Problem with passive preamp HELP!!!

[mcgreg66]

Hello DIYers,

I've built a simple design passive preamp with a 24step DIY ladder attenuator and 4pole6pos input selecor switch.

I used the switch in a simple input selector box before switching signal and ground and everything worked just fine.

I've seen many builts with all grounds tied together

Now the passive works great too with CD source....no contact bounce...no noise..BUT when when I switch to phono (from phono stage Lehmann Black Cube) it starts with loud cracking noise from the speakers..... every step on the attenuator cracks and no sound
Same thing when I change the inputs...no matter where I plug the CDP everything is o.k.
Poweramp is Scotts KT88 SE (now without volume conrol)

I have no idea what to do

Next thing is I included a 3,5mm jack ...this has no sound too...but I checked connections many times.

Inside looks like this

By mcgreg66 at 2011-09-05

Anybody can help

Thanks

Gregor

[DF96]

Where is your DC blocking? I can't see any, so I assume you are assuming that all your sources already do this. This ought to be true, but is not always so. Note that DC blocking requires both an output capacitor and a resistor to ground. DC servo is not quite as good, there will always be a small offset.

I have never used twisted pairs in this position, but I suspect that the twisting might have to be tighter than you have done.

Are you switching grounds as well as signal? Doing this can reduce hum from ground loops, but increase switching transients.

[VictoriaGuy]

Originally Posted by mcgreg66

no matter where I plug the CDP everything is o.k.

Gregor-
No matter which jacks you use for your CDP, the switch and volume control work perfectly?
Including the 3.5 mm jack?

If 'Yes' then your construction/wiring must be OK, as far as it goes.

If you have an 'ordinary' stereo pot in your parts box, try wiring that up with the output of your Lehmann, and input into the Scott. Perhaps it is a Lehmann problem?

Also, this might tell you if DC blocking (or lack thereof) is an issue- ie is the power amp going into 'protection' for some reason.

A few leads with alligator clips can be very handy for this sort of stuff.

Issues with hum /grounds can be worked out later- you are getting no signal at all?

BTW, for this sort of vol/switch arrangement, I usually tie the grounds together at the inputs. I've never had a setup with much wire, though.

[mcgreg66]

I found out that even with CD source starts very bad distorsion at around attenuator step 10-12.

So something is completely wrong with the wiring or it's the atenuator.

So I will solder a simple pot to test this.....

To the 3,5mm input socket.....
Everywhere I can measure the around 100K, but a the 3,5mm socket its only around 50k...maybe because it has only one ground and there coming two grounds (left and right) from the atenuator?!?!

Next I would try to connect all ground together...Good Idea?!

Oh god so much ???????????

In fact I realise my limited knowledge of elecronic circuits

Everything seemed to be soooooo simple.

Can someone try to explain to me what is about the DC blocking?!?

[VictoriaGuy]

Originally Posted by mcgreg66

or it's the attenuator.

I bought a similar type of attenuator from China/HongKong a couple of years ago. When I put it in a project this year, I discovered that it was faulty/miswired, or something....too complex for me to mess with. I replaced it with a 'regular' stereo pot and all was well.

So, I would not eliminate the attenuator from your 'suspect' list, yet.

[DF96]

I thought the attenuator here is DIY?

DC blocking is what coupling capacitors do. Music is AC. You only want music to pass from one unit to the next, not bias voltages too.

[VictoriaGuy]

Originally Posted by DF96

I thought the attenuator here is DIY?

Yes, I missed that. Sorry.
Impressive amount of work, that!

But there must be some simple wiring error here in the 'preamp', or the power amp must be shutting down on some inputs- it seems to me.
If so, it should be cured with wiring check and some test clip leads, and/or cap/resistor combo to confirm DC blocking.

How complex can this be?

[Mark2727]

Hello:

I am wondering:

1) If the gentleman got his passive worked out.
2) If I could get some clarification on the comment from DF96 about switching not only the signal, but also the ground. How is this implemented? And what does it mean "increasing switching transients"?

When building passives, I often experience a bit of noise. And I'm thinking this may be a problem with my grounding scheme. Usually, I just put all grounds to one central star-ground point. But, unless I remember incorrectly, the selector switch is not ground? Volume pot has a wiper, output, and ground and that ground is tied to the star ground point.

Mark

[DF96]

Some people switch the ground as well as switching the signal, so you need twice as many switch poles. I think the aim is to completely isolate unused sources. The snag is that you may be shifting, at least transiently, the ground potential so clicks can occur. I guess you can reduce this by adding grounding resistors (1M?) but I have never tried this so can't really comment.

[Mark2727]

Oh, I see. Another set of poles is needed on the ground. Well, so far, this has not been a problem with anything I've built. No crosstalk or the like.

I just question my grounding scheme in general, since I know a little about electronics, but am not even "intermediate" I would say in my skills.

Lately I have taken to NOT hooking up the ground tabs of the RCA inputs to the ground tabs of the RCA outputs directly. What I do is run them separately to the star ground. I haven't seen much of a difference.

Also I should say at times I have tried to run a separate ground from the body or frame of the volume pot but this also didn't help much.

My noise is very slight, but I feel on a passive I really shouldn't have any background noise. That's kind of the point of one, isn't it? :-)

MW