Mottlefeeder

Making slow progress, but getting there.

1.  A plywood base plate

2.  Amp and sockets fitted

3.  Plate fitted to speaker cab.

I still have to check whether the preamp clicks during switch-off with this amp, and fix it if it does.

David

The specification for a speaker usually defines the range by the frequencies at which the volume has dropped to 50%. Going slightly below the specified low limit will result in even lower volumes, until you reach the point where the speaker+box no longer behave as designed, and the speaker movement becomes uncontrolled. This could cause speaker damage.

Hypothetically, using a sub to reproduce the fundamental will put you in the audio spectrum used by the kick drum, and the result will be indistinct and muddy.

David 

1 hour ago, MrTea said:

Thanks all. Not sure I like the idea of risking the import tax on ordering the adapter from the US. I did find these though - I wonder if they're still available somewhere and would work?

https://www.musique-shop.fr/uk/steinberger-bass-string-adapter-4-string-bass.html?gclid=Cj0KCQiA-aGCBhCwARIsAHDl5x9YoCndcLGZ4232o0fuvcrxY05cpaRYf4mCUqja_tthJogr7GkMSecaAv0bEALw_wcB

Otherwise it doesn't look much difference in price on ordering some double ball end flats or the adapter. 

That looks very similar to mine, which works on my Hohner B2v, so it should work if you can find one.

Buying the adaptor brings in savings when you next want to change strings - you have more choices.

David

I've just followed the link in your original post - no, don't go for that one.

 It's a replacement for everything beyond the nut, and may not fit the neck profile, or allow you access to your truss rod. Also, there are a couple of clamp-bars on the fretboard side, which pull the strings down onto the nut or zero fret, but do not clamp them enough to allow you to safely cut them. For that, each string has to be bent through 90 degrees and fed through a hole to be clamped by the screws on the end of the headstock. Roundwounds can bend like that, flatwounds tend to split and unravel. If you look at the clamping screws in the picture, you will se that they are 90 degrees round from where you might expect them to be.

https://www.ebay.co.uk/itm/Black-4-String-Electric-Headless-Bass-Guitar-Locking-Nut-With-Screws-Hex-Key/284165012089?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20201210111314%26meid%3D33b30b56236349f5ab09652353c95d31%26pid%3D101195%26rk%3D1%26rkt%3D12%26mehot%3Dco%26sd%3D293146773871%26itm%3D284165012089%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv9PairwiseUnbiasedWeb%26brand%3DUnbranded&_trksid=p2047675.c101195.m1851 is the same part, but the pictures show it better.

David

I could have posted this as a NAD, but it's probably not of interest to most of the members.

I've now taken delivery of a Kenwood KAC-M1814. It is designed as a booster for poor car stereo systems, so it delivers 45 W x 4 into 4 ohms (or limited in some way to 45W x 4 into 2 ohms) / or bridged pairs to give 90W x 2 into 4 ohms. The first picture is the new amp compared with the 2-channel class AB amplifier that still gets dragged out occasionally. That one delivers 75W into 8 ohms or 150W into 4 ohms. That rig, with its 7AHr battery will run for about an hour, and I carry it as a back-up when we do generator-powered gigs etc.

The KAC-M1814 has good and bad points:

+  it is small and powerful
+  it has a built-in variable HPF going down to 50Hz
+  it has a built-in low battery voltage cut-off
+  it has been designed and tested to operate in hostile environments
+  it is sold by a reputable company

-  the quiescent current is 800mA. so even at low volumes, it will only run for just under an hour or so on my smaller battery
-  the power output into 4 ohms x 2 is good, but into more conventional 8 ohm speakers it is only 45w per speaker when bridged.
-  mounted on the back of the speaker cab, it is functional rather than elegant.

Just for comparison:
Ashdown MyBass + mains invertor: Quiescent current 1.7A  / average current 2A  / peak current  n/a
Kenwood class AB: Quiescent current 440mA  / average current 2.2A  / peak current 5A
Kenwood KAC-M1814: Quiescent current 800mA  / average current 1.2A  / peak current 2A

So, with an amp in a metal box, and a requirement for a larger battery (the speaker cab is designed for up to 7AHr), my small rig is coming in at 9.6Kg, and should run for about 4 hours on the built-in battery. For an all-day busk, I would probably use the more efficient Basslite 10 inch speakers, and a large leisure battery.

Back on track.

David

11 hours ago, LeftyJ said:

http://stringadapter.com/string-adapter/compare-prices/

This adapter will hold regular single-ball strings without modifications to your bass. It anchors behind the headpiece. I'm not aware of the issue @Jean-Luc Pickguard pointed out about it not being suitable for flatwounds. It might be worth contacting the company to ask though! 

I'm using one of these adaptors with flat-wound strings and have had no problems. The hole that you thread the string through is slightly dished, and the end of the locking screw is slightly convex, so it crimps the string outer against its inner, and so the string does not unravel if you remove it once it has been cut.

David 

2 hours ago, Lfalex v1.1 said:

Looks like the linkages to the drum heads are springs? Presumably they store and release energy from the bowing, giving  a sound not unlike a spring reverb, I guess.

Not certain. Just surmising.

Just checked the @upside downer link to the inventor's explanation - you are spot on.

 One lives and learns

David

I can't see anything in shot that would provide that level of reverb/echo. If you ignore that, it's just a bowed banjo.

Where's my hard hat?

David

5 hours ago, Chienmortbb said:

... it would be interesting to see the specs that Emininence published at the same time as those Basslite plots above to see whether they changed the parameters of the driver or just “smoothed” the response. 

This is the pdf I saved from the Eminence site in 2012

Basslite_S2010.pdf

On 28/02/2021 at 21:49, stevie said:

For comparison, here's an Eminence Basslite.

 

This is the graph from the Eminence web site - where did yours come from?

9 hours ago, stevie said:

I've just come across Celestion's own published measurement of the Pulse 10 chassis. It's interesting to compare it with the one I took, as it looks like it's been smoothed slightly. I also measured 1dB more sensitivity.

If they took one straight from the production line and tested it without running it in, would that account for the difference?

David

Still thinking about my options, given that no amplifier other than the reject is small enough to fit the electronics bay, and I'm now thinking of just putting the preamp in the electronics bay, and making 'plate amplifiers' to bolt on the back of the cab. That would give me the versatility of connecting up an amp appropriate to the gig, but still give me the HPF I need to protect the speakers, Also, the rework on the electronics bay metalwork would only have to be done once.

David

2 hours ago, Phil Starr said:

I've just rebuilt a small guitar combo as a bass combo and it's great but needs more power and ability to run off a battery would be great. So, I'm going through the same thought process, fortunately with @Chienmortbb on tap for advice. He's kind of talked me out of buying the module you used. I've been eyeing up some of the class D car amps as a ready built, robust alternative. I'm following your build with a lot of interest. However I have built an extension speaker to match the combo and of course you have to try it so I plugged it into my 'proper' bass amp. I'm now wondering whether just buying a TC BAM wouldn't be an easier and affordable option with a cheap inverter if ever I do need to be portable. However that wouldn't be very satisfying so I'm still thinking of your option 3 the car amplifier.

The only suggestion I can make is that I did consider using one or two 18v Li ion batteries as power supply, re purposed from my power tools. they are lighter than lead acid, you have a range of voltages available and if you could form sockets would be removable for charging. A +/-18V supply would let you get decent power into 8 or 4 ohms out of a bridged amp and you'd probably get an hour's playing or more out of them.

Anyway well done with the progress so far.

The problem with a mains amp and an invertor is the standing current of the power supplies. My 150 W and 300 W invertors have 0.5-1.0A quiescent current, and my Ashdown MyBass takes another 1.0-1.5 A (from the battery via the invertor). That's OK if we are busking all day and using a 110AHr leisure battery, but it's a third of my built-in battery capacity gone before I've amplified anything. 

Looking at car amplifiers, something like a Kenwood KAC-M1814 (two pairs of bridgeable amplifiers) would give 2 x 45 W @ 8 ohms / 90 W @ 4 ohms which would put me up in PJB territory for a twin speaker rig, and only have one set of PSU losses. My existing Kenwood class AB car amplifier - 2 x 60 W @ 4 ohms - has a quiescent current of 300mA, but takes more than class D when amplifying. Hopefully the class D unit will give the best of both worlds.

I've considered lithium batteries, but the cost of new chargers, and a leisure battery capable of lasting all day, is more than I can justify. Also, the rest of band seem to have standardised on lead acid batteries and invertors, so there is merit in staying compatible with them.

David

I seem to have walked into a dead-end here, so I'm having a rethink.

The metalwork fits the existing amplifier module, but that appears to have quality issues, so I'm returning it and I'm loathe to replace it like for like. As @Chienmortbbcommented, the alternatives may also have quality issues, and it would cost me Import duties and processing charges to find out.

The next option is to use a physically larger, and now obsolete class D module that I already have, and start again with the metal bashing. The first problem with that solution is that I then do not have that amp module available for its previous role. The second problem is that if/when that module dies, I'm back to square one.

Option 3 is to fit the combo cab with a speakon socket and a power/charge socket (the battery is inside the cab), and to mount an amp module on a plate/power bulge on the back. This keeps the obsolete amp accessible to swap between old and new projects, and allows me to swap in a newer and/or more powerful amp if and when required.

Option 4 is to go upmarket and buy an Ice or Hypex or equivalent module and a power supply to run it from 12 V. However, for the cost of doing that, I can get a car stereo Class D amplifier that is designed to do the job, and built to be robust enough for a car environment, and I can sell it on when I stop using it. 

At the moment I'm going with option 3.

David

On 18/02/2021 at 10:45, Chienmortbb said:

Some of us used a UK supplier on EBay for the grilles for the BC112 Mk3 cabs and they were made ti size an quite reasonable. I will try to find the details,

 

Here you go, you can get steel or aluminium. Alluminium is much lighter but does not take finish so well. https://www.ebay.co.uk/str/sgsmetals/Perforated-Sheet/_i.html?_storecat=7064986013

I have used this supplier for several cab grills and have been very happy with the results (he also offers a powder coating service) - 

http://www.speakergrills.co.uk

David

6 hours ago, Chienmortbb said:

It could be a number of things. The distortion could be due to toow much gain, it may also be the output coils saturating. The low cost Class D amps from Ebay/Aliexpress/Banggood etc often scrimp on comonents and that means capacitors and inductors are not up to scratch. Your preamp may also be oscillating and that could be the layout or the lack of an hf bypass  cap in the feedback of the opamp.

I assume you are using a single supply for the preamp? If the Opamp is a dual rail amp with the inputs biased to half rail (as with most onboard  preamps and stomp boxes) you may need more decoupling (smoothing on the lower part from memory), The draw on the negative rail on many opamps is higher than the positive rail and as the rails collapse you can get some really weird noises before the amp finally falls silent.

 

I'm assuming the preamp is oscillating, although it has a capacitor across the gain pot in the feedback loop. All connections to pot terminals are shielded, with the shield insulated at one end, and connected to the earthing star point at the other. The pot casing probably isn't earthed, so that is something to look into. However, I am hoping that when I get the gain structure right, the problem will go away.

The amp module is a Chinese import to Amazon UK , and is disappointingly noisy, so it may be going back. Disconnecting it from the preamp gets rid of the shut-down noise, so the problem is definitely to do with the preamp.

The preamp is powered from the 12-13 volt battery, with a TLE2426 rail splitter to provide the midpoint. There is one 100uF capacitor upstream of the rail splitter, 2 x 0.1uF from the mid-rail to the supply and ground rails, downstream of it and also as close to the dual op-amp chip as possible, and a further 0.1uF between supply and ground as close the the second op-amp chip as possible. The circuit diagram above, showing the resistive splitter was an over simplification. The only other capacitor is 10uF on the output, as a DC blocker, so as the supply voltage collapses to zero, it may be discharging into the amp module input?

Given that the splitter is designed to source/sink 40mA in order to hold the rail centred, and the circuit does not have asymmetric smoothing capacitors, would you still be looking at more decoupling on the lower side of the mid rail, or should I be looking at a bigger reservoir capacitor on the preamp so that the amp dies first?

I'm at the limit of my electronics knowledge here, so I may be talking rubbish...

David

It works - in part...

Finished the wiring up, connected the electronics panel to the speaker cab, powered it up, started to turn up the volume and I heard bass. Turned it up a bit more and I got distortion and motor-bike-engine oscillation, so that's the first problem -  the preamp has way too much gain. Also, the gain pot crackles as it is turned, which suggests a dc component in the signal? I need to check that out.

Second problem occurs when I power off - I get a loud click/crunch sound from the speakers. It's not mechanical, so I need to work out which bit of the circuit is causing it.

On  a positive note, I'm playing a 5-string through a practice amp and it's not farting out on me.

Getting there.

David

On 18/02/2021 at 11:41, RichardH said:

The prices are more reasonable than I thought. I imagine using an etch primer on the aluminium would give a better chance of a decent finish. I see that Hammerite do a direct to galvanised paint that claims to work with aluminium, too.

If you have the option of buying a pre-coated grill, I would seriously consider it. When you spray a grill, you spray from the front, and 70 % of your paint goes through to the newspaper behind. Then you spray at an angle so you don't miss the edges of the holes, and 70% of your paint goes through the holes... I reckon I spent £10-15 on paint for the last grill I painted.

David

Another day of 3 steps forward and one back...

Sorting out the wiring between the amp recess and the cab, one of my cable clips sprang off and disappeared. And where did it land - inside the speaker cab, between the speaker chassis and the speaker cone! I couldn't shake it out so I've had the grill off, and the speaker out, and while I was in there, I took out the acoustic wadding, then put everything back together again. The battery holder is now securely blocking that hole so it can't happen again, and I'm back to wiring up the amp and preamp plate. I've less room than I thought I would have, so it's getting a bit messy.

David

5 hours ago, Stub Mandrel said:

You've swiped my battery charger!

Those are based on 'power pole' connectors - they are the dog's do-do's for high current 12V use, I use them for my astro kit. I made 3D printed panel holders that allow them to be recessed possibly a bit fragile for music kit without a redesign.

That battery charger is fine for larger batteries, but I think it only goes down as far as 7Ahr. I use a CTEK charger for my audio gear - 2.3Ahr or 7Ahr, and only take the caravan battery for all day busking sessions using larger amps.

Recessing the socket was easier than I thought it would be - I'm old school with regard to plastics, so it's back to model-makers plasticard for me. I bought a 2mm thick sheet to make the speaker cab vent. Hmmm, smell that solvent.

David 

11 minutes ago, Stub Mandrel said:

I once made a little combo with four 4" speakers. Last year (or the year before...) I put a small port in it, which improved the bass response. On a hunch I removed most of the wadding inside and it improved the sound and volume - I think I had over damped it. It's worth experimenting, you can alwasy put it back in.

I've heard it said that you should snap your fingers in side the enclosure to check whether it sounds too dead. Whoever suggested that was not building cabs this small, so I think I'll leave the wadding in until I finish the rest, then experiment after that.

Moving on, my next problem is the power socket - 

Several years ago, I decided to standardise on Torberry connector for all my 12 v battery connections. Unfortunately, the panel-mount version leaves the plug protruding too far from the back panel, and likely to be broken, so I need to recess it somehow.

David

Three steps forward, one step back...

I was so engrossed in feeding battery and speaker cable through a small hole from the cab to the electronics that I forgot the speakers should be in parallel, and I wired them in series. Having fixed the speakers (8 screws) and the grill (6 screws), it all has to come  off again to be rewired. On the bright side, I was able to use a 'normal 'amplifier to drive 10Hz though the 8 ohm load to run in the speakers.

I've also fitted some sound absorbing polyester wadding in the front half of the speaker i.e. between the baffle and the brace, and also on the back wall. I can't see much reflection coming back from anywhere else.

David

Quote

 

A bit more progress - the garage is about 3-4 degrees C at the moment, so metal bashing/drilling/filing occurs in short sessions.

David

1 hour ago, Phil Starr said:

Nice work.

You probably already know this but others may not. If your speaker is following the red line for excursion then the coil has moved out of the magnetic field for part of the time, so motion is going to be non-linear for all the frequencies not just the bass. This means that there will be increased distortion across the board. Keeping the coil in the magnet gap also helps with heat dispersal. You get a lot of gains from that HPF

It makes perfect sense, but I hadn't thought it through other than for the bass frequencies.

Thanks

David

The longer term plan was to use an FDeck clone HPF giving a fixed 12dB/Octave at 35 Hz and a further 12dB/Octave filter, variable between 35 Hz and 140 Hz. Some form of limiter/soft clipping was also envisaged, although I have yet to hear a simple compressor circuit that I could live with. I also had plans for some fancy power switching to allow one socket on the back to be used for power-in or battery-charge, or not in circuit when the internal battery was in use. 

I've decided to start with some simpler options, and swap out bits if I need to: first on the list was the HPF. 

The first Win ISD graph shows the response with no filter (red), 35Hz 12 dB/Octave filter (Blue) and 40Hz 12 dB/octave filter (Green). As you would expect, neither filter has much effect on the response of this speaker enclosure, with its relatively high tuning.

Having said that, the battery sits within the speaker enclosure, and I had concerns that the enclosure response would vary depending on whether you fit no / a small / a large battery. In reality, the difference between the larger battery and no battery increases the volume by 7% which reduces the tuning by 2Hz, which results in response changes of less than 1dB, so it is not an issue. 

The second WinISD graph shows the cone movement at maximum power, using the same colours, and this shows that an HPF is definitely having an effect. The horizontal red line is the limit for controlled cone movement, not the limit for coil damage, so I probably don't need an HPF for speaker protection, but using an HPF should give me better cone control and a tighter sound. An added advantage is that by suppressing the lowest frequencies, the battery will last longer.

I already had a preamp from a previous combo amp design, which included a variable HPF, so I used that as the basis for this interim preamp. It contains three op-amps, and I configured them as a Hi-Z input buffer, a variable HPF, and a variable gain stage to provide both volume control and compensation for differing outputs from basses. The last image shows the 'finished' (cobbled together) preamp and a basic schematic.

David