I'm getting ready to order the output transformers for my impending SSE build. I've opted for larger Edcor transformers (the CXSE25-5K) but I have to choose an output impedance option. I checked with Edcor to see if they can make one with multiple taps, but they cannot.
So I must choose one impedance.
The trick is, I'm not sure which speakers I'll be using with this amp. I tend to swap out speakers fairly regularly as my collection fluctuates, so the speaker choice is liable to change over time.
That makes it tricky to choose an output impedance that will be suitable for multiple speaker types.
If I understand correctly, a 4 ohm tap is the only option if I want to drive speakers with 4, 8 or 16 ohm impedance, but I'm not too clear on this.
So I seek input and advice.
So I must choose one impedance.
The trick is, I'm not sure which speakers I'll be using with this amp. I tend to swap out speakers fairly regularly as my collection fluctuates, so the speaker choice is liable to change over time.
That makes it tricky to choose an output impedance that will be suitable for multiple speaker types.
If I understand correctly, a 4 ohm tap is the only option if I want to drive speakers with 4, 8 or 16 ohm impedance, but I'm not too clear on this.
So I seek input and advice.
Speaker impedance is a *nominal value. They are not fixed at, say, 8 ohms at all frequencies. They have a curve & what any given impedance is at any given frequency will depend on the speaker.
The transformer is simply going to reflect the output impedance to the tubes at the primary based upon it's turns ratio.
Taking that Edcor, for example. 8k output tapped.
If you put an pure 8 ohm load on the outputs, the tube will "see" 5k.
If you put 16k on the output, the tube will see a *higher impedance. Not necessarily 10k because it varies as a function of the square root of the turns ratio between primary & secondary, but higher.
Take a quick look at google images for "speaker impedance curves" to get an idea of how it varies.
The simplistic answer is that putting a higher impedance on the output will give less power but lower distortion. Vice-versa for a lower than specified value.
This is obviously a generalization.
Going with a 6ohm output is an acceptable compromise.
The transformer is simply going to reflect the output impedance to the tubes at the primary based upon it's turns ratio.
Taking that Edcor, for example. 8k output tapped.
If you put an pure 8 ohm load on the outputs, the tube will "see" 5k.
If you put 16k on the output, the tube will see a *higher impedance. Not necessarily 10k because it varies as a function of the square root of the turns ratio between primary & secondary, but higher.
Take a quick look at google images for "speaker impedance curves" to get an idea of how it varies.
The simplistic answer is that putting a higher impedance on the output will give less power but lower distortion. Vice-versa for a lower than specified value.
This is obviously a generalization.
Going with a 6ohm output is an acceptable compromise.
You're looking at it backwards. The question should be: "Are there risks in loading this amp with this impedance"?
Again, without knowing the speakers, their impedance & phase curves, there is no way to be definitive.
There are some nominal 4 ohm speakers that drop below 2 ohms at a point that makes serious demands on any amp.
All I can say is I bought 6 ohm secondary ones myself and I am pleased with their performance.
With my speakers, in my space & my choices of music.
Again, without knowing the speakers, their impedance & phase curves, there is no way to be definitive.
There are some nominal 4 ohm speakers that drop below 2 ohms at a point that makes serious demands on any amp.
All I can say is I bought 6 ohm secondary ones myself and I am pleased with their performance.
With my speakers, in my space & my choices of music.
As stated an 8 ohm speaker may be 8 ohms at one or two specific frequencies, and power levels. Otherwise they are all over the place. Most decent quality commercial speaker systems will have an impedance VS frequency graph somewhere in their specs. I included the manual that came with my Yamaha NS-10M's back in the 1990's. The graph is on the lower right corner of page 3.
the "8 ohm" Yamahas hit 6 ohms at 200Hz. 20 ohms at 82Hz, and 18 ohms at 1500 Hz. Choosing a 6 ohm OPT is probably the best option for these speakers.
I would also choose a 6 ohm OPT if you swap speakers a lot. There is no risk in running a reasonable mismatch. I would not run a 4 ohm OPT into clipping with a very light load, 32 ohms or higher due to voltage peaks that can occur when the amp hits hard clipping.
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