A real impressive performance from the LM1875 - I can see why you like them. I've gone with the LM1875 for my first foray into building a power amplifier - I'm working on a 'nested feedback' design with the LM1875 inside the feedback loop of an LM4562, which (assuming it can be coaxed into stability - I do like a challenge) should bring the already good distortion performance of the LM1875 into another league again.
That sounds interesting. You might have heard of Tom over at Neurochrome Audio who made an ultra low distortion amplifier with an LM3886 doing something similar. Lots of good information on his site. There's good info on feedback servos in Bob Cordell's power amplifier book as well.
Please show how to calculate heat-sink for a certain rail voltage and also, if possible, how to calculate fuse value and type slow-blo vs fast-blo.Thank you
Hi John, I brought and soldered the LM1875 kit together and I'd like to but this in a cabinet. Can you make a recommendation on an inexpensive power supply for this LM1875 audio amplifier board? I understand the board requires a two-rail power supply of +25VDCand - 25VDC. I looked at a transformer with a full wave rectifier that would put out the voltages, but those transformers are expensive. Any ideas would be appreciated. Thanks, Tom
Thanks John, This is a very good video. I subscribed liked and I ordered two and at this price I still have budget two buy better caps or resistors. What components would you replace for better if any in this kit?
Good videos John, concise and well-explained.... Just a daft query cos I know nothing. I didn't see any diodes, say a bridge rectifier on the power in. Does this mean that you can simply put say 25-0-25v straight from the transformer into the board? I'm looking at this board as a quasi-gaincard/clone amp, and I like the simple layout and good distortion results. The coupling cap, the dubious "audio" cap you say needs changing, would you keep the same value, or does it not matter overly much? Cheers Martin
You will need DC split rail supply for this amp, so yes, a bridge rectifier and a couple filter caps and a center tapped secondary transformer. Transformer rating no higher than 18-0-18v if you use 8 ohm loads or 14-0-14 for 4 ohm loads.
Nice video, as always, thanks. What a great little amp this is, do you know how dB amplification it has on this board? Some of these chip-amps have a bad name in audio-lala-land. Does it sound as good as well?
Amplification is 26.8 db. Frequency response is flat, noise low, distortion low, output impedance is low, It has all the qualifications of being a good amp. I don't listen to "audiophiles" because of the total BS or pseudo science some of them spew. I'm not an engineer but I listen to them.
Many may understand why you used a very substantial heat sink on the 1875, but you didn't mention heat rise on it. Do you think real-world use, open-air would be OK for sustained operation, or would an addn'l fan be needed (say, for a stereo setup with everything in a case/chassis)? This critter is pretty brutish! Thx.
This heat sink is quite large for the job. It just something I had available. However, as many professional engineers who happen to be amplifier designers say, "No heat sink is too large". Unlike a lot of consumer gear where I find the heat sinks to be marginal, I tend to oversize them for my amplifiers.
If I want to use unregulated supply of say 18-0-18 volts , I will get a peak volts of +\- 25 volts and if I want to get +\- 18 or 36 volts I should use a 12-0-12 volts transformer coz its peak output will be around +\- 18 volts or 36 volts. Am I right Sir ?
John, an old question: where did you buy the enclosure for your preamp? I am thinking about making something similar and your enclosure would fit the bill. Best, MM
I built it using acrylic (common trade names for acrylic are: Plexiglas, Lucite, Perspex, ect). There are a lot of videos on YT on how to cut and join it.
Hi there. I really like your videos and I am intrigued about this little amp. So, I bought several :) Can I use lower value film cap (4.7uf) instead of electrolytic one (10uf) I got with the kit? Thanks.
Could two of these LM1875 amplifiers be powered with two 24v switch mode power supplies? I already own two Mean Well LRS-75-24 power supplies and would like to experiment with a stereo setup using these boards. I have read about people using two switch mode power supplies in series to create the necessary +-voltage required but I'm unsure. Also the negative output of the supplies is grounded to earth... does that make series wiring impossible?
Sorry for the slow reply. If the the negative is grounded on each, this would cause a short when trying to use them in to make a dual supply. You would need supplies with isolated outputs or one with dual rails.
My question might be a lil stupid but, is there an easy way to port a simple volume knob on this board..? or do i have to make a kind of preamp between my computer and the amplifier?
here is a good start for the power supply fr.aliexpress.com/item/AC-DC-48V-2A-Power-Supply-Adpater-96W-Charger-Transformer-1A-For-POE-LED-Strip-Light/32753806260.html?spm=a2g0s.13010208.99999999.262.dHmIdJ
I have two of these that I have been sitting on for a while. do you have any recommendations for a sub channel? I want to go 2.1 channel this go around. I think I'm going to have to build a center tapped transformer because I don't see any 48v or 50v ones anywhere
i bought this kit for having a small mono amp for testing my speaker build considering your review. i did the assembly, but after watching this video i have to admit that if i slowly get the concept of dual transformer. i stil don't get what i have to buy. i mean, theorically as far as i understood i should get something around 18-0-18 3A for 4ohm loads and 25-0-25 2A for 8ohm load considering your video. but when scrolling down the comment, i see that you correct someone by saying "The 18-0-18 is perfect for 8 ohm loads. 2 or 3 amps current is fine for two of these boards.".at this point i'm totally confuse... also when searching on google i end up on component that are even more confusing like "15V-0V-15V (input and output are AC)" wich don't make much sens to me as i suppose it is suppose to be an ac to dual dc converting.. so , from here. could any one point a cost effectif power suply i could use with this board considering it's for using in mono with different speaker from 4 to 8 ohm.. idealy using RS ( befr.rs-online.com/web/ ) or any site that could deliver in europe. thw in advance, i feel like my brain gonna explode, i'm a bit desperate right now..
You must multiply the transformer's rated secondary voltage by 1.4 times to get the approximate DC output. So a 18-0-18v transformer will give you +25 and -25v in a dual supply configuration. Never connect the transformer direct to the amplifier. It must be rectified and filtered first.
Would you have any schematict to suggest for that, or a way to use a single supply. I know it's entirely my fault as i didn't pay attention to the power supply mode when watching your video. But now i feel like having two useless assembled board. any help would be really appreciate. i would totally understain if you can't of course. thx in advance in any case.
i think this video from john should solve your problem. simple schematic, two capacitor, two resistance. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Mk8TN9OjywM.html could you confirm that john?
do i need 80VA or 120VA @ 15v-0v-15v for 2 lm1875 moduls? The load is 4-8ohme. In your data @4 ohme: 40V @1.27A per lm1875 time 2 for stereo =2,52A. The 80VA transformer is labelt 15v-0v-15v 2.66A. First i think 80VA is ok for stereo but than i look at Ptot per lm1875. 50.8W per module times 2= 101,6W. Now i think 80VA is to low for over 100Wtot please help
80 VA is fine. My tests use continuous tone which music is not so the supply demands are not as serious. If the 120 VA is not too much more expensive or too large of size for your amp case, I would choose it.
I have a pair of 15 ohm 15" Tannoy speakers. From what I have seen on similar tests they should be OK but you never see any figures now for 15 Ohm. Do they sell 2 Ohm speakers ? I have seen a lot of these little amps that don't even like 4 Ohms.
The wattage at 15 ohms would be slightly more than half the power of the 8 ohm measurements. 2 Ohm speakers are rare and don't make a lot of sense in a normal "hifi" setup. I used it mainly to see how the amp would perform.
Thank you so much for the reply. I more or less guessed it was a test. The old Tannoy's I have are about 60 + years old but still sound better than anything else I could afford. They are very sensitive, they had to be as in those days amps were usually in the 10 W range. I built a stereo pair of Mullard 20 W valve amps (0.05% distortion at 20 W). I Googled it the other day. First item on The National Valve Museum, bloody ell that made me feel old. I take it then it would be OK to use a modern amp on 15 Ohm ? I'll need some sort of pre amp for the RIAA (records), etc.
So your saying that my toloroid transformer however you pronounce it is 56v Cause it has 28v 0v 28v blue black blue So your saying my supply is actually 56v
If one wanted to make a little practice guitar amp out of this kit and build it all into a box -- would that be a sensible project for this type of amp .. The biggest hurdle for me would be finding a generic power supply off ebay / ali that would take 220v (outside U.S.) and deliver the required DC - I have zero experience with "split powersupplies" - could some form of generic universal laptop powersupply be used to give this the amperage it needs ?
So my 18-0-18 toroidal transformer was a bit off the mark for powering two of these boards in stereo (8 ohms). ...There is a 50-0-50 in the shop on sale with 7amps.......? Q: Could you please tell me, What voltage toroidal transformer would you recommend for a stereo set up? (8 ohms)??
After rectification and filtering, a 25-0-25 transformer will put out too much voltage for the IC (around 35 volts per rail!). The 18-0-18 is perfect for 8 ohm loads. 2 or 3 amps current is fine for two of these boards.
I have a transformer out of an old Pioneer Stereo amp that has one ten amp secondary that gives me a rectified +/- 60VDC. That would mak a great +/- variable power supply....if I could only find a schematic or a couple of 70V input 10A buck converters
At the Maker Faire i meet a guy that builds amp on lm chips. He has a lm1875 stereo amp on 2 12v car batteries(12v-0-12v) and he has a lm3886 on 4 hawker HX 500, 2S2P for 24v-0-24V (400Kg of pure AGM Power)!! For each HX 500 he installed a 1F carhifi powercap! This supple was called THE MONSTAAR ULTRA STABILE XD by 100%fullchared(25.8v-0v-25.8v) the supple/batterie bank provides @24v-0-24v 160W per cell for 15min 3840W total. Voltage drop 1.8v per rail! @ 22v-0-22v it provides over 350W per cell=total 8,4KW @ for 15 min! And the max power is readed at 20.5v-0-20.5v over 500W per cell 12000W total for 15min OVER KILL!! this is all nice but bullshit, but with 2 lm3886 no load to max load/befor cliping the drop of the voltage per rail is
I've been wondering if two 9V batteries back to back would work. Obviously, the rail voltage would be low but would the IC run at reduced output? A 9V battery has around 500mAH so it seems like it would last for a few hours since the chip is drawing about 70mA.
If you have a decent sized heatsink and it is getting hot with no output from the amp, then you might have a problem. It is normal for the heatsink to warm up a bit with no output.
How much power i need at output of this amplifire if i have 3 speakers of 5 watt and 8 ohms resistance connected in parallel?? I got 3 ohms of resistance when i connected all 3 speakers in parallel.
Actual nominal impedance would be 2.67 ohms. You would want to use a lower supply voltage (+/- 12 volts) to keep the current down with such a low impedance.
Watts always given in rms (watts are technically not rms, but you see what I mean), continuous output before distortion starts to rise due to clipping. None of that peak, PMPO, IEC maximum instantaneous power or dynamic power rubbish.
Yes, the relative low frequency of the audio signals aren't affected much by the scope probe's parasitic capacitance. Plus with the scope not needing to work with a 10x smaller signal, I get less noise in the FFT and waveforms especially when dealing with smaller signals.
