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Where do we most often fall short with amps?
Where do we most often fall short with amps?
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Old 24th June 2003, 05:35 PM   #81
millwood is offline millwood  United States
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Default Re: Keeping your OPampTIONS open.......

Originally posted by Fred Dieckmann
I have never seen a Class B stage in any op amp and for most typical loads in a preamp they are class A or class AB.
wouldn't that depend on how you define those classes? To me, a class B is a circuitry that conducts 180 degrees, and class C <180 degrees. Class AB to me is one that is originally class A for small signals but gets pushed into class B under load.

So from that perspective, pretty much all op amps have a class B stage.
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Old 24th June 2003, 05:53 PM   #82
MBK is offline MBK  Singapore
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Originally posted by traderbam

How about power op-amps? What level of performance is state of the art these days?
Well, the whole "Chip amps" section is full of it...

I use OPA548, most people use LM3886. The LM3886 was used in the $$$ 47Labs "Gaincard" amp and I hear that this one is also used in some high class SS (either Krell or Levinson) equipment.

Power wise the chips go up to 50 Watts (3886, a bit less for 548) into 8 Ohms, performance wise many people rave about them though many implementation philosophies exist - and are being fought over . Very low DC offset. Proper bypassing and excellent PSU are very important of course, when done properly stability seems to be very good although I had my issues with PSU wiring. Never drive them into clipping . Sound better with higher Z loads (e.g. driving woofers in series rather than parallel for 16 Ohms rather than 4). Distortion figures from datasheets are around 0.01% at 1 kHz at 20W output. Open loop gains around 40 dB at 20 kHz. Slew rate around 10 v/us. Overshoot starting at 2 nF load capacitance.

Of course these are all wicked data sheet data

Since the power is somehow limited they make most sense in biamped systems. I use them in biamped dipoles after the EQ section. To me they are great and beat my previous Rotel. But I can't make any fair comparisons because the system is so customized and partly hard wired that I can't easily throw in a standard amp for comparison.

Anecdotally - I used one OPA548 to drive gheadphones in Mono . This was just a test to see if it's working. PSU was the main 375VA (unregulated) supply with 22,000 uF caps. Oh so sweet the sound! Much sweeter than my OPA2134/BUF134 "regular" headphone amp.

Next day I repeat the experiment using the 12V regulated bench supply. Still OK but sounded somehow strained, and less dynamics.

Now what do I conlude - does one need a 375VA supply to drive the couple mW of my 250 Ohm headphones "properly"?? Do I need a 5 kW PSU to drive the speaker chips????
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Old 24th June 2003, 09:47 PM   #83
traderbam is offline traderbam  Europe
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Interesting news about the 405. Do you happen to have any details about it? Perhaps we should discuss the design in a separate thread and get to the bottom of why it won a royal award for cleverness. Feedforward correction is a cunning idea...perhaps we can improve upon Quad's implementation as you imply.
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Old 26th June 2003, 08:51 AM   #84
RichardN is offline RichardN
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Default The Quad patent GB1499980 ceased a while back


Application No GB1499980


__________________________________________________ ______________________________


Date Filed 14.01.1975

Date Not in Force 14.01.1990

Date of Last Renewal 04.01.1989

Year of Last Renewal 15

Date Next Renewal Due 14.01.1990

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Old 26th June 2003, 11:21 AM   #85
ALW is offline ALW  United Kingdom
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Thanks fo that - I can't tell you how long I spent on the patent office site trying to find that patent.

It would appear then, as it's 3 years past expiry, that no one seems greatly interested commercially, as far as I'm aware.

That makes me even more interested

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Old 26th June 2003, 12:01 PM   #86
RichardN is offline RichardN
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Default It was annoying me too..

the trick came when I found a US patent for a feedforward design that referred to an earlier UK patent. That UK patent was the Quad one... Filed under "acoustical mfg ltd" .

What I'd really like to see the patent search engine add is a "show me all the patents that cite this patent as prior art". That would be an interesting reearch tool!
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Old 22nd January 2008, 09:37 PM   #87
dobias is offline dobias  United States
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Default Bear

White, of course.
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Old 6th September 2010, 06:49 PM   #88
wrinkle is offline wrinkle  United Kingdom
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People often come a cropper when using several caps in parallel, these three links show how it can go badly wrong and why:




The last one really needs to be extended to 8 or more for a wideband low impedance source.
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