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LME49710 Phono Stage
LME49710 Phono Stage
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Old 3rd October 2017, 06:12 PM   #1
simonra is offline simonra  United Kingdom
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Default LME49710 Phono Stage

Hi,
I recently completed my LM3886 chipamp and there's a bit of space left in the box that i think I could squeeze a phono stage into.
I'm going to build a version of the passive eq phono amp described in section 3.1 of this TI application note. The only deviations I'm going to make to the circuit are to reduce the gain of U1 and use a 47R and 300pF across the inputs as I use a MM cartridge.

The amplifier circuit includes U4 & 5 to 'flatten' impedance on the supply rails so I assume I can go for a pretty simple power supply (schematic attached), I've based the design on the following components:

Trafo
Bridge Rectifiers
Caps

My question is does this look like a sensible choice? I'm pretty new to this so I'd be grateful for any advice.
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Old 3rd October 2017, 06:55 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by simonra View Post
The amplifier circuit includes U4 & 5 to 'flatten' impedance on the supply rails
so I assume I can go for a pretty simple power supply.
You will want regulation after the filter capacitor for a phono preamp.
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Old 3rd October 2017, 06:59 PM   #3
simonra is offline simonra  United Kingdom
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Thanks, I was wondering whether I'd need to regulate... plus that means I could go with a higher voltage trafo and regulate down to 15V rather than undershoot.
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Old 3rd October 2017, 08:20 PM   #4
rayma is offline rayma  United States
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Yes, in fact I'd use a CRC filter before the regulator, to reduce the input ripple.
With these op amps, be sure to use good layout and local decoupling.

Last edited by rayma; 3rd October 2017 at 08:29 PM.
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Old 4th October 2017, 12:09 PM   #5
simonra is offline simonra  United Kingdom
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Thanks, I'll look into options for regulating and use of CRC... as it's in the same box as the chipamp it would be nice if I could use the 2x25V 300VA transformer that's already in there. Unfortunately I get 37VDC after the regulator with that which rules out a linear regulator and I've no experience with switching regulators.

As for layout of the amplifier, I've had a stab by making a direct copy of the TI schematic in eagle (which I need to update the gain and input R&C for). There's no local decoupling... ideally I'd like to experiment but I need to work out how I can do that.
Attached Images
File Type: png Amp Schematic.png (41.8 KB, 774 views)
File Type: png Amplifier.png (38.6 KB, 752 views)
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Old 4th October 2017, 01:12 PM   #6
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by simonra View Post
Unfortunately I get 37VDC after the regulator with that which rules out a linear regulator and I've no experience with switching regulators.
(assuming you meant 'rectifier' here where you wrote 'regulator')

If you choose a variable regulator like the venerable old LM317T then 37V input isn't a problem if your output is 15V (or so). The regulator is specified for a maximum input-to-output difference of 40V. Shorting the output might cause you to exceed that difference though - I'd put some resistors in series (as already suggested) prior to the reg's input terminal to mitigate the risk.
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Old 4th October 2017, 01:25 PM   #7
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by abraxalito View Post
(assuming you meant 'rectifier' here where you wrote 'regulator')
Doh!.. Yep I meant rectifier. Thanks I'll take a look at those regulators.
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Old 4th October 2017, 03:29 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by simonra View Post
As for layout of the amplifier, I've had a stab by making a direct copy of the TI schematic in eagle
(which I need to update the gain and input R&C for). There's no local decoupling.
The supply buffers have a 1uF capacitor at each output. Arrange the placement of the buffer and capacitor
to be close to the supply pin of the RIAA op amp. Also add a parallel 0.1uF - 0.01uF directly at the supply pin.
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Old 4th October 2017, 03:38 PM   #9
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by rayma View Post
The supply buffers have a 1uF capacitor at each output.
Yes... of course, thanks. I was looking for decoupling caps at each of the op amps and missed the obvious. I'll shuffle things around and see how close I can get them.
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Old 4th October 2017, 07:35 PM   #10
simonra is offline simonra  United Kingdom
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Thanks for the advice so far... I've reworked the schematic & layout as suggested by adding two pours for the power supply on the bottom layer and moving things around to get the RIAA op amp closer to the buffers & caps. Does this look like an improvement?
Attached Images
File Type: png Power Supply 2 Top Layer.png (45.0 KB, 680 views)
File Type: png Power Supply 2 Bottom Layer.png (27.8 KB, 654 views)
File Type: png Amp Schematic.png (44.0 KB, 215 views)
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