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Old 9th April 2009, 06:28 PM   #1
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Question Passive preamp before an integrated (?) amp

I've got an AMP6 with a built-in 20k dual-gang pot for volume control. My setup is such that the source gets split to the AMP6 and a plate amp for a mono sub. Unfortunately, this means I have to use the volume control on the plate amp as well, because I cannot control the volume using the pot in the AMP6.

My question is, can I put together a basic passive pre (using another pot) which splits the signal to the AMP6 and the plate amp (1 in, 2 out) and use it without any issues?

If it makes any difference, the plate amp has a built-in low pass filter which works fine.
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Old 10th April 2009, 12:06 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
is there a second output from the integrated that is controlled by the attenuator setting?

If not, then you can tap off a signal from the existing attenuator and feed that to an extra RCA on the back of the integrated.

Alternatively, add an extra attenuator at your source and preferably buffer the two channels. Drive both the integrated and the plate from the buffered signal.
Use the integrated attenuator to balance the volume/SPL coming from the main speakers and the bass speaker. Leave the integrated alone after that.

How would you solve source selection if the passive pre is at a single source location?
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Old 10th April 2009, 05:17 PM   #3
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Quote:
Originally posted by AndrewT
Hi,
is there a second output from the integrated that is controlled by the attenuator setting?
No. I could add an output, but that requires me to empty out the case so that the aluminum chip doesn't short anything out, and that's way too much work for my lazy self.

Quote:
Alternatively, add an extra attenuator at your source and preferably buffer the two channels. Drive both the integrated and the plate from the buffered signal.

How would you solve source selection if the passive pre is at a single source location?
So you would recommend buffering the attenuated-before-split signal, eh? That wouldn't be a problem.

There's only one source.
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Old 11th April 2009, 10:37 AM   #4
AndrewT is offline AndrewT  Scotland
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definitely buffer.
The attenuator has a variable output impedance. The next stage sees this as a variable source resistance. This can lead to an audio treble filter that varies with volume position.

The two receivers are a parallel load for the attenuator.
If one were 22k and the other 50k, the attenuator sees 15k7
A 50k attenuator has a maximum output impedance of 12k5
These two impedances will not interact well.

A buffer solves all of these resistance/impedance problems, but introduces a new one. An extra active stage between your source and the speakers. Pick a good discrete buffer that is capable of passing an excellent signal and which is easy to build.
As a starter, have a look at Pass B1 and Pedja buffer.
You can always do more complicated later if you feel the need.

Locate the attenuator for your most convenient operation.
Locate your power amps as close to the speakers as you can manage.
Connect the signals with twisted pair or shielded twisted pair. Don't spend a fortune on long and exotic cables at this stage (maybe never). The shielded twisted pair used for microphone cable is excellent.
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Old 15th April 2009, 06:08 AM   #5
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I built a simple opamp-based buffer. Sounds fine to me.

I did learn something important. The +12V rail from my computer's power supply is horrible ****. It sounded like a 4 bit MP3 encoded at 8 kbps with 1 V of extra noise, or something. Switching to a small linear supply fixed that.
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