AC/AC Adapter getting warm, alternatives?

[calli]

Hi,

I have some "vintage" musical 19" gear, a Alesi MIDI Verb II. This came with a 240VAC->9V AC Adapter which gets over 60°C. The label is long gone, so I cant even check if it is 220 or 240 or 110V... I replaced it with a "Datatronics 1.5A 9VAC" Adapter and this gets a bit over 40°C.

I suspect that the device needs the AC to generate +/- Rails for OpAmps? So it may be impossible to retrofit a modern AC/DC Adapter/Powerbrick?

Or is it just bad quality on both?

Thanks,
C!

[Zero999]

Small transformers used in AC output mains power supplies are very lossy.

You're right, the device will probably not work with a modern switched mode power supply, because it needs AC to generate +/- rails for op-amps. You could try running it off 12VAC square wave, using an H-bridge run off a 12VDC power supply, with an isolated output.

How much current does it use?

I designed a self-oscillating bridge using MOSFETs. It will also work with BJTs with a few modifications.

https://www.eevblog.com/forum/projects/dc-12v-dc-to-9v-ac-12w/msg3344898/#msg3344898

It's also possible to add current limting, with a few extra components.


https://www.eevblog.com/forum/beginners/mini-h-bridge-for-driving-vfd-filament-very-warm-to-the-touch/msg3168022/#msg3168022

[calli]

Ah, yea size matters

That fits 100% because the adapter which get 40°C is nearly twice as big. I think I am ok with 45°C which it will maybe reach in summer.

I now finally found a Schematic for the newer model, of course no power supply section there.

So the I guess that square wave trick is also used by newer devices which use DC in but definite use Opamps (as my synths which are full of TL74 etc.) Maybe it also possible (which leads to your suggestion checking the amperage) to retrofit some smal symmetrical PSU like the cheap chinese one I use in my modular. Depends which voltages I need.

Anyhow thanks for your suggestions and hints!

Carsten

[calli]

I still wonder if there is a modern plug-n-play solution for the market. Its quite common in audio-Mixers, microphone amps, effect units. These days they getting less but there is quite a market.

Is it a technical thing or not a big enough market? I imagine a switched power brick but instead of DC it gives AC...

Just googling around.... Hey there is a schematic in nearly every datasheet.... mains -> fuse -> transformer -> out  :-)

Carsten

[Zero999]

Ah, yea size matters

That fits 100% because the adapter which get 40°C is nearly twice as big. I think I am ok with 45°C which it will maybe reach in summer.

I now finally found a Schematic for the newer model, of course no power supply section there.

So the I guess that square wave trick is also used by newer devices which use DC in but definite use Opamps (as my synths which are full of TL74 etc.) Maybe it also possible (which leads to your suggestion checking the amperage) to retrofit some smal symmetrical PSU like the cheap chinese one I use in my modular. Depends which voltages I need.

Anyhow thanks for your suggestions and hints!

Carsten

I very much doubt newer models use a square wave oscillator, like the circuits I've posted. They most likely design it to work off a single power supply rail. If they do require a negative supply, a charge pump or switched mode regulator will be used to generate it.

I still wonder if there is a modern plug-n-play solution for the market. Its quite common in audio-Mixers, microphone amps, effect units. These days they getting less but there is quite a market.

Is it a technical thing or not a big enough market? I imagine a switched power brick but instead of DC it gives AC...

Just googling around.... Hey there is a schematic in nearly every datasheet.... mains -> fuse -> transformer -> out  :-)

Carsten

The only switched mode power supply I can think of, which gives an AC output, is an electronic halogen lamp transformer, but it works at a much higher frequency than the mains >20kHz, which could cause RF interference and excessive switching losses in ordinary rectifier diodes.
 
I doubt there's much of a market for this. I believe AC output mains adaptors are still allowed to use ordinary transformers, which most people with classic audio equipment will be happy with.

If you want someone to build it for you, then it will be very expensive, compared to a mass produced item, because you'll be paying for design time and it to be assembled by hand. You'll also need to provide/agree a detailed specification.

[NiHaoMike]

You could use a pair of transformers with the primary and secondary windings connected in series to reduce the losses. The main tradeoff is that it will cost more and be a lot bigger, but you could also take that as an opportunity to add noise filtering.

[calli]

Don't get me wrong. I am just curious, not reluctant. 

I see power bricks designed specialy for old computers and there is a trend to collect old electronic musical gear which gets expensive. There may be a increase in prices for such things soon, I see many of that gear sold but w/o powerbricks.

Carsten 

[KRISTOFFER]

Unfortunately most power adaptors like you describe use a transformer designed to be run at near 80 - 90% load otherwise they start getting hot when idling. This is why they tend now to be sold as 'power transformers' rather than 'mains transformers'. It keeps the cost down with respect to the size of the core and the number of turns on the windings. Under light loads they are running into saturation and getting hot. Under a high load there is somewhere for the energy to go. Imagine your car engine in normal use, then, while parked up and out of gear, put your foot to the ground and watch the engine temperature rocket up. Go for an adaptor or transformer rated to run continuously under any load if you can get one.

Looking at the circuit diagram it is using 9V AC from an adaptor to get the +5, +12 and -12 supply lines from a voltage doubler circuit. and standard linear voltage regulators. Go careful with using other adaptors (original is quoted 9VAC 830mA) since some of the ram circuits are fed with unregulated supplies direct from the doubler circuit.  It will not run on a DC supply, it has to be AC.

[calli]

Yea I am aware of all that.

Of course I am not sure that the adapter delivered with this device was a original one (the sticker is melted off over the years), but fact is that a much bigger (also 9VAC of course, 1,5A) adaptor is running cooler. I never thought  I would need that AC adapter somedays

Carsten

[calli]

Looking at the circuit diagram it is using 9V AC from an adaptor to get the +5, +12 and -12 supply lines from a voltage doubler circuit. and standard linear voltage regulators. Go careful with using other adaptors (original is quoted 9VAC 830mA) since some of the ram circuits are fed with unregulated supplies direct from the doubler circuit.  It will not run on a DC supply, it has to be AC.

I found that spot in the reset "logic". But it is clamped with a 5.1V Zehner. Or did I miss another spot?

Carsten

[KRISTOFFER]

Your quickest solution would be to get a decent 12VDC adaptor, take the rectifier and capacitor out and you will have an AC adaptor at around 9VAC.

[Zero999]

Your quickest solution would be to get a decent 12VDC adaptor, take the rectifier and capacitor out and you will have an AC adaptor at around 9VAC.

That would work, as long as it's an old, unregulated mains adaptor. Newer mains adapators use a switched mode power supply, which converts the AC mains to DC first, before feeding it into a high frequency oscillator and small transformer, with a rectifier and capacitor on the secondary side. Most old, low frequency transformer based mains adaptors also run pretty hot, even under light loads, which would lead the original poster back to square one.

Yea I am aware of all that.

Of course I am not sure that the adapter delivered with this device was a original one (the sticker is melted off over the years), but fact is that a much bigger (also 9VAC of course, 1,5A) adaptor is running cooler. I never thought  I would need that AC adapter somedays

Carsten

Generally larger transformers are more efficient and run cooler, than smaller ones. Another alternative is to modify the original circuit to use a single supply rail. An isolated 24VDC power supply, with the voltage doubler circuit in the device, replaced with virtual ground/earth/rail splitter circuit, would give +/-12V. If the current through the 0V node is under 80mA or so, use the TLE2426. Another alternative is a buffer, such as the BUF634.

https://www.ti.com/lit/ds/symlink/tle2426.pdf

https://e2e.ti.com/support/amplifiers/f/14/t/518185?TLE2426-rail-splitter-higher-current-source-sink-than-20mA-alternative-

[bdunham7]

I think you've solved your problem.  Datatronics is a solid manufacturer and I'm betting that in addition to being larger, the adapter is a lot heavier.  I look for old IT-specific AC transformers just because they are usually not as iron-deficient as the cheap crap.  40C is really a non-issue.  It is possible that the transformer is impedance protected, like a Class 2 control transformer,  you'd have to post photos/labels/model numbers to know.

[james_s]

I was going to mention the impedance protected aspect but I see he beat me to it. IIRC many of those iron transformer wall warts were about 50% efficient on a good day, and many are impedance protected rather than having a thermal fuse. It's also possible that the transformer you have was designed for 60hz in which case it will probably start to saturate and run hot on 50Hz.

[KRISTOFFER]

Is this the one ?

[bdunham7]

Is this the one ?

Since he's using it at 240VAC 50Hz, I hope not! 

[calli]

Looked different. I think it was not the original one. or the 120V AC made the difference.

[mariush]

How about toroidal transformers, should be much lighter since we're not dealing with steel plates?

Get a plastic box, an IEC connector for input to use a regular detachable cable with it,  add maybe a fuse, and a regular barrel jack cable and you have your power supply.

[HB9EVI]

I've got a MidiVerb and a MicroVerb in my guitar rack, and yes the 9VAC adapters run too hot; and it's not possible to run it from DC. I actually solved it with a toroid transformer (2x 9VAC / 25VA) suppling both devices and doesn't even run warm.
if you don't want to risk impairment of the sound quality, it makes sense to stuck with a classic 50Hz mains transformer

[james_s]

How about toroidal transformers, should be much lighter since we're not dealing with steel plates?

Get a plastic box, an IEC connector for input to use a regular detachable cable with it,  add maybe a fuse, and a regular barrel jack cable and you have your power supply.

Toroidal transformers for mains frequency are not appreciably lighter. They use a wound band instead of individual plates, they are still iron.