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Laying out preamp pcb
Laying out preamp pcb
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Old 4th December 2017, 10:46 PM   #1
belyakove is offline belyakove
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Default Laying out preamp pcb

Hi, I'm currently in the process of laying out my PGA2310 preamp pcb. I've done analog part so far and only digital connections to the board and mute relay driver left but digital connections are easy stuff to layout I think.

The circuit goes like this: (4 inputs with RF filters) -> (DG409 input switch) -> (inverting buffer with dc servo(second half of OPA2134)) -> (PGA2310 volume regulator) -> (inverting buffer to bring phase back to normal) -> (output (with mute relay))

I've read a lot of recommendations and data sheets on the PCB design but it's my first PCB and I would appreciate any critics and comments on the design.
Thank you.

P.S. Why I used smd capacitors and resistors but through hole chips is because I already have these chips. I've built the circuit on breadboard first. And PGA2310 alone costs around 30 EUR so I don't want to just throw it away.
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File Type: jpg preamp.JPG (762.7 KB, 215 views)
File Type: jpg preamp2.JPG (675.1 KB, 213 views)
File Type: jpg preamp3.JPG (597.5 KB, 208 views)

Last edited by belyakove; 4th December 2017 at 10:49 PM.
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Old 4th December 2017, 10:59 PM   #2
rsavas is offline rsavas  Canada
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A schematic would not hurt for a review :-)
1) 22K in series with the DG409, why?
2) sot-23, across the supplies?
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Old 4th December 2017, 11:15 PM   #3
belyakove is offline belyakove
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Quote:
Originally Posted by rsavas View Post
A schematic would not hurt for a review :-)
1) 22K in series with the DG409, why?
2) sot-23, across the supplies?
Unfortunately I don't have the schematics in digital form but will try to draw it soon.

1) 22k resistors is the input series resistors for inverting buffer that comes after DG409. Together with 22k in opamp's feedback forms a regular inverting circuit.

2) sot-23 is a pair of diodes on the input to prevent static electricity getting into IC

Basically input stage is the copy of input of one commercially available amp except that I've replaced 2 separate DG408 to a stereo DG409. I've attached schematics of that amp.
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File Type: png Screen Shot 2017-12-05 at 00.09.19.png (93.5 KB, 192 views)
File Type: png Screen Shot 2017-12-05 at 00.09.28.png (348.8 KB, 195 views)
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Old 4th December 2017, 11:36 PM   #4
rsavas is offline rsavas  Canada
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Okay, i see what your doing now.
DG408DG is in a soic pkg.

good luck
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Old 5th December 2017, 12:18 AM   #5
belyakove is offline belyakove
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Here's the schematic of what I'm trying to build. I've displayed only one input but other 3 are the same and connected to corresponding DG409 pins. Also I've displayed only analog part for now.
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File Type: pdf New-Schematic-2.pdf (46.5 KB, 72 views)
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Old 5th December 2017, 03:40 AM   #6
Monte McGuire is offline Monte McGuire
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Laying out preamp pcb
Looks like a reasonable design. A few details to check - I noticed that the + input terminals on U5.2 and U5.1 are not connected. Tie them to ground. Also, if U2.3, U3.3 and U5.3 are unused, tie their inputs to ground as well - even better, tie output to -input and +input to ground. That might be an artifact of the incomplete drawing that you warned about, but still U5.1 and U5.2 need to be checked.
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Old 5th December 2017, 02:11 PM   #7
belyakove is offline belyakove
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Quote:
Originally Posted by Monte McGuire View Post
Looks like a reasonable design. A few details to check - I noticed that the + input terminals on U5.2 and U5.1 are not connected. Tie them to ground. Also, if U2.3, U3.3 and U5.3 are unused, tie their inputs to ground as well - even better, tie output to -input and +input to ground. That might be an artifact of the incomplete drawing that you warned about, but still U5.1 and U5.2 need to be checked.
U2.1, U2.2 and U2.3 are parts of the same chip U2 displayed separately for convenience. So all op amps' inputs are connected. The only thing I've missed is the connection on pins 5 and 3 of U5 to ground, but on pcb they are connected to ground.
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Old 5th December 2017, 04:56 PM   #8
rsavas is offline rsavas  Canada
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What are you using to control this pre-amp?
I have some BASCOM-AVR code to offer for this type of design. Written for a Atmel xmega mcu, a character LCD, bourns encoders. Also have code for IR remote too.
Why not use NPO/COG MLCC caps for i/p filters instead of TH?
Why not place the RCA i/p jacks on the pcb? I like designing stuff that uses minimal wiring.
What ecad s/w are you using? Now I see EasyEDA, in the sch title block. I can review your gerber/drill data once you are done.
10uF filter caps could be MLCC instead of tantalum = more reliable, lower ESR, cheaper too.
Could also place some small R(100-600) at the o/p of U5 before the relays. I do wonder what it will sound like(pop/shock noise) if you directly short these o/p's. the PGA2310 mute function might be sufficient without resorting to the relay technique. Always nice to have options available when prototyping

Last edited by rsavas; 5th December 2017 at 05:19 PM.
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Old 5th December 2017, 11:18 PM   #9
belyakove is offline belyakove
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Quote:
Originally Posted by rsavas View Post
What are you using to control this pre-amp?
I have some BASCOM-AVR code to offer for this type of design. Written for a Atmel xmega mcu, a character LCD, bourns encoders. Also have code for IR remote too.
Why not use NPO/COG MLCC caps for i/p filters instead of TH?
Why not place the RCA i/p jacks on the pcb? I like designing stuff that uses minimal wiring.
What ecad s/w are you using? Now I see EasyEDA, in the sch title block. I can review your gerber/drill data once you are done.
10uF filter caps could be MLCC instead of tantalum = more reliable, lower ESR, cheaper too.
Could also place some small R(100-600) at the o/p of U5 before the relays. I do wonder what it will sound like(pop/shock noise) if you directly short these o/p's. the PGA2310 mute function might be sufficient without resorting to the relay technique. Always nice to have options available when prototyping

I'm using Arduino Nano to control the preamp. All code is already written for it including LCD, IR, encoders.

For input I've copied one commercial amp and it uses film capacitors here. This is the main reason. AFAIK capacitance of film capacitors is more stable through frequency range than ceramics and it's important in filters. Is it true?

I thought about placing RCAs on the board and it makes the most sense, I understand that. However I decided to stay with wiring for now to have more freedom with mounting inside the enclosure. I might consider actually putting RCAs back on the board. However one of the inputs will be connected by wires anyway because it will come from DAC mounted in the same enclosure.

I've used EasyEDA only for schematics. For laying out PCB I've used Sprint Layout 6.

I've used 10uF tantalum caps here as it is stated in the PGA2310 data sheet as the decoupling solution. Maybe they just used the cheapest caps that do the job, I don't know. Just used what was in the data sheet for IC.

According to OPA2134 data sheet it is short circuit "friendly". However I just checked the schematics of commercial amp that I've mentioned before and they indeed have 47R resistors between IC and relay. I will also put it there. However I've evaluated solution on a breadboard and it works without clicks and pops.

Mute pin of PGA2310 is not sufficient to eliminate of/off thump which is nasty with this IC. I've tried a lot of solutions for muting and relay one appeared to be the most reliable way to do this. It allows to make muted state the default even with no power. Signal from Arduino is needed to unmute the circuit in this case. There was a separate thread n this forum where I was fighting this thump.

Keep in mind that I'm just a novice hobbyist so everything I wrote is based on my current basic knowledge and "beliefs" and I don't claim it to be true so any critics that I can use to learn something new is appreciated.

Thank you very much for your help.
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Old 6th December 2017, 12:03 AM   #10
ohdsp is offline ohdsp
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Some random thoughts...they may or may not help!

Circuit is a little unusual, looks like the inputs switches are inside the input/feedback circuit on the opamp. You have multiple 22k resistors on the DG409 input. I would have put one 22k on the DG409 output and remove the other 22k, or replace with 0R resistors. I guess without any other reference to ground on the op-amp, given the 100nF on the positive pin to ground, that when the DG409 switches or when you power on/off it might make the on/off thump worse? Just a guess though and could be wrong.

U5.1 and U5.2 really need series resistors in the outputs (47 to 100ohms) if they are driving cables.

The integrators are interesting instead of using DC blocking caps, although you may still end up with some small offset voltage on the output which may be an issue depending on what you are driving with the outputs.

Film capacitors will be better at the values you are using that equivalent ceramics (X7R/X5R which change over voltage, frequency etc). Tantalum capacitors will be fine but they tend to be more expensive, any 10uF-100uF aluminium electrolytic would work as well.
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