AndrewT said:
I didn't think it was. I was trying to follow your suggestion, but didn't entirely understand what you wanted (a floating NFB leg on a single ended input?)
AndrewT said:
I don't have a specific source in mind, and I would not assume the source could drive a 600 ohm load, no. Most can, but I don't know how many do it all that well.
AndrewT said:
You have a point. I see where that would confuse the issue. What I meant: The input to the box would truly be balanced due to the transformer, and the whole thing inside the box (including the transformer) would be 'a gainclone'.
Even most of the folks building three op-amp buffers with instrumentation amplifiers then feed that signal into a chip amp single ended. They still call the entire thing, including the buffer, 'a gainclone'. I'm using a transformer instead of a solid-state buffer.
Perhaps I should start a new thread as 'balanced input to single ended gainclone'. Posting all the schematics was an attempt to make it clear what I was trying to do.
An aside.
A pirate walks into a bar with a giant transformer hanging out of his pants by the wires...
It's the punchline that's on-topic.
A pirate walks into a bar with a giant transformer hanging out of his pants by the wires...
It's the punchline that's on-topic.
Re: An aside.
This thing's drivin' me nuts!
(Or, a guy walks into a psychiatrist's office with a steering wheel sticking out of his pants, and says, "It's drivin' me nuts.")
xiphmont said:
This thing's drivin' me nuts!
(Or, a guy walks into a psychiatrist's office with a steering wheel sticking out of his pants, and says, "It's drivin' me nuts.")
If you have any ways to measure what you're after, perhaps it's almost time to do some breadboard experiments. The parts don't look all that expensive, even with 0.1% resistors (or add a multi-turn trimpot somewhere). And it looks pretty simple to set up the different variations that have been discussed.
Or, if you're a spice user, maybe you should first try some simulations of the various topologies. (If you're not a spice user, you could download LTspice, free, from linear.com.) For a "generic" chipamp model that might be better than nothing, there's the OP541E model from ti.com. And there are speaker system and cable models you might want to try, such as the ones at http://sound.westhost.com/cable-z.htm . Transformer modeling is covered somewhere, too. Sorry, I don't have the URL handy.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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Or, if you're a spice user, maybe you should first try some simulations of the various topologies. (If you're not a spice user, you could download LTspice, free, from linear.com.) For a "generic" chipamp model that might be better than nothing, there's the OP541E model from ti.com. And there are speaker system and cable models you might want to try, such as the ones at http://sound.westhost.com/cable-z.htm . Transformer modeling is covered somewhere, too. Sorry, I don't have the URL handy.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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No. I take care of volume control with a buffered potentiometer to avoid all the issues you guys have been discussing 😉 However, the high input impedance of the instrumentation configuration means there shouldn't be any problem using a potentiometer between the source and the amp.AndrewT said:
I incorrectly called my implementation an instrumentation gainclone. I've since just called it an instrumentation configuration with a chip amp. Technically, a gainclone is a clone of the gaincard, which is just a barbones configuration.xiphmont said:
gootee said:
Sure, and I even have most of these parts.
This discussion mostly boils down to 'I had it mostly right the first time' and a few people offered alternatives to consider (which is something I wanted, and I took some time thinking about it). It got a little sidetracked when AndrewT kept suggesting differential topologies and I thought he was suggesting some improvement to the single-ended strategy that didn't seem to make sense.
BWRX said:
Right, more actives is not a bad strategy. However I decided I was going to pursue a strategy of minimalism and a signle passive or two would do all the same for me. It is a convenient coincidence that it also means it will take much less assembly work 🙂
BWRX said:
Sure, but common usage has gone beyong calling just the replica of the gaincard 'a gainclone'; you knew what you meant, I knew what you meant, AndrewT apparently didn't, so I went into more detail to clear up the confusion. I wasn't complaining about anyone being loose with their terminology.
Anyway, for me, it boils down to choice A sans Ci or Choice B fully capacitor coupled. A scratchy pot will make either noisy, but it will make A much noisier than B. I'd want to be using at minimum a fully sealed CP pot anyway (eg, Mouser 72-PA16NP103MAB15), will that pot give me a lifetime such that I can eliminate the coupling caps in the signal path? Or is a decent coupling cap (metallized PP) at a volt and microamps just not something anyone worries about using these days? Heck, the gainclone kit, as shipped, is using a Panasonic electrolytic as Ci....
xiphmont said:
(Hm, I seem to have sold the FCs a little short, even if the tan-delta is almost 5%...)
xiphmont said:
In case anyone else is interested, that pot's datasheet is at:
http://www.vishay.com/docs/51036/p16pa16.pdf .
That potentiometer design is actually very interesting. They put the pot inside the knob, so there's almost nothing behind the panel. Cool...
So let's see... "PA" instead of "P" means it's the "professional audio" version, which makes it conductive plastic instead of cermet (and also apparently cuts the power rating in half and multplies the temperature coefficient by ten, neither of which should matter, here), and lowers the contact resistance variation from 3% to 2%. Conductive plastic is much quieter than Cermet, I believe. Good choice.
"NP" means it has a black plastic knob. Probably OK. But they do offer it in metal, too (the "NM" version), if you're interested.
"M" is the 20% tolerance. But it's a _variable_ resistor, anyway.
"A" means "linear law" instead of "logarithmic law". Is that really what you want? [But their "logarithmic law" plot looks like two straight line segments, with a breakpoint at (50% rotation, 10% of total resistance), which you might be able to get by adding a fixed resistor, if you want it.]
Regarding your original question about lifespan: They give "load life" and "rotational life" test results. But they only give one data point for each one, for which the pot doesn't appear to be too degraded, at all. So they seem to not really say how long it might last. I guess you can probably count on the 25000 rotational cycles without much change, if your power load isn't too high.
All in all, it seems like a pretty good choice, especially for $8.98 qty 1. But you still might want to do at least one search (probably for both P16 and PA16 series, maybe at diyaudio.com and at http://groups.google.com), to see if anyone who has some experience with that model line has anything to say about it.
About eliminating the coupling caps: I'd say go for it. But that's just my impulsive opinion. Here's an idea I just thought of: You could (at least) geometrically reduce the chance of total pot failure (i.e. wiper eventually lifting) by using a DUAL pot, with the elements wired in parallel. For example, you'd get a dual 20K pot and wire the two 20K pots in parallel to get your 10K pot. Then, if one wiper lifts, you've still got the other one, for double the resistance instead of infinite resistance (assuming I'm correctly imagining how you'd want to wire them). And then you'd probably notice it and realize it was time to replace the pot, before the other half failed (essentially making it failure-proof, as well as fail-safe, for that failure/amp-damage mode at least). Two parallel pots might even be electronically better than one, too; maybe sort of like a wider trace on a PCB, giving lower inductance. But I have no idea whether or not some other, possibly "bad" effects of having two pots in parallel might be more significant. My guess would be that probably nothing about it would be significant except the fail-safe feature (and the slightly bigger size and cost, of course).
Sorry to "ramble on" about that, for so long.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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gootee said:
Oh, if I linked the linear one I brainoed. I intended to link the log one. As for the plastic knob, I'll be using plastics on the back panel anyway (power, breaker, XLR gasket, lift switch).
gootee said:
Having looked over real research (eg, done by EEs whose job it is to design audio sans pandering to superstition) I see no compelling reason to avoid the coupling caps, or even the FC on the NFB leg.
For every audiophile who claims electrolytics ruin the sound, there's some other superstitious whack who claims BlackGates on all his inputs 'smooths out' the sound, revealing 'hidden nuance' and 'refined detail'. 🙂
National gave us the chip amps, they're damned good, and they recommend coupling and filter caps. I'm inclined to believe National on this one.
gootee said:
No worries, was easy to tell what you were getting at. Either way 'fail' wasn't really going to be a worry. a 470mV (worst case) 'crackle' wasn't going to hurt or bother anything except the listener ;-)
xiphmont said:
Well, I did say that that opinion was "impulsive". 🙂
I AM an EE. So maybe I should be embarrassed. Just for the record, I'm sure that my tendency toward disdain of "audio superstitiousness" is as at least as great as anyone's. And I am sure that it is safer to use the coupling capacitors, in most cases.
However, even National states that a coupling capacitor will degrade the audio quality. For example: "For best audio performance, the input capacitor should not be used.", after Equation 6 on Page 14 of the LME49810 datasheet, which I just happened to be reading, a little while ago. But it's already obvious that using coupling caps will always tend to limit the low-frequency response, in any amplifier circuit. The _only_ significant positive aspect of their use is in blocking DC (and very low frequencies, if they happen to be a problem).
So using them or not boils down to a "trade-off" decision, as does sizing them if you do use them.
Many times, an affirmative decision about whether or not to use coupling capacitors is based on the simple fact of having no way of knowing whether any possible future input device might have some "dangerously-high" level of DC present, with its output. But in your circuit, no DC at all should ever be coming in through your coupling transformer. So if your chipamp's inputs' impedances were balanced, or at least if input-current-induced voltages at the chipamp's inputs were not different by "too much", then DC offset at the output should not be a problem, except in the case of something equivalent to an "open wiper" failure in your potentiometer (and depending on the values of the input and feedback-divider resistances used, etc), in which case you would usually get a very large DC voltage at your chipamp's output pin, which would definitely be "a BAD thing".
But, if it's just a matter of not trusting the pot wiper forever, and since your "Choice A" schematic already has always-unbalanced (and varying in difference with pot setting) input impedances for the LM3886 inputs, anyway, it sure SEEMS like you could just add a fairly-large resistor to ground between R1 and the + input, in your "Choice A" schematic, to guard against an open pot wiper, and still omit the coupling caps. (You'd probably want to keep the resistor as small as possible but without sacrificing too much gain, to limit the noise produced by the resistor. Another trade-off.)
I don't know the relative failure rates of pot wipers and fixed resistors and solder joints. But even in your "Choice B" schematic, i.e. WITH the coupling caps, R2 failing open (or a solder joint in that vicinity failing open) would be at least as catastrophic as the pot wiper failing open for "Choice A".
It's very difficult to protect against all kinds of failures (or even some, or just one). I suppose you could use two parallel resistors, in place of each resistor, to get closer to "safe", in this case.
Is any of it worthwhile? Another trade-off. It's your amp.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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gootee said:
...as am I. But I come from a different set of specialties. Analog high-fidelity just isn't my forte, I'm not going to pretend it is. I have to deal with 140dB deep stuff all the time-- but only in software dsp ;-)
gootee said:
I agree there are reasons to use coupling caps carefully (eg, factor 5/50 in low end rolloff so LF attenuation isn't really a worry). The distortion figures for modern caps are damned near objectively indetectable for foil/film (which I will use for the input couple just to be sure-- it's also nearly immortal) and even the modern low-ESR eletrolytics are very good. Power supply grade electrolytics even look to have better characteristics in some ways in that they move charge *fast* for their size. Way better than big metallized (which is why I'm no longer quick to dismiss the FC).
Can I hear the difference? I'm going to build it both ways myself and listen. Golden ears are one thing I do have. Better use them while they last.
gootee said:
AndrewT already prompted me into doing the math on that, and a pot failure would just be annoying, no worse. 100-500mV is not a big deal (although the noise would be loud, the driver I'm using has thermal/mechanical properties well in excess of what this amp will be able to drive).
What I am *actually* thinking about in choice B is limiting self-noise as the pot ages. If the CP gets even a little bit 'scratchier' with age like carbon does, limiting the gain on that scratchiness is probably worth doing. I don't want the attenuator on this thing to sound like wind into a microphone in five years. Choice B cuts down on the potentiometer self-noise by 30dB in the worst case.
gootee said:
Well, if we're going to decide not to trust solder joints over time, there's no saving any design 🙂
Anyway, looking for the log version of that Vishay pot, no one stocks it. Mouser quotes a minimum order of 20 (at $20) and a 16 week lead time to get it, despite it appearing in the catalog. Bourne precision and a knob it is! The Bourne 91 series are cheaper and unsealed, but claim a 100k rotation life. The 51 series 9also available) are about 50% more expensive and sealed, but claim half the rated life.
xiphmont said:
Very good. Sorry for my "speechifying". I'm probably much farther from knowing a lot about analog audio than you are.
(Sorry if some of this is too far off-topic. I just wanted to ask you about capacitor choices.)
I have never understood the "reasoning" of the "no caps in the signal path" crowd. And thanks for the further insights about capacitors.
I have spent a fair amount of time selecting capacitors for analog signal-processing circuits, and for switch-mode power supply circuits, over the last couple of years, and have even implemented a dielectric-absorption correction circuit (similar to what Bob Pease suggested) for an integrator (to try to get the last little bit of linearity improvement for a precision ramp generator). I remember that I was never able to find a source for Teflon capacitors, and usually used polypropylene, or polystyrene, instead, when low dielectric absorption was needed. But for very small cap values, Bob Pease had a great suggestion for making Teflon ones: Just get some Teflon-dielectric coax and trim the length until it has the desired capacitance!
Regarding the "power supply grade electrolytics" that you mentioned: Do you have any specific ones in mind, that you like? I usually ended up using Nichicon's UHE series, for medium-to-low values, probably mostly just because Mouser (and Digikey, IIRC) carried them. Their 3300 uF 35V UHE-series model has an ESR spec (given for 20 degC/100 kHz) of .013 Ohms max (which is similar for the same 16x40mm size for lower voltages and higher capacitances), and a ripple current rating of 4.08A RMS (@ 105 degC/100 kHz). I also used their 2200uF/50V UHE model, which has a .017 Ohm ESR spec.
But I also used smaller UHE-series electrolytics as DC blocking caps (e.g. 220uF/10V), in audio-frequency circuits (in test/measurement equipment) where very low distortion and very level frequency response were both important, but frequencies could be as low as 60 Hz (and up to at least 22 kHz). Now I'm wondering if there is a better choice than the UHE caps. The signals, in one case, were triangle waves, typically less than 350 to 500 mV P-P, with 220uF/10V cap current in the 180 uA P-P range. Unfortunately, the cap diameter could only be 6.3mm max. In another case, the signal is sinusoidal, 5V P-P, with cap current of about 250 uA P-P. In that case, I also wonder if it's really necessary to use two caps, back to back, to form a non-polarized version, if a 50 uF value is needed.
I hope you'll share your results.
That's only if you do have the coupling caps, I assume.
I just don't know if five years would be a problem, with a conductive plastic pot. "It depends." But ten or fifteen years might be, especially if it's not sealed. Some of the guys in the TekScopes group, at yahoogroups.com, would really know for sure.
They can be saved, if you throw suitcases of cash at them.
The probabilities of problems being caused by such failures can sometimes be reduced geometrically for each level of redundancy used, in cases where it might be worth the cost. I used to work in aerospace/defense, analyzing and designing guidance and control systems (but not at the electronic board/component level, in my case). You might be surprised at the amount of effort put into planning for such scenarios, in certain types of systems. In a lot of cases, a single-point failure of _any_ kind was not allowed to cause any degradation of functionality, or near-term system survivability.
Sorry to have blathered-on for so long, again.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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gootee said:
Heh, that's funny and something I'd probably not have thought of. Too bad most coax is actually polyethelyne or polypropylene though.
gootee said:
I've usually used the Nichicons you mentioned or one of the Panasonics. Like you, I suspect that's mostly because it was what Digikey has. The hobbyist electronics scene has entirely dried up from when I was a kid. There used to be two good electronics parts stores in each of the malls near where I was a kid, now Digikey and Mouser are all that's left. I miss Olsens and Active.
I have wondered the same about polarization given that the gainclone kits are using polarized caps in their filter and output stages-- definitely not polar circuits! I've been assuming my worry is unfounded and there's something I'm missing.
(Oh, and it is odd how the current 'style' in virtually all caps is low and squat 🙂
gootee said:
My design durability criteria really isn't all that much more than "I want my stuff to survive long enough that my kids will be old enough to appreciate the workmanship." 🙂
gootee said:
No worries, I *live* for this kind of geeking.
xiphmont said:
The AC component mostly passes thru the cap so it is really only the polarity of the standing voltage that is of concern.
AudioFreak said:
Correct, it is the persistent bias of the circuit in question. However, earlier in this thread, we established the bias can be positive or negative; the DC offset is not necessarily positive (or negative); individual variation in 3886 is enough to change that.
this should not be the case.
If the front end is NPN type (or Nch) then the bias is negative.
If the PNP (or Pch) the the bias is positive.
If the front end is dual complementary National would advertise it as such and this is extremely unlikely. A bias compensated front end could swing either side of zero bias.
AndrewT said:
No, I mean: Look at the Zobel on the output stage. The DC output offset, given input bias offset variation, could end up being negative or positive in the most common designs. But the Zobel is using a polarized cap.
Even if the bias is positive,
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