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
 
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.
 
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
 
jaudio said:
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.:D

Yesssss.
Dynamics, SLAM, soundstage, transparency.:cheerful:

:cloud9:
 
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.
 
filholder said:
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.

filholder said:
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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Thanks a million Carlos,

Very kind of you. Actually dont mean to take the [email protected] but have you got an actual circuit board image of that design. I have recently aquired a old lazer printer from work and i might be a nice project to try and print my first PCB with.

If you havent fair enough i will have a go myself. Been a few years since GCSE electronics and i have a job read circuit diagrams these days but then maybe no pain no gain!!.

Thanks very much.

fil
 

nina

Member
2005-04-12 9:43 pm
Earth
analog_sa said:
The passive resistive pre represents a classical dilemma. It offers crystal clear and undistorted sound but often lacks bass definition, power and guts. ...
..
You need some form of buffering ....

Analog_sa,

I am facing the same thing as you mentioned via the passive amp. I am thinking of having a "simple" buffer opamp (unity gain, directly connect the output to i- without res) between my nOS DAC and the pot, then put a long length to the chip amp on seperate box.

As having four of LF356 on hand, will they good enough for that purpose??



:scratch:
 
AndrewT said:
I like the ClassA switch.
Can you change the ClassA setting on the fly or do you need to power down? DC pulses etc?

No, this is not meant to be switched on-the-fly.
Actually it is meant to be a pair of jumpers on the board, so that you can use whatever op-amp you like, biased or not.
I like to count with everything, and this makes the board quite flexible, or should I say "universal".

AndrewT said:
A word of warning; the ClassA setting will drop out on excessive current. Take care with high capacitance/long cables and low impedance inputs.

No problems, even with 5m cables and 10k input impedance on the amp.

This is not the pre I use on my main system, but I have several boxes laying around here...:D
 
Very interesting stuff. I fancy building a buffered based on opamps i think simply beacuse i have a few about and they seem easier to implement. Also i have read that the op-amps reject power supply noise better and since i will have to built that as well maybe it woldnt be an optimal design.

However Nukk's strip board design looks the easiest since he has given a picture which i can understand rather than a cicuit diagram which i dont. Also meant to sound good.

I am going to have to sleep on this.

Oh the other thing is what sort of size tranny would a buffer need? Tiny i guess 20-30VA?

fil
 
Hi,
a stereo buffer could draw upto 100mA if driven really hard.
This still equates to only 3w6.
However these tiny transformers have enormous regulation some approaching 20%.
You must avoid excess voltage on the supply pins when drawing quiescent current.
Your 20VA sounds about right, but go for 18v + 18v and use good regulators to control the voltages back down to +-17v or 18v if your opamp can go this high.
 
Oh the other thing is what sort of size tranny would a buffer need? Tiny i guess 20-30VA?

Yes, that would be plenty! I use 80VA because I can then take off suppies for other items like my phono stage, sub crossovers etc.

The three transistor buffer is surely easier to build than an opamp circuit but as the power supply is 80-90% of the cost, you could build an opamp buffer and the discrete transistor version and compare them both! ;)

You can find my buffer/pre-amp PSU on DD as well. Several people here have built one and found it to be very good. :att'n:
 
Nuuk,

Ok this sounds pretty good. I fancy building a decent phono stage at a later date since i am only using a NAD PP2 at the moment. If i build the PSU with a 80VA tranny to share with the phono stage how would i do it. Build your standard PSU in one box and then build seperate second stages of regulation in each of the served devices (i.e. the buffer and phono stage)?

Also if i build the transistor buffer i guess i would not need any input or output caps since all my sources and my GC power amps have then as well. I think might be the route i will go since i have some BC 547 felt over from repairing my Arcam Alpha CD player.

This does all seem very scarey, will be the first ciruit i have ever built myself and i feel like i know too little. A long way from buildin a chip amp kits which was very easy, only difficultty being the stupidly small chassis i keep building all my stuff in. I guess i will learn at some point.

Anyway thanks again

Fil
 
Build your standard PSU in one box and then build seperate second stages of regulation in each of the served devices (i.e. the buffer and phono stage)?

That's the idea! May be build the PSU in a box and have three (or more) outlet sockets so you can just plug in extra circuits later.

The PSU was one of the first things that I built so don't worry too much. Take your time and fully understand what goes where. Avoid any short circuits and you shouldn't smoke anything. Test each stage as you do it, ie output from transformer, output from rectifier, output from regulator stage 1 and 2 etc.

The transistor buffer must have the output cap - there is over 600mV going into it! Some people have found they need a cap on the input but try it without first. ;)