I will be building a LM1875 amp, based on to IC's for stereo sound.
1. Why do almost all DIY amp projects use Torroidal transformers (donut shaped) vs the normal ones found in wall warts? Will there be any performance regression if I use the other type of transformer?
2. Can the LM1875 be used with a single rail(non split) power supply? How? What are the benefits of the split PS?
3. I could also use an old laptop adapter 19V 3A, so If I split the rails, I'll get around +/-9V, which is low.
4. If I use 2 chips, can I use the same power supply rails for them, with a common ground? Or would it be better to have 2 separate transformers + rectifiers?
1. Why do almost all DIY amp projects use Torroidal transformers (donut shaped) vs the normal ones found in wall warts? Will there be any performance regression if I use the other type of transformer?
2. Can the LM1875 be used with a single rail(non split) power supply? How? What are the benefits of the split PS?
3. I could also use an old laptop adapter 19V 3A, so If I split the rails, I'll get around +/-9V, which is low.
4. If I use 2 chips, can I use the same power supply rails for them, with a common ground? Or would it be better to have 2 separate transformers + rectifiers?
1. Usage depends on situation. You can use a wall wart for a DAC or a RasPi or maybe a headphone amp but not a high powered amp. For this project you can use torroidal or a EI type transformer.
2. For LM1875 the recommendation is split power supply. However you could use a 0-48V non-split also see 20W Audio Amplifier using LM1875
See also
http://www.diyaudio.com/forums/chip-amps/50973-lm1875-lm1876-single-supply.html
3. True 19V is low.
4. I dont see the need for 2 transformers in this case but that would be ok too.
2. For LM1875 the recommendation is split power supply. However you could use a 0-48V non-split also see 20W Audio Amplifier using LM1875
See also
http://www.diyaudio.com/forums/chip-amps/50973-lm1875-lm1876-single-supply.html
3. True 19V is low.
4. I dont see the need for 2 transformers in this case but that would be ok too.
You can use any transformers you want as long as they have a sufficiently high wattage. Toroidal transformers do have an advantage because of better magnetic coupling which is more efficient and less likely to have an external magntic field interfering with your project.
Using the same power supply for two channels is quite common and will work fine for your project. Using separate power is called a mono configuration and is done because a common power supply can have crosschannel distortion and this is greatly minimized with separate supplies. But for me the little improvement hasn't justified the cost and extra work involved. I would do dual mono if I am integrating my amp with my speaker however.
If you go to chipamp.com they sell lm1875 kits in stereo and mono configurations
Using the same power supply for two channels is quite common and will work fine for your project. Using separate power is called a mono configuration and is done because a common power supply can have crosschannel distortion and this is greatly minimized with separate supplies. But for me the little improvement hasn't justified the cost and extra work involved. I would do dual mono if I am integrating my amp with my speaker however.
If you go to chipamp.com they sell lm1875 kits in stereo and mono configurations
split supply has an advantage, the thing won't require a DC blocking cap on the output.
Single supply has an advanatge, as if anything fails, DC can not get to the speakers.
Split supply is the one to go for.
Better sonics.
Single supply has an advanatge, as if anything fails, DC can not get to the speakers.
Split supply is the one to go for.
Better sonics.
The single polarity supply version uses a big smoothing capacitor in the Power Supply Unit (PSU) and it also needs a big capacitor in the output feed to the speaker.
The dual polarity supply uses two big smoothing capacitors in the PSU and does not need a big capacitor to feed the speaker.
Both are electrically the same and both require the same number of big capacitors.
The biggest difference is that the Negative FeedBack loop (NFB) of the dual polarity version isolates the capacitor effect from the speaker.
This can give a slightly better performance, particularly if the wrong size of capacitor/s is/are used. The single polarity version can be configured, but it is more complex, to include the speaker capacitor inside the NFB loop. This would negate any differences in performance between the types.
Both versions require DC blocking capacitors in the input and in the NFB loop.
Summarising:
both types can with care perform equally well.
The dual polarity supply uses two big smoothing capacitors in the PSU and does not need a big capacitor to feed the speaker.
Both are electrically the same and both require the same number of big capacitors.
The biggest difference is that the Negative FeedBack loop (NFB) of the dual polarity version isolates the capacitor effect from the speaker.
This can give a slightly better performance, particularly if the wrong size of capacitor/s is/are used. The single polarity version can be configured, but it is more complex, to include the speaker capacitor inside the NFB loop. This would negate any differences in performance between the types.
Both versions require DC blocking capacitors in the input and in the NFB loop.
Summarising:
both types can with care perform equally well.
So.. if I'm using a split power supply, I'll connect the 2 outputs of the transformer to the rectifier, which then has the 2 capacitors connected between -V & +V, with the middle point as Vground. I might not be able to source A center tapped transformer.
Since I need around +/-28VDC, I'll need 18VAC at the rectifier, so a 36V output at the transformer. How many Amps should the tranny be rated for?
Do I need fuses?
Since I need around +/-28VDC, I'll need 18VAC at the rectifier, so a 36V output at the transformer. How many Amps should the tranny be rated for?
Do I need fuses?
+/- 28 VDC is too high for the LM1875 - I wouldn't recommend going over +/- 24 VDC. I've used +/- 19..20 VDC in several projects, obtainable with a 15-0-15, 3A trafo.
Since you've already got fuses in the layout above, you might as well use them. I use fuses only on the primary side of the trafo.
Okay, if I want +/-24VDC, I'll need around +/-VAC at the rectifier.
1.Is the above layout good, expect for the fuses, which I won't be using
2.Do I really need a center tapped transformer? Can I use a normal one with 2 outputs?
1.Is the above layout good, expect for the fuses, which I won't be using
2.Do I really need a center tapped transformer? Can I use a normal one with 2 outputs?
Sorry for the worst diagram ever, but can I get away without using a center tap transformer? (the red lines show how a center tapped transformer would be connected).
In my case, I'll try and use just the remaining part of the circuit, and use the dual capacitor middle pt . to get a Virtual Ground. The issue is that I doubt the shop nearby will have a cantre tapped transformer.
In principle you need the center tap.
Creating an "artificial center tap/virtual ground" cand be done for low current needs, such as in preamps (and is done all the time on guitar pedals powered by a 9V battery) but on high current stuff it's not very practical.
Your supplier should have center tap transformers, or order one for you for practically the same cost or worst case you could use two transformers , with secondaries in series.
Creating an "artificial center tap/virtual ground" cand be done for low current needs, such as in preamps (and is done all the time on guitar pedals powered by a 9V battery) but on high current stuff it's not very practical.
Your supplier should have center tap transformers, or order one for you for practically the same cost or worst case you could use two transformers , with secondaries in series.
1. The layout is OK, but you can move things around and give more space for the input DC blocking cap - a 2.2uF electrolytic will be inferior to almost any film cap, including a Wima MKS2XL with a 5mm pitch. Similarly, the cap to ground on the feedback network can be larger than 22uF, so it would help to give space for a 10mm or 12mm diameter cap there.
2. You will need a centre-tapped trafo to ensure charge/discharge return current paths through the rectifier and primary filter capacitors (in the presence of load and signal).
I don't know why I din't think of it..
So if I use 2x 15/16 volt transformers in series, I get around 20-25 volts DC.
Is it okay to use 2 different transformers?
Btw do the transformers have to be same? What about +16V and -9V ?
So if I use 2x 15/16 volt transformers in series, I get around 20-25 volts DC.
Is it okay to use 2 different transformers?
Btw do the transformers have to be same? What about +16V and -9V ?
one can manage to make it work even in extreme cases, but for the love of god...
2 identical transformers are okay.
2 different ones could be okay.
+16 and -9 .. well could in theory work, but the -9 side will run outta juice far before the otherone, it will lead to bad things.
why not just get a proper traffo ?
sure You can be low on cash or something, but hey, its DIY, salvage if nothing else is available. essentially i BET everyone here who does ideed build stuff from time to time does salvage, i do accept faulthy electronic equipment for this purpose, switches, traffos, pots, cases, foil caps , opamps, whatever.. they come handy, and it comes for free.
2 identical transformers are okay.
2 different ones could be okay.
+16 and -9 .. well could in theory work, but the -9 side will run outta juice far before the otherone, it will lead to bad things.
why not just get a proper traffo ?
sure You can be low on cash or something, but hey, its DIY, salvage if nothing else is available. essentially i BET everyone here who does ideed build stuff from time to time does salvage, i do accept faulthy electronic equipment for this purpose, switches, traffos, pots, cases, foil caps , opamps, whatever.. they come handy, and it comes for free.
I would not run the LM1875 with more than +/- 21 volts with 6 ohm loads. Calculations might seem safe with resistive loads, however, speakers are reactive loads and you don't have the headroom with current when using higher voltages.
I was just reading the datasheet for the LM1875
http://www.ti.com/lit/ds/symlink/lm1875.pdf
If you look at pages 9 and 10 it shows PCB layouts for split supply and single supply configurations. Which means single rail voltage is certainly allowed if it is on the datasheet.
I have my own question though. I have an LM1875 kit but I haven't bought a transformer yet and I was wondering about what I need. I plan to use 8 ohm speakers and the data sheet shows that at a 20W power output that there is 20W of heat dissipation for the voltage I plan to use. A naive estimation of the power requirements would mean that I need 40W per channel or 80W total. Would a 120VA transformer be enough for this kit?
http://www.ti.com/lit/ds/symlink/lm1875.pdf
If you look at pages 9 and 10 it shows PCB layouts for split supply and single supply configurations. Which means single rail voltage is certainly allowed if it is on the datasheet.
I have my own question though. I have an LM1875 kit but I haven't bought a transformer yet and I was wondering about what I need. I plan to use 8 ohm speakers and the data sheet shows that at a 20W power output that there is 20W of heat dissipation for the voltage I plan to use. A naive estimation of the power requirements would mean that I need 40W per channel or 80W total. Would a 120VA transformer be enough for this kit?
The normal transformer requirement for a ClassAB amplifier is VA is approximately the total maximum output power to two times the total maximum output power.
I add an over-riding condition: The smallest toroid transformer for a power amplifier should be around 160VA to get a good balance of PSU voltage as output varies, mostly due to excessive transformer regulation for very small transformers.
eg.
two channels of 20W amplifiers gives 40W of total maximum output power.
Therefore the amplifiers will work with a toroid transformer from 40VA to 80VA
I add an over-riding condition: The smallest toroid transformer for a power amplifier should be around 160VA to get a good balance of PSU voltage as output varies, mostly due to excessive transformer regulation for very small transformers.
eg.
two channels of 20W amplifiers gives 40W of total maximum output power.
Therefore the amplifiers will work with a toroid transformer from 40VA to 80VA
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