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OHmmmmmm

Discussion in 'Amps and Cabs [BG]' started by blipndub, Dec 28, 2000.


  1. This is going to sound pretty pathetic compared to all the discussion about high end equipment and the level of expertise, but here's my question, well first the scenario...
    I have a Fender BXR60 (which btw i preferred to the Hartke Kickback for some insane reason) and I have a large speaker cab (2x12) that I scavenged from someone. When i run a cable from the line out on the BXR to the speaker cab i get VERY little sound. Some signal but not loud enough to even mic. I pulled apart the cab and all the connections seemed to be fine, i even experimented with wiring one speaker in case they were running in parallel and shorting out. I use the line out for recording and it sounds fine so i don't think it's an output issue.

    I'm wondering if it's a OHM thing. Unfortunately, i don't know the specs on the line out from the amp, but I think the cabinet is 8ohm. Does this appear to be the wrong signal for this type of cab?
     
  2. notduane

    notduane

    Nov 24, 2000
    Location
    "Line out" and "Speaker" or "Power Out" are 2 different critters.

    No way is the level of the Line out gonna' drive a 2x12" cab.
    Line out is usually a tap somewhere BEFORE the finals or power amp which drives the speakers.
    That's probably why you're hearin' mouse farts in the 2x12".

    Is there a "SPEAKER" out jack or "EXT SPKR" jack on the amp?
    If so, try runnin' IT to that cab. SET YOUR VOLUME LOW FIRST!
     
  3. Thanks,
    No there's not a speaker out, would i run into a pre-amp to connect to a speaker cab? Or do i need another power amp?
     
  4. dytakeda

    dytakeda

    Jul 18, 2000
    It would have to go to another preamp AND power amp. Think of "line out" as "unamplified", almost the same as coming directly from the bass. So, it would need two stages of amplification, like most bass amps provide.

    You could also use it to plug into a tuner, if you didn't want to go through a tuner.





     
  5. What are the dangers of running a "speaker out" into a regular guitar input i.e. running a head into a combo, granted of course that the combo gain would have to be low or off
     

  6. Now, you could take a lead from the existing speaker (RED positive, BLACK negative) to the extension. This would not cause any problems impedence-wise (since they're connected in series, not parallel), but you would not get as loud a result than if the speakers were parallel.

    You might find that the existing speaker has 2 sets of terminals, that way you can make up a 1/4" jack to 2 x spade cable to connect the extension, or maybe even wiring the speaker to an extra 1/4" socket (allowing you to use standard 1/4" instrument cables).

    Does the 2x12" sound better than the BXR's internal speaker? If so, completely disconnect the BXR's internal and let the amp drive the extension solely.
     
  7. dytakeda

    dytakeda

    Jul 18, 2000
    blipndub asks:
    What are the dangers of running a "speaker out" into a regular guitar input i.e. running a head into a combo, granted of course that the combo gain would have to be low or off
    =========================

    At least you realize that the gain would have to be low or off, but I would never ever try this. The danger would be blowing up the input stage of the preamp. Remember that the input stage is designed to "see" the amount of current put out when a steel string is vibrated through the magnetic field of by a bunch of wire wrapped around a magnet. In other words, not very much current and the amp's input is VERY SENSITIVE! Speakers take a lot of energy to move them big ol' coils. That's why we amplify!

    Don't do this! (Anyways, if you do, be sure and post back here to let us know what happened!)
     


  8. mmmm, I think I put you crook there, sorry!. What I described (speaker connection-wise) is actually PARALLEL...meaning that 2 x 8ohm speakers will draw a 4ohm load from the amp (if the amp can provide this, great; otherwise the amp WILL eventually fry).

    A SERIAL connection to the extension would be connecting the amp speaker cable +ve to the amp speaker, then the -ve from that spkr to the +ve of the extension cabinet. The -ve of the amp speaker cable connects to the -ve of the extension cabinet.

    I've run a head into a combo several times without any problems. You'll need to play around a lot with gain and level adjustment on each amp, though. Either a low gain on the "pre" amp, high gain on the "power" amp, or vice-versa. Very easy to get crazy distortion if you're not careful. Not the best thing to do in the long run, though.

    Best bet for speaker-level to instrument-level is to use a decent DI box - one that can take either instrument or speaker level input (or one that has an input pad/impedence selector for the input). That will take your speaker-level and turn it into line-level, perfect for feeding into a mixer or another amp.
     
  9. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    OH BOY!!!!

    Don't even think of running a speaker signal anywhere else than into a speaker!!! (no, not even into a DI-box)

    Both instrument signal level and preamp level are more or less just (low) voltage, hardly any current and thus hardly any power. (preamp level is quite a bit hotter than instrument level, but still 'without power')

    Speakers need serious amounts of power, we are speaking of 50-500 Watts (RMS) per driver! (if you leave out those little ones in small combos, but even 15W is way too much for any preamp input)

    If you want to use an extension cab with your combo there are 2 ways to go:

    1. 'external speaker out' - just plug a second cab into this one, impedance of the cab usually has to be at least 8 Ohms (only combos I've heard of that can handle 4Ohm ext. cabs are SWR Silverado and Eden Metro). If your combo has no ext. speaker out this simply means that the amp can't handle a second cab. Period. But as nil said you can connect it instead of the internal speaker. Underpowering the cab can become a problem in this case though.

    2. 'line out' or 'preamp out': you need a second power amp but not a second preamp (because this is the post-preamp signal) and the cab of cause. The unaffected bass signal would be called 'direct out', you can run this into a pa (or any other amp)

    As you seem not to know too much about electrics/electronics I would not recommend messing with speaker wiring and such.

    good luck,
    Matthias

     
  10. spoolie

    spoolie

    Nov 30, 2000
    What Matthias said. Listen to him.
     
  11. Oysterman

    Oysterman

    Mar 30, 2000
    Sweden
    EBS makes active cabinets for use in situations like this, with a built-in 300W power amp.

    Yes, they are good.

    Yes, they are expensive.

    Yes, you should consider a full rig instead, if you want more juice.
     
  12. Matthias,while I admit to not being an expert (thus my reason for asking questions of others who know more than i do!) I do have some experience messing with electronics. Trial and error, the great classroom. Please try not to sound patronizing, we are all learning from each other here.

    I mention the line out into a combo because years ago before I knew any better (than to even ask others) I ran a GK200-RCB head into my Peavey TNT 100w combo. I ran a line from the "speaker out" on the GK to the PVs guitar input, I used this rig for about a year until I bought a decent cab. I did have to be careful about distortion as mentioned earlier, but in the short run anyway it worked fine.


     
  13. I want to try and help straighten some of this out.

    This is starting to boil down to voltage, current, and impedance. All the outputs mentioned above, speaker out, line out, FX send, etc. put out a voltage. Voltage is like pressure in a water line. All the inputs mentioned above, speaker terminals, instrument in, FX return, power amp in, etc. are a certain impedance. The current that flows into these inputs is determined by how much voltage is put across its impedance. Ohms law says that the voltage = current x impedance.

    Here's my favorite analogy. Imagine a garden hose with a spray nozzle on the end. When the nozzle is closed and no water is flowing, there is pressure (voltage), but no water flow (current) (electrically speaking, an open circuit, nothing connected to an output). If you open the nozzle a little bit, a little water flows (same voltage, high impedance, little current). If you open the nozzle a lot, a lot of water flows (same voltage, low impedance, high current).

    The speaker out on an amp puts out a maximum anywhere from like 8-80 volts or so depending on power rating and impedance rating. P=I*V and V=I*R so P=V^2/R, so a 8 Watt amp rated for an 8 ohm speaker puts out a rms voltage of 8 Volts (P=8*8 / 8 = 8). An amp rated at 800 Watts into an 8 ohm load puts out 80V (800 Watts =V^2 / 8, V = 80 Volts). These amps put out this same voltage even if no speaker is hooked to the output. If you run this into the "bass input" of another amp, which is looking for less than a volt, you can easily cause distortion, and possible blow something if you turn the volume on the first amp up even a little in the case of the 800 Watt amp. Having said that, it is possible to do it and not hurt anything, but don't do it anyway.

    The input on your amp for your bass to plug into is like around a Million ohms or more and is expecting like 0.1 Volts or less. Your speakers are 8 or 4 ohms and expecting like 8-80 Volts. The line level stuff (FX loop in, power amp in, etc.) is like 10,000 ohms or so and is expecting about 1 volt. Lo-Z mic stuff is 600 ohm. You get the idea.

    The outputs for the speaker are designed to handle low impedance loads i.e. higher output currents that result from these low impedance loads. The line level outputs are designed for like 10,000 ohms or so. The safety net of design is to make your outputs capable to drive fairly low impedances, like 100 ohms or so, and then make all your inputs pretty high impedance, like 100K so they don't stress any outputs that are driving them. Then nothing smokes.

    Memorize these:

    Power = Voltage * Current
    Voltage = Current * Resistance

    notice that you can solve for voltage or current in the second equation and sub it back into the top equation giving

    Power = Voltage * Voltage / Resistance
    or
    Power = Current * Current * Resistance

    I know, that's more than you wanted to know.

    Chris
     
  14. On the contrary, thank you for taking the time for a lesson!
     
  15. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    blipndub, sorry I did not mean to offend you. I just met some guys here before who needed a fair warning not to hurt themselves or damage their equipment...

    I really like the water analogy, that's how my father explained electricity to me...

    However, running the speaker out into the instrument input is something like watering a little flower pot with a firehose :) you would have to be VERY careful....

    no offence,
    Matthias
     
  16. I wonder if anyone has experimented with a "volume soak", a device that is simply a volume control between the speaker out and the cabinet, allowing one to drive tubes at a high volume without taking the roof off, or, perhaps running a speaker out into a low impedence input like a mixer.

    I agree it wouldn't been good to give advice to someone that ends up having disastrous consequences.
    However, running the speaker out into the instrument input is something like watering a little flower pot with a firehose :) you would have to be VERY careful....[/QUOTE]
    I like THIS analogy!
    no offence,
    Matthias
    [/QUOTE]
    none taken, not seriously anyway.
    ____________________________


    [Edited by blipndub on 01-02-2001 at 04:52 PM]
     
  17. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    I think the problem of having too much power is not too common among bassists... :D

    But there is such a device for guitarists. I believe it's made by Marshall and is called 'power break' (or maybe 'power soak') - but I don't know how it works for bass and I think it isn't cheap either.

    BTW a mixer input has a very high impedance compared to a cabinet. And I still believe that this should not work, but read my signature ;)

    Matthias
     
  18. your signiture sums it up perfectly!

    Power Soak is actually what I was referring to. I don't know much about the device. Some guy was telling me about it using a Marshall and that the device allowed for the tubes to get really overdriven without the excessive volume. I wonder if a simple volume control, like something you could pick up or build from Radio Shack would work.

    Here are some consumer reviews of the Power Soak from Harmony Central

    http://www.harmony-central.com/Effects/Data/Scholz/Power_Soak-01.html

    [Edited by blipndub on 01-03-2001 at 10:11 AM]
     
  19. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    the problem is: volume is caused by (electric) power; if you don't transform the power delivered by your amp into volume (=air movement) you have to 'get rid' of it in another way: heat.
    this means you need a device wich can transform lots of power into heat without melting down, smoking off or whatever - and without influencing the signal (=your sound) too much. i guess that's why it has to be more than a simple potentiometer.
    but as i said, that's a guess.

    Matthias
     
  20. I don't think using an attenuator would work unless there were extremely low power levels involved (ie less than 1 watt). attenuators like the ones on the tweeters of bass cabs might be good for 10 watts. Using a load resistor would work for a while. You'd need a very large 8 or 4 ohm wirewound resistor, preferably with heat sinks!! Using a constant load on an amp is not a good thing in the long term though, it will eventually damage the amp. This is what happened with the early power brake things. Speakers are a reactive load, which means the impedence changes with respect to frequency and cone movement. The impedence changes because the movement of the coil of the speaker in the magnetic field produces a current in the opposite direction to the current from the amp. So the designated impedence on a speaker is a nominal impedence, not the absolute impedence. Amps are designed to be used with reactive loads and using them with a constant load will eventually damage the amp. The new power brake things have circuits in them that model speaker behavior so that they won't damage the amp. I also think that putting a constant load on the outputs of an amp would have an adverse effect on the sound of the amp, but I'm not far enough into my engineering degree to quantify it! Listen to Matthias, he makes good points:D