The subject and opinions around OPTs can become endless. In the past 2 decades, I've bought and tested many SE OPTs, including Hammond, Edcor, James, Electraprint, Transcendar, Magnequest, Hashimoto and maybe one or two unknowns. Size does matter, but then again, going too small or too large can be detrimental to the overall performance, depending on what the design goals are.
For most SET designs I've done, they are usually using either the 45 or 2A3 DHTs... some others include the 3C24 and on the larger end, Western Electric 300b. Power output starts at ~2-watts with the 45, ~4-watts with the 2A3 and 3C24 and up to 10-watts with the 300b. My design goals are wide bandwidth, low distortion and high signal-to-noise ratio. In short, S/N is probably the most important specification, especially when you only have 2-watts of output power. As high efficiency speakers need to used, any residual output noise from the amplifier will be quite audible, which ruins everything. My 45 SETs are close to 90dB S/N referenced to 1-watt output. THD at 1-watt / 1KHz is less than 0.5% and bandwidth within 1dB is from 25Hz to 50KHz at 1-watt output. This is using the Hashimoto H507S. Going to their larger 20-watt OPT can be an improvement, but not by a huge margin. For me, I try and match the OPT to the desired specifications without getting too large or too small. Then again, YMMV.
Regards, KM
For most SET designs I've done, they are usually using either the 45 or 2A3 DHTs... some others include the 3C24 and on the larger end, Western Electric 300b. Power output starts at ~2-watts with the 45, ~4-watts with the 2A3 and 3C24 and up to 10-watts with the 300b. My design goals are wide bandwidth, low distortion and high signal-to-noise ratio. In short, S/N is probably the most important specification, especially when you only have 2-watts of output power. As high efficiency speakers need to used, any residual output noise from the amplifier will be quite audible, which ruins everything. My 45 SETs are close to 90dB S/N referenced to 1-watt output. THD at 1-watt / 1KHz is less than 0.5% and bandwidth within 1dB is from 25Hz to 50KHz at 1-watt output. This is using the Hashimoto H507S. Going to their larger 20-watt OPT can be an improvement, but not by a huge margin. For me, I try and match the OPT to the desired specifications without getting too large or too small. Then again, YMMV.
Regards, KM
"match the OPT to the desired specifications without getting too large or too small."
That's the point.
That's the point.
What they hear at the same loudness could be the result of their driver struggling to swing more voltage on 7k and on loud passages going into A2 grid current.
If I may add, I've used the H203U and H203S. The H203U may have better specs and look better on a scope. But driving real world speakers, the slight edge of the H203U (EI lams) in the bottom end is offset by a more vivid midrange and richer top end overtones from the smaller C-core H203S.
I chose the H203S over the H203U for my mono hifi which usually drives an Altec 756B + 32C/806 and don't find the bottom end lacking.
Here's a video (not for sonic evaluation) just to show the pairing. 😉
The understatement of the year. I wish more audiophiles and builders put videos online, you can evaluate from them, thanks. I sent a note to an amp builder who has a youtube channel to please put up some sound files of his/her amps. I got my head bitten off as though that was blasphemous, a Tascam handheld recording of a playback could tell you a lot more than talk.
Hashimoto released two new versions 203D & 507D, it would seem somehow redesigned to benefit two stage amplifiers. The paper specifications look the same. Wonder how they differ.
http://www.hashimoto-trans.co.jp/frame/tubecateng22.pdf
http://www.hashimoto-trans.co.jp/frame/tubecateng22.pdf
The difference is phase inversion... nothing more. The standard SE OPTs from Hashimoto are inverted phase, this results in the drive to the grid of the output tube (45, 2A3, 300b, etc.) as being in phase to the output winding. So, whatever circuitry you use to drive the output tube, it "should" not invert phase to the output tube driving the transformer.
As many SE designs are inverted phase to the output tube, this would result in an inverted output at the transformer. Hashimoto simply reversed the phase on these two OPTs for that reason. My 45 and 2A3 SE amps use a 5814a as a DC-coupled cascade circuit providing ~43 dB of gain which drives the 45/2A3 output tube with an in-phase signal. The overall result is an amplifier that's in phase. Hope this clarifies the difference.
Regards, KM
As many SE designs are inverted phase to the output tube, this would result in an inverted output at the transformer. Hashimoto simply reversed the phase on these two OPTs for that reason. My 45 and 2A3 SE amps use a 5814a as a DC-coupled cascade circuit providing ~43 dB of gain which drives the 45/2A3 output tube with an in-phase signal. The overall result is an amplifier that's in phase. Hope this clarifies the difference.
Regards, KM
From experience, one can invert the speaker connections at the secondary (even with grounding) with little to no harm to the frequency response of an OPT. An OPT has already a high step-down ratio, which translates to a low-potential secondary, so little to no parasitic capacitance increase will happen.
On the contrary, one cannot simply invert the connections of an interstage transformer.
On the contrary, one cannot simply invert the connections of an interstage transformer.
In general, I would agree that inverting the speaker connections in such a scenario would not be an issue. However, using Electrostatic speakers "could" present a problem if the negative of the speaker input is also grounded and at the same potential of the amplifier power ground via the grounded power connection. Granted, it's unlikely, but possible.
Still, Hashimoto apparently consider it a scenario worth addressing, albeit to date, only the smaller 7-watt OPTs have been made available with a non-inverting version. Perhaps they will make other versions available as same, but maybe waiting to see if the smaller ones sell first.
Regards, KM
Still, Hashimoto apparently consider it a scenario worth addressing, albeit to date, only the smaller 7-watt OPTs have been made available with a non-inverting version. Perhaps they will make other versions available as same, but maybe waiting to see if the smaller ones sell first.
Regards, KM