Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Show music

Manufacturers are mostly alike in their choices of music at shows and consumers aren’t much more adventurous either. Which is interesting because at home we listen to large and diverse libraries, truncating them only for shows.

I think we hear and play the same things at shows for two reasons: it helps consumers evaluate equipment easily, manufacturers can play it safe.

Customers come to shows to hear and see the best of the best, manufacturers come to meet people and show off their products in advantageous light. It’s kind of like going to a Sunday picnic where everyone puts on their best clothes to say hello.

I must admit that I carry with me a number of non-traditional tracks I play to see what people’s reactions are, but the playing of them typically clears the room, though a few music lovers stay put, transfixed by the music.

I am always appreciative when folks bring their own – especially if it is good, new and worthy of the crowd’s appreciation.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

New technology

I haven’t spent much time cruising the show here in Newport Beach but I have poked around a bit and am a little dismayed to not see new, new technology. Perhaps it’s hiding from me. And, to be clear, it’s not like PS Audio is contributing gobs of it either.

I always hope to see fresh new ideas coming forth: a new speaker or driver technology, a new electronic invention that sets the world on fire. What I have noticed is that technology comes and goes in waves, like the ocean, and right now we seem to be in a trough, not riding a swell.

I’ll keep my eyes open for you while at the show, that is if I can get out of the room for a minute or two.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Neutral

The notion of neutrality applies to many areas: our cars when we don’t want to go anywhere, our opinions when we haven’t one, and our speakers when we want them to disappear. Yet, we know neutral is nothing more than a clever word describing a state that doesn’t exist. Cars can still move, opinions are there – just not expressed, speakers are colored by default.

The dictionary describes neutral as “having no strongly marked or positive characteristics or features.” Yet how many of us can say that of our loudspeakers? We understand from measurements none are flat and we know from listening that horns sound one way, electrostats another, ribbons a sound unto themselves, and dynamic drivers different from any of the others.

I wonder if it serves our community to describe loudspeakers as neutral when they are not and cannot be?

I also wonder if we would like them if they were?

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Tear meter

We often hear the debate about measurements vs. listening. So far the two camps are entrenched in their worldviews: if it can’t be measured, it doesn’t exist vs. if the differences can be heard, they do exist. I doubt I’ll be able to add anything of significance in this post.

But I am perplexed. One of the BHK Beta testers posted a great bit of feedback here. In it he suggested a track played on his existing amplifier (which has exceptional measurements) sounded good, yet offered little in the way of emotional engagement. He then played the same track on the BHK Signature and he and his wife had tears in their eyes, so engaging was the same music played on the BHK.

There is no doubt in my mind, and the experience of others, that some equipment emotionally involves us while others leave us unattached. Do those that believe everything can be measured envision an emotion meter someday? Or do they simply disregard the impact such equipment brings? My guess is they must ignore this data set. Acknowledging that some equipment changes our emotional reaction to music more than others would threaten many hard core measurementist’s world view – and when our worldviews are threatened we recoil, deny, ignore, or run.

But my view is simpler. I believe our ear/brains are better measuring devices than our machines, at least for now. And someday we can build a machine capable of measuring the degree of water from emotional eyes, but in the meantime, we have built-in versions at the ready. So why not just use them?

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

The difference between amps and speakers

Speakers make sound, amps make speakers sound. I know, the distinction is perhaps obvious but it occurred to me this morning the differences between devices that generate acoustic waves in our homes and boxes that use electrical waves to power them are more complex than at first glance.

Obviously a loudspeaker is very different than a power amplifier. A loudspeaker is mechanical in nature; it has electrical properties that animate its mechanics. A power amplifier is electrical; we cannot hear or see what goes in or out. Yet, as consumers we are asked to make choices based on nothing more than the reviews we read and the sounds we hear. And while we can hear sounds from a speaker, we cannot use the same mechanism to judge an amp.

Every piece of electronics in our chain is decided upon through the lens of a loudspeaker and thus colored and confused by the same. We never truly hear an amplifier, just as none of us has ever truly seen the surface of Mars; it’s always colored by a telescope or camera.

It’s instructive to remember that which we hear in our listening environments is always colored by the lens of loudspeakers, and none are neutral.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Something quite new

So now that we know how conventional amplifiers and switchable amplifiers work, what is it we did differently in the BHK Signature Mono amplifier that started this thread? We built a parallel amplifier to turn the stereo BHK Signature into the Mono BHK Signature.

A parallel amplifier simple means you take two channels of an amp, left and right, and feed both inputs the same signal, then tie the two outputs together to feed a single loudspeaker. Seems simple, right? As I had described in an earlier post, it isn’t so simple and almost never done because it can cause major problems. In a solid state amplifier, if you tie the two outputs together – red to red, black to black of both channels fed the same input – you piss off the power amplifier and have poor results. Why? Feedback. Each amplifier channel has its own feedback loop that compares what has gone into the input of the amp and what comes out, making corrections for any differences. When you tie two amplifiers in parallel the feedback loops fight each other. The exception to this rule is a tube amplifier with an output transformer. In this situation the output of the two transformers can be paralleled to good advantage because of the transformer isolating the two.

But none of this applies to what we actually did in the BHK Mono and the differences are what I believe result in the extraordinary sonic improvement, beyond what we would expect when going from a stereo to a mono build.

Bascom H. King, the designer of the BHK Signature decided to take a very different course than had ever been done before.

Instead of paralleling just the outputs and managing the feedback problem I previously described, he fashioned an entirely new topology that pairs every component, not just the outputs. So, for example, the input vacuum tubes are tied together acting as one. This is a technique I have seen used only in moving coil preamplifiers to lower noise. Many moving coil phono preamplifiers parallel multiple devices (usually transistors) together. Each time a new device is added, the noise floor drops. So, for example, adding one device in parallel drops the noise in half. But getting another such drop requires two more in parallel, then four more, then eight more and so on. But noise was not the main goal in BHK’s thinking, as the noise level of the amplifier is already extraordinarily low. Rather, sound quality was foremost on Bascom’s mind when pairing tubes, drivers and outputs together in a configuration new to power amplifiers.

And just when I thought I’d seen every possible design twist in amps, along comes BHK and shows us once again, clever design with the goal of better sound, wins every time.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Breaking conventions

When we think of a power amplifier we visualize a conventional design where the speaker is attached to the red (+) terminal and the black (-) terminal for each channel. The red is the output of the power amplifier, the black is ground. But there is a different type of amplifier called a bridged amp. In this design there are two amplifier outputs for each channel, one for the red (+) terminal, the other for the black (-). To build a stereo version of this amplifier we need four channels of amplification, one for the + and – of each channel, rather than the two we commonly use in a conventional design.

Stereo amplifiers that can be switched from stereo to mono move between conventional design to bridged with the switch, which assigns two channels into one when mono is selected. This is why a switchable bridged amplifier requires the loudspeaker to be rewired when you select mono: the speaker is placed between the two red terminals, rather than red and black. You’ve probably seen this arrangement before. Here is a picture of the rear panel of one of our older designs that featured a stereo mono switch.

Note the red lines on the back panel showing where to place the speaker cables, between the two red terminals. Also note the red line pointing to the left channel input. When the amp is switched into mono mode, only the left channel may be used for connecting to the preamplifier. This is because the right channel is now fed the same input the left channel receives, just flipped upside down – when one channel goes positive, the other goes negative. Doing this doubles the voltage and if the power supply is big enough, increases the wattage as well.

The use of bridged amplifier technology is not new and its use is increasing. But these increases are coming not from more stereo amplifiers with a stereo/mono switch, but rather from designers purpose building bridged amplifiers; like the BHK Signature which internally has four channels of amplification to make stereo.

An amplifier that is designed as a bridge cannot be bridged again for greater power. To create a mono amplifier from a bridged design, we need to take a different approach: parallel, which we will discuss tomorrow.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

The cost of a free lunch

Engineering products is a series of choices and compromises. The first rule designers learn is there is rarely a free lunch. One design choice that solves one set of problems often leads to another. And the greater the complexity of the system the fewer free lunches.

I described in yesterday’s post how power amplifiers with a switch to choose between between stereo or mono work. The technique is called bridging and it is simple to build. All the design engineer must do is provide a means of flipping the phase of one amplifier channel. This is typically handled in the input stage where the addition of a phase inverting circuit does the trick nicely. It can even be cleaner if the designer uses a differential pair on the amplifier input. This type of stage has two inputs: normal and inverted. Once flipped over, the output of one channel goes positive while the other goes negative. Between the two outputs voltage is doubled and wattage to the loudspeaker is quadrupled. But this is one of those situations where the free lunch of an easy switch feature comes with a hefty price tag.

For starters designers have the problem of power supply current. Stereo power amplifiers have enough current capacity to drive both channels to rated power with little left over. A 100 watt per channel amplifier is powered by a supply capable of 200 watts, 100 for each channel and not much more. When you bridge the amplifier the output voltage doubles but the supply cannot provide four times the current, so instead of 100 becoming 400, we run out of gas early and find ourselves limited to 200. Amplifiers that produce high voltage but clip early because their power supplies run out of steam are not ideal. But this is how the majority of switchable amplifiers are made.
Yet another problem with the bridged amplifier is impedance. Unless it is designed to be robust into low impedances, the amp can run into trouble in bridged mode. This is because from the amp’s perspective whatever load the loudspeaker presents, 4 Ohms or 8, the impedance seen by the amp is cut in half. Thus, a bridged amplifier presented with a 4 Ohm loudspeaker is actually asked to drive a 2 Ohm load. To make matters worse speakers do not have constant impedance, they dip and rise at different frequencies.

Of course I am speaking in generalities. It is entirely feasible to over-build a stereo amplifier with many more output transistors to handle lower impedances, and super-sized power supplies to keep from running out of power, but it is rarely done. More often than not, switchable stereo amplifiers are designed to satisfy the needs of stereo owners first, mono wannabes second. A good marketing technique perhaps, but a less than optimal solution for greater performance by investing in two power amplifiers, surely.

Yet, there is a third way to build a mono amplifier superior to the two practical examples I have previously described: purpose built single channel amplifier stage, and bridged stereo pair.

Let’s take a look tomorrow.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

A bridge over …

When it comes to making both stereo and mono amplifiers one solution is the easiest for manufacturers and consumers alike: the switchable amplifier. In this clever scheme customers are thrilled because they can turn their stereo amp into a mono with the click of a switch. Manufacturers are happy as well. Instead of separate models and the supporting materials needed, there is only one that doubles as stereo or mono. It’s an enticing solution to a problem few have, and even fewer benefit from. But it makes for great sales appeal.

To make a switchable amp there are two means: parallel and series. I’ll cover parallel first, series second.

Picture a stereo power amplifier as I have previously described: two identical channels of amplification, one power supply, one chassis. If you wish to make a mono amplifier all you need to do is place the two channels, left and right, in parallel with each other, creating a single channel with double the current available. But as soon as you try this you run into trouble. The first problem involves the way amplifiers share: they do not. Because each amplifier has its own feedback loop, input, output and control circuitry, paralleling the two simply causes them to fight each other, like brothers and sisters. Secondly, you get no more watts.

Remember in an earlier post I mentioned the need for marketing people to write something different on a spec sheet to sell mono amplifiers? Imagine their difficulty convincing you to buy a stereo amp with a mono/stereo switch – and neither switch position produces more watts. Your first question might be, to what benefit? But there is a simpler way that resolves both issues: placing the two channels in series, a technique known as Bridging.

To bridge an amplifier it is necessary to flip the polarity (phase) of one channel relative to the other and place the speaker in series with the two channels. What happens next is interesting. If you place a loudspeaker in series, across the two red (hot) terminals of the amplifier, the voltage across the speaker doubles if the two channel’s phases are inverted. This is because one channel is going one way while the other is moving in the opposite direction, increasing the distance between the two (measured as voltage) by twice. Like two cars speeding away from each other at the same speed, distance is doubled. When you double the voltage of an amplifier the wattage figures increase by a factor of 4. Thus, a 100 watt amplifier with a large enough power supply becomes a 400 watt amplifier. Neat, eh?

It is a cool trick but it is not without consequences, which we will cover tomorrow.

Walnut Cove, Asheville, Biltmore Forrest and Western North Carolinas Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Designing an amp

If you already design stereo amplifiers making a mono version is simple. Just remove one channel and its input and output connectors from the chassis, change its name, and you’re done. But what if you were starting from scratch? How much different would your design be than simply eviscerating a stereo model? Much would depend on the design goals of the amplifier, and many of those goals come from expectations ingrained in consumer minds, and not always for the right reasons.

When purchasing decisions are made folks typically turn first to wattage ratings on the specification sheet. It is generally expected that a mono amplifier should have more power than a stereo model, a myth perpetrated by amplifier manufacturers. Why buy a mono amplifier with identical specs to its stereo counterpart?

Power amplifiers are generally selected to match the power requirements of the loads they need to drive, though it has long been understood it is ok to go bigger, just never smaller than needed. With this in mind the challenge faced by amplifier manufacturers of the 1960s and 1970s was to give consumers the same rated power monos had in one stereo chassis. No one expected more, they were just happy when it was the same. Once the shift from monos to stereos took place and enthusiasts and manufacturers returned to mono amplifier designs to coax more life out of the system, there had to be good reasons to make the switch, and increased power seemed an obvious distinction. It is difficult to quantify more lifelike sound on a spec sheet. Yet, the requirements of loudspeakers are no different today than when the world was heard through a single channel.
If an amplifier manufacturer started a mono amplifier design from scratch, he would choose first the wattage goal, followed by the topology, lastly the mode of operation. For purposes of this post it is easiest to explain a solid state class A/B amplifier with a conventional power supply. The components are straightforward: power supply, amplifier circuit, chassis with adequate heat sinks, input and output connectors. This list is no different than that of a stereo design, just one channel short and a smaller power supply. The advantages of building the mono amplifier from scratch are cleanliness of design, and a power supply that matches the design goals set forth.

But most amplifier manufacturers don’t start with a clean sheet when designing a mono amplifier. Today’s designers almost always modify a stereo amp instead, hopefully using as many common parts as possible (so the company’s purchasing agent doesn’t have a fit). Which is why mono amplifiers usually have more power than their stereo forefathers; it is easier to use a stereo sized power supply to build the mono version.