mini-amp questions.

[newey]

I bought one of these from Guitarfetish:



It is a "mini 'tube' amp. Supposed to be 2.5 watts, not actually a "tube" anything but supposedly emulates tubish tones. It has a trimpot on the board to adjust the amount of "tube tone" one desires. A full-sized gain pot is attached as well, with a click "off" detent at full CCW.

According to the claims, it can be installed into a guitar, replacing one of the tone knobs, or it can be made into a mini-amp with a speaker. The claim was that one can install this into a guitar and plug directly into a 4X12 cab and have it sound pretty good.

It came with virtually no technical information other than a photo of the PCB with the various wiring points indicated. (I'd scan the picture, but my printer died) The connection points are labelled as follows (looking as in the photo, and proceeding CCW around the upper portion of the PCB):

1) Battery+
2) LED+
3) GND (Batt-, guitar signal -)
4) Input from guitar
5) JK+ (unamped signal out to jack)
6) SPK + (to speaker +)
7) SPK- (to speaker negative)
8) HP+ (to headphone)

Anyway, I decided to try to build a tiny 9V amp out of this rather than putting it into a guitar. But there was no indication of what speaker impedance this thing wants to see.

I emailed Guitarfetish, and got the response that "speaker impedance is a technical specification between your head and cab, and has nothing to do with our (product)."

I understand if it is installed in a guitar, that is (sort of) true. But if I'm loading this circuit directly with a speaker, doesn't it indeed matter?

I'm thinking GF misunderstood my question, but before I make a complete fool out of myself, am I all wet on this?

I have a couple of surplus guitar speakers around that could be used, an 8" and a 10". Both are 8Ω. Am I going to destroy this thing if I use one of those?

[ashcatlt]

I'm sure that they misunderstood your question. It is quite relevant for this particular product. I certainly wouldn't want to see you plug the speaker outs into the instrument input of an amplifier. In fact, if I was to install this into a guitar I'd use something other than a 1/4" phone jack for the output, just to avoid unhappy accidents.

I'd guess that if they're recommending plugging to a standard guitar cab that an 8 ohm speaker won't be far enough off to hurt anything. In any event, I think you'd be able to get a warranty claim based on the fact that they told you to do it, and failed to provide proper advice when you asked for it.

You know what else this thing might work for? Sustainer driver.

[newey]

Ash-

Hadn't thought of a sustainer driver, but you're probably right, it would work for that.

I certainly wouldn't want to see you plug the speaker outs into the instrument input of an amplifier

But that's exactly what the directions say to do if you're installing it into a guitar. It comes w/ a stereo phone jack for battery disconnect, terminal "JK" is the unamped out, SPK + and SPK- go to the jack. Supposedly can be used as an onboard pre-amp or plugged straight into as speaker.

Is there any way to test this to determine what load it can handle?



















b

[ChrisK]

Is there any way to test this to determine what load it can handle?

Yes, hook it up to a 25 Watt 10 Ohm rheostat, run it at full power, turn the rheostat down toward 0 Ohms, and when it's too much, you'll be the first to know.

Or, you could look at the ICs on it and post their part numbers (or a photo of the module). We could then search a'web and get the data sheets.

[newey]

Just a single IC on it, 2 rows of 4 pins. It reads:

20730
JRC
HO81B Or it could possibly be a "0" instead of an "O", hard to tell.

Photos aren't going to be possible without a more Macro-ish macro on my camera.

EDIT: I emailed GF back:

Andrew:

Thanks for the prompt reply, however, you may have misunderstood me. I understand that, when used as a preamp into an amplifier, it doesn't "see" the speaker impedance. But I want to make it into a mini-amp, driving a speaker, as your description indicates can be done. With the speaker load directly off the PCB, I would imagine that the speaker impedance would matter very much.

I'd love to get a schematic of this so I knew better what was going on. But one can't trace out the circuit because the pot covers 3/4ths of the topside.

EDIT 2.0: Here's the listing with the über-hyped description:

Tube Guitar Amp Circuit

[ChrisK]

Click on the part number to the right of "Datasheets" half way down the page on the left side.

search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=NJM2073D%23-ND

It's an NJR (a division of JRC) NJM2073D

It's used in a bridged configuration to get 2 Watts of output at 16 Ohms.

2.5 Watts may be optimistic. ;D ;D


Now the bad news; ya ain't gonna get even 2 Watts of DC power out of a 9 VDC smoke alarm-sized batt'ry. You need a real power supply or a beefy wall wart. I'd wanna use at least a 500 mA @ 9VDC supply.

Maybe using a 9 VDC batt'ry causes that there "tube sound".


And, at 2.5'ish'ish'ish'ish Watts of output power, the power amp might just get somewhat toasty.

[newey]

Quick work, Chris! So, optimally a 16Ω speaker?

Wall wart point duly noted. I can always do the male-to-female 9V battery clip trick and use it with a wart, or with a battery for rocking in the woods.

[ChrisK]

Quick work, Chris! So, optimally a 16Ω speaker?

Do your homework - read the data sheet.

[newey]

I did read the data sheet, it indicates 2 watts at 9V into 16Ω. Unless I'm missing something, a not unreasonable possibility . .

[ashcatlt]

To be fair, many of us haven't been reading data sheets for a living for the last 80 years .

I didn't see anywhere where it said anything like "minimum connected impedance".

I did notice that it doesn't say anything at all about running at 9V into any less than 16 ohms. Might this be a clue?

I also noticed that it seems to generate less power at a given voltage into a lower impedance. This is contrary to everything I know about power amps. Of course that is limited to the information available in screen print on the back of PA type amps, which tend to indicate that lower speaker Z means more output power.

Also, just how much voltage can we pump into a standard pedal or amplifier input before things get scary?

[newey]

for the last 80 years .

Ouch!

[newey]

I received this reply from GF today:

These like low impedance speakers- they can drive anything north of 2 ohms- So a 2 ohm speaker would be ideal for output volume- 4 ohm is OK- 8 ohm would have very little volume. Just don't use a 1 ohm PA speaker- that will fry it.

Ok, now I'm mystified.

[ChrisK]

While the specifications tell the story from the maximum perspective, the graphs are helpful in seeing the continuous function.

At the bottom of page 8, on the left side is a graph titled "Maximum Output Voltages BTL." We are using BTL since this is bridge connected.

It indicates for 4 Ohms only up to a 4.5 VDC supply. It goes up to 6 VDC for 8 Ohms, up to 9 VDC for 16, and up to 12 VDC for 32.

An observation of the 16 Ohm line indicates that at a 9 VDC supply, there is 14 VDC of output voltage. This is peak to peak since this is bridged. In reality, each output can generate a 7 VDC peak drive signal.

Since this is at 1 Khz, this is a sine wave. The R.M.S. of a sine wave is the peak divided by 2^{^0.5}. This gives a 5 R.M.S. output voltage. This correlates to 1.5625 Watts at 16 Ohms.


Moving on the the "Power Dissipation vs Output Power" graphs on page 9, we see that at a 4.5 VDC supply (the maximum recommended for a 4 Ohm load) and 10% THD,

a 4 Ohm load can have an output of 0.8 Watt at a power dissipation of 1 Watt.

an 8 Ohm load can have an output of 0.6 Watt at a power dissipation of 0.55 Watt, and

a 16 Ohm load can have an output of 0.6 Watt at a power dissipation of 0.25 Watt. If we look at this curve for 1% THD (that sine wave thing), we see that for a power dissipation of 1 Watt, we get a power output of around 1.55 Watt (which correlates with what I derived above).

So, for a given supply voltage, providing that the maximum output current limitation is not exceeded, a lower impedance results in higher output power. It also results in significantly higher power dissipation (a bad thing).


There are three limitations afoot; the absolute maximum rated supply voltage (15 VDC), the absolute maximum rated peak output current (1 A), and the absolute maximum rated power dissipation (0.7 Watt for the DIP package).


Let's start with the absolute maximum rated power dissipation (0.7 Watt for the DIP package). The "Power Dissipation" curve on page 8 indicates that the 0.7 Watt rating starts to reduce as the ambient temperature goes up above 25C. While most humans are not comfortable at 50C, it's likely that the local ambient temperature around the chip and on the PCB WILL go that high.

That means that for long-term reliability, we should operate this at about 0.5 Watt of power dissipation. For minimum power dissipation, the supply voltage should be as low as possible for proper operation as a given load.

This would be about 1 Watt maximum at 16 Ohms from a 9 VDC supply.

For the long-term survival of the IC, limit the supply voltage to 4.5 VDC for 4 Ohms, 6 VDC for 8 Ohms, and 9 VDC for 16 Ohms.

Limit the power dissipation to 0.5 Watt.

Also, just how much voltage can we pump into a standard pedal or amplifier input before things get scary?

That can be derived only from the datasheets of the affected components.

[newey]

For the long-term survival of the IC, limit the supply voltage to 4.5 VDC for 4 Ohms

AHA! 3 C-cells perhaps?

6 VDC for 8 Ohms

Or 4 . . .

The "Power Dissipation" curve on page 8 indicates that the 0.7 Watt rating starts to reduce as the ambient temperature goes up above 25C. While most humans are not comfortable at 50C, it's likely that the local ambient temperature around the chip and on the PCB WILL go that high.

Well, the guitar nut guy in my head says to put in a little cooling fan liberated from an old laptop, running on a separate battery/power supply. The more practical side of me says to forget the whole thing and make this into a pedal of some sort.

[ChrisK]

What impedance speaker do you have anyway?

[newey]

I have a 10" or an 8", both 8Ω. However, I may search for a smaller speaker/different impedance since I want it to be smaller than an 8" speaker would allow.

I've been meaning to move a guitar into my office, so 1/2 a watt or so might be a good thing. Give me something to do on my lunch hour.

[ChrisK]

You could get one at Radio Shack but it may not be of low enough quality for use in a guitar amp.

[newey]

Yeah, I don't see a pair of Celestion Greenbacks in this project's future . . .

[ChrisK]

Celestion Greenbacks

Talk about a product name predicting future price......

[andy]

Not unlike the Les Paul Gold Top, or other early guitars commanding Stratospheric prices.

On the flipside, The Blues has been cheering people up for years now!