60 WPC Amplifier for DIY Turntable Motor Drive

Got a couple of TDA7293 apms from advised eBay guy and I can say it is not a good quality of assembly and also packing is not nice either. TDAs got bent on all units and I got one that was not soldered straight. I desoldered and soldered all back. Also changed PSU filter caps to better quality once and bit higher capacity (4700uF instead of 2200uF). R12 (Pyramid's advice) changed to 8.6k (instead of 22k)and its position marked in attached image. What I'm sayng, its very affordable, but requires some fixes.
Although, I was surprised by sound quality of these amps during my test. Very interesting.
Now is my question.
I found very nice 100-120VA toroid trany in my junk with two outs: 26.2v-0-26.2v and 0-14V. I can use 14VAC to make 12VDC for SG4. Question is if I can use 26.2V instaed of 24v for TDA amps? Any negative side?
 

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Question is if I can use 26.2V instaed of 24v for TDA amps? Any negative side?

The TDA7293 is rated for 120VDC/±60VDC. 26.2VAC dual secondary will give you ~ ±40VDC. The amp may run a little warm as the DC rails are higher than they need to be, but it should work. I would use a large heat sink for the amp.

The 14VAC winding will give you ~22VDC to supply the SG4. Use a 12V regulator between the SG4 and the supply or add a decent heat sink to the 5V regulator on the SG4.
 
The TDA7293 is rated for 120VDC/±60VDC. 26.2VAC dual secondary will give you ~ ±40VDC. The amp may run a little warm as the DC rails are higher than they need to be, but it should work. I would use a large heat sink for the amp.



Thank you a lot. I used same heat-sing as your enclosure has for 2 hours. Temp is stabilized after about 40 min and raised by 5-6 deg C above ambient. I measured on the chip. It is “cool” small amp. Thanks for so quick answer.
 
Just to highlight my recent trials and tribulations of this TDA8950 Class D amp module for SG-4 speed controller.
I'm not exactly happy with this module and how its performing.
With what I have on hand to rig up this module was initially a 22V AC input, then 24V input direct to the board. At these voltage input, the DC rail is anywhere from 33VDC to 36VDC. This is on the high side, necessary or not, or testing with a oversized VA transformer isn't issue. It should work as designed out of the box but apparently it didn't. If there's ONE advantage of this model of amp board is direct AC supply input from transformer that simplify everything. The dangerous part is there's no safety measures such as fuses or anything in case anything goes awry. I have to rely on the board's relay or whatever protection circuitry on board.
With high AV voltage (22-24VAC) in, the LPF coils become finger hot and the main heatsink is very warm to finger hot after sometime in my tropical weather. On the good side, it works but I seem to think its a little over than expected. DC offset is also more than 90mV per channel and not a good thing. I also had to replace the volume pot to 10K and mono the inputs since its a mono input from SG4. I now use a 12V twin primary stepped up to 230VAC.
This is when the perplexing start to appear. Its apparent to me this module of mine do not like DC rail voltage in excess of 33VDC. The output voltage on each channel register alright with no load. When connect to single 12V winding of a twin wound toroidal (it was 15V on another test tranny) the output will build up to 230VAC and stay. Next is to connect each winding to each L&R channel, switch on the SG-4, will oscillate and shut down or a clicking sound maybe faintly heard. Its like there exist some short circuit. Next is to connect output in BTL mode, it works and constant voltage is stepped up. After this was to input a smaller mains transformer 15-0-15 to the AC input resulting with 22VDC on the DC rails. The same consequence occurred, only working in BTL mode but not in stereo mode. Its noticed that it doesn't matter if the AC input is 15 or 24VAC, no further adjustment is needed and 230VAC is still maintained. Of course, with 15VAC supply input, it seemed performing stable and lukewarm temperature. Again, I think this module do not like high DC rail voltage for it to operate. I didn't exceed any specs.

For now, I can't establish whether this TDA8950 module have some quality issues, shortcomings in the design, incompatibility for transformer output drive or flawed quality altogether. Its just my observation so far.
Considering all the above, I think I'm going to ditch this TDA8950 as a suitable amp for the SG4. This is one weird animal. Maybe its just mine.
 
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If you look at the datasheet for the TDA8950, it might explain what is happening (section 8-4, page 11). ±36VDC is near the limit for the device, and when operated in SE mode, power supply "pumping" occurs which will increase the supply rails and push the device into OVP. The inputs and outputs should be connected out of phase in SE applications and this has to be accounted for when connecting to the windings of the transformer connected to the outputs. If you don't account for this, the amp will see a nearly short circuit on the outputs because the phases are opposite. Even operated correctly, the amp is running too close to the max supply and will be subject to shut down due to supply rail pumping.

I think the answer is to run the amp in BTL mode using lower voltage supply and 12VAC input step ups on the output. In BTL mode, you would only need ±15VDC supplies which you could get using 12-0-12 AC input.
 
Bill,

I´m facing some issues with those TDA7492 amplifier boards found on Ebay and Aliexpress and I was wondering if this board would be a higher quality substitute:

Sure Electronics' webstore 2 x 50Watt @6Ohm Class D Audio Amplifier Board Compact - TDA7492

It even has a manual:

http://sure-electronics.net/2x50WattLowpowerStereoSeriesManual_20150522AA-AB32179.pdf

I have almost finished my controller, I´m just waiting for those two output caps to arrive... This Sure board has almost the same size as the board from Ebay and the replacement would be very easy.
 

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Bill,

I´m facing some issues with those TDA7492 amplifier boards found on Ebay and Aliexpress and I was wondering if this board would be a higher quality substitute:

Sure Electronics' webstore 2 x 50Watt @6Ohm Class D Audio Amplifier Board Compact - TDA7492

It even has a manual:

http://sure-electronics.net/2x50WattLowpowerStereoSeriesManual_20150522AA-AB32179.pdf

I have almost finished my controller, I´m just waiting for those two output caps to arrive... This Sure board has almost the same size as the board from Ebay and the replacement would be very easy.

Hi Altanir,
Here's my story.
I bought 2 different modules from Aliexpress, never tried any before so this was my first experience. Its affordable, looks reasonably good to me at its price point and should work.
I notice that with these modular amplifiers for SG-4 build, a 10K input impedance is preferable for fine adjustment output from the SG-4 to limit or prevent excess step up voltage to the ouput transformer. Bill also specify an attenuation network at 10k input. The TDA7492 will perform fine with 24V DC input and 12VAC output transformer. Hereon you can mono the input (out from SG-4) and a dual winding 12VAC transformer (much like stereo mode.) The amp is also be configured easily for 2 phase mode.
The downside of this amp is the input volume control is 50kohm impedance and the pot is not exactly of high quality, incremental stepped type and not some degree of variance of attenuation resistance which may result in very slight difference in output on L&R channels. I replaced and mono'ed the input replacing that pot with a 15 turn potentiometer which resulted in more consistency at the output.

My problem was I thought the TDA8950 was a better bet but resulted running it best in BTL mode. If I insist on the same type of amplifier for 2 phase, then I would have to buy 2. This amplifier is different and peculiar design, my fault for not reading the datasheet thoroughly. One input and one output is inverted. I cannot simply mono the L&R channel to mono like the TDA7492.
What I liked about the TDA8950 or higher powered ones seemed to be better build, and integrated AC/DC power input, you can connect the right transformer to the board directly. I prefer a linear PSU for any amp although some may disagree for cost and simplicity. The TDA8950 was fitted with a 10k volume pot as seen.
IMHO, these Aliexpress modules are cheaply manufactured and not the best in the market, no telling its reliability or longevityt. My experiments reveal that these work best with midway suggested supply voltage for purpose and intent. I applied near maximum DC input and resulted in more heat and other complications.
Lastly, we never know what grade of amplifier chips on board. There's no warranty and just hope it its not DOA or will work just fine indefinitely. Its all at too good to be true prices.
I don't know if its worth but I don't have any soldering station. I would love to buy an original TDA chip from a reputable source (easily available to me here),discard that onboard chip and replace it with a legit original. I shouldn't fear anything thereon.

There's one amplifier that looked so cool. Nvidia cooling fans slapped to the chip. Surely Nvidia didn't make amplifiers.
XH M254 420W X 2 high power TDA8954TH dual core 2 chip digital audio amplifier board with 2 fan-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group
Amazing!
 
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Lee,

I´m following your posts and I´ve seen your tests with another type of amplifier but this would be way over my skill level, wich is very very basic. I´m trying to follow Bill´s instructions to the letter.

I do realize and agree with you on the lack of quality of boards from unknown manufacturers in China and this is why I think this Sure manufacturer would be a good choice. It´s the only one that has a datasheet for the board itself and not only links to the chip´s datasheet. It also provides performance graphs as well as an explanation about the board layout and pins connections. The build quality seems great as well and it can be found on several reliable websites, such as the element14 you´ve mentioned.

http://www.newark.com/sure-electron...t-audio-amplifier-board/dp/53Y1827?st=tda7492

This is the bigger version.

I believe that this has all it takes to be the best option available for the TDA7492 2X50W board for our project and for 25 USD I do hope that it´s the case!
 
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Just to show my progress on the SG-4 Controller. Its not any raving beauty at this moment as I've not yet got the aluminum chassis for it which is the last piece of the puzzle. Its all installed in a disused satellite set top box chassis meantime and functional.

One thing though, I noticed a perplexing issue. Without any load connected, randomly the amp will "pumps" when I flick the 33/45rpm switch to and fro as if I were changing the platter speed. It may pump a few seconds before building up voltage to normal. I can't explain why this is happening. I will test this again when I connect to the turntable and see if the same thing happen. I'm quite sure all the connections are correct. It does play normally for hours without at any speed so far.
 

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Great job Lee! It already looks amazing.

I've received the output caps today and finished mine. Both modes (33 and 45 RPM) are outputing the correct voltage, 118 V initially and then 100 V after a few seconds, as I adjusted it.

My TDA related problems are still present, but I hope the replacement with the Sure board will be the solution. (I need to power it on after the SG4 board and it crashes when switching speed without going into stand by mode before - Easy to reset it since I've installed a TDA power switch on the back panel).

33 RPM (60Hz) is working, but the 45 RPM (81Hz) isn't. If I turn the 45 switch, the motor does not start spinning. I can only get it running If I start in 33 mode and press the up button until it reaches 81 hz, otherwise the motor won't even start. And it makes a low noise while not starting.

I've removed the original VPI motor board from the SAMA and replaced with one I've designed, without the caps. The two caps I belive are there to set the phase difference (film) and to reduce the noise coming from the motor switch (ceramic), am I right?

My motor is the Hurst PA 300 RPM used by VPI. What should I do now?
Thanks!
 

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Hi Altanir,
I think its confirmed or normal if it won't start at 45rpm for VPI tt (at least 3 that I know of). Mine won't either and this is the same result with Nigel's Controller with VPI. I rarely use 45rpm so its not a real problem to me. Just start in 33rpm and flick the switch to 45 if needed.

My VPI motor is a different variant of the same Hurst motor series for Asia Pacific export market. Its a (230VAC/50hz/500rpm), wired up in the same manner but different phase shift capacitor value. The motor pulley diameter is also different from USA region 110V/60hz decks.

Lastly, my VPI motor it was also wired with a 3kohm/10W power resistor in series with the hot wire. This resistor have a function there to drop the voltage slightly and absorb surge if plugged direct to AC mains supply. At the moment, its not any hindrance to the overall operation and will leave it there time being. I suggest there's nothing to mod or disturb your SAMA motor. Leave it as it is.

I noticed you havea SG4 Gain board pcb there. Neat thing. With my amp module I removed that volume pot and soldered in a 15 turn potentiometer so that I can accurately adjust the maximum stepped up voltage to prevent overload. My amp module is the TDA8950 module.

Your gear looks great!

Lee
 
Thanks Lee!

Good to know that this isnt a issue, just a fact.

I found the VPI SDS manual online and it says the following: "If the turntable will not start by itself when the "speed" switch is in the 45 position, set the switch to the 33 position, start the turntable, and then change to 45.".

I do face some TDA crashes swithing from 33 to 45 without going to stand by mode, but I hope the better TDA board will end this.

I'm getting a lot of motor vibration when on 45 rpm although.
 
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My TDA 8950 used to crash or pump for a number of reasons I failed to understand how this particular amplifier works. Since then its been working almost fine but not perfect. For some reason, it may randomly pump whenver I flick the 33/45 switch. Currently, I've wired the TDA8950 amp in BTL mode and connected the 12V step up windings on parallel. Its stable so far in this configuration. Not too long ago I alternatively used a TDA7498 and each 12V winding is connected to each channel. I installed a 10kohm trimpot at the input after I removed the 50kohm volume control that came with it. It worked. I've connected a cheap A LED AC voltmeter to monitor the output voltage with or without load. Any crash or pumping is easily noticed to take action should anything undesired happen. This voltmeter is intended to be installed in the new chassis. By right if everything is working and set properly, the motor should not vibrate in any way.