Quick question

[newey]

Can one of these:



www.electronickits.com/kit/complete/ampl/CANUK193.htm

Accept an instrument-level input? The stat given is "input sensitivity 100mV / 47.5KΩ". I don't know what that means, except that the resistance figure seems too low . . .

Could this be modded to work with an instrument input? Or would I essentially have a line-level input that I would have to match in some way outboard of the amp.

I'm thinking of a powered cab, more or less, for use with an effects processor or pedals. I have a 1X10" cab doing nothing useful at the moment, with a 25-watt Fender speaker mounted. Household volume levels would be all that would be required.

[sydsbluesky]

Firstly, this is a question not an answer... sorry... but it's me, so... I know my weaknesses

So... the idea is that you want the impedance of the receiving unit to be HIGHER than the output impedance of the sending unit? Okay, so this is because the sending unit will attempt to maintain the current required to power the receiving unit, and if the load is too high (low resistance) it won't be able to do it?

Now on an amp it'll fry, but on a guitar it'll just not have any source of juice so simply be unable to do anything at all?

So starting up an amp with no cab hooked up would be supplying zero ohms, which is a load to end all loads, so it tries to power it anyway and overdraws... and that's why it burns? I'm thinking there is something else happening in that instance I just don't understand, but I'm trying.


I'm looking for a thread where ash applied ohm's law... Sumgai was in on it after ash, but I can't seem to find it at the moment.



I'm a bio major... I'm better with concept and theory than technicals, but I'm working on it.

When I wired my Dean I kept thinking of it like the ATP cycle. It helped. haha.




Okay, so another question!

Would running this through a pedal help... a pedal with a buffer, I mean.

[JohnH]

newey - that seems OK for input voltage sensitivity, maybe even too sensitive in which case you can pad it down with a couple of resistors if needed.

I agree the input impedance is too low for a guitar. It should work well with an input buffer (my usual JFETs would do it), or after a stompbox/processor that is not true bypass. These have quite low output impedances and will drive that citrcuit fine

John

[sydsbluesky]

Apr 23, 2010 0:12:11 GMT -5 JohnH said:

n. . . It should work well with an input buffer (my usual JFETs would do it), or after a stompbox/processor that is not true bypass. These have quite low output impedances and will drive that citrcuit fine

John

I'm chalking that up as a small victory.

[newey]

OK, so maybe it's not such a "quick question" . . .

John-

thatseems OK for input voltage sensitivity, maybe even too sensitive in which case you can pad it down with a couple of resistors if needed.

What would be the real-world result of "too much sensitivity"? I guess I'm not clear on what "input sensitivity" means.

And I was unclear on whether "100mV / 47K Ohms" was 2 separate parameters (i.e., sensitivity and input impedance both), or whether it meant "100mV divided by 47K Ohms".

If we say that 47K is more or less a line-level input, could this be set up with 2 inputs, one for line sources and another for instrument level? (I mean in an "either/or" fashion, not both at once)? Or would a separate preamp circuit be needed to do that?

[JohnH]

newey - those numbers are two different parameters. If im interpreting rightly, then the 100mV sensitivity means that an input voltage of 100mV can drive the amp to its max output. A lot of electric guitar playing probably results in outputs of that value, but it can also be a lot more on transients and strumming - I have noted voltages above 1V, even up to 3V on an outlandish nutz series wiring. So the risk is distortion on loud parts. But it could well be that the built-in volume control is able to control that, effectively making the amp less sensitive. Hopefully it is near enough to the input of the circuit that there is nothing before it that will distort first. But even if there is an issue with levels, two resistors, or a trim-pot will bring it under control.

The 47k is impedance or load that the amp puts on whatever is plugged into it. We would not want to load guitar pickups with pots of this impedance, nor dierctly connect to an amp input of this value. Normal guitar amp inputs are 500k or 1M. The result of plugging directly into this with a normal guitar would be dullness of tone, as when plugging directly into a mixer with no buffer or DI box. But if this device is always used with a processor, then it will be fine. Also, an acoustic guitar, or any active guitar would be fine.

So I reckon go for it, and stand by to add about a 100k trimpot to the front, if needed.

John

[ashcatlt]

syd - you are correct that a lower input Z wants more current from the source. Unfortunately, that's about where you lose it.

1) No speaker connected is not 0Ω, but (approaching) infinity. This is not a large load but a very small one. From what I've read tube amps can handle one state (0 or infinity Ω) and solid state amps can generally handle the other. I don't remember which is which, and don't rightly understand why it would hurt any amplifier to have no current draw on its output...

2) a passive pickup does generate current, but there's not much there, and the voltage is also generally quite small. You can't hurt these by leaving them open nor by shorting them. This is a good thing when it comes to wiring. As was stated above, the real "danger" of overloading a pickup (with a Z that's too small) comes from the fact that this Z goes in place of the R in the big RLC filter that we call a guitar circuit. Reducing the load impedance reduces the cutoff of the high-pass filter, and can cause a significant loss in "treble" response.

Like John said, a buffer before this amp fixes the impedance issue. If whatever processor you've got ahead of it has an output volume adjustment, you'd be all set. You'll want to be careful distorting this thing. Unless it's got some protection circuitry you could end up sending those nasty square waves to the speaker. That's a great way to melt a voice coil.

[sumgai]

My turn!

First, I was all set to blab out "Harmony! Synchronicity! Beauty! My work here is done!!"

Sadly, a moment's reflection aborted that feeling of euphoria. Here's why:

Yes, everything stated above is correct, with the obvious side-step of ash correcting sydsbluesky on one point, but for the most part, it's all cool. Where it breaks down is that you have all overlooked one rather measly little point: newey's linked device is a power amplifier - get that? it's only a power amplifer..... there's no preamp!

What this means is much worse than a simple impedance mismatch.... it means that there's absolutely no tone control at all - the frequency response is flat across the entire spectrum (or so the claim goes). The resultant tone from this thing, if fed by a "mere" buffer circuit will be unusable, to be charitable about it.

But not all is lost! For 20 bucks (USD) assembled, this is a very good value. The money saved here can be put towards any number of devices that will juice this thing right up. For instance, while I don't use 'em, I've seen enough of them to know a Line6 Pod will drive this thing just fine. A Johnson J-Station is about the same thing, if you happen to have one of those, or run across one on the cheap.

In brief, anything with tone controls (and that is meant for a guitar) will probably drive this amp, and make it sound at least reasonable. Even older Digitech/DOD and Zoom units will do the job. I know because that's what I used before I got into using Roland gear. But I'm morally certain that a standard stompbox, no matter what the bypass circuit used, will not make this amp sound good, unless it's one of those new Boss pedals with the Fender Legacy nameplate (the FDR-1 and the FBM-1).

In summary:

Can one of these ccept an instrument-level input?

The short answer is no, it can't. The longer answer is yes, it can, but only if you use a quaified pre-amp device between the guitar and this amp.

HTH



sumgai

[sydsbluesky]

Apr 23, 2010 10:35:02 GMT -5 ashcatlt said:

syd - you are correct that a lower input Z wants more current from the source. Unfortunately, that's about where you lose it. . .

Okay, I'm good with that on my end... That was the part that wasn't a blind guess. But hey, credit where credit is due!

Apr 22, 2010 22:41:52 GMT -5 sydsbluesky said:

. . .Would running this through a pedal help... a pedal with a buffer, I mean.

I had that one, too.



Oh, and then leave it to sumgai to blow the thing out of the water! Good stuff.

[ashcatlt]

I agree with sumgai to a point. This is (from the description) a clean, flat amp with no onboard tone control. You will probably want to add some color to it on the front end.

The caveat I've got is that the cabinet to which you intend to connect it, being a Fender brand 10", is not by any means a hi-fi, full-range speaker. It is, in fact, what we would call a woofer in any other situation. It will have that "warm fart" problem that ChrisK used to talk about. It will tend to roll off a big pile of the "treble" which escapes attenuation by the guitar's own low-pass filter network.

All this to say that "speaker simulation" of any sort would likely be redundant. Not to say it might not sound okay, but you're definitely not going to get this 1x10" cab to sound like a 4x12" closed back Marshall box, and it quite possibly won't be necessary at all if you're looking for a clean sound to begin with.

[ashcatlt]

Apr 22, 2010 22:41:52 GMT -5 sydsbluesky said:

I'm looking for a thread where ash applied ohm's law... Sumgai was in on it after ash, but I can't seem to find it at the moment.

Did you find it? It was your thread, dude! Just down a couple on this forum.

[JohnH]

Following on from Ash - I quite often plug straight into my PowerBlock amp, with all tone modifications bypassed or at zero, so the amp is giving a flat output all the way up to past what I can hear. This goes into a guitar cab, which is the main thing that colouirs the sound and makes it sound good. That set up is good for a crystal clean clean, (or with tweeters switched on, for acoustic). Then, the other boxes come in between amp and guitar for different tones.

[sydsbluesky]

Hot damn, Ash.

Hidden in plain sight.

[newey]

I didn't mean to confuse anybody- Yes, it's a power amp, no preamp onboard. To be used with a processor or pedal of some sort.

And clean is good by me.

I was mainly interested whether an instrument input was feasible, for FX gear that doesn't have a line out, or for pedals. But I can certainly live with a line in only.

For $20 assembled, I'm going to pull the trigger on this and see what I get. The kit to build the same amp runs $15, so I can't see not buying the assembled version for $5 more. I'm debating battery power as well.

[sydsbluesky]

Battery power? You could stick your speaker in a backpack and walk around town with the turtle amp!

There's a crazy dude downtown that does that every summer...

But really, though. Sounds very portable. Is this your plan?

[JohnH]

Hell yes! But you have to do the roller blades too.

This is Crazy Eddie, from Venice Beach SoCal:



go 'Crazy Newey' - midwest style: Guitar with backpack-amp and roller blades, but in the snow with board shorts!

John

[sydsbluesky]

And for the love of the gods please remember to take plenty of pictures.

[newey]

Sounds very portable. Is this your plan?

I don't know, it's a possibility. But at 20 Watts, I'm figuring it's going to be pretty power-hungry. Maybe just use a wall wart and be done with it.

[sumgai]

newey,

A pair of 9v "transistor radio" batteries will probably not be able to deliver enough oomph to make this thing sound out at all, or weakly at best, but even at that, they'd die off in a couple of heartbeats. However, a trio of 6v lantern batteries would probably do the job for several hours before winding down.

18v wall-warts are not common fodder...... But even at 12 volts, you should be able to make the bedroom windows rattle a little bit.

Just a thought.



sumgai

[JohnH]

Talking of 18v, how about a reused laptop power supply? Most users have several lying around from previous laptops, and they are fairly stable and used to giving out 50W or more.

John

[cynical1]

We just scrapped about 50 NIB Lenovo batteries/power supplies along with about 25 laptops when we changed vendors...man, I wish I'd have known...

HTC1

[newey]

Talking of 18v, how about a reused laptop power supply?

I thought of that last night and ran down to the basement to dig an old one I have out. But it's 19.5V, so I'd have to step it down a bit. But, I can probably scavenge one that's 18V or less, too.

And a trio of lantern batteries, without the ability to be recharged, doesn't sound too economical. Also sounds heavy . . .

[ashcatlt]

A laptop power supply is likely to be noisy. Some of these switching type supplies actually make an audible (like acoustic) whine. You'll need to build some kind of filtering involved. That extra volt and a half might just come in handy.

[sumgai]

Hehehe, forget the laptop brick..... just use the laptop battery itself! ;D

That's usually good for 50-70 watts for at least an hour and a half, and a laptop usually uses all of it's output capabilities, due to the screen (the biggest load) being always on. This amp, when idling or at low volume, won't draw nearly that much juice, which translates to the batteries lasting longer than expected.

Keep in mind that as batteries charge and discharge, they use up a finite amount of life expectancy. Used laptop batteries might not hold up for very long, comparatively speaking. But then again, they probably cost about the same as the oats that have been through the horse.

The idea of the laptop brick itself is also good, and very cheap. If you're concerned about the over-voltage condition, just insert two diodes (1N4001 or larger) in series with either lead, and that will drop 1.3 volts right there. Cheapest thing you can do, guaranteed.

And I don't know where ash gets his supplies, but I've never heard one whine. And I've got no less than five of them right now, two of them being at least three years old, and they're all silent as a submarine at sea. Even so, if you get one that does make noise, it's cheap and easy to replace, no?

We could get fancy here, but I think we've covered the simplest, cheapest and most reliable possibilities, don't you?

I say go for it! Crazy Newey, bring it on!!



sumgai

[ashcatlt]

Okay, the switching supply I could hear was actually for a cheap portable DVD player. Even when the device itself was off and just charging, there was a high frequency whine that came from the supply itself. It was very much like the sound of a CRT that's left on when the attached computer is off. Of course, I can (or could in high school) hear dog whistles.

I'm more worried, though, about the electrical noise - the AC wiggles that are going to ride on that DC power and find their way into the amplified output. There may be some power supply filtering built in, but you won't know until you've got it in hand.

I think that if you intend to run it from anything other than battery you'd ought to expect to add some filter caps. There will be some voltage dropped across this network, so it might not be such a bad thing to look for something a little bit extra big.

If it comes down to it, I've got a milk crate full of orphaned wall warts, I'd be happy to box up a couplefew and send them your way.

[newey]

I pulled the trigger on this amp last night, so it'll soon be meandering my way. The search for a used laptop battery and/or power supply now begins in earnest.

I'm also thinking of adding an onboard pre. To the extent that this is going to be portable (assuming use of a battery), it would be nice to have an "all-in-one" unit so as not to have to lug around a stompbox all the time.

There's a schematic floating around somewhere for a clone of an EH LBP-1. I could build one of those and add it to the front end of this thing. I'd then have 2 knobs- pre and post gain, but still no tone control. Wonder how easy/tough it would be to add a tone control to a circuit like that?

Another thought would be to use the guts out of one of these:



Which does have an unbalanced ¼" input, in addition to the XLR. That might give a little "tube warmth" to the signal, as it's got a 12AX7 aboard.

[JohnH]

Can you post that preamp circuit?

Or if you are interested in something designed to suit, I have several JFET based designs that do clean to dirty crunch, and tone control is not a problem. How much electronics construction do you want to do?
This is my simplest and best sounding, and all written up:

guitarnuts2.proboards.com/index.cgi?board=fx&action=display&thread=4709

As drawn, it does zero to very loud clean, or two settings of crunch.

It would need a tone control added, which could (probably best)be just a single knob, or a two or three knob stack. Also, it could be configured to run at 18V. A further stage could add a dedicated pre gain control.

john

[newey]

From the Free Information Society (www.freeinfosociety.com)


The one in the upper left is the schematic I was thinking of- looks simple enough. This would be a first as I've never built anything on perfboard before. Years ago, I had an LPB1 and liked it a lot. It sounded good, and was the only stompbox I've seen that had a plug as an output instead of a jack, so one could plug it right into the amp input, thus eliminating the need for an extra cable (but also no footswitch).

Thanks, John, yours looks good as well.

[JohnH]

newey - the image did not appear , but I i found it through its image properties.

Thats a simple one transistor building block amp stage, suitable for clean boost. It has a gain of about 20 to 24 db, or x10 to x15 with a 9V supply. That is likely to clip with a guitar, in a raspy way since its a standard silicon transistor. Some attenuation at the front would fix that. Its seems OK to drive the input impedance of that amp (i tested on 5 spice), but it has more gain than required - in fact according to your power amp spec which states 100mv sensitivity, you don't need much gain at all from a guitar signal. The circuit would also need to be changed to work off a higher voltage supply

Lets say given that you want a simple circuit, not significantly more complex that that, then I think we can do better.

My suggestion would be to plan the project in two stages. First, everything without the pre-amp, and wire the power amp directly to a jack. Test it through a stomp box to act as a buffer and see how the sensitivity is with a plain guitar signal. Does it clip at max guitar volume or not? Hows the raw tone? hence you can decide if you need gain, or tone control etc. Then I can help determine the right simple preamp to suit.

John

[newey]

That sounds like a good plan- see what I get before going further.