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Old 19th December 2015, 09:14 PM   #1
SGK is offline SGK
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Default Self's "5532 Low Noise Unity Gain Balanced Input Stage"

Howdy

I have been working on a board for Self's "5532 Low Noise Unity Gain Balanced Input Stage" (Fig 18.28 of Small Signal Audio Design 2nd Ed.) but using the lower noise quad LM4562, i.e. the LME49740. Attached is a pic of my circuit (right channel; the left channel is the same obviously).

My circuit incorporates unbalanced input as well. I am aware that there will be a volume mismatch between balanced and unbalanced inputs but this is something I am prepared to live with. (I do not expect inputs to be regularly swapped back and forth between balanced and unbalanced and so any relative volume difference won't be an issue.)

I have a question regarding the grounding of the HF filters etc before the unity gain buffers (U$1 and U$2 in the circuit). The discussion in Chapter 18 of the aforementioned text has each of C1-C2, R3-6 (references to part numbers in my circuit) simply grounded. It seemed sensible to me (a novice) to ground, at least, the HF filters R1/C2 and R2/C1 via Pin 1 / Pin GND on the XLR connector, i.e. directly to chassis, rather than letting any RF enter the circuit (signal) ground. I just continued to ground R3-6 the same way. I am, however, unsure whether this is a sensible approach. Perhaps I can get some "best practice advice" from people here.

I continue to fine tune the board layout/routing, but I have attached a pic of where I have things currently. The board is 5cm x 120cm and the idea is that all the connectors are hard-wired to the board and so support it in place. I have endeavoured to keep signal routing to the top layer where possible and the power for the quad op amps on the bottom layer (each using the other layer only where necessary). I felt a four layer board necessary (well, three layers but it seems that's rare) to get a decent ground plane. Therefore, the middle two layers will be ground planes (not yet added in the board so as to not clutter the pic I have attached). All vias extend through all layers and so vias from top or bottom layer to GND will link both of the middle layers.

If anyone has any constructive comments on the layout I would appreciate that as well.

Thanks in advance!

Steve

PS: I know the LME49740 is 'end of life' and stock from Mouser disappeared over the last couple of days, even while I had some in my basket. However, I have enough for my purposes and in any event perhaps the LMP8674 is a worthy substitute (it seems to have even better noise specs).
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File Type: png Balanced line input circuit.png (73.9 KB, 1314 views)
File Type: png Balanced line input board.png (65.7 KB, 1266 views)
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Old 19th December 2015, 10:09 PM   #2
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I use KiCad PCB and find that if I route the supplies and grounds manually, this vastly improves the odds that I won't despise the final flooded and filled "planes" after autorouting. I just use min linewidth traces for the hand routed supplies, and let the flood fill make em fat, later.

For example if I connect up all the ground pins with hand routes on the green layer, then when I flood-fill the green layer with ground, I know that there won't be any pins that connect to ground through long meandering traces with one or more vias.
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Old 19th December 2015, 10:58 PM   #3
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Given the power supply is separate you shouldn't have many problems with ground loops.
A pre amp is low current so doesn't generate much ground bounce.

You can use a copper pour to fill in the ground plane or not, I suspect it wont make much difference.
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Old 20th December 2015, 02:06 AM   #4
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Why the lmp8674 vs the opa1612? I realize it's a bit more $$ and not a quad, but is that an essential part of the design? Seems a more performant part.
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Old 20th December 2015, 02:17 AM   #5
dewardh is offline dewardh  United States
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Why bother? Why not just use a THAT 120x chip and be done with it?

THAT IC Selection Guide
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Old 20th December 2015, 08:08 AM   #6
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@Mark, I can do the same in Eagle but if you look at all the traces in blue (bottom layer) from power and signal traces these slice up any bottom-layer ground plane and hence I thought it better to have a separate ground plane(s) on the internal layers.

@nigel, perhaps you are right but boards are so cheap I was thinking 'play safe'

@derfnofred, I haven't looked at the OPA1612 but a quad saves an enormous amount of board space and routing complexity. There's also op amps such as the AD797 (I have some limited previous experience with this rocket fuelled op amp) and the LT1028. The only quad I have found with specs similar to the venerable LME49740 (LM4562 quad) is the LMP8674 but I have not done a particularly extensive search as I have a bunch of the LME49740.

@dewardh, I started thinking about the THAT products having read the material on Elliot Sound Product's website but was steered heavily away from such implementation as being 'dated' and 'not low noise'. Have you read Self's material? The goal is to get a balanced input stage that has a noise level as good as or better than an unbalanced input.

Anyone have a view on my grounding to XLR Pin 1 / GND / Chassis versus to 'regular (signal / circuit) ground' question? (If you look closely at the middle left of the circuit you'll see I have labelled the ground differently, as PE, allowing me to route it separately from GND elsewhere (RCA unbalanced input shield, power GND, GND at voltage dividers ahead of differential amplifiers).)
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Old 20th December 2015, 09:29 AM   #7
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Dear Steve,

don't forget to separate the ground plane for each channel to be able to do a dual mono amp.

BR, Toni
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Old 20th December 2015, 09:38 AM   #8
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Hi Toni. Does that require using two separate power supply entry points (and caps) given 0V is brought to the circuit from the PSU? Presumably it does.
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Old 20th December 2015, 09:40 AM   #9
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Quote:
Originally Posted by SGK View Post
My circuit incorporates unbalanced input as well.
I think you should make a provision to ground the unused input pin when you use the SE mode.
Through a resistance representing the SE source R expected.
Otherwise you'll muck up the CMRR which, in a SE circuit, really means changing the gain.

jan
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Last edited by jan.didden; 20th December 2015 at 09:42 AM.
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Old 20th December 2015, 09:46 AM   #10
astx is offline astx  Austria
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Quote:
Originally Posted by SGK View Post
Hi Toni. Does that require using two separate power supply entry points (and caps) given 0V is brought to the circuit from the PSU? Presumably it does.
Think if you would build a mono amplifier. Put two of them in one case. Dual mono!

BR, toni
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