I'm not crazy about regulating power amp rails. I'd think you are better off with a 18-0-18 volt 100va transformer if driving 8 ohm loads. Personally I'd not run the 1875 any higher than 46 volts (+/- 23v rails) with 8 ohm loads. Speakers are reactive loads and you are asking for problems (current limit) when pushing this chip any harder.
With 0 volts output, the IC can handle about 3.5 amps according to the datasheet. With reactive loads, you can be at the doorstep of current limiting!
With 0 volts output, the IC can handle about 3.5 amps according to the datasheet. With reactive loads, you can be at the doorstep of current limiting!
I appreciate your input johnr66. One thing that should be said about my build is that it is sort of a "junkyard dog" project meaning that everything is either free or salvaged. I'm not expecting Grade A sound out of this project, it's more of a learning experience.
On the topic of the current that the regulators might need to handle I did sample both LT1085s and LT1084s. The LT1084 should be able to handle a bit more than the LT1085 and if something does start smoking at least I did not pay for it
On the topic of the current that the regulators might need to handle I did sample both LT1085s and LT1084s. The LT1084 should be able to handle a bit more than the LT1085 and if something does start smoking at least I did not pay for it
I wonder if there exists a heatsink-able active device that could be used as a series element and lose 3v per each rail (so that you get your 28 down to 25-ish)?
P.S.
I've made solo LM1875 amplifiers in two flavors:
Grandpa's hi-fi version, in the earliest part of this thread
LM1875 Turbo II, a more durable high dynamics party sound version, in my signature line.
Which would you like better?
P.S.
I've made solo LM1875 amplifiers in two flavors:
Grandpa's hi-fi version, in the earliest part of this thread
LM1875 Turbo II, a more durable high dynamics party sound version, in my signature line.
Which would you like better?
Well I have mentioned the LT1085/1084 several times. As far as I can see by reading the datasheet it should work fine provided it has the proper heatsink. The minimum dropout voltage for the 1085 is around 1.5V working at it's maximum operating current/temperature. Both of these should be avoided provided it is heatsinked properly and the amplifier isn't on full blast all the time, which it won't be in my home.
Is there any reason you can see why the LT1085/1084 would not work?
Once again my transformer has 2x21VAC @ 1.8A, non centertapped. This would power TWO LM1875 chips using a double bridge config to get +/- rails. One regulator per rail to get the voltage down to a more suitable level.
Is there any reason you can see why the LT1085/1084 would not work?
Once again my transformer has 2x21VAC @ 1.8A, non centertapped. This would power TWO LM1875 chips using a double bridge config to get +/- rails. One regulator per rail to get the voltage down to a more suitable level.
A Zener diode, or a transistor assisted Zener diode, or a capacitance multiplier with the Vout set to 95% when voltage is a bit low changing to a Zener limited Vout when voltage is high, or a JLH ripple eater.
My advice is to skip the regulators and build out the supply as is. It will be operating near or over the chips 60v limit because the transformer's unloaded voltage may be high. If necessary, add a few 3 amp diodes in series to bring the supply voltage down to 28 volts per rail with amps quiescent (~100ma). With a 1.8 amp trafo, the rails will be pulled down below 25v when the ICs are delivering high current anyway. To mitigate some of the effects of loudspeaker reactance current, add a 1 Ohm resistor in series with the outputs (audiophiles may shun me for that but it works). This will keep your salvaged parts project cheap.
I thank you all for taking your time and I'll look into all of the things you suggested. Might be I skip the regulators, might be I don't. Time will tell.
Now begins the search for heatsinks for either the LM1875s/1085s or both. I was thinking salvaged Intel processor heatsinks. I've found it hard to find any info their thermal resistance but I'm guessing ye olde "bigger is better" will work here.
On the topic of regulators, I'm going to ask my main man at unversity if I can look through the workshop which is where all the goodies are kept. Might be I find something nice!
Build thread will come, just gotta source everything.
Now begins the search for heatsinks for either the LM1875s/1085s or both. I was thinking salvaged Intel processor heatsinks. I've found it hard to find any info their thermal resistance but I'm guessing ye olde "bigger is better" will work here.
On the topic of regulators, I'm going to ask my main man at unversity if I can look through the workshop which is where all the goodies are kept. Might be I find something nice!
Build thread will come, just gotta source everything.
She is almost that strong.
This will work after only slight modification.
I have Lm1875's in 2 flavors, grandpa's hi-fi in the first few posts of this thread, which is NOT durable enough for this application; and, LM1875 Turbo and Turbo II (schema in signature line) which are able for it.
Several series diodes may be applied between:
Bridge rectifier plus first caps (as one unit). . . then series element such as cable or 1 or more diodes (like your example of several diode drops). . . then power supply reservoir tank/capacitance (like 4x2200u). And then following the tank/reservoir capacitance, a series element such as a cable and, at your option, an additional diode drop, may be applied. Use only as much as needed to guarantee certain safety.
This structure is near ideal but will take some care and could result in either wonderful quality or exploded parts, at your choice.
I am willing to discuss this option farther, because of what it can do, not because of what it shouldn't do.
P.S.
The orthodox output resistor for a chip amplifier maxes out at 0.45 ohms in order to prevent negative output impedance that is not good for tone, and therefore we are sure that a modicum of resistor may be helpful; however, the 1R spec mentioned may be slightly excessive and such excess looks like there may be a needed repair at some other point in the circuit, which will need some homework before proceeding.
There is NO big fell swoop that I would approve of; however, you have my blessing to apply it more slightly, if it is of benefit. It is just that we should never apply a band-aid instead of a repair. Indeed we should apply some caution, but after the proof, more power to you and full steam ahead!
P.P.S.
I would agree that, in this case, we should probably abuse some diodes that can withstand it rather than abuse a chip that will not withstand it. And, yes, I would agree that such usage is entirely possible and that it is not impossible to do it well.
P.P.P.S.
The main question remains to be solved, in that we have some voltage to dump, probably shouldn't dump a lot of current, and that I don't know of exactly what parts we should use for this task. I do know that they should be heat-sink-able so that we don't cook our capacitors.
You can incorporate the extra diodes right in to the bridge itself. You can keep quiescent voltage down a bit by adding a couple power resistors across the supply, such as a 330 ohm across each leg. Gator clip the PSU together and take measurements.
I not all for this bodgery to make things work, but I can understand if you don't want to spend the cash.
I not all for this bodgery to make things work, but I can understand if you don't want to spend the cash.
I think that a low voltage drop capacitive multiplier circuit is probably the answer, but mainly because those big power transistors are 1). heatsink-able and 2). have voltage drop.
Keantoken has a capacitive multiplier page here: http://startfetch.com/keantoken/content/Kmultiplier.php
It looks as if it could be fitted with sturdier devices with enough current capacity to run LM1875 amplifiers.
This way we lose the extra voltage and lose some power noise, but without losing a lot of current.
Keantoken has a capacitive multiplier page here: http://startfetch.com/keantoken/content/Kmultiplier.php
It looks as if it could be fitted with sturdier devices with enough current capacity to run LM1875 amplifiers.
This way we lose the extra voltage and lose some power noise, but without losing a lot of current.
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D44H11, D45H11 http://www.onsemi.com/pub/Collateral/D44H-D.PDF
But, what is the suggested zener value to use?
But, what is the suggested zener value to use?
Or add a 4.7uF or larger cap across the secondary to act as both a load and filter.
Well, we needed just a 3v~4v drop.
So, 3v zener series to 1n4004 (or could use a 1n5819 schottky series to the zener?), this diode string added parallel to C1 (and again at C2)?
(for reference to others reading)
An externally hosted image should be here but it was not working when we last tested it.
