RF filter for a transformer input line amp

[ruffrecords]

IC2, IC1 is gone. There really is no problem with noise, since I only use it as a line input with rarely more than 10db of gain (just for mixing).

Can you be a bit more precise about this noise/oscillation? Can you see the oscillation on a scope? What frequency is it?

Cheers

Ian

 

[living sounds]

Lower series resistance will be better from a noise standpoint.  Take me out back and shoot me if I'm wrong, but I don't see it doing anything negative to the transformer tbh. 
The actual and reflected dcr and impedance on the secondary are all in parallel with your 1K1 loading resistor and then all that is in series with the series R of your input filter.  I would keep the whole lot under 1K myself. 
As I said at the start, you might be able to just up the value of the shunt C to 1800pF or 2200pF and add a smallish 200R or so series R.

Well, the resonant peak was very obvious (with no series resistor). I will test this some more...

 

[Winston OBoogie]

Well, the resonant peak was very obvious (with no series resistor). I will test this some more...

I do think you should put a resistor in, just thought that you could try scaling the resistor down and the cap reciprocally up.

 

[john12ax7]

BTW, if a low resistance feedback resistor is a problem for the op amp to drive, what about voltage followers?

R3 is a pot, what would be the lowest limiting value, 100 ohm perhaps? You would need to drive R2 in series with R3 ll -IN (which would be large), plus this entire value is in parallel with the next stage input impedance.

A follower would only see -IN (which is large) in parallel with the next stage.

 

[Winston OBoogie]

R3 is a pot, what would be the lowest limiting value, 100 ohm perhaps? You would need to drive R2 in series with R3 ll -IN (which would be large), plus this entire value is in parallel with the next stage input impedance.

A follower would only see -IN (which is large) in parallel with the next stage.

He says his usual max gain is 10dB.  So with a 1K1 feedback, gain set R of 500R gives A = 3.2/10dB.
Next stage, if I understand his new layout correctly, is 47K so, ignoring -tve in which has negligible effect, gives us a load to be driven of over 1K4.  Over twice the minimum load specified on datasheets.  I  honestly don't see this a problem. 

Since we threw away 10dB of level in the input transformer and are now adding it back in, any help with added noise in the input stage would be welcome if it were my desk.

Peace out.

 

[john12ax7]

Even at 1.4k I wouldn't force a 5532 to drive something that low if it wasn't absolutely necessary.  We'll have to agree to disagree on that one.

 

[Winston OBoogie]

Even at 1.4k I wouldn't force a 5532 to drive something that low if it wasn't absolutely necessary.  We'll have to agree to disagree on that one.

Do you have data? 
True,  THD with a 600 ohm load at high levels on a 5532 is certainly not great, around .1% @20V.  Self shows this improving considerably by 820 ohms.  Alternatively,  Sam Groner displays THD at 600 ohms, 2K2, and overall xnsfer function.  Best case on that op amp with max V and no load seems to be a decade better than with 600 ohms and is some.  .01% THD or thereabouts.  Difference between this and 1K4 doesn't seem to amount to anything worth worrying about.  Nothing I can hear anyway.

On the other hand,. Raising the feedback by 2 to a 2K2 value gives us a gain R of 1K for a closed loop A of 10dB.
And 1K is equivalent to 4.2 nV/rt Hz of noise.  I can hear this in an input stage of a desk.

 

[Winston OBoogie]

On the other hand, this isn't my desk so I'll bow out of this one

Peace

 

[john12ax7]

Look at the spectral plots for +20 at 600 ohm.  20+ dB higher,  but even worse is lots of upper harmonic hash.  At what loading will this sufficiently decrease? I don't have a definitive answer.

But I do agree it's not our mixer,  best thing is to measure and listen for yourself to decide what the best trade off is.

 

[Winston OBoogie]

Look at the spectral plots for +20 at 600 ohm.  20+ dB higher,  but even worse is lots of upper harmonic hash.  At what loading will this sufficiently decrease? I don't have a definitive answer.

But I do agree it's not our mixer,  best thing is to measure and listen for yourself to decide what the best trade off is.

Yeah it's pretty knarly at 600 ohm load for sure  
Self says it's ok at 820, Groner's charts seem to show it'd be OK by about 1K.  But the last time I played with 5532's I broke out in hives so I don't visit often

Good luck Living Sounds, you'll get it sorted

 

[living sounds]

Here's my current state of the input circuit.

Less than 0.1 db deviation within the audible range and no more than 9 degree of phase shift at a gain of 10 db. The 27R resistor has been in the circuit before, just added it to the schematic. Appears to be completely stable at full gain (VR = 0 Ohm). Probably could get away with an even lower value of C6, but would have to order the caps. At high gains frequency/phase response gets a little uneven, but I hardly ever need this much gain.

 

[living sounds]

Can you be a bit more precise about this noise/oscillation? Can you see the oscillation on a scope? What frequency is it?

It happens in the EQ stage down the line at high EQ gain settings at the peaks of the EQ curves.

 

[Winston OBoogie]

So with the most recent circuit you posted, it's now stable?  Or is except at very high gain settings?
Is the transformer back in the path again?

 

[living sounds]

So with the most recent circuit you posted, it's now stable?  Or is except at very high gain settings?
Is the transformer back in the path again?

Yes, stable with the transformer in the path, at any gain and at any setting of the EQ stage following it.

 

[Winston OBoogie]

Yes, stable with the transformer in the path, at any gain and at any setting of the EQ stage following it.

OK good job and good to hear

 

[living sounds]

OK good job and good to hear

And thank you guys! After my vacation I will mod the remaining 31 channels.  8)