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Impedance matching

Posted: Mon Apr 04, 2011 11:40 pm
by MissionBrown
Hi Folks,

I built an isolation cab recently, my amp is a Princeton Reverb, which requires a nominal load of 8 ohms. My speaker cabinet that I place inside the isolation cabinet is 16 ohms.

They seem to work together ok, and a lot of sources on the net say it shouldn't ruin everything. I can crank the amp and not annoy the neighbours.

One thing that I have noticed is that when I've got my headphones on and the amp is still in the clean range, I get a little bit of distortion. It doesn't sound like tube clipping and my mic preamp is set well below the overload threshold.

Could the distortion be coming from the mismatch between expected output and actual load?
I haven't had a chance to try it with an 8 ohm speaker and am seeking other users impressions before I lay out some cash in the wrong area.

The Mic I've use is an SM57 which seems fine, though I did buy it 2nd hand.

Re: Impedance matching

Posted: Tue Apr 05, 2011 9:28 am
by olrocknroller
I'm no electronics guru, ;) but in my experience, it all depends on how beefy the output stage of the amp is. A lot of amps now are built with impedance-critical output circuitry, because it makes the amp cheaper to build. Just because it's a guitar amp does not mean it's a heavy-duty amp! As a result of this, I stick to the impedance recommendations in the manual, and on the back of the amp. I often notice that the better (?) amps allow a range of impedances, even providing a switch to match the amp to your speakers...these are the ones I seek out when planning to connect to my own cabs!

My guess is that nothing will "blow" at 16 ohms, although it may not sound as good as it should, but at four ohms you could seriously fry the output stage...

Easy fixes...put a dummy 16 ohm load in parallel...that will match the impedance back to 8 ohms...

or build a new isolation cab for a twin 16 ohm speaker cab... :lol:

Re: Impedance matching

Posted: Tue Apr 05, 2011 10:27 am
by Dennisthe Menace
I'm not sure I'm understanding this correctly, but:

1) If you are using the SM 57 as a vocal MIC, then you will need to make sure that the MIC Preamp is
going into "EFFECTS RETURN," therefore bypassing the Fender's Preamp section. Anytime you take
a Preamp, and feed it into another Preamp, you will have 'overload signal.' This does not mean you
will have full over-driven signal, but you might experience the 'peachfuzz' tones through the cans.

2) You might want to check the metal 'wind'screen and check for any insane clogging from spit deposits :shock: .
....If the MIC checks out, and you DO HAVE the Preamp going into the Line In/FX Return input with the Preamp
set below the overload limit, well sometimes a cable not making a good connection can cause the same symptoms.
....Does the Preamp have a battery and if so, when was the last time it was changed? I'm sure you know all these
things, but every now and then, I catch myself still making some of these same mistakes, even after all these years! ;)

Re: Impedance matching

Posted: Tue Apr 05, 2011 1:48 pm
by Mr. Bill
Dennis, I think what he meant was that he put the mike inside the isolation cabinet to pick up the guitar amp output. But your overload suggestion is a valid point, especially inside a sealed box, where the mike is getting hit by sound from many different angles.

The output section of the Princeton is designed to see an 8 ohm load. By using a 16 ohm speaker, the two output tubes will now "see" twice the normal impedance load. This will cause the amp to work a little harder to make the same amount of output. I wouldn't think that this would be enough to cause the amp to distort, but that would depend upon how loud you were actually turning up the amp. If you turn down the volume control of the amp, does the distortion stop? When you turn up the volume, does the distortion just increase or does the signal break up in some other abnormal way? What sort of speaker cable are you using from the head to the cabinet?

Isolation cabinets are great for what they are, but they do add their own set of problems sometimes. When a speaker is played in a sealed box, there needs to be enough air movement inside to not interfere with the speakers basic vibration patterns. Best case is that the speaker's frequency response will just be limited, worst case is that there will be a reverse voltage produced by the speaker that is fed back into the amp's output stage.

Olrockandroller's suggestion of adding a 16 ohm resistor will fix the problem if it is being generated by the mismatched load, and by way of heat, it will also reduce the overall volume of internal isolation speaker as well.

Re: Impedance matching

Posted: Tue Apr 05, 2011 4:11 pm
by MissionBrown
There is a fair bit of room inside the box, and it doesn't sound like the amp and speaker are inside a small area(unlike my previous attempt back on 1999). And it's definitely not air tight.
The cab dampens the overall volume from the amp so that I can record a decent guitar sound without making all of the windows in the house rattle. It's far from silent.

With the 16 ohm speaker, I get a decent sound at about three on the volume.
Actually it's nice at 2, but it won't do any overdrive if I hit harder on the strings.
The distortion is sort a crackle, it's kind of like the sound of a mixing desk overload, but that's been ruled out as I can set the mic preamp volume to a level where the amp is barely audible. I've only tested it with my Zoom H4N recorder as it's just easier to travel with headphones between rooms. I'll need to try with my mixing desk too.

The speaker cable is 6 meters of a generic 2 core from the amp to the cab: and inside the cab I've used 3ft of hifi speaker cable which I figured would be ok over this short of a run.

I have noticed the crackle when using another shorter (6ft) speaker cable from the amp to the box. This problem isn't evident when I use my 5 watt head, but it doesn't get as loud and it's got provision for a 16 ohm load. I may try running it on the 8 ohm tap to see if that's where the problem lays.

Another option I haven't tried is using a different Mic. The only other one I have is a Rode NT2a condenser. It's got a DB pad and has been reported to suffer no distortion at over 110db.

There was an issue with one of the Mic jacks, so I bypassed that and soldered a new lead directly to the one which leads to the outside world. Currently I am leaning toward swapping the 16 ohm speaker for an 8 ohm from my local 2nd hand dealer. They offer a 3 days return period if you don't like what they've sold you.

Lastly I might have a problem with my headphones. It's not evident on the computer, but I can't rule it out yet either.

Re: Impedance matching

Posted: Wed Apr 06, 2011 10:20 am
by Mr. Bill
It sounds like you've spent a lot of time working this out. Keep eliminating things one at a time and you'll figure it out.

I agree, the fastest test would be to swap for an 8 ohm speaker, as this would seem to be the most likely suspect. Is there any way that you can open up the box and listen to the 16 ohm speaker directly at high volume to see if the distortion is actually coming from the amp/speaker?

The 5 watt head (I assume class A single ended type) may not react the same as the PR to a load mismatch.

The speaker cabling all seems fine, just try and keep the cable runs isolated from the input lines.

Re: Impedance matching

Posted: Wed Apr 06, 2011 6:26 pm
by MissionBrown
I'm going to swing by the music shop on the way home from work to pickup a loan speaker and give it a test.

I could test the amp with the lid open, but I think the rattling of the cutlery in the kitchen might drown out the distortion :lol:

It's in there because our doors are narrow and the path to the lounge is partially blocked by a cupboard. The other reason is that the floor in there is slate and has no space underneath to vibrate (unlike the lounge or bedroom).

Will let you know how it goes :)

Re: Impedance matching

Posted: Wed Apr 06, 2011 9:20 pm
by MissionBrown
Does anyone see the benefit of going down a size of speaker?
I have a 12" extension cab, but I'm wondering if 10" would move significantly less air and reduce the sound pressure level?

Also, does it need to be attached to a baffle or can I just throw the driver in the box as is?

Re: Impedance matching

Posted: Thu Apr 07, 2011 3:07 am
by MissionBrown
ok, I've done some more experimenting.
I ended up using a 10" 8 ohm G10D-25 Celestion speaker with no baffle(it was just wedged into place by a couple of pillows, and didn't sound too bad.
Still quite bright for my taste and at 25 watts it would probably burn out if I had the volume up for a long time.
After about 15 minutes, the magnet was slightly warm to the touch.

The crackle was still evident with my portable recorder, so I plugged into my mixing desk.

Channel 1
No sound.
Try channel 2
No sound
Channel 3
Some fiddling with the mic lead and I get a sound.

So it seems that my mic lead needs a little bit of maintenance.
They're a few years old, haven't seen heaps of action, but they've been used a bit over that time.
I might have to open my mixer up and clean the mic sockets too, this house is bad for dust and damp :(

I put the 12" cab back in the box and Immediately it sounded a little weird. I'm sure it's because the DC resistance is about 18 ohms over the length of the cable and connectors.
It's like the OT just isn't in love with the mismatch and possibly the cabinet isn't snug in the padding.
This config did not bring the crackle back.

So I'm leaning toward a Weber Z matcher and/or building a baffle for a 12" speaker so there's a little more room for the air to move inside the cab.

Re: Impedance matching

Posted: Thu Apr 07, 2011 10:35 am
by Mr. Bill
If the magnet got hot, I'd investigate the possibility that the amp is going into high frequency oscillation. If the output wires are too close to an input line, etc. the amp can start to oscillate at frequencies too high to hear, but not too high for the amp to reproduce. This could also cause the distortion that you are getting.