Question about Ohm/impedance settings

The Scarecrow · Post by **The Scarecrow** » Thu Mar 17, 2005 7:58 am

Okay, a question about Ohms/impedance, specific to my own setup. I'm a bit of a dope when it comes to electrics and so forth for things like this, so wanting to make sure I get some advice on the Ohm setting before I do anything to my amp that may damage it.

My DSL 100 has three output jacks, a dedicated 16Ohm which negates the other two, which are switchable between 4 and 8Ohms for mono/stereo. I have traditionally run the 16Ohm jack into my quad, also set for 16Ohm, as my 1960AV cab has two imputs, switchable between stereo and mono where in mono, one runs 160hm, the other 4Ohm or 280w. When in stereo, they both put are 8Ohm/140w per channel.

I have found the 160hm setting does blunt the bottom end a lot, and gets flabby and there's excessive muddiness at moderate volumes - I understand running it stereo will take a lot of that away, and tighten it up a bit. What I want to know if if I can run the DSL in stereo i.e. 2x speaker cables running into the quad, to get 140w each channel/left and right side speakers? Obviously the quad would be set for 8 Ohm for stereo, but what do I set the amp to?

Sorry if this is confusing! Advice appreciated!

Post by Bg » Thu Mar 17, 2005 8:20 am

so your amp has two other sockets that can be switched to 8 ohms stereo..... that would be the setting I'd go for. You are definitely going to get more power coming down from 16 ohms - most amps power rating is at 4 ohms, as you increase the impedence the amp pushes out less power! so a 100 watt amp may only be pushing 25 watts at 16 ohms

The Scarecrow · Post by **The Scarecrow** » Thu Mar 17, 2005 8:52 am

So just so I understand, I can run two speaker cables, at 8ohms each into the two jacks on the rear of the quad, also at 8ohms and this will be sweet right?

philipnz · Post by **philipnz** » Thu Mar 17, 2005 9:04 am

No
You can run a single 16 ohm quad through the 16 ohm output.

You can run two 16 ohm quads through the two sockets with the selector set to 8ohms. (2 16's in parallel are 8ohms)

You can run two 8 ohm speakers throught the two sockets with the selector set to 4

Also most amps wont mind a 100% increase but wont tolerate any decrease. So you can plug a 8ohm speaker into a 4 ohm output - but not the other way around

Post by Bg » Thu Mar 17, 2005 9:06 am

buggered if I know

if in doubt don't.....

Kloppsta · Post by **Kloppsta** » Thu Mar 17, 2005 9:31 am

just out of interest, I run my 16 Ohm Orange quad through the 16 Ohm output on my DSL and I find there is PLENTY of bottom end, clarity and no "flabbiness" at any volume.

I honestly don't believe many people would be able to "tell" what Ohmage you are running with your AMP & CAB, just by listening. I experimented with LOTS of cabs before settling on the Orange. The two best cabs I have ever owned were 16 Ohm cabinets (loaded with Celestion V30's)

http://www.termpro.com/articles/spkrz.html - makes for an intersting read

The Scarecrow · Post by **The Scarecrow** » Thu Mar 17, 2005 9:40 am

Kloppsta wrote:just out of interest, I run my 16 Ohm Orange quad through the 16 Ohm output on my DSL and I find there is PLENTY of bottom end, clarity and no "flabbiness" at any volume.

I honestly don't believe many people would be able to "tell" what Ohmage you are running with your AMP & CAB, just by listening. I experimented with LOTS of cabs before settling on the Orange. The two best cabs I have ever owned were 16 Ohm cabinets (loaded with Celestion V30's)

http://www.termpro.com/articles/spkrz.html - makes for an intersting read

I think the makeup of your previous cab (the mesa) and the orange are factors, thicker wood than in the Marshall one, but from memory, you always had a tight bottom end, probably due to that Mesa, it was a nice sound.

I got the 1960av because it had better mids, which was what I was after, so I run the deep switch on the DSL to compensate. It makes the speakers sound "bigger", but at lower volumes (i.e. not being cranked past 4, which is still very very loud!), there's too much low end, much the same way tube amps sound crap when not cranked hard.

Probably differences in our tone too, I wager I use more gain and mids than a lot of people (both are usually set to 8+) do. I'd agree that most people can't tell the difference in tone, but I'm traditionally tone-fussy.

ash · Post by **ash** » Thu Mar 17, 2005 10:05 am

Its not the impedance that causes the sound problem, its the series/parallel relationship of the speakers' wiring inside the cab. Switching to stereo gives you two parallel pairs in parallel, but the 16 ohm setting gives two parallel pairs in series or vice versa, which apparently has a lower damping factor and subsequently looser sound.

I think Phil's assesement is the right one. Two 16Ohm speakers add up to 8 Ohms in parallel. Two 8Ohm pairs in parallel via the two speaker leads present 4Ohms to the amp's output transformer.

The power output isn't going to be affected by the impedance setting on a valve amp, because the output transformer accounts for those impedance effects, unlike a solid state output stage which would lose efficiency with excessive load.

Post by Bg » Thu Mar 17, 2005 10:47 am

its funny but I read awhile ago that the impedence attenuation is actually worse in valve amps than it is in solid state....

ash · Post by **ash** » Thu Mar 17, 2005 12:19 pm

MW Amp dude tells me it can be if the taps in the output tranny aren't suitably matched to the speakers. There's one sure way to find out... maybe on Sunday?

ant · Post by **ant** » Thu Mar 17, 2005 7:25 pm

Don't quote me but I'm not sure the marshall is actually stereo? Sure you might have two output jacks but they are in parallel and completely different from a stereo poweramp. If you are asking can you plug a jack into each output and then into the 2 jacks on your cab, You could but it will be almost exactly the same anyway and it sounds like the impedance wont match up.

Obviously the numbers on the impedance selector are there for a reason, with a valve amp you can usually 'mismatch' up or down 1 notch fine((ie 8ohm speaker can be run 4 or

but the sound will be different, which is what matchless do - essentially running a load one step lower than most other 2or4xel84 amps.

The power output isn't going to be affected by the impedance setting on a valve amp, because the output transformer accounts for those impedance effects

This is most definately wrong trainwreck express amplifiers run 2xel34 into a 6600 ohm primary impedance for 36watts, marshalls run 2xel34 into 3400 ohm for 50watts, go figure.

Some people agree that using the highest ohm cab your amp can support is best as it uses all the turns on the secondary winding of the output transformer, I belive the effect would be tiny.

One tip - dont run guitar cables to your speaker cabs!! The conductors are usually tiny and won't handle the huge currents needed to power the speakers get proper 2 wire speaker cable or such, it does make a difference. HTH

ash · Post by **ash** » Thu Mar 17, 2005 7:53 pm

Ant, how much effect do you reckon the capacitance of a co-ax guitar cable would have on the speaker output?

ant · Post by **ant** » Thu Mar 17, 2005 8:16 pm

It depends on how long the run is.. but coax will reduce the highs somewhat which could be good in some amps (solidstate some valve amps). I just suggest you try both, it seems hard to come across many 'complete' cables, but its easy/cheap to DIY just match up the two ends

On the weber speaker site there is a wire gauge calculator that it recommends for different wattage / ohm cabinets. Can't find it off hand but I calculated for a 5watt amp/8ohm speaker and it recommended 18 gauge wire which is pretty thick, thicker than a guitar cable conductor anyhow.

Rog · Post by **Rog** » Fri Mar 18, 2005 7:33 am

One of the problems with using instrument leads as speaker leads is this:

The small wires used for instrument cables present a significant impedance to current flow, compared with the larger wires more commonly used for speaker leads.

One effect of greater impedance to current flow is generated heat. It is possible to generate enough heat to degrade the insulation of an instrument lead, by using it for speaker loads. Once the insulation is sufficiently degraded, a short-circuit path can be exploited by the current (current likes to go the easy route). This short circuit current will destroy amplifier components very quickly.

Since current I equals the square root of the power in Watts divided by the impedance in Ohms, a 50W amp will allow around 3 and a half amperes to flow into a 4 Ohm load.

This is roughly equivalent to trying to run 8, 100W lamps with instrument cable. As you can imagine, the results would be not nice.

DC4 · Post by **DC4** » Fri Mar 18, 2005 1:06 pm

Yep, Rog has it nailed...