LM1875 PCB, Which To Use

[Iván Francisco]

It's better a regulated PS to drive just only a tweeter?
Thanks

[danielwritesbac]

Originally posted by Paswa


daniel, please explain it a litle or draw or edit my attached pcb image to show me the solution.. I can't figure it out

Well, I can't figure it out either, but, here's a guess:

EDIT: This I made so that you can listen to the amp before you order PCBs. The design is comptable with plain vero (phenolic) and experimenters board, which has pads like this: http://www.futurlec.com/Pictures/EXPBRD.jpg so you try different components without getting "tangled" up.

[danielwritesbac]

Or this? Maybe it'll give someone a head start on making one up complete with active preamp? Anyway, I think there might be room.

[danielwritesbac]

Originally posted by Iván Francisco
Can I drive a pair of tweeters with your design, in a triamp project, without a dc protection circuit?
Thanks

Ivan, most of the designs at this thread have the extra 2 caps on the left side. One decouples the NFB and the other decouples the Input. That is DC protection already.
Its a slight hassle on component selection for those two parts, but a small collection of suitable candidates (for those two parts) can resolve that in a short timeframe. Its especially easy on a triamp speaker project.

The design pictured above can handle 28v rails (good diodes with a 36v center tap transfo).
That amp mates with 6" (almost xmax) to 8" woofers fairly well. It also does, midranges, tweeters, and the lot.

[Iván Francisco]

Hi Daniel,
thanks for the explanation, it was crystal clear.

[Iván Francisco]

This is the layout to make a home made pcb, is quit good?

[Iván Francisco]

I forgot to mention a couple of things:
The 22k fb resistor goes under the board, it can be surface weld, the same with the 100n decoupling caps (near the chip).
Question: the 22u cap "must" be non polarized? voltage:16V good enough?
Dimentions: width 33mm, deep 25mm
PS: sorry my language guys, you know where I leave.

[danielwritesbac]

Originally posted by Iván Francisco
This is the layout to make a home made pcb, is quit good?

It looks nice.
I see two possible problems.

The load that is driven that drives the chip is 22k. Its not hooked up that way. Maybe an SMD will fit horizontally?

Ah! Here goes.

The input circuit goes:
Source/preamp (pass),
potentiometor or 22k (load),
1k (pass),
1m (load),
1uF (pass),
22k (load)
LM1875 (pass)

You can exchange the positions if the 1uF or 1k if you have a preamplifier.

---------------

Next up, a small technicality. The input filter cap, marked square, means poly. That's fine, but 2.2uF might pass more bass than the 22uF cap in the NFB, and you don't want it to do that.
Here's some options.
a) Input, 4.7uF electrolytic--NFB 22uF
b) Input, 1uF poly--NFB 22uF
c) Input, 2.2uF poly--NFB 47uF
Any one of those will have the input filter cap pass less bass than the NFB, which is what you want it to do.

See the attached spreadsheet for reference. Dial in LM1876 because its calculations will be quite similar to LM1875.

[danielwritesbac]

Originally posted by Iván Francisco
Question: the 22u cap "must" be non polarized? voltage:16V good enough?

The NFB cap can be polarized. All of mine are.

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Here's your gain settings for NFB:
Gain for preamp, 22k vs 1k,
Gain for line level source, 33k vs 1k,
Gain for computer source, 40k vs 1k,
Gain for headphone level source, 44k, vs 1k,
*Its possible to increase the values in proportion (to each other) for a difference in sound, yet at the same gain setting. The spreadsheet above can calculate that.

----------

On the 16v cap question, well, that "seems" a bit small to me. The above examples will deliver only a small voltage into that cap. However, you may wish to interview a variety of capacitors.
Most of my personal preferences for this component have been 100v caps, because I have used whatever sounds nicest to me.
Different brands can give you variety; however, different voltage ratings of the same brand can also give you variety.
I think it takes a comparison of a small collection of samples, for best results.

[danielwritesbac]

Originally posted by Iván Francisco
voltage:16V good enough?

The LM1875 can operate on DC voltages from 8v to 30v (abs max 32.5v); however, normal operation is 25vdc rails.

Normal:
Normal voltages for LM1875 are given by the common and inexpensive 36vct (18+18) transformer. You need at least 36va for each LM1875. At this voltage, that's 1 amper (minimum) for each LM1875. At the low price, monobloc format (+dynamics) is cost effective.
After power supply and rectifier, you get average 26vdc rails.
Expected speaker output power is 28 watts.

EDIT2: The higher voltages are workable for 22 watts in 16 ohm speakers.

EDIT: If at 16vdc rails, LM1875 produces 10 watts for 8 ohm speakers or 15 watts for 4 ohm speakers. So you decide.