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Messages - Roly

Well there's an application I never thought of.  :cheesy:  Just keep in mind that when using a brushless fan that they seem to need a bit of current to get moving initially, but once running they will go down real slow before stalling.

A couple of things to note from the LM3914 data sheet are that operation in bar mode may start to get a bit erratic below 200mV RHI to RLO and while this circuit goes well under that no problems have been encountered, however closing the span even more may encounter non-linearity.

The other point is that having low values for the span and zero pots gives a significant improvement in the internal temperature coefficient of the resistive divider, and this will be come progressively more important as the working span is reduced.

Be interesting to hear how it goes.
It may sound obvious, but the key to faultfinding is to methodically isolate parts of the signal chain to identify which part contains the problem. 

In this case you already suspect that you might have speaker trouble, so a test you can do is try connecting your amp output to a different speaker cab and see if the fault clears.  If it does it suggests you have a speaker fault, if not then it is back up the chain somewhere, say the amp itself or your signal source, guitar, stomps, leads.

Assuming the fault disappears when using a different speaker cab; apart from the cone being damaged there are a couple of other faults you can get in speakers, damaged voice coil (inside the magnet), and the braids that carry the signal from the speaker terminals across to the back of the moving cone.

A dislodged voice coil generally makes the speaker sound "choked" and thin, while a frayed braid can produce clean low levels but may "fart" when driven hard (which sounds like what you're experiencing).

A couple of test you can do (with the amp off) both involve moving the cone by hand, but this must be done gently, and with the fingers distributed around the cone so you don't distort its shape.  The first is to press evenly around the central dust cover and listen for any scraping sound.  It should move freely without any scraping, if not this is a sign that the voice coil may be coming apart and fouling the magnet gap (but if you distort the shape of the cone you may cause a good coil to scrape anyway).

The second test is to disconnect the speaker from the amp and clip an ohmmeter onto the speaker terminals and again gently move the cone forwards and backwards.  The reading should remain quite steady, but if it shows any intermittancy you may have a frayed or broken braid.  In some cases you may also get crackles and even loud buzzes or blurts.  You can also try (gently) wiggling the braids, and again the reading should be stable and there should be no crackles or blurting.

A damaged voice coil means the speaker is basically stuffed and needs to be replaced or re-coned.

Damaged braids are easier; they almost always break just behind the speaker terminals, and if you look behind using a small mirror and light you may be able to see where it/they are frayed.  The simple fix is to solder the frayed ends back together (the braids normally have a fabric core that holds them in alignment).  Done carefully this can be an effective repair and give the speaker several more years of life.  If you do find that you have a broken braid it is generally much easier to remove the speaker for repair than try to do it in place in the cab; just watch you don't damage the paper cone in the process.  HTH.
Quote from: phatt on May 11, 2012, 06:56:46 AM
Someone once noted that 90% of electrical problems are found in connections. :o
Not active devices. I tend to agree ;)


I dunno about "90%" but it sure is a very high percentage, and this sounds to me more like a dirty connection problem than the noise gate itself.

@Covers4Christ when did you last clean all your plugs and sockets?  Never?  I carry a small bottle of metho and a packet of (smoking) pipe cleaners with a plastic twill.  Folded in half and dipped in the alcohol, then scrubbed into your sockets, and finally used to give the plug stems a good wipe over too.  Almost always fixes these mysterious problems.

If the problem persists after a good cleaning, start taking your plug covers off and looking for stray strands of shield or broken solder joints.

Whoops!  {Up too late.  xP }  Yes, the tempco of a silicon junction is -2.1mV per degree C, NOT 20mV as I stated above, however the mid point of the zero and span pots is 0.625V and the RHI and RLO are about 20mV (10 degrees) above and below, giving a range of about 20 degrees, depending on how you set it up.

Yes the LM3914 gets warm but it's rated for over a watt (1.36W strictly), and when it's at its worst case is when the cooling fan is going hardest.

50mV/-2mV/deg = 25 degree span.

It is also quite possible to re-arrange this sensing input to use a transistor set up for a DC gain of (say) ten and use the transistor to amplify its own change of Vbe by taking the signal off at the collector when the tempco will then be 20mV/degC, (or by whatever you set the gain to).  This may need some capacitive bypassing to stop it amplifying stray AC noise as well.

The main problem with using a sensor with a small tempco is that any stray noise turns up as "chatter" at the control output and is normally met by using a Schmitt trigger with some backlash, but this tends to widen the control range.  In the application above there is some chatter apparent as the control moves between steps, but it is minor and hasn't been a problem.

Really the main problem with this circuit is organising a suitable supply where the only available voltages are the output stage supply rails, perhaps 35 volts, which is a bit too high and has to be limited by a three-pin regulator.  Where I have applied this I have used a power resistor between the supply rail and the input of the three-pin regulator to drop the bulk of the supply voltage before it gets to the regulator input (a so-called "economising" resistor).

In the case of a LM7812 12 volt regulator the minimum input voltage needs to be about 4 volts above the output, so the resistor value is chosen to drop the available supply to 12+4=16 volts at the maximum fan current, 120mA.  For a 35 volt rail this is 35-16 = 19 volts, and at 120mA requires R=E/I = 19/0.12 = 158.33 or 150 ohms at P=E*I = 19*0.12 = 2.28 watts, say a 5 watt resistor.
When in doubt, throw it up against the universe and see what sticks.

Tests on some 50 watt solid-state amps in the workshop.
Percentage watts


[td}Jade 100ke (commercial)
Homebrew "Red"164%100%62.9%
Homebrew "Black"155%100%64.4%

Not too surprising that the overbuilt homebrew amps did somewhat better than the commercial one.

None of these amps are characterised for 4 ohm operation so the prospective 4 ohm power output was derived from measuring the output voltage at the same current point as 8 ohm clip level, and is therefore a bit dodgy and may be somewhat less.  All values corrected from real load values to exact resistances; all 1kHz sine.

The only valve/tube amp to hand "smoked" during warmup after being dragged out of long term storage, before any figures could be obtained (damn EL34's again).
Quote from: erokit on April 06, 2012, 08:16:27 PM
Interesting factoid. I don't remember where what equation this based on. But doubling the speaker impedance from the amp output impedance reduces the wattage by 31%. So an 8 ohm 80w amp into a 16 ohm speaker is 80 * .69 = 55.2w.


Nope.    :(

Power Law.

P = E^2 / R

E-squared remains the same because it is determined by the amplifier supply rail voltages (the output voltage does change slightly due to amp internal supply resistance with heavy loading, typically less than 0.5 ohm, but this is minimal until the amp is being seriously overloaded anyway).

Therefore P varies inversely as R.

Double R, half P; half R, double P.

Worked example, 80 W in 8 ohms

P = E^2 / R

80 = E^2 / 8

E^2 = 80 * 8 = 640

E = 640^0.5 = 25.29 volts RMS

Set R = 16 ohms

P = E^2 / R

P = 640/16 = 40 watts

Set R = 4 ohms

P = 640/4 = 160 watts, then smoke because the output current is now double what the amp is rated for.
Quote from: joecool85 on May 03, 2012, 11:52:55 AM
Quote from: Roly on May 02, 2012, 11:27:30 AM
Here's my indicating proportional "thermofan" design that has been used in several amps including a re-creation of an Acoustic 360 by J.C.Maillet;

VERY cool!  If you don't mind, I'd love to have you start a thread here about this and include schematics, diagrams, pictures, whatever you can.


Design starts with a problem, and in this case the problem was a small home-brew PA that had been dropped on me to fix because it "has a couple of problems" and "doesn't work very well".  These would soon turn out to be major understatements.

The mixer portion needed a serious scrub up, mainly replacing 741's with TL071's, but the power supply and power amps were such a misbegotten mess that the only option was to totally rebuild them in a different case.  Since it was a "love job", to keep costs down the metal case from an old gutted VHS VCR was used.

The original unit didn't have a heatsink worthy of the name, about a square foot of 1/16" ali flashing screwed down to the wooden case floor, so again the workshop junk heap provided a length of pre-loved real heatsink.  But as the rebuild was coming together it became obvious that the heatsink would have to go inside the VCR case - somewhat less than ideal.

This in turn meant that the heatsink would need to be blown by a fan (again, dragged out of the junk pile), but the amount of noise it made led to a long ponder.  What was really required was some way of controlling the fan so that it provided cooling in proportion to the need.

A search of the InterWeb turned up any number of "bang-bang" or on-off fan controllers ranging from highly dubious to elegant overkill, but no proportional controls.

I eventually concluded that I would have to roll my own, and decided on using silicon as temperature sensors combined with a linear LM3914 "Dot/Bar Display Driver" (aka Line-Of-Light driver) as the heart of a proportional fan controller.  In bar mode the accumulating current drawn through the LED's could come via a fan, giving proportional current control.

The Controller

I chose this IC because its input side is floating and can be referenced over a wide range of zero and span settings; and because the LED driver outputs are programmable current sinks that can also be easily set anywhere between 2mA/step and 30mA/step.

This current mode output is an important property since it makes the circuit quite insensitive to supply voltage variation, and means the indicating LED's can be left out or doubled up (or more) in series, and that a second fan can be used in series on a supply up to 25 volts without any circuit changes.

The LM3941 has a stabilised reference voltage output of 1.250V available which is used to energise the zero point and span point settings, and in this case provide a stable bias voltage for the temperature sensor.

The zero and span are set by two twenty-turn trim pots in series, and while ordinary trim pots will work their setting is quite sensitive and tends to be a bit fiddly.  These are tweeked pretty arbitrarily so that only the first LED or two come on when idle at a moderate ambient temperature, and that the last LED comes on when the heatsink is getting too hot.

In this IC the reference output also doubles as a setting for the output current sinks which each sink ten times the current flowing out of the reference output.  Here the output current was set to about 20mA/step which relates to full fan current of 120mA a bit over half scale.

This current is set firstly by the value of the zero and span pots themselves - at 500 ohms each they present 1k ohm to the reference, drawing 1.25mA and therefore setting the output current to ten times this current; secondly the temperature sensor draws roughly another 1mA, so the overall setting is around 20-25mA/step.

The Sensor

One of the properties of a silicon diode is that it has a temperature co-efficient of about -20mV -2.1mV per degree C, that is the "on" voltage falls by 20mV -2.1mV for every degree C it gets hotter, and it's pretty linear from 0 to 100 degrees C.  Since the base-emitter junction of any transistor is also a diode, and since some transistors come in packages with handy screw holes, and since we don't need to know accurately what the temperature actually is, a specific temperature sensor IC isn't required.

I used a couple of BD139's because I wanted to monitor the temperature of both the power transformer and the heatsink, and they were to hand.  The mounting hole made them much easier to attach than ordinary power diodes (such as the EM410, 1N4001, etc) which are another option.  These two were connected in parallel so the hottest one rules, and there is no reason why this couldn't be extended to monitor several points if desired.

It is also possible to stack diodes or transistors in series to get a larger voltage swing with temperature if tighter control is needed, 2 = -4.2mV/C, 3 = -6.4mV/C, etc, but just for amp cooling applications I have only used a single diode drop and found that sufficient.

This voltage is input to the LM3914 on its "Sig In" line.  Because the collector and base of the temperature sensing transistor are held at the reference voltage (1.250V) the emitter voltage rises from 1.25-0.65 = 0.6 to 1.25-0.6 = 0.65 volts over the temperature span.  The Zero or RLO input is set for around 600mV and the Span or RHI input set for around 650mV.

The Display

The LED display is actually optional, 'tho I would at least include it inside for diagnostics even if not mounted on the front panel.  There can also be more than one display if the LED's for each step are wired in series.  I have used a range of LED's from dull green. bright green, through amber, brighter orange, to red and very bright red.  At typical on-stage distances it is difficult to see how many LED's are alight, so the change in colour and brightness give a good indication at a distance of how hot things are running.

The Fan

The fans used have been the modern electronically commutated type, nominally 12 volt at 120mA, but there is no reason why a brush-type fan could not be used.  These particular fans were recovered from dead PC power supplies scrounged free from local computer repair shops (and 'tho normally filthy, full of still useful bits).

The fan voltage is the main limitation to the supply voltage for the controller since the LM3914 itself will work up to 25V, so two 12 volt fans can be used in series if desired.  This circuit should also be capable of running two fans in parallel although this hasn't actually been tried, and it is worth noting that experiments indicate a rule of thumb that fans provide 90% of their cooling effect in the first 10% of their rev range -  a little draft has a big effect, and the law of diminishing returns sets in early.

In the event that electronic noise from the fan operation gets into the audio circuitry an electrolytic cap of suitable rating, say 100uF/25V, can be placed directly across the fan.  Similarly with the control circuit itself, although in practice the switching between segments is so slow that no "zippering" noise has been evident (as it sometimes is in VU meter applications of this chip).

Supply Voltage

Naturally if 120mA at 12 or 24 volts DC is available then supply is easy, but in practice it has been necessary to use a three pin regulator and supply side dropping resistor from the main amp positive supply rail.  The exact arrangement depends on what is available in each situation.


Construction is quite non-critical and has been normally done on strip or dab board, with the LM3914 and trim pots mounted on one board, ribbon cable to the LED's mounted on another board on the front panel, and the temperature sensors on flying leads and suitably insulated with heat-shrink.  A fairly important point is that the zero and span setting trim pots are accessible with the circuit mounted in place.

Here is one possible layout that I've used;

{errata - LED shown in wrong direction}


In operation the fan generally doesn't start rotating until things have warmed up a bit and three or four LED's are alight.  Once started however the fan will continue to run from very slow to full speed depending on the temperature.  In use it was found that something pretty gross had to be done to get the top LED's to light, such as driving the amp hard with a towel blocking the air paths.


A possibility that hasn't been tried is to use the highest indication as a thermal cutout or limiter as last-ditch amp protection.  This could be done by including an opto-coupler in series with the hottest LED to limit or mute the audio drive, or to operate a relay as a supply cutout.

Acoustic 360

When applied to the re-creation of the Acoustic 360 bass amp by JC Maillet it was desired to run three fans and this required the modification of the output circuit so that they were run via current mirrors.

In summary, this simple circuit has proven very effective in providing fan assisted cooling in solid state amplifiers, while minimising fan noise problems.

Add: 120729
Thermofan test in Acostic 361+ clone build
Quote from: spud on December 15, 2010, 12:45:13 PM
Also, what size would be appropriate for a LM1875 chip amp implementation running about 15 - 20w? Probably 24v or so.  I figure the small 1 inch one would suffice since it's rated at 2.15o C/W/3 or am I not understanding the rating they give? 

Just to take these numbers as an example.

Amps power output = 20 watts
Heatsink rating = 2.15 degrees C per watt

Assuming the amp has a pretty typical efficiency of 50%, then the heatsink has to dispose of 20 watts.

20 * 2.15 = 43 degrees C

This is the temperature rise at full power over the ambient temperature.  Normally we would assume 25 degrees C ambient, but on-stage under lights a much safer assumption is 40 degrees C.  To get the actual operating temperature at full power we add the rise to the ambient;

40 + 43 = 83 degrees C.

We haven't accounted for the thermal resistance of the chip to case, or case to heatsink (via any insulating washer), so we can guess that the actual chip temperature will be closer to 100 degrees C.

As a rule of thumb the absolute maximum chip temperature shouldn't be hotter than 100 degrees C, so this setup would be shaving it a bit too fine for comfort (or overall chip life), so really a heatsink with a much lower thermal resistance is required.

The quoted thermal resistance of 2.15 degrees per watt also assumes optimum mounting of the heatsink, fins vertical, in genuine free air, and not exposed to any external heat source (such as stage lights).

Heatsinks are made the shapes they are for good reasons, thick where the devices(s) mount so they conduct heat well out to the thinner fins.

A couple of "rules" of heatsinking are that you always need more heatsink than you think; and quoted thermal resistances tend to be optimal, or even optimistic.  On the plus side, even a small amount air movement from an under-run fan makes a large difference to heatsink performance; however even a strong blast of air won't turn a seriously undersized heatsink into a wonder heat dissipator.

First design for sufficient passive cooling, then add a thermally controlled fan to cover those nasty extreme situations that crop up.

Here's my indicating proportional "thermofan" design that has been used in several amps including a re-creation of an Acoustic 360 by J.C.Maillet;
Amplifier Discussion / Re: Light Bulb Limiter
May 02, 2012, 10:13:29 AM
Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

"If you do have a suitable bench supply - This is much easier! Slowly advance the voltage ..."

He's talking about having a variable DC bench power supply, not a Variac, and bringing the DC on the newly built amp up slowly.  As you will see just below there he shows a very conventional split rail supply.

To go with my light bulb limiter I have a collection of globes of various wattages ranging from a 10 watt "pygmy" pilot light up to 100 watter, but even testing valve/tube amps I rarely use anything higher than the 40 watt - when you need limiting at all it is generally at a pretty low level.

I also have a couple of blown 3AG fuses with a 150 ohm 10 watt resistor soldered across each, and I normally use these when servicing solid-state amps, particularly rack amps, as they often have exposed fuse clips for each supply rail and I can just plug them in.

The general line of attack with an amp that used a SMPS would be to first isolate the SMPS from the amp and see if it is functioning correctly on its own ('tho it may need a small load to fire it up).

If so, then a power resistor or two can be used between the SMPS and the amp, but if you can't get something reasonable out of the SMPS then you have to start by fixing that first (and SMPS repair is a whole topic on its own).

Variacs are certainly handy, but over a lifetime of servicing I've never really felt the need to get one.  Since they are actually a transformer they are still a pretty "stiff" supply and a limiting lamp is a much more effective way of restricting the power input, which is what you really want to do to avoid frying anything.