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Old 6th July 2008, 04:17 AM   #21
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"Exactly.
Stability is something to strive for?"

I'd use one or maybe both of them. The transconductance is pretty low and the threshold voltage too so it will not work too bad like this though. But I agree - vbe multiplier and/or LTP CCS would be a big improvement!



"Not sure ur english isn't causing a mis-translation here.
Within reason these Mosfets have a negative tempco.
No thyristor I am aware of inside it... ?
Maybe you meant thermistor??
If so, then you mean negative temperature coefficient. "

Nope, I meant thyristor. I don't know what it is inside the transistor that causes this though. At about 200 deg C Tj the gate started drawing current. On the transistor I tested some kind of thyristor action clamped gate-source to 2V or so if the gate current exceeded 8mA.

If the transistors are mounted on a reasonable heatsink this will probably not happen though. The one I tested had the heatsink at 160 degrees which is waay more than will ever occur in an amp. When the heatsink was kept cool the current decrease with temperature was enough to keep dissipation down.

So I don't know if I can really agree that the temperature coefficient is within reason I'm pretty sure this is the reason why the datasheets don't show any data for temperatures over 75 deg C.

"Except for one important detail - clipping of high frequencies on an ESL sounds rather awful. Doesn't matter much if it is voltage or current clipping, imho... you'll hear hard clipping, and hear the increase of distortion with level without much difficulty."

I won't disagree with that. What is the capacitance of the ESL as seen from the amplifier? If 2 ohms is purely capacitive at 20000Hz then that's like 4uF. With 7A peak current that would give a maximum slew of about 2V/uS... You are right - this doesn't look to good.


"Ummm?
Not no capacitance though.
You scared me, and I pulled my Hitachi book just to check.
The devices called for on the K side are the 2SK1058, spec'd at 600-900pf per gate. (usually around 800pf on a good day)"

But that capacitance doesn't swing the whole output at +-50V as it's the gate-source capacitance. It will only swing +-10V - and that is if the amp is close to current limiting - effectively reducing gate source capacitance to 100pF as seen by the VAS. Gate-drain capacitance is only 8pF! Total capacitance of output stage as seen by VAS: ~300pF. 28mA into this -> 100V/uS. Bad devices will be worse of course OTOH gate voltage swing will be less than +-10V if driving an 8 ohm load.


"I guess it depends on how slow you want your amp to go maybe?
I doubt that the amp as shown will slew anywhere near 100v/us.
More like 10v/us.
Dunno.
The usual is a coupla watts or more to shove a row of those around... but ymmv. I don't have the spec's handy on the VAS xistors... my guess was that they were smallish..."

It seems the slew rate limiting thing here is the peak output current because of the insane capacitance of ESL:s. More output devices in parallell would actually increase the slew rate into this load! If the amplifier is presented with a more "normal" load it should be very fast though without it.

It seems amps with conventional VI limiters for current limiting won't be very good either. Amps capable of 300W or so still might have only about 6A available around 0V output. Depending on the connection the current might reduce even more on the opposite swing - not very much current at all can be supplied when slewing in the positive direction at negative output voltage and vice versa.

"Anyhow they show a 3 amp fuse on the rails to the outputs... so it ain't makin much power..."

3 amp RMS per rail -> 4.2 amp RMS on output. 140W continous sinewave into 8 ohms or 70W RMS into 4 ohms continous sinewave. Seems like a good value for the amp. If more current and power at low impedances is needed then more output devices are needed too.

An my conclusion is:
BJT:s are hard to beat because they cost about a fifth of what lateral mosfets do and allow very high peak currents if we want! One 2SC5200 or FJL4315 will easily do 15A - which we need 2 laterals (or maybe even 3 because the current limit reduces so much when they get hot) to equal. The bipolars have much higher Gm giving lower distortion for the same NFB and lower rail losses giving more output power - especially with low impedance loads.

Laterals are still cool though being almost indestructible with minimal protection circuitry. Too bad they are so expensive and won't provide very high peak currents.
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Old 6th July 2008, 05:00 AM   #22
taj is offline taj
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Quote:
Originally posted by Greg Erskine


... my understanding that the copyright is only the printed material, not the actual schematic. So you can't photocopy it or scan it but redrawing the schematic is OK.
Hi Greg, (With all due respect...)

I don't want to dwell on this, as I've expended too many brain cells on this topic already, but your understanding of copyright is "the internet version" which only exists in the denial/justification prone minds of cyberspace denizens.

The version of law that most countries have agreed to respect and abide by, is significantly more complex and protective.

But I stand down. It's not a topic I want to argue.

..Todd
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Old 6th July 2008, 07:52 AM   #23
patch is offline patch  United States
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>>>
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.
<<<<

You know, I have read that opinion here before. I have read others who have repeated that same sort of allegation about Mr. Sloane's work.

I have not seen a factual basis for that opinion.


Where did that opinion about simmulation come from? Mr. Sloane himself?

I'm guessing not. I bet guys here simmulated a design, or tried to build one or more of them, and found errors in the book. I sure won't use this site's search engine to track that down...don't have that kind of time.

I do recall Mr. Pass mentioning something a few years ago about hearing one of the complex Sloane designs, that someone had built, and that it sounded good.
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Old 6th July 2008, 11:35 AM   #24
jcx is offline jcx  United States
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unstable VAS bias:
Unstable VAS current in amp from Slone book


a simmed "solution" - the added circuitry should at least help point to the problem

optoisolator VAS bias for Comp Diff?
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Old 6th July 2008, 02:09 PM   #25
patch is offline patch  United States
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Ack, I do see. Thanks so much jcx.

As for one fix...

>>>
I'm sure I'm about the bazillionth person to rediscover this technique...
<<<

Count me the bazillionth-and-oneth.
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Old 6th July 2008, 06:17 PM   #26
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Yo, megajocke!



I'd not use a 3 amp fuse myself... various reasons.
Including their intrinsic non-linearity.
And not enough juice, imho.

Most "good" amps will crank one heck of a lot more current out than what you suggested. But for simple DIY stuff, better safe than sorry.

oh, yeah... BJTs do many things very well.
The thing that I do not like about them is that they "hard" clip - which is audible, very audible. Mosfets for whatever reason seem to be a bit "softer" on the edges (of the clipped waveform) and do not sound the same (better to my ears) when clipped... try to build an amp that does not ever get driven into clipping, get back to me on that one...

also, beyond a certain temp, the Mosfets I've worked with from Hitachi go postive tempco... so I am unsure what you found.


...it's all an art of compromises, one has to pick and chose the compromises based on the intended outcome (if you happen to know a priori what it will be) or the serindipitous outcome, or perhaps the unfortunate outcome...

Anyhow, the bottom line advice to first time builders without much experience, and without an experienced person to hands-on advise, and without much test gear is to build something known to work for the intended application, and preferably something that has a PCB and/or parts kit available so that you get a known outcome. Either that or do like some people did, which is to build Nelson Pass' first ZEN amp, which had almost no parts - but I recall several people coming to me with non-working home built Zen amps!!

_-_-bear
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http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 7th July 2008, 04:46 PM   #27
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Quote:
Originally posted by bear
Yo, megajocke!



I'd not use a 3 amp fuse myself... various reasons.
Including their intrinsic non-linearity.
And not enough juice, imho.
Yo! Nonlinearity? It's in the drain so shouldn't do much difference - and I'd worry more about the temperature dependence of the transistor in that case. Channel resistance which is much higher than fuse resistance might change over a 2:1 range over the working temperature of the transistor... I'm not sure the transistor will be able to blow a fuse much bigger than 3 amps either as it will only be on about 50% of the time.

Quote:
Most "good" amps will crank one heck of a lot more current out than what you suggested. But for simple DIY stuff, better safe than sorry.
Hmm, I were thinking about amps with very high power. Home amps of reasonable power outputs usually only cut out the relay after a short delay on overload detection instead of using active current limiting circuits. High peak currents are available at all output voltages in both directions on those amps for a short while at least. Bigger amps like for PA (where a little distortion is better than the amp cutting out for 5 seconds) and some big home amps use VI limiters though, some with delays, some without. Those are probably not optimal to use. Even normal speakers can demand unexpected peak currents, more than Vcc/Re actually on the right (wrong?) waveforms!

Quote:
oh, yeah... BJTs do many things very well.
The thing that I do not like about them is that they "hard" clip - which is audible, very audible. Mosfets for whatever reason seem to be a bit "softer" on the edges (of the clipped waveform) and do not sound the same (better to my ears) when clipped... try to build an amp that does not ever get driven into clipping, get back to me on that one...
Are you talking about lateral fets or the switching types? Could it possibly have something to do with their high resistance and interaction with the load? BJT circuits might clip at almost the same voltage both at high and low currents. What do you think of clip limiters? Fast attack gain decrease with reasonably fast release activated by detecting when the feedback loop loses control. Alternatively, overkill output power could be used instead...

Quote:
also, beyond a certain temp, the Mosfets I've worked with from Hitachi go postive tempco... so I am unsure what you found.
Interesting, at what currents and voltage? Did they blow up? I didn't test the bias stability on high temperatures, only what happened at overload. If I remember correctly I only tested the P channel part, the N might be different.

Quote:
...it's all an art of compromises, one has to pick and chose the compromises based on the intended outcome (if you happen to know a priori what it will be) or the serindipitous outcome, or perhaps the unfortunate outcome...

Anyhow, the bottom line advice to first time builders without much experience, and without an experienced person to hands-on advise, and without much test gear is to build something known to work for the intended application, and preferably something that has a PCB and/or parts kit available so that you get a known outcome. Either that or do like some people did, which is to build Nelson Pass' first ZEN amp, which had almost no parts - but I recall several people coming to me with non-working home built Zen amps!!

_-_-bear
I'd say the PCB layout is as important as the schematic - there are lots of nonobvious things that can go wrong there. Getting multiple channels on the same supply hum-free seems to be a common problem too.
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Old 8th July 2008, 10:35 PM   #28
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I don't know amplifier design but did build Randy Slone's Optimos, the early version of the schematic is also published in that book. Note that 11.1 is the first amp while the Optimos is the last amp in the book.

It sounds fabulous. About 2 months ago, I took my Optimos 130w monoblocks to compare to my friend's Conrad Johnson solid state class AB power amplifier (retailed at $9,000?) in his $50,000 system driving his Thiel speakers. Both my friend and myself thought that the power amplifiers performed at the same level of competence.

Regards,
Bill
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Old 8th July 2008, 10:40 PM   #29
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Bill, is your book newer than the 1999 version? I have that one, and the Optimos is not in it. The Optimos was a later improved amplifier compared to the ones in the book, as I understand it. He used to have schematics of it on his website, but removed it when some company bought the rights to it and started manufacturing commercial units (don't know what company though).
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Old 9th July 2008, 02:23 AM   #30
Karl71 is offline Karl71  United States
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Christer -

Slone is the designer for ZUS amplifiers, a company located in British Columbia, Canada. ZUS is selling the Opti-Mos amps.

Karl
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