Author Topic: Cheap DIY High Power/Voltage AWG Buffer Amp  (Read 1524 times)

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Offline mawyattTopic starter

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Cheap DIY High Power/Voltage AWG Buffer Amp
« on: February 05, 2021, 07:36:25 pm »
For those looking to buffering the output of an AWG to higher levels of power and voltage with a cheap DIY solution, here's an option. It's based on the cheap TDA7293 audio amplifiers that cost ~$5 on eBay. This is basically a PCB copy of the basic schematic from the TDA7293 data sheet, but includes a bridge rectifier and a pair of 2200uF filter capacitors rated a 50VDC. You can power the PCB from an AC source (center tapped transformer), or use a dual DC supply like we did with our new SPD3303X-E (our first use of this triple output lab supply).

This amp is designed for audio use it has an AC coupled input with a low frequency cutoff of ~7Hz and the feedback is arranged for unity gain DC, but with an AC gain of 33X with a lower cutoff frequency of ~5Hz. The overall low frequency corner of ~4Hz, and the output is DC coupled. Boot-strapping is employed to allow higher + output voltage swings but this has a lower corner frequency of ~3Hz where the output limits by ~2.5V from peak. Amplifier bandwidth is limited to ~100KHz on the high end since this is a core audio amp chip, so don't expect higher frequency performance.

The TDA7293 also has a nice turn ON/OFF feature which ramps the output signal up slowly during Power ON and downward even slower during Power OFF without introducing any harsh glitches or significant DC offsets as shown below in next thread.

We modified the PCB to allow direct input DC coupling and remove the AC coupled feedback so the gain extends to DC, which is required for a good AWG buffer amp to handle the various waveform including DC. Now this amp handles all sorts of waveforms with DC offsets, and/or just direct DC.

The modification is very simple with just 2 soldered jumpers as shown in the image in the mid upper right PCB backside where two short wires are soldered across the component leads.

Considered the TDA7293 cost ~$5, this PCB audio amp seems a bargain!!

Anyway, hope this is helpful for anyone wanted to buffer the AWG for higher power/voltage use.

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
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Offline mawyattTopic starter

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Re: Cheap DIY High Power/Voltage AWG Buffer Amp
« Reply #1 on: February 05, 2021, 07:38:56 pm »
Power ON and OFF output waveform characteristics.

Best,

Edit: Added a couple Bode Plots, third 10-1MHz, fourth 10-10MHz, 5th is a pulse response, and 6th 2-Tone IMD with both tones @ 10Vrms.
« Last Edit: February 06, 2021, 02:00:36 am by mawyatt »
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
The following users thanked this post: THDplusN_bad, ch_scr, macaba, Bad_Driver

Offline hthalljr

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Re: Cheap DIY High Power/Voltage AWG Buffer Amp
« Reply #2 on: March 25, 2024, 07:21:10 pm »
Thanks, Mawyatt, for this tip: it just  might work for my application.

I have an experimental electrochemical cell that I'm driving with a variety of AC waveforms. Depending on temperature and concentration, the cell has a resistance of 1 to 3 ohms, with considerable capacitance. I've been driving a Fosi Audio TDDA7498E amplifier with a Siglent SDG1032X arbitrary waveform generator, mostly sine and square waveforms, from 1kHz down to 5 Hz, .5 to 5 amps RMS, maximum peak-peak voltage of +- 6. Below about 20 Hz there's considerable distortion of a square wave, even into a pure resistive load, but I'd like to extend the frequency range down to DC. Fosi engineers inform me that their amplifier can't be modified for DC operation.

I found a TDA7293 amplifier board on Amazon that appears to have a layout identical to the one you used:
 https://www.amazon.com/TDA7293-Digital-Amplifier-Channel-12v-50V/dp/B07KNV8SDG.

Questions:
1. Will your modification allow the amplifier to work into a load as low as 1 ohm?
2. You show DC +, neutral, and - connections into the three-terminal board input.  How do I connect an AC supply?
3. Will this transformer do the job? 110/220VAC to 30VAC 3000mA 3A Center Tap Power Transformer 15V-0-15V.
 https://www.ebay.com/itm/125642129393

Thanks,
Tracy Hall Jr
Provo Canyon, Utah

 

Offline Tjuurko

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Re: Cheap DIY High Power/Voltage AWG Buffer Amp
« Reply #3 on: March 26, 2024, 03:31:07 am »
TDA7293 may have problems on DC due to the use of "bootstrap capacitor".
 

Offline mawyattTopic starter

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Re: Cheap DIY High Power/Voltage AWG Buffer Amp
« Reply #4 on: March 26, 2024, 03:23:15 pm »
You will need a transformer of at least 6 amps rms to supply a 5 amp squarewave, likely more like 7 amps minimum. You'll also need a very large filter capacitor for the supplies, much more than the 2200uF on the PCB.

Also you'll need a massive heatsink, which the Amazon link doesn't show, the one we utilized has a small heatsink, too small for your requirements tho.

As mentioned above, the TDA7293 uses a "Bootstrap" technique to allow higher output voltages relative to the supply voltage, this won't work well at lower frequencies and certainly not a DC. Recall you'll need to remove the input AC coupling to the TDA7293 for DC use, pay close attention to the input DC offset. Also note the true power rating, it's not 100 Watts!!


Study the data sheet carefully as these weren't designed for DC operation, so likley some issues will arise.
https://www.st.com/content/ccc/resource/technical/document/datasheet/4f/18/a6/c8/21/33/41/8d/CD00001887.pdf/files/CD00001887.pdf/jcr:content/translations/en.CD00001887.pdf


My recommendation based upon these limitations and your requirements is to get a pair of 8~10 amp high current bench power supplies (the cheap SMPS types will probably be OK), one for the + supply and one for the - supply. Adjust the current limits to just over the expected load current and the voltage limits just enough to supply the load voltage at the load current, this should minimize the TDA7293 power dissipation.....which will still be quite high and require a massive heatsink.

You should consider getting a few of the TDA7293 PCBs as likely you'll smoke a few, they are cheap tho :-+

Also, take a look at the LM3886, this is a DC rated Power Amp type.

Good luck!!

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 


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