Home Made Guitar cab, one speaker is significantly louder.

[ioginy]

Hey all.

I made a Cabinet to go with an amp i built a while back and I have noticed a little issue. First, here are the specs:

Open back cabinet
Speaker 1. - 1964 Celestion G15M (50 watt, 16ohm, 55c/s bass response)
Speaker 2. - NEW Celestion G10 Vintage (60 watt, 16 ohm)
They are wired in parallel for the 8ohm amp load.

It first came up when I played the low E string on my neck pickup. I got a buzzing sound from the amp. After a little play and touch I found it was coming from the 10". There are no cracks in the cone and everything looks great, it just seems to be overdriving a bit.

After playing a bit more I noticed that the 10" was doing WAY more work than the 15". In fact, i could put my head up to the 15" and not have a problem but had to pull away when I went to the 10" because it was significantly louder.

I tested the g15M on its own before using it in the cab, and it works like a charm. The cone is in great shape and the sound is clear.

Any ideas what is going on, and how I might be able to fix it?

[antiquekid3]

Have you compared the DC resistance across one speaker to the other? My guess is the 15" has a much higher DC resistance than the 10".

Kyle

[ioginy]

I will give it a check, thanks.

If that is the case, how can I rectify the problem without actually replacing the speaker?

[antiquekid3]

Alright, here's the way I'm looking at this...you have two speakers. One could be more efficient than the other. One could have a DC resistance higher than the other. If the one putting out more sound has the lower resistance and is more efficient, putting them in series would help alleviate the problem. Since P=(I^2)(R), it will have less power than the larger resistance and less efficient speaker, which should help balance the power. So you might try putting them in series and sticking them on the highest ohm tap to see what happens.

If that doesn't work and you're more concerned with matching the speaker volume than efficiency, you might consider sticking a dummy load in parallel with the louder one.

Also, you could try hooking the louder one to the 4 or 8 ohm tap and the softer one to the 8 or 16 ohm tap. I don't know if mismatching the impedance is a good idea, but I don't think you could hurt it finding out...I'm pretty sure matching the impedance is only to maximize power delivery.

Good luck!

Kyle

[ioginy]

The amp has only an 8ohm out. The transformer has a 4ohm tap as well that I have not hooked up as an option (though is could be done very easily). Either way, both speakers are rated at 16ohm. As far as I know, mixing with the output resistance can do some serious damage to my output transformer. Wired in series would give me a load resistence of about 32ohms, which would probably do terrible things to my output transformer. I believe even individually hooking one to the 4ohm tap and the other to the 8 ohm tap could do some damage. If this can be done with guarenteed safety, I'll give it a whirl. Volume isn't my concern, I just want them to work evenly.

[ioginy]

I just did a little research and it appears that it is not a catastrophic situation running a 16ohm speaker from an 8ohm tap, however from a 4ohm tap it could cause some problems. it seems that there is about a full step of lee-way when it comes to mismatching, but it is not the greatest idea.

[ioginy]

Scratch that. It appears I have fallen prey to one of the common misconceptions about speaker load mismatching. There is a very informative thread here that anyone who is mixing loads should read.

http://forum.orangeamps.com/viewtopic.php?f=3&t=37029

[nyazzip]

i have wondered about this for years now, and never really got what i consider a straight or clear or scientific answer. seems like nobody really freaking knows. certainly guitar players don't.
i did successfully run an old 4 ohm Traynor into a 32 ohm load(unbeknownst to me) at low to moderate volumes for 6 months though...also i have run amps for short intervals with no load plugged in....
but yeah i have never really seen an agreement on this.
i sum it up like this, and it is probably erroneous: the closer you get to open/infinite resistance, ie no speaker plugged in, (or a 32 ohm speaker plugged in), the worse it probably is.

[Geek]

....Traynor....

N'uff said 8-)

Cheers!

[dhuebert]

Well... I read the orange amps posting about impedance matching. Sounds like a lot of BS to me.

Listen:

The output transformer, or any transformer for that matter, is like a big spring. You put energy into the spring by compressing it, and you let energy out of the spring by releasing it. Some of the energy is lost as heat. It is the same exactly with the transformer: electrical energy is put into the transformer and an electric field is produced, electric energy is removed and the field collaspses giving back the energy applied minus the energy lost as heat. This field collapse BTW, is how the coil in you car generates the voltage it does.

Now lets move to the front suspension of your car. The spring is preloaded by the weight of the car, in an output transformer we can think of this as the DC current that flows thru the primary winding at idle. A field is generated but the energy is never given back because DC dosen't change; hence: a preload. As you drive, the spring is compressed and uncompressed over unevenesses in the road. In an amplifier an AC signal is applied to the primary with the changing field generating a signal in the output winding. Aside: The shock absorber in the car is akin to the damping factor in your amp, they do the same job. (the British call shock absorbers "dampers" and you want the damping factor to be just right to make the suspension work).

Now lets think about load: 4 ohms, 8 ohms, 16 ohms. 1000 pounds, 1500 pounds, 2000 pounds. The effect is to make the springs operate over a different area of their compression range. If the load is well matched to the spring, everything will work as designed, the ride will be smooth. But if you put a 4 cylinder into a car designed for a Olds 455 or visa-versa the ride quality will suffer. The same applies to speaker impedances, if it is designed for 4 ohms and you connect 16 ohms to it, the sound quality will suffer. Within certain design limits tho, you won't break anything. With impedances, if you put a 4 ohm load on a 16 ohm tap, tube life may be shortened by the extra current required to drive the primary to the voltages demanded, but I doubt it. (if you halve the resistance, the current must double to produce the same voltage. V=IR) And if you put 16 ohms on a 4 ohm tap you are actually unloading the transformer, making its job easier. ( and the tubes might last longer, but I doubt it) The sound quality, however, will suffer.

Now we come to the damage part. The reason the coil in you car produces the 10s of thousands of volts it does is because the field is in freefall collapse when the points open. This is the same effect as disconnecting the load on a tube amp with a signal present. The secondary field goes into freefall collapse, the field lines cut across the primary and since there are so many more primary windings than secondary, thousands of volts are produced on the primary, zorching your tubes. Just like taking a big crowbar to your front suspension and levering out the spring; it will take a big chunk out of your head as it passes into the next county.

Finally: Look at the graph entitled Operation Characteristics here:

http://frank.pocnet.net/sheets/093/5/50C5.pdf

Plate current goes down as load goes up as you would expect. Power output peaks and drops off slightly with increasing load. And distortion drops to a minimun when load is well matched to output device. The range of resistance used here is 500 ohms to 5000 ohms; a tenfold difference. In our discussion of speaker impedance we are going from 4 ohms to 16 ohms, a fourfold increase, well withing the design limits.

Note also there is no disastrous region of the graph.

To conclude: Impedance mismatching is an inconvienience for your amp but accidental load disconnection is a disaster.

Next week: The Oscillator or Taking the Shocks Off Your Car.

2 weeks: Frequency Response or Driving Faster Over Washboard Can Make it Smoother.


Don