Bryston 875HT
8-Channel Power Amplifier
Bryston Ltd., 677 Neal Drive, P.O. Box 2170, Peterborough, Ontario, Canada K9J 7Y4. Voice: (705) 743-5325. Fax: (705) 742-0882. E-mail: info@bryston.ca. Web: www.bryston.ca. 875HT eight-channel power amplifier, $5195.00. Tested sample on loan from manufacturer.
Reviewing a Bryston product is usually a simple and straightforward matter because it’s most unlikely that there is anything wrong with it. The only thing wrong in this case is the price—the 875HT is a wonderful amplifier but not very good value at $5195. There are, for example, 12-channel power amps by B&K, ATI, and others in the $1500 to $2000 range that may not quite equal the Bryston in specs but aren’t functionally inferior and have four more channels. I have the greatest admiration for Bryston engineering, but this product isn’t competitively positioned in the market. The serial number of my sample was 000012, and the 875HT is not a very new model. Doesn’t that tell you something?
One of my main reasons for wanting a review sample of the 875HT was to try it out on my Linkwitz Lab “Orion” speaker system, which can use four amplifier channels per side for the four drivers (see the February 2005 review on this website). The Bryston drove the Orions flawlessly, but was it any better than my old 6-channnel McIntosh MC7106, with the two woofers per side paralleled? I heard very subtle differences, probably due to the different woofer hookups; unfortunately my ABX double-blind comparator cannot switch between two multichannel amplifiers, so an objective sonic determination could not be made. Basically, it was six of one and half a dozen (well, eight actually) of the other.
Of course, a much more common application of the 875HT would be to drive all the speakers in a surround system. The amplifier could accommodate even a 7.1 system with an unpowered subwoofer, using one channel on each speaker. A 5.1 system with a self-powered subwoofer could use bridged channels on each of the left, center, and right speakers, and single channels on the surround speakers. Alternately, the amplifier could power four separate stereo systems in four different rooms. The permutations and combinations are almost endless.
The Design
The layout of the 875HT is simplicity itself. The eight identical power-amp modules run parallel from front to back. The abutting heat sinks form a ventilating grid on top. The back panel has eight unbalanced RCA jacks, eight balanced XLR jacks (without push tabs), eight speaker terminals (accepting dual banana plugs/spade lugs/bare wires), and switches for bridging adjacent channels to form four mono amplifiers. The front panel has nothing but an on/off switch plus eight LEDs to indicate power in each channel. No other bells and whistles.
[Dr. David Rich, our erstwhile technical editor and favorite EE authority, is supposed to contribute a circuit analysis/critique to this review. I shall append it when I get it; meanwhile I want to post the body of the review forthwith.]
The Measurements
The small-signal frequency response of the Bryston 875HT I shown in Fig. 1. The deviation from total flatness is only –0.025 dB at 10 Hz and –0.08 dB at 20 kHz, but the gradual rise to a maximum of 0.045 dB at 9 kHz is somewhat unusual (although inaudible, of course).
Fig. 1: Frequency response of one channel at 1 watt into 8Ω.
By far the most important power-amplifier measurement is distortion vs. power output. Fig. 2 and Fig. 3 show the THD+N of one channel of the 875HT into 8Ω and 4Ω loads, respectively, with inputs of 20 Hz, 1 kHz, and 20 kHz. The three curves tend to coincide in the best amplifiers, including Bryston’s stereo models, so the significant separation seen here is a little bit strange. The measurement bandwidth was extended to 80 kHz for the 20 kHz distortion curves, to include the 2nd, 3rd, and 4th harmonics; therefore, obviously, more noise was included. The 20 Hz curves, on the other hand, are normally expected to lie right on top of the 1 kHz curves. Even so, the measurements meet and exceed Bryston’s specs as printed in the owner’s manual and as certified on the individual checkout sheet that comes with each unit—except at 20 kHz, where the shortfall is minimal. This is still a very low-distortion amplifier.
Fig. 2: THD+N vs. power of one channel into 8Ω at 20 Hz (green), 1 kHz (blue), and 20 kHz (red).
Fig. 3: THD+N vs. power of one channel into 4Ω at 20 Hz (green), 1 kHz (blue), and 20 kHz (red).
The FFT spectrum of a 1 kHz tone at clipping level into 8Ω, shown in Fig. 4, indicates that the THD is dominated by the 3rd harmonic, all other harmonics being of significantly smaller amplitude. Even the 3rd harmonic, however, is at an extremely low level.
Fig. 4: Spectrum of a 1 kHz tone at approx. 90 watts into 8Ω.
The single-point noise of the amplifier, with a measurement bandwidth of 22 Hz to 22 kHz and referenced to clipping level into 8Ω, was –107.7 dB. That’s an excellent figure and it accords well with Bryston’s specs.
Fig. 5 shows the crosstalk (i.e., separation) between two adjacent channels at a level of 1 watt into 8Ω. Adjacent channels are more susceptible to crosstalk than widely separated channels, and the measurement here indicates flawless performance. (Bryston appears to provide no crosstalk specs.)
Fig. 5: Channel separation (adjacent channels) at 1 watt into 8Ω.
One of the most revealing amplifier measurements is the PowerCube test. This test, more commonly seen in Europe, measures the ability of an amplifier to drive widely fluctuating load impedances, such as presented by certain loudspeakers. As far as I know, The Audio Critic is the only American audio journal to publish PowerCube measurements. The instrument for the test is made in Sweden; it produces repeated 1 kHz tone bursts of 20 ms duration into 20 different complex load impedances across the amplifier (magnitudes of 8Ω, 4Ω, 2Ω, and 1Ω, phase angles of –60°, –30°, 0°, +30°, and +60°), registering maximum output at 1% distortion. The graphic output of the instrument shows the 20 data point connected to form a more or less cubelike polyhedron. The test really separates the men from the boys when it comes to real-world loads rather than just resistances.
The PowerCube of one channel of the Bryston 875HT is shown in Fig. 6. Interestingly enough, it is not even as good as that of the dirt-cheap Behringer A500 (see the December 2005 review on this website). The power supply cuts out at 31.6 volts into 8Ω (125 watts), which at least it is able to deliver into all phase angles at 1% distortion, and then fades out significantly at lower impedances. I expected a more gentle slope of the top of the polyhedron, i.e., more power into the lower impedances, especially 4Ω. The power supply of each channel appears to be rather limited.
Fig. 6: PowerCube of one channel with two channels driven. The three axes are output in volts, impedance in ohms, and phase angle in degrees.
The Sound
An amplifier with the measurements of the Bryston 875HT has no sound of its own. To talk about the soundstaging, midrange immediacy, etc., of such an amplifier is audio-cultist nonsense. The sound of the 875HT’s output is the sound of its input.
Conclusion
The 875HT is a classic Bryston power amplifier with slightly compromised power supplies for its eight channels in comparison with Bryston’s stereo (two-channel) models. Even so, it can be trusted to perform flawlessly in any kind of multichannel installation. The only problem is the inflated price.
—Peter Aczel
The Bryston 875HT: Topology and Afterthoughts
By David A. Rich, Ph.D.
Engineering Consultant
Editor’s Note: Like so many other distinguished members of the engineering community, David Rich has a cavalier attitude toward English syntax, punctuation, and spelling. I spent only a limited amount of time editing his stream-of-consciousness writing and therefore cannot guarantee flawless copy.
Bryston is the only company I know of that puts its schematics on the Web so you can see the full schematic of each unit. Since it has been a long time since I looked at a Bryston, let’s have a brief review of what’s inside all their offerings.
Bryston amps are fully discrete, including the balanced-to-single-ended converter (one per channel) that precedes the main power amplifier. In bridged mode they use one of the discrete amplifiers for the balanced-to-single-ended conversion and the other for the phase inversion needed to create the bridged outputs. Please do not confuse a bridged amplifier with a fully balanced amplifier, which has many advantages over a bridged amp. The only fully balanced amplifiers known to me are the products of Spread Spectrum Technologies (James Bongiorno’s company).
Each Bryston amp is unique in that the stages that provide current gain also supply voltage gain. The voltage gain is to 4.75, set by a nested feedback loop. The closed-loop voltage gain is possible because transistor stages inside the feedback loop are not emitter followers. Recall that the emitter followers used in the back end of a standard amplifier supply no voltage gain.
What is the topology Bryston uses to provide voltage gain? In the first stage, it is a common emitter amplifier. This drives a composite Darlington, with the collectors of the Darlington tied to the amplifier’s output. For those of you into such things, the nested feedback loop is taken at the speaker output and returned to the emitter of the common-emitter amplifier. In other words, current feedback.
In larger Bryston amplifiers, an additional very novel compound transistor circuit follows the Darlington, but that stage is missing in the 875HT. Putting voltage gain in the output stage allows the preceding stages, which provide the bulk of the open-loop voltage gain of the amplifier, to be run from regulated power supplies. The regulated rails are at a lower voltage than the unregulated power rails connected to the output stages (they need to swing only 25% of the output voltage). In a normal amplifier, with the output current-gain stage having a gain of only 1, the preceding stages would have to be run off the power supply that supplies the output rails, lest the amplifier clip at a lower voltage set by the clipping point of the voltage-gain stages.
Gain in the output stage and the nested feedback loop reduce distortion in two ways:
(1) the nested feedback loop, which has a wider bandwidth than the complete feedback loop;
(2) the voltage-gain stages ahead of the nested feedback loop only need to swing at 20% of the output voltage swing. Since distortion is the result of nonlinearity, the less an amplifier signal moves the less distortion occurs.
So why doesn’t everybody do this? I believe the primary reason is that the Bryston topology could oscillate into some loads and when clipped, unless the engineering talent behind the amplifier really understood all the third-order issues that could bring about the oscillations. I am convinced the Bryston engineers have that understanding.
The Bryston voltage amplifier stages are less complex than those of other amplifiers we have looked at that produce very low distortion at 20 kHz. First, no circuit to linearize the second voltage-gain stage is included in the Bryston. Douglas Self has written extensively on distortion mechanisms in traditional power amplifier designs. (2002. Audio Power Amplifier Design Handbook. 3rd ed. Newnes.) He identifies the need for an emitter follower before the second gain stage, or the use of a cascode transistor as part of the second gain stage, as key circuit elements to reduce distortion.
Second, the differential pairs are biased by resistors, not an active current source. This can result in a reduced power-supply rejection ratio, but this is less of a problem with the regulated supply rails of the Bryston. We have a remaining problem, however, that is more subtle—the common-mode signal at the differential-pair input is suppressed less than if the current source were in place. A reduced common-mode rejection ratio of the differential pair can lead to distortion. Are these two circuit simplifications the source of distortion we see in the Audio Precision curves? Probably not, since the same circuits are used in the Brystons with higher power, which yielded some of the lowest numbers we have seen. The regulated rails and reduced signal swings that are unique to the Bryston appear to overcome the downside of the simplified front-end circuits. So why does this amplifier have more distortion than a typical Bryston amplifier? (They specify this amp will distort by a factor of two over the more powerful Bryston models.) The answer is most probably that the number of amplifier stages inside the nested feedback loop is reduced by one, and the stage eliminated is an interesting topology proprietary to Bryston. I also note that the amplifier’s noise is not state of the art. Noise dominantly comes from the differential stage. Spread Spectrum Technologies again appears to be in the lead here.
The output stage uses a foldback current limiter. This can cause trouble with early activation of this limiter into an inductive or capacitive load. The PowerCube shows this amp does not have that problem. Modern amplifiers for home applications use IV sensor circuitry which is independent of the amplifier itself. Complicated versions of these circuits can almost calculate if the output transistors are in their safe operating region. If you refer to the spec sheet of a power transistor, you may see multiple Safe Operating Area (SOA) curves. One will be for dc operation and the remaining ones, which allow more power to be dissipated, are only for short periods of operation. Complex external protection may allow the amp to deliver more current for a short sine-wave burst (what the PowerCube generator drives into the amp) before the amp is shut down for exceeding the dc SOA. Some amplifier manufactures are taking advantage of this and are bring back the dynamic power spec, although current FCC regulations were designed to prevent this sort of thing.
The problem with external current- and voltage-limit protection is that when the protection is activated, relays in series with the speaker terminals open (the Bryston approach does not need the relays, which could cause a slight decrease in amplifier reliability) and the amp shuts down. You have to power-cycle it or press a reset button to bring it back to life. Fine for home use but unacceptable for a rock concert. The amplifier also includes clipping indicators, again as a warning to the pros that the amp could be getting ready to cut out because of the IV current limiter or, worse, come to a thermal shutdown (cycle time on this fault would be longer).
The unit has two power transformers (one for four channels). Each transformer has two 33,000 μF primary filter capacitors. In low-priced AV receivers rated at similar power output, we see much less primary capacitance. This is one reason we do not see FCC power ratings for all six or seven channels driven on these receivers, the other reasons being the transformer’s ability to supply current and the size of the heat sinks to dissipate the energy lost in the class B amplifiers. On the other hand, Bryston does not specify an FCC rating for this amplifier driven into 4 ohms. It does so for its other products.
Also, I note that Bryston specifies a maximum current rating for the ac line coming into the unit at 14 amps, which is just under the rating for a standard wall outlet (leaving 1 amp to supply everything else on the same ac loop that may serve several rooms in your house). This is an indication that the amplifier can drive significant current to the speaker load. Look at the back of some other 7-channel amplifiers and see what they give as a maximum current spec. A final point on the same subject is that UL would not approve an amplifier that can draw much more current. 125 watts into 4 ohms times 8 channels is about it. Amplifier manufacturers selling 300-watt channels times 7 will not pass UL, and you can see the absence of the label on these amplifiers. In reality, you would need to hang a 30-amp 220-volt fuse on these amplifiers to prevent them from popping a fuse. These giant amplifiers do not do this in real life because they never go near full power, at least not simultaneously on all channels. Why has no multichannel power amp been offered with the correct power cable—you guessed it, the high-end dealers would go crazy with thoughts of reduced sales. Imagine them having to say, first call your electrician and then come back. These same high-end dealers will of course happily sell you power conditioners that do nothing for well-designed amplifiers. The Bryston is especially well-designed with respect to the ability to reject power-supply noise, since the voltage-gain stages are regulated. (No, tweaks, the last thing you want to do is regulate the output stage, which would only limit the voltage that the amplifier can swing under dynamic conditions.)
Finally, I do want to point out that honest AV installers (the ones without the store front and no need to sell useless junk at 50 points profit) may well bring a separate ac line into your home-theater rooms. That’s not a tweako move but a recognition that all the fancy lighting and electronics (including multiple subwoofers) may indeed need to be fed by multiple supplies. Tip-off to a rip-off—any AV installer who tries to load up your installation with high-end wire, power conditioners, or high-end amps should be shown the door. The majority will not do so but they still have to make a living. What you should see instead on the final bill is a specific charge for theater design, which may be billable at four-figure rates by an acoustician with an advanced degree and a significant portfolio. Designing a good home theater is hard and expensive. Designing a poor one loaded up with power conditioners, etc., is easy and expensive.
Should the Bryston 875HT be part of your home theater? I really do not think so. It is overpriced for home applications. Instead consider multiple Bryston 4B SST’s (full Bryston topology with an FCC rating of 500 watts per channel into 4 ohms and a much lower price per watt), driven by multiple 15-amp lines. This is certainly overkill in the way a BMW 7 Series is, but it is not like paying the same price for a Pinto with a special carburetor that is claimed never to need fuel. Those who want a Ferrari should look up the Spread Spectrum Technologies Ampzilla 2000. At $2750 per channel ($19,250 for seven!) it is double the price per watt of the Bryston, and like the Ferrari may not be altogether reliable, for example for use in sound reinforcement applications. Those who don’t have cash dripping from their pockets are directed to 7-channel amps with UL labels and FCC power ratings into 7 channels at 4 ohms. The maximum power will be no more than 150 watts into 4 ohms, all channels driven, if UL is on the back. This should cost you about $1500 to $2000.