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Project Specific Discussions => Magnetics => Topic started by: Holger on April 30, 2020, 03:58:43 PM

Title: Transformer combo comparison in modular tube pre amp
Post by: Holger on April 30, 2020, 03:58:43 PM
This post is related to my modular tube pre amp system
https://groupdiy.com/index.php?topic=67518.0 (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
(http://audiofor.tv/diy/2020/sowter%20ip%20sowter%20op.jpg)
Sowter/Electro Mag
(http://audiofor.tv/diy/2020/sowter%20ip%20EM%20op.jpg)
OEP/Electro Mag
(http://audiofor.tv/diy/2020/oep%20ip%20EM%20op.jpg)
All
(http://audiofor.tv/diy/2020/all.jpg)
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
Title: Re: Transformer combo comparison in modular tube pre amp
Post by: EmRR on April 30, 2020, 04:11:23 PM
Thanks for showing this.
Title: Re: Transformer combo comparison in modular tube pre amp
Post by: ruffrecords on April 30, 2020, 05:41:38 PM
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
Title: Re: Transformer combo comparison in modular tube pre amp
Post by: Holger on May 01, 2020, 10:14:52 AM
I did some THD+N measurements @ 4 and 10 dBu OP with my Neutrik Minilyzer today.

(http://audiofor.tv/diy/2020/tx%20meas.png)
Title: Re: Transformer combo comparison in modular tube pre amp
Post by: ruffrecords on May 01, 2020, 06:09:59 PM
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