Copyright 1980:	EdwardT.Dell, Jr. Editor and Publisher
Reprinted by permission of:	Audio Amateur Corporation 305 Union St., P.O. Box 876 Peterborough NH 03458-0876

David Vorhis and I reviewed the Hafler DH-101 preamp in TAA 3/78, p. 33. Since then the preamp has been well received in the marketplace and is generally considered the best sound value for the dollar spent.

Now Hafler's second major product is upon us: the DH-200 stereo power amp. It is a neat, relatively small black box containing two 100W into 8 ohm rated stereo channels - no frills or embellishments, just the basic necessities. Fig. 1 shows the front panel.

The DH-200 is unique in that, to my knowledge, it is the first American production amplifier to use complementary MOSFETs in its output stage. Over the past several years much has been written about power FETs for audio. In the DH-200 these devices begin to provide the performance factors expected of them.

The DH-200 is available either in kit form at $300 or in factory wired form for $400. Hafler introduced the amplifier in the summer of 1979; delivery of wired units slightly preceded the kit versions. Audio Amateur received an early unit for review, before the kits had started rolling; consequently I can comment only indirectly on the kit and its relative complexity. Incidentally, after writing this review I bought the sample unit.

As for the kit, it is not difficult; the manual lists only 77 construction steps. As the interior view in Fig. 2 shows, the two amplification channels are contained on the two left-right heat sink assemblies. These heat sinks form the end "panels'' of the amp and mount the four MOSFET devices (two paralleled complementary pairs), along with lower level circuitry on each of the two PC cards. The chassis pan mounts the power supply components and the minimal interconnecting wiring.

Hafler supply the kit with amplifier heat sink modules prewired and tested, which simplifies construction and of course gives the builder confidence. Construction time is said to be 4-5 hours. 

THE CIRCUIT

The DH-200 circuitry is completely complementary push-pull and drives the MOSFETs with conventional bipolar transistors for amplification. An interesting feature afforded by the MOSFET properties is that the DH-200 has none of (nor does it require) the conventional volt-amp limiters, because the MOSFETs act simply as low valued resistors when turned on and have self-limiting properties (no thermal runaway).

The amp uses a fairly heavy power transformer, feeding a 25A full wave bridge, which develops +/- 60V (unregulated) to feed the two channels. For protection it has an AC line fuse, AC line thermal breakers which sense heat sink over-temperature, individual V + and V - fuses to each channel, and within-the-loop speaker fuses. Excessive temperature shuts down the amp altogether and lights the panel indicator (shown in Fig. 1).

Component quality level is consistent with the price asked. This means you see no esoteric parts, aside from MOSFETs. Resistors are mostly carbon film, except for a couple of power wirewounds. Aluminum electrolytics are used for bypassing and signal coupling, and a fairly large number of disc ceramics is also used.

The output binding posts are the finger-busting-to-tighten nickel plated type, and the input jacks are conventional tin-plated phono connectors. High level interconnecting circuits use stranded #18 wires.

BENCH TESTS

On the test bench the DH-200's performance is among the most impressive I have seen. It handles with ease just about everything you throw at it. Since most of my measurements tend to show very low distortion and appear to reveal no particular pattern, I'll summarize most of them focusing on those I feel to be significant in one way or another.

Hum and noise are quite low for both channels, measuring 82.5dB (R) and 85dB (L) below 1 watt with the input open. With a more realistic source termination these figures will improve a few dB. For a 100W output add 20dB to the S/N. As these are wideband measurements, what one actually hears will sound even better with most present-day signal sources. I measured both channels' gain as 26dB, which translates into a nominal 1.4V rms input sensitivity (for a 100W into 8 ohm output).

In short, the DH-200's distortion levels for both THD and SMPTE IM are extremely low. THID, for example, is basically near the residual level of a Sound Technology oscillator/analyzer at all but the highest frequencies and power outputs. At 20kHz at 100W of output it gets as high as 0.013%, and at a 50kHz 100W level it measured 0.04%! For levels from 1 to 100W below 20kHz it remained as a rule well below 0.01%.

That this amp can deliver such clean high level power at frequencies as high as 50kHz is, I am sure, due in large part to the MOSFET output stage. Many bipolar output amps will either self-destruct or melt fuses when asked to deliver 100W above 20kHz; I do not recommend such testing to the inexperienced. The DH-200 appears to shrug off such stresses and happily deliver clean watts.

	The waveform photo in Fig. 3 gives some insight into the virtues of the MOSFET output. It is a dual trace shot of the amp's output (top) and distortion produces (bottom) while delivering 100W into 8 ohms at 10kHz. Not only is the distortion (+ noise) low at a 0.005% level, it also does not exhibit the very strong "spike'' type products associated with typical crossover distortion for such test conditions.
	SMPTE IM measurements on the DH-200 revealed some of the lowest readings I have ever encountered on my analyzer. Over a 45dB range from 3mW to 100W (into 8 ohms) IM was never higher than 0.05% for either channel, and this was at the low end where noise is actually dominant (not true IM). At 100W output levels, both channels read 0.002% (equipment residual). The DH-200's IM specification is less than 0.005% from 100W down to 1W; at the 1W level we measured 0.003%, so the amp easily meets its specs here.
Perhaps one reason the higher level distortion readings are so good is that the DH-200 has a conservative power rating. I measured outputs of over 130 watts (8 ohm) from both channels at the clipping level. Into 4 ohms I was able to get about 250 watts from a single channel! When this amplifier does clip, it does so rather neatly and cleanly, relatively speaking. For example, Fig. 4 shows a clipped 20kHz triangle wave into 8 ohms. Overload recovery is fast and clean, with minimal sticking.

	Fig. 5, a square wave response picture taken at a low level, illustrates the DH-200's frequency response.    In this mixed sweep photo, the square wave's left half-cycle portion is at 10uS/cm. The expanded right portion shows the risetime of 2.5uS at 1uS/cm. The 2.5 uS small signal risetime is exactly as the amp's specifications state.
	Low frequency response is illustrated by Fig. 6, a photo of a 100W output level 20Hz square wave.

At high level, high frequency square wave outputs, the DH-200 holds up very well. Figs. 7a and 7b show the (+) and (-) slopes of a 12kHz square wave at a 100W into 8 ohm level. Again a mixed sweep presentation is used, with the expanded scale at 1uS/cm. As these two waveforms indicate, during the waveform's initial rise and fall some slewing is evident, at about 30V/uS for both (+) and (-) slopes. This is in fact the DH-200's specified slew rate.

While these photos and the discussion of the high frequency THD suggest a healthy HF output capability, the real evidence is in Fig. 8. This photo shows the DH-200's output waveform driving 8 ohms in parallel with 1uF, at an 80V p-p level at 100kHz. As you can see, there is a small (but damped) overshoot with about 20uS of ringing, which indicates good stability, into a very difficult load. You will appreciate the difficulty if you consider the amp's 13V/uS slew rate while driving the 1uF capacitor. To provide this rate of change in voltage across 1uF requires a current of 13 amps!

Most bipolar amps simply give up or oscillate when subjected to such a torture test. Typical protection circuits will simply not allow such high currents to be delivered. One notable exception is the Double 400 amp, and I recall noting how the extra output current improved that amplifier over the stock Dyna 400 (see TAA 2/77, p. 48). However, even that amp was not quite as fast as the DH-200 shown here, indicating it has less current available.

In their literature the Hafler people discuss at some length the output characteristic of this amp; apparently they feel it an important virtue in controlling undesired components ''kicked back" from the speaker. They rate the damping factor as 150 at 1kHz and 50 at 10kHz. I measured a damping factor of about 400 at frequencies below 1kHz, dropping to about 150 at 10kHz.

 

LISTENING TESTS

For listening tests I placed the DH-200 in my system, which consists otherwise of a PAT-5/WJ-lA, (TAA 3/79) Magneplanar MG11 A speakers, Linn Sondek LP12 turntable, Grace 940 arm with Denon 103D cartridge, and Audio Interface CST-40 transformer. Speaker, phono, and head shell leads are Fulton Musical Industries, and I use the 28' ''Gold'' speaker leads. (The MG-IIA speakers have been modified with gold plated binding posts, and the tweeter fuses are bypassed.) My listening room is roughly 24 x 26 x 7' with heavy carpet and acoustical tile ceiling.

The wall immediately behind the MG11 A's is covered with a burlap acoustical screen, and the remaining walls are paneled. My listening position is 15 feet away, equidistant from the speakers.

On hand for direct comparison with the DH-200 was an Audionics CC-2 amplifier, which is similarly priced. The CC-2 has appreciably less power than the DH-200, rated at 70 watts versus 100 into 8 ohms, but at lower levels it is not an unfair comparison. The circuit topologies are very similar except for the output devices, and the CC-2 runs + 45V supply rails while the DH-200 has + 60V supplies. Since some critics have in the past reviewed the CC-2 quite favorably, I felt the comparison with the DH-200 would prove interesting.

I had had the CC-2 on hand nearly a year at the time of thes6 tests, so I understood its sonic merits and faults fairly well since either it or another copy (used in a modification study) had been in almost daily use. I would sum up the stock CC-2 as a medium level performer with a relatively inoffensive but not spectacular overall sound. It is not a bad amp by any means, but neither is it a super star.

The CC-2's high frequency range is soft and recessed, which gives it a subdued sound. The lower range is reasonable, but does not go quite as deep or have as much impact as I'd really like. But considering the size of the power supply, it is really not at all bad.

The CC-2's midrange is perhaps it's least distinguished region, as it never seems to come alive or have as much depth as it should. In my room and with MG- 11 A speakers, which are notoriously power hungry, it sounds underpowered. Now this is not necessarily all the amp's fault, as 70W driving a pair of MG- 1 1A's in a large, carpeted room is asking for trouble. At any rate, clipping often occurs on high dynamic range recordings at good playback levels. Operating a bridged pair of CC-2s eases this situation appreciably, of course but sacrifices aural detail.

Given that they use similar circuits but differ in power output, one might think the DH-200 and CC-2 would sound similar except when pushed to high levels. This is not at all the case, however. The difference between the two is quickly apparent even at low levels. The DH-200 is more open and transparent, and brings you closer to the real sound of music. This was my first and strongest positive impression, and the one I feel sets it apart from the CC-2 (as well as many others, I should add). On certain material, the reproduction can be quite life-like and natural.

However, all is not exactly perfect with the DH-200. After listening awhile I realized I could hear some HF sizzle. This results in a discontinuity of sound between the upper midrange and lower treble regions. While the midrange seems reasonably clear, the treble becomes blurred and loses focus.

Feeling this problem was a potential limitation of the amplifier, I told Ed Gately, Hafler's president, of my general observations after my listening tests. He informed me that since our sample was built both sets of input differential pair transistors had been changed to a different type. He sent me a set of the new transistors; I installed them and retested the amplifier.

Since changing eight 3-lead transistors takes more than a few seconds, I could not evaluate the new transistors with a rapid before/after listening comparison. I did what I thought best: played the amp for several hours continuously before the change, made the change, reinstalled it, and listened again. Unfortunately, I could detect no obvious change in the sizzle.

At this point I tried some experiments with the remainder of my system, to note how different ancillary components might influence the sound of the DH-200. Substituting a pair of Precedent Audio MZMOD 2 speakers (which use dynamic drivers) tamed the sizzle noticeably, as did less exotic cabling. For example, substituting the more conventional inexpensive plastic jacket/molded plug input cables, and #12 solid copper wire cables with nickel plated banana plugs at the output made the sizzle less prominent. However, while this mitigated the sizzle, it also reduced the degree of ambience displayed and tended to make the sound image more two-dimensional. It would seem however, that the audibility of the sizzle depends somewhat upon the components with which one uses the amplifier.

To give further insight to the sound of the DH-200, it should be noted that the sizzle is by no means an overwhelming defect. But after one notices it as a source of unnatural sound, it does stand in the way of maximum enjoyment of a piece by obscuring the finer HF detail. In terms of the sound character, the sizzle gives a roughened sort of crispness or hashy HF sibilance on a speaking voice. Plucked strings are reasonably good in terms of initial transient attack, but have a fuzz and haze added on top. This HF problem is evident on various other types of program material as well, and seems to brighten the sound. It is not a true brightness, of course (as a high end frequency response lift would be): the hashy HF non-linear components added to the sound create an apparent brightness.

While noticeable this HF sizzle does not totally intrude on the overall sound of the amp. The sense of "aliveness'' holds up very well at high levels. Given a program content with good hall ambience, the DH-200 reproduces much of that ambience.

There seemed to be some loss of image detail and depth through the spectrum compared to my reference amp (the modified CC-2), with the reproduced sound appearing slightly coarser in texture. This is definitely not a major difference in degree. Were the HF sizzle problem not present, this type of comparison would be (relatively speaking) more important. And I certainly can't say the broad range coarseness is unrelated to the sizzle, so were the sizzle not there perhaps I would perceive the mild coarseness differently.

I would rate the DH-200's bass reproduction as about average. I feel it will satisfy most people, but those who like the deep, powerful floor-shaking type of bass may find it falls short. And I made my comparison through a direct-coupled preamp; preamps with more conventional HF rolloffs or rumble filters will sound subjectively thinner. I would observe as I did above about the CC-2's bass, that the DH-200 doesn't sound bad when one considers it has only 10,OOOAF per supply line of supply capacitance, or an energy storage of 36 joules. The popular audiophile trick of out-boarding a filter bank to increase the PS capacitance would probably soup up the DH-200 substantially as regards bass impact (and I'm sure people will do it routinely on this amp!).

Having discussed in fair measure what we can hear from the DH-200, I'd now like to put it in some perspective for you. I know I've been somewhat critical of the amp, but that's the purpose of a review; after all, I'm just reporting on what I hear.

Ed Gately challenged me to compare the amp with any other I wished, and I think I understand the nature of this challenge: it's not so much that the DH-200 will whip all comers, regardless of cost, but rather that it can realistically be compared to them. I don't feel the DH-200 is the near state-of-the-art amp that some have termed it; but this is certainly no put-down when you consider the price differences. OK, so the DH-200 can't go one-on-one with the $1500-2000 giants and win. But for $300 (or $4001 it is still one whale of a lot of amplifier; I would say as it stands it would still be a good value for many dollars more. And that is what is important to us as audio amateurs.

To put my comments on the DH-200's sound reproduction into a proper perspective, I don't think you should be too strongly deterred from considering the DH-200 because of the factors I noted. Overall it still generates an exciting sound which is powerful, detailed, dynamic, and musical to a high degree. This is largely what we want in an amp. As to whether or not you would be bothered by the sizzle, I recommend you go to a showroom or friend's home and listen. Better yet, borrow a DH-200 if you can, and see if it's a problem on your speakers.

MANUFACTURER'S COMMENT

At the kit price of $300, the DH-200 will soon become an audio legend and leave a solid state mark comparable to the ST-70 of David Hafler's Dynaco days. For the audio amateur, it is about as close to the ideal thing as could be imagined. I understand a rack mount kit option will be available. A bridging operation kit will also appear some time in the future, and I look forward to this one with particular interest.

It will be interesting to follow this amp's history. There's no doubt in my mind it will be a standard modification vehicle, just as the ST-70 was (and still is). It is gratifying to see that the Hafler people are interested not only in the audio kit market but also in the aural experience. And that is what counts the most about this amp.

When designing the DH-200 it was our desire to design an amplifier which would be in the 1980's what the Stereo 70 had been in the late 1960's. We feel we have fully reached that goal.

Walt's review mirrors the findings of other reviewers except in regard of his subjective listening tests. Other reviewers including those at the Audio Critic, Absolute Sound, Sensible Sound, etc. have not heard Walt's "sizzle.''

By the time this gets into print, the bridging kit to convert the amplifier into a 300 watt mono amplifier will be available, and we hope the rack mount kit will also be available.

Thank you for a very fine review.

"I believe the report to be fair in most respects in view of the fact that the Hafler is compared extensively against our CC2.

''First, I consider both amplifiers to be excellent. Our approach was to produce a highly listenable product with application potential in a large number of systems. As Mr. Jung suggests, the CC2 is known for producing a rather 'laid-back' (not spectacular) sound. As he also suggests, associated components play an important part in the over all system sound. We find the CC2 to be an asset in many lower cost systems using loudspeakers such as the LS3 5/a, the DQ10, the Mordaunt-Short Pageant and others. The CC2 is also effective with higher efficiency studio monitors with a tendency toward brightness. I think the DH-200 likely to be excellent with loudspeakers which are a bit reticent at the top end.

"Mr. Jung points out the excellent clipping characteristics of the Hafler as well as its ability to produce sustained high frequency energy without taxing the output stage. Although the CC2 is a bi-polar design, it too will produce rated output for extended periods at 20kHz without current limiting protection circuitry and performs well into loads as low as two ohms.

"We have incorporated a new transformer into the CC2, providing nearly 95W into 80 loads, single channel, and over 80W , both channels driven.''

CHARLES WOOD,

Audionics of Oregon
Suite 160, 10950 SW 5th Ave.
Beaverton OR 97005