1073 custom build initial test, noise at -80, and strange thing that fixes it

[wells2p]

<1073 custom wire wrap build> Did first actual listening test this weekend. Sound truly amazing.
Haven't finished EQ harness yet so tested with EQ bypassed.

Once it is done I'll post build pics along with screenshots of some sweeps

the only oddity was the -75 and -80 db positions had a horrific RF squelch and the strangest thing ever seems to solve it.
If I merely take a jumper wire and touch one end to the -80 terminal of the sensitivity switch and leave the other end dangling (e.g. not grounded) the noise vanishes completely.
I really don't know what to make of this. I've double checked all the caps for the boost and pre amps and am certain the issue isn't there.
Further proof being if I turn the sensitivity switch off and use jumpers to simulate -80 it is dead quiet
In any case my next step is to figure out what that single wire touching the -80 terminal is doing and make the appropriate fix.

 

[Bo Deadly]

That's just an oscillation. Whenever you have high gain combined with the reactance of some inductance or capacitance, you run the risk of creating an oscillator. The reason it stops when touching the parts is because you're changing the capacitance.

It's just a layout issue. There are a few very important rules about laying out parts. One of the more general and probably most applicable to your issue is that 0V wires should all converge at a single point at the last filter cap of the power supply like the trunk of a tree. If you have more than one 0V path to the PS or if you simply branch at the wrong spot, the extra resistance / inductance of the wire (even as small as it is) can cause oscillation.

Make a drawing of were your 0V lines are running physically. Then try changing removing it and replacing it with a thicker wire at a different point. If the behavior changes, that's confirmation that you have a grounding issue.

But it could be other things. Just because your wires are connected as specified in the schematic, doesn't mean the circuit will work.

Post a pic of your build.

 

[wells2p]

That's just an oscillation. Whenever you have high gain combined with the reactance of some inductance or capacitance, you run the risk of creating an oscillator. The reason it stops when touching the parts is because you're changing the capacitance.

It's just a layout issue. There are a few very important rules about laying out parts. One of the more general and probably most applicable to your issue is that 0V wires should all converge at a single point at the last filter cap of the power supply like the trunk of a tree. If you have more than one 0V path to the PS or if you simply branch at the wrong spot, the extra resistance / inductance of the wire (even as small as it is) can cause oscillation.

Make a drawing of were your 0V lines are running physically. Then try changing removing it and replacing it with a thicker wire at a different point. If the behavior changes, that's confirmation that you have a grounding issue.

But it could be other things. Just because your wires are connected as specified in the schematic, doesn't mean the circuit will work.

Post a pic of your build.

This is fantastic info!! I will post a pic in the next day and thank you

 

[wells2p]

hi Bo

I have attached a few pictures, as well as two 20 second videos explaining a few things including B- star connection which is isolated from the chassis and the chassis cable which connects to my PSU where the two are joined. I also point out where I am able to suppress the noise.

Btw the first electronic project I have a built in my life And it was not from a kit - I sort of made it up as I went along using wiring diagrams. Point being I am under no illusion that it was built properly - in fact I am surprised I am only having this one issue although I haven’t posted any sweeps yet.

Thanks again for your help

 

[Bo Deadly]

hi Bo

I have attached a few pictures, as well as two 20 second videos explaining a few things including B- star connection which is isolated from the chassis and the chassis cable which connects to my PSU where the two are joined. I also point out where I am able to suppress the noise.

Btw the first electronic project I have a built in my life And it was not from a kit - I sort of made it up as I went along using wiring diagrams. Point being I am under no illusion that it was built properly - in fact I am surprised I am only having this one issue although I haven’t posted any sweeps yet.

Thanks again for your help

So two more important rules:

1. Source Currents Match Sink Current:

Current flowing in a wire generates a magnetic field which can induce currents in nearby wires which can then be possibly amplified resulting in noise or instability. To minimize this effect, wires delivering current are placed physically close to the wire returning that current. This causes the magnetic fields to cancel out. This becomes more important with higher currents (this is why heater wires in tube amps are twisted together for example).

Your build does not follow this rule. There are many unpaired wires. This means that you have large gaps where currents can run in a loop. That is not good. Even if your circuit is stable, you will get cross-talk, hum, noise, ...

2. High impedance or high gain signal lines must be shielded. As explained in rule 1, EMI can induce currents in wires. This is particularly true of high impedance wires where tiny currents equate to higher voltages (see ohms law). And of course small voltages can be amplified into higher voltages. If these wires are longer than a few cm, they must be shielded. And the shielding must be quite good. So a two conductor with 100% wrapped shield is called for.

It's not clear that your build follows this rule. In particular, the gain control switch has 5K signals with potentially very high gain (80dB). The bundle of wires connected to that switch is very long. And it's not clear that it's shielded. I suspect it is or your circuit would almost certainly not work at all but it could be a lot better.

 

[wells2p]

fantastic.

So to answer the shielding question the following wires coming from the switch are shielded.

1. to the boost amp,
2. back from the boost amp,
3. to the pre amp,
4. connecting the switch to the Rx Gain of the preamp.
5. From the mic input transformer
6. From the line input transformer

Additionally the wires from the output transformer to the phase switch and from the phase switch to the output XLR are a Belden 2 wire shielded twisted pair. This is all per the Neve documentation. Where I did screw up (I think) was to extend the leads from my shielded wires 2 - 3 inches instead of having very short leads and butting them up against the connection. I did this to make life easy.

The long length is really between the input trannies and the switch as I don't see how I can get the switch any closer to the board. I could relocate the trannies closer to the switch however that would be easy.

Regarding the unpaired wires I assume you're referring to the solid 22AWG going between the boards. I did this to make it look cool and darn I wish I would have known lol. I will simply unsolder those and redo them as twisted. I did twist all the wires going to and from the EQ lift switch. Also I WAS going to use relays to shorten the run for EQL and Phase but I decided against that.
In any case I'll switch the unpaired wires to twisted and retest.

Last but not least...Is there any reason not to just put a cap on the switch and call it a day? I tried a few values last night and seems to work. I know this is just a band aid and not addressing the root cause but just thinking if my whole architecture isn't viable it would be an easy way to make it work. Doing so last night seemed to have no negative effect on the sound.

Thanks a million!

 

[FIX]

Is the switch make before break?

Looks like maybe not. Switching gains would leave the feedback open for a moment, creating a chirp or thump. That could be the trigger...

 

[wells2p]

Is the switch make before break?

Looks like maybe not. Switching gains would leave the feedback open for a moment, creating a chirp or thump. That could be the trigger...

very interesting idea indeed! and would be easy to test. Thanks for the idea

 

[wells2p]

Found this article. Seems like a good overview of oscillation, testing of, and correcting

https://www.syscompdesign.com/2017/09/05/curing-circuit-oscillation/

 

[FIX]

very interesting idea indeed! and would be easy to test. Thanks for the idea

If they aren't the amp can go open loop in between selections.

 

[wells2p]

Found it! B- issue at T5. Not knowing any better, I daisy chained everything along the ground plane instead of
Creating a ground bus. i added a heavy gauge solid wire as an enhanced ground and voila all problems hum hiss everything better