A friend read my ULTIMA blog and emailed me, are you able to upgrade the preamp you built for me and also what do you think of preamp output transformers (OPT)?
That's interesting. I did not want to include a preamp OPT as it would be costly. Since this is a cost-no-object design, I say why not? Preamp OPT itself is a much more difficult design than a normal power OPT or capacitor. It's easier to design a good capacitor or power OPT but preamp OPT is another different story at all.
A very, very good and wide bandwidth will be needed to do this job. If that is used, benefits are plenty. Here are a few recommended preamp OPTs you can choose:
Sowter type 9650
http://www.sowter.co.uk/specs/9560.htm
Better than 10 Hz - 60 kHz, 7k5:600
FEATURES A very large core to ensure a high bandwidth and low output impedance. High grade grain oriented silicon iron (M6) core for minimal losses and harmonic distortion. Multi section winding ensures minimal leakage inductance for an exceptional bandwidth. Low copper resistance which permits an output impedance of 203 ohms when driven from a valve with rp = 3000 ohms and a secondary load of 600 ohms.
Hashimoto HL-10k-6
http://www.eifl.co.jp/index/export/hashimoto/hashimoto.html
20 Hz - 45 kHz, pri:sec = 10k : 600/150
Tamura A-4714
http://www.eifl.co.jp/index/export/export2.htm
5k/60mA: 600/6 Ohm
Some other excellent OPTs vendors are not mentioned, like Lundahl, Magnequest, Tango, AudioNote UK, and etc. You can select from them as well. You decide!
