Using +/-70V on a LM 3886

[Pery]

Hi there !!

I have a very good power supply of +/- 70 Volts.

Could I use it to make an audio amplifier for 8 ohms speaker using LM3886 chip ?

I wonder if anyone has experience in this project. I would like to get around 60 watts RMS or so.

Thanks for any help.

Pery

 

[markiemrboo]

+/-70v is way too much for an LM3886. You should be looking more around the +/-30v mark. Unless you mean 70v from rail to rail, which would be +/-35v? That would be OK for an 8 ohm load according to the datasheet, and would give about 50W.

 

[pchw]

I have 2 salvage toroids from Radio Shack's Accurian. Each is 28.5x2 at 2A, that should make it a 110VA rating. Will it be enough to drive one channel of BrianGT's LM3886 kit? My speaker is rated at 6ohm. As you can see that I am trying to fit the kit into the Accurian

Thanks,

 

[Arx]

I have 2 salvage toroids from Radio Shack's Accurian. Each is 28.5x2 at 2A, that should make it a 110VA rating. Will it be enough to drive one channel of BrianGT's LM3886 kit? My speaker is rated at 6ohm. As you can see that I am trying to fit the kit into the Accurian

Thanks,

That voltage is probably a bit on the high side, and power is probably a bit too low. It would probably work, but maybe not well, depending how much power you're hoping for.

-Nick

 

[jaudio]

I have 2 salvage toroids from Radio Shack's Accurian. Each is 28.5x2 at 2A, that should make it a 110VA rating. Will it be enough to drive one channel of BrianGT's LM3886 kit? My speaker is rated at 6ohm. As you can see that I am trying to fit the kit into the Accurian

Thanks,

This is from Brain's user guide page 16

"Many have successfully used 160VA transformers, while the 220VA range seems to be adequate for almost all stereo implementations, not straining the transformer. The price point between 220VA and 330VA, however might lead one to purchase the larger of the two. Anything above this could be considered frivolous for a stereo pair, unless one happens to be on the shelf or in a surplus vendors stock"

 

[pchw]

Yup, I read the manual and saw this paragraph.
From the VA rating's perspective, 2 of these toroids will be equivalent to 220VA. The Accurian is only rated rms 25wpc at 50-20KHz, the chipamp should be a big improvement regardless I just need to be careful and don't get too excited and crank up the volumne

 

[Panelhead]

What do you have to lose.

I think that either the voltage needs to be lower, or the current higher though. The 2A current is a real limit. 5 amps would be much better.
The 28 volts AC would be perfect for a paralleled LM3886 amplifier and 8 ohm loading. You might get 80 watts or so with a higher current transformer.


George

 

[pchw]

I must apologize to Pery for threadjacking.
If your xformer is indeed 70VAC x 2, then it will be too high. What voltage is yout country?

 

[Pery]

LM 3886 using +/-70Volts.

Hi,

I was reading the LM 3886 specs and is not possible to use this voltage of +70V, 0 Volts, - 70V !!

In my country , Brazil, Rio de Janeiro , the voltage is 117V AC 60Hz (wall outlet).
I was wondering to make a mono amp using a power supply of an old Marantz 300DC.. together LM 3886. I see now that this is not possible. The appropriate voltage is +/- 25V to 35V.

Question solved .

Thanks

 

[pchw]

Hi Perry,

So, it is like US, 117V 60hz. Now, second question, does your transformer have dual primaries (120 and 240). If so, you can wire it to use the 240 primary. That way the secondary's voltage will effectively cut in halves, i.e. 35VAC x 2. Then you use input choke instead of input cap. That way you should be able to use your tranny, just some thoughts, see whether this will work.

 

[Malquis]

I built once a two channel amp with this little beauties running at
+42 0 -42 V and they sounded reeaaaaaaaly loud and got extremely HOT so I decided to lower the voltage as I didn't wanted them to die.
Those toroidal transformers rating +-28V 2A would be really nice to use since the amp will ask for the power and the transformer will do an effrot to deliver it to the amp and as W=VxA the voltage will drop to deliver that wattage at a determined current obiusly depending on the speaker resistance you'll use.
For eg. 50[W]=((28Vx1.41)x2)x A --> A=0.63A, but if more current is needed eg. low resistance, to deliver the 50W the voltage will drop, so I would use this transformers with a nice regulator and making them work at +-28V so there is a marging for some volts to drop.
for +-28V AC 50Hz after rectifier and filterin you 'll have +-39.5V per rail so you'll have a nice margin to work with at the input of the regulator maintaining some nice +-28V at the output of the regulator delivering the current your litlle guis ask for remembering that W=VxA

 

[Dxvideo]

I applied 2 x 28v transformer to a LM3886. As I remember the DC value was about 38v on rails. But I think its not a healty value for supplying. Because they were very very hot even without any input signal.
The best voltage I tried is +/- 24v regulated. Now I am using that value for my LM4780 gc. Theyre in comfort, cool and bassy!

 

[gtforme00]

Pery,

How much current can the 70-0-70 power supply source? I know you are thinking of a chip amp, but if it interests you, that power supply might be good for a high power discrete design. Like the super leach amp here: http://www.delta-audio.com/Leach-Clone.htm

Just a thought,
David

 

[Bing Yang]

Can LM12 Cascoding design work for LM3886?

I have the same issue and did not find an answer.

However, I just came across an article in the most recent issue of AudioXpress. The author used an LM12 with a cascoding output stage so that high rail voltage can be used. The solution seems to be simple. One can find that in National Semi's application note AN446. Can LM3886 be used with the scheme? I have spare output transistors.

Bing

 

[john65b]

First, I assume it is a 50VAC transformer with a Full Wave Bridge Rectifier to get the +/- 70VDC rails.

Instead of a Full Wave Bridge Rectifier (4 diodes), can we just use a Full Wave Rectifier setup (2 diodes) and get the 35V Rails? Isn't Full Wave DC Rectification Voltage = .7*VAC and Full Wave Bridge DC Rectification Voltage = 1.4*VAC?

I see FWBR circuits and FWR circuits all the time in tube power supplies, correct?

I may be an idiot, but am I bold in asking this question?

 

[Pery]

Using +/- 70V on a LM 3886.

Thank you everyone,

My problem was to replace the right amplifier board on my power amplifier Marantz 300DC which has dual power secondary supply with +/- 70V.

I was thinking to produce a chip amp inside of the power amplifier to replace the old one which is bad conditions.

Now I see that is not apropriate with a LM 3886.
I have to find another solution , because is impossible to repair the original main board of 150 watts RMS.

Pery

 

[lovechild]

I have a chip amp with 70V rails, it's the Sanoy STK4048 chip in the standart application from the datasheet,
you'll find it here. The chips are out of production, but ampslab does still sell these old Sanyos.

I can't comment much on the amp though, I got it used and it has a problem with the input stage that needs to be fixed first,
but you will find some info using the forum search.

best, LC

 

[consort_ee_um]

The way to go with high voltage supplies is to use the LM4702 driver chip + some output transistors see Glenns page:
http://home.pacific.net.au/~gnb/audio/lm4702.html
The chip will take +/- 100V