Protecting a speaker from DC - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th February 2013, 03:34 PM   #1
diyAudio Member
 
Join Date: Sep 2009
Location: Pretoria
Default Protecting a speaker from DC

Hi All.
I have recently completed Rod's P3A amplifier. Used his PCB for it.
I would like to add some DC protection for my speakers.
Now I have built those uPc1237 Ic circuits for some amplifiers. They work great, and the added turn-on delay is nice.
But, I was wondering what advantages/ disadvantages would a simple electrolytic capacitor in series to the speaker have.
Why isn't it done with split supply amps??
Does it affect the sound?
  Reply With Quote
Old 14th February 2013, 06:09 PM   #2
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
Erm...Split supplies were developed precisely to eliminate the output capacitor. You can do as you please with adding a capacitor - even taking the NFB point to the output side to include it in the loop but there is now little point in having dual rails.

There are dozens of threads here about capacitors, output capacitors and what's bad about them. Some folks would eliminate all reactive components from amplifiers if it were possible. If you want to start yet another unending thread about capacitors in amplifiers you will be going about it the right way - Just watch this space.
__________________
regards
  Reply With Quote
Old 14th February 2013, 06:16 PM   #3
RJM1 is offline RJM1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Titusville, Fl.
"Why isn't it done with split supply amps?" Because you would have to use a non polarized capacitor because you would not know if the amp failed and put out a positive or negative voltage. These are almost impossible to find in the thousands of uF size and would be very expensive. "Does it affect the sound? " Yes, all signal coupling capacitors add distortion.
  Reply With Quote
Old 14th February 2013, 06:16 PM   #4
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
You would need a bi-polar cap so that means two caps back to back of twice the value you want the equivalent "single" cap to be ... bulky.

(Ian mentions moving the feedback take off point to the other side of the cap to include and correct the "bad" effects of the cap... if designed in from the start then yes, a good idea, but otherwise what about the DC conditions ))
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 14th February 2013, 06:55 PM   #5
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
Quote:
Originally Posted by Mooly View Post
..... but otherwise what about the DC conditions ))
Yes, I see the loophole. Apologies.
I should have said "as there is now no point in having split rails." or clearer.
__________________
regards
  Reply With Quote
Old 14th February 2013, 07:26 PM   #6
diyAudio Member
 
Join Date: Feb 2009
Location: UK
The Quad floating ground idea gives the same DC protection as a series cap, but is still a DC coupled output as far as the amp and speaker is concerned. However, I suppose that if you have 100,000uF of PSU capacitance it wouldn't help all that much. In the days when 4700uF was enough for a 100W amp, it was a very elegant idea.

Virtual ground in power amp applications
  Reply With Quote
Old 14th February 2013, 07:51 PM   #7
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
You could use a single cap on the output of a split-rail amplifier. As far as I can see you risk it exploding if the amp fails and the capacitor is then reversed biassed. But so what.

I have found a single cap not to affect the sound of a split rail amp except with very careful listening. Use a large value cap so that there is very little a.c. voltage across it to minimize distortion. Use a good quality cap such as Nichicon gold.

The only other issue I'd worry about is that electrolytics are designed to have some d.c. across them, it helps keep their internal structure properly 'formed'. You could achieve this by mis-balancing the amplifier to produce a little intentional d.c. at the output, but this might affect sonics depending on how it's implemented.

I don't know if the impact on sonics is too critical - worrying about your speakers is likely to impact your enjoyment of the music more. The smell of a burnt out voice coil is guaranteed to spoil your day !
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 14th February 2013 at 08:00 PM.
  Reply With Quote
Old 14th February 2013, 08:29 PM   #8
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
I have a single supply amp, the ST120 with djoffe bias mod, with an input and an output capacitor(3300 @ 80v). On my Peavey SP2-XP speakers, it sounds just like the CS800s split supply amp with no input or output capacitors, at 1.5 Vpp level. Both sound better than the "legendary" stock ST70 tube amp with new caps and output tubes. A new CS600s was $999 at the local store 2 years ago, so decent protection circuits like it has, are not cheap. I paid about $200 for the CS800s with several problems, like tripping the breaker sometimes when I turn it on (1998 main caps) and burnt up input resistors.(probably somebody plugged a hot guitar amp output in it and burned them).
I tried putting 10000 uf back to back electrolytic caps between the speakers and the output transistors on the CS800s. It sounded funny on top octave Steinway piano, which I know what it is supposed to sound like. (I've got one installed between the speaker stands). It also sounded funny on tinkly bells and brush cymbals. I'll have to rely on the speaker disconnect relays and the overcurrent detection current transformers of the CS800s to protect the speakers. I don't like funny sounds.
I've just spent over a month fiddling with the Peavey PV-1.3k split supply amp trying to install something better for DC protection than the SCR crowbar that melts the lands on the PWB instead of tripping the breaker and protecting the speakers. The detection circuit is working, but the lands of the PWB are not tough enough to carry 10000 uf of energy @ 85 V. I think it would be easier to build a couple of channels of single supply amp- like the MJR7-4 or G amp than what I am doing. Have just bought the parts to try the G amp. Speaker disconnect relays are also notorious for sounding funny, particularly after several years of dirt gets on them. Also, 8 to a dozen parts have to work right for the speaker disconnect to work. The number of speakers at the local musician's consignment shop witn new cheap bass drivers, and the number of amps with shiny new output transistors, is evidence that blown amps blowing speakers is not a rare phenomenon.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 14th February 2013 at 08:41 PM.
  Reply With Quote
Old 14th February 2013, 08:47 PM   #9
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
I have a hard time trusting output relays to protect speakers in off-the-shelf amplifiers because they don't always implement it properly. In most cases the relays are there to control turn-on and turn-off noises rather than guarantee your bass drivers will survive an output stage failure. The issue I've seen is mostly that the relays are underated for use with inductive loads, and there are very few relays that are adequately rated and available for purchase so it's unlikely a commercial amp has used one with sufficient rating unless it's a low power amplifier to start with. Secondly, the relay needs to be a two-way relay, the speaker terminal needs to switch from being connected to the output of the amplifier, to being connected to ground. This ensures that any d.c. arcing is quenched quickly when the relay tries to disconnect the inductive load it's connected to.

I have a project on the bench which is a commercial amp with inadequate output relays. I am considering adding additional speaker protection.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 14th February 2013, 09:03 PM   #10
diyAudio Member
 
Join Date: Jan 2011
Location: Fosser, Aurskog-Holand, Akershus, Norway.
When taking the sonic influence in mind, I actually would prefer a speaker protection based on the u1237, or similar, wich uses an relay to connect the speakers.
The alternative with capasitors would require four capacitors with a value of each the double of the PSUs total capasitance to give a neglectiable influense to the properties of the amp. This as we will need to put two in series, aka Bipolar, not to decrease theese properties.
__________________
Sooner or later you end up with TANDBERG
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Protecting the speaker output using a microprocessor tauro0221 Chip Amps 54 3rd September 2012 05:49 AM
Any DIY Speaker Builders in MD/DC/VA - Need $1000 DIY Speaker Recommedation Dkalsi Multi-Way 12 10th March 2011 12:48 AM
Protecting Ribbons exurbia Planars & Exotics 1 14th April 2010 03:24 PM
Protecting treble driver from DC? AndrewT Multi-Way 5 25th February 2006 03:33 PM
Protecting speakers against dc with mute? soundNERD Chip Amps 11 21st March 2004 11:52 AM


New To Site? Need Help?

All times are GMT. The time now is 07:16 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2