PE Subwoofer Pre-Amp - Exposed

[Millstonemike]

It's missing the node dot in the schematic, should be on the same connection where R4 and R5 meet.

Correct. If its not connected, I use a "c" like shape to denote a connection doesn't touch another. Is that really old school? With R7 removed, the schematic looks like this:

[thekorvers]

Very nice job Mike. Good research and write-up.

I bought one of these a little while ago to check out, but had not done anything with it yet. I do dislike having to hunt down parts and having to modify it just to make it work properly. I prefer things that work properly right out of the box.

I agree that PE should contact the supplier/manufacturer and specify the modifications needed. At least they should offer a parts kit for the modification at modest cost or suffer much reduced sales now that the word is out.

[thekorvers]

Correct. If its not connected, I use a "c" like shape to denote a connection doesn't touch another. Is that really old school

When I studied electrical engineering ( in Europe a long long time ago) schematic standard was when 2 lines cross without a dot at the junction, no connection is made. When 2 lines cross and there is a dot at the junction, a connection is made. The only place I see this in your schematic is to the right of C8, where it says Nominally 1/2 Vcc. So this schematic is not very consistent, something I see a lot these days.

I miss proper protocol and standards of the old days, not only in engineering, but in life in general. Things are often done so sloppily nowadays. (I think this may be a senior citizen's rant).

[Millstonemike]

When I studied electrical engineering ( in Europe a long long time ago) schematic standard was when 2 lines cross without a dot at the junction, no connection is made. When 2 lines cross and there is a dot at the junction, a connection is made. The only place I see this in your schematic is to the right of C8, where it says Nominally 1/2 Vcc. So this schematic is not very consistent, something I see a lot these days.

I miss proper protocol and standards of the old days, not only in engineering, but in life in general. Things are often done so sloppily nowadays. (I think this may be a senior citizen's rant).

I only put a dot where a signal point is called out. From Wikipedia:

[thekorvers]

I am talking of a time when there were no home computers or CAD (Computer Aided Designs). According to your post then, I am going by the Old style CAD symbols, probably adopted from the same symbols in earlier schematics before computers were available. I still think that the best and safest way to represent a connection is with a dot. No ambiguity.

By the way I was not disparaging your drawing of a schematic as I assumed that this was a schematic from a manufacturer. Regardless where the schematic came from, I think you did an excellent job analyzing this and finding a solution for the problem. I commend you for it!

[Millstonemike]

I am talking of a time when there were no home computers or CAD (Computer Aided Designs). According to your post then, I am going by the Old style CAD symbols, probably adopted from the same symbols in earlier schematics before computers were available. I still think that the best and safest way to represent a connection is with a dot. No ambiguity.

By the way I was not disparaging your drawing of a schematic as I assumed that this was a schematic from a manufacturer. Regardless where the schematic came from, I think you did an excellent job analyzing this and finding a solution for the problem. I commend you for it!

It was my schematic. I agree with you on the dots, less ambiguity. They're just a PITA. It's an additional step to add each of them.

[thekorvers]

The schematic looks pretty professional, something a manufacturer would put out. So you reverse-engineered all this. Pretty impressive. .

[tomzarbo]

1

Okay. I'll give Gordy a few weeks to report back.
Thanks again, sir.

TomZ

[tomzarbo]

Dos Post

[Drewdwn93]

Great write-up, I'm glad I found it. I was wondering if there is any way to bypass (or delete) the low pass section altogether (other than having RP2 fully clockwise)? I already have a sub pre-out on my soundcard (SB Omni). I only need the gain because the line-out on the SB Omni is too weak.

[Millstonemike]

Great write-up, I'm glad I found it. I was wondering if there is any way to bypass (or delete) the low pass section altogether (other than having RP2 fully clockwise)? I already have a sub pre-out on my soundcard (SB Omni). I only need the gain because the line-out on the SB Omni is too weak.

Remove C5, C6 and R7. That will effectively make the LP stage a pass through buffer. Your sound card's LF output should be split to to drive both the L & R input channels of the pre-amp. The sound card LF output will need to drive the resulting 5K impedance load of the pre-amp's input.

[Drewdwn93]

So, I'm still leaving R8, RP2, and R6 in the signal path, right? How will the orientation of RP2 affect the circuit after removing the crossover caps and bias resistor? Or does it not affect it at all since it is a unity gain stage? Could I not simply pull my sub out from pin 1, rather than going through the additional unity gain stage? Also, I have no problem changing some of the components/values to make it work with fewer components in the signal path, if it's possible. Also, when you say to split the sound card LF output, can I just use a jumper at LIN and RIN? Thanks for your help, I'm still very new to this stuff.

[Millstonemike]

So, I'm still leaving R8, RP2, and R6 in the signal path, right? Yes. How will the orientation of RP2 affect the circuit after removing the crossover caps and bias resistor? Theoretically, it shouldn't. But 50 K pots are prone to noise. If you like, you can jumper the pins on each of the pot's gangs making it effectively 0 ohms regardless of rotation. You can remove the pot as well with the added jumpers in place (see the pic, below). Or does it not affect it at all since it is a unity gain stage? Could I not simply pull my sub out from pin 1, rather than going through the additional unity gain stage? You'll want to use the existing sub output to take advantage of the decoupling cap, C7. This will avoid DC bias in the signal from entering the next stage / amp (though your sub amp may also have one, it's a good safety measure to keep it in the circuit). Also, I have no problem changing some of the components/values to make it work with fewer components in the signal path, if it's possible. I would leave R6 and R8 in the signal path lest the unity gain stage present a very low impedance load to the output of the variable gain stage. Also, when you say to split the sound card LF output, can I just use a jumper at LIN and RIN? Yes. Thanks for your help, In the words of the immortal Mr. Miyagi, "welcome". I'm still very new to this stuff.

[tomzarbo]

Gordy, did you ever get to test this out? How did it do?

TomZ

[Mike E]

Gordy, did you ever get to test this out? How did it do?

TomZ

X2

if it works like it is suppose to now, there is a sure amp i want to get for some a sub build.