Review of the Hypex UcD180AD amplifier modules

Introduction

This little experiment began when I dug out my old Harman Kardon Citation 12 stereo power amplifier which had been passed down within the family from my uncle to my dad. My dad used it for several years until I inherited it in the late 1980's but later I bought an NAD 2100X amp in 1992. Then my brother used it for a while and I finally got it back but at this point on of the pre-stage transistors in the right channel began to have problems. After listening to the good channel I realized that even though this might have been a decent amp in it's time (late 1960's) it used way too much feedback and had a very grainy and harsh sound. So I had to decide whether to restore it and sell it on Ebay (because I wouldn't use it) or strip it and try to find a kit that would work with the transformers in the chassis. This amp was very conservatively rated at 60wpc (I read somewhere that someone measured it to be capable of 180wpc before any significant distortion showed up) and the power supply transformers are quite big and heavy. It is a dual mono design with a very nice looking chassis. My dad called it an "anchor" since it is so freakin' heavy for a 60wpc amp. So I figured that it would be a great foundation for a new dual mono amp, I just had to find the right amps to use with the power supplies.

I measured the power supply and determined that the output was +/-39VDC and went seaching for kits that would work with this voltage level. I narrowed the choices down to a few chipamp designs (LM3875, LM3886, LM4870), some kits based on the Tripath digital amp chips (www.41hz.com Amp1B bridged per channel) and the Hypex UcD180 digital amplifier modules. In the end I chose to go with the Hypex UcD180AD modules (which I got from www.diycable.com) based on a lot of recommendations and their reputation. The UcD180 modules will work with 30VDC to 45VDC power supplies so the 39VDC that the Harman Kardon puts out would work well. I went with the more expensive "AD" modules which use higher grade opamps. In the end it cost me around $300 for the required parts since I already had some of the parts in my basement stockpile and didn't have to buy transformers or a chassis. I would still like to try the Amp1B kit based on the Tripath TA2022 chip which provides 60wpc into 8 ohms and may do so in the future.

So now that I decided which amp to go with I began working on the power supply. There is a schematic of the circuit below. I replaced the electrolytic power supply caps with some larger (and newer) 10,000uF Panasonic electrolytics. I added 0.1uF/100V ceramic snubber caps across the bridge rectifier. I went through several revisions of circuits for DC offset protection and muting features. In the end I didn't go with the DC offset protection circuit because it could only hurt the performance and the Hypex modules have built in overcurrent and overvoltage protection so the only time the DC offset circuit would trip is if the amplifier module failed. So in the end I just went with a simple startup muting circuit which mutes the modules while the power supply capacitors charge. I tried a few versions of the muting circuit (and had a bad zener diode at one point) so there were a few issues before. However, this final circuit shown below works fine during startup and shutdown. During shutdown the sound plays for a few seconds after the unit is unplugged but then mutes gently and instantaneously before the sound gets a chance to fade or distort.

Test setup

When reviewing amplifiers I always use my Yamaha A-700 integrated amplifier as a reference point for comparison. I try to keep all other variables constant in the comparison so the only difference is the amps. I will discuss this in more detail later on. I use a relay switchbox to switch the speaker connections between the amps. I use my Asterion speakers for all amplifier comparisons as well. The biggest weakness of the Asterion speakers is harmonic distortion below 200Hz which makes the bass and lower midrange less defined. The weakest link in this audio chain is the equiment used in front of the amp and I expect to get some criticism regarding this and my evaluation results. However, I feel that as long as those variables are kept constant between amps that my analysis is still valid. I still expect criticism that my source equipment doesn't have enough resolution to reveal the details that each amplifier can convey. You'll have to read more to hear my arguments regarding this.

My CD player is a modified Marantz CD-67SE and you can read about the modifications at this link. I'm using a Marantz SR4500 receiver as a pre-amp in "pure direct" mode with the digital output from the CD-67SE so I'm using the internal D/A converters in the SR4500. I'm working on a passive preamp using an Alps 50k motorized potentiometer so I can use the analog output of the CD-67SE and take advantage of those modifications previously discussed. However, I have compared the sound between the analog output of the CD-67SE and the pre-out of the SR4500 using the digital signal from the CD-67SE. I hooked both outputs into my Yamaha A-700, matched the levels and compared them using Sennheiser HD-580 headphones. They definitely do sound different but the differences are much more subtle than I would have expected. The sound from the SR4500 seems a bit more focused and detailed with greater dynamics and crisper highs. The sound from the CD-67SE analog output is more three dimensional, smoother and has fuller bass. The difference in the bass sound is probably the most noticeable difference and probably infuences some of the other impressions regarding detail and imaging. I have a feeling that the Burr-Brown opamps that I modded the CD-67SE with had a lot to do with this because this is one of the main things that changed when I installed and evaluated them. So based on this comparison I would think that the SR4500 pre-out would provide higher resolution due to the less spacious soundstage and thinner bass. The key is that the source equipment was kept constant between amps so no extra variables were added to the equation. I may prefer the sound of the CD-67SE analog output with the Alps pot after long term listening due to the smoother and more spacious sound which may be less fatiguing but for this evaluation I used the pre-outs from the SR4500 due to more volume control flexibility.

Hypex UcD180AD versus Yamaha A-700

So using the setup described above of the Marantz CD-67SE cd player sent digitally into the Marantz SR4500 receiver in pure direct mode and then sent from the pre-outs to the Hypex and Yamaha amps. A relay switchbox selects which amp is connected to the speakers with an LED to indicate which one was hooked up. A wired pushbutton switch is used to trigger the relays from the listening position and it is impossible to tell which position the switch is in. For blind tests I just cover the LED on the relay box. The levels between the amps were matched perfectly by ear after a lot of experimenting. I spent three days comparing the amplifiers and on the final day my friend came over to give his impressions. For most comparisons listening levels averaged in the 80-85 dB range (my ears and speakers can't tolerate much higher levels than that). My friend thought that any differences would be more pronounced at higher levels but I felt that since I never listen at those levels it didn't make much sense for me to try to evaluate them in under those conditions. I use the Yamaha A-700 in direct mode and "auto class A" mode (which basically turns it into a space heater). One nice thing about the CD-67SE is that it has an A-B repeat feature that allows you to repeat a particular section of a song over and over which makes it easier to evaluate differences fairly.

Over the first two days I compared the amps with a lot of different CDs. I found it very difficult to tell the difference between the amps but began to generate opinions. The problem was that even though I knew what to listen for when comparing the two amps after a while of evaluating while knowing which amp was playing I still couldn't consistently distinguish between them in a blind comparison. I would often pick the wrong amp during blind tests. It wasn't until I used Phil Keaggy's "Lights of Madrid" CD to compare the amps that I was able to correctly pick which amp was which in a blind comparison but even then it was extremely difficult. With this CD the Yamaha sounded a bit more "forward" and detailed compared to the Hypex. I noticed this mostly with Phil's classical guitar and I guess being a classical guitarist myself helped but I later realized why this revealed the differences. It wasn't until my friend noted similar differences that I realized that I was describing them differently. He said that the Yamaha sounded like it was more dynamic and after thinking about it I would have to agree with him. It was the plucking of the guitar string that revealed the differences for me and since the Yamaha was (ever so slightly) more dynamic these notes seemed more forward and detailed to me.

My friend began evaluating the amps by first listening to the Phil Keaggy CD where he came up with the comment about the difference in dynamics. However, he didn't attempt to do a blind comparison at this point. He next listened to the CD layer of the Dire Straits "Brothers in Arms" SACD and struggled to hear a difference but still didn't attempt a blind comparison. In an attempt to hear differences in tonality he asked to listen to the Cranberries "No Need to Argue" CD but felt that the amps were tonally identical. It was at this point when I asked him to do a blind comparison. The results were inconclusive because he often picked the wrong amp. I felt that this wasn't the best recording to expose any differences between the amps. I recommended that he listen to the Innocence Mission's "Befriended" CD next because I felt the resolution was much better. During this comparison there was a point where he correctly picked which amp was which on one particular track and stated that the difference was in the dynamics but it was still extremely hard to pick out. He later tried more blind comparisons and was once again unable to distinguish between the amps conclusively. In the end he concluded that he really couldn't confidently distinguish between the amps.

So based on my analysis the Hypex UcD180AD modules and the Yamaha A-700 are extremely similar in terms of imaging, soundstage, tonality, etc. but the Yamaha might have a slight edge in dynamics when used in auto class A mode. At this point I know a lot of you will say that my system doesn't have the level of resolution to be able to tell the difference between these amps. You may have a point but I'll still say that this is the most similar amplifier comparison I've done to date. I've compared the Yamaha A-700 to several other amps and the differences were night and day compared to this analysis with the Hypex modules. The second most similar amp I've compared it with was my friend's McIntosh MC-240 tube amp but there was still no confusing the two in a blind comparison (as to be expected but the similarities were greater than I expected considering the McIntosh is a tube amp). Other amp comparisons include my Marantz SR4500, an older Pioneer receiver, a low power SET amp and a few others but the differences were blatently obvious. I even compared the Sonic Impact T-amp to my Yamaha to notice very significant differences that were easily distinguishable (however, after seeing the response plots for the T-amp I suspect some of the differences that I heard were due to the slightly boosted top end of the T-amp). My T-amp review can be found here. I'm curious if Tripath's TA2022 chip (60wpc) will reveal similar properties but that's an experiment for the future.

Conclusions

I know that this review might stir up a lot of controversy among the Hypex fans but I try to be as objective as possible within my means. I admit that my associated equipment isn't the greatest and I haven't had the chance to audition many truly high end amps in my system but still feel that my analysis is valid. Don't think that because the UcD180AD modules sound very similar to my Yamaha A-700 that they aren't good. The A-700 is my favorite amp and I have yet to top it although the Hypex amp is just as good and doesn't heat up the room like the Yamaha does. I was really surprised at what I discovered during this evaluation but have read good things about both the Yamaha A-700 and the Hypex modules. My friend said to use the Yamaha in the winter and the Hypex in the summer.

I would appreciate some feedback regarding my review above. If you have compared the Hypex or T-amp to other high end amps I would like to know what your impressions are. My email can be found under the "About Me" section. I realize that amplifier preference is highly subjective which is why I like to focus on the differences and not comment on which is better because different people have different tastes. Please share your experiences with me because my main goal is to learn and maybe find an amp that I like better than the Hypex UcD180 modules or Yamaha A-700.