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Old 10th April 2004, 01:46 PM   #1
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Default Grounded Grid pre amp-- help please

Hi everyone, I'm new to this forum and I have an interseting question that I hope someone out there has an answer to.
First of all. I sent away for the book by Bruce Rosenblit " audio reality". Very good read and it also has a number of diy projects in it. One that took my fansy was the Grounded Grid pre amp.
So off I went to source parts and start building this amp. It contains three 12au7's and three very basic Hammond transformers and resistors and caps, etc....So once altogether I get this rather small but nasty humm going through my speakers. I traced it to the pre amp and it is (I think) a 50hz ground loop.
The amp also has a 10 ohm resistor from the main ground and connecting to the to the circuit. Which I'd never seen before.
Could someone please help out here as I'm rather desperate to get rid of this humm.
Thanks heaps: andrewe1
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Old 10th April 2004, 02:21 PM   #2
SY is offline SY  United States
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A description of your grounding scheme might be useful. Star grounds? Bus?
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Old 11th April 2004, 03:04 PM   #3
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Default Re:gg preamp:To SY

Hi SY, I'll tell you what I know from reading the information given to me. As follows:
Its a grounded grid gain stage in which the cathodes are held at ground also using a cathode follower to compensate for the low input impedances. The cathode follower has a high impedance.
It uses three power supplies: 1 for the filaments which are 12vdc supplied. 2 . is the -200volt supply. 3 .is the =200volt supply. All dc.
For good linearity and efficient coupling between the stages, the cathode resistance needs to be large (@51k) and is connected to the -200 volt supply. The negative supply is at ground potential!?
The postitive is at well.
If you can make sese of this...fantastic.... as I am still leaning and the book stated that this would be a faily easy project....hmmm welll.
Anyway if you need to know more ,please let me know. Also, could you get back to me as soon as you can.
Thanks heaps
andrewe1
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Old 11th April 2004, 03:30 PM   #4
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Hi,

How did you do the layout?
Did you use a PCB, breadboard or point-to-point wiring?

If there are several powerxformers have you tried turning them around to cancel out induced hum?

I have a copy of the circuit which is a classic GG stage.
Naturally you won't solve problems like this by just looking at a diagram but you could try earthing the preamp chassis and see if that helps.

Cheers,
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Old 11th April 2004, 03:48 PM   #5
SY is offline SY  United States
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I wasn't being clear enough. All grounds are not equal and ground loops are almost inevitable when you build your first circuits. What would be good to know is HOW you have each of the grounded elements connected to "ground."
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Old 11th April 2004, 10:38 PM   #6
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Hi,

Quote:
The amp also has a 10 ohm resistor from the main ground and connecting to the to the circuit. Which I'd never seen before.
The way this is drawn on the diagram is confusing, wrong even.

The 10 Ohm resistor as shown is actually shorted out if you follow the diagram blindly.
It is supposed to put L and R channel ground to ground through the 10R straight at the RCA connectors.

The rest of the grounded components seem to be at chassis level as far as I can tell.

Cheers,
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Old 12th April 2004, 11:02 AM   #7
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Default re: ggpre amp

Once again, thanks all for your help.
SY. The amp has been grounded at two points. The mains earth comes in and goes directly to the chassis. The other ground is the inputs and outputs which are grounded through the 10ohm resistor. All linked together. I have tried to cut down on as many ground points as possible to minimise loops but I dont know?
The tranies are as far away as possible from the audio side of things, at least 7". I've tried turning the tranies around but it doesnt make any difference. It is a clear case of earth loops with a little rf interfearance.
I think also what maybe a good idea if I take a few shots of the layout of the amp for you to go over it see what you think. Maybe that would be a good idea?
Thanks
andrewe1
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Old 12th April 2004, 04:21 PM   #8
Mike L. is offline Mike L.  United States
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Hello,

Did you know that Bruce has a forum set up at his Transcendent Audio website? He is very good about answering diy builders questions. Just be as concise as possible when stating your concern.

Mike L.
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Old 12th April 2004, 05:16 PM   #9
SY is offline SY  United States
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A picture would be helpful indeed. It sounds like you need some work on the grounding scheme- lots of grounds can be good as long as they attach at a common point ("star ground").

I haven't seen the schematic, but I'd guess that the 10 ohm resistor goes from the input jack (isolated from the chassis, I hope) back to chassis ground, just to break up any loops in a stereo setup. You want, in general, all power supply and all signal grounds to tie together at only one point, the chassis ground.
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Old 12th April 2004, 07:46 PM   #10
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Did you earth the heater supply as well?

You should have

from R10 to earth
from R14 to earth
from rca input and rca outputs to earth
from pots earth
from d4 and c9- to earth
from d2 and c5+ to earth
c6+ and c7+ to earth
z2 and z4 to earth
c10- and c11- to earth
c1- to earth


So you should have a lot of wires all running to one earth...that one earth is connected to chassis via 10R.....

And earth from mains is connected to chassis at same point at the 10R resistor..

And you should have no humm...
But what I described is on the schematic as well ofcourse...however...I personaly always have 1 star earthing point...where I take a wire from each point as indicated above....

In other words...NOT a wire from D4 to c9 and then to earth BUT a wire from D4 to earth and a wire from C9 TO EARTH..small difference...but a difference none the less.
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