This page is under construction. It will be expanded as time permits. For now it may look like a random collection of thoughts related to OPT’s. That’s because it is!
I will attempt to explain where much of the confusion comes from with regard to output transformers and their power ratings:
The problem lies with the way many transformer vendors rate the power capability of their transformers. Many state the facts, but they are not fully understood, and some just flat out lie! There is no real standard for specifying the power handling capabilities of an output transformer. The power handling capability of an output transformer is not constant over the audio frequency range. The lower frequencies require more metal in the core and more windings to provide enough inductance. This means a bigger physical size, more iron and copper, and therefore more money. The unfortunate truth of the OPT puzzle is that size really does matter, and the cost / performance curve is pretty constant with a very few outliers. If I had only one ruler to measure OPT performance it would be weight.
Many output transformers (especially the Chinese ones found on Ebay) are specified at 1000 Hz. This is pretty useless for approximating performance with music. So, that "50 watt SE OPT" from Eastern Audio that weighs 5 pounds is like most other 5 pound transformers, a true 8 to 10 watt OPT. Some are specified over a range of frequencies, the specifications are correct, but require some thought about their application.
Hammond actually specifies this in the text on the 125 series web page. Note the following:
- Designed for general purpose or replacement use (not Hi-Fi), in single ended, tube output circuits.
- Frequency response: 100 Hz. - 15 Khz at full rated power (+/- 1db max. - ref. 1 Khz).
- For full frequency response (20 Hz. to 20 Khz.) - see our 1627-1642 series.
Note the words "not HiFi", this does not mean that the 125's will suck when used in HiFi applications. It means that they will not work at their rated power levels. Note the words "Frequency response: 100 Hz. - 15 Khz at full rated power (+/- 1db max. - ref. 1 Khz), This means that you can get 8 watts through a 125 CSE at 100 Hz, losing 1db of signal compared to 1KHz at an unspecified amount of distortion. Now, I have tested the Hammond 125CSE pretty thoroughly. It will handle under 1 watt at 20 Hz without undue distortion. This doesn't look too good, but how often do you listen to 20Hz? The lowest note on a bass guitar is in the 40Hz range where you can get 2 or 3 watts. If you try to crank 8 watts through a 125CSE at 40 Hz, it just wont happen. You will get out a very distorted 2 or 3 watt signal.
The same situation exists with the budget Edcor transformers. They are rated for operation above 70 Hz. Their specifications are a bit optimistic too. I couldn't crank 15 watts at 70 Hz through an XSE15-8-5K transformer. It works about the same as the Hammond 125CSE. Neither the budget Hammonds or the budget Edcors or any OPT in the 1 pound (weight) range will accurately reproduce music over the full 20 Hz (or even 40 Hz) to 20 KHz range at any power level over about 2 watts.
There are two situations where a 1 pound OPT works good. The first is a powered subwoofer situation where the main amp doesn't see much use below 80Hz, and the other is where the speakers have no real response below 80Hz. My Yamaha studio monitor speakers are dead below 70Hz so the 125CSE sounds reasonable on these speakers. I can still hear saturation effects on bass heavy music if the volume is turned up, but it is not terrible. You can't play techno on 15 inch woofers with these, it sounds gross even when turned down.
OK, the tiny little SE OPT's that sell for $20 to $50 may not sound that good on a pair of15 inch wall shakers, is there any reason why you shouldn't invest is some 11 pound Hammond 1628SEA's. Well yes there is. It takes energy to constantly change the direction of all the magnetic particles (simplified explanation) inside the core of those big transformers. This means that big transformers are generally not as efficient as small ones. Yes your precious output power is wasted in magnetic and resistive losses. It also means that some fine audio detail may not even make it through the transformer. These effects can be minimized by the use of more efficient materials, but at considerable cost.