Jerry R.

OK, next try... My first post in this thread was on April 28 2018, but no response... In 2017 I designed and build a current-drive system: - 2-way acoustic suspension loudspeakers, without crossovers and with (sorry, no horns) Scan-Speak drivers, - a 4-channel current-drive amplifier with an output impedance of 1380 Ω, - a 2x4 channel DSP/DAC for EQ, crossover and Dirac Live. I used woofers with non-conducting voice coil formers to avoid back-emf eddy currents (the other back-emf that also creates modulated noise). The amplifiers are based on an existing design for a voltage-drive amplifier of which I changed the feedback into negative current feedback and I changed the output stability filters (I would like to build an amplifier with a complementary cascode configuration). In the DSP I used filters and Linkwitz-transformers to create phase coherent crossovers (based on the ideas of Duelund), and to extend the bandwidth to full range. It's a full-range 2-way system (from just below 20 Hz). Since it was operational I gave away my voltage-drive system, no voltage-drive for me anymore... I'm open for questions, about the basic principles behind current-drive and about my system.

No, by far, it has an output impedance of 0.2Ω. The F1 and F1J come close (80Ω and 60Ω). In my opinion with a damping factor of lower than 1% you are getting there... More than 800Ω (400Ω) for an 8Ω (4Ω) loudspeaker. I build an amplifier with an output impedance close to 1400Ω. In my opinion a damping factor higher than 100 is voltage-drive (ideal is ∞) between 10% and 10 is power-drive (ideal is 1) and smaller than 1% is current-drive (ideal is 0), the rest are in-betweeners.

Hi Chris, I think that (Klipsch) horn speakers benefit from transconductance amplifiers. But, I think that amplifiers with an output impedance of about 40 Ω are not realy transconductance amplifiers (or current-drive amplifiers). Those are more power-drive amplifiers, with an output impedance around the impedance of the loudspeaker (within a factor of 10). So, amplifiers without feedback and valve amplifiers are all (more or less) power-driven amplifiers. A real transconductance or current-drive amplifier has an output impendance of 400, 4k, 40k or more Ω. You read and mentioned already the paper of Esa Meriläinen and learnt that in the small-signal domain of drivers there is a huge benefit of current-drive. I don't know whether the Klipsch drivers have non-conducting voice coil formers (bobbins). If they have, the benefit is even greater (as you could read in his paper). I think in the large-signal domain around resonance a horn system is probably inherently better than other systems. But because of the elimination of electrical damping (one of the non-linear distorting effects of voltage-drive) you have to cope with the rising response around resonance (with -real- transconductance Qts equals Qms of the driver!). EQ does the job. You also need EQ for the rising response on the high-end due to the elimination of inductance effects (damping and non-linearities). But if a horn is used (far) below resonance there is a drawback. Around resonance a driver is sensitive for second harmonics of frequencies half of the resonance and for third harmonics of frequencies a third of the resonance (so, with driver resonance at 60 Hz, the driver is sensitive for second harmonics of 30 Hz signals and for third harmonics of 20 Hz signals). This goes for every driver, but with a closed system you can realize enough acoustic damping of the resonance and of the sensitivity. I don't see how you could do that with a horn system. On the other hand, I do wonder whether a horn is ever used so far below resonance... So, I think that every driver thats work with an electromotor (F = Bl · I) benefits from transconductance / current-drive in terms of distortion, bandwidth and stability (no destabilizing effects of heating voice coils). (I don't know how piezo-drivers, electrostats and other exotics react to current-drive.) But... But traditional crossover don't work with transconductance! They need a redesign or elimination. Esa explains how to build passive crossovers for current-drive. Even better is to use an active configuration. You can use DSP for driver correction, time alignment of drivers, crossover, time alignment of loudspeakers and for room correction. Leaves the problem of availability of (real) current-drive or transconductance amplifiers. I build one myself (it is no rocket science, just changing the feedback, and the book of Esa helps), build my own crossoverless 2-way system (sorry, no horns) and bought a 2x4 channel DSP system with DAC. The sound is crystal clear, soundstage is great and stable and an active system is very efficient. I have 4x 80 W available, but I wonder if I every used 1 Watt per channel... Send me some Klipsch-horns and I will tell you how they sound. :-) How do you go on your quest for lower distortion and better sound?