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Old 15th October 2013, 01:21 AM   #1
Bigun is offline Bigun  Canada
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Default do all Amps need dc-protection ?

I've started to work with MOSFETs instead of BJT power devices. I've produced smoke from burned resistors, sparks from shorted outputs, blown fuses, crackling sounds even. But the MOSFETs keep on trucking.

Modern parts appear to be very robust.

If the outputs are over-rated and the amplifier is stable and used for domestic (read - not abused) use then what would cause the outputs to fail ?

And if they never fail why would I ever be concerned about dc on the output.

And so why bother with dc-protection ?

I notice that Hugh's latest NAKSA 80 is MOSFET output and he says no dc protection is needed.

Is the only reason left the old 'so I can sleep at night' argument ?
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Old 15th October 2013, 01:30 AM   #2
GoatGuy is offline GoatGuy  United States
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[1] you protect against large ("unconstrained") DC output to protect the output devices from run-away thermal overloading.

[2] You protect against DC output (period) to save down-wind devices ... such as your woofers, or any other load that doesn't have a capacitive intermediary filter.

[3] You protect against output overloads in general for operating safety and reasonable fault tolerance for things that shouldn't ever normally occur, but which have a sizable probability of occurring one or more times over the life of the amp.

[4] You protect against DC output to quench the circuit in case a MOSFET actually 'breaks open' or other latched-to-supply problem.

Those are decent reasons, each in their own right. Doing it because "its not hard", and "the added circuitry doesn't impinge on the actual nominal output" are the more usually cited reasons.

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Old 15th October 2013, 01:35 AM   #3
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What about perfectly serviceable outputs and a DC offset caused by a small signal device failing, or a passive like a capacitor failing? Even a poor solder joint can lead to DC on the output without a component failure.

In truth I don't personally worry about it too much. New construction is only on in my immediate presence and as it proves itself I become more passive in monitoring it. Neither of my old Brystons have any DC protection and they frankly are not unlike some of the DIY efforts in design.

Build it, test it, torture it and enjoy what survives .
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Old 15th October 2013, 01:45 AM   #4
danny92 is offline danny92  Portugal
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Hi Bigun,

That's right, nowadays the transistors are very tough, but you must pay attention to several things:

-The devices should dissipate no more than 3/4 or rated power
-The maximum current (Ic/Id) should be also lower than the indicated value, by 2 or 3 A
-And Vce/Vds max should be at least |Vcc|+|Vee|

The protections you should include are:

-VI limiter (read the Michael Kiwanuka papers) to prevent damage if output is shorted
-DC protection relay (DC at the output could kill the speakers), use a comparator with a very low frequency low pass filter and a speaker relay
-Thermal shutdown relays, activated when high temperatures are detected in the heatsink, you could do this with a comparator, with a known voltage, and a thermistor voltage divider
-You could also use a overvoltage protection at the input

In my opinion all amplifiers should have this protections, because if you don't have them the devices could enter in stress and damage could occur, and the device will never be the same, even if the device isn't destructed it could have damage and the life of your amplifier could be shortened. I think the power amplifier should be a reliable equipment, and to last many years

Best regards,
Daniel
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Last edited by danny92; 15th October 2013 at 01:47 AM.
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Old 15th October 2013, 01:47 AM   #5
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I have used many diy amplifiers without DC protection for many years and I have never had a problem. Will I put it in my diy amplifier? the answer is NO!!!
is it good to have it in amp? the answer is YES!!!
To me less is better(kind of way). if you are "really" afraid to burn
a driver if DC goes to the speakers and speakers are "really" expensive...USE IT!.
in my case I have small Polk audio RTi A1 and "IF" I get some DC voltages to drivers and burn them...I would just order some new drivers...So far I have been using amplifiers for a long time (nine years) without DC protection and I have NEVER blow or damage a speaker. I do not use fuses for speaker either
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Old 15th October 2013, 01:56 AM   #6
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Quote:
Originally Posted by Bigun View Post
Is the only reason left the old 'so I can sleep at night' argument ?
How about the "those speakers took a lot of time and cost a lot of money to build" argument.....

To each his own - but I'm not putting my speakers at risk - period.
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Old 15th October 2013, 03:03 AM   #7
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These amps don't need DC output protection: Retro amplifier 50 watt circuit
Leak Stereo 30, Stereo 70, Delta 70
Armstrong 6xx
Dynaco ST120, ST80
the G amp by Sakis
They already have it with a capacitor in series with the speaker biased at half the supply voltage. These have no negative supply, and have quasi-comp outputs instead of complimentary.
MacIntosh some number that had transistors and output transformers. Output transformers are sufficient speaker protection.
Others, IMHO, yes. My speakers cost 6x what the amp does, plus 2x for driving down to get them nearly a full day. One shorted speaker lead, either 1/4 phone plug or whisker off a lead in a binder post, doesn't matter how good your bias current and VI limiter sensors are working.
Less risk, of course, for Speakon connectors.
Running maybe 20-30 watts for four hours my ST120 got hot enough to melt solder and an output wire sprung up and hit the cover. Blew 2 output transistors. The speaker was fine. Cost <$30.
This PV-1.3K, by contrast, has melted the traces to the crowbar triac several times in its life, to look at the damage in that area. Caused by a cold solder joint on an op amp socket from the factory. This really needs a speaker disconnect relay, which I am attempting to install now. (up to 30 parts now. A very scary project stopping >50 amps @ 170 volts without sounding funny at low volume or costing $100 in parallel FETs + heatsink when the amp only cost $55. )
10000 uf electrolytic capacitors back to back on the output, sound a little funny at high frequencies. (bells, top octave piano). As my ears hear pitch instability on the piano, I suspect the effect is high frequency intermodulation distortion. To install eight 2200 uf 200 v speaker block capacitors, besides the weight the 1.3k doesn't need, and the $50 cost , I can't find the cubic space in there.
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Last edited by indianajo; 15th October 2013 at 03:20 AM.
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Old 15th October 2013, 07:03 AM   #8
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Quote:
Originally Posted by lanchile View Post
is it good to have it in amp? the answer is YES!!!

I would consider having dc protection if it was only a question of added complexity. Unfortunately, it generally involves a relay in series with the outputs and i have never tested one that didn't sound horrible in that position.

Crowbar type protection seems like a much better alternative.
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Old 15th October 2013, 08:51 AM   #9
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What MOSFETs are you using? Laterals? These are pretty rugged, but I have blown verticals before especially TO-220 ones.
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Old 15th October 2013, 09:29 AM   #10
AndrewT is offline AndrewT  Scotland
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Herein the UK we have the Distribution Board MCB (breaker or fuse if really old) Then a plug top fuse.
That is the first stage of protection.

I then add a second stage of protection by fitting a separate close rated fuse to each transformer.

The third stage to 20th stage of protections are optional.
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Last edited by AndrewT; 15th October 2013 at 09:45 AM.
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