Audio and Chassis ground in an 8 channel preamp

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

Whoops, good call! I'll have to fix that. Thanks.
JohnP
Posts: 6
Joined: Sun Jun 22, 2014 12:19 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by JohnP »

JR Wrote:
yup base of q8 probably goes to collector q5
Yes, and base of Q7 goes to collector of Q9 (not emitter)

JP
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

Correct again. For what it's worth, this layout is based off the free 2520 schematic that is floating around which after cross checking it with the GAR2520 was found to be almost identical. Obviously that part of the schematic is incorrect. Thanks for taking the time to look into it for me.
Attachments
api-2520-schematic-1-638.jpg
api-2520-schematic-1-638.jpg (62.61 KiB) Viewed 16574 times
JohnP
Posts: 6
Joined: Sun Jun 22, 2014 12:19 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by JohnP »

rp3703 wrote:
Obviously that part of the schematic is incorrect.
Well then, check the connections to C5... (and which value to use)
Also, if you want a real matched pair for Q1-2, look at the SSM2212 (SMT-Only though...)
MAT12 (thru-hole) prob too expensive. MAT02 still available as NOS

JP
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Audio and Chassis ground in an 8 channel preamp

Post by JR. »

I was about to criticize the bias string for having only one diode drop, until i read the parts list...

D1 is actually two diodes in series. :lol:

JR
Cancel the "cancel culture", do not support mob hatred.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

Like I said, that 2520 schematic is nearly identical to the GAR2520. The schematic I drew has all the correct parts(although with the two mistakes listed earlier). I am reworking the layout and will post that with a corrected schematic soon.
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

I re-laid out the DOA portion of the PCB after having the two earlier mistakes. The traces between each component are shorter than with my previous layout. I may still try to work on the output path since it is a little long. Below are the 3D versions of the front and back of the PCB’s as well as the actual trace layout and the corrected schematic. I came up with a solution to my audio -/chassis ground issue. I placed the ground plane(audio -) on the top of the PCB with all the components and the 48V connections which connect to the chassis on the bottom of the PCB. They combine with each other only at the main power connection from the PSU. Don’t know why I never thought of that before. I checked the connection to and from C5 and it is drawn on the schematic exactly like it is laid out on the GAR2520. Thanks for the help finding all the mistakes. Hopefully there are no more.

I also wanted to get some opinion on the power supply connections to the DOA. Since power supply 0V only really needs to run to those four caps, should I move those caps closer to the connector to the PSU? My thought is that this will keep PSU return away from the audio-.

Also, I have my mounting bolts from my output transformer tied to the ground plane. Would it be better to tie that to the chassis ground instead?
Attachments
AMS-312-SCH.jpg
AMS-312-SCH.jpg (189.08 KiB) Viewed 16541 times
AMS-312 B-side.png
AMS-312 B-side.png (72.81 KiB) Viewed 16541 times
AMS-312 F-side.png
AMS-312 F-side.png (105.6 KiB) Viewed 16541 times
AMS-312 Layout.jpg
AMS-312 Layout.jpg (253.35 KiB) Viewed 16541 times
rp3703
Posts: 10
Joined: Tue May 31, 2016 2:55 pm

Re: Audio and Chassis ground in an 8 channel preamp

Post by rp3703 »

So it looks like you were right about C5. I got in a hurry and sent the PCB off for manufacture. When I got it back and put one together, sure enough, it didn't work. After retracing my work, I realized that C5 was supposed to go to the base of Q2, not the collector. Cutting the trace and adding a jumper got it up and running. Other than that and two resistors that were labeled in the wrong spot, I'm pretty proud of myself for not messing up anything else. Thanks for checking my work though. You saved me from a much bigger disappointment than what I ended up with. As far as any grounding issues go, which was the reason I came here in the first place, it is quiet so far. I'll have to get all 8 channels assembled and put it to use for a while to see how it performs. Thanks again for your help.
Post Reply