|Max dB with Doug Blackburn
Back Issue Article
The Amplifier: Whats Inside? What Makes a Difference? Whats Overlooked? Part Three of Three
For the final installment of "The Amplifier" Im going to discuss the mechanical package. This may sound like the least interesting of the three installments, but I assure you it will not be boring. Ive just scratched the surface of how important the mechanics and mechanical resonances of components and wires are in previous Max dB articles, in my reviews, and in Talk Online. This is the first time Ive dissected a single component and discussed many of the mechanical-package concepts and the importance of addressing mechanical resonances.
Before I start, heres is a repeat listing of the components which make up the subsystems of an amplifier along with the Max dB article that covered each topic.
Covered in Part One, September 1998:
Covered in Part Two, September 1998:
Low-level gain stages
Covered in Part Three, November 1998:
The first thing that should be understood is that the mechanical package of any component is responsible for about 1/3 of the sound of that component. You may have never read this anywhere else before, so it may be difficult to swallow. The other 2/3 of the sound of a component is about evenly divided between the design of the power supply and the design of the rest of circuit.
You can take identical components, build identical circuit boards and install all the parts in different cases with different hardware and the resulting components will sound different. There is no way to stop this. The reason the sound will change is that mechanical resonances affect sonics -- what you hear when you listen to your system is affected by how the manufacturer dealt with mechanical resonances in the products you use.
Some words about the unspoken "problem" with monoblock amplifiers
Mechanical resonances are such a sonic influence that it is actually quite difficult to make two monoblock amplifiers that sound exactly alike. This is one of the "unspoken truths" that amplifier manufacturers, at least the observant ones, dont often admit publicly.
We all assume that monoblock amps are the standard to aspire to -- and that is true if the amps are identical in every way. But the further the two mono amps deviate from one another, the more the advantages of having mono amps are buried by the problems. There are plenty of places to deviate: how tight the circuit board standoffs are, how tight the binding posts are, whether each component on the circuit board touches the circuit board or sits up off of the board supported by its leads, how tightly the transformer is mounted, panel thickness, panel curvature (prior to installation), how tight the panel screws are, slight differences in the vibration of the transformer, how tight wire ties are which hold wires together, routing of wires, length of wires -- the list goes on and on. A single-chassis stereo amplifier reduces the effects of mechanical resonances because both channels are influenced equally by the same resonances. Im not saying a stereo amp will sound better than a pair of monoblocks, but there is an advantage to a single chassis that causes a struggle for manufacturers of monoblocks.
Your only defense is to be highly observant when listening to mono amps. If you sense something is not quite right, it probably isnt. The differences arent going to be big enough to be completely obvious from the first moment you hear the amps. But over time you will realize something is not quite right. You may never be able to say for certain exactly what it is, but you will eventually notice. Its far better that this happens earlier rather than later.
The key is uniform distribution of mechanical resonances
Products with "clumped" resonances will never sound as good as products where the designer took the time to distribute resonances across the audio frequency spectrum as much as possible. However, there will always be some limitations to how effectively you can spread out resonances. Doing nothing is rarely the right answer.
Every change made will change the sound heard, but that sound is not necessarily going to be better. Tuning a component is filled with pitfalls. The designers who do pay attention to mechanical resonances are going to end up with a more sophisticated, refined sound that will elude products which are designed and assembled with little or no thought to resonance control. Actually, I want you to think "resonance redistribution" when I write "resonance control." You dont necessarily want to kill all resonances by embedding the entire product in asphalt or pouring the box full of visco-elastic polymer with a resonant frequency lower than 3Hz. Brute-force deadening of all resonances with heavy absorbent or damping materials is usually worse-sounding than effective resonance redistribution -- at least when we are dealing with electrical/electronic components. The one exception to this seems to be the power transformer. The more effectively isolated from the components chassis it is, the better the component seems to sound.
The faulty four: rubber, plastic, nylon, steel
Several years ago, Michael Green told me "When you are trying to make a good-sounding audio component, the more rubber, plastic, nylon and steel you can get out of it, the better it will sound." I immediately went home and removed the rubber pads that were on both sides of the toroidal transformer in my amplifier. Unbelievable. The amp sounded unquestionably better without those pads. Leaving the transformer resting on the chassis is very dangerous, however. A later experiment raised the transformer on a sand-filled baggie, and the sound improved again. A little later I found a very small inner tube and used it as an air cushion under the transformer -- another improvement over the sand bag. In my moded CD player, I replaced the nylon circuit-board standoffs with brass screws -- a significant sonic upgrade. In fact, Michael Green recommended brass as the replacement material for the rubber, plastic, nylon and steel if possible. The resonance characteristics of brass are the most musical of any metal -- which is why musical instruments with metal in them usually employ brass. Aluminum is the second choice if using brass is not possible.
Keeping these principles in mind, lets begin our exploration in more detail.
Case/chassis To minimize panel resonances, panels should be thought of as listening rooms. The thickness, width and height should not be even multiples of each other. The less similarity there is in these dimensions, the more spread out the resonances are. If a panel has a slight curve, it will sound different when screwed down to the chassis than if it were flat. This can be used to good advantage when it is not possible to change the shape or size of a panel that doesnt sound particularly good. Damping panels can help or hurt the sound. If damping is undertaken, it should be "controlled" damping. A double-layer chassis of aluminum and copper sounds very different than a chassis of aluminum panels the same thickness as the aluminum-copper sandwich. Lower-cost components in steel chassis can end up sounding a lot better if they are transferred into an aluminum chassis -- impractical for 99.9% of us. If you have a component of moderate cost in a steel chassis, the best thing you can do for the sound is some controlled damping, perhaps using FunTak flexible adhesive on the bottom of the top cover and on the top of the bottom of the chassis. An isolation base with a damping platform is another big boost for mass-market or high-end component performance.
The best-sounding case would be asymmetrical, but it would be difficult to sell to customers because of its lack of beauty and symmetry. Concessions to appearance can/should be done in ways that minimize the deleterious effect of symmetry.
Feet Usually manufacturers slap on some nondescript rubber feet without much regard for the sound of the component. Can you blame them? Some audiophiles will mess with six to ten different kinds of feet under each component trying to find the magic combination. The fact is, there is no magic combination, no universally "best" foot. All feet are all different-sounding because they all have different mechanical resonances. You think your brass cones are a little too pushy in the upper midrange? Want it to go away? Put electrical tape, one layer, right on top of the cones and put them back under the component. Upper-midrange push is gone. Whats going on? Why do feet sound so different -- just from a piece of tape? Its all part of the mechanical resonance control and redistribution game. You dont have to use expensive feet to change the sound of your amplifier. Wood blocks change the sound as much as $200+-per-set cones. Not that the cones and blocks sound the same -- they dont. And each different type of wood will sound different (dont bother with pine though; it doesnt sound very good). The magnitude of change produced by $200+ cones and inexpensive wood blocks can be very similar. Brass blocks, aluminum blocks, cones, wood blocks, soft rubber pucks -- they all sound different. No one of them is "perfect," so dont obsess over them and dont overspend on them. The expensive devices dont guarantee better results. Nor do inexpensive devices necessarily sound worse. If you are a DIY kind of person, your ideas about what might sound good as feet just might work great. Its definitely worth experimenting with. Just avoid rubber, plastic, steel and nylon.
One thing about feet: If you use an isolation and damping setup of some kind under a component, the feet become less important. Different feet will still sound different, but the magnitude of the difference will be smaller. It isnt that the feet become less important, its more of an issue that when you isolate the component and couple the feet to a damping device of some kind, the feet dont have as much work to do. When there is less work for the feet, their influence is less than if the component were sitting on a typical shelf or amp platform.
By the way, no matter how hard manufacturers of cones try to convince you that cones are mechanical diodes and vibrations can travel through them in only one direction, dont believe it. Cones are coupling devices, period. They couple components to the shelf. If the shelf vibrates up and down, the component is going to vibrate up and down. Cones cannot possibly stop two-way vibration. However, cones will sound very different than a cylinder of the same diameter as the top of the cone. This is because of the contact patch difference, not because turning a cylinder into a cone somehow makes it impossible for vibrations to travel up the cone. A large contact patch picks up more vibration, and a small contact patch picks up less vibration, above a certain frequency anyway. But both the cylinder and the cone are coupling the component to the shelf, and both pass vibrations in both directions.
Screws, nuts, bolts, washers The two operative factors with these fasteners are the materials they are made from and how tight they are. Here again, brass sounds the best if it can be used in a given application. However, brass screws are brittle and not particularly strong. For commercial products, brass screws cannot usually be a consideration -- they just arent heavy duty enough. Most screws are going to be steel because of strength and cost.
You can control how tight all the screws and other hardware are though. I think as you experiment, youll find that everything sounds better if the screws are snug but not tight. This is a problem for manufacturers. They want products to ship without components falling off or coming loose because screws werent tight enough -- a pretty reasonable goal. They really do have to make all the screws tight. You, the owner, dont have to leave them that way. You can loosen and then snug each screw to get a nice improvement in sound quality for free. Loose screws dont sound good. In this case, "snug" means tight enough to not loosen up while the amp (or other component) sits in your listening room. If you do loosen then snug screws in an amp or other component, be sure you tighten them before shipping the component anywhere.
Circuit board standoffs As mentioned before, any time better materials like brass or aluminum are used, you will get better sound quality. This can be a problem in some designs. If you are thinking about doing this with your own amplifier and you did not personally design the amplifier, I would recommend that you be extremely careful about what you do. Shorting out a circuit board by installing conductive brass standoffs in place of an insulator could result in some very expensive repairs. Common nylon circuit-board standoffs really sound bad enough to make using a substitute worthwhile. If brass is not an option, ceramic spacers are one possibility if you can find appropriate lengths.
Heat sinks (if solid state) Heat sinks in solid-state amps are one of the major sources of bad solid-state sound. Imagine 15 or 20 heat sink fins all lined up, all with the same thickness, the same depth and the same height. Each dimension is repeated, stacking identical resonances on top of each other. A heat sink like that has a very significant effect on the sound of the amplifier. What to do about it? Not that hard really --.. slit the heat sinks along their length at random points so that no two fins have the same length. If you really want to remove the heat sink from imparting a strong sonic signature, have them custom-made with different lengths, asymmetrical nesting, different thicknesses, and different depths to the fins. Resonances will be amazingly well distributed and the sound of the amplifier will benefit quite noticeably. That kind of heat sink isnt going to be possible in low- to medium-priced amps though. Only the amps up in the $3500+ range are going to have a budget big enough to support custom-made heat sinks. Bonding a copper plate to the internal side of the heat sink is another way to redistribute resonances for much lower cost.
You want to know a reason some people like the sound of tube amplifiers more than solid-state amplifiers? Because tube amps dont have heat sinks. Yes, heat sinks can sound that bad. How many solid-state amps have you ever seen that had anything done to the heat sinks to redistribute all those resonances? Not many, Id wager. That hard amusical glare, the closed-in sound from the upper midrange on up, the lack of transparency, dry white sound -- you dont suppose these have anything to do with all those identically resonating heat-sink fins, do you?
If you have an amp with a bunch of identical heat-sink fins, you should definitely experiment with some damping of the fins. The bottom edges are the safest places to apply damping materials. The top edges are the worst place to apply damping. Heat rises, even in metal, and the tops of the heat sinks are the hottest part where the most heat transfer takes place. So do your damping at the bottom edges of the fins. You can try a home insulation product called rope caulk -- a sticky, heavy, deadening, putty-like material used to fill cracks around window frames. Wrap some rope caulk around the bottom of each fin, using a different amount of caulk on each fin to vary the dampening.
Tube sockets The forgotten tube socket. Too many are high-temp plastic. Ceramic sockets generally sound better. However, the best-sounding tube socket is the socketless socket. Hollow pins rise from the circuit board to accept the tube. Theres no molding around them to hold them in place, nothing to keep them from being bent over during a tube change. Kind of a radical approach, and many manufacturers wont even consider this due to potential service/maintenance problems and cost. However, these nifty hollow-tube arrays rising from a circuit board sound better than traditional tube sockets.
Mounting methods and materials for the transformer and other mechanical parts Holding the transformer in place during shipping is a major sonic roadblock for amplifiers. Amps have the biggest and heaviest transformers of any audio component. To ship an amp, the manufacturer has to be 100% certain the transformer stays exactly where it is supposed to stay. With something that heavy, the manufacturer really doesnt have too many choices.
However, you may be able to improve things once the amp is a permanent resident in your system. This isnt for the amateur, however. You could do great harm to yourself, your family or your home by messing with the transformer and causing a problem which does not appear until later. As mentioned earlier, transformers sound better if they are isolated from the chassis. Sand bag, inner tube, Vibrapods -- use your imagination, but dont use anything conductive! And if you do isolate the transformer, dont forget to lock it down again later if you move or ship the amp.
Binding posts and RCA jacks When mounting the RCA jacks and binding posts, over-tight never sounds as good as snug. However, snug can be problematic, especially with binding posts. Any post that is too loose is liable to spin when tightened or loosened, and this is unacceptable. So you have to be careful if you loosen RCAs or binding posts. "Stressless" binding posts are being sold by Cardas, and these offer some serious advantages -- the posts in these things arent even locked down! And you dont have to rotate a nut on them to tighten the connection. You actually tighten a thumb wheel which squeezes the spade connector tight against the post -- but you do not add tension stress to the post. This design of binding post goes a long way to solving all the problems associated with traditional binding posts.
"Stressless" RCA jacks also exists but they require a different mindset on the part of the designer. In fact, these stressless RCA jacks fix two problems at the same time. "Stressless" RCA jacks solder directly to a circuit board. This eliminates the locknut needed to hold chassis-mount RCA jacks to the PCB. At the same time, the wire to connect the chassis-mount RCA to the circuit board is eliminated. The RCA connects directly to the circuit without wire. The designer must plan for this kind of RCA from the very beginning because retrofitting is rarely possible. Most PCB-mount RCA jacks are terrible mass-market items. But here again, Cardas is one of the leaders, with several different PCB-mount RCA jacks with high-quality plating and a large surface contact area for the center pin.
Wire ties, etc. Most amplifiers have all the wires very neatly bundled with military precision and routed though the amp. This may look great, but it isnt the best-sounding way to organize wires. The component in question will sound better if the wire ties are removed and the wires spread out from one another and prevented from touching the sides of the amp as much as possible. Most of the big-name amp makers neatly and dutifully bunch wires together and carefully route them -- harming the sound of the amplifier at the same time. Wonder why point-to-point wired amplifiers always seem to sound so good? Wires arent bundled together and tied with wire ties. Wires are all different lengths and they are generally running through air and touching nothing except at their ends. Use care unbundling wires though. If two wires are twisted together, do not untwist them. This was done on purpose. Keep unbundled wires away from large ceramic resistors, which may run hot enough to melt some kinds of wire insulation. Avoid routing unbundled wires near sharp sheet metal edges, sharp screw heads, etc.
Im out of space already! Theres so much more to say about mechanical resonances and their effects. Ive really only touched the surface with this article. However, the information presented here is only an introduction to mechanical resonances and their control. Future columns will explore the subject in more detail and include experiments that you can try.
External tuning of the amplifier(s) can be very rewarding -- sonically, too. Using isolation and damping bases like the Bright Star Air Mass and Big Rock and others are very effective for amplifiers and I strongly endorse their use. Dont over-tighten those binding posts when attaching your speaker cables. Experiment with tuning the amplifier by placing a wood block, sand bag or other material (not rubber, nylon, plastic or steel) on the top cover of the amp. Almost anything you put on top of (or under) the amp will change the sound you hear.
If you are shopping for an amplifier, look for evidence that the manufacturer is addressing mechanical resonances to some degree. If the amplifier is inexpensive, under $1800 for example, it is unrealistic to expect too much mechanical resonance control from the manufacturer. However, there are a few designers of lower-cost products who take some serious, though not always obvious mechanical-resonance-control measures. As price increases beyond $1800, you should expect increasingly sophisticated mechanical resonance control. At least some of these more obvious controls should be quite easy to see. Unfortunately there are still more amplifier manufacturers ignoring the importance of mechanical resonance controls than there are manufacturers employing effective mechanical resonance controls. Youre the buyer, you have the last say. Whats it going to be? The cool-looking big-name amp with absolutely no attention to mechanical resonance control or the smaller companys well-tuned but perhaps ordinary-looking product? Paying attention to mechanical tuning does not relieve pressure on the design of the power supply or gain stages. But it does show that the designer is keeping abreast of the latest thinking about what makes amplifiers (or other components) sound the way they do and that he has chosen to exercise some influence over it rather than take whatever comes with no attempt to improve things.
See you next month.
Copyright © 1998 SoundStage!
All Rights Reserved