Simple Chip Amp for P to P wiring

[danielwritesbac]

Originally Posted by Juergen Knoop

depends...use an auxiliary supply and you don't 'need' to have any drop at all. However ripple supression maybe poor, but voltage limitation is all you want or not?
Regards

Yes, I want only to squash the power supply's peaks so that the bass notes play in proportion.

[Juergen Knoop]

eh Daniel, we don't speak the same language and I'm not referring to my awful english.
You may want a stiff regulated power supply without giving away any headroom?
A supply that doesn't sag under load, and yet provide the maximum voltage the chipamp can eat?
Regards

[coldcathode]

Juergen,

with all do respect to Daniel sometimes he does "Sprecht Shiza" LOL

In any case, Power Supply "ripple rejection" / "stiffening" IS one area where TUBES and SS are very similar.

Daniel,

In all seriousness you have to think of ripple rejection/capacitance and Voltage lowering as three interrelated objectives.

In the tube world the PSRR of tubes is not as good as chips. So we rely upon "smoothing" the DC as much as possible. This is imperative in just about ALL tube designs regardless of application.

I often use a very old set of analogies that I learned a long time ago.

Think of electricity as WATER, the pressure is Voltage, Current is VOLUME.

Think of the output (sound) as a fixed size jet that can be turned off and on rapidly (like a fuel injector in a car) We want to be able to supply the WATER needed for ANY of the frequencies of the "jet". Low frequencies require MORE water but the PUMP (the Power Supply) takes TIME to produce a given volume/pressure. The HIGH frequencies are pretty easy since they require very short pulses at a given pressure and thus MUCH less Volume (current). BUT, low frequencies need the equivalent Pressure but MUCH more VOLUME.

So we have a pump (transformer) it has "pulses" at a specific frequency. (120Hz here in the US using a bridge) this travels down pipes or hoses to the different components. We also use the water to "activate" the jets valve. Any pulses on the water line are then transfered to the jet at that frequency regardless of the control circuit. Pulses above a certain threshold will result in water being spurted out of the jet. We need to limit the pulses so the jet functions as we want it to.

So we place "accumulators" (capacitors) in the line they fill up with a quantity of water to the pressure of the pump. While full if the jet calls for equal or less Volume than the pump delivers we get the supply from the pump not the accumulator. If it calls for more we get it from the accumulator until the accumulator is "exhausted" and we are then relying on the pump again.

Just use HUGE accumulators correct? Well that works to some extent because given infinite capacity we get perfect smooth endless supply of water. BUT, we cannot afford infinite capacitance because there is a limit to them imposed by its resistance (ESR) and cost, size etc.

The best solution is always a compromise. Since we have a pulse with a defined frequency (time) we can then "filter" it out by using a resistor and capacitor together to IMPEDE the pulse. (cannot eliminate it but can cut down its amplitude).

If the PUMP can supply a reasonable amount of Pressure and Volume then we simple place a filter in series that has a reactance suitable to impede the pulse.

This involves reducing the PRESSURE (voltage) thru the resistor in proportion to its resistance.

Since you have EXCESS voltage you can lower it by placing an RC filter in the line. You can use some of the calculators available online. Heres one to help.

You will have to do all the "playing around" with it but, most of the designs I have seen for chip amp supplies seem to have no rhyme or reason with which the are designed for ripple rejection (probably because of the high PSRR of the chips). Neverless I see no reason why we cannot "clean up" the supply further if you have excess voltage to spare.

A 1R resistor coupled with a 10,000uF cap to ground attenuates 120hz ripple by 18dB. You only lose 3V at 3AMPS RMS. Two of these in series should get rid of 6 volts and cut down on the ripple.

These need to be BIG resistors. BUT you could use larger caps and smaller R's to achieve similar results. Example: 0.5R and 30,000uf = loss of 1.5V at 3A and 21db of 120hz attentuation.

Now that we have that under our belt, how that relates to sound. Bigger caps equals TIGHTER bass, since we can supply more current more quickly.

Smaller caps = "muddier" bass or "SAG" sometimes (like in a guitar amp) "SAG" is desireable. In the "Audiophile" world it is much less desireable.

So the moral of the story:

Look at most tube amp power supplies, most will call for a Transformer with an output voltage at least 25% higher than the B+ requirement. Then the excess voltage is burned off in the RC or LC filters. Capacitance requirements are proportional to the current demand.

Typical TUBE PS might be something like this
1K5 --> 40uF --> 100R --> 150uF -->load @ 100mA we lose 160V but attenuate 120hz by like 70db

For SS current demands we need to factor the Resistors DOWN and the capacitors UP.

so for similar ripple rejection @ 3 amps and loss of 1.5V we could do this
0R5 --> 30,000uF --> 0R5 --> 30,000uF --> 0R5 --> 30,000uF --> load

By using "trios" of paralleled 10,000uF caps there is some advantage in that the ESR's act as filters also.

So you see that IMHO there is no substitute for capacitance in the PS and within reason you can never have enough.

BTW, you need to watch the cutoff frequency of the filters since it will also lower your bass response if it gets to high (above 10hz or so) since the output is AC you do not want to limit the ability to supply the current.

Sorry for the long winded post but it actually helps me to think this all thru from the beginning.

[danielwritesbac]

This calculator, from your link above, confuses me a bit, because. . .

It seems to combine the desirable, a 19db drop in noise, with the undesirable, the more current = the larger the voltage drop. My current draw should vary between idle and max on some music sources. Problem!

Question: Does the calculator represent an increase (worsening) of voltage fluctuations, or does it represent voltage fluctuation that would be present anyway, whether or not you used an RC? Sinking the voltage, only to have it sink yet more under load, is pointless when it is already fairly clean DC voltage.

Problem: Right now, the power supply seems able to convert signals from well below the 60 cycles, now up to the ghz range, including flipping the DC meter on open key transmitter, vacuum cleaner running from same outlet, etc. . . get a signal and you get yet more DC. Not cool! Noises are all converted to DC--really clean DC, along with a whopper of a caveat--about 10% of its total output is "weak" DC (top 10% of voltage has insufficient amperage). This is wrong. Its really unseemly for a bass amplifier. How to make only strong DC?

[danielwritesbac]

Originally Posted by Juergen Knoop

. . .
A supply that doesn't sag under load, and yet provide the maximum voltage the chipamp can eat?
Regards

Yes.

It seems plausible that this would make an ample amount of very pretty bass along with BPA200 and 30+30vdc rails. At least it seems to be worth a try. Heatsinking may be a small challenge, but its one that I can do.

Maybe someone can relate this comment:
Not using any weak power, may make the sound of strength.

[danielwritesbac]

Hey guys, are we trying to make something like this: Excellent 2 X LM3886+6N11Tube Audio Amplifier Board - eBay (item 190349126840 end time Nov-22-09 20:26:20 PST)

[danielwritesbac]

Well, this test round was a wild guess. I've no idea what a T-network is for anyway. Here's the DC offset pictured at 10mv.

[Juergen Knoop]

Originally Posted by danielwritesbac

Hey guys, are we trying to make something like this: Excellent 2 X LM3886+6N11Tube Audio Amplifier Board - eBay (item 190349126840 end time Nov-22-09 20:26:20 PST)

is this somekind of business proposal?
Anyway...this pcb is not DIY-friendly, because of the difficulties of heatsink mounting AND a man should show his tube, instead of hiding it in an amplifiers case.
Let the tube punch through the top of the case like an...you know!
Regards

[danielwritesbac]

First test was this Building a Gainclone chip amp with T-network feedback. by Franz G, Joe Rasmussen and Nick Whetstone.

I just adapted it for LM3886 and connected it to the power supply posted earlier. A 4.7uF 250v polyester (normally seen on tweeters) was added to the output of that power supply at V+ to V- to short noise that would have otherwise increased with frequency. Caps at the chip pins are 470uF//100nF (perhaps they should be 330uF and/or perhaps the input dc block cap should be slightly smaller).

Curious results:
1). First time that I've ever heard an LM3886 not screaming, shouting, or otherwise misbehaving like a poorly compensated op amp. Yes, it sounds perfectly acceptable. Very decent large soundfield too. And the pretty bass sound of LM3886 is present (otherwise, I think that nobody would spend so much time fooling with this little chip).
2). Far too much gain!!! Not even the weakest MP3 player needs so much gain. I'd be the last person to complain about too much gain, but this really is too much.
3). Practically no power--it didn't play a bit louder than LM1875, until. . .
4). Since there's no NFB cap, I tried an output cap, and ended up with partial success up until the output cap trial and error became just like a copy of one rail of the power supply. . . then (only then) there was appropriate output power. Apparently, 0mv dc offset works much better than 10mv dc offset. Weird.
5). Maybe there is too much bass. You certainly wouldn't need a bass booster with this one.

The T-network design had too much gain and a few other oddities. It sounds surprisingly nice.

[danielwritesbac]

Originally Posted by Juergen Knoop

is this somekind of business proposal?
Anyway...this pcb is not DIY-friendly, because of the difficulties of heatsink mounting AND a man should show his tube, instead of hiding it in an amplifiers case.
Let the tube punch through the top of the case like an...you know!
Regards

For that application, the 6N11 somehow looks insufficient. Perhaps this: 6L6 may be more interesting?