In general - there are different possibilities to add a DC protection like fuses and relais controller like TA7317.
But I prefer only an ultimate solution without relais contacts and fuses between the power amp output and the speaker terminal.
What would be such a solution?
Is there a simple solution that would give DC protection but won't impede on the musicsignal? Probably not otherwise my amps would have such a scheme.
Maybe I should browse for such a solution in this forum first.
I think fuse in the power supply line works reasonably well.
I size it at ~ 50% of Ipk for maximum power into nominal load resistance.
Some size it smaller at 50% of Irms for maximum power into nominal load resistance.
This fuse must be F type and must not be over-rated or a substantial current could flow through the speaker if the amp goes faulty.
eg.
F2A or F2.5A in each supply rail of a push pull solid state amplifier delivering ~100W into 8r0.
I size it at ~ 50% of Ipk for maximum power into nominal load resistance.
Some size it smaller at 50% of Irms for maximum power into nominal load resistance.
This fuse must be F type and must not be over-rated or a substantial current could flow through the speaker if the amp goes faulty.
eg.
F2A or F2.5A in each supply rail of a push pull solid state amplifier delivering ~100W into 8r0.
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My Thresholds SA/1 have the standard 20 Amps Fastacting (Bussmann GBB ceramic series) railfuses reflecting it's continiously amp-rating according to the manufacturer. They drive Acoustat electrostatics that are 4 Ohms nominal but in reality taper of in the hights to under 1 Ohm. Especially when a female voice starts to sing hard the powermeters on the amp move significantly to the right.
I can of course lower them to 8 Ohms F.
F20A is what I would put in for a 1200W into 2r0 amplifier.
Keep in mind that an F20A fuse will pass a continuous 20A of DC current for quite a long time before finally blowing. The speaker will probably be dead by then.
F20A can pass 40A of DC current for a few to many seconds, depending on tolerances.
Okay, I'll just ordered 7A Very Fast Acting ceramic fuses to replace the 20 amp versions that are in place now.
That's a lot saver I think. The SA/1 delivers 600 Watts at 2 Ohms according to the graphs that Nelson Pass supplied for the review of the Audio Januari, 1987 issue. Strange thing is though that the reviewer (Leonard Feldman)stated: "Although I did perform some measurements of power output at 2 Ohms, it soon became clear that if I tried to push the amplifier to its limits with that type of load, I would blow its fuses." So that's why Nelson supplied those 2 Ohms graphs for this review.
So even with the standard railfuses the amp could not be pushed to its limits. I have several German reviews of Threshold poweramps that confirm this finding and do not give a specification for 2 Ohm other then a dash (-) at that rating. Railvoltage is approx. 63 Volts for these amps.
But it's a bit peculiar when you have an amp that can deliver lots of current to low loads but is curtailed by its fuses.
I'll try them out anyway...
It's not peculiar.
Very high output currents tend to be very fast transients, not sustained sinewaves.
An amp that can deliver very high transient currents does not need to also deliver that same current output into a test load on a long term test.
It sounds like the amp is rated @ ~350 to 400W into 4ohm speakers.
The peak current into a 4r0 load will be ~14Apk = ~10Arms.
If you use 4ohm speaker then an F7A or F5A fuse is suitable.
This fuse will allow sustained high power bass currents to pass and similarly will allow fast transient currents to pass without blowing.
I don't know if you can buy F7A you may have to go to either F5A or F6.1A or F8A.
I know that the dynamic nature of music produces fast transients that would in turn require sudden bursts of current. But why do the American audio highend companies that manufacture amplifiers like Krell, Levinson, Rowland and Pass all produce amps with very stiff powersupplies with almost no dynamic headroom? Isn't that then a waist of resources. I believe Carver and Harman Kardon produced cheap amps that could boast enormous loads of instaneous power when needed.
The rating for this amp is 40 amps continuously and 150 amps peak. So I suppose the hardware can sustain such a output otherwise this rating is useless. That's probably why Threshold decided to incorporate 20 amp railfuses each for the positve and negative portion to underline the potential of this amp although I think your reasoning is valid also.
I just ordered ten FF7A (FF = very fast acting devices) to try out.
I was just wondering how the reviewer of Audio could claim that he was about to blow the FF20 amp fuses, when he tried to measure the 2 Ohm output.
I believe that "stiff" supplies sound better than limp supplies that sag under load.
I also believe that "dynamic headroom" is a measure of the badness of a power amplifier, rather than what the advertising would have us believe.
I note the ratio of almost 4:1 of transient peak to continuous current. That makes a lot of sense in the context of what you said about big US made amplifiers.
I also believe that "dynamic headroom" is a measure of the badness of a power amplifier, rather than what the advertising would have us believe.
I note the ratio of almost 4:1 of transient peak to continuous current. That makes a lot of sense in the context of what you said about big US made amplifiers.
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My solution is to snip (cut) the secondary GND terminal of the power transformer after make all necessary modifications
(see attachement, third schematic).
Please note - necessary modify not work for beginners and no using of "low cost" capacitors (no use of high CV product) !!!
And one (only by full range mode clearly) disadvantage there is present by too low internal spaces for the new caps: to high -3db cutoff frequency through the 6 db high pass function.
If your Acoustat electrostatics currently works as full range loudspeakers, you must add and adjust an appropriate subwoofer
Please note additional, above solution is only for existing amps useful. The royal way for new amplifier projects is the use of non symmetrical voltage supply (one supply range = GND at the same time) and a high quality electrolytic capacitor between output and speaker terminal (always better than relais contacts and fuses). Any few manufacturers uses even a transformer, but this solution is much more expensive than a very high quality computer grade cap.
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I know what you mean, the perfect voltage source is an amp that doubles it's current with every halving of the load it has to drive. I has no Dynamic headroom at all.
Saying that, do we need this kind of behaviour when amplifying music?
Heavily biased Class A designs of course need a lot of hardware to sustain the quiescent current drain on their powersupply and the generated heat needs to be deflected by massive coolingfins.
A moderate AB design could perhaps need a lot less hardware to deliver the musical transients you speak about.
A story about the ability to handle short bursts of power to handle musical peaks:
At the januari 1984 CES in Las Vegas, Phase Linear demonstrated that their DRS-900 power amplifier with the same 150 Watts continuous rating as a Threshold S/300, did not clip when high transients of current where asked to be delivered. The Threshold S/300 showed visible clipping on the oscilloscope when it had to deliver the same high transients of current as the DRS-900.
Thanks for your solution.
My Acoustats are Fullrange ones and adding a subwoofer would probably introduce other problems to deal with.
Yes, but if create solutions for all problems the sonic transmission is much more better in all respects than presently. To do this is my main work.
Unfortunately this isn't to realize with "Plug and Play"
In the meantime I have prepared several smaller amps (mostly integrated amps) in the kind as described in post #13 without any trouble, despite the fact, that I must halved the values of capacity - cause twice of the voltage strength is necessary).
For big amps like this
http://www.diyaudio.com/forums/pass-labs/142576-threshold-s-1000-series-ii.html
there are good caps available in the meantime from Munforf, made by FTcap:
http://www.mundorf.com/english 1.1/kondensatoren2.htm
For big amps like this
http://www.diyaudio.com/forums/pass-labs/142576-threshold-s-1000-series-ii.html
there are good caps available in the meantime from Munforf, made by FTcap:
http://www.mundorf.com/english 1.1/kondensatoren2.htm
Your playing with fire lowering the fuse value - easier to blow with a low impedance load and when they do the output goes to the remaining DC rail voltage - not nice. A 2 pole breaker works better, both poles open at the same time.- John Curl likes that approach too. You'll have to experiment to see which value doesn't trip with your load.
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