New Speaker Hiss

[Crook36]

Hi,

back again with more questions. I finished my amp6 and it is very good (to my ears).

I bought some new PSB speakers, more efficient 93dB, 6ohms. When i put my ear to the tweeter the hiss is much louder than on my old more inefficient Tannoys (almost inaudible before). The hiss is related to the vista windows volume control. High the volume, higher the hiss. Changing the foobar volume does not increase the hiss, it seems directly linked to the pc volume. Don't get me wrong sat in my listening chair i cannot hear it at all, but it is still irritaing the hell out of me.

Any ideas on this please? Thanks, Mark.

[krilli]

The PC soundcard could be noisy, I know mine is. It could be the amp ... is it in a nice shielded box? The cables twisted, shielded Etc.? In short, what measures have you taken to clean up in-amp noise?

[Crook36]

Hi Krilli,

The amp is in a stainless/aluminium box. The cables are not twisted. The input stage is silver/teflon, the output is 2.5mm copper, just normal jacketed cable. The input/output and power cables do not cross each other. Maybe it is just the soundcard. It is a Creative Soundblaster USB job, but i'm ordering a Beresford 7520 dac later this week hopefully it will improve things (but maybe make things worse!).

Could ferrites on cables make things better?
Could it be gain related, i intended to use the AMP6 with an MP3 player and used 22/82k on the in/feedback? Now i am getting the DAC i am worried the gain will be far too high.

[krilli]

I think there's some stuff one can/should do w.r.t. the signal wires, but I don't know and can't advise specifically.

That gain seems rather high, especially with such sensitive speakers. What's the loudest you've set the MP3 player to? I think in general, you can trade out the noise if you lower the gain. For low noise, AFAIK, gain should be as low as possible - but no lower

Also, what kind of power supply do you use?

And is the hiss as bad with other sources?

[Crook36]

Hi,

i can have the windows volume to about 50% or so, but usually it is very low. I think i'll start by changing the feedback resistor values to something lower and see how i go from there. If not i have some good quality sheilded coax (CT100 or something like), i suppose i could try with the sheild floating on the output and see how it is.

I use a 12v smps, but i have a 13.8v regulated bench supply knocking around so i can change and listen. This was my first DIY project and i had no desire (or knowledge) to mess around with my own transformers and mains electricity!!!!!

As for source i have nothing else to use. I moved to PC/FLAC, so didn't use a volume pot on the amp. My cd player has no volume control and my creative vision mp3 player has no volume control when i use the A/V out/RCA adaptor.

[Saturnus]

PC soundcard (regardless of quality and price!!!) are notorious for being hissy. What you really need is an USB soundcard or an external DAC, though I have seen some people having good results by shielding the output opamps on the soundcard itself but that will only cure a minor part of the problem.

But for now just adjusting the gain to a more reasonable level for PC use will do.

[omishra]

1. In the boxed amp, if you switch off amp - short the inputs to ground - switch on amp again and listen same thing then amp is noisy. Wiring, shielding can help here.
2. Otherwise, noise is at input source. If its not increasing with volume level(e.g. muted source, increase volume) then its residual and can be lowered. For that I decreased input level to 1/3rd with fixed resistors (voltage devider) in each channel. As a result, you have increase the input volume level a little bit. Just to improve SNR.
Otherwise change noise source is better option.

[Crook36]

Well i seem to have solved the problem. Got rid of the soundcard and got a Beresford 7520 DAC which is quieter. Changed the output cabling to a good quality sheilded coax. I also put Ferrites on all my external leads/cables. At the same time as this i changed my interconnects to VDH D102 III and speaker cable to Chord silverscreen (although i still say twin and earth is just fine). Changed the op amps in the 7520 from NE5532 to either THS4032 or dual OPA627 on a BB adapter (depending how i feel).

So the whole thing sounds quieter and the general quality feeding the amp is massively increased. Unfortunately i changed so much in my set up at the same time i have no idea where the improvements really are.
[edit] I also wired the amp and the dac together which made it quieter again (must be something to do with the ground).

It was probably the 8 GBP of ferrites that did it though. Ha ha ha.

Mark.