hiss in preamp balanced output SSM2142

[meaghers]

I've finally gotten my preamp to a semi-assembled state and am having noise problems. I am using pcb's from Rod Elliot.

I have traced down a rather loud hiss to the balanced output of the preamp. I believe this because when I disconnect power from the SSM2142 balanced output line drivers, the noise goes away. Additionally, when I disconnect signal from the SSM2142's the noise is unchanged, indicating it is not coming from the preamp circuit.

I have used the suggested circuit for the balanced output as presented in the chip's datasheet, utilising 0.1uF ceramic caps as decoupling. The datasheet boasts highly of the chip (link here), thus I do not believe this to be simply the best it can do. This is such a simple circuit, what could go wrong?

The hiss is quite loud and always present, no matter what gain state the preamp circuit is in.

Any help? Where do I start tracing this problem? I am still fairly new to DIY and unfortunately do not possess a scope.

Many thanks; pictures to follow shortly

[Minion]

What are you useing as a preamp before the Line driver?? Got a schematic?? what type of Grounding scheme are you useing ?? what type of Power supply and how much filtering??

[scott wurcer]

Do you have a schematic of your actual circuit? Sometimes these sythesized transformer circuits can be made to have a huge gain by improper loading.

[meaghers]

the whole preamp:

the balanced outputs:

schematic from datasheet:

@scott: I have used the circuit above as detailed in the datasheet for the chip... it's very simple!

@Minion: I am using P88 from ESP, details on this web page. As for grounding scheme, I have the power supply (P05a from the same site) grounded to the chassis at one point, and the preamp circuit and the balanced output grounded to the chassis at a separate point. I have played with a few different grounding points and have found no difference.

As for the power supply, details are here , and my big electrolytics are 2200uF (thats what you meant right?)

thanks for the quick replies guys!

[Minion]

You should have all ground wires comeing to a common point and then bolting that to the Chassis not 2 seperate points....

also you are P2P ing the ballanced line Driver stage , because of this you could be picking up interferance because of the Extra long leads and crossed wires, Pluss the extremely long distance of the ballanceing stage from the PSU....
Since the Curcuit is so simple and should work I believe it is probably the fact that your are Point 2 point wireing it....

Have you thought about maybe getting a Piece of vero board or even just makeing a PCB and putting your Ballanced line driver stage on that....?? If you do this I would add a couple bigger Power caps as close to the IC pins as possible in the 10uf to 100uF range.....

If you want I could Quickly design a PCB for you if you wanted to etch it at home or for a couple bucks I could etch it for you....

something like this maybe??


Cheers

[meaghers]

I have tried grounding everything to a common point with no noticeable change in noise floor. I have thought of using a small project board for the output, but I really can't see the very very small amount of wire affecting things much at all, can you really? There is 1" of wire at most extending from each leg of the chips.

Would it help if I got a vero board and put the balanced circuit nearer to the psu? I think I will try that...

[Minion]

There seems to be quite a bit of wire going from the PSU to the SSM Chip , with that much of a distance from the PSU you should try adding a 10Uf cap as close to the IC Pins in Paralell with the 0.1uf ceramic caps and see if that helps with the Noise....

also have you tried it with the Top of your Chassis closed?? this should shield the curcuit from interfernace to some degree....

[meaghers]

Yes I have tried with the chassis closed also with no difference. I will try adding the parallel caps as you suggest. Thanks so much for your help. If that doesn't fix it, I went and got some vero board and will try putting the ssm closer to the psu on the board.

[meaghers]

I have only 22uF 35V electrolytics available at home, will these work? In which polarity do I mount them?

[edit: I see that the schematic you have so generously posted for me gives polarities for the caps, I will follow that. Thanks!]

[meaghers]

Well the 22uF electro's didn't do anything ... I'm trying the vero board now...



by the way Minion I believe you may have pins 1,2 and 7,8 mixed up on your pcb layout. According to the datasheet, pins 1 and 2 are joined together to one phase, and 7 and 8 are joined to the other phase. On your layout, I think you have 1 and 8 together, and 2 and 7 ... ? Did I see that right?