this is related to some specific transformer combos.

Recap: I've built a modular pre amp system based on Ian Thompson-Bell's TwinLineAmp.
The transfomer combos are

Module 1 Sowter 1948 1:10 IP, Sowter 1461 2k4:600 OP
Module 2 Sowter 1948 IP, custom wound ElectroMag 2k4:600 OP (there is a thread about this transfomer in this forum)
Module 3 OEP A187A15C 1:10 IP, EM 2k4:600 OP

I tried the OEP because I was annoyed about the long Sowter lead time (six weeks at that time, 12 weeks nowadays)
The OEPs are available within two days so I gave them a try.

I've recently discovered RoomEqWizard REW, so here are the measurements.
EDIT: measured with Antelope Orion 32+, amps set to line level input
First freq response: Sowter/Sowter

Sowter/Electro Mag

OEP/Electro Mag

All

Conclusion freq wise: the Sowter/ElectoMag combos looks most promising, the OEP/Electromag is not so bad, but IIRC the
THD was much higher compared to the other combos (0.1% for Sowter IP compared to 0.5% for the OEP)Edit: wrong, see below

 

[emrr]

Thanks for showing this.

 

[ruffrecords]

Looks like you have the same HF roll off problem with the OEP input transformer that I have observed. OEP are looking into this.

The 10Hz hump with the sowter:sowter combo  is probably the higher inductance of the output transformer resonating with the output cap

The EM output transformer has a higher insertion loss than the Sowter because its winding resistance is higher.

At what frequency and level did you measure distortion?

Cheers

Ian

 

[Holger]

I did some THD+N measurements @ 4 and 10 dBu OP with my Neutrik Minilyzer today.

 

[ruffrecords]

These results show the problem with the Sowter output transformer at low frequencies and high levels. I clearly shows it saturating at 20Hz ant +10dBu. This is due to the mu metal in its core. The EM being made simply of M6 steel shows a much more modest rise in distortion at low frequencies and high levels, at the expense of slightly higher mid band distortion.

Cheers

Ian