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A simplified universal differential or single ended phono preamp
A simplified universal differential or single ended phono preamp
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Old 29th July 2018, 06:11 AM   #301
EUVL is offline EUVL  Europe
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DIY is hobby that takes time (to enjoy).

I decided to discard the heatsink and dump the heat to the case instead.
So on the "back" side, I added a board for PS caps and R-C for the output followers.
The side facing the preamp is of course a flooding Gnd plane.


Patrick

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Old 7th August 2018, 02:25 PM   #302
mondogenerator is online now mondogenerator  England
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Quote:
Originally Posted by Destroyer OS View Post
Um, no.

RCA ground to chassis is a complete failure a lot of the time. It really depends on whether it's to the input or output, and what's between them. "pin 1" is a failure of an argument if you think every RCA ground should be direct to chassis. It just isn't that simple.
For what it's worth (coming from a relative novice)

I always use isolated RCA connectors, if I'm going to use them at all.

Ground currents are far more easily routed.
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Old 16th September 2018, 05:23 PM   #303
MagicBus is offline MagicBus  Greece
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Quote:
Originally Posted by scott wurcer View Post
This is a universal differential or single ended output to compliment the phono front end. Bob Cordell used IC op-amps to provide gain and additional RIAA time constants here only gain is needed. It has been suggested that the impedances of the original front end could be scaled to get 40db of gain directly. Since this involves a lot of recomputing and checking I leave ths as a later experiment.
For the first circuit the options are:
1) Simply use one of the outputs with choice of phase. I recommend gain of 5 (14dB) to bring the gain to 40dB. Of course you could use just one 1512 for max economy.
2) As shown this is fully differential in and out so the gain is now 46dB total. This also has a 3db noise advantage, the 1512's are effectively in parallel for noise since adding uncorrelated noise does not care about phase.
3) Not shown but it is possible to add a simple servo from each in-amp output to the reference pin (5), a simple op-amp integrator will do.
The second circuit sums the two in-amps with no extra components for a dedicated single ended output, giving again 46dB of gain and 3dB less noise. All these options could easily be a few jumpers or dip switch. We published this circuit as a design idea in EDN over 30yr. ago and it actually won the $150 best of prize!

Hello,
I hope my question is not way off topic... I'm trying to build a high gain stage with the THAT INAs as shown in the above post. I've already built a quick prototype and I need to address the dc offset. It is very likely to end up with coupling capacitors and there is no problem with that but since I'm in the designing phase I thought to try a DC servo as well. I found a schematic supposed from an older datasheet - attached - but my knowledge only allows copy and paste so, I would like to ask if this is the right circuit to use. Also, it would be more practical to use a double opamp in DIP8. I would appreciate any recommendations.



Thanks in advance.
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Old 16th September 2018, 08:08 PM   #304
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by MagicBus View Post
Hello,
I hope my question is not way off topic... I'm trying to build a high gain stage with the THAT INAs as shown in the above post. I would appreciate any recommendations.
Try the circuit in the app note, it works well. The exact op-amp you use is not going to matter that much.
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Old 16th September 2018, 08:28 PM   #305
MagicBus is offline MagicBus  Greece
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Nice! Thank you very much!
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