Impedance question

[quickbird]

In the catalog post I see listings for the Solos II 120W 2x12 combo 20x26.5x11.5 2xSunn 128G 2 channels 11 knobs 4 inputs.
This would imply that the original speakers in parallel would yield a 4 ohm impedance.
I have picked up a Solos II with 16 ohm Celestions in it.  One of which was blown.
Am I correct in assuming an 8 ohm load (2x16 parallel) on an amp that was designed to drive 4 ohms is actually safer for the amp, ie. will not require as much load but will be lower output.

I thought I read somewhere here that the amp was rated for 8 ohms btw.

[bigobassman]

The first thing to do is to determine the minimum ohms for the head.  For maximum performance, a matching ohms cab should be used.  Normally, an amp is not strained running at it's stated minimum ohms rating.  You'll notice the difference between an 8ohm cab 4 ohm head configuration and a 4ohm cab 4 ohm head configuration.   

[quickbird]

This is a combo and according to the info I find on it, it came with 2 each 8ohm speakers and the amp is rated to 120w. Therefore I assume the amp is at 4 ohm impedance assuming the parallel wiring of the two speakers.  I do not know the minimum impedance this amp is rated for and also not sure how to figure it out.

So I presume what you are driving at is that I should use impedance loads that the original design was made for to get best results.   

If I were to use 2 each 4 ohm speakers impedance would be 2 ohms and that would draw more current to get the same sound output from the amp. Stressing the amp electrics.

If I were to use 2 each 16 ohm speakers impedance would be 8 ohms and the current draw to get the same sound output from the amp would be less than  the Pair of 8 ohms speakers but I might be sacrificing clarity since true impedance varies over frequency range of the speaker.

I also expect that any external cabinet I attach via the external speaker jack should match the internal speaker impedance either way or risk other issues.

[bigobassman]

You've got a pretty good grasp of what I'm saying.  While using a 4 ohm cab with a 2 ohm minimum head will work, and even work really good, the maximum performance is indeed achieved with "matched" amp and cab(s.)  Any better amp maker, and of course Sunn is one of them, would design the amp to perform at their advertised minimum ohm and not over work it.  Yes, you must factor the ohm rating of any extension cab that you add.

On a side note and examples - Saturday, I picked up a 1982 Peavey Mark III head and matching 2-15 cab.  I've been looking for something along these lines to add to the collection of "vintage" bass amps.  Anyway, the head is a 2 ohm minimum.  The cab is a 4 ohm.  The head states that with a 4 ohm total cab, the power is 210 watts.  But with 2 ohm total cab(s,) the power goes up to 300 watts.  Another example is an Ampeg SVT-350 with a 1510HE cab.  That head runs 200 watts with an 8 ohm cab, but pumps up to 350 when used with a 4 ohm load.   

[quickbird]

Thanks for the followup.
I have been researching and the last bit about wattage increases tells me I also understand another thing.

As the load impedance decreases say from 8 to 4 ohms, the wattage output of the amp inversely increases.  What you have to be careful of, from what I've learned, is that if you choose to go below the minimum load rating of the amp, the max wattage you draw may be too high for the amp to handle and as they say, you let the smoke out. 

That said, if you increase the load impedance from 4 to 8, the wattage output of the amp likewise inversely decreases.  I believe this will preserve the amp's integrity, but there may be a sacrifice in sound reproduction as true impedance varies over the frequency range.

So for my SOLOS II, the speaker output jacks are in parallel.  I assume that with the factory internal speakers being 8 ohms that they are loading at 4 ohms.  An extension cabinet should also be rated at 4 ohms.
So to go back to your original response, this amp can drive at 2 ohms safely if all the assumptions hold true. I think I should replace the speakers with two 8 ohm units and ensure the amp is running optimally.

Kudos

[bigobassman]

You're welcome for my input - that's what the forum is for.  I think that your bottom line is to be sure of the minimum rating for the amp.  Take what we've discussed and make the proper applications.  Good luck.

[pickinatit]

So to go back to your original response, this amp can drive at 2 ohms safely if all the assumptions hold true. I think I should replace the speakers with two 8 ohm units and ensure the amp is running optimally.
Kudos

IMO (and that's all it is) If I were going to make assumptions, I would assume that the amp was set up "optimally" (is that a word?? haha) to begin with.  2 X 8ohm speakers  for a 4 ohm load driven by a 4 ohm amp.  The additional 4 ohm speaker would reduce the load to 2 ohms.  I would never assume that the amp can handle that.
Why not take it to a tech or subject to some test equip. & be sure before you take any undue risk.
One assumption you are making though,  the 8 ohm cab should certainly be alright. 

[quickbird]

good thought,  I "assume" since there is external speaker output that does not turn off the internal speakers when attached that the amp can handle 2 ohms.  It looks like there was a sticker or something on the back at one point, but don't know.
Anyway, I do not intend to install an external so I will avoid that context.

The current 16 ohm Celestions that are in the cabinet are LOUD. They don't sound full until they are loud enough to make your ears ring.  

I am contemplating post processing the pre-amp output and piping it back into the amplifier input to lower the preamp levels.  Any suggestions?

In a larger living room with volume at half mast this thing kicks it out nicely.
I have a Peavey Renown 112 that is a hi wattage unit ,but it just seems flat compared to this thing.


Meanwhile, I search for 8 ohm speakers.
A couple images

[loudthud]

The 128G was an 8 ohm speaker. Sunn part number 81-0126-00-90. It was also used in the A212, 612SR, 612S, 412SP, 412L, Stage 212, and Beta 202. The Solos II was probably designed for a 4 ohm load because it uses circuitry not unlike the Concert Series. I'll see if I can confirm that the Solos II and Concert used the same power transformer.