OTL vs. speaker impedance curve?

James Romeyn · « **on:** 31 Jan 2008, 11:20 pm »

I'm considering driving a speaker whose impedance curve is pictured below, w/ an OTL amp:

Would appreciate a description of the expected relationship between the impedance curve & the system FR curve. I'm especially interested in the estimated shape & height (in dB) of the FR peaks.

What about the 5.3-Ohm dip? (Amp manual requires 8-Ohm minimum.)

Is it correct that an optional output transformer will have flatter FR w/ the downside of lower resoultion & transparency?

Thanks all!

*Scotty* · « **Reply #1 on:** 10 Feb 2008, 06:02 am »

ro7939, Without knowing what the circuit design of your amplfier is or its manufacturer it will be hard to answer your question. Here is a link to the formula used to calculate amplifier output impedance. http://www.transcendentsound.com/amplifier_output_impedance.htm
This is a link to a Stereophile review of the Transcendent OTL T8 with measurements of the response curve of a simulated speaker load and the impact of the output impedance on the frequency response. In this case in is minimal and probably inaudible.
http://www.stereophile.com/tubepoweramps/1197trans/index5.html
Whether you have objectionable deviations from flat response from your combination of speaker and amp depends on the amplifiers design,a transformer coupled output stage does not guarantee a flatter response curve. Here are links to measurements of three different transformer coupled amplifiers with very different response curves.
http://stereophile.com/tubepoweramps/1004lamm/index4.html
http://stereophile.com/tubepoweramps/1206luna/index4.html
http://stereophile.com/tubepoweramps/1206luna/index4.html
An OTL design avoids the increased distortion in bass the of a transformer coupled design due to transformer core saturation and low frequency phase shift problems.
Scotty