Battery powered rig

[Mottlefeeder]

After the recent North West bass bash, lurksalot commented that I should start a build thread for my small battery powered amplifier in case anyone was contemplating a similar folly.

Some of this has already been posted on another forum, but since photobucket has blanked out all the images, it seemed to make sense to start at the beginning with a new thread.

I play a lot with acoustic guitarists, either busking, or at open mic nights, and my existing combo, a Hartke Kickback 10, was adequate, but rather heavy, and limiting because I had to sit by the power socket so as not to trail cables across the floor.

My initial specification was based on the Hartke, with a bit of Phil Jones Briefcase thrown in - about 100W, one 10 inch speaker, one 7 AHr 12 volt battery and 2-3 hours of use per charge.

I had already built a couple of Jack 10s. so I had a 10 inch speaker to hand - an Eminence basslite S2010, and for a small box, WinISD suggests a volume of 0.9 cu ft (25.5L)and a port tuned to 58Hz. Eminence offer similar figures on their website.

The Mk I was a simple reflex box with the preamp, amp and battery mounted on a plywood plate that could be carried separately, or bolted onto the back of the cab. The amp is a Kenwood car stereo module, containing a switch-mode power supply to give positive and negative voltage rails, and a 60W @ 2ohms pair of class AB amplifiers, so it gives a genuine 120 W into 4 ohms bridged.

It was unwieldy and cumbersome to carry when bolted together, and a pain to work with in a crowded room when loaded in in bits. Also, the battery life was OK for an acoustic evening, but for a day of busking I needed more power, so I had another think.

The MkII had the same volume and porting, but incorporated the amp in a recess in the back, space for three 7 AHr batteries in the base, and a space for the preamp in the front. The woodwork suddenly became a bit more complicated -

And the final result.

This one was easier to carry, but the amp, which normally runs cool to the touch, now runs too hot due the lack of circulating air. At this point I started wondering whether I could find a class D amplifier module, and redesign the cab around that.

The Mk III used a class D stereo amplifier chip on a ready made pcb - readily available from ebay. Just for a change, I made the cab smaller to accomodate the amp and speaker in the same volume as the Mk I speaker.

Having recently aquired an Ashdown MyBass 550 in a trade, I was keen to be able to use it with this pair of speakers, so I added extra speakons and switches to the back of the cab so I could connect either the internal amp or the external amp to the speaker, or connect the internal amp to an external speaker. Almost inevitably, while setting up in a hurry on a dark stage, I managed to connect the output of the MyBass to the output of the battery amp - the MyBass won.

At that point I decided that I should go modular, and have separate heads that clipped onto the top of the cab. No more switching options to get wrong, just clip the head on before you leave home and you're good to go.

This is the Mk IV - the current version. The Mk III cab was butchered to convert it into a MK I style cab, and kitchen-cupboard kick-plate fasteners were fitted to anchor the clip-on amplifier modules

The battery combo:

The mains combo:

This is the battery powered amp and battery with an earlier version of the preamp.

I need to take some more photos before I post the next installment.

David

Edited March 22, 2018 by Mottlefeeder
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[naxos10]

Impressive work sir.

[jimbobothy]

+ 1, great stuff and thanks for sharing 😎

[Pea Turgh]

Nice!  I’d often thought about using car amps. Nice to see it implemented, and so well too!

[cheddatom]

very impressive!

[baldwinbass]

Genius

[stevie]

Fascinating.

[Mottlefeeder]

Some time ago, FDeck (of HPF fame) suggested that if you knew what kind of sound you wanted, and didn't need a lot of fundamental (e.g. for DB) you could do your back a favour and use smaller cabs. This was one of the triggers for this design. I used the HPF of a Fishman Platinum Pro, and the HPF of a graphic EQ to identify how much bass loss was too much, and concluded that provided that the response got down to 70 Hz, it did not sound bass-lite (to me).

Having identified the cutoff frequency, the next problem was what to do with the frequencies below the cutoff, where the speaker would be unloaded. After a few more evenings playing with WinISD, I found that a 12dB/Octave filter at about 55Hz would reduce the cone excursion below the resonance and keep it within Xmax, even with an input signal equivalent to 120W on the output.

This WinISD screenshot shows the frequency response of a 25.5L box, with (red) and without (green) the filter, and two larger boxes for comparison - maximum bass extension (blue) and optimised by WinISD (grey).

This WinISD screenshot shows the cone movement for a 120 W signal. Everything without the filter overloads above 40Hz. Given that a bass note is a mix of fundamental and harmonics, this may not be a problem, and I have played bass through these cabs without the filter, but I feel safer having the filter in circuit.

Finally, this WinISD screenshot shows the power handling of the various cabs. the two 25.5 L cabs have identical traces, but the other two both have considerably worse power handling in the 50-100Hz region, where most of the energy of the note is likely to be. This prompted me to stay with the smaller volume cab with an HPF.

I hope this image contains all the information required to build a cab. If not, please let me know.

David

Edited March 30, 2018 by Mottlefeeder
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[SpondonBassed]

Thank you.

I like the last image best.

[Mottlefeeder]

I've been putting off posting this. My problem is that all I have are the sketches and notes that I used to build a prototype, and I do not have the skills (nor the motivation) to redraw everything so it is as clear as the instructions you would get with a kit. If there is sufficient interest, and someone wants to take that on, I wouldn't have a problem with that.

This is the layout of the current battery powered head.

 

At the back of the head, the feed to the amplifier is fused, and the feed from the battery is fused, so regardless of whether the amp is being run from an external battery, or the internal battery is being connected to a an external power source, they have some protection. I transport the head with the external battery-out and amp-in connectors disconnected, so that I cannot arrive at the venue with a part-discharged battery due to the power switch being knocked in transit.

Slightly left of centre is a stereo class D amplifier pcb capable of giving 22 W into 4 ohms per channel on a 12-13 volt supply, or about 10 W into 8 ohms per channel. Each channel contains two amplifiers in bridge mode, so you cannot bridge them to obtain more power.

The pcb is designed around a TA2020 chip originally made by TriPath (and marketed as 'CLass T' just to confuse things). Various pcb designs are available, but I can only comment on this one. The sensitivity is high enough that you can plug an active bass directly into it, so if you have on-board EQ, you just need this pcb amp, a speaker or two, a battery (hopefully with a fuse) and you are go to go. Alternatively, you can buy an off-the-shelf amp and preamp in a box.

Googling [Tripath amplifier pcb] found this supplier of the amp module that I used -
https://www.ebay.co.uk/itm/MKIII-Tripath-TA2020-PCB-25watt-Class-T-amplifier-UK-/251464814688?clk_rvr_id=1520242820360&utm_medium=cpc&utm_source=twenga&utm_campaign=twenga&utm_param=eyJlcyI6MCwicyI6OTcyMDIzNywiY2kiOiIwMzhhMTZkNDRlYWJmMDNmNjZiODRkYzNjZWUzYTQ1NSIsImkiOiIyNTE0NjQ4MTQ2ODgiLCJ0cyI6MTUyNTQ1NzQ4MCwidiI6Mywic28iOjE1MDAsImMiOjE0OTcwfQ%3D%3D&rmvSB=true

Power supply
To the right of the enclosure is the power supply pcb. This is the circuit and layout.

           

 

 

The stripboard tracks run along the longer dimension, and are only shown where they are needed; links run across the shorter dimension; cuts in the track are shown with an x.

This provides a fused feed for the preamp circuits, and a mid-supply voltage rail so that the op-amps can run with plus and minus supplies. The chip used is a TLE2426 which will be difficult to get cheaply unless you have an account with CPC or RS Components. An alternative would be a pair of dividing resistors each parallelled by a capacitor, as discussed here - http://sound.whsites.net/project43.htm

Input circuit
The front right pcb provides input buffering and gain, muting, and a feed to the tuner jack socket on the back of the amp. This is the circuit and layout.

There are two input sockets, for active and passive basses. Plugging in to the passive (right) jack socket feeds through the left socket switch contacts and into the first stage op-amp. Alternatively, plugging into the active (left) jack socket opens the switches and places an attenuator in the signal path, bringing the level down to match that of a passive bass.

The two jack sockets are standard Cliff or similar, where the jack socket contacts cross the tube where the jack plug fits, and are physically lifted by the insertion of a jack plug. These sockets are available with pcb pins, but they are not at 0.1 inch spacing, and do not fit stripboard. However, if you take one with solder tags, cut off half of each solder tag, and squash the remaining bit into a crude pin, it will be pretty close to the spacing you need, although the stripboard hole will need to be enlarged.

The first stage op-amp feeds a buffer to the tuner jack socket, and feeds the filter pcb via the mute switch. The other pole of the mute switch is used to change the supply polarity to the front panel LED, so it shows red when muted and green when live.

Filter circuit
The front left pcb in the first image contains the  variable HPF, variable LPF, volume control and buffers to feed the line out and amplifier pcb. This is the circuit and layout.

The dual op-amp 1A and 1B is used in two standard filter Sallen Key circuits giving a roll-off of 12dB/octave. The HPF is optimised as a Butterworth filter, giving a sharp cutoff and good transient response while the LPF has a more gentle transition, intended to take out fret and string noise. The HPF operates between 30 and 120Hz (copied from FDeck's design), while the LPF operates from 20KHz down to 200Hz (copied from a Walter Harley design from 2000).

I prefer to use Omeg conductive plastic potentiometers because I like the way they feel, but they have a gap of 2 rows between the two tracks. Conventional carbon 16mm potentiometers have a gap of 1 row between the two tracks, so if you want to use them you will need to modify the layout slightly. The authors of the original designs both recommended antilog potentiometers and lamented their lack of availability, settling for a log pot working backwards instead. I have used linear potentiometers and they work well over the important part of the range, but antilog pots are now readily available in carbon, although not in conductive plastic.

The dual op-amp 2A and 2B is used to feed the amplifier module at the right level, and to provide a buffered line output.

On both the input and filter pcbs, there is a link to allow ground-lift for the tuner, line out and amplifier out in case there were earth loop problems. I had no problems, so the earths can be hard-wired in.

Apart from the supply-splitting chip TLE2426, and conductive plastic dual 100K potentiometers, all components are available from www.bitsbox.co.uk and the conductive plastic potentiometers from rapidonline.co.uk.

David

Edited May 5, 2018 by Mottlefeeder
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[Woodwind]

@Mottlefeeder this is a fantastic thread!

 

It pre-empts a query I was going to post later in the month.

I'll definitely be making a cab akin to your MK1 version

[rubis]

Hat tipping stuff Mottlefeeder, I'd be very happy if I could have produced that out in the garage (or kitchen)!