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XM4 Initial Impressions

post #1 of 53
Thread Starter 
OK first let me preface this with the statement that I am not the audiophile that many of you are, and my impressions are based on, shall we say, more of a layman's point of view.

My experience with headphone amps is limited to the Headroom Micro with the desktop module. I use this amp with the Micro DAC at home (FLAC, MP3) and alone with my iPod (Cryo Dock) at work. My main cans are the Sennheiser HD-650 for home listening, and the Etymotic ER4p IEMs for work.

I listen to all sorts of music, but for testing I used Norah Jones "Come Away With Me". I also used a variety of rock music such as Nickelback and Green Day since that's what I often listen to at work. I know what I like, but I'm not always good at describing it.

I wanted to get a dedicated work/travel/mowing/whatever rig so I could leave the Micro amp at home. To this end I purchased a set of Sennheiser HD-580 headphones and a Practical Devices XM4 amp (I'd use the Etys mowing and traveling). I am very happy with the HD-580s as you can read in my thread in the headphone forum.

I like quality gear. I'm willing to pay more for the good stuff because I'm rarely disappointed when I do. I'm also pretty good at figuring out when "more money" does not equal "better".

The XM4 is an adorable little amp. I swear that was the first thing I thought when I opened the box. The packaging is minimalistic, but I paid for an amp, not a pretty box, so I'm fine with that. I'll spare you the details about it's look and finish, other than to say it's a good looking little amp.

When I first plugged in the IC cable, I noticed something odd. The cable was not sitting perpendicular to the faceplate. Indeed, I could move the cable freely back and forth and up and down. With some pressure I could impact the signal. Ick.

Out of the box the little amp drives the HD-580s, but the volume is at about two o'clock with normal listening volumes. No problem, open the amp and you can pop on some jumpers to boost the gain. Simple enough, but a switch would be better. In fact I'm surprised there's no switch for gain. This thing has variable defeatable crossfeed, defeatable bass-boost, auto power-off (4 settings!), an LED dimming feature (nice!) and a friggin' flashlight mode. A gain switch would have been nice. Anyway with the gain up, the volume is now at about 9:30 and I'm rockin'. The problem is that with the gain increased, there is an audible hum when using my Er4ps.

Actually - the hum got me thinking - it sounded like a good old fashioned 60Hz hum, and sure enough it was - the hum exists when the amp is plugged in. Since you can charge and play at the same time (assuming rechargable battery of course), I would do that - a lot. While on battery only, there is a slight hiss, but nothing I could hear with music playing.

The bass-boost is a nice feature, because to my ears the amp sounds a tad anemic. The problem is that with the base-boost enabled, I think it sounds too boomy. I am not a bass-freak and boomy bass is a sure way to turn me off. Between the two, I like the base-boost on.

The Cross-feed feature seems to be one of those love/hate things. On my Micro amp I love it - no - I *adore* it. It's subtle, but when you turn it off you miss it. Beautiful! On the XM4 the crossfeed is adjustable which is a nice touch. The problem is no matter how I adjust the crossfeed, I hate it. First the volume is cut significantly with crossfeed enabled. Second, it seems to sound even more anemic. Bass-boost to the rescue right? Not so fast. It's almost as if the cross-feed lowers all sensitivity, but the bass-boost only boosts the bass. Now it's even more boomy.

The volume seems to be uneven somehow on my copy. On low volume, when I slowly increase the volume, it increases first in the left, then the right. Also with the gain up and the ER4ps in place (low gain phones to be fair), I cannot turn the volume to "off". There is always sound. Once at normal listening volume this is not an issue, but now I wonder if the left side is louder than the right...

Now with all the negatives, I can almost certainly say, that if I did not have the Micro amp to compare it to, I would love it to death. The Micro has ruined me though.

For the price ($165 shipped with rechargable battery and adapter), it's a nice little amp. I'd be happier if it had better jacks, but otherwise the build quality is pretty solid. It can't compare with my Micro amp, but it's not designed to.

For my money, I'd spend the extra money and get a Hornet or one of the other premium amps. If that's not an option, the XM4 might be just the ticket.

post #2 of 53
Thanks for the review.
Assuming it is about the same as an XM3, I will suggest rolling the OPA2134 Opamp to an OP2227.
post #3 of 53
This was one of more pleasant reviews. Good Job!
post #4 of 53
I just got one delivered this week. I think you have something wrong with you input jack. I can't wiggle the IC cable at all. I agree with you about the cross feed. I can't hear it do much besides lowering the volume. My biggest complaint is that the bass boost button is to close to the headphone jack. I'm using the amp with a HD580 as well. I've tried a Grado plug adapter and a Radio Shack adapter. Both push on the bass boost. I wish the buttons were on the back.
post #5 of 53
Originally Posted by ClieOS View Post
Thanks for the review.
Assuming it is about the same as an XM3, I will suggest rolling the OPA2134 Opamp to an OP2227.
I'd really recommend trying the OPA2107 as well, I'd say it performed better than the OPA2134 in every way. I didn't find the OPA2227 to be as much of an improvement, but YMMV. Heck, they're both pretty cheap to try.
post #6 of 53
An AD8620 is also a good alternative.
post #7 of 53
I would first recommend you contacting the Seller and see about getting a replacement. That jack could become problematic in relatively short time.

Nice job with the review!
post #8 of 53
Thread Starter 
Thanks all. I'm not so sure the jack is "bad", but rather "inexpensive".

I really wanted to love this amp, but I think I've plummeted off the cliff of high-end quality and this amp just isn't what I'd call "high-end quality".

My current thinking is that I'll return it (they offer a 30-Day money back policy) and put the money towards one of Ray Samual's amps.

post #9 of 53
Really good review. I've been on the fence about getting this amp, but I think I'll continue to wait for a Hornet now.

In Skylab's review of the XM4 he noted the loose jacks too so a lot of them must go out that way.
post #10 of 53
Just purchased the XM3 from a fellow Head-Fier. Sounds pretty similar to my experience. The headphone out jack is just fine but cable shifting can cause the input side to short out. This makes it useful as a transportable amp but not as a true portable.

I'm also using HD580's. I've noticed that with the amp in the High Gain position when I get to about 1:30 on the volume knob the amp is driven into distortion.

Overall the sound is cleaner than the headphone jack on the receiver with more controlled and present bass and it's a must for bedroom listening with just a source component at my disposal.

Just ordered an AD8620 to see if the gain issue can be addressed and to see if sound can be changed up a bit.

Running off the supplied 9V NiMH now. I'm also going to pick up an external power supply to see if the increased voltage addresses the gain stage issue.

Overall it's a nice build quality and a good sound. For the $75 I paid with the ability to change up the Opamps I'm more than happy to play with it for a while until I can justify a home tube amp which I suspect is the perfect mate for the 580's.

Oh as a side note I tried them out with the Sennheiser CX-300's out of the iPod Nano and noticed an improvement in depth and detail though not terribly dramatic. I would definitely want something a bit warmer for those particular headphones. It's obvious that a line out cable and a few tracks converted to Apple lossless are necessary to really hear what's going on there.
post #11 of 53
We've just completed qualifying a new supplier for the jacks for the XM4. The new jacks are consistently tight and have less wiggle than the old jacks (with the old jacks, most were tight, but some suffered from varying degrees of "looseness").

All units shipped after May1, 2007 will have the new jack. Thanks go to GAD for flagging this issue.

post #12 of 53
Thread Starter 
Nice to see the manufacturer here addressing an issue!

Any chance of swapping out the version I have with one of the newer ones?

post #13 of 53
Yes, no problem. Send me an email and I'll set it up.

post #14 of 53
Nice customer service there.

AD8620 is in the mail...

Impressions to follow.
post #15 of 53
OK so I am officially blown away. I had assumed that the transition would take many hours of burn in to be effective.

Attention Practical Devices.... If you want a giant killer for high impedance cans look to the AD8620 for your solution.

First the amount of power available to the headphones with a simple chip change is fairly staggering.

When going with the Stock OPA Op Amp there was little difference between the Amp and my headphone output jack on the STR-DA3000ES. Now that the chip is changed I might as well be listening through ear-muffs through the stock receiver output.

There is a slight grain to the signal that was not present before but knowing what I know about burn in I'll be willing to bet this issue alleviates itself.

The signal is simultaneously more detailed, more open, and more accurate. I thought perhaps I would sacrifice soundstage for accuracy but I'm wrong. Intrinsically I should have known this, accuracy in the high frequencies is the father of soundstage.

I'm terribly excited at this point I will keep this amp as a portable solution and begin to look at tubes for the home.


Any experience to lead you to believe that there's another op-amp with a larger soundstage?!?

Your call so far as it relates to a Sennheiser HD580 is nothing but dead on.

I love everything about the change but I could stand a bit more warmth. If threre's a solution that provides the improvements of the AD8620 with a more tube-like character... I'm all ears.
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