Phono preamp grunding questions

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Tyimo

Member
2002-11-16 3:43 pm
Hungary
Hi!

I would like to ask for some help here.
I built 2 MM phono preamp from kits. One is discrate and with PSU, another is with IC and with battery. When I try to use them with my JLH or ZenLite amp I hear a light noise already on idle and it gets stronger on higher volume too. The noise is not a "humm", it is a "brzzzz" kind. :)
But! When I try these MM preamps with my factory HarmanKardon amp there is absolute zero noise!!!

I tried the followings:
- connect the LP ground to MM ground and without connection too,
- connect the PSU with and without the wall earth,
- earthing the case, and not earthing too,
- RCA's grunds insulated from each other and connected together the grunds too.

I have no idea what could be the problem.
One more info: the HK amp is in another house by my parents.
I also tried the MM preamps with my factory Yamaha amp by me and ther is the noise! Could it be something with my house electric system?
Is there any rule of thumb for phono preamp grunding and connecting?

Greets:

Tyimo
 
Tyimo,

one possibility could be induction from
mangetic stray field from transformers
etc. Try to move your preamp somewhat
around when they are connected in your house.
(Thow it is somewhat unlikely that two different
circuits show the same symptoms). Be sure
not to put your preamps directly on top of your
amplifier.

When running the preamp on battery it is
independent from your electical system at home.

So shorting the phono inputs of your preamps,
as analog_sa suggested, is a good idea to find
the source of your trouble.

Did you check all connections on your pickup cartridge
also, including grounding? Often there is a shield connection
from the ground of the cartrige to the metallic arm just
close by to the cartridge connectors.
 

Tyimo

Member
2002-11-16 3:43 pm
Hungary
Hi Schnullimaus!

Thanks for the tips.
I already checked my LP Player and the cartridge. It was O.K. with my HK integrated phono stage.

Another :
When I touched the case of the MM preamp ( the battery operated) it was hummm, however the RCA's and the PCB was totaly isolated from the case.
In the weekend I will try to short the inputs.
Tyimo
 
Every amplifier has an input. Therefore it has an input impeadence. Every amp also has an output and an output impeadence, pre-amps are also amps. All I am saying is that the output impeadence from one stage must match the input impeadence of the next stage to achieve best results. This might not be the actual problem, but it sure wouldn't hurt. Maybe the HarmonKardon input Z is closer to the output Z of MM stages. Could it be isolation issues or PS issues? Hard to tell excactly from a few words. Phono stages have hi gain and are suseptable to noise easily.
Impeadence matching is a major issue when designing cascaded amp circuits.
 
Sounds like the hum or buzz is from the turntable - some of the weirdest problems I have firgured out were turntable hum problems.

Since you shorted the inputs to the preamp (correct?) and there was no hum I would try shorting cables in the turntable if possible and/or shorting the cartridge pins.

I've seen bad or poor cables, bad grounding, 'bad' cartridges, relative position of equipment, and just bad luck with combinations cause hum/buzz - and several weird things that I've forgotten cause this.

Good Luck

Bill

Just reread some of what you did, and comments
1- Very unlikely to impedance matching :)whazzat: NOT true that in and out impedances have to match - it depends on what you are trying to do - run long lines and avoid reflections - Yes, many other situations - No. And very little to do with hum - well I guess the impedances involved might make it easier or harder to pick up hum, but in this case matching is not going to cure. Look on Self's site for good explanation of grounding, ground loops and the like.
2- Could be ground loop or some strange interaction with the equipment, but seems unlikely since two preamps do it and one is battery which should get rid of mains ground loops.
3 - It works at your parents - suggests the turntable, cartridge, and preamps are fine.

That leaves some kind of 'enviromental' thing at your house (which would mean that if you took it to a shop they would find 'no problem' - boy I used to hate those)
So try different positions of various equipment - try a completely different part of the room, a different AC socket (I've run across house wiring faults causing hifi hum problems), a different room. As I said good luck.

Bill
 
Hi Bill!
Thanks! I think it is something magic.
One time I had some similar, but reversed problem: I tried my Hiraga Le Monster amp at my friend and there was big humm and fuzz and any kind of noise, but at home the amp was deadly quiet and superb....
I will post next the schematic of the battery powered preamp, maybe you will find something wrong..

Greets:

Tyimo
 
Hi!

Here is the info about my battery powered little-simple MM- preamp.
One more note:
In the kit there wasn't the component for C9 ( cap from pin8 to grund), so I didn't built in. It is shown only in the scmatic.
Could this cause the noise problem? What cap should I use for it?

Thanks:

Tyimo
 

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The capacitor mentioned is to make the power supply 'look like a short circuit' ie like a voltage supply at high frequency and needs to be as close to the supply pins of the op amp as possible. The type has to be good at HF not always what is considered 'good' at audio frequency - with high speed op amps it helps prevent oscillations - do not know how important it is for the 5532 (I've never really played with the 5532 much) which I believe is not a super fast op amp. Oscillating circuits can give strange problems - can't hurt to add the cap and see what happens, try a small ceramic (npo?) or polystyrene, check a 5532 data sheet see what it recommends. I have seen people put 'this cap' directly soldered to the power supply pins, across the top of the IC.

Battery power supplies while seeming a great idea can be a little tricky - batteries are NOT perfect voltage supplies - sometimes you're lucky and sometimes ...

Having said all that I sspect that your problem is not this at all since did you not say that you had the same problem with a completely different preamp.

On a completely different note the design of the battery pre is not at all like what I would do - even if usig a 5532, coupling caps, feedback and so on - of course it might sound great. Not the place to go into that, and to close I somehow suspect that the pre is not your problem.

Bill
 
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