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Old 6th September 2009, 05:52 PM   #21
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Default Here the corrected image.

Try to make a "refresh" cicle into your brain folks...the "stablishment"...things we use to do without think about, polutes our minds.

It is not possible that the passive can be worse then the active..we know building and testing that each active stage distorts a little...so..not having active you will be rid from that.

regards,

Carlos
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Old 6th September 2009, 05:55 PM   #22
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Carlos, my beliefs are based on tests that I've done.

Switches and potentiometers doesn't distort the signal? You need to measure and listen again.
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Old 6th September 2009, 06:01 PM   #23
PMA is offline PMA  Europe
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Do not waste your time Andre, it is useless. Many have never heard a high quality preamp, but will argue.
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Old 6th September 2009, 06:07 PM   #24
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Default Are your talking about yourselve Pavel?

oh!...i understood...many...may be others.... i see indefinite subject.

I also feel pitty with folks that never tried passive and go believing active is the solution.

Do not believe me....believe in yourself, but please guys, test your beliefs into real world not to go deliranting all life long.

regards,

Carlos

.................................

You know André..one day i will build something to listen....first i have to learn how to hold the soldering iron.
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Last edited by destroyer X; 6th September 2009 at 06:14 PM.
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Old 6th September 2009, 06:31 PM   #25
Glowbug is offline Glowbug  United States
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Quote:
Do not believe me....believe in yourself, but please guys, test your beliefs into real world not to go deliranting all life long.
Definitely, but sometimes, there are situations where active preamplification is needed. Just as there are times where it's not.

Are you running a turntable straight into a power amp?
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Old 6th September 2009, 09:06 PM   #26
fotios is offline fotios  Greece
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By chance, i was faced with a similar case in my last power amplifier. I had in my plans to include a signal attenuating pot in input, for direct connection of CD players in amplifier. The reason was that i know that many people it preffer this.
From my experiments, proved that a potentiometer connected directly between the input socket and the input of amplifier is not so easy. The input of amplifier consisted from some R-C filters ( Π or T ) before the LTP. A variable resistor (like the pot) it is tuned with those filters when its value is changed and produces unwanted signals. In the begining, i used a 50KΩ pot in input, which has an apropriate value to give a relativelly high Zin. By injecting square waves in input (i use allways square waves during the development of a project) i observed in my DSO strong overshoots during the turning of potentiometer. From luck, when i connected the signal generator (which has Zo=50Ω) directly to the input of amplifier the overshoots disappeared. After this, i found that only a pot of 1 to 1,5ΚΩ as much does not cause overshoots and oscillations. The existence or not of the input DC blocking cap (either electrolytic or polypropylene or 2,2μF or 22μF) does not matters, the result it is the same. To the report, the input impedance of my amplifier without resistor termination to GND, it is arround 15ΚΩ. The issue is found in input impedance. If we use a 50K pot, then we can have a good Zin in the cost of oscillations. If we use a 1K pot, then we can have a clean SQUARE signal in the cost of very low Zin. The solution it is a buffer between the input socket and the input of amplifier. Thus, we can terminate the input of buffer to GND with a 50ΚΩ resistor, and in the output of buffer we can place a volume pot with the correct value of 1ΚΩ. Τhe pot is connected as usually, the two extreme terminals accross the output of buffer and GND, and the wipper terminal in the input DC blocking cap. But a very small ringing remains in the rising edge of SQUARE wave. This can eliminated completelly by connecting a 220 to 270 pF cap after the DC blocking cap and GND, in the cost of increase the rise time of square wave by 0,2μs. In my case, i had an initial rise time at 60Vpp of 1,78μs and after the changes 1,8μs.
Summarizing: Against my initial thoughts that the addition of a simple level control pot in the input was nothing, this proved very complex finally. In practice, now and after 2 weeks of extra work, inside my amplifier exists as well a preamplifier powered from a special (zero ripple) seperate supply. The good thing it is that, i can use the buffers as subtracters to have and balanced inputs.
So, IMHO to connect a source like a CD directly in the input of amplifier, there is the need of a buffer to seperate the input socket high impedance from the low input impedance of the main amplifier. The buffer it is a form of preamplifier.
Of course, you can connect a CD player in an amplifier without input level controls, as you can manage the volume from the remote control. Most Hi-Fi or Hi-End comercial amplifiers does not includes input level controls. But you must be very cautious for the Ζο of CD player to not exceed the 1KΩ.
I suppose that the 90% of DIYers they use only sinus waves or music (!!!) to check their projects. Square waves, although are not related with music, are very usefull to reveal concrete technical details of the circuit, mainly the TIM distortion which caused from the wrong compensation of Cdom, the wrong compensation of the feedback loop, the wrong input filtering and the existense of the smallest oscillation.

Greetings
Fotios
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Old 6th September 2009, 09:58 PM   #27
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Default Hello Glowbug, for sure i am not running phono cartridge directly into a power

amplifier...level is too low and phono needs strong equalization.

Only already pré amplified line out signals from CD players, DVDs, Satelite Receivers, Television Line out connectors and so on...the most used audio sources.

For a turntable, you need, at least a RIAA equalized phono pré amplifier.

.................................................. ..................................................

Hello dear Fótios

I already knew, in advance what you would say...i know your ideas about...thank you very much by your important contribution here.

Karolus
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Old 7th September 2009, 08:09 AM   #28
Calvin is offline Calvin  Germany
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Hi,

Quote:
Many have never heard a high quality preamp, but will argue.
Agreed, since they mostly will not have listened to a decently constructed passive stage!
"Need for an preamp" is that sweet fairytale grandpa used to tell from those old days, when he was a Young boy and signal sources didn´t provide for enough signal level.
Decently constructed means not a classical Pot-circuit. Besides the rather low quality of contacts (wiper) of many pots the highish und not constant output impedance is not what we want.
But many switched resistor networks that show a constant output impedance are high in Zout value which either reduces the allowable cable length considerably and/or reduces the bandwidth too much. No wonder those sound ruther dull.
You need to design a constant Zout and low Zout(<<2kOhm) network.
When You compare such a network with and without active circuitry (which is the case with my preamp) You will find that adding active circuitry distracts from the music signal in almost any case (those cases where activation actually improved things were cases in which the in-out-impedances were quite off of the norm or where unusual long cable runs (>10m) were required).

jauu
Calvin
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Old 7th September 2009, 09:01 AM   #29
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Calvin, to get Zout of less than 2kOhm, what is your Zin?

What do you use for active circuitry?

André
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Old 7th September 2009, 09:32 AM   #30
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Default Zin and Zout around 40K...this is what i use as CD out can hold that

also amplifier input circuit can accept without load too much.

To use lower Zin you should ask Calvin, as he seems that knows more than i know and he can provide us some schematic.

Mine use 33K in series with the potentiometers... trimpot (33K) in the input and the potentiometer in series (50K).

As you have those resistive lines in parallel, you will have reduced total impedance that will be sensed by CD output and power amplifier input.

Mine worked fine, but for sure other ideas can be even better, i am not selling the circuit idea, i am just saying that we can do it passive, avoiding active stages..and this, obviously and super of course, must be better than active..as you have to manage resistances (impedances) only to adjust level...so....no active is needed, as you do not need to amplify nothing, you need to control.

CD output has already nice output impedance..and that impedance matches the power amplifier input (reasonable matching for several audio amplifiers..and you can tweak yours to match perfectly)... Cd output level is high...from 1 to 2 volts RMS into the modern ones..this also matches the power amplifier input sensitivity, that can also be adjusted to optimize.

In the reality, all we need is a potentiometer and a series resistance..to each channel.... inductance and capacitance is minimized using short wiring and distance between wires..small box..control box.

regards,

Carlos
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