• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ohms FAQ

#941
AE1 said:
You are entirely correct in your observation but the OP is mixing cabs - as many of us do
Even if we use mixed cabs from the same manufacturer there will still be mismatch in characteristics
We are stuck with using what we have so have to make the best of it
Click to expand...
No, there are manufacturers that do engineer mixed cabinets to work together without mismatching. It's up to the player as to which choice to make.
 
#944
bcbassman said:
I once combined an 8 ohm and a 4 ohm cabinet with my Ampeg PB250 head and it didn't like it. It shut off during the gig and I had to finish the gig by unplugging one cabinet. Live and learn.
Click to expand...
That would result in a combined 2.67 ohms, an illegal load for your amp. Your are lucky that the amp went into protection rather than failed.
 
  • Like
Reactions: Jim Carr, BassmanPaul and SteveIII
#945
If it is a horn with a crossover circuit, the circuit is impedance
matched to the 4 ohm driver circuit and you would have to modify
that or disconnect it. Since is is at least 2 octaves higher in operation than
the drivers, it has no effect on the driver impedance.[/QUOTE]

A similar concern is wiring 2x10" Hartke aluminum drivers with a 5" from a 4.5 4x10 XL having 8 ohm total. Must have been wired with two sets of two 8 ohm speakers in parallel then those two sets wired in series. I have two 10" 16 ohm aluminums from another XL cab I can run parallel for 8 total, so the 5" will work, right? Please advise if I am not correct or need more information on this. Thanks.
 
#946
I think we need a lot more detail, including crossovers and so on. There are all sorts of ways you could connect that up and destroy drivers and maybe amps too. There's also the matter of sound quality!

"Must have been wired with" is an assumption I would be exceedingly wary of. There's also the issue that if the setup had the right amount of HF delivered to live in a 4*10, its probably going to be delivering far too much in a 2*10.
 
#947
JimChjones said:
I think we need a lot more detail, including crossovers and so on. There are all sorts of ways you could connect that up and destroy drivers and maybe amps too. There's also the matter of sound quality!

"Must have been wired with" is an assumption I would be exceedingly wary of. There's also the issue that if the setup had the right amount of HF delivered to live in a 4*10, its probably going to be delivering far too much in a 2*10.
Click to expand...

Hey, Jim, My assumption is based on that wiring scheme seemed to achieve the cab's 8 ohm total. But then, that's why I'm here - my ignorance of the mechanics. I haven't started tearing down the 4.5 cab yet, but it's all original from the wiring to the crossover for the smaller driver and all else. Actually, I'll be using the 4x10 cab but with two 16 ohm 10's and two 8 ohm 10's with two separate circuits, one including the 5". When I get inside the cab I will check the crossover and 5" details. Would the small 5" not be an 8 ohm item? The other post I quoted from states the horn operates at least 2 octaves higher than the 10's having no affect on their impedance. OK, I just opened the cab and one end of a 2.2k 250 volt MET CH is soldered to the inputs - side, the other end of capacitor is wired to + on the 5". From - on 5" back to inputs + terminal. The 10's wired parallel, series. Discounting sound quality, which is no trouble to adjust with different values of capacitors (is this correct?) wiring two 16 ohm drivers for 8 ohms and attach the high frequency in manner as was with the 4.5 will safely operate?
 
#948
rcgtrix said:
If it is a horn with a crossover circuit, the circuit is impedance
matched to the 4 ohm driver circuit and you would have to modify
that or disconnect it. Since is is at least 2 octaves higher in operation than
the drivers, it has no effect on the driver impedance.
Click to expand...

A similar concern is wiring 2x10" Hartke aluminum drivers with a 5" from a 4.5 4x10 XL having 8 ohm total. Must have been wired with two sets of two 8 ohm speakers in parallel then those two sets wired in series. I have two 10" 16 ohm aluminums from another XL cab I can run parallel for 8 total, so the 5" will work, right? Please advise if I am not correct or need more information on this. Thanks.[/QUOTE]
Not at all true. The reason the horn has no effect of total impedance is because of the crossover (high pass on t he horn) which raises the effective series resistance as the frequency falls. This combines with the inductance of the low frequency drivers which naturally rise in impedance as frequency falls.

The crossover is not "matched" to the impedance of the low frequency section, the sensitivity at the crossover point is. Generally for bass cabinets, there is no low pass filter on the low frequency section.

The horn does not operate at least 2 octaves above the low frequency section, otherwise there would be a hole in the overall response big enough to drive a truck through.
 
#949
rcgtrix said:
Hey, Jim, My assumption is based on that wiring scheme seemed to achieve the cab's 8 ohm total. But then, that's why I'm here - my ignorance of the mechanics. I haven't started tearing down the 4.5 cab yet, but it's all original from the wiring to the crossover for the smaller driver and all else. Actually, I'll be using the 4x10 cab but with two 16 ohm 10's and two 8 ohm 10's with two separate circuits, one including the 5". When I get inside the cab I will check the crossover and 5" details. Would the small 5" not be an 8 ohm item? The other post I quoted from states the horn operates at least 2 octaves higher than the 10's having no affect on their impedance. OK, I just opened the cab and one end of a 2.2k 250 volt MET CH is soldered to the inputs - side, the other end of capacitor is wired to + on the 5". From - on 5" back to inputs + terminal. The 10's wired parallel, series. Discounting sound quality, which is no trouble to adjust with different values of capacitors (is this correct?) wiring two 16 ohm drivers for 8 ohms and attach the high frequency in manner as was with the 4.5 will safely operate?
Click to expand...

Your assumptions are somewhat amusing, but at least your conclusion is correct. Just put the new drivers in, wired in parallel for 8 ohms and you are probably better off than messing with anything else (given your previous comments)
 
#950
agedhorse said:
Your assumptions are somewhat amusing, but at least your conclusion is correct. Just put the new drivers in, wired in parallel for 8 ohms and you are probably better off than messing with anything else (given your previous comments)
Click to expand...


Thanks, agedhorse,

Glad to be entertaining. Maybe I'll just use the 5" as a plug on the baffle. Though it's good to know that it's doable. I'm sure your knowledge must be dead-on - just repeating from earlier post, or maybe I missed it. But, I should not attempt too much, huh? I do appreciate the help.
 
#952
Good explanation. So my two 8 ohm cabs should be fine with my head having a minimum load of 4 ohms. My question, as they only have one output, can I run a Y connector out of the amp to split into the two cabs? Seems simple enough but I want to be very careful as to not do something stupid.
 
#953
jerund said:
Good explanation. So my two 8 ohm cabs should be fine with my head having a minimum load of 4 ohms. My question, as they only have one output, can I run a Y connector out of the amp to split into the two cabs? Seems simple enough but I want to be very careful as to not do something stupid.
Click to expand...
Yes, that will be fine.
 
#954
Electrically that's correct, but I really wouldn't mess about with non standard cables if I could possibly avoid it. If the head has only one speaker socket then if at all practical I'd put a second parallel socket in each of the cabinets. Rather unlucky for both amp *and* cabs to have only a single connector.

Indeed unless I was on a very restricted budget while I was at it I would install two jack sockets and two speakons in each cab, and be thoroughly future proofed.
 
#957
JimChjones said:
Fair enough. I'm maybe over concerned about keeping the cable collection standard and simple.
Click to expand...
Over the years I have destroyed several fine amps to a state of uneconomic repair as a result of intermittent speaker connections. The amps had internal auto protection circuitry.
The problem is the use of phono plugs and sockets for speakers. Noting phono plugs were designed for voice only telephones, phono plugs can suffer from thermal cycling resultant from the relatively high current experienced with solid state amps - e.g 5 Amps at 4 Ohms for 100 watts, compared with milliamperes from a guitar pickup.
Originally the internal phono plug conductor contacts were screwed together but nowadays are only staked (riveted) for low cost, so any loss of contact pressure from temperature cycling or corrosion can result in an open circuit. When this is intermittent it can wreak havoc.
Then there is the socket (jack). The most common style has a sidewards bearing contact which relies upon side pressure on the body (usually earthed) terminal which can result in poor contact - the effect is worse with oversized holes and undersized plugs - e.g 6 mm instead of 6.35 mm. That can be partly offset by aligning the socket so that the spring contact sits horizontally and at the top. This configuration assists to increase contact pressure by the downwards weight of the speaker cable.
I have found that when the speaker cab is pushed close to a wall the heavy and stiff Fig 8 speaker cable deflects the phono plug/socket sidewards and that can inadvertently damage the contact assembly or reduce contact pressure. Speakon connectors require even more space in that regard.
The solution lies in more care with plug and socket selection and matching - and speaker placement - sometimes difficult on a small stage.
If a single speaker burns out mid gig disaster strikes anyway.
Where possible I now use two speaker cabs in parallel with separate independent cables to each so that if one loses contact the other remains in circuit. Many amps provide two separate speaker sockets that support this configuration.
Also I always carry a spare amp and speaker.
MURPHY's LAW says "if something can go wrong it will"
 
#958
IvanMike said:
You should never run the amplifier below its minimum safe load - doing so will most likely damage the amplifier, and void your warranty!! For example, for an amplifier rated at 4 ohms minimum safe load you could run it with one 8 ohm cabinet,
Click to expand...

Thanks for taking the time to break this down. So I'm trying to understand this and just want to ask for clarification. I have a cab with a 500 watt/8 ohm rated load/speaker in it. So I could safely run a 500 watt/4 ohm solid state amp with this cab with no issues? If I understood this at all I think that's right.

Appreciate the info! :thumbsup:
 
Last edited:
#960
Possibly helpful tool

http://tinyurl.com/sccbfuz

  • Falstad circuit simulation to show combinational load of two cabinets
  • Ohmmeter on left shows load
  • Parallel or Series switch
  • MultiSelect switch to change load for each cab
  • Right click to edit a load resistor to change its value
  • Hover over a load resistor to see it's power dissipation, compare this to other load resistors
  • Pinch zoom works in Chrome and FireFox
  • This is a DC approximation
  • Switching in the voltage source will also show load if you hover over it

The Falstad circuit simulator also includes
Crossover under "Circuits" -> "Passive Filters" -> "Crossover"
Class-D amp under "Circuits" -> "Op Amps" -> "Amplifiers" -> "Class-D"

GNU Licensed: sharpie7/circuitjs1

Always do final testing in Reality
 

Latest posts

Top Bottom
Back