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Old 14th January 2012, 06:17 AM   #951
210 is offline 210  Germany
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I think, amp-feetback (typically?) has the hardest job to do,
because it has to correct all the misbehaviour of the circuit.
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Old 14th January 2012, 09:54 AM   #952
godfrey is offline godfrey  South Africa
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I tend to feel sorry for the input stage, because it gets blamed for everyone else's mistakes.

Internal mutterings of a feedback amp:
Feedback network: Oh look; the output stage is clipping. Let's go beat up the input stage again, that should fix it.
Input stage: sigh....
2'nd stage: O no! What they gonna do this time, switch me off or saturate me so bad it hurts?

Happy weekend, everyone!
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Old 14th January 2012, 11:12 AM   #953
tvrgeek is offline tvrgeek  United States
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the 20's were really good for the price. Most of their line is. Really good value. There is a big step up available and they must be 15 years old. Drivers have gotten better. The S2 drivers look really impressive on the slick sheet. Maybe worth a hear. There are no pure beryllium domes available for DIY. I know I am getting somewhere when I build speakers that are in all ways better. Close last time, but no there yet. My last crossover put my Dayton/Vifa pair somewhere between my Q1'a and the 20's. My cabinets are better, crossovers better, but I need a big step in the driver department which means bucks.

Back to amps. Smaller amps tend to sound better to me that big ones, in the range I can even dream of. Thinking why. Less gain and what that means to the linear range of the devices ? Gain of 25 not 35 for example.
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Old 14th January 2012, 11:57 AM   #954
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Quote:
Originally Posted by tvrgeek View Post
Back to amps. Smaller amps tend to sound better to me that big ones, in the range I can even dream of. Thinking why.
Here's a hypothesis for that. Small amp means small transformer, less capacitance to mains. So lower magnitude of common-mode RF currents flowing around the big loop between amp and pre (if its a poweramp) or amp and source (if its integrated).
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Old 14th January 2012, 12:06 PM   #955
Pano is offline Pano  United States
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Large amps have too many parts. The sounds get lost and confused along the way.
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Old 14th January 2012, 12:12 PM   #956
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Hi,

Quote:
Originally Posted by abraxalito View Post
Here's a hypothesis for that. Small amp means small transformer, less capacitance to mains. So lower magnitude of common-mode RF currents flowing around the big loop between amp and pre (if its a poweramp) or amp and source (if its integrated).
We can easily lay this one to rest.

Only amplifiers of incompetent design will use generic mains transformers that allow large leakage from the mains (common-mode or differential-mode).

It is quite trivial to specify suitable means to minimise this for any kind of transformer size. It is both common and law in medical device designs so it is really nothing out of the ordinary or esotheric...

Ciao T
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Old 14th January 2012, 12:25 PM   #957
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Hi,

Quote:
Originally Posted by tvrgeek View Post
Back to amps. Smaller amps tend to sound better to me that big ones, in the range I can even dream of. Thinking why. Less gain and what that means to the linear range of the devices ? Gain of 25 not 35 for example.
I take it here you mean "lower power" when you write "small"?

I know this chap, his 2W SE Tube Amp has a DF (Dunker Factor) or 25 and is the size of a large side-table, it is most emphatically not a "small amp".

If for low power, there are several mechanisms we may include.

A lower power amplifier requires lower voltage supply rails. Until rather recent times generally, the higher the rated breakdown voltage of a transistor the poorer it's linearity (Beta, Gm), the lower it's Ft, higher parasitic capcitances and so on.

Equally, when using a single output pair a lower power amplifier can use transistors with lower rated collector current and power, again, this generally meant the ability to use parts with more favourable parameters for audio use.

Paralleling output devices can introduce it's own difficulties and so amplifiers with single output devices MAY be preferable to having to parallel devices, but this case is much less clear-cut.

There are of course exceptions to every rule and we nowadays get quite good highish voltage transistors etc. even compared to 10-15 Years ago, so most of these rules can now be thrown out of the window now and you can always make a high power amp by bridging two lower power ones, with some added potential benefits (which are not always realised of course).

Ciao T
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Old 14th January 2012, 12:42 PM   #958
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Originally Posted by ThorstenL View Post
Only amplifiers of incompetent design will use generic mains transformers that allow large leakage from the mains (common-mode or differential-mode).
I did say RF, explicitly. And RF is not going to fall in the category 'large'. Differential mode leakage is inevitable if you want to, you know, draw power from the mains.

Quote:
It is quite trivial to specify suitable means to minimise this for any kind of transformer size. It is both common and law in medical device designs so it is really nothing out of the ordinary or esotheric...
Medical safety has spec for RF leakage currents? If so, do you have a link, I'd like to follow up.
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Old 14th January 2012, 01:13 PM   #959
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by abraxalito View Post
Here's a hypothesis for that. Small amp means small transformer, less capacitance to mains. So lower magnitude of common-mode RF currents flowing around the big loop between amp and pre (if its a poweramp) or amp and source (if its integrated).
If so, wouldn't 2 smaller( or multiples) transformers be better than one big one ...?

Quote:
Originally Posted by ThorstenL View Post
Hi,



We can easily lay this one to rest.

Only amplifiers of incompetent design will use generic mains transformers that allow large leakage from the mains (common-mode or differential-mode).

It is quite trivial to specify suitable means to minimise this for any kind of transformer size. It is both common and law in medical device designs so it is really nothing out of the ordinary or esotheric...

Ciao T
Whats considered "generic" ....?
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Old 14th January 2012, 01:23 PM   #960
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Quote:
Originally Posted by a.wayne View Post
If so, wouldn't 2 smaller( or multiples) transformers be better than one big one ...?
I've made a few measurements and using two smaller transformers in parallel gave higher capacitance to mains than a single larger one within a particular construction style. I hesitate to generalize to between all transformer styles - R and EI core types offer less capacitance overall than toroids.
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