Cheap DIY High Power/Voltage AWG Buffer Amp

[mawyatt]

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,

[mawyatt]

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.

[hthalljr]

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

[Tjuurko]

TDA7293 may have problems on DC due to the use of "bootstrap capacitor".

[mawyatt]

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,

[hthalljr]

Do you think this board might do the trick, using two DC power supplies, as you suggested?

https://www.amazon.com/LM3886-Power-Audio-Amplifier-OPA445/

- Tracy

[mawyatt]

Link doesn't work!!

Best,

[coppercone2]

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

What can you do with AC and a cell?

I am interested in electrocleaning and polishing. I did some crude experiments with a brush connected to power supplies and a variac also. AC seems OK for like pre weld cleaning or something on aluminum to get some crap off.

I would be interested to know what else you can do with AC and electrochemistry. I think its a good rust removal method for mild steel, because it does not make grinding dust, even if it makes some fumes, and its quiet. As long as you neutralize and watch power levels to prevent splatter.

[hthalljr]

Sorry for the dead link. I must not be using the link utility on this message board correctly. Highlighting the link and invoking the "insert hyperlink" icon inserts before it and after it. The  preview option doesn't work for me, nor does the link in the posted message. The only way I can use the link is to copy and paste it into the address field of my browser.

Description, on Amazon: "LM3886 Power Audio Amplifier Board OPA445." It has one input terminal, one output terminal, and a terminal for V-, Gnd, V+, which I would connect to two DC power supplies. The LM3886 is attached to a heat sink having 7 fins and is 52mm wide and 82mm long (the same length as the circuit board). A thin white films appears to electrically isolate the heat sink from the LM3886. The fins appear to be about 25 mm high.  It has two large electrolytic capacitors and one smaller one. The seller provides NO specifications, and there is not yet any customer feedback on the product. Price: $60, which I don't mind losing, if you think it's worth a try.

https://www.amazon.com/LM3886-Power-Audio-Amplifier-OPA445/dp/B0CC25P36T

Per the TI data sheet,
https://www.ti.com/lit/ds/symlink/lm3886.pdf?ts=1712261061713
the LM3886  takes a 20-90 volt input (+- 40 typical) and has a minimum output load of 4 ohms, which is a concern, as my load can go as low as 0.5 ohms. However, the load power is never above 5 watts. The typical application circuit shows total harmonic distortion + noise only down to 20 Hz, with an 8 ohm load. Presumably this is because THD+n is of little interest below 20 Hz and does not indicate a 20 Hz low frequency limit? The heat sink is at V- potential unless electrically isolated.


- Tracy

[hthalljr]

Oh dear, I guess I need to disclose my disreputable purpose!

It's a cold fusion experiment. DC codeposition of palladium and deuterium from a solution of Pd(Cl2) and LiCl in D2O has been shown to produce anomalous excess heat, but it consumes large quantities of palladium, so the deposition has to eventually end to recycle the precious metal. I'm trying to use alternating current between graphite electrodes to first deposit a thin layer of Pd(Dx) on one electrode, then strip it from that electrode while depositing it on the other.  There is so much capacitance in the circuit that at higher frequencies I'm just heating the electrolyte without actually depositing any material. So I calibrate my calorimeter with a sine wave at 1 kHz, where there is no deposition, then lower the frequency until I observe evolution of D2 and O2 gasses, which usually accompany metal deposition. With my setup, I need to go below 5 Hz to see gas evolution. My Fosi audio amp is only rated down to 10 Hz, but I can push it as low as 1 Hz. However, I think I'll need to go as low as 0.1 Hz.

The instrument that allows me to obtain accurate current, voltage, and power measurements over a wide range of frequencies is the "Joulescope."  Until I discovered it, I was flying blind and actually embarrassed myself when I announced a false measurement of excess heat.  (I discovered my mistake when I did a control experiment, substituting H2O for D2O, and got the same result). You can't just measure current and voltage with "true rms" meters and multiply them together, because the capacitance introduces a phase difference between them. The Joulescope actually shows the excursion into negative power when the electrolytic double-layer unloads and returns its energy to the circuit.

- Tracy

[coppercone2]

Interesting I have pdcl2 for via copper plating. After a lot of failures with echem I never bothered to try the dubious process in favor of a eyelet staking kit. I have seen weird phenomena with ball lightning before so I find cons fusion interesting. I don’t have heavy water though

[mawyatt]

Recall Pons and Fleischmann seem to have made similar errors!!

Best advice wrt the LM3866 is get one and try it out, it's cheap enough!!

Yes simply multiplying AC voltage and current, whether rms or not,  doesn't always convey the correct answer. This is because the "assumption" is the voltage (or current) is dissipating into a resistive load. What really matters is the apparent Power Angle, or Real Power and Imaginary Power, where the Real Power does work and the Imaginary doesn't. This is a serious problem with the Power Grid as the Reactive Power (Imaginary) causes losses in generation and distribution but doesn't do work (Real) at the loads. Much effort goes in to correcting the Grid Power Factor (Power angle close to zero) which relates to the quality of the Power involved.

Anyway, search here as we may have designed an AWG Power Amp around the LM3866 and created & procured a PCB for such, although don't recall ever assembling one.

Also, vaguely remember way back a massive meltdown in one the labs involved with researching Cold Fusion here in US and another in USSR. So be careful!!

Keep us posted on your progress!!

Best,

[mawyatt]

We did design a version of the High Current AWG Buffer Amp based upon the LM3866.

https://www.eevblog.com/forum/testgear/hv-buffer-amp-for-awg/msg4939390/#msg4939390

Best,