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Old 27th May 2008, 09:14 AM   #1
jerryo is offline jerryo  Isle of Man
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Default Pre-amp considerations.

I have been reading a lot of information regarding the use of either active or passive pre-amps, and I have yet to find any agreement as to which is best. I am building, well I am about to build a pair of mono block power amps (Lynx) to which I need to add a pre-amp with remote volume control. Question; which is better, to have an amp next to each speaker giving the shortest speaker leads using an active pre-amp, or having the amps together and having the shortest interconnect leads using a passive pre-amp? What other alternatives are there? I would welcome any ideas and opinions on this. Thanks in advance for your input.
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Old 27th May 2008, 10:12 AM   #2
Mooly is offline Mooly  United Kingdom
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Hi,
Many possibilities and many will have their own strong views on the matter. Unless you have to run many meters of speaker cable personally I would go with all the amps in one place.
Do you need remote control of input selection as well ?
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Old 27th May 2008, 12:05 PM   #3
AMV8 is offline AMV8  United Kingdom
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Hi
I am also building a pair of lynx monoblocks - when I collect all the parts.
To help you make a decision on active or pasive pre amp you need to look at the output impedance of your sources and the input impedance of the power amp. You need a relatively low output impedance feeding into a high input impedance. This is normally easy to achieve with an active pre amp. However this is difficult, and often impossible, to achieve with a passive pre amp unless you operate it with a buffer stage.
It is normally best to keep the speaker cable short and have a longer run of interconnect between the pre and power amp. If the pre amp output impedance is low ( around 100 ohm ) and the power amp impedance is high ( around 10k ohm ) an interconnect between the pre and power amps of 5 meter should not be unduly affected by capacitance in the interconnect. However if the interconnect is longer or the interconnect has a high capacitance then you may experience high frequancy attenuation.
So I find it safest to use an active pre amp with short speaker cables and longer incerconnect cables.

Don
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Old 27th May 2008, 01:10 PM   #4
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How about both? IMO short low level interconnects are desirable with either a passive or active preamp. Short speaker cables are also desirable, but you can compensate for distance with wire gage to some extent. Not so for the low level runs- go for low capacitance. You mention a remote volume control, so why not split the difference with a couple feet of low level and a few feet of high level? The choice of a passive pre involves many things, including how loud you like to listen. I find most active preamps have far more gain than I'd ever need, so passive works just fine. The safe rule is low output impedance from the pre, and high input impedance to the power amp, but it's really more complicated than that. If the input impedance to the power amp is flat over frequency, the value is far less important, and the only penalty is signal level. That's often the case, but the only way to know is to measure it. I run a 7kohm stepped attenuator, so the typical output impedance is around 3k. That feeds a flat load of about 10-20k with no problem. EMMV (everybody's milage may vary, not just yours!)
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Old 27th May 2008, 02:07 PM   #5
jerryo is offline jerryo  Isle of Man
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Hello Conrad, Mooly and AMV8, thanks for your replies. There are a couple of factors affecting the choice, one of which is my wife who has a great dislike for thick speaker cables (interconnects are a lot easier to hide) so having the power amps under the speakers has two advantages.(one sonic and one quieter life)! Also, I recently took my current pre amp (a pretty bad NAD thing) out of my system so the dvd player that I am using at the moment feeds straight into my NAD power amp. The effect was like taking a blanket off the speakers. The dvd player has a built in volume control but its replacement does not, hence the need for a pre amp with remote volume control. I would prefer to add as little circuitry as possible back into the signal path. There are quite a few choices with regards attenuation methods; any preferences? I have been looking at the kits from Dantimax (passive and active) which look like they might do. Thanks again.
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Old 27th May 2008, 04:58 PM   #6
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Why not build a preamp with buffered output ?
Let it have a gain of 2 and output impedance of 50 Ohm.
Then just add 50 Ohm resistor across the inputs of the power amps to get gain = 1 again.

Use any interconnect cable you want or better a 50 Ohm Coax cable, e.g. RG233

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Old 27th May 2008, 05:05 PM   #7
AndrewT is online now AndrewT  Scotland
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Default 50ohm interconnect

I hadn't thought about a terminated Coax for audio.
Does anyone else have a view?
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Old 27th May 2008, 05:30 PM   #8
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Old 28th May 2008, 07:20 AM   #9
Mooly is offline Mooly  United Kingdom
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Default Re: 50ohm interconnect

Quote:
Originally posted by AndrewT
I hadn't thought about a terminated Coax for audio.
Does anyone else have a view?
Not just interconnects, speaker cable as well !
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Old 28th May 2008, 08:44 AM   #10
AndrewT is online now AndrewT  Scotland
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Default Re: Re: 50ohm interconnect

Quote:
Originally posted by Mooly


Not just interconnects, speaker cable as well !
How?

What is the characteristic impedance of the cable you recommend?
What series resistor will you fit at the power amp end?
What load resistor value will you fit at the speaker end?
What effect will the speaker impedance have on the terminating resistor?
What power will be dissipated in the terminating resistors?

Again, How?
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