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A few questions on building up an amp
A few questions on building up an amp
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Old 22nd February 2019, 08:34 AM   #1
isnotdynamic is offline isnotdynamic
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Default A few questions on building up an amp

I have a vague plan to assemble an amp/DAC from parts I have here and some bits I've ordered. I'm pretty clueless on electronics so I need some experienced eyes to make sure I don't do anything too dumb.

What I have is a USB DAC, A hybrid ladder/series stepped attenuator, a transformer and I ordered a pair of TDA7293 mono amp boards with on-board rectifiers along with some assorted bits to complete everything.
I want to use the dac and attenuator as the preamp feeding the TDA boards.
Here is a pic of the attenuator

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here are the ebay amp boards

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The goal is Computer > DAC > Attenuator > TDA7293
The transformer is a 160VA dual secondary center tap toroidal. I assume I can run the amp modules in parallel from the transformer? Eventually this will have a small computer like a raspberry pi in it as a self contained music unit feeding the DAC but for now I just need to know if this will even work as described.

Last edited by isnotdynamic; 22nd February 2019 at 08:58 AM.
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Old 22nd February 2019, 10:48 AM   #2
avtech23 is offline avtech23  Australia
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A few questions on building up an amp
Don't see why it couldn't work.

What voltage transformer?

One thing - don't be tempted to power up the amp modules without heatsinks attached....

There is some useful information here: Project 127 (and plenty of learning material if you are interested in learning a bit more).
The ever growing project list.. :NAP140 Clone, NAD 3020i & 3225PE, JLH1969, TDA2050, Fender 5F2A Princeton, NE5532 Pre, SHPA. : VSQC Phunk Style, Retro 50W, Filmosound 601. : Apex AX-14, TGM-10, TDA7265, LM4780, B1 Korg (Last in line)...maybe
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Old 22nd February 2019, 11:17 AM   #3
isnotdynamic is offline isnotdynamic
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The transformer is rated at 2x 28V 2.6A, just below the limit for the TDA7293.
I have the heatsinks ready as well as a fan, just in case. I was going to use the transformer with a pair of lm1875's but the voltage would be way too high after rectification and it was only $10 to get a pair of TDA7293 boards. Doesn't need a separate rectifier either, so extra saving!
I'm trying to use up some of the stuff I have laying around and make something useful with it.
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Old 22nd February 2019, 10:14 PM   #4
indianajo is offline indianajo  United States
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Your transformer has two 28v @ full current windings. Your boards have two Ac inputs with one common. So the boards are set up to have the windings set up as a 56 V ct transformer (two center windings tied together & connected to com). As peak voltage is 1.4 X the full current rating, the voltage of this transformer would be -39 0 39 peak minus 1 for the rectifiers. The datasheet says the IC is good for +-50v. However your 2.6 A would be shared between the two amp boards. 1.3 A x 56 v would be 72W input, less on the output.
The details of your mounting will determine whether this project will oscillate or produce music.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
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Old 23rd February 2019, 02:57 PM   #5
isnotdynamic is offline isnotdynamic
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As these boards are suitable for anything from 12vac up, I'm going to run the transformer wired for 240v so It'll have 2x 14.2v outputs. After rectification it should be around 20v or so with the same 2.6Amps if I understand things correctly. I think that should be a good compromise for now and keep things stable while providing sufficient output for my needs until I get a better suited transformer.
Does this sound like a workable solution?

Last edited by isnotdynamic; 23rd February 2019 at 03:03 PM.
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