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Old 6th September 2009, 09:38 AM   #11
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Default Do not feel yourself offended..but i also think we (humans) use to fool ourselves

say....if we believe pr amplifier has a good brand, or good parts inside..well constructed, expensive, famous and so on, we use to "make our minds" about performance.

An A to B testing... a blind one...may change your evaluation.

But first, you need to match all levels...the bypass key into the pr amplifier and the pr amplifier output level must be matched...same level and all adjustment flat and filters off.

Doing this way, and other guy operating the pr amplifier bypass switch (provided in some amplifiers or constructed by yourself) you may "feel" such difference once again... and can do your evaluation once again.

Not knowing if the pr amplifier is inside the audio chain or not, just listening the flip of the switch, not knowing if the audio is direct or througth the pr amplifier (blind test) you will confirm/dennie your evaluation.

I am absolutelly sure you will love the direct audio (no pr amplifier into the audio chain - pr amplifier bypassed).

The problem is that people do not do, usually, those things...and levels are different, this confuses evaluation, some folks understand higher level entering the power amplifier as "nice sound"...others not...a matter of preference the presence of dinamics or the compressed audio (alike FM transmissions...you know that, i am sure you know that difference)...others does not appreciate...levels adjustment, levels matching is super important into the audio quality result...matching brands use to result better... say...a NAD CD to a NAD power amplifier or pr amplifier, use to perform better than a different brand..impedance matching and level matching.

Well.... i lot of beliefs i had in the past had colapsed when facing A to B blind testing (blind is the most important thing... we cannot know what is playing..or our empathy/preference/beliefs and prejudices will influence evaluation)

regards,

Carlos
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Last edited by destroyer X; 6th September 2009 at 09:52 AM.
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Old 6th September 2009, 09:55 AM   #12
sandyK is offline sandyK  Australia
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Quote:
Originally Posted by AndrewT View Post
A pre-amp is required to interface the source to the receiver.
It can have gain or pass on the signal substantially unaltered.
It can have an attenuator.
It can have some form/s of EQ and/or filters.
It can receive and distribute many signals.
It should send on the signal irrespective of the loading of the receiver and connecting cables.
It can also be used to isolate the earth side of components using Switch Mode PSUs, so that the sound quality degradation arising from the 100nF capacitor between Input and Output sides of the SMPS can be greatly reduced. The annoying mains hum/hash due to this high impedance source of mains voltage is ADDITIVE when several SMPS type devices such as DVD/STB etc. are all in situ, whether switched on, or not, and can also result in a nasty "bite" when inserting/removing input cables to an earthed amplifier.

Carlos
Also, how do you propose to keep relatively similar volume control settings ,
when using a source such as Digital TV, where the maximum transmitted audio level is -20dB ? Without a preamplifier, the power amplifier would need to have an undesirably high amount of gain, and S/N of high level sources would be compromised due to the need for excessive volume control attenuation.
SandyK

P.S.
I also agree with Andre Visser.

Last edited by sandyK; 6th September 2009 at 09:59 AM. Reason: afterthought
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Old 6th September 2009, 10:00 AM   #13
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Default Alles gut Calvin...you have full agreement on that

This thread is a provocation..some folks loves challenges...the afirmative that pr amplifier serves for nothing is a terrible provocation that serves to people to think about once again...we do not use to think about things that are already blessed by the use....if everybody uses if because it is needed..this is the way people (also me, of course) think about.

The interesting thing, is that if people try to defend the pr amplifier...well..during the brain storm produced to defend the pr amplifier, some of them will see the real thing.

The main problem that impeaches people to learn/evolute/change their minds is our own Ego/pride... try to imagine.... John Curl pr amplifier has one million hits..imagine those people, how they will feel when perceive pr amplifier is not needed...they will not feel very confortable.... and this is the main reason people is resistant to check that stuff.

Imagine those guys testing A to B.... a blind test..other guy operating the switch.... they may feel not happy or confortable knowing they have been fooling themselves for so long time.... having worries for something not even needed... this is terrible....what they do?

They do not even test that stuff..they go believing and say to themselves...

- "this destroyer x is an idiot"

Really?

regards,

Carlos
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Old 6th September 2009, 10:36 AM   #14
AndrewT is offline AndrewT  Scotland
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post10 attachment looks like a (passive) pre-amp to me.
It receives a source signal.
It sends an output signal.
It attenuates the signal on the way through.

I used a potentiometer version of this with input switching exclusively for about a decade and accepted that the treble got filtered and worse the filtering varied with the pot setting.

It does not meet
Quote:
It should send on the signal irrespective of the loading of the receiver and connecting cables.
the output impedance is far higher than permissible for most receivers and cables.

I did without any pre-amp for a few weeks to assess a new amplifier. A low output FM tuner was connected directly (900mm cables) to the power amp inputs. A bit inconvenient when answering the phone or the door bell.
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Last edited by AndrewT; 6th September 2009 at 10:39 AM.
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Old 6th September 2009, 02:14 PM   #15
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Default Interesting the treble losses Andrew T

Well....i could not find the same result you had... and i will be thinking about the reasons that can produce those problems and interferences.

Adjusting power amplifier sensitivity to 1 volt rms we usually have not door bell entering...also i think door bell does not play too much to be a problem.... door bell magnetic field captured to some cable (aerial) will result in microvolts..and this will not drive an amplifier to loud levels...also door bell does not use to be ringing constantly into a family home.

Unless we adjust amplifier sensitivity to 5 milivolts to work together phono cartridges,we should not face problems with our power amplifier... but i think this was not your case, as phono cartridge really needs a pré amplifier because the RIAA equalization...phono needs a two stage pré amplifier and cannot work with a passive pré amplifier and a high sensitivity adjusted power amplifier.

Is your door bell some horn used against air raids?...something very strong with high consumption having strong magnetic field irradiating from the wiring?

heheheheh..... as you count your foot steps while dancing..why not to have a professional, industry alarm as a door bell?... those second world war motor driven alarms, may be exotic and interesting to some people.

Carlos
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Last edited by destroyer X; 6th September 2009 at 02:20 PM.
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Old 6th September 2009, 02:47 PM   #16
AndrewT is offline AndrewT  Scotland
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I think you misunderstood what I meant by "A bit inconvenient when answering the phone or the door bell."
The direct feed from FM tuner into fixed gain power amp means there is no method of attenuating the output. The average level when fed this way was about 1W to 2W into 89dB/W/m speakers.

If I had to answer the phone or answer the door bell then the only way to avoiding SHOUTING at the caller was to turn off the amplifier and wait a couple of seconds until the amp went into "mute" mode.

The loss of treble from a passive pot when feeding short cables and the parasitic capacitance of any power amp is very noticeable if you have a reference to compare to. For that decade I did not realise just how much I was losing. But it had one big advantage, as I turned up the volume towards -6dB the treble filter moved downwards further into the audioband and thus avoided some of the HF clipping that makes music and voice sound so bad when played at too high an SPL.
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Old 6th September 2009, 04:15 PM   #17
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Default Yes.....now i understood what you said

also the losses into high frequencies makes a lot of sense to me too.

the CD output capacitor goes in series with the volume pot to ground...and this creates a high frequency filter.

Of course we can use resistances in series to increase the impedance the CD player output "will see"....or will face.

regards,

Carlos
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Old 6th September 2009, 04:31 PM   #18
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Default Well boys...with pr amplifier, or without pr amplifier..the fact is that

Brazilian music drive us to shake our bodies!

http://www.youtube.com/watch?v=JIM36EkPr50

Enjoy!

Carlos
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Old 6th September 2009, 04:42 PM   #19
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Carlos, the reason I've tried listening without my pre-amp were because I also thought it can only sound better, it didn't. I've also tried a 'high quality' transformer volume control, apart from the switching clicks while adjusting volume, it sounded quite good, I still prefer to use my pre-amp.

I believe in equipment with flat frequency response and low output impedance, adding series resistance can only have a negative effect on SQ.
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Old 6th September 2009, 05:33 PM   #20
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Default Try not to believe and to do tests... real answers are sometime different compared to

some previous beliefs....of course, sometimes not... sometimes confirms our previous beliefs/thougths.

Series resistances, considering standard impedances of 47k or even more, do not disturb treble, the opposite...maybe this was what you have said...i do not know..sometimes english confuses me a lot.

We need a coupling box, or a control box...all inputs goes to attenuating trimpots..this way you can match levels into all your inputs, always reducing the level and having, to compensate, a sligtly higher sensitivity into your power amplifier.

The input trimpots sets levels to each input... Line, DVD out and so on... this goes to mechanical switch.... that send the audio to the main volume potentiometer, and you can have two potentiometers, one to each channel... that's the way i use... same way heathkit has used, down the seventies, with one of their famous power amplifiers.... each input going to trimpots...rigth and left trimpots..then to a switch that selects the input that will be sent to the volume pot and then to the power amplifier input.

Nothing changes, the opposite..audio is more clear, less distorted, less mufled... goes unchanged from the CD output or other audio source, to the power amplifier without load the CD output, without load the power amplifier input and all passive...no supply, no transistor, no electronic switching..nothing to disturb.

Resistances, placed in series, increases the impedance the CD output will see (feel or be influenced by)...also into the control box output increases the control box impedance perceived, felt, sensed, by the power amplifier input.

Simple..it is always better...switches and potentiometers does not distort signal, does not introduce harmonics, noises or annoying things.

I forgot to delete the input green marks.... imagine they do not exist..only the ouput series resistance sets the minimum impedance the power amplifier input will sense.

regards,

Carlos
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Last edited by destroyer X; 6th September 2009 at 05:46 PM.
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