DIY Audio Projects Forum

4S preamp project
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Author:  Gab [ 06 Dec 2018, 21:06 ]
Post subject:  Re: 4S preamp project

Is this high res enough ?

Author:  Jakub [ 10 Dec 2018, 05:13 ]
Post subject:  Re: 4S preamp project

Can I be interested in a wiring diagram? Would you like to build by your (:

Author:  Gab [ 10 Dec 2018, 14:50 ]
Post subject:  Re: 4S preamp project

Jakub wrote:
Can I be interested in a wiring diagram? Would you like to build by your (:

I’m not shure if I understand exactly what you meant, but here is the link for the power supply schematic and I have uploaded the shematic for the amp and buffer sections.
Original 4S writeup :

Author:  Gab [ 12 Dec 2018, 13:48 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
Does the ground lift switch make any difference in the noise?
Is the noise the same in both channels?
Can you pos a high resolution picture of you wiring?

What do you think of my grounds ? I’m reading the write up on grounding and shielding and I start to realize some errors. I did not wire things up so the signal and power grounds are seperated...

Author:  Gab [ 18 Dec 2018, 09:35 ]
Post subject:  Re: 4S preamp project

Anybody ?

I could also add this : the noise I hear is also there with no input connected to the preamplifier. It’s also present with no volume at all, and does not change when the volume is increased

Author:  Suncalc [ 18 Dec 2018, 13:23 ]
Post subject:  Re: 4S preamp project

Gab wrote:
Anybody ?
Ok. Let's walk through this logically. The noise has to be coming from somewhere. So let's review what we know.

1. Noise is only present on one channel (left).
2. The ground lift switch does make some difference.
3. Swapping tubes makes no difference.
4. Noise is independent of the volume setting.
5. Noise is independent of input or output loading.

Number 1 indicates that there is a difference electrically between the left and right channels. There are several possibilities: grounding differences, cold solder joints, bad components, improper power connection. Number 2 indicates that it is likely not coupled noise from the environment or connected equipment. This can be verified by swapping left and right inputs and outputs and making sure the problem stays in the same channel of the preamp. Number 3 eliminates a bad tube. Number 4 says that it's not high side coupling of the input, but a ground problem is still possible. Number 5 helps confirm 4.

Now your input wiring is VERY complex. There are lots of prospective ground paths. It would be helpful if you could show us as as built schematic including the ground paths. I can't tell from the pictures how you wired the inputs.

Another thing to consider is that you have 4 independent noise sources in your amplifier (i,e, LEDs). Since you are likely switching these in and out with the same switch you're using for input selection, there is possibility for problems in this area. If you could draw out in detail the entire input switching mechanism, that would help. It would also hep to see the exact power supply diagram including how you are supplying the LEDs. It is important that their high frequency noise is kept out or the audio circuits.

From the picture I can't tell how the LEDS are wired into the circuit. However, it look like they are fed after the first inductor. This may be a problem as the inductor is required to choke out the high frequency noise (which may be modulated with the 120Hz ripple. You really need to take a look at the noise on the B+ with your oscilloscope and see if you can identify the primary frequency.

I await your reply.

Author:  Gab [ 18 Dec 2018, 17:15 ]
Post subject:  Re: 4S preamp project

I stopped at the office supply store on my way home and bought some graph paper. I'll try my best to draw the input wiring clearly. I'll come back with something later tonight.

Swapped left and right inputs yesterday. The noise is still in the left channel at the output either way it's connected.

The LEDs are fed by the exact same wire(s) that brings B+ to the anodes. there is a 47k 2w resistor behind the terminal strip that is the closest to the input switch. A red wire runs from the output of the 47k resistor back to the ''power status'' LED. The power for the other 3 LEDs also comes from there, through a small black wire into the input switch. The input switch has 3 positions, (actually 4 but I only use 3) and switches 3 circuits at the same time : left, right, and the LED supply that comes from the same nearby 47k resistor. The audio signal goes from the input switch to the volume pot and from there to the first stage of the preamp. I hope it gets clearer for you.

Finally : I don't exactly know how to use my scope. could you explain how I should connect it to see the noise of the B+ ? I know, it's dumb. I knew it would be usefull somewhere in the process, but I dont know how to use a scope that old and with as much functions as this one. I am used to small handheld flukes we used to diagnose hall effect wheel speed sensors on cars...

Again, I want to thank you. Your help is appreciated and I learned A LOT because of you !

Author:  Gab [ 18 Dec 2018, 17:53 ]
Post subject:  Re: 4S preamp project

Here is the drawing of the switch and leds

Author:  Gab [ 18 Dec 2018, 17:59 ]
Post subject:  Re: 4S preamp project

You can see the 47k 2W resistor and the wires that goes with it

Author:  Gab [ 18 Dec 2018, 18:45 ]
Post subject:  Re: 4S preamp project

I tried to disconnect the only ground of all the LED circuit to se if removing this part of the circuit makes any difference. But it changed absolutely nothing to the sound of the amp. The same noise is present on the left channel.

Am I right to think that this test shows that the problem does not come from the LED part of the circuit ?

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