Amps; open and closed backs ???

Ray Anderson · Post by **Ray Anderson** » 18 Jun 2011 3:43 am

Hey guys Mr. Greenhorn here, I have a question on what's up with open and closed back amps? What does that have to do with tone? And what is the best choice? Really would like some input on this before I buy another amp.

Jason Hull · Post by **Jason Hull** » 18 Jun 2011 4:04 am

Neither is better than the other, just different.
I like my closed-back amp because it has better bass response. I don't know how to describe the difference in sound, other than to say that open-backed amps sound more...open.

Tommy Boswell · Post by **Tommy Boswell** » 18 Jun 2011 4:20 am

In my experience most guitar amps are open backed.

I can tell you that if you are interested in trying different speakers in your amps, then open back is the way to go. That's because any time you put a speaker in a closed box, it has to be the exact right one to work acoustically with the volume of the box and the size of the ports, if any. With an open-back amp you only need to match the impedance and power-handling capability of the speaker.

Ray Anderson · Post by **Ray Anderson** » 18 Jun 2011 4:23 am

Are you saying that an open back would be more suitable in close quarters and closed for longer distance and more direct. Would that be a proper analogy?

Jack Stoner · Post by **Jack Stoner** » 18 Jun 2011 4:46 am

Not really. I have an open back and the comments I got from another steeler that was sitting in the back of the room was that it had very good "projection".

A speaker or how it's housed is only one part of the overall sound.

Georg Sørtun · Post by **Georg Sørtun** » 18 Jun 2011 5:01 am

Measure the shortest distance between front and back of speaker membrane for a given open-back cabinet, and compare to speed of sound through air. You'll see that below a certain frequency the more and more non-directional positive-polarity vibration for lower frequencies coming out of the front, gets attenuated (canceled out) by the reverse-polarity vibration coming out of and around from the back.

In short: regular, "normal" size, open-back cabinets can't deliver really low frequencies, no matter how big (size) the speaker or how much effect the amplifier can deliver to the speaker.

Closed-back cabinets have a different "deficiency", in that the cabinet must be very large to prevent the air "locked" inside the cabinet from trying to hold the speaker membrane stationary. Takes a lot of amp-effect to move/vibrate a large membrane in a small, closed, cabinet, as the air inside has to be compressed/depressed so especially low frequencies gets attenuated and effect burned off as heat.

Larger and more efficient closed cabinets are impractical for anything but PA systems, as few if any musicians want to haul the necessary-sized cabinets around.

Smaller and more "handy" closed-back cabinets that sounds good for guitar/steel, are usually not really "closed", as tuned "bass-pipes" and/or "bass-holes" in one form or another create natural delays and phase-shifts to "increase the distance" between front and back of the membrane(s) in a compromise between size and low frequency reproduction capability. With the right tuning towards the lows, quite small speaker element(s) in a small, closed-back, cabinet can be made pretty efficient.

For a guitar or steel amp the attenuated lows in regular open-back cabinets usually isn't a problem, as guitars and steels don't go very low into the bass-range. The natural and uneven slope towards lows and the "phase-play" and "break-up" in the lower region, is also what most players have become used to and "play on" - it is part of the "natural sound of the amplified instrument" for most.

What sounds best - open or closed or somewhere in between - depends on the individual player's ears. Whatever you like, is "best" for you.

Dave Grafe · Post by **Dave Grafe** » 18 Jun 2011 8:28 am

There you go, Ray, Georg has the subject matter well covered there with not much left to say.

My own preference when using a single amp/speaker is to go open backed, however when I use my Fender BDR rig I stack the open-back amp on the closed-back extension cab and get the best of both worlds.

Jonathan Cullifer · Post by **Jonathan Cullifer** » 18 Jun 2011 9:14 am

I have heard closed-back cabs that sound excellent from the side or rear (including my own). The cabinet's construction and materials have a lot to do with it.

I am a fan of bass from the cabinet, as opposed to bass from the speaker itself, and the cabinets I have fit that well. I also like the cabinet construction of the Nashville 112.

Ray Anderson · Post by **Ray Anderson** » 18 Jun 2011 10:08 am

I tell you of a truth I value Georges analogy of this matter, I just hope there will not be a pop quiz. LOL The reason I'm asking is I have a Marshall AVT 150 that I like very well and I was wondering what I would need to do to make it "steel" friendly, it si a single cab with 12" speaker and has great efx on it. So feel free to chime in and voice your opinion and suggestions. I have played steel through it but does not sound quite like my Nash 400.

Ray Anderson · Post by **Ray Anderson** » 18 Jun 2011 10:27 am

Another question for you guys, Does Sho-Bud make a good amp? I'm looking at a Sho-Bud with a JBL 15" D130. What's your opinion on this and what would be a fair price for such an amp that is in fair condition but plays strong.

Don Drummer · Post by **Don Drummer** » 18 Jun 2011 10:47 am

I use two Evans amps each with a 10 inch speaker. I suspect the closed back feature makes them sound quite "punchy" overall. I use them in all situations.

Dave Grafe · Post by **Dave Grafe** » 18 Jun 2011 12:33 pm

It is no surprise that the amps don't sound alike, Ray, as the Marshall is not really designed for the steel so much as shredding lead guitar, while the NV400 is specifically designed as a steel guitar amp.

The hotter pickup of a pedal steel guitar may be too much for the Marshall to cleanly digest at any useable levels, but then again it may be just the dirty sound you are after, not for me to say...

Georg Sørtun · Post by **Georg Sørtun** » 18 Jun 2011 7:09 pm

I kind of like my NV112s, but that amp goes deeper and suits my extended/uni PSGs better when I bypass the eq section with its fixed (built-in) high-pass filter. It's a decent little amp that is quite well voiced for my PSGs when run "flat".

FWIW: back in the days when I wanted "real power", I used a "horn-loaded, dual transmission line / slave bass closed construction" with a single 7 inch long-throw KEF speaker element driven by a 70 Watt TIM-free amp - all my own construction. Not big but heavy, it could deliver clean full-range (HiFi quality) sound at levels that made medium sized PA systems scream unintelligently and blow speakers.
(Transmission lines are rarely found in cabinets for el-instruments because they are complicated to build and tune and make cabinets heavy, but they can increase low-freq efficiency with a few hundred percent compared to all other methods.)
Would be fun to fire it up again one day ... or at least check if it has survived 15 years storage in the tractor-garage

Bobby Snell · Post by **Bobby Snell** » 19 Jun 2011 12:11 am

That AVT is only 150 watts SS power, so it won't have any near the headroom of the Nash400. And the AVT was designed to have the poweramp effect a saturated tube when the amp goes about 3/4 up on the master, so it is going to react quite differently in a band situation.

You may try a different, more efficient speaker, voiced more for full range...the stock speaker is maybe the weak link in these amps. The AVT's, with a pre-amp tube are good amps. I've got a 50watt single 12" combo that I can use for low volume gigs. If the gain and master are kept around half-way, it's fine for steel, but that's with no drummer or brushes, quiet guitars, etc. If you stacked two 150's that may equal a Nash 400

Tony Prior · Post by **Tony Prior** » 19 Jun 2011 4:13 am

Tommy Boswell wrote:In my experience most guitar amps are open backed.

You mean like Fender Bassmans, Bandmasters, Tremolux, Showmans, Dual Showmans ? New Bandmasters, Supersonics, Roc Pro, Marshall stacks... etc....

Just having fun !

Both good, both different, better bottom end on the close cabs, more Hi end on the opes...there is a ton of amps in both camps..

Jason Hull · Post by **Jason Hull** » 19 Jun 2011 5:41 am

Georg Sørtun wrote:Closed-back cabinets have a different "deficiency", in that the cabinet must be very large to prevent the air "locked" inside the cabinet from trying to hold the speaker membrane stationary.

The closed-back amp that I use (a Traynor) is completely closed and has an over-sized cabinet (relative to the one 12" speaker). It has a LOT more bass than my (open-backed) Deluxe.

The Deluxe has the same wattage, a slightly different circuit, and a smaller cabinet, so this may be comparing apples to oranges. Open-backed amps seem to work well in a greater variety of situations, but I pull out the closed-back amp when I want to punch them in the guts!

Tommy Bannister · Post by **Tommy Bannister** » 1 Jul 2011 4:09 am

hi guys and gals,Ihave a uestion that any one of you might be able to answer! PLEASE
The stock PEVEYnv 112's over here in England come with a 12" blue marvel speaker,I HAVE CONSIDERED PUTTING 12"BLACK WIDOW SPEAKERS IN BOTH MY TWO PV 112'S,DOES any one of you think that this idea would increase the tonal effect overal,ALSO WHY did Pevey decide to put blue marvel 12" speakers in the 112's in the first place? PEVEY USING A DIFFERENT MANAFACTURE OF SPEAKER IN THEIR AMPS DON'T ADD UP IN MY MIND!!!!!
thanks in advance for your replies get back please
anyone
tommy from UK

Tim Whitlock · Post by **Tim Whitlock** » 1 Jul 2011 8:07 am

I can't speak much to closed back cabinets. I can tell you that with an open back cabinet, like a Fender Twin, you get a lot of sound coming out of the back. With the right amp placement, the sound coming out of the back will reflect off floors and walls creating that big Fender 3-D effect. I struggled for a long time to figure out why on some nights my Twin sounded like God's own amp, when on other nights it sounded very ordinary. I believe the answer lies in the way the open back cabinet interacts with the available nearby reflective surfaces.

Side note: I once experimented with splitting the Twin into separate head and closed back speaker cabinets, but I missed the big sound from the open back combo cabinet and switched it back right quick.

Dave Grafe · Post by **Dave Grafe** » 1 Jul 2011 9:06 am

The phenomenon that Tim W. is what creates the big difference between the two types - open back cabinets "sing" while closed-back ones sound "tight" - not necessarily a scientific quantization but one that has stood the test of time...

James Morehead · Post by **James Morehead** » 1 Jul 2011 9:56 am

Tim Whitlock wrote:
Side note: I once experimented with splitting the Twin into separate head and closed back speaker cabinets, but I missed the big sound from the open back combo cabinet and switched it back right quick.

So why didn't you use open back speaker cabs and keep the "head" configuration?

My twin is in a head, and I have a pair of 15" cabs--open back. The weight is managable, yet the sound is there.

Place one or both speakers a few inches from the back wall, (if your stage is setup that way), gives you that 3-d effect Tim likes(and I do too).

Tim Whitlock · Post by **Tim Whitlock** » 1 Jul 2011 1:39 pm

So why didn't you use open back speaker cabs and keep the "head" configuration?

There were some other factors. I had some closed back cabinets laying around so I tried them out first with the head before committing to buy or build new ones. I put the head in an old Dual Showman Reverb cabinet but it was very tall and bulky by itself. The weight of the combo was not really an issue - mostly I thought I would like having the head next to me so I could make control adjustments on the fly. I guess I could have removed the back panels off the speakers but in the end I found the whole process cumbersome and went back to the combo. The combo is a little easier to get in and out of the van and takes up a bit less room.

James Morehead · Post by **James Morehead** » 1 Jul 2011 2:32 pm

Hey Tim, Yeah, I can dig the hassel.

My cabs are small and easy to toss around. I love my head cab next to me on stage--easy to tweek, and I set an extension cab up as my monitor and mic it. If we have a big stage, I bring both cabs. The downside is an extra piece to carry, so an extra trip to the van.

The combo is alright, but I hate to jump up to tweek it. Hate to lift it, too. So pros and cons.

Tommy Boswell · Post by **Tommy Boswell** » 2 Jul 2011 6:17 am

Tony Prior wrote:
Tommy Boswell wrote:In my experience most guitar amps are open backed.

You mean like Fender Bassmans, Bandmasters, Tremolux, Showmans, Dual Showmans ? New Bandmasters, Supersonics, Roc Pro, Marshall stacks... etc....

Just having fun !

Both good, both different, better bottom end on the close cabs, more Hi end on the opes...there is a ton of amps in both camps..

Yeah, I forgot to say "in my limited experience". However, my experience with tuned-ported (closed) PA speaker boxes is less limited...I've built a few. And I've read here on the forum about people swaping out speakers in closed cabinets. If you don't match the electromechanical parameters of the driver to the cabinet design, you're going to lose something, particularly the low frequencies.

James Yoder · Post by **James Yoder** » 2 Jul 2011 3:04 pm

Guys I'm going to build a open back cabinet for a 12 inch speaker. What should I use? I'm powering it with a MB200.