Safe to Use DC Instead of AC Power Adaptor for Topping L30 II?

[mike7877]

(photo from an ASR user's [broken, I believe] L30 II which I found in a different thread)


Bottom left: AC power in
Immediately to the right of AC power in: 2x Rectifiers
Immediately to the right of 2x rectifiers: Bulk Capacitors (obviously lol)

I looked up the model for the rectifiers and they're Schottky barrier rectifiers (datasheet link at bottom of OP), rated for 3.0A, forward voltage 0.85V (pretty high for a Schottky, no?).

If this amp is half-rectification of the 15VAC waveform, I'd be able to hook up a DC source at like 95-97% of 15*(1/0.70707) VDC, and the device would work using just one rectifier, right? Max power output might be affected, but likely not practically speaking using 250 ohm headphones that are a bit lower-than-average sensitivity at non hearing damage levels?

If it is, in fact, full rectification, would there be any harm done by giving it ~21VDC? I have a bench power supply so can limit the current if that'd help keep things safe (ie. set max current to 0.750A, connect, power on L30 II, see max current is being drawn continually and no operation from L30 II, safely power down. Or better: set 0.750A, connect, power on, working!!! Increase max current to 3.0A)


The reason I want to do this is I want to use it semi-portably. Right now my only option to power it is a 300W "sine wave" thing I got from Amazon. It an excellent device - it should have 400W stamped on its back - but idle current could be better... With supply at 12.5V, average current is 0.7A - I say "average" because peaks are 1.2 when the AC voltage output is its highest (170V or whatever). I'd really like to be able to use an energy efficient buck-boost converter I have going straight from my battery pack. It's 93-97% efficient, smaller, lighter... If not, it's not the end of the world. Though I didn't originally plan to use the L30 II portably, I was unaware that its adaptor only provided AC... Barely anything has an AC input on it these days! I would have had second thoughts for the L30 II if I'd known, but, now, after thinking on it, I do think I would've bought it anyway.

Will L30 II pull through for me? lol




Link to datasheet:

https://www.mouser.com/datasheet/2/80/20190806171106-2504905.pdf

 

[solderdude]

You cannot use DC as input as you will only feed one voltage rail (positive or negative) and you need both.
Positive half of the incoming AC feeds the internal positive DC rail, the negative half of the incoming AC feeds the negative DC rail.
Applying DC thus will result in just one DC rail being present and the amp not working.

For this to work you will have to use a dual rail DC power supply and feed it directly to the 2 big capacitors (so have to open up the amp).
There are no benefits to be had though other than portable usage as in the end the max. output power is determined by the DC voltage and current limits of the TPA6120.

Using 2 batteries (of the correct DC voltage) connected directly to the 2 caps will work but is not recommended unless you have some insurance the battery voltage differences remains small (so not recommended, one could use a similar power scheme as the O2 amp).

A way out of this could be to use a single DC voltage and switch polarity quickly (creating a square-wave). That would be much more efficient than using a sine-wave and the internal rectifier will work well on this.
Very little losses.

Another option could be to use a DC/DC converter with wide range input and dual voltage output that feeds the 2 capacitors (requires disassembly).
That will cost some efficiency though.

 

[jasonhanjk]

Get 3 lithium cell in series and gets about 12VDC.
Use 2 battery pack and you get +-12VDC.
Now you have a portable amp.

 

[mike7877]

Get 3 lithium cell in series and gets about 12VDC.
Use 2 battery pack and you get +-12VDC.
Now you have a portable amp.

That's not a bad idea

 

[mike7877]

You cannot use DC as input as you will only feed one voltage rail (positive or negative) and you need both.
Positive half of the incoming AC feeds the internal positive DC rail, the negative half of the incoming AC feeds the negative DC rail.
Applying DC thus will result in just one DC rail being present and the amp not working.

For this to work you will have to use a dual rail DC power supply and feed it directly to the 2 big capacitors (so have to open up the amp).
There are no benefits to be had though other than portable usage as in the end the max. output power is determined by the DC voltage and current limits of the TPA6120.

Using 2 batteries (of the correct DC voltage) connected directly to the 2 caps will work but is not recommended unless you have some insurance the battery voltage differences remains small (so not recommended, one could use a similar power scheme as the O2 amp).

A way out of this could be to use a single DC voltage and switch polarity quickly (creating a square-wave). That would be much more efficient than using a sine-wave and the internal rectifier will work well on this.
Very little losses.

Another option could be to use a DC/DC converter with wide range input and dual voltage output that feeds the 2 capacitors (requires disassembly).
That will cost some efficiency though.

What about two individual, isolated DC supplies (from the same battery), one to each cap?

 

[solderdude]

That'll work but requires opening up the amp and is basically the DC-DC converter solution I mentioned.
Use a wide input range intended for 12V inputs or you can even use a DCDC converter running of 3.6V for instance.
Note that when using Lithium cells you need to ensure a proper charging scheme, certainly when using more of them in series.
Risk of fire/explosion is real.
Of course battery packs with accompanying chargers is an option but much more expensive and cumbersome than using a 10W DCDC converter.

 

[mike7877]

That'll work but requires opening up the amp and is basically the DC-DC converter solution I mentioned.

True lol. I think what I'll do is replace the AC input jack with a 4 conductor jack, internal wires soldered to the diodes and cap legs (one each). Then make an adaptor for the AC to hit the two positive pins (diodes) so I can use AC at home. Or just get a second 4 pin male connector to connect the 15VAC adaptor directly to the diodes (through the 4 pin input jack)

 

[solderdude]

a 4-pin connector could be used.
Wiring suggestion could be:
pin 1 = common/ground (in some plugs like mini-XLR this makes contact first).
pin 2 = original AC input middle pin.
pin 3 = + voltage rail (directly to corresponding cap)
pin 4 = - voltage rail (directly to corresponding cap)

When connecting to AC use pin 1 and 2
when connecting dual rail DC voltage use pin 1, 3 and 4.

 

[wwenze]

Is the AC adapter dead lol

You can use an inverter... But are there inverters that output at ±20V? Maybe there are. Food for thought.

 

[mike7877]

Is the AC adapter dead lol

You can use an inverter... But are there inverters that output at ±20V? Maybe there are. Food for thought.

The adaptor isn't dead lol. The inverter I talked about having in the OP is a 12V 300W sine inverter. It has a 0.7A idle draw though, and I believe about only 75% efficiency for the bottom 30-40W.

I don't think I'm going to go down that route because I'm fairly sure what you describe is something only made in China with new old stock capacitors and fraudulent FETs, shipped for free across the ocean in 7-10 days, maybe 2 weeks, because China claims it's [still] a developing country so the US pays the cost of shipping for all packages under a low fairly high dollar amount [yes, "developing", even with their own entire space program and satellite launching service, yeah with those 10,000 miles of high speed rail connecting all the biggest cities and dozens and dozens and dozens of smaller cities, yes the place with the biggest army possibly ever...]

Something like that could randomly malfunction, upping the voltage 10x to 150VAC, and, of course, it'll be when I'm caught somewhere in the rain, wearing headphones :/

 

[Roland68]

That's not a bad idea

The voltage on the L30 II is internally stabilized to 2 x 15 volts DC. So you need a higher voltage than +/- 15 volts. The unstabilized voltage is around +/- 23 volts.
You should also check whether the protection circuit requires the AC voltage.

 

[mike7877]

The voltage on the L30 II is internally stabilized to 2 x 15 volts DC. So you need a higher voltage than +/- 15 volts. The unstabilized voltage is around +/- 23 volts.
You should also check whether the protection circuit requires the AC voltage.

I think it's higher - the L30 II specification for output is 37V peak to peak, so 18.5V per side.

To verify I checked its manual, and for 300 ohms it's rated for 560mW 0.1% THD+n, which requires 13VAC (peaks at 18.33V)

I think the TPA6120A2 (output chip) needs its supply to be about 1.5-2V more than its output voltage, meaning its supply would need to be about 20V. Since 15VAC is 21.15V, that doesn't leave much room for voltage droop, which might be a problem because this thing is also rated for 7W output (takes at least 10 to make) - the voltage will probably droop then, and more than a volt. Since this high power rating is at lower voltages (16/32 ohm), this wouldn't be a problem if the regulation is dynamic, dependent on the voltage available from the bulk caps. There may be a set maximum (that'd be smart), but when supply drops, regulation might just be the lowest point of the bulk caps' ripple minus 1V. Or something

 

[Roland68]

I think it's higher - the L30 II specification for output is 37V peak to peak, so 18.5V per side.

To verify I checked its manual, and for 300 ohms it's rated for 560mW 0.1% THD+n, which requires 13VAC (peaks at 18.33V)

I think the TPA6120A2 (output chip) needs its supply to be about 1.5-2V more than its output voltage, meaning its supply would need to be about 20V. Since 15VAC is 21.15V, that doesn't leave much room for voltage droop, which might be a problem because this thing is also rated for 7W output (takes at least 10 to make) - the voltage will probably droop then, and more than a volt. Since this high power rating is at lower voltages (16/32 ohm), this wouldn't be a problem if the regulation is dynamic, dependent on the voltage available from the bulk caps. There may be a set maximum (that'd be smart), but when supply drops, regulation might just be the lowest point of the bulk caps' ripple minus 1V. Or something

It's not about belief.
The TPA6120a2 has an operating voltage of +/- 5 - +/- 15 volts and the "absolute maximum ratings" are 33 volts, i.e. +/- 16.5 volts. This voltage is only a safety buffer for overshoots and is unsuitable for continuous operation.
The supply voltage of the TPA6120a2 in both the L30 and the L30 II is +/- 15 volts, which has already been measured in both.
If you don't believe it, just open it up and measure it yourself.