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opc 8th January 2012 11:38 PM

The Wire - Low Power Ultra High Perfromance (LPUHP) 16W Power Amplifier
10 Attachment(s)

Hi Guys,

Well, here we go again... but this will definitely be the last in the series for at least a year while I work on something a little more involved.

What is it? It's basically a version of The Wire headphone amp on steroids that is meant to drive your loudspeakers! You're looking at either a differential or SE input, followed by 8 parallel LME49600 buffers. This gives roughly 1.42 ARMS of output current, and a voltage swing or roughly +/- 15 V or close to 11 VRMS. If you're driving an 8 ohm load, it's good for about 15-16 watts before distortion creeps above 1%. It has 18dB of gain, so you get full power output into 8 ohms with about 1.3VRMS input. The gain is easily adjustable to suit your needs.

What is it good for? Well, if you have reasonably high efficiency loudspeakers, and don't need ear bleeding levels of output, then this is bar-none the best measuring amplifier I have ever seen or used. It also has something special and a little unique about the way it sounds. There's something strangely romantic about it, like a really good low power SE tube amp, but without the fuzzy haze that usually comes along with that category.

How did it come to be? As with several of my other projects, I had some empty space on a PCB panel and couldn't let it go to waste, so I designed this and laid it out in a few days, and a little to my surprise, it just kinda worked! I also had been thinking about the concept of it after having used the BAL-BAL version of The Wire headphone amp to drive some 100dB/2.83V/1m loudspeakers I was measuring and testing. With the BAL-BAL headphone amp, the output was almost high enough, but I could still get it to clip if I pushed it too hard. That's basically how this thing was born.

Each board is a complete monoblock amplifier, including the power transformer, a fully regulated +/-18V PSU, balanced or SE input, a heatsink for the output buffers, and the loudspeaker outputs. You just build it, plug it in, and go. It measures 2.5" w x 6.8" l x 1.8” h so it will easily fit in a variety of extruded aluminum enclosures, making it a very easy project to complete.

How about technical merits? Well, with lower output power, and very low idle dissipation, a fully regulated supply is actually realistic to implement. Add to that the extremely high PSRR of the amplifier itself and you get a noise floor that sits at about -147dB with a few 60Hz (and associated harmonics) creeping up to -130dB. I was using this with a driver that was 103dB/2.83V/1m and I couldn’t hear anything with my ear right against the cone. It’s that quiet.

As for distortion, it’s the first time I can honestly say “un-measureable” for a power amp. The measurements are attached, but if you loop the AP back into itself, you get the exact same thing, so there’s just nothing there. Same applies to IMD.

How about frequency response? Well, if you use the LME49990 as specified, then you’ll be ruler-flat to well past 100kHz at full power without any problem. Trying front-end op-amps with higher slew rates will likely get you much higher.

So what are the downsides? It’s simple… power and cost. You don’t get a huge amount of output power, but generally speaking you get more than enough for most applications. With very high efficiency speakers (95-100dB) you’ll get painful levels without any problem. With high efficiency (90-95dB you’ll get very loud levels without any problem. If your speakers are lower efficiency than that, you start to give up output. As for cost, the output stage is a little on the expensive side. It’s close to
$100 just for the buffers and op-amps, and the total cost if you order all the parts from Digikey is about $205 plus taxes and shipping per channel.

This leads me to the next question… I have 50 boards total, and I’m not sure if I should bother offering kits or not. I would need everyone to order a kit, so if the numbers are 50/50 it’s not really worthwhile. I’ve settled on $20 per board for the PCB, and if you include all the parts with taxes and Paypal, it’s basically $239 per channel including the PCB. That’s by far the most expensive kit I’ve ever offered, but on the plus side, it’s genuinely complete, including the transformer and every required part.

Is there any interest in this? Is there anyone out there looking for a reference level amplifier with lower output power? As always, questions and comments are welcome!


opc 8th January 2012 11:46 PM

9 Attachment(s)
Attached are the measurements. As mentioned above, looping the AP directly, input to output, gives exactly the same thing except for the noise floor which is the amplifier itself. Any of the distortion measurements fall into the realm of "un-measureable".


opc 9th January 2012 12:08 AM

3 Attachment(s)
Attached is the schematic, the layout, and the BOM.

mwmkravchenko 9th January 2012 12:32 AM

Looks very interesting!

Maybe a lab amp for torture testing speakers on my "test bench" (computer desk in reality).

qusp 9th January 2012 12:33 AM

yeah i'd be in for a couple of pcbs to try on my tweeters and maybe show off at the next headfi meet for low efficiency planar magnetic headphones, saying 'you call that a headphone amp?' 'THIS is a headphone amp!!'

as far as full kits though hmm, its not something that i need in a hurry so i would probably just buy the parts in dribs and drabs with other orders as well as using stuff already in my parts bin, probably even my own transformer as i have a couple that would suit, so a kit wouldnt be of much interest to me on this one.

nice work BTW, i knew this was coming, but seeing it, it just looks so over the top lol

qusp 9th January 2012 12:40 AM

whats limiting the power btw, the dies themselves? for instance if you silver epoxied the buffers to a cpu fan forced heatsink could you get it up a bit more, to say 20W into 8R with 3vrms input?

hochopeper 9th January 2012 12:41 AM

This looks like just the thing my tweeters have been waiting for.

Count me as down for:


IanAS 9th January 2012 12:54 AM

Hi Owen,

Can you suggest why it sounded worse when you paralleled two LME49600 for the headphone amp rather than having one?

opc 9th January 2012 12:54 AM

It's really the current limit on the LME49600 that limits the power. You get about 250-270mA peak from each buffer, so the overall current limit happens at between 2-2.16A peak for the 8 buffers. If you used a higher voltage op-amp, and the LME49610 then you could run the rails right up to +/-22V which would net you about very close to 20W into a 9 ohm load.

The LME49600 actually allows quite a bit more more current during short bursts, but rating power than way can quickly become misleading.

If you needed more power but wanted to take a similar approach, then you'd need to look at bridged outputs with about 40 buffers. That would give you roughly 26 VRMS and 3.3 ARMS into 8 ohms for roughly 85W. The cost of that would be pretty obscene though :)


Wolfsin 9th January 2012 01:04 AM

Wow, regardless of how much my hearing might deteriorate I could use that to drive my Senns :-) Can it be partially populated in terms of buffers -- I mean until I really need 'em?

On the serious side, I would be in for a couple (maybe three) boards of your design if all they did was heat my garage; not in kits. There is a lot of ten+ year-old Aragon iron in the closet barely broken in, testimonial to how little the speakers get used.

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