Does any power amp have a current limiting device?

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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
 
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.
 
Dear Sir,
thank you very much indeed for your extremely kind and valuable reply.
Let me be more precise between your statements.

>
ashok said:
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
 
pinkmouse said:
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
 
pinkmouse said:
:D 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
 
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.
 
Dear Mr. Ashok,
thank you so much fro your extremely kind and precious reply.
Let me please reply to you hereunder.

>
ashok said:
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
 
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
 
phase_accurate said:
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

Dear Sir Charles Lehmann,

Absolutely agree on your first point, but I think Mr.Jan Didden and Mikeks wouldn't..agree because they think overbuilding the output stage is wastage of resources and could be cured with SOA limiter[Their opinion not mine]
But I would favour Overkill in output stage in pro-audio amp..

Yes , the switching amps would take anything resistive to highly reactive without any threats, provided an overcurrent protection implemented along them....

sincere regards,
K a n w a r
 
AX tech editor
Joined 2002
Paid Member
Workhorse said:


Dear Sir Charles Lehmann,

Absolutely agree on your first point, but I think Mr.Jan Didden and Mikeks wouldn't..agree because they think overbuilding the output stage is wastage of resources and could be cured with SOA limiter[Their opinion not mine]
But I would favour Overkill in output stage in pro-audio amp..

Yes , the switching amps would take anything resistive to highly reactive without any threats, provided an overcurrent protection implemented along them....

sincere regards,
K a n w a r


Well, the reason for the overkill is given as if SOA doesn't work with reactive loads. But it does, that's actually the REASON for SOA protection. Now, if you need more SOA area to drive your speakers than is available in your output stage, obviously you need a bigger output stage or multiple parallel devices. I don't see why anyone calls this overkill. It isn't, it is just common sense engineering.

If on the other hand someone would use multiple devices in an unfounded fear that SOA protection is not enough, either the SOA is incompetently designed, or the available SOA is not enough (see first paragraph) or the person is irrational. And yes, in this last case it IS a waste of resources.

I would be willing to change my view though if someone comes up with a situation where SOA protection, well designed, would be insufficient. Since you Kanwar seem to like to project an air of expertise, that should be easy for you.;)

Jan Didden
 
Does any Power amp have a current limiting device?

HI Beppe,
First off, It is important to see that current is not the only factor: If the amplifier is clipping, the speaker cone is stationary at maximum current, which equates to DC flowing through the voice coil. This will destroy the voice coil, as no cooling is available.
The optimum is a soft clipping characteristic, combined with overvoltage and overcurrent protection.
Have a look at http://www.hypex.nl, they do a number of excellent Class D amplifiers, with all the stuff you'll need to build a complete power amp, with all the protection you'll need for minimal cost. A UcD180 module is only 60 Euros....

I built my amp with UcD180's and it sounds better than any Krell I have ever listened to, I have listened to those and Mark Levinson, MacIntosh and numerous others.
Have a look at the Class D forum as well, it is full of good advice. Cheers, Arthur.
 
Hallo Flachi !

Hoe gaat et ?


Jan

I don't mean to say that SOA protection wouldn't work. It is just that there are amps out there whose devices and protection are dimensioned for nominal power into a resistive load only. If both are dimensioned with real-world (i.e. non-real loads to make things complicated ;) ) loads in mind then there isn't a problem.



Regards

Charles
 
janneman said:



Well, the reason for the overkill is given as if SOA doesn't work with reactive loads. But it does, that's actually the REASON for SOA protection. Now, if you need more SOA area to drive your speakers than is available in your output stage, obviously you need a bigger output stage or multiple parallel devices. I don't see why anyone calls this overkill. It isn't, it is just common sense engineering.

If on the other hand someone would use multiple devices in an unfounded fear that SOA protection is not enough, either the SOA is incompetently designed, or the available SOA is not enough (see first paragraph) or the person is irrational. And yes, in this last case it IS a waste of resources.

I would be willing to change my view though if someone comes up with a situation where SOA protection, well designed, would be insufficient. Since you Kanwar seem to like to project an air of expertise, that should be easy for you.;)

Jan Didden

Dear Jan,
I think you misunderstood the overkill concept,
The reason for overkill is not to overcome the SOA limitation, but to extend the capability of the Output stage as whole to project the minimum stress level on the output devices whenever a situation of a specific stress is encountered and thereby doing so would eventually increases the life of the equipment in field operations.....everyone likes to have a minimum stress level whether its a FET/BJT or you & me...its reality....

If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for....

SOA protection..ofcourse a well designed one is always sufficient but if you offer a "LITTLE EXTRA" in terms of output devices, it must not be regarded as a resource wastage...

K a n w a r
 
AX tech editor
Joined 2002
Paid Member
phase_accurate said:
Hallo Flachi !

Hoe gaat et ?


Jan

I don't mean to say that SOA protection wouldn't work. It is just that there are amps out there whose devices and protection are dimensioned for nominal power into a resistive load only. If both are dimensioned with real-world (i.e. non-real loads to make things complicated ;) ) loads in mind then there isn't a problem.



Regards

Charles


I think you misunderstood. SOA protection is designed to safeguard the output devices, WHATEVER loads you throw at them. So, if the SOA is set up correctly, no imaginable load will destroy them.

Jan Didden
 
AX tech editor
Joined 2002
Paid Member
Workhorse said:


Dear Jan,
I think you misunderstood the overkill concept,
The reason for overkill is not to overcome the SOA limitation, but to extend the capability of the Output stage as whole to project the minimum stress level on the output devices whenever a situation of a specific stress is encountered and thereby doing so would eventually increases the life of the equipment in field operations.....everyone likes to have a minimum stress level whether its a FET/BJT or you & me...its reality....

If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for....

SOA protection..ofcourse a well designed one is always sufficient but if you offer a "LITTLE EXTRA" in terms of output devices, it must not be regarded as a resource wastage...

K a n w a r


Well, it is all very strange to me. So we design an output stage for a design load regime. Then we design a protection system to make sure they will not fail. And then we throw in something extra? For what? Lower the stress? Does that mean you expect it to fail unless you throw in the extra? That means your design wasn't sufficient in the first place. Very, very strange...

Jan Didden
 
So, if the SOA is set up correctly, no imaginable load will destroy them.

Ahhhhh, now I see whre we misunderstood each other !! I didn't want to say that they won't protect your output devices, far from that. But they sometimes do that too early i.e. they restrict output power into reactive loads if the output-stage is dimensioned on the cheap and the SOA protection dimensioned accordingly. I don't critisise SOA protection as such but cheap undersized output stages.
There are amps that offer the same nominal power into resistive loads but differ in perceived power into real-world loads. One of many reasons can be the above.

BTW: With switching amps, max current limitation is about the same as SOA limitation.

Regards

Charles
 
Dear Jan,

You still Misunderstood....its extremely Strange

THE SOA Protection is designed to Safeguard the output devices when encountered with reactive, or simply say Unsafe Loads..which would otherwise damage the devices.....And the output will not be destroyed in case of well designed SOA protection....because SOA Limiter is always there to safeguard the devices...No destruction is possible with any type of load....until your SOA protection isn't damaged


The Fact is that when you add a little extra, this increases the Long period operation reliabilty in high temperature conditions and ease the output as a whole....Because the reliability of a BJT/FET device is always inversely proportional to its operating temperature...
Thereby when we Parallel More Pairs of output devices ..it simply Increases the Available DIE-AREA and complements the Heat Dissipation over Larger surface area of Heatsink and ensures cooler running of device through increased heat removal from the device die area attached to the heatsink and boost the reliability


K a n w a r
 
AX tech editor
Joined 2002
Paid Member
Workhorse said:
[snip]If 4 pairs of output devices with SOA protection are sufficient for handling a given reactive load, then adding 2 more pairs[total six] wont be stated as wastage, rather it should be seen as an added advantage to lighten the stress level even more..... which would always have a positive effect on the operation of the amp.... which is designed to operate in professional arena rather than in home environment...
I would only consider it as a wastage if the output devices pairs exceeds the six pairs to something eight or even nine pairs of devices, because in that case it is clearly evident that the designer has no understanding of what SOA protection is meant for.... [snip]

(sigh) OK, so you say that adding a few extra pairs have a pos effect on the operation of the amp. What it it, please? Do you mean longer life time? How much longer? Do you have any information on how much longer? Or is the few pairs extra just something that 'feels good' ?

I also have no idea why adding 2 pairs is a Good Thing, while adding 4 extra pairs shows a misunderstanding of the SOA meaning.


Jan Didden
 
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