A Tale of Three LM1875 amps - Page 3 - diyAudio
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Old 28th February 2008, 06:07 AM   #21
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Hi ttan98
Thanks for that, I was lucky my Dad was a carpenter so I picked up a few hints along the way.
Got any rain in Melbourne yet, I was down there a few weeks back for a break (my wife and I love Melbourne) and it looked pretty dry, the drought has well an truly broken in our little town.
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Old 28th February 2008, 09:29 AM   #22
ttan98 is offline ttan98  Australia
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Quote:
Originally posted by Zero One
Hi ttan98
Thanks for that, I was lucky my Dad was a carpenter so I picked up a few hints along the way.
Got any rain in Melbourne yet, I was down there a few weeks back for a break (my wife and I love Melbourne) and it looked pretty dry, the drought has well an truly broken in our little town.
Drizzle over the last 2 days, not down pour, good enough for the garden though but still not sufficient for the dams.

This summer is not so dry enough rain to stop my plants dying, 2 plants and 1 tree died last summer.
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Old 28th February 2008, 03:25 PM   #23
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Before this thread gets completely hijacked by open baffle speaker discussions (I'm also considering an open baffle midrange, so I'm eager to learn more about it, but not on my amp thread!) and Australian weather reports, let me respond to Daniel.

I was mistaken about Rin; I don't understand the purpose of the 1k series resistor. Okay, so now I know the 22k is Rin. Thanks for clearing that up. In that case, I probably won't mess with it. I intend to try the 330pf in parallel with Rin next.

However, the rest of your babbling about resistors leaves me totally confused. You really need to work on your communication skills. I mean that in the nicest way possible. I didn't understand a single thing you wrote, and I've noticed this tendency in some of your other posts, and similar complaints from other readers, so I know it's not just me. I do appreciate your contributions, but I admit I ignore certain parts of them.

Regarding the speaker ground: once you jumper around the original PCB ground trace, you must then break that trace somewhere between the speaker/zobel path and the large coupling cap or you will have dual paths to ground, and that's probably not good! Don't worry about permanently altering the PCB, because you'll never consider going back to the original layout. If you do, for some reason, you can simply jumper the break you made.

I'm almost tweaked out on this little amp, and pretty soon I'll have two identical amps to try in stereo. Glad you're all enjoying the journey along with me.

Peace,
Tom E
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Old 28th February 2008, 03:48 PM   #24
AndrewT is offline AndrewT  Scotland
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Hi Mad,
you should (must?) fit a pair of filters on the input.
The High Pass to block DC coming from the source and a Low Pass to attenuate the Radio Frequency interference that comes in through the cables.

The 1k0 series resistor and the 330pF capacitor from line in to signal ground are your RF (low pass) filter and are a single pole (-6db/octave) passive filter. They cut the treble by 3db at their turn over frequency.
If Rs (source impedance) is 100r then the total R in series with the input signal is 1k0+100r=1k1. The C is 330pF.
The filter frequency is (F-3db) 1/Pi/2/C/R=438kHz. Quite high. This could usefully be made a little lower, possibly as much as one octave lower, but at 200kHz you may notice a slight change in timbre of the highest harmonics.

The DC blocking capacitor can be fitted in the source equipment or in the next amplifying stage. Most builders put it in the power amp input and in the preamp input.
This is a series line input capacitor with a resistor to ground (Rin=22k).
If you use a 10uF series capacitor with 22k Rin the F-3db=0.7Hz

You should ensure that these two passive filters set the bandwidth of your amplifier.
If your amp cannot properly handle 0.7Hz, then you must move the high pass filter up until it becomes the bottleneck. i.e. make the capacitor lower in value, or add a smaller cap to the source equipment.
If the source has a 2u2F cap then the effective capacitance is now 2u2+10uF=1u8F and f-3db=4Hz. This is noticeably bass light with most wideband speakers. Experiment with these two filters to find what suits your ears and your equipment. BUT, remember that bandlimiting rule.
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Old 28th February 2008, 04:59 PM   #25
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Andrew:

Thanks very much for your response. I find your posts to be technically astute and yet mostly understandable by even a novice such as me.

There is a DC blocking cap present. Supplied in the kit was 1uf electrolytic which would yield a 7hz -3db. I found that a little bass shy even on my small speakers, so I replaced with a 2.2uf film cap. I like that sound a lot more.

Regarding RF filter, there is no provision for it in the QK50. Based on posts I've read in other LM1875 threads, I purchased 330pf ceramic caps to add a filter. I did not understand filters at that time, but I think I'm beginning to see it now. With the 1k series resistor supplied with the kit, the 330pf from input to ground would yield 438k -3db point. If I change the 1k to 1.5k (I have lots of extra resistors of different values, but no other caps on hand), I will get a low pass filter with a corner around 300k. That seems like a good compromise to me. Is all that correct?

One of the problems I have had in reading all these threads is the bewildering array of labels for various components in the simple circuit. The terms Cin and Ci and Rin and Ri and Rb mean nothing to me. I look at the NSC schematic, and all R's and C's are numerically labelled. On top of that, my kit also uses numbers, but they're slightly different. When I compare the two schematics, I can figure out which components are equivalent, but when someone says Rin or something similar, I have no idea what they mean. I'm beginning to understand, but it's been difficult.

Thanks to everyone for your patience in explaining these concepts. I promise I will put all your advice to good use and build an amplifier you would be proud of and pleased to hear.

Peace,
Tom E
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Old 28th February 2008, 05:16 PM   #26
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by madisonears
..... I purchased 330pf ceramic caps to add a filter. .............. With the 1k series resistor supplied with the kit, the 330pf from input to ground would yield 438k -3db point. If I change the 1k to 1.5k
yes increasing the resistor value will lower the RF filter frequency.
What is the source impedance? If it's <<200r then changing the resistor to a higher value makes sense.
If Rs >>500r then retain the 1k0 or 1k5 and listen to the result. But it might be worth trying 2k0 and then paralleling an extra resistor to lower it to listen for any difference.
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Old 28th February 2008, 07:54 PM   #27
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Hi madisonears
Let me apologise for hijacking, thinks got out of hand, so sorry.

Zero One
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Old 28th February 2008, 08:12 PM   #28
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Zero One:

No worries, mate! Your contributions are worthwhile, even if not always on topic. You've helped a lot with my amps, and I'm looking forward to learning more about the open baffle configuration.

Peace,
Tom E
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Old 28th February 2008, 08:44 PM   #29
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Quote:
Originally posted by madisonears
One of the problems I have had in reading all these threads is the bewildering array of labels for various components in the simple circuit. The terms Cin and Ci and Rin and Ri and Rb mean nothing to me.. . .I'm beginning to understand, but it's been difficult.. . .
My apologies. I really do try to be as "organized" as possible. Its just that I can't help but think of the many interrelationships, rather than thinking that one component does only one job.

EDIT: Thanks for the compliment!

Click the image to open in full size.

This is from LM1876, (the lesser power stereo LM1875 version with spike noise added). The picture shows the National Semiconductor resistor names corresponding to the Overture Design Guide spreadsheet.

You've probably already figured this out, but here's a handy chart anyway:
R1=Rb -input
R2= an optional noise-prevention
R3=Rin -input (the optional 330pF cap parallels this load)
R4=Ri -nfb
R5=Rf -nfb
R6= the output zobel resistor (values valid from 2.2R to 5R)
C1=Cin -input
C3=Ci -nfb
C4= the output zobel capacitor (values from 0.1uF to 0.22uF)
X1=Audio Input -Purpose of this "additional load" is noise-prevention.

Input bandwidth:
The input load with the most influence on sound is the load closest to the chip, "inboard" of the input filter cap. For purposes of compatibility with the design guide spreadsheet, this is still referenced as Rin. This is R3 on the K50 kit.

Relationship at the input:
Rin (k50's R3), representing a load, and Rb (k50's R1) in-series resistor, represent a relationship of an almost wide open potentiometer.
These two resistors are partners.
Simply put: 1k with 22k is a common figure, and so is 2.2k with 100k. Those are merely baselines (starting place).

Passive preamp:
The good news is that the K50 kit comes ready to use with a passive preamp or for use as pot+source. That's because it has a complete (also called "armored") input circuit.
So, your purchase of this kit was very smart.

On the passive preamp topic, lowering Ri (R4) down from 10k, will be louder. At some point, gain boost will also boost the sonic signature, badly. Perhaps the "almost unnoticable" borderline is 4.7k (when partnered with 150k).

Active preamp:
Past a certain amount of gain, then hiding the amplifier's sonic signature (by voicing) becomes mighty difficult. This is when you choose to add another amplifier--an active preamp, and the point is that neither one has much gain, and so neither highlights its own sonic signature to excess.
If using an active preamp, then C1, X1, and R2 (passive pre input circuit components onboard K50) aren't required if those safety features are, as expected (not guaranteed), already provided by the active preamp.

Bridged:
A potential way around the problem is using bridged mode. As seen in the vastly award-winning miniature Tripath amplifiers, Bridged amplifiers apply equal and opposite force to both speaker output poles. This also happens to apply equal and opposite force to the amplifier's sonic signature. Other potential benefits include increased soundstage and power handling/output, depending on application. I think that this is either unexplored for LM1875 or a well-kept "shop secret" in case this potential is realized.

Relationship between the input and the NFB:
There is a relationship between Rin (k50's R3) and Rf (k50's R5).
Perhaps those wiser than myself can explain it.
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Old 28th February 2008, 09:13 PM   #30
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Quote:
Originally posted by madisonears
At the same time that I moved the speaker ground, I also replaced the crappy polyester zobel cap with a nice quality MKP, and that also probably contributed to the improved sound.
Problem. The stock K50's Zobel has invalid values of 1R + 0.22uF. That zobel is audible. Any change will be heard inside the audio band. Valid resistor values start at 2.2R for partnering lousy caps, and at 3R for partnering quality caps.

If you increased the quality of that cap (decreased the ESR), then expect to see brown spots, or smoke/fire, forming on R6, the zobel resistor. Have I done this? Make a guess--just one.

There's no way to tell which of the changes quoted above resulted in the improvement. But, I'd sure like to know.

P.S. Its good to remove the zobel before voicing the amplifier.
A higher quality, lower ESR cap at the zobel with only 1 ohm resistance, will have depressed the treble. That's abusive to the amplifier (too much load) and its a masking practice. Instead. . . Decreasing the upper treble may also be done by "sliding" the NFB values down (smaller numbers both resistors) by 16% to 20%. Decreasing the "middle ground treble"/"upper midrange" can also be done by increasing the onboard capacitance from 220uF to some larger figure. As a last step, after voicing, add the zobel back on, but not within the audio band.
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