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Old 3rd October 2007, 10:27 PM   #1
RyanW is offline RyanW  Canada
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Default Is a preamp necessary?

I am trying to determine the best means of adding volume control to my system. Do I need a preamp or can I just use a dual gang pot as a voltage dividing attenuator?

Initially I look at Elliots P88 and see that the pot divider is between an input and output divider so I figure that the buffers are necessary.

Next, I look at Greg Ball's SKPre and I see that there is no input buffer... just a voltage dividing pot followed by the preamp.

Finally, I look at Elliots P37 and he has the preamp followed by the voltage dividing pot.

I am trying to figure out if the following is correct:

1. Any standard source (PC, DVD, TV, etc...) will be capable of directly driving a 10k or 100k log pot voltage divider. Is this correct?

2. A standard source will have enough output voltage so that the power amplifier gain stage will reach maximum if the source is not attenuated. True?

3. Adding amplifier circuits when they are not necessary is just another source of noise. If the output voltage of the source is adequate then there is no need to amplify the signal and a volume pot is the "minimalist" option.

4. Does an input and output buffer offer any benefit?

I appreciate all the input. I am sure that I am missing something here, but I am just a NooB trying to understand why I am building what I am building rather than just building for the sake of it.

PS. If number 2 is incorrect, I am planning to build Elliots P09 which has 6dB gain at the output and I'm sure I could increase this further. What I am trying to figure out here is whether I need to use P88 or if I can simply place a voltage dividing pot in the signal path ahead of the X-over circuit. Or rather... Which method would provide optimal results?
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Old 3rd October 2007, 10:43 PM   #2
gni is offline gni  United States
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Quote:
1. Any standard source (PC, DVD, TV, etc...) will be capable of directly driving a 10k or 100k log pot voltage divider. Is this correct?
Most devices can drive a 10kohm load. . .and want to. . . .most
electronics today have a generally low output impedance (47ohms
to 600ohms) and really need at least a 10kohm load to work
and couple the voltage correctly.

Quote:
2. A standard source will have enough output voltage so that the power amplifier gain stage will reach maximum if the source is not attenuated. True?
Depending on the source: some can drive an amplifier to full
output. . .some cannot! Some amps have higher input voltage
sensitivity so a source with higher output is necessary. I have
a portable minidisc that cannot drive my amps to full output. . .
many amps need in the neighborhood of a 1V to do this. . .
some tube amps need 2V or higher. . . some very sensitive amps
only need 500mV to go full scale. . .depends on your amp.


Quote:
3. Adding amplifier circuits when they are not necessary is just another source of noise. If the output voltage of the source is adequate then there is no need to amplify the signal and a volume pot is the "minimalist" option.
I would go on that. Why add more electrical circuits if you have
enough voltage to drive your amp.


Quote:
4. Does an input and output buffer offer any benefit?
Isolates different stages. . . transformer can do similar job. Impedance
matching is another function. . .sometimes they can reduce noise
in a system.

You might need some gain if driving an x-over. . .almost always
some insertion loss. . . you will want your voltage back to unity
at least. . .6dB of gain might be nice to match levels of two
different amps if they don't have level adjustments on them.

Try the system passive at first. . .if insufficient gain. . .add the
preamp.
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Old 3rd October 2007, 10:58 PM   #3
RyanW is offline RyanW  Canada
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Thanks gni,

When you say that most sources need at least a 10k load are you saying that the input impedance should be >=10k or <=10k?
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Old 3rd October 2007, 11:31 PM   #4
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by RyanW
Thanks gni,

When you say that most sources need at least a 10k load are you saying that the input impedance should be >=10k or <=10k?

In general terms, when driving a 'voltage signal', the smaller the output (driving) impedance and the greater the load (receiving) impedance, the better it will be.
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Old 4th October 2007, 12:05 AM   #5
gni is offline gni  United States
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Yes. . . bigger load impedance to a certain extent is better for
a voltage transfer function.

Thank you Gordy for stating it in a better way. . .

[Source (low Z)] ---- > >----[Load (high Z)]

[47ohms] ---- > >---[10kohms]

In the old days we used power transfer(telephone 600ohm). . . that
was using matching impedance. . .we don't really do that anymore
in audio.
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Old 4th October 2007, 01:39 AM   #6
RyanW is offline RyanW  Canada
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Thanks for the input guys. I understand the theory behind low source impedance coupled with high input impedance... I just though there may be some practical limitations to this. Perhaps limiting the input impedance to say 50k or 100k would be a practical limit vs. infinite.

I would be interested to hear the input from the guys, who I'm sure exist, that would argue the that having input and output buffers on either side of the volume control improves performance. There must be an argument for the other side... No? As you elude to gni... Input and output buffers "Isolate different stages. . . transformer can do similar job. Impedance
matching is another function. . .sometimes they can reduce noise
in a system." I'm not sure I fully understand how this works. Is there somewhere I can research this? How do I determine whehter I need impedance matching or isolation of different stages? Is this cut and dry or is it more of a tweak for when things aren't working as they should (ie. ground loops, etc...)

I appreciate all the feedback from all of you gurus that help out NooBs like myself.
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Old 5th October 2007, 09:47 AM   #7
Gordy is offline Gordy  United Kingdom
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In a nutshell:

a) use a buffer at the input of your preamp if you need a really high input impedance, otherwise consider routing directly to your volume control

b) use a buffer before your volume control if the operation of the volume control without a buffer will cause an unacceptable change in system operating conditions

Hope this helps.
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Old 5th October 2007, 10:59 AM   #8
SamL is offline SamL  New Zealand
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I am using 10K DACT attenuator as volume control for almost 10 years. Started with Plinius SA100 power amp and now AKSA 100n power amp. The system sound nice, smooth, fluid and I can sit down and listen for hours without any fatigue. However, it is not capable to play loud. Turning the knob pass 11 and you can tell it starting to strain. Right now, I am thinking of getting a preamp. Well mainly bec of movie. Most 5.1 chn when sum to 2 chn will sound very soft and the passive preamp is not able to push up the volume.
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Old 5th October 2007, 06:18 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
most amplifiers (both power amplifier and pre-amplifier) prefer to see a low source impedance. They perform better and reject interference better.
A volume control has a variable output impedance. This varies (roughly) from zero ohms to {pot value/4}.
The only reason to add a unity gain buffer is to convert the high output impedance of the volume control to the low source impedance that the next stages wants/needs.

If the system cannot reach maximum volume with all the sources then amplification can be considered. It is relatively easy to change the unity gain buffer to +6db or maybe as high as +12db.

I cannot listen to any of my sources at near maximum volume. Either I have too much gain for the sensitivity of the amps or the SPL from the output is just too plain LOUD.
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regards Andrew T.
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