Re: Where to insert HPF and LPF.
« Reply #20 on: November 29, 2011, 03:25:35 PM »
Hi,

After some testing and experimentation I managed to get all the cutoff frequencies I need for the LP and HP.
However, I noticed a big increase in noise (about 15db) in the HP filter.
I'm pretty sure this is caused by the cable I run from (-) of C82 to a board where I have all the switchable caps (for different HP Fcs) and then back to the (+) of C82.

I tried to keep the cable as sort as possible but I still get an unusable SNR.
Are there any solutions to this problem?
(I believe it's a common topology and I've seen many units that have cables running from the caps to the switches.)

thanks
Mike



abbey road d enfer

Re: Where to insert HPF and LPF.
« Reply #21 on: November 29, 2011, 05:24:31 PM »
However, I noticed a big increase in noise (about 15db) in the HP filter.
What noise? Hum, buzz or hiss? If it's buzz, that may be due to the fact that one of the wires goes to a very sensitive node (the base of VT1). This wire should be thoroughly shielded.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Where to insert HPF and LPF.
« Reply #22 on: December 02, 2011, 12:46:51 PM »
What noise? Hum, buzz or hiss? If it's buzz, that may be due to the fact that one of the wires goes to a very sensitive node (the base of VT1). This wire should be thoroughly shielded.


It looks like being an issue with the base of VT1. The frequency analyzer shows me a big peak at around 50Hz.
The whole circuit is not in an enclosure yet but I doubt I'll get 15db less noise in a box.
Before adding the HPF the noise difference between bypassed and 'in' was less than 2 db.
I'll investigate more andd come back with more questions..
In the meantime feel free to suggest solutions  ;D

abbey road d enfer

Re: Where to insert HPF and LPF.
« Reply #23 on: December 02, 2011, 01:08:23 PM »
What noise? Hum, buzz or hiss? If it's buzz, that may be due to the fact that one of the wires goes to a very sensitive node (the base of VT1). This wire should be thoroughly shielded.
It looks like being an issue with the base of VT1. The frequency analyzer shows me a big peak at around 50Hz.
The whole circuit is not in an enclosure yet but I doubt I'll get 15db less noise in a box.
I disagree with that. You have large lengths of sensitive antenna picking up all available garbage.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Where to insert HPF and LPF.
« Reply #24 on: December 02, 2011, 01:11:34 PM »
I disagree with that.

Glad to hear!
Will a PCB trace have less noise than an equally distance wire?

abbey road d enfer

Re: Where to insert HPF and LPF.
« Reply #25 on: December 02, 2011, 07:20:22 PM »
I disagree with that.

Glad to hear!
Will a PCB trace have less noise than an equally distance wire?
Depends on the size of the track compared to the size of the wire. A 16mil track has about the same capacitance as an AWG #26.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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