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In Reply to: Certainly,.. Try 'Em, You Might Like the Dukes... posted by Interstage Tranny on March 13, 2007 at 07:19:12:
Interstage Tranny, it may have been a circuitous route getting here, but this is definitely more of what I'd hoped for. Fantastic insight into these amps. I thank you. (I actually was thinking when deciding to buy these I may eventually "fiddle" with the feedback, floating the speaker ground (-) by a 1 ohm resistor and taking feedback off between that and the (-) of the speaker, to create a constant-current amp.)I look forward to using them to drive midrange horns without a passive x-over network. Based on what I've read here and in the post linked to, I believe with the OPT taps as they are and how that will reflect the lower impedance of the speaker back to the power tube stage, and I'm surmising the the feedback loop, this amp may exhibit 'impedance follower', a.k.a. 'constant current' characteristics naturally (without my "fiddling" necessary).
I also base this theory off a comment made in the thread in the Tube DIY forum linked to by Steve O of "I would think the OPT is too small for good bass, however, it beat the pants off of both of my ARC d90 and d70mk11 amps, no contest there." I think what was happening there was the bass response was following the impedance curve, with the voltage varying with frequency and more power provided at high impedance of the speaker's resonance and much much less the few octaves above it. I'll actually be measuring that with my Terrasonde ATB-3 by feeding a fixed voltage frequency sweep into the amp and measuring the input sweep's output voltage being delivered to the speaker as influenced by the complex impedance. I suspect it will -NOT- be a fixed voltage to the speaker but rather to vary as the impedance varies, following the impedance. Anyway, that's just conjecture at the moment. I'll know for sure after i get the amps and do some measurements.
As mentioned with my desire to experiment with constant-current amplification to single drivers (specifically, ones that exhibit a frequency response as somewhat an inverse of the impedance curve) this is actually a desirable behavior to me (I won't be using passive crossovers). It's a highly undesirable behavior from an amp when passive crossovers are used however, as the power delivered is not controlled by the driver alone but by the driver and crossover together, making a 'false' emphasis usually in the midrange where crossovers combined with the speaker often produce in impedance hump. Additionally with the amp's active impedance being relatively "high" on the 25V or 70V taps when a signal is fed being, and with that in series with the x-over circuit, it's no longer negligible compared to the speaker's impedance and it ends up essentially 'moving' the crossover point. (this, from what I've read regarding difference between typical super low-impedance amps and high-impedance constant-current amps ; I haven't measured this myself.)
On a different note, I had first thought these were amps made for theaters but learned here they were ['budget'] sound reinforcement PA amps. Do you know approximately in what years these were built?
Also, so then no one has every had any exposure to the Bogen / National Theater Supply amp I asked about in a thread posting before this one? (titled [mis-spelled] "Hoping for a History Leason #1: Bogen / NTS-1125") While this thread's been incredibly enlightening and all I'd hoped for educationally, discussion on that other amp I just purchased in that thread hasn't gotten any love... ;)
-steve
(p.s. I just read this back; woops! thanx for reading this marathon message of mine)
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Follow Ups
- Ah, just what the Dr. ordered.... - m8o 09:54:52 03/13/07 (0)
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