Does any power amp have a current limiting device? - diyAudio
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Old 11th December 2005, 01:24 PM   #1
beppe61 is offline beppe61  Italy
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Default Does any power amp have a current limiting device?

Dear Sirs,


I understand that most of the commercial amps have some sort of circuit/device that limits the delivery of current to the load (speakers).
I read that in order to safeguard the life of the output devices some designer overprotect them limiting the peak current too much sometimes.
I would like to know if anyone has ever eliminated this circuit and what has been the outcome.
I wonder how this circuit is built.
If I could find this circuit in the schematic I would try to by-pass it in some way.

Thank you very much.

Kind regards,

beppe61
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Old 11th December 2005, 02:31 PM   #2
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1. You are making a very general statement.
2. Yes some amps limit current more than what some users want.
BUT maybe the designer wants it that way.
3. If you are a DIY guy or FIDDLER of equipment , the current limit might be a blessing in disguise.
4. From your statements it looks like you "might" not be a techie guy. In that case do not alter anything unless you want 'holy smoke'.
5. Specify the amp in question to determine if it has been "over" protected.
6. Current limit must have prevented many DIY'ers amplifiers from becoming toast!

So be VERY cautious about what you want to do. If you have a scope and other test equipment , you could possibly do it - assuming you are a techie.
I think not many people would like to help you do this by remote control on an amp they know nothing about. If you have a specific amp in mind , ask again!
Cheers.
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Old 11th December 2005, 02:55 PM   #3
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I agree with Ashok. Beppe, if you want an amp that can supply current, then you would be best off building or buying something specifically designed to do so. May I suggest a Krellclone?
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Old 11th December 2005, 06:16 PM   #4
beppe61 is offline beppe61  Italy
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Dear Sir,
thank you very much indeed for your extremely kind and valuable reply.
Let me be more precise between your statements.

>
Quote:
Originally posted by ashok
1. You are making a very general statement.

I understand the problem is quite complex, unfortunately.
But it is my opinion that amp manufacturers should help customers providing them with figures useful to assess the driving ability of their products.
From what I have gathered here and there this value of max peak current delivery is a very important one and very seldom is in the specifications.
They feel bad to declare let's say a 5 A of max peak current when other amps can deliver 20 A ?
They fill a page with data of no real interest.
Better to say nothing. Or let me try it with my speakers.
I have compared with my speakers two amps:
1) 90W/8ohm ; peak current = ?
2) 45 W/8ohm; peak current = 20A
There is no comparison in driving ability between the two, being 2) a world better than 1).
I bought of course 1).

2. Yes some amps limit current more than what some users want.
BUT maybe the designer wants it that way.

No discussion about this. But they must declare it to customer.
I want an amp powerful and reliable.
Maybe I am asking too much.
My friend gave a look inside amp 2).
A very minimalist one with off-the-shelf components.
And with an output stage made out of just two darlingtons/channel.
As I said really a world better, at least with these beasts of Dynaudio speakers.

> 3. If you are a DIY guy or FIDDLER of equipment , the current limit might be a blessing in disguise.

Let's say for a moment I am not a DIY guy (i.e. the reality).
May I know the current deliver of an amp I want to buy or it must remain a secret until I test it with my speakers?
Here in Italy I have to buy before listening.
I have the home full of bad buys made on the basis of specifications.

> 4. From your statements it looks like you "might" not be a techie guy. In that case do not alter anything unless you want 'holy smoke'.

I see the point. I do not want to destroy anything for sure.

> 5. Specify the amp in question to determine if it has been "over" protected.

The amp is a Samson Servo 260.
At full power the woofers of the Dynaudio just mumble.
Something like a velvet fist in a ... velvet glove.
The sound is not bad at all just the lower part of the audio spectrum (<100Hz) is just missing with my speakers.

> Current limit must have prevented many DIY'ers amplifiers from becoming toast!

The amp 2) could be a breeze to clone indeed.
It has about 10 transistor /channel and a very remarkable sound.
But this is not my point. My point is that the common specs are not useful to establish the driving ability of the amp.
Max peak current maybe it is more useful, from what I have read.

> So be VERY cautious about what you want to do.
If you have a scope and other test equipment , you could possibly do it - assuming you are a techie.

I am not a techie. I am just a disoriented audio hobbyst.

> I think not many people would like to help you do this by remote control on an amp they know nothing about.
If you have a specific amp in mind , ask again!
Cheers.
To end my ramblings, I understand that modding an amp without knowledge could destroy it.
If only these value of peak current were stated more clearly in the specs these could be very useful IMHO.
Even if I decided for the DIY route, only the UcD modules datasheets report the max current available from a module.
But which is for instance the max peak current of an AKSA amp?
10A, 20A, 30A ?
I could not find it anywhere..
I know the sound is fine but what about its driving ability with demanding speakers?
Of course to be sure I should buy a Krell and stop any discussion.
They could drive even a door.
Unfortunately the original Krell is out of my reach and the clone is too difficult to build.

Thank you very much indeed.

Kind regards,

beppe61
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Old 11th December 2005, 06:26 PM   #5
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by pinkmouse
I agree with Ashok.
Beppe, if you want an amp that can supply current, then you would be best off building or buying something specifically designed to do so.
May I suggest a Krellclone?
Dear Sir,

your suggestion is very wise, but my skills as a DIYer are very limited.
I need an amp in the kit form like for instance an AKSA amp.
I am afraid that the driving ability is quite different though, and I happened to like very much a deep and powerful bass response.
Anyway I also think that the Krellclone is just too complex.
Are you aware of amp in kit form with a good delivery of current?

Thank you very much indeed.
Kind regards,

beppe61
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Old 11th December 2005, 06:29 PM   #6
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Quote:
Originally posted by beppe61
...They could drive even a door...

Quote:
...and the clone is too difficult to build...
Really, it's not. If I can build it, anyone can. It looks complicated, with lots of components, but all it takes is a bit of care and attention.
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Old 11th December 2005, 07:32 PM   #7
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by pinkmouse
Really, it's not. If I can build it, anyone can.
It looks complicated, with lots of components, but all it takes is a bit of care and attention.
Dear Sir,

thank you so much for your extremely kind and valuable reply.
I see that you are a UK citizen so I feel the urge to state that some of the best example of amp design come from your land.
Congratulations.
Some years ago I had a Cyrus One, then a Rotel, a Cambridge.
Now I have fallen in love with another UK amp ( I would like to say the name but I am waiting to purchase one maybe in the ebay.co.uk).
They are monos, 45W/8ohm but with a great current delivery (22A max peak current ! each).
They made my current hungry Dynaudio sing, and for the very first time my system was enjoyable.
The friend who lent them to me, has opened it and found very few components. Again a very simple even minimalist approach.
Think of 10 transistor per channel. A single darlington output pair.
I really do not know about copyright law about audio design.
But that would be a design to clone.
I looked at the schematic of the Krell. It is really to much for me.
I am a very unexperienced DIYer.
I do not know of any other kit with a great current delivery.
I understand that the company has ceased its activity nowadays.
This is indeed a pity because their designs were very good indeed.

Thank you very much indeed for you very kind support and advice.

Kind regards,

beppe61
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Old 12th December 2005, 02:02 AM   #8
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Hi Beppe61 ,

You needn't worry about copyright etc. As long as you copy the design for personal use it should be OK. The problem arises only if you use it for commercial purposes ( meaning you start building copies and selling them for gain ).
A very large number of DIY guys are making copies of amps they like but cannot afford to buy ! In any case at the cost they make it , they will never make it commercially viable .

So you can go ahead and copy the amp you like. No one can stop you. However study the Krell clone carefully and any other kits available before picking one.

In addition , email Hugh at AKSA to find out about the current capability . He might even suggest something if you need higher current capability ! Maybe that amp IS your solution.

Cheers,
Ashok.
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Old 12th December 2005, 10:25 AM   #9
beppe61 is offline beppe61  Italy
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Dear Mr. Ashok,
thank you so much fro your extremely kind and precious reply.
Let me please reply to you hereunder.

>
Quote:
Originally posted by ashok
Hi Beppe61 , You needn't worry about copyright etc.
As long as you copy the design for personal use it should be OK. The problem arises only if you use it for commercial purposes ( meaning you start building copies and selling them for gain ).

Thank you very much indeed. Now I am feeling better.
I have absolutely no intention to use them for commercial purposes.

> A very large number of DIY guys are making copies of amps they like but cannot afford to buy !
In any case at the cost they make it , they will never make it commercially viable.

I know this too well. One starts to save money and ends spending 2, 3 times the budget.

> So you can go ahead and copy the amp you like. No one can stop you. However study the Krell clone carefully and any other kits available before picking one.

I have the old Krell stereo amps (the KSA50, 80 and 100) in the greatest esteem indeed. The mono version even higher as well, of course.
They were powerful, with a prodigious bass and nice full-range sound. They drove anything and have been the basis of the Krell success worldwide.
I would like very much to be able to clone one but as I said looking at the schematic posted here of the KSA50 I find them beyond my ability, that is very poor.

> In addition, email Hugh at AKSA to find out about the current capability .
He might even suggest something if you need higher current capability !
Maybe that amp IS your solution.

Surely taking into account that is a complete kit with a very well done users' manual it should be easier to build.
I will try to assess its driving ability (my recurrent nightmare after my choice of Dynaudios as speakers).

Cheers,
Ashok.
Dear Mr. Ashok,
thank you so much for your extremely kind and precious advice.
Kind regards,

beppe61
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Old 12th December 2005, 11:46 AM   #10
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Regarding overcurrent protection:

Limiting current to a specific value is not going to help much in terms of device protection.
The most effective protection circuits are limiting current as a function of output voltage (so called SOA protection) and are therefore often employed. Their downside is that they work perfetly into resistive loads but not so into real loudspeakers where current and voltage can be out of phase and cause the circuits to limit much too early.

There are only to solutions to this problems:

1.) Overdimensioned output stages for which the protection can be more generously dimensioned. These can also help to increase the output stage linearity.

3.) Using amp principles that don't need SOA protection but only fixed-value overcurrent protection: Switching amps !

Regards

Charles
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