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Old 1st April 2011, 03:36 PM   #111
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Quote:
Originally Posted by simon7000 View Post
In power amplifier input pairs I place power resistors next to the devices. These allows me thermal control of the offset from the bipolar input transistors! I also use lm34s to set the reference level!
You don't really need power resistors, I could imagine 1/8 Watt ones glued to TO92's maybe even in the current mirror so you could use duals for the input devices.
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Old 1st April 2011, 03:48 PM   #112
Bonsai is offline Bonsai  Taiwan
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While we all run around designing circuits that are DC coupled in the interests of the ultimate in high fidelity, guys like Doug Self designing mixers are just as busy slapping 5534's and 5532's down with dozens of electrolytics in the signal path between the mic and the hard disc where the music is stored.

2 or 3 electrolytics in the signal path between your source (CD, phono or whatever) and the power amp will not alter the sound irretrievably for the worse. Depending on your pursuation, that was already taken care of in the recording studio, or it is, as I suspect, of no consequnce in the big scheme of things.

The differences in CD recordings (noise, sound stage, dynamic range, instrument placement, mic set-up etc) are huge and in our discussions on these pages, we seem to completely ignore this.
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Old 1st April 2011, 04:59 PM   #113
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Bonsai, there are hi end engineers and there are mid fi engineers. Buy mid fi, if you don't know or care about the difference, but please let us in 'hi end' be able to explain ourselves properly.
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Old 1st April 2011, 05:43 PM   #114
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Quote:
Originally Posted by scott wurcer View Post
You don't really need power resistors, I could imagine 1/8 Watt ones glued to TO92's maybe even in the current mirror so you could use duals for the input devices.
Scott,

In amplifiers such as Jan's Pax where current conveyors are extensively used, the mismatch of the positive rail and negative rail sources placed demands on the input stage to keep the output offset close to zero volts. By using thermal excitation the current sources can be tweaked in to match each other.

The current trend in solid state designs is to try and get complimentary parts to match as closely as possible.

So where you have the ability to actively monitor a device's parameters you can make some adjustment.

It is just another tool in the advanced designers tool box.

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Old 1st April 2011, 07:47 PM   #115
RCruz is offline RCruz  Switzerland
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I am using a modified Meridian 101 preamp that has no series capacitors in the signal path.

As a matter of fact, it is basically a double opamp LME49720 configured to have 16x gain placed after the log pot.

I have no DC offset on the output just by using a carefully built + - 15v PSU using special series regs.

The mods I did on the Meridian resume basically to the opamp swap and the new psu.... I did not alter the stock configuration so it never had any caps in the signal path (I mean I did not invent anything... I am only explaining what I found inside the preamp)

Now I am learning about DC servo and Bob explained (thank you Bob) it must be placed in the signal path... It seems we are trying to avoid caps but adding more items in the signal path.

(I always try to avoid opamps because they rely so heavily in negative feedback and have loads of components only needed for it´s safe functioning)

Can we use DC Servo using only discrete components ?

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Old 1st April 2011, 08:01 PM   #116
SY is offline SY  United States
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Quote:
Originally Posted by RCruz View Post

Can we use DC Servo using only discrete components ?
Yes, but you'll not gain any performance advantage. It's pretty easy to set up an IC opamp servo that is effectively out of the way at audio frequencies and requires no fancy components.

The distinction between series and shunt in signal paths as far as claimed audibility is... well... not logical. And as usual, totally unsupported by any evidence.
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Old 1st April 2011, 09:40 PM   #117
RCruz is offline RCruz  Switzerland
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What do you mean by using an opamp out of the way at audio freq ?

Do you mean the opamp is operating at very high freq (or at dc level) and that can not be heard ?

About the distinction between series and shunt parts... of course all parts leave their own signature but IMO a series cap is restrictive and a shunt cap is aditive. (Soundwise)
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Old 1st April 2011, 09:42 PM   #118
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Unfortunately, Kirchoff's Law doesn't care about your opinion.

If the servo doesn't have to handle high frequencies and the AC at its output is divided down and/or filtered (while maintaining DC gain), you can use pretty cheap opamps and caps with no performance penalty.
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Old 1st April 2011, 10:18 PM   #119
RCruz is offline RCruz  Switzerland
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IMO There are no perfect components... all of them leave or add some signature.

Kirchoff´s law "The algebraic sum of currents in a network of conductors meeting at a point is zero" works as an idea so we can visualize the effects of currents inside a schematic but what I meant was:

series cap is restrictive = these caps somehow limit bandwith and caps are not very linear so freq response is always affected.
for shunt cap (I meant decoupling cap) is aditive = these tend to add bandwith by reducing impedance at certain frequencies but at the signature cost

About the opamps in the signal line.... that might be bad. No two opamps sound the same.
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Old 1st April 2011, 10:36 PM   #120
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Quote:
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Can we use DC Servo using only discrete components ?
Some do (and temp compensation).
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