Impedence question!

 

Silver Member
Username: Paulfolbrecht

Post Number: 104
Registered: Dec-05
Got a pair of bi-ampable Gallo Ref 3s. Lower tabs which run the 2nd woofer voice coils specify 4ohm impedence. Plan has been to run these with Rotel 1066 in bridged mode which puts out 150W/ch *but specifies minimum 8 ohm impedence*.

My thought is that as long as I don't run the amp too hard I should be fine. I understand that the speakers could pull a lot of current at 4 ohms and overheat the amp, but, again, I do not think I will be taxing the amp too heaving at all at moderate listening levels.

Someone please let me know if this is not a great idea. I am auditioning these used speakers. IF I keep them, buying Gallo's decided sub amp to run the 2nd voicecoils would also be an option. (But I'd rather not, as the 1066 currently has very light duty powering only my cinema surrouns.)

Paul
 

Bronze Member
Username: Eld

Texas

Post Number: 89
Registered: Dec-05
Paul,

Not a good idea running 1066 in bridged mode on 4 ohms, at least that is what I got from Rotel email regarding this. :-) Rotel didn't give me the exact reason. You are better off running it in single mode with +/- 100 watts at 4 ohms.

Good luck.
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7783
Registered: May-04


What is "single mode"? Only one channel running?
 

Bronze Member
Username: Eld

Texas

Post Number: 90
Registered: Dec-05
Jan, I'm surprise you had to ask.

I'm sure Paul knew what I meant.
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7785
Registered: May-04


Maybe he does, but I still don't.
 

Silver Member
Username: Paulfolbrecht

Post Number: 105
Registered: Dec-05
Jan likes to tease but he knows 100x as much as me about audio so I tolerate it. What is your opinion, Jan?

These lil Gallo speakers are the f---ing stuff, I can tell you that. I think the JA rm25s (damn fine themselves) are going on audiogon. And I come out financially ahead again.
 

Bronze Member
Username: Eld

Texas

Post Number: 92
Registered: Dec-05
Jan, ask me if I care? :-)
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7786
Registered: May-04


I'm not teasing, I've never come across the term "single mode". The question I asked seemed capable of being answered with a yes or no. I guess no one really knows what they've put in print.


The difference between 100 watts stable and 150 watts unstable is not much of a question. If the amplifier is unstable at 150 watts, what do you expect to gain from the extra few watts?


 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7789
Registered: May-04


PF - I really don't understand this sentence, "Got a pair of bi-ampable Gallo Ref 3s. Lower tabs which run the 2nd woofer voice coils specify 4ohm impedence." Possibly if you could explain what is happening more clearly I would have a better answer. Otherwise, if you don't plan to run the amplifier so hard that it will be stressed, are you really likely to run at wattages higher than 100 watts? If the answer is no, what is the advantage you perceive in running the amplifier in the bridged mono mode? Just out of curiosity, how is this amplifier bridged? What do you do to make that work with this amplifier?


 

Bronze Member
Username: Eld

Texas

Post Number: 93
Registered: Dec-05
Jan,

Like Paul, I thought you were pulling my leg.

Since bridge mode is two channels per speaker, then yes, single mode is one channel per speaker. Is there another configuration?
 

Silver Member
Username: Paulfolbrecht

Post Number: 106
Registered: Dec-05
The amp does only 60W non-bridged. So it is 60 vs 150. Considering that Gallo's dedicated 2nd voicecoil amp is 240W/ch, I figured 60W is probably not too close to optimal.

Probably this sounds strage b/c these speakers are strange. The 10" woofer actually does have TWO voicecoils, the 2nd of which *can* (it's optional) be driven by a dedicated amp. Gallo sells such an amp but any amp can be used.

Amping the 2nd coil bring bass response down to 22Hz from 34.

My concern was/is that the impedence of the 2nd coil taps is specified as 4ohm which is half the min impedence the Rotel amp is speced at in bridged mode.

I played them for 2 hours last night setup like this and the Rotel amp never even got warm, and it does sound good. Getting that extra 12Hz adds a lot, of course. I listened without the 2nd coils amped with my Sunfire sub in the mix and then with the coils amped and no sub. Prefer the latter.

Paul
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7794
Registered: May-04


Going from 60 watts stereo to 150 watts mono explains why the amp is unstable at the lower impedance.




"Considering that Gallo's dedicated 2nd voicecoil amp is 240W/ch, I figured 60W is probably not too close to optimal."


I assume the Gallo amp is mono and is meant to drive only the second voice coil of the single woofer. If that's the case, I would still opt for the more stable connection. I assume you are running this speaker off your tube amps. Though large amounts of stable power on tap will always be a nice option, the relationship you need to consider is how much power your bass amp provides versus the amplifier driving the main system. Try both connections and listen for which you prefer.


 

Silver Member
Username: Paulfolbrecht

Post Number: 107
Registered: Dec-05
Well, it's (the Gallo) a 2ch stereo amp, 240W/ch.

"Going from 60 watts stereo to 150 watts mono explains why the amp is unstable at the lower impedance."

Could you perhaps expand on that? What exactly is meant by "unstable"? Is this something that would affect the sound and/or something that could potentially damage the amp?

BTW, I am using a nifty inline low-pass 50Hz filter between the preamp-Rotel for these inputs. This is how you get away with not using Gallo's dedicated amp, which, at $900, isn't exactly cheap. These are the same units 6moons used when reviewing these speakers originally, which was before the Gallo sub amp existed.

Paul
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 7802
Registered: May-04


I see, so the bass amp replaces your main amplifier for the woofer. Is that correct? I assume you are using the F-mod filters. Their frequency cut off point is typically higher than what is stated on the literature. They sell an adjustable unit which allows experimentation with various cut off frquencies and I would suggest that unit rather than a fixed value filter.


However, I'm not certain why the F-mods are needed for this application. Is the bass amp connected directly to the woofer without any crossover being employed? Any connection to the internal Xo should provide a low pass filter for the woofer.


A transistor amplifier has long been considered "ideal" when it will double its power output with each halving of the impedance load. This has changed slightly over the years and many designers would prefer about a 1.5 times increase in power for 1/2 the impedance. To more than double the wattage when facing a four Ohm load would indicate a slightly less heavily regulated power supply. As the amplifier is asked to deliver more current into the lower impedance, the circuit could be exceeding its current/temperature limits. At some point the amplifier will act as if it has a dead short on its outputs. It may also be nothing more than a "protection" the amplifier manufacturer feels better stating rather than explain why the amplifier keeps blowing output transistors when hooked to wierd loads. As long as the amp remains no more than warm there is probably no danger.


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