[kingpage]

Can a 9V wall wart power supply produce +/-9V? I seem to have seen some people refer to amps with a 9V power supply (not batteries) having +/-9V or something similar. I understand from the resources online and schematics that 9V is split into +4.5V and -4.5V through a virtual ground circuit.
 
Also, my HA11 amp has a 9V wall wart, and some 7809/7909 regulators. Does that mean it regulating +9V and -9V even though it's only a 9V power supply? I know you can't do that with a 9V battery in portable amps such as a Cmoy.
 
Some specs of desktop headphone amps state +/-15V, does that mean they are actually using a 30V wall wart (which is not exactly common) or their own rectifiers/transformers?
 
Any clarification would be great!
 
 

 

[00940]

Two possibilities:
 
- It's an AC wallwart. You can then make use of a voltage doubler to get a dual PS. See: http://sound.westhost.com/project05.htm for example.
 
- It's a DC wallwart. You can then make use of a dc to dc converter. There exist many providing various output voltages for various input voltages.

 

[tangent]

Quote:

I seem to have seen some people refer to amps with a 9V power supply (not batteries) having +/-9V or something similar.

 
Yes, it's a common confusion.  As you have already found, if it's split in a linear way, you get +/- 4.5 V, not +/- 9 V.
 
 
Quote:

Some specs of desktop headphone amps state +/-15V, does that mean they are actually using a 30V wall wart

 
Not in the way you are thinking of it, no. They are probably using a true dual supply. That is, two independent supplies, tied together only at their ground point. A single supply that is split, by contrast, is still a single supply.
 
Don't expect to find some small set of simple configurations behind all this. There are many, many different ways to make a power supply. They all have their pros and cons, which is why they all still exist.

 

[kingpage]

Thanks.
 
Looking at the datasheet of L7809CV which says it outputs 8.55V-9.45V but the minimal input voltage is 11.5V, I wonder why it is used in a circuit with a DC wallwart of 9V instead of something like the L7805CV. What if I want to increase the total voltage to 18DC, how should I do it? Should I substitute 7818/7918 regulators for the current ones or should I do nothing since it's only +/-9V from that supply? I thought I knew what to do, but now I'm still trying to figure out the best way to mod it.

 

[00940]

Quote:

Thanks.
 
Looking at the datasheet of L7809CV which says it outputs 8.55V-9.45V but the minimal input voltage is 11.5V, I wonder why it is used in a circuit with a DC wallwart of 9V instead of something like the L7805CV. What if I want to increase the total voltage to 18DC, how should I do it? Should I substitute 7818/7918 regulators for the current ones or should I do nothing since it's only +/-9V from that supply? I thought I knew what to do, but now I'm still trying to figure out the best way to mod it.

Pics found of the HA11 on the internet show a label "AC 9V" at the wallwart jack. With 9VAC, it's possible (barely but possible) to get a bit less +/- 12VDC, as shown in the first link I gave you above.
 
I wouldn't touch anything. If you change the input voltage, you also risk to blow up caps which probably aren't rated for twice the voltage.
 
 
Here is probably how it's inside your amplifier, before the lm7809-7909.
 

 

[kingpage]

How could I miss that small detail??! It's AC, of course! I just assumed it's DC all the way. Thank you so much for point that out. No wonder the power was rated at 1.8VA instead of 1.8W, and I thought it was a typo.
 
The caps inside are 470uf and 47uf respectively rated at 25V.
 
Here are some photos I found online. Unfortunately, I haven't managed to open the case yet, due to the irregalur hex screws used as both 2.5mm and 3mm allen keys don't fit. The hex screws are ever so slightly smaller than 3mm in size. I probably need one of those with ball ends.

 
Now, there's no need for a 18VDC wall wart anymore, since I was thinking of getting +/-9V from it, but it's already +/-9V which is mentioned by numerous people. I guess that's what got me confused in the first place. VAC wall warts are much more difficult to buy, never mind they are more expensive than their VDC counterparts. I did find a few 12V and a 16V one on eBay. Although +/-9V is double the minimum requirement of the opa2107 chips, a bit more power wouldn't hurt (except my wallet).

 

[kingpage]

Can a 9VAC wall wart provide +/-12VDC? The peak for a 9V RMS is 12.7V, so is it as easy as substituting 7812/7912 for 7809/7909 for me to get close to +/-12VDC?
 
Even though the dropout voltage is 2V, the actual value could be lower. If I change only the regulators, I'd be getting around +/-11~11.5V which is pretty good without buying another voltage wall wart (12VAC or 15VAC), right? Is there any heed I need to take?

 

[00940]

It doesn't seem reasonable.
 
First: 9VAC gives you 12.7V peak. But the diode drop will reduce it to 0.7V, which means 12V peak.
Then: you'll have some ripple. 12V peak is a maximum at which the caps are charged, but the voltage will sag as capacitors empty in between charges. In a quick sim, the sag is as bad as 0.4V, for a 5ma load and 470uf caps. We're now at 11.6V.
And finally: your transformer is rated for 9VAC for a given main voltage. But your 110V isn't always 110V. It can vary by 5% up or down, sometimes even more. You have to take it into account 95% of 11.6V is 11.02V.
 
As you see, 9VDC is the maximum regulated voltage you can get with conventionnal regs, with a 2V dropout. You could use low drop out regulators (they have a drop out around 300mv) to get 10.5V or so, but it would be a bit more difficult to implement.

 

[kingpage]

Double post.

 

[kingpage]

The wall wart is rated at 9VAC with a 200mA load and an input of 220V 50Hz, but in Australia the AC power supply is 230V; in many places it's 240V (reported by some people online). I'll wait until I get my hands on a multimeter to test the voltage of the wall wart.
 
Low dopout regulators, interesting. Any down sides compared to standard ones? Difficult implementation as in a change to the circuit layout?
 
Will the use of higher value filter caps slightly increase the voltage by any chance? I'm thinking of 2200uf at 25V instead of the 470uf. But of course, the main aim is to reduce ripple.

 

[miskatcitnamor]

Someone will correct me if I am wrong but no increase in capacitance will result in an increased voltage.

 

 

[00940]

 
Quote:

The wall wart is rated at 9VAC with a 200mA load and an input of 220V 50Hz, but in Australia the AC power supply is 230V; in many places it's 240V (reported by some people online). I'll wait until I get my hands on a multimeter to test the voltage of the wall wart.
 
Low dopout regulators, interesting. Any down sides compared to standard ones? Difficult implementation as in a change to the circuit layout?
 
Will the use of higher value filter caps slightly increase the voltage by any chance? I'm thinking of 2200uf at 25V instead of the 470uf. But of course, the main aim is to reduce ripple.

- It's possible that you'll get 5 to 10% more voltage out. Still not enough i'm affraid to get +/-12V reliably after regulation.
- Low drop out regs are usually more expansive, will maybe come in weird packages, will require circuit layout change and additional components. Furthermore, you'd probably need adjustable ones as you want non standard voltages out.
- Bigger caps would reduce ripple and thus give you a bit less voltage sag. But you won't get much that way.
 
 
Honnestly, I wouldn't bother. +/-9V isn't too bad for such a simple amp, unless you have old 600ohms headphones.

 

[kingpage]

There are some interesting facts from this site. According to the author (and his testing), linear wall warts have higher than rated voltage under lower current loads and fixed voltage regulators can still work with lower than required voltage. I suspect my amp will need no more than 25-30mA of current, so I hope it will be enough for a stable +/-12V. I will confirm this when I manage to open the impossible case first.


 
+/-9V is not bad, but isn't +/-15V optimal for the operation of opamps and SQ? +/-12V is a good compromise.

 

[00940]

 
Quote:

There are some interesting facts from this site. According to the author (and his testing), linear wall warts have higher than rated voltage under lower current loads and fixed voltage regulators can still work with lower than required voltage. I suspect my amp will need no more than 25-30mA of current, so I hope it will be enough for a stable +/-12V. I will confirm this when I manage to open the impossible case first.
 
 

 
- Sure, a transformer that is not fully loaded will put out more voltage. How much vary a lot though and it is best to measure it. With a voltage doubler and big caps, you're loading the transformer a lot more than you think though.
 
- Looking at the graph you provide, the lm7812 stops regulating when the input voltage is less than 1.8V or so above the output voltage. When you get under that, the input-output differential is always 1.37V, that is the drop due to the serie transistor inside the regulator.

 

[tangent]

I think the transformer loading issue is what's happening here. If the amp doesn't draw anywhere near 200 mA — that being 9 V divided into 1.8 VA — the wall wart may well put out enough voltage even under normal wall voltage sags to maintain good regulation.
 
If the regulators ever do drop out, they don't just fall off a cliff. All that happens is that additional ripple gets through, which later elements in the circuit will cope with. It just increases the chance of hearing wall voltage hum.