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Old 20th December 2011, 12:28 AM   #1
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Default Parallel LM1875, 2 chips per channel, in 5 minutes ea? Can bridge if 8 ohm speakers.

I wish to show a real 2-chip parallel LM1875 point to point done quickly and easily.

Reasons to do it:
The little chip is hi-fi, clear, easy, inexpensive, and it isn't hobbled by the harsh clipping of the spike system (which is absent), so Parallel LM1875's "RMS power" output is a strong competitor to single larger size chip amplifiers. A further difference is the slightly dark voice which is helpful in applying an NFB cap for its advantages to speaker safety, longevity and fun dynamics, yet with the outcome of a level frequency response.

Higher power applications:
This tiny parallel amp can be used for 4 ohm speakers without bridging, or you could add bridging for 8 ohm speakers.

My application:
I, personally, need this clean little high current amplifier in order to interview candidates for bridge adapter. It is a real amplifier. Your input on the electrical circuit(s) is welcomed and desired.

The photo shows small size wire bender pliers (small rounded needle nose pliers) safely bending the pins so that we can attach the power rails.
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File Type: jpg Parallel LM1875 p2p wirebender.jpg (112.4 KB, 811 views)

Last edited by danielwritesbac; 31st March 2012 at 11:02 PM.
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Old 20th December 2011, 11:20 PM   #2
Einric is offline Einric  United States
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I'm very interested in this parallel design.
Could you please post a build sheet with a sketch diagram?
I have a line on some aluminum extrusions that would accept two of these nicely
for a pretty sweet dual mono compact amplifier.

Thanks
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Old 21st December 2011, 01:06 AM   #3
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Yes, this is great. I was stalled out while trying to find the heatsink.

If possible, use the pre-insulated transmitter type extrusion.
It has a very thick black shiny non-conductive coating. Tabor makes most of them.
The really bent teflon frying pan on sale at the dollar store may also work or may need a heat spreader bar added.
After install, just check with the ohmmeter between tab and heatsink to insure no conductivity to electricity--tab is v- rail.

And hey, I've got to find my heatsink before soldering, because it is a rigid assembly and it is also necessary that the bolts like up with the spaces, not the fins. . . because I'm using bolts, not self tapping screws. I just don't have self tapping screws that fit the shoulder washers. So, I'm temporarily stuck.

Last edited by danielwritesbac; 21st December 2011 at 01:17 AM.
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Old 21st December 2011, 01:26 AM   #4
Einric is offline Einric  United States
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I am a radio technician and as such I have access to strange extrusions for radio parts.
One such radio I work on has aluminum extrusions that are 3/16 thick aluminum rectangle tube approximately 3x4.5x6 with raised ridges similar to heatsink fins on the two narrow edges.
They are coated in some sort of thermal conductive non-electrical conductive black finish that is very handsome.
They are tapped at both ends for 4 M6x5mm screws.
I think it would make a wonderful 2 parallel LM1875 mini monoblock for 4Ω speakers.
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Old 21st December 2011, 10:23 AM   #5
AndrewT is offline AndrewT  Scotland
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If one accepts the design thought that a single 1875 driven to it's maximum capabilities is at it's limit on an 8r0 load, then would it be sensible to use more than an 8ohm as the nominal speaker load for each single chipamp?

If so, then I suspect that a dual could probably drive a 6ohm speaker and a triple could probably drive a 4ohm speaker.

I raise this design issue because the 1875 is stated to have a minimum output current of 3Apk and a typical output current of 4Apk (when output voltage is >10V below supply voltage, this is the loaded supply voltage while delivering that output current). This is further confirmed in the text as an active 4Apk current limiter that operates when Vce is low and is designed to trigger at lower currents when the Vce is higher.
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Last edited by AndrewT; 21st December 2011 at 10:40 AM.
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Old 21st December 2011, 10:54 AM   #6
AndrewT is offline AndrewT  Scotland
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Lets' take a +-30Vdc supply absolute maximum supply that runs at a normal (quiescent) +-27Vdc.
Take an 8r0 load and drive to maximum output.
The expected maximum voltage is ~4V less than the loaded supply rail voltage. Expect that normal +-27Vdc to drop to +-24Vdc when under load. Subtract a further 4V for losses through the chipamp. Maximum sinewave output voltage is ~20Vpk and current is ~2.5Apk. So far output current limit is OK.

Now move to a reactive speaker load where transient current demand can approach 3times the resistive load demand.
For a 4Apk of short term transient demand, expect the supply voltage to not droop as in continuous sinewave testing.
The normal Vsupply stands at +-27Vdc and the 4Apk limit applies when chipamp Vdrop is 10V, we have 17Vpk available at the speaker terminal and 4Apk limited current capability.
The effective impedance is required to be ~4.25ohms. This is only a transient:continuous 4:2.5 ratio of currents, i.e. 1.6times the resistive load current peak.
We are at ~ half the potential peak demand of short term transients of 7.5Apk.
Basically the chipamp can drive an 8r0 load as specified in the datasheet.
It cannot drive an 8ohm severe reactance speaker load when using reasonable near maximum supply and speaker operational conditions.

Substitute 12r0 and 12ohm speaker and you will find that the 1875 comes much closer to being able to drive a higher impedance reactive load. 4:1.67 gives ~2.4times into reactive short term transients. 2.4times for 12ohms is much better than 1.6times for 8ohms

This short design exercise has not yet taken account of the reducing output current limit when Vce of the output devices is higher. National don't specify how that IV limiter operates.
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Last edited by AndrewT; 21st December 2011 at 11:02 AM.
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Old 21st December 2011, 12:24 PM   #7
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Thank you for the information.
I am at odds with one of the figures. It is that 30. Please don't do that.
The amplifier drives 4 ohm speakers conditionally, and those are not the conditions. Firstly, do not start at +-30vdc. That hinders longevity. Please replace that with +-27.5vdc or less (whether the amplifier is attached or not).

Last edited by danielwritesbac; 21st December 2011 at 12:53 PM.
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Old 21st December 2011, 12:45 PM   #8
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@AndrewT What do you think of "PI filter overdo" (a larger "R" in CRC) as a safety current drop mechanism? And, if that seems wise then I wonder what is the "R" that should be on the schematic?

Apparently, several LM1875 applications need a little bit of added current limiting for safety.

Last edited by danielwritesbac; 21st December 2011 at 12:50 PM.
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Old 21st December 2011, 01:36 PM   #9
Einric is offline Einric  United States
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I will be running 18v secondaries so even at 4Ω the 1875 should be able to remain nice and safe especially when 2 are in parallel.

I will be looking at building these on a perfboard since my proposed enclosures have a slot at the bottom to accept PCB.

SO, I'm looking at a minimal component count setup for perfboard that I can order from Digikey.
Digikey has a great supply of 1875 and I can get the special size perfboard from them.
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Old 21st December 2011, 03:27 PM   #10
AndrewT is offline AndrewT  Scotland
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Dan + Ein,
put your numbers in and check what to expect for current capability and current demand.

Dan look again, I said Normal quiescent supply voltage of +27Vdc.
You are complaining that is too high and yet suggest +-27.5Vdc !
Can't you read english?
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