Salas hotrodded blue DCB1 build

[Salas]

Originally Posted by KT

Awesome! Thanks.

I will look at the board closer when I have it in front of me. Can it be done with one straight cut across the board, or are there components that need to be cut around?

This is good new for keeping those LDRs cool while reaping the benefits of the hotrodded power supply.

Great!

Like in the attachment. Feed 15V DC at the 100nF cap's pins under its pads for the output relay to work. Even raw DC will do. Watch the polarities by following the whole board as depicted. Feed +10-0-10VDC regulated under nearest pads that used to continue from the "Vout" auxiliary feed out taps connector. Cut carefully, use a vise, don't listen to any Cat Stevens while cutting. Its a relatively thick good quality FR4 board.

[mantha3]

I got my PCB1 from Teabag and I'm waiting on the kits so I can buy a kit from teabag.

Question - Advised to use a 10K Attenuator. Are you to run the Input RCA to the 10K Attenuator and then the Attenuator to the PCB1 OR are you to run the Input RCA to the PCB1 and then the PCB1 to the Attenuator?

I think the Attenuator run to the PCB1 and the PCB1 out. The thing I'm confused about... I would think you'd want a high 100K Attenuator in front of the PCB1 and if the attenuator was after the PCB1 then the 10K makes sense...

you want high input impedance so the buffer is easy to drive (so a 100K attenuator makes sense). You want low output impedance so the buffer can drive hard (so a 10K attenuator makes sense after the PCB1)

[Salas]

Its a DCB1. You need RCA in to 10K-20K pot in front of the DCB1. Much high impedance pot will interfere with the JFETs capacitance and will compromise rise time spec. Its an audible degradation.

[mantha3]

I hope I don't upset anyone saying this... Not trying to offend. I am coming from running tubes. I was thinking a plus of a buffer was a higher input impedance...

I have a source that has an ouput impedance higher than the 10-20K... Perhaps this isn't a good fit? A buffer with this low of an input impedance seems odd.
input Impedance: 200K Ω / output impedance: +/-180 Ω is something ballpark that seems better for a buffer... Say you have a source with a 10K output impedance... with the 10 to 1 rule you'd want the input impedance at 100 K I thought.

I am learning more about impedance... I'm pretty new. Not trying to offend here.

Thanks for the time Salas and all

[mantha3]

More on my question in regards to the 10 to 20K input impedance possibly being low...

A BURSON buffer has Input impedance: 500 KOhms with Output impedance: line out 15 Ohms @ line level. This input impedance makes more sense to me... Not trying to sell or say the Burson is superior... Just noting the input impedance difference.

I like what the PCB1 has on the output impedance.. Just wonding if I have any way to get the input impedance up or if I'm worried about something that I should not worry about.

[Salas]

It has 220K input impedance as a buffer stage. When using a pot that puts series impedance in the equation, then it has an optimum range. In that case when needing attenuation also, its best to feed the high impedance source directly to DCB1 and use a 5K pot on its output. DCB1 can have 1 MegOhm even if the 220K input to ground resistors are changed to 1M. Any buffer won't have more input impedance than the pot if used before it. Here its the high capacitance JFETs used that pose a best bandwidth pot value limit VS BJT input buffers.

[mantha3]

Ah, ok

220K input impedance sounds excellent! I'm kind of new to this as you probably can tell by now. Thanks again for the help Salas.

Just so I don't screw something up like I'm known to do

To get the 220K:
1 - RCA inputs wire to DCB1 Input
2 - DCB1 ouput wire to 5K pot/attenuator and pot/attenuator wire to RCA outputs

I do this and won't get the audible degradation you explained above?

Thanks again. You are quick!

[Salas]

Yes do that. And let us know how it went for you.

[mantha3]

One more question... I'm using this as a pure buffer... So I actually don't really want the attenuator at all.. Just a high input impedance like the 220K and then the output as low as possible.. I plan to build this with two pairs of RCA outs to drive two amps.

skip the pot entirely? tweeks to the PCB parts?

[Salas]

Skip the pot entirely then. No tweaks needed.