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Published: Mar-11-2013, updated: Aug-23-2018

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Feeding/charging a low-current draw device such as the Kameleon amplifier from a powerbank is not going to work with most powerbanks.

The reason for this is that a powerbank has an internal battery of 3.7V.
In order to get 5V from an output there is a step-up converter present in the powerbank that puts out around 5V.
Such a circuit draws current even when not loaded with anything. This would drain the battery even when not charging anything.
The solution is to switch off the internal DCDC converter when current draw drops below a certain value.
As most phones and tablets draw quite a lot of current when charging (everyone needs fast charging) the DCDC converter will switch on when enough current is drawn.
That charging current will drop when the, to be charged, device is (almost) full.
When that minimum required current is reached the DCDC converter will switch off as the device is charged and the powerbank itself won’t be loaded any more.
Depending on the ports(1A and/or 2A) and circuits used the output switches off somewhere between 50mA and 200mA.
When the current drops below that threshold current the output(s) of the powerbank switch off and thus stop supplying 5V.

This is a problem when you want to use the powerbank to charge or power low current circuits such as the Kameleon amplifier.
In my case the to be powered device draws less than 50mA.
The output of the powerbank senses a low current draw and thus either does not switch on or switches off again after 10 to 20 seconds.

A solution would be to add a resistor that ‘bleeds’ a current just below the minimum required current.
BUT that would draw say 80mA/h and drain the battery a bit more as well as dissipate heat (0.4W).

USB keepalive.JPG

My solution is a simple circuit that ‘draws’ very short 130mA pulses every few seconds.
Of course I am not the first to think of this but thought I’d share my solution with DIY’ers that also may (have) run into this.
As these pulses are very short (small duty cycle) there is just a 10 mA/h current drawn and the output stays ‘on’ so 5V will remain present untill it is removed from Powerbank.

The circuit is shown below:

Schematic Powerbank Dummyload.png


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rechargeable battery depletion voltage monitoring circuit

Using rechargeable 9V batteries in C’Moy type amplifiers can be beneficial in saving the environment and battery costs.

Most will use their amplifier till the sound starts to distort or it stops playing. This is no problem for single use alkaline cells that are disposed off once depleted. What many not may realise is that rechargeable batteries should not be used below a certain voltage limit.

For the familiar and widely used PP3 9 Volt block battery this voltage is around 6.3V. The nominal voltage of these batteries is around 8.4V as it basically consists of 7 small 1.2V batteries on top of each other. Under charging the voltage may even be 10V.

The opamps used in most C’Moy amplifiers still function normally even at 5V and when not playing very loud it may still sound great without distortion. For rechargeable batteries the damage is already done. Lifespan can be shortened considerably when it is discharged below 6.3V. This is because those 7 cells inside are in series and if one of them is of lesser quality it will be depleted before the others are and can receive receive a reverse voltage because the current drawn by the load still flows through a depleted cell. That cell will have a much shorter capacity and each time it is recharged and used again the capacity gets less and less fast and the battery is dead within much less than the usual 500 cycles.  It may not even reach 10 cycles.

This problem gets bigger when 2 or even 3 batteries in series are used. When 2 batteries are used till the amp stops playing the total battery voltage will be around 4V thus 2V per battery. This problem is even bigger for amplifiers having 3 of those batteries on board. As stated NO problem for alkalines as they will be thrown away in any case but rechargeable batteries become single use types almost immediately.

The best way to counter this problem is to make a circuit that switches off the batteries all together below a certain voltage. Those circuits are complicated though and have not seen many circuits using this trick except the well known O2 amplifier !

The handy DIY’er may build it and implement it in their design but not everyone is as handy with the iron. A simple circuit consisting of only a few components may be of help.

The simplest version would be a zener diode in series with the power LED (if present) where the power LED goes ‘off’ below a certain voltage limit.This requires a changed value of the series resistor and a added zener diode and does not draw more power as well (handy for battery operated devices).
In this case the power LED simply goes off when the target voltage is reached.

Small problem… once the power LED is off it is hard to see whether the amp is still on.. and when left on (as it appears to be off) the battery can still be damaged.

A variation is to add a red LED in series with the power LED and switch it on when the battery(ies) have reach their minimal ‘safe’ voltage level.Now you will find yourself warned by a (not very bright but still visible) red LED at the front and also the power LED will still be on. When left unattended or not watched while playing you still run the risk of over-discharging the batteries but chances are slimmer of that occurring.

A circuit that can be build into an amp is given below:

voltage monitor small

The schematic above can also be viewed / downloaded in higher resolution as a pdf.

It can be easily configured for 3 different battery voltages and questions and or remarks can be made in the FORUM

This circuit may save quite a few rechargeable batteries from being thrown away in the bin prematurely when the warning LED is viewed and the amp is switched off and the batteries recharged AS SOON AS POSSIBLE. Lifespan will decrease when batteries are stored in (almost) depleted condition so recharge them as soon as possible.

Note when using this circuit on a single 9V battery the time frame both LED’s will be on is very short. In this case LED1 may NOT be a blue color LED because of the relatively high operating voltage of blue LEDs.

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Very subtile crossfeed (X-feed) , a simple schematic for use between source and input headphone amplifier:
(own design, built circuit)

In some case where op-amps are used as ‘followers’ or ‘buffers’ and only 1x gain is used (when the output and – input of the op-amp are connected) some people like to change a complicated circuit (which an op-amp in reality is) for a simple emitter or source follower believing this will improve sonic properties. For those that like to do this a small tutorial. NOTE this circuit can NOT replace an op-amp when it is used as an amplifier and WON’T improve performance. Also it can NOT drive low impedance loads. Why bother to post ? I had it lying around doing nothing.

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Stax SRD-6 SB schematic and modification so it becomes mains powered (230V AC only)

Headphone amplifier protection circuit
This circuit can be added to an existing (headphone) amplifier and will provide power-on delay, fast switch off and DC protection for the headphones/speakers.
This schematic can be found as a PCB kit (for 9VAC operation only) on e-bay.

an article about interlinks (by R.A. Cooper):
NOT very suited for subjectivists

They look like sanitary pads which you have to wrap around your cables resulting in noticeably better sound quality.
Miracle Wraps .. brilliant or snake oil … you decide.

Do you want to get rid of the virtual earth in the AMB M3 headphone amplifier ? This can be done by using a symmetrical power supply and omitting the virtual ground. A Tutorial for it.

After a heated debate about WAV versus FLAC I tried to use a simple method to check for differences in contents. A completely pointless exercise but to some it may contain some info they find useful.

Some test I ran with my DIY headphone measurement rig. It will be updated when I have done more tests.

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PX625 and MR9 mercury batteries are no longer available. Here is a tutorial on how to make a battery adapter for these batteries.

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