Topic: Rolls PB223 mod?

[Chuck]

So, here's where I'm at with this so far: (see attached .jpg drawing)

I plan to remove C3 and C4 as they are no longer needed. I don't like to see those electrolytic capacitors in my signal path either.
I'm going to build in a switchable high pass filter, using the existing existing 12v/ 48v DPDT switch.

I'm thinking that the logical place to add to single pole filter (-6 db per octave) is after the transformer. Though, I've seen it done on the microphone side too. If anyone has any thoughts on this, I'd really appreciate hearing them.

By replacing R9 with a smaller value resistor, I could make a 75 Hz to 125 Hz high pass filter using quality capacitors in the .1 - 2 uf range for C*. Since I don't know the impedance of the transformer output, I'll be guessing, with trial and error, on the exact values needed. I wonder how to acheive good quality control on capacitor values? A 10% fluctuation in value, could have a huge impact on the matching of the channels.

Again, I'd really appreciate any input, if anyone sees any problems or has suggestions on making this project better.

[Church-Audio]

So, here's where I'm at with this so far: (see attached .jpg drawing)

I plan to remove C3 and C4 as they are no longer needed. I don't like to see those electrolytic capacitors in my signal path either.
I'm going to build in a switchable high pass filter, using the existing existing 12v/ 48v DPDT switch.

I'm thinking that the logical place to add to single pole filter (-6 db per octave) is after the transformer. Though, I've seen it done on the microphone side too. If anyone has any thoughts on this, I'd really appreciate hearing them.

By replacing R9 with a smaller value resistor, I could make a 75 Hz to 125 Hz high pass filter using quality capacitors in the .1 - 2 uf range for C*. Since I don't know the impedance of the transformer output, I'll be guessing, with trial and error, on the exact values needed. I wonder how to acheive good quality control on capacitor values? A 10% fluctuation in value, could have a huge impact on the matching of the channels.

Again, I'd really appreciate any input, if anyone sees any problems or has suggestions on making this project better.

You need a capacitance meter to match them, you can also use software and a computer sound card and do a loop back into your computer to see the roll off of you hpf in action. Here is an example of one of my hpf designs in action.

This is the frequency response of my st-9100 preamp the red line is HPF on the blue line is with the HPF off with computer software you can see the roll off of your design ( this does not mean it will sound good your ears are the best judge of that. But it is a starting point, I can use this program to make sure left and right are exactly the same frequency response and if the caps are off value they will not be I hand measure all of my caps so that left and right are linear. A capacitance meter can be purchase for about $99 get a B&K they make great ones it really does not mater if its 100% accurate as long as the two caps are reading the same on your meter your golden.


Chris Church

[Chuck]

Unfortunately, I'm not inclined to buy a special meter just to measure the caps.
I must have half a dozen meters, but none measure capacitence.

[Church-Audio]

Unfortunately, I'm not inclined to buy a special meter just to measure the caps.
I must have half a dozen meters, but none measure capacitence.

I can measure them for you and send them back if you want. What valiues do you need? I might have them here.

[Chuck]

Unfortunately, I'm not inclined to buy a special meter just to measure the caps.
I must have half a dozen meters, but none measure capacitence.

I can measure them for you and send them back if you want. What valiues do you need? I might have them here.

Awesome offer, Chris... +T
Let me work some things out and I'll definately get back to you.

Do you think I'm on the right track here, adding the filter after the transformer rather than before it?
With a 10k resistor and a .15 uf cap, I get a corner of 106 Hz. Does that sound like a good place to strat? ToddR suggested between 70 Hz and 120 Hz with a single pole filter.

[Church-Audio]

Unfortunately, I'm not inclined to buy a special meter just to measure the caps.
I must have half a dozen meters, but none measure capacitence.

I can measure them for you and send them back if you want. What valiues do you need? I might have them here.

Awesome offer, Chris... +T
Let me work some things out and I'll definately get back to you.

Do you think I'm on the right track here, adding the filter after the transformer rather than before it?
With a 10k resistor and a .15 uf cap, I get a corner of 106 Hz. Does that sound like a good place to strat? ToddR suggested between 70 Hz and 120 Hz with a single pole filter.

I use a 330nf cap for a 10k input and that works well. I try and stay away from 100hz I find it eats to much of the warmth up. I like 80hz or even 70hz as a HPF when you get into 100hz your eating up the warm range of 150 to 250 as well these are critical areas. Sparkie is a good source for knowledge about cap values, I just change things until I like the sound of it because its about sound in the end, not just values.
It depends on a lot of things like the transformers your using and what the input impedance is. Simply strapping a 10k resistor to a cap although it does work might not sound great. You really need to just change the value of the coupling caps in the first stage of the preamp, I use a DPDT switch and "swap" the coupling caps out in the front end of my preamps. I find that is a better solution.

[Chuck]

So, you are suggesting not using the R/C circuit after the transformer, but rather just using the DPDT switch I have to engage a different value capacitor without the tie down resistor on the microphone side? Currently, the DC blocking caps are 47 uf electolytics. If I make (2) 330 nf capacitors switchable on Pins 2 ands 3, you think that would be the way to go? For some reason, I thought the tie down resistor was needed to correctly time the capacitors charging to the correct frequency. I'd need a quad throw switch to make that work, as that microphone side is balanced. I want to get rid of the 47 uf eletrolytics, as I think they are needed anymore, now that I have a transformer there to block the DC to the pre-amp side. .

[Chuck]

I finally, today, figured out what I'm going to do with this project 

I'm going to make it a three (maybe four) channel mixer!
I have always wanted a cheap/small/ easy way to mix several sources.

My new plan...

-Use the existing phantom power circuit to power a third (or fourth) microphone.
-Add in two line input jacks.
-Add a passive "blend" control to mix the extra microphones (brought up to line level with the transformers) with the line level source.

So, I could run my regular rig. Patch the line outputs on the PMD671 to the inputs on this new box. Then mix in a third microphone (typically an omni) for a three microphone mix. I'd then run this signal (line in) to a DAT.

This way, I'd have my usual two microphone recording and then a three (or possibly four microphone) mix as well.

The blend contol could be a simple 10k pot (tied to ground on one side) that feeds the microphone signal (at line level) equally to the line outputs.

To me, a device like this would actually be useful.

[Chuck]

Can someone tell me if using (4) 9.6 v NiMH rechargables is not as good as using (6) v SLAs in series to provide the cleanest phantom power? Is the chemical reaction that creates the DC cleaner with the SLA cerlls as opposed to the NiMH? I am really itching to get going on this project.

Also, is there any benefit in put large capacitance in parallel with the batteries, like used in automotive sound systems?

Thanks,

Chuck