Battery-powered "Busking" amp.

[darwindeathcat]

Yeah Brymus, I meant Hz. I edited it to say Hz now. Sorry...

Slimbo, that Lepai amp module looks interesting, and CHEAP! It says class T, that means it's using a Tripath chip. Someone would have to take the plunge to see if it sounds good though.... On e-bay usa, they are going for around $24 with shipping. (http://shop.ebay.com/i.html?_kw=Lepai%20TA2020&_fcid=1&_localstpos=85282&_sc=1&_sop=15&_sticky=1&_stpos=85282&gbr=1) Not a bad price at all for 20 watt X 2 channels, already assembled.

Here's the specs:

Features:

   * Tripath power IC.
   * Mini size design is good fit for home audio, car,motor bike and other audio device.
   * It has flash rainbow light in the front panel and very colorful at night.
   * Aluminum alloy case makes it looks more beautiful and good to distribute heat.
   * Compact Size: approx. 115mm (D) X 140mm (W) X 42mm (H)

Specifications:

   * P.M.P.O: 2X180W
   * R.M.S: 2X20W
   * Input power:12v 2A DC
   * Input sensitivity: 200mv
   * Input impedance:47k
   * Frequence response: 20Hz-20kHz
   * Speaker impedance :4-8ohm
   * Minimum THD :<0.05%
   * Signal noise to ratio: >100db
   * RCA input : 1 slot (mp3/cd available)

Follow this link to see a gut shot. It has an op-amp based preamp, and build wuality looks pretty good! http://cgi.ebay.com/Lepai-TA2020-Class-T-Amp-Mini-Stereo-Amplifier-Adapter_W0QQitemZ290369860539QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item439b6473bb

[Slimbo]

http://www.diyaudio.com/forums/class-d/90500-lepai-t-amp-ta2020.html

There is a huge 50 page thread on these amps in the DIYaudio forums, adding inductors and higher quality and different value components, bypassing the tone controls etc. One guy had a hand drawn schematic which I neglected to save. I don't think these chip amps can be paralleled or bridged, so I'm thinking the best way to set this up is send the same signal to both input channels and connect 2x8" drivers. I have ordered one, hopefully it will be here next week.

I think at the start I will experiment with the gear I have before trying out that reverb PCB from OCT Distribution, though I did email them and they answered and said they will be happy to freight one to me overseas, so that's excellent. Always good when a company replies like that, thanks OCT.

I have a Sure SM58, never tried harmonica through it, (any comments on the suitability of this welcomed), I will run it through a Sansamp Bass Driver to the amp.

I have also found a 12V 4AH SLA battery. I need to find out what voltage this puts out at full charge, because I think some guys had blown the chip from running of a charging car battery at just above 14v.
I also wonder if 4AH will be enough, at full noise drawing 2 amps we get around 2 hours? How will the battery like that kind of power draw?

So many questions!

[darwindeathcat]

Thanks for that link! It's a lot of reading, but I will definitely skim through that... Yes, the way to go is to split the input signal, and send it to both channels. Best is to have some small series resistance on each channel input after the split (1kohm on each leg of the split will do). Then send the output of each channel to it's own speaker. If it's truly 2x20watts, then doing it this way, the amp will be putting a full 20 watts through each speaker, making about as loud as a 40 watt amp. The amp will be loud, anyway.

Yes your sm58 will sound AWESOME for harp. It's own of the go-to dynamic mics that pros use (that and the sm57). It won't sound like a bullet mic (they use ceramic or crystal elements), but IMO, you are better off with a dynamic mic for a solid state amp anyway. Especially if you are using that Sansamp first. It will let you get any kind of sound you want. Should be a killer harp rig!

I do not know too much about the battery stuff. That's why I started this thread, actually... I'm pretty sure that class T amps draw MUCH less power than other chip amps of comparble wattage, so you might get quite a few hours out of it. Remember that you are not going to be blasting at full volume all the time, so you should be able to get a bit more time than the math tells you when calculating with peak values. Using RMS power consumption values should get you a closer estimate. Given what I've been able to learn about batteries, that one you picked sounds like it will work fine. I'm not sure what kind of power supply circuitry one should use for battery operation. I suppose at minimum, some small series resistance (100 ohms?) and at least one large cap to ground (about 100uf or so) couldn't hurt. Make sure these are rated for reasonably high power. Prob 1 or 2 watts for the resistor, and at least 20 volts on the cap. Someone else can better answer these questions though...

[darwindeathcat]

Here's a link to the announcement and discussion of that sweet octoboard reverb module from diystompboxes. My comments and replies from Frank are at the end:

http://www.diystompboxes.com/smfforum/index.php?topic=79388.0

[Slimbo]

I now have the amp, it is a small circuitboard and wont take up much room at all. Next step is to decide what size and how many speakers. I need 2, one for each channel, and I'm thinking 8" drivers, but this is only based on "I seem to recall reading that a few well known harp players prefer 8's". Their size and weight fits the design criteria, as long as they sound good. I think Adam Gussow talks about his combo's, one tube, one SS, both with 8's in his 2nd youtube lesson, but I may be wrong. I think the best design I have seen is on Bill Fitzmaurice's site with the fairly new XF Guitar Cabs.
Link here:

http://billfitzmaurice.net/XFCabs.html

The smallest is 2x10", I asked on the forum regarding the possibility of scaling it down for 8's. Bill replied "You can, though it will take quite a bit of graphics design skill to do so." My graphic design skills are high school geometry, but if its just a matter of getting the 4 angles correct on the speaker surround parts I think I could eventually figure it out, and scaling down the rest of the box will be easier, good enough for prototype purposes. All this should be ok as long as Im not missing some very crucial points, like the lengths of the inside dimensions cant be the same as the 1/4 wavelength of the 8th harmonic, or the change in box volume will mean the formally crystal clear highs will now sound like mud.

edit: check this out, very interesting. Open back, sealed, or ported?

http://www.webervst.com/sptalk.html

[Brymus]

Cool keep the updates coming.

[darwindeathcat]

Yes, we use 8" speakers because they break up quicker (ie. at lower volume) plus they aren't too bass heavy. I use speakers salvaged from old electronics. So far, the best I've found are 8" speakers from old organs. Great Tone! Some people report that the cheap Jensen MOD series 8" sound good for harp, and others will swear by Webers. I get my vintage speakers for about $5-10 bucks each, versus ~$25 for the cheapest new speakers, but you definitely will have to put the time in and search around to find those old drivers. Read the reviews of various speakers at this blog: http://bluesharpamps.blogspot.com/ He reviews both Jensens and Webers, and is thought to be a reliable source.

And yes, Adam mentions the 8" speakers in that vid.

[darwindeathcat]

I found a reply direct from Adam from his own Modern Blues Harmonica forum where he discusses why he likes smaller speakers for harp:

Isaac:

The only thing I need to add to your interesting post is that some of the distortion--for the best overdriven amp sound, I mean--needs to come from the speaker cone. It's generally easier to get this from smaller speakers (6", 8", 10") rather than larger speakers (12", 15"), but the key thing is that the magnets need to be relatively small. A Hartke 4 x 10" bass speaker cabinet will sound like crap for harp because the speakers have huge magnets, precisely so that they WON'T distort when you're running loud low frequencies through them.

Amplified harp sounds best when the various sorts of overdrive--preamp, power amp, speaker--accentuate a certain high-mid frequency range. One reason old tube amps sound great, I suspect, is because they (ideally) have original speakers, with old paper cones. I remember playing through a tweed Champ, mid 50s, with the original speaker. Incredible sound.

I have two Mouses. They're identical, except that one has a plain capped speaker and the other has a speaker with a projecting lip of some sort--about 1" high. A mini-loudspeaker coming off the cone. Much, much better sound.

I didn't know that the ideal in a solid state amp, for harp, is to overdrive the preamp but not the power amp. That's interesting.

[Slimbo]

Very interesting, thanks for that. So it may be a matter of starting a speaker collection and swapping them around untill the best sound is found.

I have been thinking a lot about the sound I want out of the amp, but I only have what I have heard on YouTube and downloaded mp3's to go off. I purchased a few of Adams lessons, I have downloaded a lot of the YouTube ones of Adams and Jon Gindick, and I have instructional material with cd's, my favourite being Brendon Power's "Play Irish Music on the Blues Harp". From these examples, I prefer the "clean" sound of the harps. When Adam demonstrates a technique, then plays it through an amp, it sounds harsh and grating. I am certain the little mike he may be using when recording the lessons is not doing the sound justice and it sound a million times better in real life, but I still like the sound of a undistorted harmonica. I think the amp should have the ability for a bit of overdrive, but I dont know if it is going to be as important in this amp as it would be in a amp designed for electric guitar. I kindoff pictured this amp would be used with acoustic guitar more that electric, maybe some electric bass (only because I already play bass, not sure how it will handle low frequencies).

Anyone care to comment on the need for distortion, and how much, for harp?

[darwindeathcat]

The best solid state harp amp I have currently is one I constructed from an old radioshack PA amp I bought for $8 at the Goodwill. It's a "Realistic" brand, model number MPA-20. Very importantly, it's an older one, and the circuitry is 100% discreet (ie. it uses power transistors in the poweramp section, not a chipamp). The newer MPA-20's use a chipamp, and don't sound nearly as good. The only modification I did was to remove the "phono" section (I'm not feeding it input from a record player!), and to change how the negative feedback loop get's tapped to the output transformer. Originally, the NF loop was connected to the 8ohm tap of the OT. I removed this tap, and added two new leads, one to the 16ohm and one to the 4ohm tap of the OT. I made it switchable. When you go to the 4ohm position, it get's crunchy and breaks up early. It's also a bit louder. When you go to the 16ohm tap, it's a little quieter, but MUCH cleaner, even cleaner than it originally was. Both settings have their purposes, and I think both sound really good. This amp also has two input channels (with two separate pre's and two separate volume controls), so it's a really versatile amp. I can plug both harp and guitar into it, or harp and a footdrum (porchboard). BTW, this amp also sounds killer with my cigar box guitar (it's an acoustic with a piezo pickup).
I built this PA into a custom cabinet I threw together with singe 25 watt 8" paper-cone speaker with a smallish magnet. It's one of a pair that came from an old organ. The BEST harp speakers that I've ever found.
Anyway, the point of all this is to say that PA amps are a really great place to look for nice solid state Harp amps. PA amps have been used for years for harp (eg. look how everyone loves those old Bogen PA heads), and a nice solid state one will work as well or better than a tube one. That amp you are buying (the Lapai) is basically a PA amp. I'm not very familiar with those tripath chips, but everyone seems to think they sound good for instrument amps. You are probably on the right track. If you don't get THE sound you want out of it at first, then you'll need to start experimenting with Preamp circuits until you find one that works well with that particular chip. I suggest starting with something like the Fetzer Valve or the Tillman pre (both very simple one-transistor designs), and then move on to something like the Peppermill or Blue Magic (relatively simple two-transistor designs). Any of these preamps should let you dial in a relatively clean tone, or get a nice mild overdrive.

[Slimbo]

Thank for the tips re preamps. If I can do overdrive easily and keep it cheap, no reason not too.

I have wired the amp to a 4Ah 12v SLA battery, it arrived charged from new, I havent used it long enough to comment on how long it will power it for. I hooked a Onda mp3/4 player and it sounded terrible. Distorted and not loud at all, nowhere near the volume needed for an amp that will do some outdoor work. I then tried a laptop as a source and what an improvement, the amp sounds pretty good, actually really good. With the mods the guys are doing in the link in my early posts, this would be one sweet hifi amp. Tone controls are not great, but they can be improved. It has run for maybe 5 hours at low volume at work here and the battery has dropped maybe 0.2V, to around 12.6. No heat that I can feel anywhere in the amp, the laptop power supply is warm. So a very positive first impression, I think this amp will be up to the task.

Next step it to hook a mike up to it. I have also found a delay pedal so I may get some idea what the reverb will sound like. What is the difference between delay and reverb anyhow?

Stay tuned, and please post comments.

Edit: I'm answering my own questions. Does it get any sadder than talking to yourself on an internet forum? Anyhow, http://www.harmony-central.com/Effects/effects-explained.html

[darwindeathcat]

Good to know that the amp is working! Sounds like you will definitely need a preamp though, and perhaps one with significant gain. Maybe even a lm386-based one? I know Armstrom on this form had success building a harp amp with a lm386-based pre. Look for the "smokey", "Ruby", "little gem", and "noisy cricket" schems. Those'll do ya well.

[Slimbo]

 I think the best choice will be the one that has the most tone shaping. The Noisy Cricket Mk II looks like the leader with a grit switch, gain, volume and tone. Smokey's got gain and a tone switch, Ruby's and Little Gem have volume and gain pots. I have had my eye on the Tonemender as well, so naturally I searched for it in this forum and found an improvement, the PhAbbTone box, by the username phatt. Might have to breadboard a few.

[armstrom]

one thing to keep in mind is that the LM386 won't break up as nicely at high voltages unless it's driving a low impedance load like a speaker. If you run the output directly into the input stage of a power amp (which is usually on the order of 10-50K ohms rather than the typical 4-16 ohm speaker impedances) you should lower the supply voltage to ensure you get good breakup if that's what you're after. The datasheet provides plots of the maximum output voltage swing vs. supply voltage for various loads. Unless you're driving a speaker you want to look at the plot for infinite load. To mimic the kind of clipping (maximum output swing) you would get when driving an 8ohm load with a 12V supply you will need to lower the supply voltage to about 7.5V. Of course, you could also add an 8ohm resistor to ground on the output to simulate the load as well. I did not try that with my setup though.

[Brymus]

I think the best choice will be the one that has the most tone shaping. The Noisy Cricket Mk II looks like the leader with a grit switch, gain, volume and tone. Smokey's got gain and a tone switch, Ruby's and Little Gem have volume and gain pots. I have had my eye on the Tonemender as well, so naturally I searched for it in this forum and found an improvement, the PhAbbTone box, by the username phatt. Might have to breadboard a few.

Check out my TDA1519b practice amp,it uses a lm 386 in the pre amp for its clipping behavior.
And went from a Dr Z tone control to a Marshall TMB in the last version.
Be sure you read the 386 data sheets and get the lowest power one (325 milliwatts)
I use that pre amp for tone shaping as a pedal in front of my other amps too.The last version can get any sound from chimy clean to over the top gain.
Think of it like a TubeScreamer feeding a Marshall tonestack feeding a Distortus Maximus.
And you have an idea of the gain available.