New Amplifier Design and Configuration - MAYBE

randyc · #1 03-27-2010, 10:22 PM

OK, there's really nothing new under the sun, so it's said. But bear with me for a few moments for the sake of discussion ... A couple of days ago, one of our forum members (LPDeluxe) generously gave me a nice old Dynaco vacuum tube stereo amplifier and two studio monitors.

My intent was to convert the Dynaco, which my memory suggested was a low-distortion 70 watt amplifier, into a high quality guitar amplifier with a 15 inch speaker. That speaker is something that we discuss here frequently and with enthusiasm. I wanted to make the conversion, build a cabinet (with speaker, of course) and then return the resulting amplifier to LPDeluxe as an expression of gratitude for the studio monitor loudspeakers. That's the introduction to the story.

One of the many drawbacks of these old tube stereo amplifiers is the aspect ratio, as seen from above. These devices are not as narrow as a guitar amplifier chassis, whether used in a combo or in a head configuration. Another drawback is the weight - TWO output transformers instead of a single one - in addition to the normal power supply transformer.

I evaluated some of the "Thiele-Small" numbers for several fifteen inch speakers so that enclosure dimensions could be estimated. A sealed or ported enclosure would be very large (like my infamous Showman cabinet) which - surprise - also translates to heavy. I am not expert at speaker or enclosure design so I simply applied the manufacturer-provided parameters to a freeware application, downloaded from the net. (I actually used two free programs, as a sanity check.)

An open back configuration is about the only option for size and weight considerations. Personally I've not seen 15 inch speakers configured in this way although they have been manufactured, generally for steel guitarists. Most large loudspeakers are in sealed enclosures probably due to the desire for enhanced bass response, which isn't necessary for guitar. (My 12 inch open-back combo enclosures - single and dual - seem OK to me in that regard.)

There is an immediate weight penalty of 32 pounds for the Dynaco amplifier which suggests a two-piece construction combo: head plus cabinet. The two components, carried separately, need to be latched together, quickly and easily. The reason for latching them together will be clarified soon.

I envisioned a neodymium (light weight) 15 inch loudspeaker, like a Jensen MOD15-120 on TOP and the Dynaco amplifier on the bottom of the stack (keep the center of gravity low, where it should be). The Jensen has about the right characteristics for an open back configuration, I think, and it's inexpensive for a 15. The ultra-linear configuration of the amplifier and the crisp response of a neo speaker should be a fine combination for clear jazz articulation (and chord melodies) without distortion.

The most compact enclosure design occurs when the speaker is angled at about 15 to 30 degrees downward - that angle accommodates the typical stereo amplifier outline dimensions best from packaging considerations. This is also a good configuration for adjusting the amplifier level slightly higher than normal for those who enjoy a slight amount of distortion or compression (increased amplitude gets the vacuum-tube amplifier sound while angling the speaker downward tends to reduce the volume). This might also enhance bass response of the amplifier although I don't see that as a problem.

In a situation where a little more volume may be required, the assembled head/cabinet can be tilted back (like the chrome-plated legs on Fender Twins) which focuses the loudspeaker straight ahead. The sketch below indicates what I had in mind.

At about this point in the concept, John (LPDeluxe), e-mailed me the model number of the amplifier and I researched it on the internet. Rather than a 70 watt amplifier, this model is a 35 watt device. That modified my thinking regarding the complexity and weight of a two-piece amplifier with about 2 dB more output power than a Deluxe Reverb. I talked this over with John and he made the point that the amplifier was unconditionally mine; he expected nothing in return and in fact had plenty of amplifiers already, thank you.

After looking over the schematic and noting that the configuration was the desirable ultra-linear circuit (which requires special output transformers) I decided that I just couldn't modify this old beauty for any reason whatsoever. Further, I decided that the Dynaco, at 35 watts, would be a much better monitor amplifier than the 20 watt solid-state amplifier that I currently use. So I'm going to retire my current inexpensive monitor speakers and amplifier, replacing them with the two Carvin monitors and the Dynaco that John will be shipping in the near future.

I love the idea, I know that this is going to be a good-sounding monitor combination. It will be nice to divest the headphones which those of you who are studio-knowledgeable frequently tell me are not a good idea anyway. But I'd started to feel some enthusiasm for a new amplifier mechanical configuration. So I'm thinking about another new amplifier design - one that would be lighter and slightly more powerful than the Dynaco. (Some may recall the "Mini-Bassman" that I designed a few years back and posted on the forum last year. An upgraded version of that amplifier would be a nice fit for the new mechanical design that I'm considering.)

The current dimensions of the concept amplifier are: overall height 27 inches, width 18 inches. The height can be reduced since I don't have to accommodate an existing amplifier design. Weight for the speaker enclosure is estimated at around 20 pounds and slightly more for the amplifier enclosure. The two pieces can be latched together and tilted, as mentioned above, for versatility.

The cabinet material would be a laminated structure consisting of 1/2 inch MDF (or particle board) and 1 inch foam. The combination is fairly light and the foam will assist in dampening resonances (although the open back shouldn't promote too much internal excitation). I've checked price and availability for the foam and it's reasonable. I can laminate the two materials and then machine them as one piece.

Whether or not this project will ever be constructed is not known. It's interesting to me but - like John - I have an abundance of amplifiers and since I no longer play to an audience other than wife and kids, any number greater than one is excessive.

This post is intended primarily for entertainment and to stimulate conversation regarding the subject of lightweight amplifiers and different methods of construction. I may design a 70 watt (or thereabout) amplifier that would be suitable for the project, although it may not be built. If I do so, I'll post it here as a design example and go through the various design steps and procedures for your interest, including manufacturers' part numbers.

PS: if the amplifier looks like it's going to tip over backward, it's because the center of gravity hasn't yet been precisely established. It will mainly be determined by the transformer locations since the loudspeaker is very light ~ 10 pounds.

billkath · #2 03-28-2010, 07:39 AM

Quote:

The most compact enclosure design occurs when the speaker is angled at about 15 to 30 degrees downward - that angle accommodates the typical stereo amplifier outline dimensions best from packaging considerations. This is also a good configuration for adjusting the amplifier level slightly higher than normal for those who enjoy a slight amount of distortion or compression (increased amplitude gets the vacuum-tube amplifier sound while angling the speaker downward tends to reduce the volume). This might also enhance bass response of the amplifier although I don't see that as a problem.

Have a rethink about this part, Randy. The reason that speakers don't face down is not aesthetics-it's reflections. The LAST thing you want to do is point it at the ground. Not only will the sound bounce off whatever surface you are pointing it at, but will absorb some frequencies whilst boosting others on the scatter , depending on the material it is facing. In No Way will it absorb sound-the ground. All it'll do is make your amplified sound not be true to what you think you should be getting, and I know that is hugely important to you as an EE.
What you perceive as a bass bump standing beside the combo will NOT sound like that a couple of feet away-that's just a static node.

The reason that some guitar amps let you tilt them is precisely because sound is getting absorbed and scattered by people at shows with low stage areas. It's not an ideal thing to do, as the sound reflects off the ceiling, but careful tilting can make it a little better-aiming it between the crowd and the ceiling as much as possible.

randyc · #3 03-28-2010, 11:50 AM

The speaker is only tilted fifteen degrees It's an old trick, Bill, the idea originated with Neil Young about fifty years back. I used the same trick back during the eighties when I had only ONE amplifier to do it all ... pointing my Bassman cabinet down toward the floor or back toward the wall allowed a bit of distortion for pop music without overpowering the audience. (Also enhances bass response. Tone controls can make up for any deficiencies.)

May not be a good idea for recording but works fine for live performance. Of course, there's an easy fix if the result isn't satisfactory - just tilt the thing back at twenty degrees so the speaker is at a normal angle - that's really the intentional configuration. See the sketch on the far right in the original post. (The reason wasn't for accoustic considerations, the speaker tilt is to cut a few inches and a few pounds off the cabinet.)

Thanks for your suggestion !

Cheers,
randyc

lpdeluxe · #4 03-28-2010, 12:11 PM

John here. I've got the Dynaco in the back of the Toyota for the trip to the local shipper tomorrow. As I've said to RC before, I think it'll make a dandy monitor amp -- when it was part of my living room stereo, it had wonderful stereo imaging. I looked it over a little while ago and noted that it has a TMB tone stack, as well as "low," "ceramic" and "high" phono inputs. Quite a piece of work. There's also a little silver sticker on the back that says "Factory wired" -- I seem to remember that they were available as kits, and, in fact, that early Sunn amps used Dynaco kits to power their bass gear (later, after Sunn grew large enough to hire their own engineer to design circuits, they continued to buy their transformers from Sunn).

It's a hefty one, too: substantial transformers in a small package. Randy, I'm really interested in seeing what you come up with.

billkath · #5 03-28-2010, 12:11 PM

Nah, mate. That design will give you nothing but severe comb-filtering. The speaker baffle being that far back from the front of the cab, and angled like that is only going to give you hardship. Any sound is going to bounce straight back up off the bottom hard edge of the cabinet, causing severe interference patterns and comb filtering. Apart from that, the angles at the back of the cab are going to reflect back the 180 degree out of phase sound from the back of the speaker, even with an open-back design, and cause even more phase cancellation. As the angles are not parallel either, any reflections that do come through are going to be wildly out of phase, giving you tons of nodes.

If you want to angle the speaker, and I don't recommend it, but if you do-you'd need to angle the sides of the cabinet to the same angle that you are going to place the baffle. That's mean you overhanging the front of your amp, otherwise the amp is going to cause the same thing.

First reflections, Randy-it's all about first reflections. Your bassman is not designed like that.

billkath · #6 03-28-2010, 12:31 PM

Here's your pic, with a roughout of the reflections. Notice the rear relections as well, remembering that they are 180 degrees out of phase.
The relut is uncontrollable peaks, troughs and phase cancellation-awful to listen to.

15_amp.jpg

randyc · #7 03-28-2010, 02:05 PM

Hi Bill,

Many thanks for the picture, you didn't have to go to all that trouble. I understand angles of incidence and angles of reflection - they taught me all of that stuff at the University some time back.

If you were to draw the same angles on any older sealed Fender cabinet, you might conclude that they wouldn't work very well but somehow those guys muddled through, though not a single designer had an engineering education at the time.

The Showman cabinet is a good example: 18 inch JBL, the magnet is about 2 inches from the bare back of the cabinet (not an ounce of absorbent material, no baffles, no port). You'd think that the rear reflected wave would virtually cancel cone movement because the dimensions are so shallow. But it works just fine and we're still using a lot of those old designs today.

BUT because you're enthusiastic about this and because it's not unknown for me to err, here's what I did:

I have two small (1 x 1 x 2 feet) Marantz monitors. I angled them (on top of the oak cabinet where they are placed) similar to the angle in the sketch and listened to the result.

Frankly, I can't discern any difference at all (although I'm sure that it could be measured). As Noted previously, the human ear is not a good measurement instrument and the human brain is not a reliable processor.

Nevertheless, I do appreciate your responses and I'm definitely not disinterested in them. That was the entire point of posting the sketch: to elicit the opinions of others in case I miss something. (Which, BTW, seems to happen more and more often these days, LOL !)

Also, please note that the entire inside of the speaker enclosure is foam, not fibreboard - the entire structure is a laminate to conserve weight and dampen vibration. I think that I mentioned that in my first post but perhaps I didn't emphasize the concept adequately.

(BTW, a member of a U.S. forum pointed out the similarity of the concept to the old Fender 75 - which is a 75 watt tube amplifier with an open-back 15 inch speaker enclosure. He said that it was the cleanest amplifier he'd ever played. The 75 didn't have an angled speaker, it was a conventional LARGE combo. I hope to do better than the 70 pound Fender. Speakers are lighter and so are transformers, not to mention two pieces instead of one.)

Cheers,
Randy

randyc · #8 03-28-2010, 02:08 PM

Quote:

Originally Posted by lpdeluxe

John here. I've got the Dynaco in the back of the Toyota for the trip to the local shipper tomorrow. As I've said to RC before, I think it'll make a dandy monitor amp -- when it was part of my living room stereo, it had wonderful stereo imaging. I looked it over a little while ago and noted that it has a TMB tone stack, as well as "low," "ceramic" and "high" phono inputs. Quite a piece of work. There's also a little silver sticker on the back that says "Factory wired" -- I seem to remember that they were available as kits, and, in fact, that early Sunn amps used Dynaco kits to power their bass gear (later, after Sunn grew large enough to hire their own engineer to design circuits, they continued to buy their transformers from Sunn).

It's a hefty one, too: substantial transformers in a small package. Randy, I'm really interested in seeing what you come up with.

I found the entire manual on the internet yesterday. It's a high-quality scan and I've reproduced (by hand) the schematic. It's remarkably close to the 40 watt design that I posted last year. Especially the power stage, which uses a dual pentode/triode, as in my design, rather than dual triodes (every other guitar amplifier ever made).

I'm anxious to hear the Dynaco through those Carvins. Maybe one of my kids can use the little solid-state monitor amplifier and my current speakers. Both are so very proud to be in their own apartments this year (instead of in the dorm).

Cheers John,
Randy

P.S. sent you a couple of e-mails regarding the Club situation.

billkath · #9 03-28-2010, 02:43 PM

Quote:

If you were to draw the same angles on any older sealed Fender cabinet, you might conclude that they wouldn't work very well but somehow those guys muddled through, though not a single designer had an engineering education at the time.

The guys who designed them knew enough about speaker about speaker and cabinet design, though, Randy. They knew enough about baffle construction not to place it back in at an angle,and to absolutely keep the leading edges-all the edges- of the speaker away from first refletion points. That's why no speaker cabs are designed like that. Trial and error and math. 2 inches from the back doesn't matter as much as 1 inch from the front.

Quote:

The Showman cabinet is a good example: 18 inch JBL, the magnet is about 2 inches from the bare back of the cabinet (not an ounce of absorbent material, no baffles, no port). You'd think that the rear reflected wave would virtually cancel cone movement because the dimensions are so shallow. But it works just fine and we're still using a lot of those old designs today.

Absorption and the rear of the speakers are not the most important thing. Cabinet design is all about compromise. Any absorbant material is only going to absord certain frequencies anyway, which is why most cab designers only put in enough to kill any shrill harmonics. In any event, you'd only use absorbant foam in ported design-not sealed units.

Quote:

I have two small (1 x 1 x 2 feet) Marantz monitors. I angled them (on top of the oak cabinet where they are placed) similar to the angle in the sketch and listened to the result.

You are missing the point. They already in cabinets. If your cabinets were designed like those, and you just wanted to point the cabinet in a different direction I'd have just been warning you that using the speaker cab like that can have unforseen downsides regarding sound elsewhere in the room. That's not what I'm talking about-it's the basic design that is fatally flawed.

Quote:

Frankly, I can't discern any difference at all (although I'm sure that it could be measured). As Noted previously, the human ear is not a good measurement instrument and the human brain is not a reliable processor.

At normal room monitoring level-83dB, you wouldn't. It's precisely because of our ears that these things matter.

Quote:

Also, please note that the entire inside of the speaker enclosure is foam, not fibreboard - the entire structure is a laminate to conserve weight and dampen vibration. I think that I mentioned that in my first post but perhaps I didn't emphasize the concept adequately.

I did note that. That doesn't affect the front of the speakers, and the large angle and reflective surface. Dampening viration will in no way stop sound waves travelling around the inside of the box in the critical 80hz-3.5khz range, and bouncing through the cone to cause comb filtering.

randyc · #10 03-28-2010, 02:49 PM

Bill - I think you're getting overly concerned about:

(1) a project that may never happen

or perhaps

(2) an assumption that I am ignorant of these concepts

Your criticisms have been noted with respect, OK?

randyc · #11 03-28-2010, 04:09 PM

Quote:

Originally Posted by a member of the U.S. forum

Have you heard of a cabinet designer by the name of Bill Fitzmaurice? If not, you might enjoy viewing his site: XF Guitar Cabs

Just got around to looking at that site and I was astounded to see the identical speaker configuration that's in my favorite jazz amplifier: this Sunn "Beta" that I purchased new in 1981 ! In fact the above 15 inch configuration is similar to the Sunn (and to Fitzmaurice's enclosures) except they are dual speaker configurations.

Another example of how there's nothing new under the sun. Or in this case under the "Sunn" since the Beta preceded Mr. Fitzmaurice's design by several decades.

PS: this is a nice followup on LPDeluxe's observation that Sunn used Dynaco power amps in their initial product offering, a fact that I've also noted in the Sunn corporate history.

randyc · #12 03-28-2010, 05:52 PM

OK, it's raining and it's not football season so I decided to make a couple of measurements regarding the concept. Didn't have a spare 15 inch open-frame loudspeaker handy but I do have a 6 inch version (thanks to ZT Amplifiers, who sent me a spare loudspeaker when the "Lunchbox" review was being conducted).

With the aid of a few machinists tools and accessories, I set up the speaker at approximately 20 degrees tilt to my workbench. I placed a foam pad under the speaker (1/4 inch instead of 1 inch - scaled down proportionally with the speaker being tested). The setup was configured (as near as I can tell) to the concept drawing of the 15 inch enclosure.

I didn't enclose the speaker because too many variables would be introduced - I'm only trying to determine the effect of the tilt and the "lip" under the speaker.

Here's a photo of the setup, SM-57 microphone positioned about 6 inches from the speaker cone and a laptop computer handy to record the data:

Here's a photo of the reference configuration - speaker in free air with microphone positioned as before:

And after processing the data, here is the difference between the two configurations, from 100 Hz to 10 kHz:

There is a measurable difference between the open air configuration and the tilted configuration with foam pad and particle board (the workbench surface) underneath. It's not of practical significance, in my opinion. The variation is uniform and averages to a level that suggests little audible difference between the two responses. (Recall that most people can just detect 3 dB difference in power levels while those with exceptional hearing can detect perhaps 1 dB.)

It's nice to conduct experiments like this quickly when the correct components and equipment are handy - rather than spending several hours constructing a computer model for simulation (although that can be fun too).

PS: the small "ripples" in the overall frequency response are room reflections - the measurement was made in my lab which is only 12 x 12.

goshawk · #13 03-28-2010, 07:07 PM

OK, I'm not an engineer or any kind of designer, but I do have a question looking at the design. What are the implications, if any, of having the speaker angling down toward the components like that? Is it going to introduce more vibration than in a comparable design with the speaker on the bottom? And will that have any impact on the tubes and solder joints?

randyc · #14 03-28-2010, 07:34 PM

The speaker isn't angled toward the amplifier, it's just very slightly off vertical (about 20 degrees) while the amplifier is directly beneath the speaker cabinet. With amplifier and speakers in separate enclosures, the vibration effect on the amplifier would be considerably less than a typical combo, where the tubes are directly behind the speakers in most configurations. (Also the enclosure is intended to be laminated from foam and MDF, so it should largely be self-damping.)

A possible concern in this particular configuration - if the amplifier is further reduced in size - is heat dissipation. The typical open back combo gets good convection circulation, because of the internal volume of the cabinet. A head configuration doesn't stay as cool. If I strove for the most compact layout, the transformers could be smaller but might have to be cooled to maintain good performance. The addition of a pair of small, quiet fans would take care of that problem.

Good questions, thanks for asking.

(These old vacuum tube designs can be quite rugged. The Russians were still flying vacuum tube electronics in their fighter aircraft as recently as the late 1970s while western military aircraft had started transferred technology to solid-state a decade and a half previously. FWIW, the transistor was invented in 1948.)

billkath · #15 03-28-2010, 08:06 PM

Randy, Mate-I just don't want you to waste time and money, that's all. I'm really not trying to get on your case-I'm just giving you some advice. I'd be horrified if you took offense-I know well your background and capabilities. I myself took my degree 31 years ago, and have been a professional studio engineer since then. It's just advice. You take it or you don't-I'll not be upset.

As to specifics-

The pic of the first amp--That Sunn -is a stereo speaker configuration that has been designed to be used at angles. It-like Bose speaker arrays- has been especially designed with SPECIFIC angles to take ADVANTAGE of phase cancellation and reinforcement. It only works with more than one speaker. The Leslie speaker is another version of the two speaker/phase alignment concept. The 2 (or more) speakers HAVE to be at a set angle-the same angle that we use for X/Y micing congurations. You can't do it with one speaker, however, and that Sunn wouldn't sound too clever if one speaker was blown-disconnect it and see-put a dummy speaker load on first, on the speaker you disconnect (You know that- ~I just put that in in case anyone not versed starts running valve amps without a load).

Pic 2-unless you are set up with a cabinet, you will not be able to recreate what a speaker cabinet does to the amplified sound. No offense, but it's not going to tell you anything, apart from how the speaker performs. All the sound, front and back, of the speaker are dissipating throughout the surrounding area. What you have, in effect, done is make an infinite baffle, front and rear. The same concept as an open backed cabinet, but applied to the front as well. The energy is dissipated throughout the room. No nodes of significance will be found.

The resonances of the baffle, sides and top of the cabinet play a hugely significant roll in how a cabinet "sounds". The speakers, on the other hand, not so much.

Now-I have no horse running in this race. It matters (as Billy Connolly says) not a jot to me if you want to go ahead with this with your original design . All I'm saying is that if you were to design a standard, rectangle or square shaped open backed cabinet, with the speaker baffle in standard configuration, and invented a device to tilt the whole cabinet forward or back, you'll successfully do what you want to do.
If you'd rather do it your way, that's fine with me. I'd just ask you to do one thing-once you have the amp built, test it by A/B ing your cabinet design with any standard cabinet. You'll soon see what I mean.

randyc · #16 03-28-2010, 09:05 PM

HI Bill,

No I don't take offense, it just seems that you and I are at cross-purposes and I wonder why you're spending so much time on this trivial discussion?

Personally, I found the idea interesting because:

(A) The original thought was to construct an amplifier/cabinet for my generous friend, John, to repay him for the pair of studio monitors.

(B) It's fresh in my mind and I'm always interested in any concept that can make an amplifier smaller and lighter.

(C) I've been considering a 4 ohm cabinet with a single speaker to mate with my Bassman for several years (the dual D-130s were long ago converted to PA speakers) and my only remaining uncommited cabinet is the 8 ohm Showman. This might be the appropriate vehicle.

FWIW, you're incorrect about the Beta, it isn't a stereo amplifier like my Gibson RVT-79 (which IS a stereo amplifier with angled speakers but they are angled in the opposite direction from the Sunn configuration). You're also incorrect about why the Beta's speakers are angled. It's to increase the angle of projection; there's no intentional phase cancellation or phase enhancement in the design. The two radiating patterns slightly overlap at the center but the intent is simply to increase the amount of area over which the volume level is reasonably consistent. As Fitzmaurice puts it: " ... widens high frequency horizontal dispersion by up to 40 degrees compared to a flat baffle".

But let's not get off the track, I didn't suggest that any advantage could be gained by an angled single speaker except for the ability to "crank" the amplifier a bit because of the volume that is lost by inclining the speaker downward. The concept of tilting the speaker and considering a laminated, self-damping material was to produce a smaller and lighter cabinet.

As I stated in our first exchange on this topic, although there will be measurable differences, I'm convinced that there will be minimal audible differences in the sketched cabinet configuration. And you are convinced that the result will be <ahem> less desirable - so there it is.

I don't mean to sound overly defensive about my modest ideas but how I occupy my time, my thoughts and how I spend my disposable income are very personal decisions

I did invite comment and you provided quite a bit which I respectfully considered. I think, however, that we've reached the point of diminishing returns, don't you?

I simply don't know you well enough to accept suggestions without question, no offense. Perhaps that day will come but in the meantime I'll probably just muddle along like I've done for the past 65 years, LOL.

Cheers,
Randy

billkath · #17 03-28-2010, 09:45 PM

Just because it's interesting to me-that's all. I enjoy detailed discussion about things in this area.
No Worries-we'll leave it there.

randyc · #18 03-28-2010, 09:58 PM

OK, thank you, Bill.

You are obviously experienced in many areas where I have zero knowledge (but lots of curiosity). I hope that you'll share some of that experience when I get around to formulating the right questions. And thanks again for correcting me on the Fender input impedance error.

Cheers,
Randy

billkath · #19 03-29-2010, 07:13 AM

Of Course, and the best of Irish luck if you decide to go ahead with the project-I'm sure you'll make it work fabulously.

Talking of fabulous-This morning I did the sound design for a live music video for an up and coming young grunge rock band-Kidss, done in our Opera House. The production company said they'd lay on all the gear, and there wasn't any need for me to bring my OB stuff- They'd suppliment the theatre's gear with some bits more appropriate for recording. I was naturally a bit nervous-one person's idea of what's good enough is not necessarily the same as someone elses. I needn't have worried-top notch stuff.
Anyhoo-as nearfields they'd supplied a pair of Unity Audios "The Rock", instead of the normal pair of Adam's.
Oh-these are fantastic! Talk about accurate-the bass end was as tight as-a tight thing! Beautiful silky top end, solid mids.
Take a look at them, and a listen if you're browsing your local music store.
Unity Audio

Pretty cheap as well-about 1000 bucks street price a piece. Adams better run!

randyc · #20 03-29-2010, 02:52 PM

You see, that's what I meant about specific experience. I understood about one in three words of what you described; the rest was inferred but not comprehended

My knowledge of microphones is limited to this: use the SM-57s for the guitar amplifiers; use the SM-58s for the vocals. Pitiful stuff, LOL !

randyc · #21 03-31-2010, 03:33 PM

The project has morphed into the design of a 4 ohm, single 15 for my Bassman, as mentioned previously. I spent a considerable amount of time yesterday looking at 4 ohm 15 inch speakers (there aren't a lot of them, 8 ohms is far more popular).

After crunching numbers for a while, I found a Carvin speaker that looks perfect for the application - and about 60% of the price of the closest contender ! Best estimate for total cabinet weight is on the order of 32 pounds (presumes the use of particle board + foam laminate originally noted). Interestingly, according to the Thiele-Small numbers that Carvin quotes, I can make a sealed design very close to the size limitations I'd been considering for the open-back model. Will update if this starts looking real.

BigDaddyLoveHandles · #22 03-31-2010, 03:53 PM

Randy, I'm curious about your speaker choice. Did you consider these 4 Ohm 15" speakers:

Eminence Commonwealth 15 (225W)
Weber NeoMag 15 (75W)
Weber Cali 15 (Ceramic) (80W)

I've been reading good things about Weber's NeoMag 15's "D-130 sound". In Eminence, I prefer the lighter Legend, but it is 8 Ohm. And with the Cali, you can choose your dome and cone type, too.

By the way, the Fender Jazz King (140W) was an example of an open backed 15" combo; little brother to the 200W Steel King. It had the Eminence Legend in it.

wizard3739 · #23 03-31-2010, 04:06 PM

Randyc, I thought you might be interested in this eBay link for an enclosure. I have also considered building my own speaker enclosure.

wiz

randyc · #24 03-31-2010, 04:10 PM

Yes, looked at the Eminence, nice speaker but unsuitable for small enclosures. Weight is the prime consideration which goes directly to size.

Webers are not being considered, they don't provide enough information to estimate the performance.

Open back was the original choice and may still be - it would be nice to have the option of sealing the enclosure, though. Open backed configurations are dependant almost totally on speaker choice. One can get some flexibility in response with a sealed enclosure, making the choice of speaker a little easier.

One problem that I'm working on at the moment is the foam material - the composite that will make the cabinet light and self-damping. Most of this material is frequency dependant, absorbing more and more energy as frequency increases. That's not good because a lot of the harmonic content that defines "tone" would be lost. My goal is a cabinet that is reasonably flat (within a 5 dB window) from 80 Hz to 3.5 kHz (which is asking a LOT from a 15 inch speaker).

Ideally, a foam material that is more absorbent at LOW frequencies would be nice, to compensate for the large speaker rolloff at higher frequencies and to dampen the natural resonances of the enclosure dimensions (which are going to be in the range 400 - 2000 Hz).

It's fun, so far, may not ever get built (depending on other priorities) but I've wanted to get that Bassman back into service for years. I do occasionally hook it to the speaker in the Ampeg Reverberocket (I added a special jack for that purpose). It sounds good but the impedance mismatch limits the output power. The result is a VERY HEAVY Fender Deluxe, LOL !

PS: If I had a functional brain, I'd write a check for $199 and get this: http://www.carvinguitars.com/product...SX1502&cid=199