Project 11.1 from Slone "High-Power Amplifier" Book
Does anyone have experience with the "11.1" project from the Randy Slone book, "High-Power Audio Amplifier Construction Manual"? It is the first and simplest amplifier in Ch. 11. What is the wattage? Can it handle a minimum load of 2.5 Ohms presented by an e-stat? Slone says early on that all of the amps in the book will drive a 2 Ohm load, but only temprarily. The nominal load is 6 Ohms.
BTW, I love the book. I like his approach and design philosophy. I am anxious to get building.
Re: Project 11.1 from Slone "High-Power Amplifier" Book
I often play my amp into 1ohm but dont turn it right up.
Not sure if this is the amplifier book I remember with that sort of name or not... if it is the same one, avoid! Otoh, it may be a different one. :)
Perhaps if you post the schematic, people could make intelligent comments, or unintelligent comments about it.
Generally speaking, if the driver can handle enough current, you can increase the number of outpoot devices, and so be able to run more current into your load.
In order to run a lower Z load you need to run more current into the load. Take the power supply rail and figure out what the current requirement into the load impedance will be, then work backwards to look at that SOA of the outputs... then you will know the answer to your question for any specific amplifier.
The 11.1 is a simple amp intended as a beginners project. It has 50 V rails and a single pair of 2SJ162/2SK1058. The max Id rating of those is 7 A, which you reach already at 14 V output for a 2 Ohm load. Obviosly you could reach several times the max Id with that rail voltage. You could lower the rail voltage to maybe 20 V (which is probably safe considering Vgs and other losses) or use parallel output devices as Bear suggested, but it is hardly worth it for a simple amp that not even the author thinks is very good.
That's what I have read. I don't intend to run the 'stats very loud. The 2.5 Ohm impendence occurs at around 12 KHz.
Why should I avoid the book? I've read past posts regarding Slone's works, and I know that there are conflicting opinions regarding his positions and designs. What are your objections?
So, do you think I could use the 11.1 amp with the 'stats? The builder told me that Rotels, Adcoms, and NAD units have had no trouble with his panels. Would a higher power design in the Slone book be more appropriate in this application?
Thanks to everyone so far for your imput. Keep it coming.
"max Id rating of those is 7 A, which you reach already at 14 V output for a 2 Ohm load. "
Which is 10V RMS for a whopping 50W at 2 ohms.
For better results, reduce the main supply voltage to like ±30V and use a higher voltage like ±40V for the front end. It still won't put out any more power, but now it won't be a space heater.
And even 30 V rails are enough to make the amp suicidal with a 2 Ohm load, especially since 11.1 is intendend as a very simple project and doesn't have current limiting or any other form of protection.
I believe that the advice to stay away largely stems from the fact that one of the more complex amps exists in simulation only and if built would blow up on first application of power.
"And even 30 V rails are enough to make the amp suicidal with a 2 Ohm load,"
If he was driving a regular speaker I might agree with you, and drop the main rail even lower.
The E-stats only drop that low at very high frequecies. With the low program content and high peak-to-avereage ratio up there I think he may be safe.
" especially since 11.1 is intendend as a very simple project and doesn't have current limiting or any other form of protection."
An output fuse and a thermal switch on the heatsink are generally sufficient for lateral MOSFETS.
The Hafler XL-280 used three pair of outputs on ±63V main rails with a higher voltage for the front end, it drove 2 ohms quite well. If we cut the ±V in half the dissipation will be cut by about 75%, so one pair ought to do (with his speakers).
I said, IF that is the book I recall...
post the schematic, is what I said??
Anyhow, it is usually difficult to make a silk purse out of a sow's ear?
A better idea might be to start with a project that is known to work for your application? That is one that has sufficient current capability for the power level that you want to run at?
In general terms, ESLs are fabulously low sensitivity speakers, which means in practice that you want to throw the maximum voltage swing you can muster at them.
IF you have an ESL with a dip down to 2 ohms, then you might want to reconsider how you match to them? That might be the reason that you have such a dip? The place for that is in the ESL section here... :D
But, assuming you are going to need to drive the 2 ohm part, as has been mentioned it only is a big problem if you have to put significant energy there for a while - in terms of blowing up the amp - but in terms of practical listening, you will not like the sound of your amp clipping at all when you run an ESL!! Very annoying, imho.
That's one reason people prefer tubes on ESLs in some situations!
Anyhow there are a number of amps that have been suggested on DIYaudio where people have made up PCBs for the driver part, and there are the Pass designs - some of which will drive 2 ohms all day - and there are known commercial kits that range from blank PCBs to full parts + PCBs and instructions.
For a first ever building project, I'd suggest staying within the range of something that is well known and relatively easy to put the parts together so that it actually works.
The JC-3 amp thread might be worth a look too - or was the JC-2? I get the nomenclature mixed up. Someone made a PCB up for that, and it is a Class A amp that should not mind the low Z load, assuming the proper number of outpoot devices...
These are just some ideas...
Oh, there are also some Opamp based amplifier projects that might work just fine for you! There's an app note that uses a new ultra low distortion opamp as a front end for a power amp: National Semiconductor App Note AN-1645 you probably also want to look at AN-1490 which is referenced in the first one.
No reason that I see that you can't bridge a pair and get substantial voltage swing... and maybe even bias them up into near Class A...
So, P = I^2 R
100 = I^2 2ohms
100/2 = 50 = I^2
= 7 amps
BUT, you want 100 watts (for example) at 8 ohms!
That works out to be 200 @ 4 and then 400w @ 2 ohms!
So let's refigure:
400/2 = I^2 = 14 amps.
Since about a 200w @ 8 ohm amp seems to have about the right voltage swing to run most ESLs, that means even more current available! (whoa!) (but the good part is that some transistors today have ample current and voltage ratings)
Now in practical terms, you're not going to actually pull this current running an ESL, but the ouput devices need to have the SOA so that they do not blow up in use - the ESL looks like a capacitor as a load. (rather reactive, not resistive)
So, the amp also needs to be stable into this load. (important!)
The reason that your ESL is dipping down to 2 ohms is that there is some combination of factors, first the reduction of Z as frequency goes up, and probably in this case (if it dips and comes back up) a resonance of the secondary of the ESL driver transformer with the panel itself (or some other component added in...) causing an impedance dip, and doubtless a phase shift WRT to frequency.
You want an amp that will not blow up or clip before you reach normal listening levels WRT the power level in the majority of the spectrum - which works out to be the voltage swing required when run through the ESL's transformer necessary to deflect the diaphragm sufficiently (on the order of 5kv!). Ok then?
Put another way, to run most ESLs you want an amp with an ample SOA, and that means current capability, PLUS the most voltage swing you can manage - that equation usually means a big, high power amplifier in practice.
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