GC passive pre-amp dissapointment - diyAudio
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Old 2nd June 2005, 04:59 PM   #1
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Angry GC passive pre-amp dissapointment

Hi,

Ok i built a set of monobloacks based on Brian GT's Lm3886 boards. They have been playing through a NAD C350 for a while and have sounded absoltutely great. However i thought that using the pre-amp section of the NAD, good kit though it is, must be in someway muddying the sound. I thought that a passive pre-amp was in order but since the hifi is also used for movies etc. decided to a remote controlled passive pre-amp based on the Lite Audio MV-01 boards. These are very reasonably priced and fitted my needs.

I finally finished this project over the weekend. The passive pre is wire entirely with home grown audio solid silver teflon insulated wire. Basiclly tried to make all wiring as short as possible. As my main source I am using a heavily modded Arcam Alpha 5+.
Anyway to cut a long story short the sound now sound less dynamic, the bass wollier, less slam, less impact. I am very dissapointed. Now since this is passive i have go any capacitor to bed in so thi sis it.

What should i do. From readin here i guess the issue is the impedence. The preamp was built into a tiny case and i havent got any space to put in an active stage. Can i buid a seperate buffer stage and what would people recommend.

I know there is the massive GC Preamp Suggestions thread but there dont appear the be any boards for sale.

Any advice greatly recieved.

Fil
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Old 2nd June 2005, 05:10 PM   #2
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The passive resistive pre represents a classical dilemma. It offers crystal clear and undistorted sound but often lacks bass definition, power and guts. Even if impedance requirements are satisfied. Getting the passive as close as possible to the power amp (preferably in the same case) is often good but the sound may remain gutless all the same.

You need some form of buffering so the pot does not have to drive any length of cable directly. Many possible solutions but very hard to find one which sounds truly transparent.
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Old 2nd June 2005, 06:10 PM   #3
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Quote:
Originally posted by analog_sa
You need some form of buffering so the pot does not have to drive any length of cable directly...
...and the low input impedance of these bipolar-input (power) op-amps.
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Old 2nd June 2005, 06:30 PM   #4
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Thanks guys,

So would a buffer in a seperate box but hardwired into the preamp be OK? Has anyone got a ciruit based on a high quality op amp, say Ad825 or the BB 627 which I could implement on say verio board?

I thought i way i was doing it was the same as the most people would seem to use just GC and the either a stepped attenuator or a pot. But after listening to it for so long thorugh the NAD C3350 pre it does sound very flat.

Are there no buffer kits?

Fil
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Old 2nd June 2005, 06:35 PM   #5
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Quote:
Originally posted by carlosfm


...and the low input impedance of these bipolar-input (power) op-amps.

What is the input impedance of the lm3886?
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Old 2nd June 2005, 06:48 PM   #6
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Quote:
Originally posted by jaudio
What is the input impedance of the lm3886?
It's not specified in the National datasheets, but for bipolar-input op-amps it is usually around 300k and lower (instead of several Mohms of the fet-input op-amps).
But the problem is that to prevent dc-offset you have to use a low value input resistor resistor to ground.
Bipolar-input devices are very sensitive to this value, while the fets are very immune to this.
That's why a bipolar-input op-amp varies dc-offset according to the position of the volume pot.

I say like I'm repeating for years: the amp needs a pre.

Yesssss.
Dynamics, SLAM, soundstage, transparency.

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Old 2nd June 2005, 06:48 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
I stopped using passive pre about 20 years ago but recall being very pleased with the sound.
100k pot into 2m of 50pF/m cable rolled off the top end very slightly but bass seemed very natural.
Could your existing pre have a big bass hump that leaves your new sound seeming thin?
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Old 2nd June 2005, 06:53 PM   #8
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Its not just bass though, the bass is looser but also it about the general presentation. It just lost a lot of impact all acroos the spectrum.

Even my girlfriend who basiclly think i am off my head with this audio stuff said it sounded boring and for once she is right.

Please guys what are my options.
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Old 2nd June 2005, 06:56 PM   #9
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Quote:
Originally posted by filholder
So would a buffer in a seperate box but hardwired into the preamp be OK?
Yes.
But ideally you should put it inside your passive pre, after the pot, with short wires.

Quote:
Originally posted by filholder
Has anyone got a ciruit based on a high quality op amp, say Ad825 or the BB 627 which I could implement on say verio board?
Try this.
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Old 2nd June 2005, 07:10 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
I like the ClassA switch.
Can you change the ClassA setting on the fly or do you need to power down? DC pulses etc?
A word of warning; the ClassA setting will drop out on excessive current. Take care with high capacitance/long cables and low impedance inputs.
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