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Old 3rd November 2011, 10:29 AM   #21
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Discovery: The feedback resistor located on the original model from years ago, measures at 114k. The first post should have specified feedback resistor in the range 100k~114k.

Second discovery: Hand cramp from the ohmmeter! It absolutely doesn't matter if you buy 1% metal film, 5% carbon or any combination thereof--they all lie. It takes at least 100 resistors to find some that match.

Third discovery: Resistors with weird values are fantastically easier to match--If you buy a value that's only available for 1% resistors, you get 2% resistors. However, if you buy any "standard" value, you may get a whole bag of 10% randomness with a 1% paint job. For example, if you want 100k metal film, buying 99k metal film could be easier to match in half the time.
If only someone would have told me that earlier.
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Old 3rd November 2011, 10:58 AM   #22
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Quote:
Originally Posted by AndrewT View Post
. . . When the PSU is running at full voltage the bulb filament is hot and bleed current is set by the hot bulb parameters.
after shutdown the voltage falls and the bulb cools and the resistance drops and the bleeder tries to pull a more constant current rather than an exponentially reducing current. When down to just a volt or two the bulb will be drawing ~3times the current that a fixed resistor bleeder would be able to. . . .
OMG!! Thank you! This explains the odd little DC powered light bulbs cabled nearby the rectifier in some receivers.

My LED does something different. Its backwards to your example, but it shunts more dirty dc than clean dc. As they are run up to 1/3rd capacity, they will "attack" any noise. For example the 1156 LED replacement bulbs that mistakenly set RV's on fire because most RV converters are half wave (12vdc+6vac=18v and overrun). Perhaps I should have used a more durable 1/2w LED instead of the 1/6w LED?

I'd sure like to try that light bulb trick too.

But I'm onto working on the amp, and point-to-point has proved challenging. I can't figure out how to make the power rails even/same length and don't know if that is even necessary. If a straight bus is run across three chips, the center chip will have the highest exposure to the power supply cable. Is that a concern?
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Old 3rd November 2011, 11:56 PM   #23
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Default Amplifier photo, regular parallel stretched out to make triple parallel

Here's what I came up with for power circuit. I just stretched the Parallel LM1875 out to make room for triple parallel. It is still tiny.

Does it really need 6 of radio grade 100nF?
Will this arrangement do for power circuit? Suggestions?
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File Type: jpg LM1875 Triple.jpg (109.8 KB, 433 views)
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Old 4th November 2011, 05:26 AM   #24
gootee is offline gootee  United States
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It might be coool if you mounted everything on a blank pcb, dead bug style, and used the pcb as a ground plane. Or, if it's more convenient, and it's a one-sided pcb, mount the stuff on the fiberglass side and drill holes to the other side for grounds.

Wow, imagine using a two-sided board for a single-supply circuit so you could have both power and ground planes. But you might need to etch or otherwise remove small "anti-pad" areas for holes to the other side.
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Old 5th November 2011, 08:03 AM   #25
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Quote:
Originally Posted by gootee View Post
It might be coool if you mounted everything on a blank pcb, dead bug style, and used the pcb as a ground plane. Or, if it's more convenient, and it's a one-sided pcb, mount the stuff on the fiberglass side and drill holes to the other side for grounds.
Chicago style? I'll bet your radios are excellent!
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Originally Posted by gootee View Post
Wow, imagine using a two-sided board for a single-supply circuit so you could have both power and ground planes. But you might need to etch or otherwise remove small "anti-pad" areas for holes to the other side.
Minimum etch, shielded? WOW!!

Your insight amazes me. Although I want to to do a single supply parallel LM1875 on a different thread soon. . . I'd also like to complete this split rail amp here, And, I'm worried about it.
Actually, I'm stuck.
From the photo of post #28, the 100nF//100nF//100nf section looks like a clumsy generic guess gracelessly scaled up three times. Isn't the center extraneous? From here, what it looks like I should do is 100nF between each chip (100nF//100nF per rail). What do you think is best? Isn't there anything more graceful than painting it over with a crowd of 100nF caps? That doesn't look either optimal or specific. Is 100nF always the correct value, no matter how many there are? Help?

Thank you!
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Old 7th November 2011, 06:34 AM   #26
gootee is offline gootee  United States
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Are you asking about the power supply bypass caps, at the chips?

See the section on "Power Supply Bypassing", here:

Analog Devices : Analog Dialogue : PCB Layout

From that, it looks like the bypassing should go from power pins to LOAD GROUNDS; i.e. load ground should be connected right at the bypass caps' connection to the ground plane.

But notice that the classic "alternate connection" topology, from rail to rail, sounds like it might improve distortion performance, at the cost of higher-voltage caps.

As you can see from the link, you might want to use multiple values of caps, in parallel, maybe especially if high-speed (fast-changing) energy will be present. Maybe you could think fast audio edge (rise/fall) times for that (and scale up the cap values), but RF and other high-frequency junk will also be present, basically always. On another note, I don't know offhand if it might also be beneficial to use multiple larger-value caps in parallel, to better-cover bass/mid/treble audio frequencies. If in doubt, or since I know that you like to experiment, you could try .01, 0.1, 1, 10, 100, 1000, just for example, or maybe the classic 1-2-5 sequence "translated" for this purpose: 0.1, 0.22, 0.47, 1, 2.2, 4.7, 10, 22, 47, 100, 220, 470, 1000. Yeah, that looks like overkill, heheh. But then again I've never TRIED it. I guess I'd at least consider adding one or two more cap values, between your 0.1 and 470, just to see if your golden ears can detect a difference.

After all, the main "signal path" is the CURRENT that goes from power supply through chipamp then speakers. And I sometimes think of the bypass caps as a point-of-load power supply that has to supply at least the transients for that current, assuming we don't want the supply rails' and grounds' conductors' inductances to cause voltage disturbances when we try to instantaneously allow current to flow through them (and/or just not have the current be available precisely when demanded, as the chipamp opens and closes the "valves" between rails and ground). And if the transients are not available precisely when needed, then phasing and time-alignment might vary with frequency, which might mean the difference between good and exquisite. But that's all just guesswork, at this point.

At any rate, the point I was going to make was that the problem that you usually should watch out for, when choosing multiple bypass cap values (especially the small values), is that certain cap values might form a resonant LC circuit with the inductance of the supply rail (or ground) conductors (and whatever else contributes to the lumped inductance they see), at a frequency that can get excited by something that might happen to come along (fast edges, RF, switching spikes, etc), and tend to cause ringing or maybe even oscillation. It's a minor point, but sometimes it can bite you. I guess maybe you could apply fast-edged square waves and look for tendencies toward trouble, with a scope.

Last edited by gootee; 7th November 2011 at 06:40 AM.
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Old 7th November 2011, 09:55 AM   #27
AndrewT is offline AndrewT  Scotland
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Originally Posted by gootee View Post
I sometimes think of the bypass caps as a point-of-load power supply that has to supply at least the transients for that current,
I consider this an important fact that many have either forgotten or never researched to become aware of.

The FAST output is supplied by the tiny bypass caps if they are fitted in the correct location to provide almost instantaneous current.
The Medium spead output is supplied by the decoupling, again located so that they can meet medium speed demand.
The Slow output is supplied by the smoothing capacitors. Because these signal are slow changing, then the smoothing can be remotely located from the output devices.
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Old 8th November 2011, 03:11 AM   #28
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Quote:
Originally Posted by gootee View Post
Are you asking about the power supply bypass caps, at the chips?. . . At any rate, the point I was going to make was that the problem that you usually should watch out for, when choosing multiple bypass cap values (especially the small values), is that certain cap values might form a resonant LC circuit with the inductance of the supply rail (or ground) conductors (and whatever else contributes to the lumped inductance they see), at a frequency that can get excited by something that might happen to come along (fast edges, RF, switching spikes, etc), and tend to cause ringing or maybe even oscillation. It's a minor point, but sometimes it can bite you. I guess maybe you could apply fast-edged square waves and look for tendencies toward trouble, with a scope.
Yes, that pile of yellow radio grade caps is certainly capable of making a powerful disturbance in their passband. If that wasn't the purpose, then the amp and its schematic needs repaired to a more user-friendly arrangement.
And that's what I'm asking about.
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Old 8th November 2011, 04:52 AM   #29
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Quote:
Originally Posted by gootee View Post
. . . But notice that the classic "alternate connection" topology, from rail to rail, sounds like it might improve distortion performance, at the cost of higher-voltage caps.. . . I guess I'd at least consider adding one or two more cap values, between your 0.1 and 470, just to see if your golden ears can detect a difference.. . .
An interesting difference for rail to rail is that its not hard wired to the speaker return's powerful broadcast. So, rail to rail maybe really good for pre drive?
I will add a single Cornell Dublier, "Mallory" SEK 105c 4.7uF 250v electrolytic, rail to rail at exactly the point where the power supply dc cable attaches to the amplifier. Thanks for the reminder!
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Old 9th November 2011, 02:25 AM   #30
gootee is offline gootee  United States
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Quote:
Originally Posted by AndrewT View Post
I consider this an important fact that many have either forgotten or never researched to become aware of.

The FAST output is supplied by the tiny bypass caps if they are fitted in the correct location to provide almost instantaneous current.
The Medium spead output is supplied by the decoupling, again located so that they can meet medium speed demand.
The Slow output is supplied by the smoothing capacitors. Because these signal are slow changing, then the smoothing can be remotely located from the output devices.
Yes. But don't we then still have to wonder about fast, LARGE transients? Maybe I should look at some actual music signals with fast-and-large transients such as snare rimshots or bass drum kicks, with wav files as inputs to an LT-Spice simulation of a power supply and output stage, that also includes all of the parasitics (and compare the response to an ideal one, maybe), with varying numbers of different-sized capacitors, and maybe especially larger number of small ones (again, with most all parasitics modeled). Either that or I could set up a physical circuit and dust off a good scope and try capturing some one-shot events to compare (or just use a pulse train as input).

This is another one of those cases where the science indicates tendencies but reality could be anything from "those tendencies are less than negligible" to "wow that makes all the difference in the world". I guess the first thing to do is dust off my back-of-the-envelope calculator, or try some quick-and-dirty spice comparisons of extreme cases. But, while spice and envelope backs are nice, even if I see a difference I still almost-certainly won't know if it would be audible or not. Maybe at that point I could hand it off to some willing golden-eared type. It's a bit frustrating to contemplate.
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