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Preamp for TPA3251 - Building suggestion
Preamp for TPA3251 - Building suggestion
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Old 31st October 2019, 12:55 PM   #1
cyruz is offline cyruz  Italy
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Join Date: Oct 2019
Default Preamp for TPA3251 - Building suggestion

Hello everybody!

I'd like to build a preamp based on some items that I found online, so it would be nice if I can use some help... I apologize beforehand for the triviality of some question that I'm gonna ask


Preface: the source DAC is an ES9023 and should accept a minimum load of 5k Ohm and the 3e-audio TPA3251 board I own declares 10k Ohm for unbalanced input. My need to have a preamp in the middle is just to protect the speakers. Sometimes I'm in the clouds so I want to avoid damages.

1. I tried to understand if I could use some passive volume control but I'm not able to determine the exact output impedance of the DAC, so it's hard to choose a pot that would be fit for the job. What would be the best alternative with the smallest impact in terms of sound clarity?

2. I found this stereo buffer preamp module (I don't know if I can link the shop), would it be a good alternative to n.1?

Click the image to open in full size.

based on 2 AD OP275 opamps. Specs says: Default Gain of 18dB (modifiable) and Input impedance: 100-130kohm.

How I determine the right gain and the pot impedance? I own a 10k series attenuator (Eizz), do you think it would fit for the setup? Would it be a concrete step-up from an ALPS pot?

3. I know that to minimize interferences I should keep the cable from/to the RCA short and use shielded cables. Do I have to use a STP cable and put the braid to ground on the case or it's better to use a single shielded cable connecting the braid to the RCA ground pin? Do the ground pin of all the RCA connectors have to be joined together and grounded on the case?


Thank you very much for your help!

Last edited by cyruz; 31st October 2019 at 02:38 PM.
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Old 31st October 2019, 08:25 PM   #2
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
The circuit for that preamp has a gain of 0dB or 20dB or 29dB, switchable. Its input impedance is 100k. The circuit diagram shows NE5532's, not OP275, which makes sense as NE5532's are better and cheaper.

Output impedances are nearly always low(*), the DAC's specs say don't load more heavily than 5k (ie 5k or higher input impedance is required for the next stage). It has an opamp output buffer so its low impedance output. The reason not to load it heavily will be to prevent distortion rising above datasheet specifications as the opamps overload.


(*) except valve equipment.
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Old 1st November 2019, 09:27 AM   #3
cyruz is offline cyruz  Italy
diyAudio Member
 
Join Date: Oct 2019
Quote:
Originally Posted by Mark Tillotson View Post
The circuit for that preamp has a gain of 0dB or 20dB or 29dB, switchable. Its input impedance is 100k. The circuit diagram shows NE5532's, not OP275, which makes sense as NE5532's are better and cheaper.

Output impedances are nearly always low(*), the DAC's specs say don't load more heavily than 5k (ie 5k or higher input impedance is required for the next stage). It has an opamp output buffer so its low impedance output. The reason not to load it heavily will be to prevent distortion rising above datasheet specifications as the opamps overload.


(*) except valve equipment.
Hello Mark,

thank you for the feedback. The shop advertises the use of OP275 and a default gain of 18db. Is that wrong information?

Do you think this buffer is worth?
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