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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
$3,60, that's excellent!
Nicko, you might be interested in my application: a solid-state speaker protection relay for hi-fi amps. I made a prototype with some smaller FETs and got 0.005% THD at 100W. The devices mentioned here should hopefully push it down below 0.001%.
The gate capacitance doesn't really matter because it only needs to switch once at power on and again at power off.
My motivation: After 6 months or so, tarnished speaker relay contacts are already the biggest source of distortion in my new Blameless build.
Registered Member #1538
Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28
Interesting device, indeed. I always thought one could "scale up" the channel width of a MOS-FET easily, reducing Rds_on and increasing current carrying capability. Or may be they use two dies of some smaller device for economical reasons?
Steve, may I ask how you implemented your solid-state speaker protection relay? I am currently reading the "Audio Power Amplifier Design Handbook" by Douglas Self which mentions the benefits of a speaker protection relay disconnecting the speaker from the output. He explains that finding a proper relay not prone to corrosion of the contacts is important. While I like the idea of using a mechanical switch because it should in principle not ad any distortion (or at least not more than the speaker terminals), it seems to be difficult to find a relay made and rated for this purpose. A solid state solution which depends less on part properties not given in any datasheet might be beneficial, if it dose not introduce significant distortion. I already had the vague idea of using two back-to-back N-Channel FETs placed exactly where the relay would go, but I didn't try anything by now (and since I don't have the required equipment for THD measurement it wouldn't be of much use either).
Or is there a better way I didn't think of yet?
And by the way is the phrase "Blameless build" by chance in any way linked to Douglas Self? He likes to call specific amps this way...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Timo,
I'm also a big fan of Douglas Self and I have that book too. My "Blameless build" is just that, a hi-fi amp that I built based on his Blameless concept. For more details go to scopeboy.com and see "The Selfless Amplifier" in the projects section.
My idea was just what you said too, two N-channel FETs back to back where the speaker relay was, driven by a photovoltaic isolator. The Rds(on) of fets is now so small, I figured that even if it was nonlinear, the distortion had a chance of being negligible.
I built up a prototype and tested it on my old Tek distortion analyser, which has a residual of 0.002% on a good day. I used a 120W power amp and 4 ohm dummy load to force current through the switch, and used the analyser to look at the voltage across the FET switch's Rds(on). It had quite a lot of third harmonic distortion, maybe 0.5%. But it was a pretty small voltage, so the math worked out nicely.
The FETs that gren found have even lower Rds(on) and cost less than the ones I used, so I'm excited.
I have a bad feeling that there may be no speaker relay guaranteed to stay linear for ever. You can gold-plate the contacts, but it gets blasted off the first time the relay has to clear a fault. When Douglas Self avoids writing about a problem, you know it must be bad!
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Steve Conner wrote ...
Hey Timo,
I'm also a big fan of Douglas Self and I have that book too. My "Blameless build" is just that, a hi-fi amp that I built based on his Blameless concept. For more details go to scopeboy.com and see "The Selfless Amplifier" in the projects section.
My idea was just what you said too, two N-channel FETs back to back where the speaker relay was, driven by a photovoltaic isolator. The Rds(on) of fets is now so small, I figured that even if it was nonlinear, the distortion had a chance of being negligible.
I built up a prototype and tested it on my old Tek distortion analyser, which has a residual of 0.002% on a good day. I used a 120W power amp and 4 ohm dummy load to force current through the switch, and used the analyser to look at the voltage across the FET switch's Rds(on). It had quite a lot of third harmonic distortion, maybe 0.5%. But it was a pretty small voltage, so the math worked out nicely.
The FETs that gren found have even lower Rds(on) and cost less than the ones I used, so I'm excited.
I have a bad feeling that there may be no speaker relay guaranteed to stay linear for ever. You can gold-plate the contacts, but it gets blasted off the first time the relay has to clear a fault. When Douglas Self avoids writing about a problem, you know it must be bad!
I think these should be able to take a decent amount of power and still make nice contact even at low levels :o)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Mercury contactors are nice. But I looked at a data book, and the cutaway pictures suggested that the load current passed through an iron plunger. (The plunger is pulled downwards by the magnetic field from the operating coil, this displaces the mercury upwards closing the circuit.)
Conductors made of iron can generate a lot of distortion because of their non-linear magnetic properties. This was shown in another of Douglas Self's experiments.
Still, I guess it would be worth getting a mercury switch to try out.
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What does a $1,600,000,000 corporation do when it can't figure out how to lower RDSon?
