Intel on mic pre low-cut circuits!

[djmiggymigz]

hmmm trickery!! I actually just opened up a name brand cloud lifter today to see what their circuit looks like in comparison to the DIYRE version as well as my copy.. ALL THE SAME. They used all smd components but it was the same exact LSK389, R values, everything. That was cool to see. Especially since they sell for $150 and I'm about to make 2 for about $40/each!

Back to my question, does this work for the (about) 80Hz rolloff I'm going for. Not sure if this is the same. On the rockscience schematic the cap(s) are in series (i think) but in this idea I'm inserting it directly when the switch in the on position. Advice/critiques?

 

[gyraf]

..you get very high termination impedance (the 150K) for frequencies below your lowcut point. At the expected high gains of a mic pre, this would probably result in excessive low-frequency noise..

/Jakob E.

 

[djmiggymigz]

..you get very high termination impedance (the 150K) for frequencies below your lowcut point. At the expected high gains of a mic pre, this would probably result in excessive low-frequency noise..

/Jakob E.

hmm well that's precisely what I DONT want my low cut switch to do!! What could/would the remedy be? Does the cap need to live elsewhere?

Pretty sure I can also just get rid of the 150k tho as that's from CAPI's schematic as a loading resistor prior to the 2520

 

[gyraf]

I don't see your full schematic, so I don't know your options..?

 

[djmiggymigz]

Oh my bad here it is thus far.

 

[gyraf]

Your output attenuator seems to be constant-(low)-impedance, hpf would perhaps be best placed between output transformer and attenuator

 

[djmiggymigz]

Okay question time! How would that work since after the output transformer the signal is once again balanced? Where I have it right now I only have to interrupt one path but after the output I'd have to somehow filter both the hot and cold signal right?

Also, it probably isn't a huge difference but I think I want the filter prior to the 2520 gain stage so that I can benefit from the extra headroom of cutting the sub frequencies out.

 

[djmiggymigz]

Does anyone have or maybe can point me in the right direction of a preamp schematic that already has a Lowcut filter in place? I'm curious to check out how it's implemented in other circuits

 

[djmiggymigz]

Is the R3 loading resistor needed for the RC filter circuit to function? Looking into adding another switch to switch the loading resistor in/out of circuit. But not sure if the R3 being out will change how the RC filter functions

 

[gyraf]

If you want your filter before opamp, you need specified and not too high impedance at that point. I don't see room for that without sacrifice of other parameters.

 

[djmiggymigz]

Okay, so if you were building this you would put the lowcut after the output tx before the trim?

 

[ruffrecords]

Okay, so if you were building this you would put the lowcut after the output tx before the trim?

Just to put this in context the 312 was a 500 series mic pre designed for use in API mixing consoles. Its output would most likely go to a patch bay and the return would feed an API EQ module which most likely included the HPF you require. This in turn would feed a routing module which sent signals to various mix buses.

Bottom line, there was never any intention to include a HPF to the 312 when it was designed so any later attempt to do so may be problematic. Both the source and load impedance at the input circuit are not particularly well controlled aso the actual HPF frequency you get may vary during use and the other potential problems fitting it to the input have already been described.

Because the source and load impedances are well controlled st the output transformer/attenuator junction (in other words not unduly influence by what you connect to it), the output would be a better place to ad a HPF.

Yes, the signal there is balanced so you really need a balanced HPF (two capacitors instead of one)

Cheers

Ian

 

[djmiggymigz]

Just to put this in context the 312 was a 500 series mic pre designed for use in API mixing consoles. Its output would most likely go to a patch bay and the return would feed an API EQ module which most likely included the HPF you require. This in turn would feed a routing module which sent signals to various mix buses.

Bottom line, there was never any intention to include a HPF to the 312 when it was designed so any later attempt to do so may be problematic. Both the source and load impedance at the input circuit are not particularly well controlled aso the actual HPF frequency you get may vary during use and the other potential problems fitting it to the input have already been described.

Because the source and load impedances are well controlled st the output transformer/attenuator junction (in other words not unduly influence by what you connect to it), the output would be a better place to ad a HPF.

Yes, the signal there is balanced so you really need a balanced HPF (two capacitors instead of one)

Cheers

Ian

Okay yupp thanks for the added perspective there, that made the challenge click in my head! I suppose the original design also wasn't trying to increase headroom with the goal of pushing the tx's/opamp harder either! Hmm that makes me wonder if it'd be better to simply focus on the pre and then get some sort of pultec style 2-band Hi/Low eq rocking in a unit that lives right next to my pre!

Regardless, I'll look into the option of adding the filter after the output tx. I'd still love to have that option onboard the pre. Probably gonna have trouble/confusion/questions figuring out the balanced version of this but I'm going to try before asking! I suppose the same circuit works with two matched caps, 1 for both + & -, going out to the attenuator.

 

[ruffrecords]

Okay yupp thanks for the added perspective there, that made the challenge click in my head! I suppose the original design also wasn't trying to increase headroom with the goal of pushing the tx's/opamp harder either! Hmm that makes me wonder if it'd be better to simply focus on the pre and then get some sort of pultec style 2-band Hi/Low eq rocking in a unit that lives right next to my pre!

A better and more flexible solution IMHO.

Regardless, I'll look into the option of adding the filter after the output tx. I'd still love to have that option onboard the pre. Probably gonna have trouble/confusion/questions figuring out the balanced version of this but I'm going to try before asking! I suppose the same circuit works with two matched caps, 1 for both + & -, going out to the attenuator.

For this you either need to learn a bit of basic electronics maths or use this on line RC filter calculator:

https://www.omnicalculator.com/physics/rc-filter

I cannot see the values of the variable resistors at the output on your schematic but it is a reasonable assumption they are intended to look like a 600 ohm load. So use 600 ohms as R and set the frequency to 80Hz and out will pop the capacitor value. Maintain balance you need to put on capacitor in each of the two output legs of the transformer so their value will need to be twice what the calculator says. And you will need a double pole switch to switch the HPF in and out of circuit.

Cheers

Ian

 

[djmiggymigz]

Okay that actually doesnt seem as complicated as I thought it might be. The double pole makes a lot of sense to me. I've been messing with the digikey RC filter calculator trying to figure out how that relationship works (as well as reading up on rc filter theory). Thanks once again for the response!

Didn't realize that the rc filter would affect impedance enough to cause issues at the 2520 input! Reading up on impedance now and seeing how far over my head it is. Gonna take a while to understand this one (along with many mistakes most likely!!)

Agreed though, the dedicated EQ unit would probably be way more useful and helpful

 

[djmiggymigz]

Okay just found this after looking into "balanced RC Filter LowCut Filters". Would this be a rather drag and drop filter using the assumed 600 Ohm resistance of the attenuator as the R value?

I'm also attaching a quick screenshot of that section of my schematic with the caps in place (but not switched quite yet).

https://ethanwiner.com/gadgets.html

Based on this I calculated for about an 80Hz rolloff w/ 3300pF caps.

Why would I need to double the cap value from the calculator? Is it because with the caps in series the effective values halve? Even though at this point in the circuit the caps would be living on 2 separate traces?

 

[gyraf]

For 80Hz into 600 Ohms I get 3.3uF

http://www.muzique.com/schem/filter.htm

As the two are effectively in series, we need double that in each side - so two 6.6uF (aka two 3u3's in parallel in each line)

Switch between 3.3uf and 6.6uF, and you'll get 160Hz and 80Hz