Bosoz hiss problem

[gchrist]

I have recently finished my Bosoz preamp. You can find some details here:

Bosoz

Although my power amplifier is completely silent when I connect the Bosoz preamp I can hear a faint hiss in the background. With the power amp on closing the Bosoz completely eliminates this hiss. So the hiss comes from the Bosoz.

When the music plays you cannot hear it but in low passages it is there. I have not included a capacitor stated in the articled between the AC input lines but I have instead used a dedicated IEC inlet with an onboard filter. All the other parts are standard from Digikey as described in the article. I have only changed the rectifier diodes and placed MUR860 instead. The pcb's are from Kristijan.

I have used a 4gang sfrenice p11 pot at the input. I could not find a suitable pot for the output (5K). So I am using P1 and P2 as a volume control. The hiss is the same with the volume fully closed or open.

I have used cat5e cable for my connections. This is not a shielded cable. Maybe the problem lies here.

Where to you think this hiss comes from?
If you have built a Bosoz does it also hiss?
How can I troubleshoot this small problem?

I had found in the past a schematic - wiring diagram for the Bosoz published by an asian site on this forum. If someone knows the link please post it. I am wondering if my grounding scheme has something to do with this.

Thank you
George

[gchrist]

I have also been thinking about the following:

At the input selector I am only switching the live rca input lines. I do not switch their shields. I have connected all the shields of the input RCAs together and soldered then at the GND point of the pcb. Of course this way when I listen to a particular source I have connected not only its ground but also all the grounds together to the gnd point of the pcb.

Should I have soldered the input grounds to the switching selector also? Should the shields of the input RCAs switch also?

Thanks
George

[Taco]

Your setup is almost the same as mine, that is also a IEC inlet with filter, the same rectifier diodes. And standard caps in the circuit. My bosoz has also some noise, but it's only hearable when I put my ear against the speaker and turn the volume (only 1 pot at the input) way up. I think the noise comes from the mosfets.

Like you I only switch the hot signal not the ground. The only differences is I use shieled cable for input and outputs.

[gchrist]

How did you post your picture above?

I cannot find a way to post my pictures.

On page 7 of the original artical Nelson states that if one uses the P3&4 pots he can also attenuate circuit noise also. Taking into account that I cannot find any 5k four gang volume pot on the market can I use a 10k pot instead? Some people say that this will cut the high frequencies. Has anyone tried this.

Today afternoon I will disconect any signal going into the pramp board and see if the noise persists at the output. If it does then the noise does not have anything to do with my input selector scheme. The noise is generated by the circuit.

Thanks
George

[till]

I have an attenuator only at the output of my BZLS (see picture here: http://home.tu-clausthal.de/~tpa/relais/index.html ) and i can hear hiss only in distances < 20cm from the horn shown in my avatar, and only at the highest volume settings of the preamp. Do you use a power amp with much gain? Try the attenuator at the output, i don´t need one at the input.

[gchrist]

Till,

I have searched the forum and came to the conclusion that I am using P1 and P2 as volume controls while P3 and P4 are better suited for this purpose. So:

If I take the 10k pot from the input and install it at the output then:

1) With what resistors should I substitute P1 and P2 at the input. The input signal is from a cd player.

2) What resistors should I install at the output, and where, so that the 10k output pot can work as a 5k pot?



George

[till]

I left P1 and P2 away, you may want to use trimpots here in case you have a signal source with too high level. Also you could set your pot there for preferred position, take it out, measure, and mount 2 fixed resitors instead to emulate it.

For the output pot: i guess 10k works as good as 5k here. You could put fixed 10k in parallel, but it may show not a normal log characteristic then. Try out.

[gchrist]

Thank you Till,

I will try these mods and reply back.

[hifimaker]

Hi Gchrist,

A lot of folks use trimpots on the input to set the level and then measure the trimpot value and replace it with a resister.

Some people are placing the input resister on the input of the source selector as well. This allows the preamp to have a source dependant value for P1&P2.

The benefits of DIY.

-David

[Nelson Pass]

The Mosfets have a finite amount of random noise - you are
correct to use the output resistance as the volume control.
You can also reduce the noise by reducing the intrinsic gain
of the circuit with Source to Source resistance.