parallel feedback caps

[subterfuge]

Yamaha CRX-E150, pre-amp section w/ tone controls. What is puzzling me here is, why TWO parallel caps in the feedback loop (C763/C768)?

Wouldn't just one around 50pF (47pF) do as well, or is there some deeper reason I'm not seeing here? And while we're at it, why not even something lower, like 10pF?

I mean, with the given values, 1/2*phi*R*C gets kinda odd results for what I thought it should have in this part of the ckt. Then on the other hand, I probably just can't calculate this the right way.

[Donk]

Hi,

Your calculation is OK: 470k//50pF gives 6.8kHz (-3dB). Strange!
Maybe the schematic's value isn't correct about 100pF-100pF.


Regards,

Donk

[Zen Mod]

Originally posted by subterfuge
Yamaha CRX-E150, pre-amp section w/ tone controls. What is puzzling me here is, why TWO parallel caps in the feedback loop (C763/C768)?

Wouldn't just one around 50pF (47pF) do as well, or is there some deeper reason I'm not seeing here? And while we're at it, why not even something lower, like 10pF?

I mean, with the given values, 1/2*phi*R*C gets kinda odd results for what I thought it should have in this part of the ckt. Then on the other hand, I probably just can't calculate this the right way.

they have drekload of 100p caps

two in series can have tighter tolerance than just one

they teasing us


........ as you see - plenty of solutions ; do not underestimate first one .

[subterfuge]

Originally posted by Donk
Your calculation is OK: 470k//50pF gives 6.8kHz (-3dB). Strange!
Maybe the schematic's value isn't correct about 100pF-100pF.

Yes, that's what I calculated also. Ah, and the values have been confirmed, I've taken the thing apart and measured them.

Originally posted by Zen Mod

they have drekload of 100p caps

two in series can have tighter tolerance than just one

they teasing us

........ as you see - plenty of solutions ; do not underestimate first one .

The first one is what I thought at first. Tolerance I didn't think about ...teasing, don't they always.

How about phase, nothing to do with that on this part?

[myhrrhleine]

Are they really parallel?
Or both used for a 2nd order response?
Is there a resistor lurking near by?

[subterfuge]

Originally posted by myhrrhleine
Are they really parallel?
Or both used for a 2nd order response?
Is there a resistor lurking near by?

The caps C763 and C768 are in series to each other and parallel with R757, placed in the feedback loop of the op-amp shown in the schematic. The one I drew myself btw, checked and verified to be true to the actual circuit, but missing the PSU connections for the sake of clarity.

Filtering they sure are, like the whole section there, being part of the tone controls. But how much should they filter out? I would've placed the cutoff point somewhere above 20kHz, which would mean way smaller values for those caps. But those C756 and R755 throw me off on my calc. I see a band pass here also, but should I?

Suggestions for calculations and all gladly taken, thanks for the replies so far people.

[DSP_Geek]

Look at the whole circuit: the 470K resistor is also in parallel with the network of VR701, R704, R705, and VR702, which off the top of my head (too much work to calculate this properly) looks like about a few K in series with R759, so the effective resistance at signal is not far from 10K because the impedance of C760 is negligeable at signal frequencies.

[myhrrhleine]

Sorry,

The viewing mode I use doesn't show attached schematics.
I was going by the post text.

But, as to your question, yes one 50PF would work.
Also, The network is in parallel with the EQ

[subterfuge]

Originally posted by DSP_Geek
Look at the whole circuit: the 470K resistor is also in parallel with the network of VR701, R704, R705, and VR702...

Ah, yes. I have calculated few pages worth of cutoff points for every separate part in this section, with different tone pot settings also (both max/both min/both in the middle). It's just that done like that, I got confused about the workings of the whole.

It seemed simplest just to bypass the whole tone ckt, rather than try to figure out something "neutral" or such. Then again, can't really carve the front panel of this box for any kind of extra control, there's a big PCB right against it (like there are all over this tiny box).

So now I'm back to thinking how at least to try and make the best out of what is already there.

Originally posted by myhrrhleine
Sorry,
The viewing mode I use doesn't show attached schematics.
I was going by the post text.

It's OK...don't know if my explanation was that good though.

Looking back...I didn't mean my remark about the schematics to be snotty, but just to clarify that it's been checked to be right. I've read way too many schematics myself which proved not to be that accurate when checked against the real things.

Apart from possible copyright issues(?), drawing these myself, I can also at least try to make them as simple and clear as possible.

Thanks for the replies. Any insights welcome, as always.