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Converting a BA Sx to battery power -- is it worth it?

Started by JLT, March 19, 2015, 09:19:31 PM

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JLT

Quote from: JLT on April 05, 2015, 04:20:39 PM
Update on my own project:

The Radio Shack unit turned out to be wonky, so I ended up ordering two kits from Alltronics: a preamp kit and an 18 watt amplifier kit. Got them yesterday, but won't be able to put them together until next week. Found what was described as a "sub-woofer" in a thrift shop for $2.50, and I'll see if that works as a speaker.


Update to my update: I built the Alltronics kits and hooked them up to the "woofer" and found that the woofer just couldn't handle the power. I hooked up a speaker from a compact stereo and that worked better, but started distorting  with the volume control on "2" out of "10." Do I need to drop another resistor between the amp and pre-amp to lower the gain? Or should I just get a speaker rated for 60 watts rms and see how that works?

And speaking of speakers, would any sub-woofer or woofer do, or should I get one specifically for a bass guitar amp? I do have an EQ box I can run the signal through to optimize the sound. The speakers I'm looking at are:

http://www.ebay.com/itm/Single-8-inch-8-ohm-Home-or-Studio-Woofer-Replacement-Speaker-Custom-Cabinet-/201028325758?pt=LH_DefaultDomain_0&hash=item2ece38d17e

http://www.ebay.com/itm/8-Woofer-with-Poly-Cone-and-Rubber-Surround-70W-RMS-at-8ohm-DJ-PA/261566982131?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D30251%26meid%3D3710a194a5454fe9aabf1a8d2dcbfc77%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D201028325758&rt=nc

and FWIW, the schematic for the preamp is at

http://www.alltronics.com/mas_assets/acrobat/KIT_98.pdf

and the power amp:

http://www.alltronics.com/mas_assets/acrobat/KIT_105.pdf

I currently have a 20K pot installed between them for a volume control. That's not specified in the on-line schematic for the power amp, but is included in the one I got for the kit.


Roly

Quote from: JLTcouldn't handle the power

Which is likely to be the case with just about any consumer/domestic speaker these days that you pump a genuine 15Wrms into.

The preamp has a problem, very low input impedance, only about 60k without the optional 10k killer resistor.  Passive guitars don't like.  You need at least a buffer of some sort (e.g. FET) to get it up to at least 1Meg.

It also could have more gain than you need and you can moderate it by increasing the value of R6/100 ohms to say 220 ohms.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

JLT

Quote from: Roly on May 01, 2015, 12:10:09 AM
Quote from: JLTcouldn't handle the power

Which is likely to be the case with just about any consumer/domestic speaker these days that you pump a genuine 15Wrms into.

The preamp has a problem, very low input impedance, only about 60k without the optional 10k killer resistor.  Passive guitars don't like.  You need at least a buffer of some sort (e.g. FET) to get it up to at least 1Meg.

Thanks for your response. Can you give me a little more information on this ... what kind of FET, and how to wire it in, and what other components might be needed to support it? If you could just direct me to a schematic, I'd be grateful.

I omitted the "killer resistor" as specified in the kit instructions, since I wasn't going to using an electret condenser mike. Instead I'm using two piezo pickups, as furnished by JJB electronics.

QuoteIt also could have more gain than you need and you can moderate it by increasing the value of R6/100 ohms to say 220 ohms.

At this point, I'm not sure how much gain I'll need, since that will depend on the speaker. I'm assuming that a speaker with a 70-100 watt (RMS) rating will need a true 15 watt output to drive it, at the very least.

Which prompts me to ask that question again: are either of the two speakers I cited earlier going to be good "fits" for this project? If not, where should I be looking.

Again, let me thank you for your response. I didn't expect to need such hand-holding, and I'm grateful for your help. I get questions all the time like this in my own areas of expertise (lutherie, etc) and I know how exasperating such questions can be.

Roly

Quote from: JLTInstead I'm using two piezo pickups

In which case the input impedance will be way too low, 2.7Megs being more typical.  Will sound very thin and trebly.



N-channel FET such as an MPF102 or 2N3819 (o.n.o.).  Drain to V+, Source to ground via "RS", that 10k resistor (see, it came in handy for something!) and to the input.  2.7Meg resistor from Gate to ground, "RG".

Shouldn't need C1 and I think your amp already has a cap at C2 (input).


I'm talking voltage gain which should not be confused with power output.  15 watts into any speaker is going to make quite a racket.  Whatever maximum power a speaker is rated for it will still produced quite a noise even with only one watt.

I'll leave speaker recommendations to JMF.
If you say theory and practice don't agree you haven't applied enough theory.

JLT

Quote from: Roly on May 02, 2015, 11:47:59 PM
Quote from: JLTInstead I'm using two piezo pickups

In which case the input impedance will be way too low, 2.7Megs being more typical.  Will sound very thin and trebly.

Definitely not what I want for a bass amp!

Quote


N-channel FET such as an MPF102 or 2N3819 (o.n.o.).  Drain to V+, Source to ground via "RS", that 10k resistor (see, it came in handy for something!) and to the input.  2.7Meg resistor from Gate to ground, "RG".

Shouldn't need C1 and I think your amp already has a cap at C2 (input).

Exactly what I needed. I'll have to see if I have that 2.7 Meg resistor in my big bag o' parts. And if the cap at C2 is the same as the 100 nF C3 on the Alltronics schematic, you're right, and I can dispense with that. As for the FET, it looks like Radio Shack doesn't stock it, so I'll see if one's available at our real electronics store here in Sacramento (Metro Electronics).

Quote

I'm talking voltage gain which should not be confused with power output.  15 watts into any speaker is going to make quite a racket.  Whatever maximum power a speaker is rated for it will still produced quite a noise even with only one watt.

I'll leave speaker recommendations to JMF.

I'll await his input before I order.

J M Fahey

Thanks for the trust vote :)

Both speakers will work properly, pick one.

As of "it doesn't handle the power" it's not exactly that but simply power amp clipping.

1 W makes a big noise if using a guitar, 15W is not *that*  much for a Bass, because:
* human ear is much less sensitive.  Big time.
Ever wondered why alarms, beepers, sirens, etc. *all*  operate at high mid frequencies?
That's because we are genetically tunes to them.
Low frequencies? ... not that much.
Only animals I know who prefer them are whales.
* speakers lose tons of sensitivity for every octave they go down, as in 8:1 per octave

So we have reduced ear sensitivity coupled to reduced speaker efficiency ... bad combination.

That said, your amp will be among the best *portable* 15W amps found in the Market.

That distortion you hear can easily be tamed by setting volume slightly lower, I suggest you use an A/Log/Audio volume control instead of the one which you are using now, which I guess is B/linear.

Commercial amps use an automatic limiter/compressor to do so all the time and minimize user tweaking, but you won't find EBay kits with that feature , the only option is to fully clone a commercial MI Bass amp, which *is*  doable, but leave it for a future project, because it involves designing your own PCB and Layout from the ground up, better go step by step.

Just to show that it's not for the faint of heart, have a look at this Fender BXR25 schematic.

http://support.fender.com/schematics/bass_amplifiers/Bassman_25_schematic.pdf

The limiter/compressor is based around variable gain UC7 , a CA3080 ; the distortion detection circuit which drives it is made out of everything below and to the right of it; for reference including Q2, U8a (TL072) , half a dozen diodes and capacitors, a dozen resistors .

So you'd better be your own "human limiter" , set the amp just to clipping (if you want to maximize power, that is)  and play louder/softer by yourself picking *just*  what's needed.

And in the middle of a hot jam, letting it growl a little won't hurt, being surrounded by loud instruments is not the same as playing alone.

In a nutshell, your battery amp will be *very*  usable, just not a wall breaker.

Roly

Quote from: JLTDefinitely not what I want for a bass amp!

Oh, I had forgotten that detail.  IMO piezo pickups are totally unsuited to bass guitar by their inherently bad low frequency characteristic, whatever the input impedance, even infinity (which we can do for you sure  :) ).  Are you trying to pick up a signal from some sort of acoustic bass?

1Meg is typical for a magnetic pickup, so that will do for the time being (or series 3x 1Meg).

There has to be a DC blocking cap between the buffer output and main amp input, but you only need the one.

Get whatever N-channel small signal FET's they do stock, a source follower is pretty non-critical.

The maximum power handling (WRMS) and sensitivity (dB/Watt @ 1m) of a speaker are not really related.  A speaker may have a high power rating yet also have a high sensitivity, or any other combination.

But JMF is the real expert here - he actually builds guitar speakers.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

JLT

Quote from: J M Fahey on May 04, 2015, 05:02:06 AM
Thanks for the trust vote :)

Both speakers will work properly, pick one.

<snip>

In a nutshell, your battery amp will be *very*  usable, just not a wall breaker.

That's what I want. I have another amp for large venues.

JLT

Quote from: Roly on May 04, 2015, 05:46:05 AM
Quote from: JLTDefinitely not what I want for a bass amp!

Oh, I had forgotten that detail.  IMO piezo pickups are totally unsuited to bass guitar by their inherently bad low frequency characteristic, whatever the input impedance, even infinity (which we can do for you sure  :) ).  Are you trying to pick up a signal from some sort of acoustic bass?

I am indeed. The bass is a Dreadnaught I converted into a (very) short-scale acoustic bass. I've played it through other bass guitar amplifiers with good results. Not as pure a signal as one I'd get with magnetic pickups, but a warmer, more resonant sound.

The peizo pickups were the ones recommended for the application by JJB, and I've had good luck with their other products, as well.

Here's the link to that product:

http://www.jjb-electronics.com/prestige-220.html

Roly

Quote from: JLTHere's the link to that product:

...and not a word about the need for high input impedance with piezos, all the more so with smallish ones like this.

I'd be thinking of a high impedance FET buffer as close to the piezo elements as possible, built in, beltpack, or FET lead.

This circuit would also work.
If you say theory and practice don't agree you haven't applied enough theory.

JLT

Quote from: Roly on May 06, 2015, 06:14:11 AM
Quote from: JLTHere's the link to that product:

...and not a word about the need for high input impedance with piezos, all the more so with smallish ones like this.

I'd be thinking of a high impedance FET buffer as close to the piezo elements as possible, built in, beltpack, or FET lead.

This circuit would also work.

I'll keep that in mind, but I'll start with the circuit that Roly suggested. The cable FET may not be practical, since phantom power may not be available, and it may not be necessary. I'll find out by and by.

I ordered the FET today from Jameco, as our last real electronics store was out of stock, and they won't restock because they're going out of business. I am bummed. There'll still be Radio Shack and Fry's Electronics, but they aren't the sources they used to be.

JLT

Believe it or not, this project is still live, although I haven't done much in the last month due to other projects, sick family, and life in general.

But the speaker has been acquired and mounted in its cabinet (a re-purposed 1950s-era record player), the electronics are ready to mount, and a lead-acid 12-volt battery has been used to power it, along with an external power supply (120 vAC in, 12 vDC out).

One final question for those who have been so kind as to put up with me until now: I'd like to arrange it so that, when the external power supply is used, it also re-charges the battery. Something tells me that simply connecting the hot leads of the power supply and the battery together isn't going to work for that, and that some sort of isolating circuitry or voltage regulating circuit would be required (as it is in cars).  The question is: what sort of circuitry would be required for this, or is it even possible? As usual, schematics would be a big help.

J M Fahey

It is possible but the power supply voltage, under load, must be higher than 12.6V , is that so?

In the simplest way, you run the amp straight from the battery, and the supply has, say, 15V output.
In that case you use a resistor between supply and battery which is always charging it, whether you play or not, and lets pass some current, usually called "1/10C" , where C is the (C)apacity of the battery in Ampere Hours.

Suppose yours is 7AH so C is 7
Then charging current is 7/10=0.7A
Resistor is (15-12.6)V/0.7A=3.5 ohms (search for a reasonably close one, not critical at all) , and dissipation will be at least: (15-12.6)V*0.7A=1.68W .

In the real World your battery may be over discharged and have only 11V so resistor might dissipate (15-11)V*0.7A=2.8W

In practice, you will use a ceramic resistor between 3 and 4 ohms and capable to dissipate 5 to 10W`(they are inexpensive anyway).

These calculations were made with a 15V supply, repeat using the values of your actual one.

That is a safe current which will not stress your battery or the charger and is easy to calculate.

It has only 2 problems, both quite bearable and which of course can be expected from such a simple circuit.

1) it's a sort of slow charger , because since it's not controlled, you try to err on the safe side, so although it would charge your battery in 10 hours in  practice it will take a couple extra ones.

2) it "never stops" so it can overcharge your battery if you forget to unplug it for a couple days.

There are intelligent chargers which supply higher current at the beginning (so they charge faster)  and automatically turn off when the job is done, but of course they are somewhat more complex (not NASA complexity, of course)

An intermediate solution is to feed the battery regulated and current limited  13.8V which has been proven to be both faster and safe if you forget to turn it off.

Really there's many options and schematic on the Internet, but the simple solution posted first works quite well, it only requires an intelligent owner ;)  who checks voltage every few hours and remembers to turn it off when done  :)

JLT

Quote from: J M Fahey on July 01, 2015, 12:06:44 AM
It is possible but the power supply voltage, under load, must be higher than 12.6V , is that so?

Um, no. It says 12 v at 5 amps ... it's the kind of replacement power supply for laptops; the eBay URL is http://www.ebay.com/itm/121606465910

So I think I'd be better off another power supply with a little more voltage, as you recommended. Any suggestions as to sources, etc.? Remember, this is a "budget" project.

Quote

It has only 2 problems, both quite bearable and which of course can be expected from such a simple circuit.

1) it's a sort of slow charger , because since it's not controlled, you try to err on the safe side, so although it would charge your battery in 10 hours in  practice it will take a couple extra ones.

2) it "never stops" so it can overcharge your battery if you forget to unplug it for a couple days.

There are intelligent chargers which supply higher current at the beginning (so they charge faster)  and automatically turn off when the job is done, but of course they are somewhat more complex (not NASA complexity, of course)

An intermediate solution is to feed the battery regulated and current limited  13.8V which has been proven to be both faster and safe if you forget to turn it off.

Really there's many options and schematic on the Internet, but the simple solution posted first works quite well, it only requires an intelligent owner ;)  who checks voltage every few hours and remembers to turn it off when done  :)

You're asking  a lot of me, my friend! But I'll try! And thanks again for your help.


J M Fahey

Your power brick is probably regulated 12V, perfect to run your project ... but it won't charge the battery.

To charge it you need to have current flow into it, so you need a higher voltage (plus a limiting resistor)  than what the battery has.

As a reference, a charged lead acid battery has 12.6V DC across its terminals , already higher than the 12V supply, but it's even worse: probably because some wicked sense of humour , the battery demands *even higher*  voltage, while charging.

If you feed it:

* 13.8V : it will charge well, its own internal voltage rises beyond 12.6V but it practically stops "pulling" current from the charger when it's full, that's why it's considered a "safe" voltage.
At the same time,  besides having regulated 13.8V it does not require much, it "self limits" so it's a very popular battery charging voltage.

* as large as possible current from alternators (in cars) to speed up the process, you don't want to run your engine for 12 or 14 hours just to charge the battery, so they feed it a lot.
Fine, but there is the possibility of bursting it easily, so car regulators are set to cutoff above hair rising 14.4V  :o
That's why some car power amps, VHF transceivers, etc. are specified rated power at 14.4V , it's what's available in a car with the engine running.

* as you see, battery voltage rises when charging way above 12.6V , that's why my example used a 15V DC supply.

Try to search for some power brick similar to the one you found, but either in 13.8V (I bet you'll find a few) for simple self controlled charging or higher (up to 18/19V DC) to use with a limiting resistor.

Or maybe you find a ready made battery charger for about the same price, just plug and play :)