Pre-Amp - LM1036 + NE5532 Clipping and Distorted Bass

RayAkmal

Member
2016-03-11 6:05 am
Hi guys,

I bought this pre-amp.
LM1036 NE5532 Stereo Preamp Preamplifier Treble Bass Tone Board DIY Amplifier | eBay

I found no schematic, It's not clear what 'Power Seat' is.
I found the DC input has no polarity (Center pin can be positive/negative).

I tested it with 12V 1.5 A power supply (SMPS and Linear PS). The sound is good, detailed and clear but when I crank up the volume and bass control more than halfway the bass starts to clip and distorted.

Anyone has experience with this pre amp board? Is this clipping normal? Is there something I can fix/mod to get rid of the clipping? Thanks. your help would be very much appreciated.
 
By the looks of it, this board expects AC input (i.e. a plain transformer), enough for a bridge rectifier to be turning it into at least 15 V= (so maybe 12 V~? 3-5 VA would be plenty), which is then regulated down to 12 V=.

I would, however, still expect the circuit to work decently even with 12V= input only. There might be little more than 8-9V= to work with, but that should be plenty for the LM1036 which was developed for automotive use and about 8 V anyway, and a '5532 should work OK, too.

Maybe the power supply isn't as simple as I thought then. There are two 100µ caps at power input, are those simply connected in parallel?

"Power seat" is just a screw terminal that can be used instead of the power socket if you want to put the thing in a box with a bare transformer to wire it up to.
 

RayAkmal

Member
2016-03-11 6:05 am
@sgrossklass
Nice, that 'power seat' will be usable to power my VU meter.
Yes, those capacitors are connected in parallel.
Yes, I think so. A 12 V 1.5A power supply is more than enough for this chip. With 16 V power supply, expect bigger THD (0.03% vs 0.3%).

@AndrewT
That's correct, based on datasheet, with Vcc = 12V 35 mA, typical max output is 1.0 Vrms, so that clipping at 0,3 V is not normal. In spite of this, I'm still gonna use this pre-amp inside my DIY boombox, but the controls will be pre-set/fine-tuned and can't be changed.

Thanks for your help guys
 
If this board requires an AC input, there will be no mid point with a DC supply because there will not be enough generated DC voltage or maybe they use AC coupled capacitor rectified feed for mid point, that is all assumption of course.
At the bottom of the page is "Ask a question". Why not ask for power requirements this will bring you all closer to why it doesn't work properly.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
If you scroll down to the Item Specifics block, "Power Supply 12-15V DC" couldn't be more clearly stated and as AndrewT posted. I think though, there is a TO252 device which could be a generator for the negative rail or simply a supply splitter. There are several versions of this basic design - most with through-hole parts, AC supply and standard 3 terminal regs. This finished module is SMT and looks to be intended for plug-and-play single supply use and for use with headphones. Maybe that's why the output voltage is low?

As you have it working, I guess you found the polarity is clear from the under side view which shows the ground or negative terminal of the "power seat" or power supply screw connectors nearest the edge. Dunno what would happen if that connection was reversed as I don't recognize a safety blocking diode. Maybe that 4-terminal device is a diode bridge on the board nearby :confused:
 

RayAkmal

Member
2016-03-11 6:05 am
Guys, I don't know much about electronic, I tried to understand your posts.
Btw. I re-soldered the potentiometers, cleaned DC input contacts.
The clipping and distortion are 'magically' gone for now.

@AndrewT. I need to learn about 4 Vac equal 11.3Vpp & Non rail to rail op amp :)
@JonSnellElectronic. It's clear this board need DC input.
@Ina Finch. The DCin center pin is connected to the nearest the edge of power connector.
I connects the center pin for DCin to positive polarity of 12 Vdc power supply.
That 4 terminal device near DC in connector looks like a bridge diode, can be found here on another board (D1).
I'm going to use this board, also confused about polarity of dc supply.
Wholesale Product Snapshot Product name is Wholesale Brand New 2pcs Panel VU Meter Warm Back Light Recording&Audio Level Amp with driver+Free shipping-10000598
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Good, then I assume the LED lights up when the centre pin is +. Alternatively (when viewed from control side), it would power up with the left side screw connector + and right side -.

The bridge rectifier on the VU meter kit is bigger, different mounting and shape. Unless you can see the +, -, ~, ~, codes marked on the preamplifier device, you probably can't be certain whether it is a diode, a pair or 4 in a bridge arrangement.

Have you tried the preamplifier with headphones or earbuds connected?
 

RayAkmal

Member
2016-03-11 6:05 am
LED lights up when the center pin is + or - It's weird. :D
Sorry, I can't see the +,-, ~,~ codes on D1 even with a magnifier.

After re-soldered the potensiometers and clean up dc connectors distortion is gone. Before this, I've tried with different earphones and earbuds, it clipped/distorted.

I made a mistake in thinking the input for VU meter board was 12 Vdc. Actually it needs 12 Vac. You are correct, D1 on the VU meter board is a bridge rectifier. (A Supplied schematic that came with the board, says it needs 9 Vac).
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
You're in luck, since 9VAC rectifies to about 14VDC with light loads. You should be able to apply about the same power supply DC+ from your preamp to the + terminal of C6 and the supply will still be regulated as intended. You would simply be feeding DC in after the rectifier this way, assuming it is sufficient for the 3-terminal regulator - possibly a 7809 or LM317T type. I would prefer to see the schematic to make sure that is going to be safe first, though.

Re: the weird power supply on the preamp -
Its likely with SMT product designs, that the preamp has an on-board DC/DC switched mode regulator. That allows a number of possible supply arrangements that conventional diode supplies and linear regulators can't do, like reversed polarity or universal input connections. Without knowing the schematic, its just guessing though.
 
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RayAkmal

Member
2016-03-11 6:05 am
I was curious so I tested this VU driver with 12 VDC, connected to power amp output, the result is the VU meter needles moved out of scale.

The manual says I don't need to adjust VR or anything if I use this VU driver with the accompanied VU meters.

Do you think I should adjust VR1 and VR2 trimpots or I better change the supply with 9VAC?

btw...Am I correct that trimpot CW adjustment add resistance?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
'Can't offer much help without a schematic. As DIYs, most of us who contribute, try to avoid ready built and even kits, as the Ebay sellers now give nothing at all of the design in print and it means reverse-engineering the boards to assess their value and use them as you wish. Unless we are also currently playing with these kits or similar circuits, we probably won't understand the problem - other than from an analysis of how the circuit operates and with what components.

To start somewhere:
What is the part number on that regulator chip? That's the large 3 pin device with a metal tag for bolting to a heatsink. It helps to know what DC output is expected and what really is there at the output pin. Google the part number as "XXXX pinout" for the connection details.

Remove the inputs from the preamp. You could have an issue with polarity being reversed and the power supply ground being different on the preamp to the meter. Try reversing the preamp dc supply. Short (ground) the meter inputs with something like a 1k resistor or connect the inputs to another working amplifier as specified and test again.

Yes, DIY requires a box full of miscellaneous parts - some you may never need but others you can't live without ;)
 

RayAkmal

Member
2016-03-11 6:05 am
'Can't offer much help without a schematic. As DIYs, most of us who contribute, try to avoid ready built and even kits, as the Ebay sellers now give nothing at all of the design in print and it means reverse-engineering the boards to assess their value and use them as you wish. Unless we are also currently playing with these kits or similar circuits, we probably won't understand the problem - other than from an analysis of how the circuit operates and with what components.

No problem. I understand that. Anyway you and others have been very helpful and I really appreciate that.

To start somewhere:
What is the part number on that regulator chip? That's the large 3 pin device with a metal tag for bolting to a heatsink. It helps to know what DC output is expected and what really is there at the output pin. Google the part number as "XXXX pinout" for the connection details.

It's a L7812CV regulator Chip. Can't understand much reading the datasheet :)

Remove the inputs from the preamp. You could have an issue with polarity being reversed and the power supply ground being different on the preamp to the meter. Try reversing the preamp dc supply. Short (ground) the meter inputs with something like a 1k resistor or connect the inputs to another working amplifier as specified and test again.

Yes, DIY requires a box full of miscellaneous parts - some you may never need but others you can't live without ;)

Noted. I will wait till I have a 9VAC transformer and 1K resistors. I will resume later if time permits. I have several DIY projects right now. This boombox can wait for awhile :)

Once again, Thanks for all the help.