Thursday, October 10, 2013

Simple Optical Compressor for bass guitar - construction

An extremely simple yet capable compressor circuit is published by Elliott Sound Products, here:

Compressor Mk I

I built the circuit exactly as-is (in an Altoids tin). 

Care was taken to optically couple the bulb to the photocell (LDR), using a perspex rod, wrapped in smooth tin-foil.  The perspex came from eBay:

as did the miniature "Grain of Wheat" bulbs:

This was put together, and sealed in heatshrink tubing, to make it fairly lightproof from external light sources.  The rest of the components came from (free postage) or from my junk box.

Here's the final assembly.  The green wire provides grounding (shielding) for the tin-foil, as per the original article description.

Here's the finished article.  The Altoids tin was really too small, so I had to cut notches in the lid, which rather spoiled the look.

The duck tape on the bottom of the box provides insulation.  The whole tin is effectively coupled to the amp "earth" via the jack socket outer rings.

I soon discovered the following:
  • Using jack-sockets to carry high-current, low-impedance amp output signals is a very dumb idea.  I knew this before, I just kind-of forgot it in the urge to get everything to fit in an Altoids tin.  Even with the amp output muted (but powered), I got a fat blue spark when unplugging the box.
  • The "grain of wheat" bulbs don't really get into their proper operating range of brightness when you have an efficient speaker cabinet (like a one) - even when the variable resistor is turned right up.
My amp has an FX send / return loop, so I was using that for the input side (on the left).  The two speaker sockets (on the right) were there in case a "pass thru" was needed, although again my amp has two speaker-outs, so no need to use that in practice.

I was a bit concerned about the way the tin was earthing together the FX send/return loop with the "-" side of the amp output.  It worked fine for the two amps I tried, but what would happen if it was connected to an amp in bridge-mode where the "+" and "-" sides were both driven?  Clearly not ideal.

Compressor Mk II

 Mk II was going to be built in a proper case, with a Speakon plug on a trailing lead (so it simply cannot be plugged in wrongly), and would use a high-efficiency LED with suitable dropper resistors instead of the bulb (so as to be more effective at low amp output levels).

It would also have electrical isolation between the amp out / speaker side, and the delicate FX send/return loop (which will be used for a shielding / ground connection for the case).

Here it is:

The Speakon cable enters through a grommet, is double tie-wrapped to the floor of the case, so even your clumsy drummer friend shouldn't damage it.

The variable resistor is a 5 watt, 500 ohm type which I found on eBay:

You'll notice a pair of 47 ohm fixed resistors, in parallel, on the green input wire next to the variable resistor.  In parallel that makes around 24 ohms.  The purpose of that, is to prevent the last little bit of the variable resistor's track dissipating a lot of power.  In fact it's hardly necessary as the LEDs use less power than the bulbs.  I used two small resistors in parallel because they have a low power rating (I think 1/4 watt) and didn't want them to get too hot.

Because LEDs can "flash" much faster than a bulb can, and only respond to current in one direction, we need a "rectifier and smoother" circuit.  This is what the small diode and big  capacitor does.

The capacitor is 470uF, 25v (from the junk box).  Voltage rating is important, as your amp can produce quite high peak voltages.  Make sure you use at least a 25v rated one, higher if you can - preferably 63v.

The output of the capacitor feeds two separate LED circuits.  The first one has 2x 220 ohm in parallel (effectively 110 ohm) feeding the high-efficiency LED that is inside the shrink wrap.  Because the LED light output is very directional, I did not use the perspex rod - I just put the LED right up against the front face of the LDR.

The second circuit goes through 470 ohms to a small front-panel LED.  The idea being that you can see what the compressor is doing from the outside.

The final part of the circuit is the same as the Elliott original.  A 33k resistor couples the input side (amp:  FX SEND) to the output side (amp:  FX RETURN), forming a potential divider with the LDR.

With the lid off in the dark, you can see both LEDs are working together.

The case was sprayed a non-Ashdown blue, and a nice quality collet knob from CPC/Farnell added.

The final circuit seems to work well - the component values will need some tweaking to suit your amp / cabinet combination.  If you want a circuit that is less sensitive to amp/cab variations then you might want to look at the other Elliott pages.

Circuit diagram:
Free download in PDF
and ExpressPCB .sch format (R-click / Save As) .. software available here

Thursday, July 11, 2013

Sony Vaio VPCEB4L1EWI - Heatsink clean and refit

This time, it's a Sony Vaio VPCEB4L1EWI, with i3-380M CPU.  About two years old.  Running Prime95 stress test, the cores were reaching 85 deg C and the fan was rather noisy.

1.  Strip off the back plate, undoing all the visible screws, taking careful note of where they all go back - there are several different sizes and types.  The back plate then snaps off - be careful with the small white lugs on the upper side here.  One came off despite me taking care. 

Always wear an anti-static wrist strap, connected to a metal part of the laptop (e.g. the USB connector) when working.

2.  Here is the heatsink / heat pipe / fan assembly removed.  The grey heatsink paste had set quite hard.

3.  Clean up the CPU top surface carefully with meths, a cloth, and cotton buds. 

 4.  Clean all the old paste off the heatsink.

5.  To do a really good job, "flat" the heatsink down with a diamond file, cleaning the copper dust off regularly.  I kept applying a few drops of meths as a non-greasy lubricant while working.

After a few minutes you should be able to get a near mirror-finish on the heatsink.  Don't over do it - copper is a very soft material.

7.  Apply a new heatsink paste (I used Arctic MX-4), and re-assemble the machine.

Measuring the core temperature with "CPUtemp" and running "Prime95" stress test, the peak core temp reached 68 deg C instead of 85, and the fan ran much quieter as a result.

Friday, June 07, 2013

Building a quick & easy website .. pros and cons

Just finished the first draft of a website for my wife's home tutoring business.  I used the free web site builder tools from, and they're not bad.  A few things we came across were a bit frustrating however:

  • Once you've chosen your template, the background image (in our case, of the coloured pencils) seems to be unchangeable
  • There's no "undo" button, so use the "Save and Publish" option frequently to avoid losing work
  • When you create a new menu item (on the top row, in our case), it throws away work you've just done on your current page, so save it first!
  • Working with images is somewhat frustrating in terms of positioning, resolution etc

There are also good things about the sitebuilder:  The sites look good on iPhone and Android devices as well as on a PC / Mac.

I also noticed that when running this auto-generated page through W3C Validator, it finds 5 "errors".   Hmmm.

I looked around for help pages, or a forum on this website builder too, to no avail.  Anyway I should probably be using something "proper" like Wordpress, Joomla or even good old Dreamweaver.  Thoughts /comments appreciated.

Monday, February 04, 2013

Improving an Ashdown Little Giant bass amplifier

I have an Ashdown Little Giant amp.  Several reviewers have mentioned that this amp is not at loud as it should be (e.g. here).

It all depends on whether the output levels coming from your bass are enough to drive the output stage fully.  I have an active bass, but even with the input set on max, to get a decent output level I need the output volume control set to around 3 o'clock.

While there are workarounds available, e.g. use an effects send/return through a pedal which has some gain, I wanted a simple 'no extra boxes' solution to go with my lightweight cab.

A nice chap called Wilfried Klaas has come up with what I consider to be the perfect solution to this - adding an extra small preamp (gain) stage inside the amp itself. His web page (in German) describes this improvement.

Wilfried was very helpful and supplied me a ready-made amp board for around €15.00.  This was fairly straightforward to fit.  I've recorded the process of testing the amp, fitting the board, and re-testing the amp.

Incidentally - my amp, purchased off Ebay, started off life as a green-front, LG350.  However something strange had been done to it, the fan was wired to run continuously off the 12v supply, rather than being connected to the variable-speed fan output.  Eventually the power module failed, and it went back to Ashdown, who replaced it with a Powersoft Digimod 1000 module.  Only one side is connected and used, so I have effectively an "LG500".

1.  Measure the amp's existing output

First, I hooked up the amp's instrument input to a sine-wave source.  I used a 1kHz, 0dB digital source which you can get hold of from here.  This was loaded as a .wav file onto an IPod.  The output from the iPod was checked on a scope; above 90% volume there were some signs of clipping (presumably the iPod's headphone output stage).  This input level was just enough to light up the 'peak' light on the LG's input stage.

The amp output was connected to a 4 ohm dummy load, which I made as follows - two 8.2 ohm resistors on big heatsinks, connected in parallel, as shown here.

For the actual test, these were placed on a metal tray, on a wooden board, as they will get hot if you run the amp into the loads for more than a few seconds.

With a sine wave input, the amp produced a maximum output of around +/- 60 volts peak, shown here:

 The RMS power in a sine wave is

    (V^2 / R) / (sqrt(2))  =  (3600 / 4) / 1.41 = 636 watts

After a few seconds,  some kind of power limiter kicks in, presumably in the power amp module itself, and the output falls to 44 volts peak, which is

     (1936 / 4) / 1.41 = 343 watts.

So the amp can produce more than the rated power (for short periods), the problem is down the lack of gain.

2.  Fitting the "Wilfried Klass" buffer stage

Firstly, switch the amp off and unplug the mains lead, leaving the load connected to dissipate the internal power, and take the top off the amp case.  The cooling fan has a 3-pin connector which looks very similar to PC-type cooling fans, this is easy to remove.

Inside of Little Giant amp

The white wires on the bottom left carry mains electricity, and even the +/- 60v output from the amp could deliver a shock, so always work with the power off.

The module requires + / - 12v DC power and an earth, which are obtained from the green connector following Wilfried's instructions.

Power connector On Digimod 1000 module

The next step is to find and break into the grey ribbon cable that goes from front to back. , on the right side of the amp.

The third wire from the bottom carries the preamp signal.   This needs to be picked out of the ribbon and cut into.  I used the scope at this point to check that the sine wave signal was indeed on pin 3

before I cut into it.   I broke into the cable carefully using a craft knife:

The buffer circuit is then connected up, with the input coming from the front panel, and the output going towards the back panel:

 I then powered up and tested the amp was working, but this time maximum output was reached (before onset of clipping) at a lower setting on the output knob.  The limiter behaviour is the same as before.

Finally, the buffer circuit was fitted into the insulating sleeve supplied by Wilfried (red colour in the picture below).  I used double-sided sticky tape to secure this to the inside of the case, and tie-wraps to keep the wiring in place.

Having just used the amp in a band rehearsal, I would say it's a great improvement - performance volume levels can be reached, while there's still some 'reserve' on the output knob.  There is no appreciable increase in the level of hiss or hum from the amp (it's pretty quiet both before and after the mod).

So thanks to Wilfried for making an OK amp (available for reasonable money on EBay) into a great little amp.

Tuesday, January 08, 2013

Helping my son revise for a maths test .. most of the questions seemed a bit dull and unlikely to catch a boy's imagination, so I made this one up for him.

A crazy sniper shoots a high velocity bullet vertically upwards.  The muzzle exit velocity was previously measured on a firing range, using an accurate timing device, to be 980m/s.

Use the simple equations of motion (known as the SUVAT Equations .. available on thickipedia here) to calculate

A) the time the bullet takes to reach the top of its trajectory
B) how high it goes
C) bonus mark : could the gun have shot a hole through the Red Bull Stratos balloon just before Felix jumped?