Lunchbox amp from ZT... 200 watts and under 10 pounds!

[Steve A.]

I heard about this from a friend and checked it out at the local music store:

ZT Amplifiers - The Loudest Little Amps in the World

The sucker delivers the power although I did think that the sound was too midrangy (good for high gain leads to cut through the mix but IMO not that well-suited for blues or jazz guitar). I got it anyway ($259 street) mainly because the designer Ken Kantor had offered to send a custom EPROM to someone on the Gretsch forum. Long story short: I got the custom chip in my amp and love it, but they did epoxy the chip to the socket at the factory in China so it could be a problem for the end user to replace the chip himself.

I was amazed that the EPROM has only 8 pins but that it has so much control over the sound of the amp. Well, it is like the software instructions for the DSP chip (which does all of the heavy lifting).

I did have a second concern with the amp: there is not a lot of clean headroom in the amp. If you turn the volume all of the way up and you keep the gain control in the clean range it does not push the amp anywhere close to full power; you have to push the gain control into a moderate overdrive to get some decent power from the amp. Hopefully there is a fix for that, too. That portion of the circuit is analog and Ken said that there are two parts that should increase the clean headroom (I am still awaiting further instructions on that).

In any case, I've been having a lot of fun with the amp. I got it mainly for sitting in at jams or with other bands- without having to haul around a huge amp to be heard. But for the hell of it I did play a full gig with it last Sunday night, with and without a small ext cab, and it worked fairly well. Both running direct into the amp and using my pedal du jour- the Boss ME70.

Steve Ahola

[tubeswell]

So lets cut to the chase Steve, what's inside it? got any pics?

[Steve A.]

So lets cut to the chase Steve, what's inside it? got any pics?

I just found the memory card for my digital camera the other day so I guess I might as well take some gut shots of it- although I strongly doubt that anyone could build a clone of it.

And I guess I ought to record some sound samples, too.

(And I thought that I was going to get to take the day off and go sailing! Back to work.... kee-rack!)

Steve Ahola

[Enzo]

He has a descriptive white paper on the site, but it is not too informative.

The 200 watts is peak, he claims about 120 watts continuous.

The output is bridged, that means no part of th output is ground. Make sure the barrel of your speaker cord plug - if you use an external cab with it - never touches any grounded things, like chassis.

DId he use chip amps or discrete transistors for the output stage?

[Steve A.]

He has a descriptive white paper on the site, but it is not too informative.

The 200 watts is peak, he claims about 120 watts continuous.

The output is bridged, that means no part of th output is ground. Make sure the barrel of your speaker cord plug - if you use an external cab with it - never touches any grounded things, like chassis.

DId he use chip amps or discrete transistors for the output stage?

Enzo:

Thanks for the warning about bridged outputs! Here is what Ken had to say about amplifier wattage ratings:


Understanding Amplifier Power

Nobody has come up with one number that can fully represent the power capability of a guitar amp. RMS is a very conservative approach, but it misses the issue of dynamic power and real world musical signals. At the other extreme, Peak Power can readily be inflated to the point where it is not a reliable indicator of anything. In fact, the actual power of any given amplifier varies widely at different frequencies. Every amplifier company has their own procedure for stating the power capability of their products, which ideally should be done within the boundaries of accepted technical standards and legal requirements. Here is our thinking on the matter, which we believe results in specifications that are both conservative and useful.

The internal speaker of the Lunchbox is nominally 6 ohms, with a DC resistance of 5.3 ohms. Electrically, the amplifier output stage runs differentially, using a 48V rail. The output stage can source in excess of 41Vp or 28Vrms into the speaker load. This yields the following typical specs:

Maximum Instantaneous Peak Power: 280W
Musical Peak Power: 236W
RMS Power: 130W
Sustained Average Music Power: 200W

[TD_Madden]

The Rev. Billy G used his for tomorrow night's "VH1 Storytellers" episode.

[Steve A.]

The Rev. Billy G used his for tomorrow night's "VH1 Storytellers" episode.

So does the Lunchbox allow you to look into the future, too? I will have to read the manual more closely!

Steve

[Enzo]

I take nothing away from his product, but yammering about peak power and all that is a standard marketing ploy in audio. You can say those exact same things about any amplifier.

My example is the 900 pound piano. If you grasp under it and yank upwards, you can get it off the ground for an instant. That is peak power. But you can;t lift it and hold it. That is continuous power.

[Steve A.]

I take nothing away from his product, but yammering about peak power and all that is a standard marketing ploy in audio. You can say those exact same things about any amplifier...

That quote was hidden in one of the publications on his site. Although the 200 watt figure is used, I believe he refers to the actual sound output more often in his sales literature: over 120dB on the main pages- and over 134dB at 1 foot in the Advanced User Guide.

Actually I should have read his explanation about amplifier power before putting 200 watts in the title of the post here- but it is rather impressive to show the little amp to someone and tell them that it is 200 watts...

Steve Ahola

[teemuk]

I take nothing away from his product, but yammering about peak power and all that is a standard marketing ploy in audio.

Everytime I see a manufacturer marketing their product with peak power I instantly remove the safety switch.

IOW, I also have a bit of distrust on that product. It's a class AB amp with a linear power supply and therefore cannot be quite efficient. Yet, all the electronics are packed inside a sealed cabinet. The rear panel is the heatsink but there are no fins so it will be terribly inefficient in heat dissipation. I just have to wonder how long that amp will last when being cranked full up because I imagine the output devices are really toasting in there.

Comparing this thing to typical 100W amps with big power transformers and plenty of heatsinking I find the small size of the cab a bit disturbing too.

[jrfrond]

I have seen the Lunchbox in live action for the last two months. The guitarist in my band is Billy Penn, of Penn Amplification and 300Guitars.com. He is a die-hard tube guy, and went nuts over these, so he now uses the Lunchbox onstage in our fairly-loud and energetic wedding band. He still has trouble wrapping his head around the fact that it sounds and feels so good, is pretty damn loud and so compact AND reliable. He beats the piss out of the Lunchbox for four or five hours at a time and it has never hiccuped.

This prompted me to buy one myself. I have to say that these little things border on revolutionary and have major cool factor. Sure, we've seen OTHER good, compact amps like the G-K 250ML, but this one is just "right". Trust me.

[H Chandler]

I contacted Ken Kantor and asked him how they got the 200 Watt number and he answered:

The 200W number is based on peak voltage. Other numbers we list, for more sustained signals, are based on a THD of under 10%. Just FYI, the >minimum< THD you will find on the LB, at any power level, is about 1%.

200 Watts is the equivalent RMS power available for 100mS tone bursts from 250Hz to 2500Hz. The actual driver impedance is used in the calculation.

I also asked about continuous power and got this answer

The answer is that it depends:there are sophisticated thermal limiting/SOA mechanisms built into the amp, that make testing it the way you suggest [using a 1kHz sine wave into a dummy load]not very straight forward. The longer the signal is applied, the lower the apparent "RMS" power will be. If you measure it within 0.5 seconds, you will see an unclipped RMS power approaching 130W. At the other end of things, the RMS will tend downward towards about 60W, steady-state.

----
Is this standard practice for rating solid state amps? Is this why there is so much confusion comparing tube vs solid state power?

I have a question to you guys that have heard this amp. How does it sound compared to a clean Fender Deluxe Reverb with a JBL speaker for playing clean solo chord melody jazz?
It seems like with such a small box the lows would just not be there.

[Steve Conner]

OK, so what we have here is a 60 watt amp. The remaining 140 watts are marketing. Not really surprising, if it could deliver 200W continuously, I wouldn't rate the chances of that 8" speaker surviving for long, and of course there is the previously mentioned lack of cooling fins.

I don't believe there is any standard practice for rating solid-state amps, but there is a kind of de facto standard for good quality PA amps. Basically, the power figure on the front is what you can get unclipped - "RMS" - into a dummy load, but the transformers and heatsinks are too small to keep that up indefinitely. So on a sine wave test it would overheat and cut out after a couple of minutes. However, music isn't a continuous full power signal, so in practice they work just fine.

Poor quality PA amps are rated the same, except on the sine wave test they explode instead of cutting out.

The Lunchbox designers say in their white paper that they use some fairly aggressive distortion processing to make it sound louder, so I wouldn't be too optimistic about its performance on clean jazz.

[tedmich]

60W + 10lbs = LM3886+SMPS

BFD....

[tedmich]

I take nothing away from his product, but yammering about peak power and all that is a standard marketing ploy in audio. You can say those exact same things about any amplifier.

My example is the 900 pound piano. If you grasp under it and yank upwards, you can get it off the ground for an instant. That is peak power. But you can;t lift it and hold it. That is continuous power.

Enzo you reminded me of the 3mm displacement "weight lifting" feats of the late guru Sri Chinmoy

this is my favorite, it was on the wall of his followers veggie restaurant in seattle


~34 x 45lb plates on each side...plus bar (?) = +7000lbs... with one arm!
[
As to amps, why not tout the dB peak like the white noise pulse "burp" favored by the "competition car stereo" crowd?
YouTube - Car audio competition