Hi everyone, i hope you are having a nice day.
So after building my first lm1875 amplifier that was single supply i was very happy with the sound quality, But it does have some issues mainly due to the power supply.
So now i want to build my first proper amplifier in a proper chassis.
Basically i have a toroidal transformer that was free from some junk electronics. It has 2x16v 5 amp windings and some low current windings that i could use for a protection circuit.
After rectification with one 6800uf capacitor per rail i get 24vdc per rail and with around 1.5 amp load it drops to around 21vdc per rail
That seems ideal for lm1875, But i want to use 4 ohm load so i am slightly worried.
Although the amplifier is going to be used with markaudio chn-50 speakers that are rated at 7w so its probably never gonna reach the current limit.
I am planning on using this chassis or something similar:
https://www.audiophonics.fr/en/alum...iy-box-100-aluminium-260x166x70mm-p-9500.html
And these boards with genuine parts
https://www.ebay.com/itm/20W-HIFI-M...p4429486.m46890.l6249&mkrid=711-127632-2357-0
So do my plans seem ok or should i consider something else ?
If anyone has any better amplifier design for these voltages and loads pleas tell me, But it has to be a design that has pcb designs available.
So after building my first lm1875 amplifier that was single supply i was very happy with the sound quality, But it does have some issues mainly due to the power supply.
So now i want to build my first proper amplifier in a proper chassis.
Basically i have a toroidal transformer that was free from some junk electronics. It has 2x16v 5 amp windings and some low current windings that i could use for a protection circuit.
After rectification with one 6800uf capacitor per rail i get 24vdc per rail and with around 1.5 amp load it drops to around 21vdc per rail
That seems ideal for lm1875, But i want to use 4 ohm load so i am slightly worried.
Although the amplifier is going to be used with markaudio chn-50 speakers that are rated at 7w so its probably never gonna reach the current limit.
I am planning on using this chassis or something similar:
https://www.audiophonics.fr/en/alum...iy-box-100-aluminium-260x166x70mm-p-9500.html
And these boards with genuine parts
https://www.ebay.com/itm/20W-HIFI-M...p4429486.m46890.l6249&mkrid=711-127632-2357-0
So do my plans seem ok or should i consider something else ?
If anyone has any better amplifier design for these voltages and loads pleas tell me, But it has to be a design that has pcb designs available.
Don't worry just use that transformer with confidence.
2 x 16V 5A* is OK (you can't go wrong with 160VA). The LM1875 chip is current limited anyway.
*Volt, Ohm, mA, kHz, µF, W etc. Please!
2 x 16V 5A* is OK (you can't go wrong with 160VA). The LM1875 chip is current limited anyway.
*Volt, Ohm, mA, kHz, µF, W etc. Please!
The TDA1875 is quite robust and can handle up to 60volt supply or +-30volts.
+-24volts looks fine to me. (16volts AC will produce 22.6volts with full wave bridge rectification.
The formulae is 1.414 x AC for bridge or 0.707 x AC for half wave.
+-24volts looks fine to me. (16volts AC will produce 22.6volts with full wave bridge rectification.
The formulae is 1.414 x AC for bridge or 0.707 x AC for half wave.
Better calculate with a margin. In reality it will be more like 2 x 17V unloaded or with light load. Always measure real life voltages to verify all is according plan. The rectifier diodes (2 x Uf) are often forgotten and should be distracted in calculations.
Like the 24V already measured, all is fine.
Like the 24V already measured, all is fine.
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Ok, Thanks.
All the reading around got me worried because some people don't even recommend using lm1875 for 4 ohms at all...
Also what size filter capacitors should i use ? Would for example 1x6800 uf per rail be enough for 2x lm1875 amps.
Also is there a good speaker protection circuit available ? There are some on ebay but if something more reliable is available i would gladly pay a little more.
I had a scary experience with amplifier failing and outputing dc in the past...
All the reading around got me worried because some people don't even recommend using lm1875 for 4 ohms at all...
Also what size filter capacitors should i use ? Would for example 1x6800 uf per rail be enough for 2x lm1875 amps.
Also is there a good speaker protection circuit available ? There are some on ebay but if something more reliable is available i would gladly pay a little more.
I had a scary experience with amplifier failing and outputing dc in the past...
Those 2$ China boards are quite good...Cheap and easy .
ALL the parts included are garbage though. Buy 1st world ones.
Your torroid is Fine...
Do Not need Any protection circuits. IMO that is a paranoia driven affectation. Audio Condoms imo 🙂
Personally been doing this audio nonsense since 1967... no condoms.
Resultant 1875 amp built will be decent, while not quite 'high end' tho.. all claims aside.
It will be Fine on your chinese driver.. pleasingly so.
ALL the parts included are garbage though. Buy 1st world ones.
Your torroid is Fine...
Do Not need Any protection circuits. IMO that is a paranoia driven affectation. Audio Condoms imo 🙂
Personally been doing this audio nonsense since 1967... no condoms.
Resultant 1875 amp built will be decent, while not quite 'high end' tho.. all claims aside.
It will be Fine on your chinese driver.. pleasingly so.
I wouldn't run any chip amp without some sort of output protection ie. Crowbar circuit or some other protection if you run expensive drivers. I've seen quite a few chip amps fail with DC at the output and take out drivers. Its cheap and easy to at least fuse the DC rails a little higher than the peak current required so you at least have some sort of last resort failsafe.
And for good reason.
All analysis above concentrated on voltage, an easy task for1875 which stands much higher, none about current which is its Achilles' heel.
Maximum/peak current out is 4A, protection turns on at some 4.2 to 4.4A or so.
Possible peak current into 4 ohm, using your supply and assuming 2V drop per output device would be 20V/4 ohm=5A
Clearly way beyond suggested.
1875 is actually a robust very well designed 8 ohm load amplifier,with ample margin, including normal impedance drops.
4 ohm? .... it may be usable in your particular case, with 7W speakers, moderate level background music use, just be careful and do not rise volume beyond that.
When trying to drive 4 ohm loads, besides not being able to, 1875 overheats big time.
Notice when trying to put out meager 15W out, with +/-20V rails, it will dissipate 20W (worse than Class A 😱) while any normal Class AB (other amps) would be some 5-7W
they are clearly saying:"do NOT use it with 4 ohm loads"
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Not much bad will happen if one plays at normal volume with 4 Ohm speakers. Dissipation is high however.
But it would be better to choose another chip for 4 Ohm loudspeakers.
But it would be better to choose another chip for 4 Ohm loudspeakers.
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Well he is using a fun 3" full range driver with about 3.5 mm Xmax
and 7 watt nominal rating.
So average listening wont be more than 2 watts
And likely linear distortion is around 5 watts.
Basically it is well known not to expect more than 15 to 18 watts
from a 1875. And it does have thermal shut down.
Chip amps are rather straight forward.
Basically with a stereo pair on a 120 to 150 VA transformer
Anytime your going below 6 ohms just use a rather healthy heat sink
and enjoy life.
Looking at the driver Re might be 3.4 ohms
But with typical impedance rise and a Reflex.
Load is much closer to 6 ohms.
For having the parts on hand and the load well defined.
Just use a large heatsink.
people like to go overkill with power supplies, in reality 6800u per rail is enough.
There is surge current at startup so the rectifier gets a quick punch all its life.
But 20 or 30 amp rectifiers are pretty common. So 6800u is fine.
If you did run 2 parallel caps for capacitance over 10,000
then the start surge is a concern.
Again people get all silly with soft starts and relays.
But if you under 250 VA for transformer. Just a basic thermistor
for soft start.
and 7 watt nominal rating.
So average listening wont be more than 2 watts
And likely linear distortion is around 5 watts.
Basically it is well known not to expect more than 15 to 18 watts
from a 1875. And it does have thermal shut down.
Chip amps are rather straight forward.
Basically with a stereo pair on a 120 to 150 VA transformer
Anytime your going below 6 ohms just use a rather healthy heat sink
and enjoy life.
Looking at the driver Re might be 3.4 ohms
But with typical impedance rise and a Reflex.
Load is much closer to 6 ohms.
For having the parts on hand and the load well defined.
Just use a large heatsink.
people like to go overkill with power supplies, in reality 6800u per rail is enough.
There is surge current at startup so the rectifier gets a quick punch all its life.
But 20 or 30 amp rectifiers are pretty common. So 6800u is fine.
If you did run 2 parallel caps for capacitance over 10,000
then the start surge is a concern.
Again people get all silly with soft starts and relays.
But if you under 250 VA for transformer. Just a basic thermistor
for soft start.
What would be a good option ?
I am open to discrete transistor designs too, But it has to be a complete project with pcb designs available.
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Markaudio CHN-50 impedance plot looks like 6ohm minimum and rising to 20khz https://www.markaudio.com/online_shop/ch/chn-50/ so I don't know why people are talking about 4ohm problems; it's irrelevant for this driver.
The 1980's Maplin disco amp had a fuse on the output of the amp.
It saved me a set of speakers when at a gig I tripped over speaker lead and pulled it out.
Luckily just the fuse blew so a quick change and back to full volume again.
Recent thread LM2876 done parallel since 3886 is hard to get.
Likely might not need to parallel if your looking for a pre made board.
Far as I can remember 2876 will do little more power than 1875.
And has mute function similar to 3886.
So believe pin to pin same as 3886.
So can drop a 2876 into a 3886 board.
if im correct @pbilous should likely have better info than me.
and of course his recent design would do more power since parallel
all in all single chip as well have more power/ safer operating area
using the transformer you already have.
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You can try other ready populated boards, there are transistors, Class D, other chip amps and so on.
Look at the usual sites, I will not name them.
And of course the classic TIP based amplifiers, those may be also built.
Speaker details? It seems they are 6 ohms, so no problem for 1875, but its high wastage of power as heat is not efficient. Must be better chips out there.
Please describe the speaker volume, and room size / height, and treatment if any.
For long term protection from hearing loss, it is best to stay below 100 dB volume level. Bear that in mind, no point in using too much installed power.
Look at the usual sites, I will not name them.
And of course the classic TIP based amplifiers, those may be also built.
Speaker details? It seems they are 6 ohms, so no problem for 1875, but its high wastage of power as heat is not efficient. Must be better chips out there.
Please describe the speaker volume, and room size / height, and treatment if any.
For long term protection from hearing loss, it is best to stay below 100 dB volume level. Bear that in mind, no point in using too much installed power.
Relatively well known wideband with lots of pre designed enclosures.
Manufacture safely rates it at 4 ohms since Re is 3.4 ohm
But majority impedance curve is 6 ohms and up
He pretty much called it in the Original Post it is 7 watt thermal.
So linear distortion be way below that maybe 3 to 5 watts max.
LM1875 with average to large heatsink, is gonna drive it all day.
amplifier Efficiency not really a issue since average use will be 3 to 5 watts.
Seems the transformer is about 100 to 150VA so would be rather
healthy supply for planned use.
Mark Audio CHN-50
7 Watts is kid stuff for the 1875.
Play with cap size in power supply for best bass.
Start with 2200, and go up to 6800, min 35 or preferably 63 V rated.
Play with cap size in power supply for best bass.
Start with 2200, and go up to 6800, min 35 or preferably 63 V rated.
TDA2050 is very similar to LM1875, on many parameters, but has more output current available.
This video give some insights regarding both chips including behaviour on 4Ohms load :