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1W mini-amp built-in to bass. Thoughts?

#44
UncleFluffy said:
80 ohm phones should be fine unless they're crazily inefficient. There's at least 6V p-p available to drive them which gives about 1/2 a watt. If voltage gain is a concern the LM386 data sheet shows how to boost it up above the 20 this circuit is running at.
Click to expand...
I don't follow your math. 6 V p-p ~= 2.1 V rms. The load is 40 Ohm (two 80 Ohm, driven in parallel). From P = V*V/R, I get 110 mW, or 55 mW per ear.

Regardless, BeyerDynamic DT770 Pro (80 Ohm version) are rated at 100 mW maximum (per ear), so I think the LM386's output will be adequate.

Whether operating the LM386 at a gain of 20 is good enough would depend on the output level of the pickups. Do you know the range of output level that one encounters among popular pickups? Obviously, increasing the gain of the LM386 will aggravate whatever noise problems it may have.
 
#45
PhiDeck said:
I don't follow your math. 6 V p-p ~= 2.1 V rms. The load is 40 Ohm (two 80 Ohm, driven in parallel). From P = V*V/R, I get 110 mW, or 55 mW per ear.

Regardless, BeyerDynamic DT770 Pro (80 Ohm version) are rated at 100 mW maximum (per ear), so I think the LM386's output will be adequate.
Click to expand...

You're correct. My math was sloppy. As you point out, I neglected the single-ended drive and the parallel wiring of the phones.

PhiDeck said:
Whether operating the LM386 at a gain of 20 is good enough would depend on the output level of the pickups. Do you know the range of output level that one encounters among popular pickups? Obviously, increasing the gain of the LM386 will aggravate whatever noise problems it may have.
Click to expand...

This is intended mainly for active basses so I haven't concerned myself too much with unbuffered pickup levels. It works fine with a couple of passive basses I have here but (a) levels vary, and (b) the loading of the amp may affect tone.
 
#46
UncleFluffy said:
Yes, that's a trade-off for board size. It would be better up at at least 100u, preferably more. I'll see if I can source a non-polarised electrolytic that's small enough to make it work then update the schematic accordingly.
Click to expand...
Regarding the need for the large DC blocking capacitor at the LM386's output, have you considered an alternate approach, creating a virtual ground (@ 0.5 Vbatt), to which the headphones jack's sleeve connects? In which case only the input to the LM386 need be AC coupled, which requires a much smaller value (and size) capacitor.

Unfortunately, it appears that virtual ground integrated circuits, such as TI's TLE2426, don't sink or source quite enough current. However, I vaguely recall designing a discrete equivalent decades ago.
 
#47
PhiDeck said:
Regarding the need for the large DC blocking capacitor at the LM386's output, have you considered an alternate approach, creating a virtual ground (@ 0.5 Vbatt), to which the headphones jack's sleeve connects? In which case only the input to the LM386 need be AC coupled, which requires a much smaller value (and size) capacitor.

Unfortunately, it appears that virtual ground integrated circuits, such as TI's TLE2426, don't sink or source quite enough current. However, I vaguely recall designing a discrete equivalent decades ago.
Click to expand...

That's possible but I suspect that it would end up taking up as much board space and costing more than just using the big blocking cap, and if we're going to a virtual ground why not bridge instead - the complexity is about the same?

The other reason is that it's safer to keep the jack sleeve at ground potential in case this is fitted to a conductive surface.
 
#48
UncleFluffy said:
That's possible but I suspect that it would end up taking up as much board space and costing more than just using the big blocking cap, and if we're going to a virtual ground why not bridge instead - the complexity is about the same?

The other reason is that it's safer to keep the jack sleeve at ground potential in case this is fitted to a conductive surface.
Click to expand...
Agreed, bridging would be even better. Normally, with a different signal in the Left and Right channel (and only three wires) it would not be an option, but in this case, where you drive the left and right speaker with a mono signal, it's doable. The main problem is that current bridging ICs that I could find seem to operate from 5.5 V (max.).

Your point about mounting the headphone jack to a conductive surface which may be grounded is well taken, and would apply equally with a bridge driver circuit, although it could be overcome by mounting the headphone jack with an insulating shoulder washer.
 
#49
PhiDeck said:
Agreed, bridging would be even better. Normally, with a different signal in the Left and Right channel (and only three wires) it would not be an option, but in this case, where you drive the left and right speaker with a mono signal, it's doable. The main problem is that current bridging ICs that I could find seem to operate from 5.5 V (max.).
Click to expand...

There's a number of all-in-one bridge amp chips available which could do the job, but in the end simplicity is the #1 goal for this design.

PhiDeck said:
Your point about mounting the headphone jack to a conductive surface which may be grounded is well taken, and would apply equally with a bridge driver circuit, although it could be overcome by mounting the headphone jack with an insulating shoulder washer.
Click to expand...

Yes, that's true but it still leaves the headphone wiring less assumption-tolerant than I'd like.

Thanks for the ideas though - these would be very useful in a higher wattage version or if the speaker was internal instead of external.
 
#50
UncleFluffy said:
I've been noodling around this evening and have breadboarded up a simple 1W amp that's small enough to be fitted inside the control cavity of a bass.

<snip>

- is this useful to you?

- what features would make it even better for you?

Much thanks...
Click to expand...
2 words - Blue Tooth. To the computer, to a receiver/ D/A converter on the amp or soundboard, to the iPhone ... If it can be small enough to wear on my ear & powered with a tiny rechargeable battery, it will fit in the cavity, no problem.
 
#51
251 said:
2 words - Blue Tooth. To the computer, to a receiver/ D/A converter on the amp or soundboard, to the iPhone ...
Click to expand...

Heh. That's doable, but I do digital + software for my day job so try not to touch the stuff when I'm noodling around for fun instead of $.

If anyone did want to take it on I'd guesstimate that you could build yourself one for under $50.
 
#56
Ladies and Gentlespoons ...

DSCN1628.jpg


(First prototype build, no sound clips as yet, got to run to the corner store and buy beer, jam session in 10 minutes).
 
#57
Update: guitarist was a no-show, so I have been noodling around with this for an hour while drinking the beer. Initial thoughts using it "in action":

- Even with a 220u output cap it's very bright. That's actually because it's fairly flat up to 20k+, but that may not be what people expect to hear.

- It's loud and you can hear a lot of detail in your articulation. That's probably good for a practice amp.

- This may be down to the hacked-up wiring job I did for testing but it picks up a lot of hum and RF. Might need to add a little filtering.

If anyone wants one, it's a viable DIY project now, but it could stand another round of tweaking.
 
#58
Updated:

Final schematic

schem.png


Note: C3 is a non-polarised electrolytic.

Frequency response

freqresp.png


If you want it to roll off earlier, increase C5.

Noise

noise.png


Note: the 60Hz bump is almost certainly hum pickup in the test harness.

Distortion

thd-amp.png


The THD graph looks scary, so I tested a piece of wire as a control:

thd-wire.png


Hrm, the wire still has the spikes at 1kHz, 2kHz and up. An artifact of the cheapo Behringer outboard ADC/DAC I'm using? If you subtract one from the other the distortion figures look pretty good.

PCB artwork

nopre.png


NB: This artwork isn't for the board pictured above, this is for the revised schematic in this post.
 
#59
Finally :)

One caveat though. The recording setup looked like

DSCN1630.png


and there is this monstrosity 20 feet away outside the window:

DSCN1631.png


so the bass was picking up a lot of electrical noise.

Link Removed

As a reference point, with just the cable plugged in but no bass the amp is practically silent.

Ibanez SR535, flats, preamp flat, pickups blended 50/50:

Link Removed
 
#60
The SR535 is my wife's, so maybe not the best bass to do a test install on. Instead, I did an initial test on "Bernie The Beater" - a heavily abused Dean Edge 09 I keep in the office to practice on while the computer's busy baking stuff.

DSCN1635.png


Amp and battery on the control cavity cover. (Bernie is passive, so needs a battery adding. Fortunately, there's plenty of space.)



DSCN1636.png


Cover screwed back down.



DSCN1640.png


The headphone jack is hardly noticeable.
 

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