LionChief Curcuit Board with 4 Ohm load?

@SteveH posted:

Here's a link to the datasheet for the amplifier chip that this module uses: PAM8403

Not much info on the board mfg.'s site HiLetGo PAM8403 Super Mini 5V 3W*2 Digital Amplifier Board USB Powered Volume Control

Maybe I missed it, but I'm not seeing it's input impedance.  

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

THEY DON'T STATE IT

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

1. To get a better idea of what we're dealing with, could you measure the resistance across it's input connections with the volume control potentiometer fully clockwise and again with the pot fully CCW?      THE POT VARIES THE OUTPUT NOT THE INPUT
2. Do you have an assortment of 1/2 or 1 Watt resistors in the 1 to10 Ohm range?  YES
3. How about an assortment of 1/4 Watt resistors in between 100 and 1kOhms?  YES
4. What values of potentiometers would you have on hand?  I HAVE A LOT OF DUAL TAPER 10K AUDIO POTS FROM ALTOID TIN HEADPHONE AMP PROJECTS I USED TO DO.  ALSO SOME 10K TRIM POTS

Feel free to PM me with answers to 2, 3, and 4.

John

Good to know.  So what's the measured input resistance?  I'm guessing around 10kΩ, but it would be good to measure it and also make sure that the pot doesn't affect this reading.

@SteveH posted:

Good to know.  So what's the measured input resistance?  I'm guessing around 10kΩ, but it would be good to measure it and also make sure that the pot doesn't affect this reading.

No reading even at the R x 1 scale regardless of pot position.

John

It makes sense you wouldn't see a reading at the lowest scale.  What the highest resistance scale you tried?

For the heck of it I put a 4 ohm wirewound resistor in series with a 4 ohm speaker.   As I suspected the volume was even lower.

John

@SteveH posted:

It makes sense you wouldn't see a reading at the lowest scale.  What the highest resistance scale you tried?

Yes, of course.  Around 40kΩ Steve.  Regardless of pot position

You have an Ohms symbol on your keyboard?

John

Catch 22- trying to measure an amplifier input impedance (AKA complex resistance) using a multimeter. Why? The meter supplies DC, and many amplifiers specifically use isolating capacitors that block DC. You need a frequency generator and sweep the input across an audible frequency range and THEN you might get a result.

The tip is the name impedance- not resistance. Impedance is used implying a complex state (range of frequencies) VS DC steady state- what most meters are only capable of.

@Vernon Barry posted:

Catch 22- trying to measure an amplifier input impedance (AKA complex resistance) using a multimeter. Why? The meter supplies DC, and many amplifiers specifically use isolating capacitors that block DC. You need a frequency generator and sweep the input across an audible frequency range and THEN you might get a result.

The tip is the name impedance- not resistance. Impedance is used implying a complex state (range of frequencies) VS DC steady state- what most meters are only capable of.

Vernon, of course you're right.  Impedance is a vector: either between resistance and capacitance or resistance and inductance.

However, in my experience, one can sometimes get an approximation (within 20-30%) of the impedance of an audio device's input by measuring resistance.  Usually they're in the vicinity of 150, 600, 1k, 2k, 10k and sometimes 40kΩ or higher.  John's measurement will give me a better idea where to start with experimental T-pad resistor value suggestions.

@Craftech posted:

Yes, of course.  Around 40K Steve.  Regardless of pot position

You have an Ohms symbol on your keyboard?

John

Omega and other special characters can be accessed on my computer, like so:

Not sure how to access these special characters on my phone though...

@SteveH posted:

Vernon, of course you're right.  Impedance is a vector: either between resistance and capacitance or resistance and inductance.

However, in my experience, one can sometimes get an approximation (within 20-30%) of the impedance of an audio device's input by measuring resistance.  Usually they're in the vicinity of 150, 600, 1k, 2k, 10k and sometimes 40kΩ or higher.  John's measurement will give me a better idea where to start with experimental T-pad resistor value suggestions.

Omega and other special characters can be accessed on my computer, like so:

Not sure how to access these special characters on my phone though...

Nice.  Thanks.  Never noticed it.

John

From a 1997 article by Tom Engdahl entitled Connecting speaker signals to line level imputs:

Suitable for 1W - 50W amps.  Signal attenuation 20dB

Adjustable output level

If you want to adjust the output level then replace R2 with 1 kohm trimmer. This makes it possible to make the output level to be adjustable from 0V to line level.

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

I may try it.

John

@Craftech posted:

From a 1997 article by Tom Engdahl entitled Connecting speaker signals to line level imputs:

Suitable for 1W - 50W amps.  Signal attenuation 20dB

Adjustable output level

If you want to adjust the output level then replace R2 with 1 kohm trimmer. This makes it possible to make the output level to be adjustable from 0V to line level.

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

I may try it.

John

John, I'd say go for it with the 1kOhm in the R2 position, starting at full attenuation and then slowly increase the voltage.  Maybe obvious, but just in case it's not, with the trimmer, the Line Out would be connected to the center tap on the potentiometer instead of between the resistors as shown in the original schematic.

@SteveH posted:

John, I'd say go for it with the 1kOhm in the R2 position, starting at full attenuation and then slowly increase the voltage.  Maybe obvious, but just in case it's not, with the trimmer, the Line Out would be connected to the center tap on the potentiometer instead of between the resistors as shown in the original schematic.

Thanks Steve,

I ordered a 1K trim pot (I only had a lot of 10K) so I used the fixed resistor.   The HiLetgo amp had the same volume output with two different 4Ω speakers as the original 8Ω stock setup.

Then I tried an LM386 board I had ordered and it fried.  Really low quality.  Those are going back.

Right now I am experimenting with a tiny Adafruit 2.5W Class D mini mono amp.   With the trimmer all the way up there was an increase in volume with one of the 4Ω speakers.  I haven't tried the other one yet.

I'll keep everyone posted.  The 1K trim pots will be here on Tuesday.

By the way, the AC - DC buck converter is fantastic.  You can dial in precise DC voltages from track power.

John

John,  Based on your experiments, it seems like the LC amp's output is already low enough that the R1{10kΩ} R2{1kΩ} L-pad is attenuating the signal too much.  If you still have the 3 + 3 W HiLetgo, you could easily devise a lesser L-pad attenuator with -3, -6, -12, and -18dB connection taps providing the output signal.

Since you already have a good resistor assortment and understanding of audio terminology, I'm going to recommend that you use something like the resistor ladder shown in Figure 2 within this Article Attenuators for Measurement by Jonathan Novic.  Note, that the 1Ohm resistor values shown in Figure 2 are representative and can be substituted (see Table 2) with other values such that each resistor in the ladder has the same value.

@SteveH posted:

John,  Based on your experiments, it seems like the LC amp's output is already low enough that the R1{10kΩ} R2{1kΩ} L-pad is attenuating the signal too much.  If you still have the 3 + 3 W HiLetgo, you could easily devise a lesser L-pad attenuator with -3, -6, -12, and -18dB connection taps providing the output signal.

Since you already have a good resistor assortment and understanding of audio terminology, I'm going to recommend that you use something like the resistor ladder shown in Figure 2 within this Article Attenuators for Measurement by Jonathan Novic.  Note, that the 1Ohm resistor values shown in Figure 2 are representative and can be substituted (see Table 2) with other values such that each resistor in the ladder has the same value.

Although the article was written to address the effects on high wattage amps the idea sounds good, but if I am attenuating the signal too much either a trimmer (which is arriving on Tuesday) or trial and error might be more practical than a ladder.

But see if this sounds right:

The Lionel amplifier circuit probably puts out 2W at 8Ω which is 4V.  That's around +14dBu which varies.  My guess is that the average level of announcements, chuff, horn, and bell is around +4dBu which is exactly LINE LEVEL.   So either no attenuation is necessary or very little to avoid possible distortion.

John

@Craftech posted:

Although the article was written to address the effects on high wattage amps...

Not just high watt amps, the L-Pad design principles discussed in the article apply to any amplifier circuit in the audible frequency range.

@Craftech posted:

...So either no attenuation is necessary or very little to avoid possible distortion.

John

I think this is likely, I'm now thinking somewhere between -3 and -9dB may be about right.

As far as testing with the ladder, I'd use one of these project breadboards or without that, a series of alligator clip leads.

Also note: the ladder doesn't have to necessarily have to be made with an individual resistor in each of the locations shown in the diagram.  For example, where two 1Ω are in series between taps, a 2Ω could be substituted.  There are several locations where substitutions could be made to yield essentially the same circuit for testing purposes.  When making substitutions, as long as the ratios between Rseries and Rshunt remain the same, the circuit attenuation is the same at each tap.

I have not seen suggested yet is Lionel's separate Amplifier board. PN# 8007-110 or 691-8018-T25. S&W Train parts lists the 8007-110 in stock.

@SteveH posted:

I think this is likely, I'm now thinking somewhere between -3 and -9dB may be about right.

Since the Lionel amplifier circuit's output is adjustable with the LC remote would that not act as a variable attenuator that can be left alone once the ideal output level is found?   Adjusting is cryptic enough that it is unlikely to be accidentally changed.

John

@Chuck Sartor posted:

I have not seen suggested yet is Lionel's separate Amplifier board. PN# 8007-110 or 691-8018-T25. S&W Train parts lists the 8007-110 in stock.

My concern with that is that the DC ground of the Lionchief amplifier is NOT the same as the separately powered and half wave rectified old school amp board. In fact, I'm thinking the problem exists even with these other amp boards and may have been why the LM386 popped.

The lionchief board rectifies ALL power coming in through a full wave bridge. Then in theory, the DC common (mistakenly called ground in some cases) would not be equal to frame ground. Older Lionel electronics often used half wave rectification, such that AC frame common was actually DC common too in those circuits.

This is a problem- that straight line assumes that the power source for the amplifier is at this same DC common potential.

@Chuck Sartor posted:

I have not seen suggested yet is Lionel's separate Amplifier board. PN# 8007-110 or 691-8018-T25. S&W Train parts lists the 8007-110 in stock.

Thanks Chuck.

The railsounds amp might be a good option as an auxiliary amp, but I think it is larger than the tiny ones I am working with.  I also have some stick on weights inside the tender and a DIY back-up light circuit.

John

@Vernon Barry posted:

My concern with that is that the DC ground of the Lionchief amplifier is NOT the same as the separately powered and half wave rectified old school amp board. In fact, I'm thinking the problem exists even with these other amp boards and may have been why the LM386 popped.

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

There have been other complaints in the user reviews about quality issues with those particular boards.   So that can't be discounted.

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

The lionchief board rectifies ALL power coming in through a full wave bridge. Then in theory, the DC common (mistakenly called ground in some cases) would not be equal to frame ground. Older Lionel electronics often used half wave rectification, such that AC frame common was actually DC common too in those circuits.

This is a problem- that straight line assumes that the power source for the amplifier is at this same DC common potential.

From Tom Engdahl's article from which the circuit diagram was taken:

Notes on using the circuit

Remember to check that the circuit is connected in the right way every time you use it. If you mix up speaker out and speaker ground connection, you can end up shorting speaker output to ground, which can damage your amplifier and/or other equipment connected to it. If you want to redice the risk of damaging anythign then put 10 ohm resistor between the circuit and speaker ground connector (this will make sure that short circuit impedance is always at least 10 ohms, though it can introduce sometime hummign problems because of increase grounding impendace).

John

@Craftech posted:

Since the Lionel amplifier circuit's output is adjustable with the LC remote would that not act as a variable attenuator that can be left alone once the ideal output level is found?   Adjusting is cryptic enough that it is unlikely to be accidentally changed.

John

That could work.  Although, I can see the possibility of an inadvertent press of the announcement button while simultaneously attempting to throttle up from stop.

Once you discover the right amount of attenuation, constructing a final design L-pad would only require two resistors.

@Chuck Sartor thanks for the tip about the Lionel's separate amp board [PN# 8007-110 or 691-8018-T25].  I did find it on the S&W Parts Site, but there's no detailed info about it there or on Lionel's parts site.  Do you happen to have any specs for it?

Sorry, no specs. Track power in, speaker input and then out to the speaker.

@SteveH posted:

That could work.  Although, I can see the possibility of an inadvertent press of the announcement button while simultaneously attempting to throttle up from stop.

Once you discover the right amount of attenuation, constructing a final design L-pad would only require two resistors.

@Chuck Sartor thanks for the tip about the Lionel's separate amp board [PN# 8007-110 or 691-8018-T25].  I did find it on the S&W Parts Site, but there's no detailed info about it there or on Lionel's parts site.  Do you happen to have any specs for it?

I seem to recall somewhere that it's output is 5W, but I don't know if that is into a 4Ω load or an 8Ω load.  EDIT:  That can't be right, sorry.  Here is the board in an American Flyer PB-1 dummy.

https://www.lionelsupport.com/...26-48117Complete.pdf

It is connected to a 600-8152-536 speaker.  I looked up that speaker.  It is 16 Ohms and it is only rated at 0.2W.

John

@Craftech posted:

From Tom Engdahl's article from which the circuit diagram was taken:

Notes on using the circuit

Remember to check that the circuit is connected in the right way every time you use it. If you mix up speaker out and speaker ground connection, you can end up shorting speaker output to ground, which can damage your amplifier and/or other equipment connected to it. If you want to redice the risk of damaging anythign then put 10 ohm resistor between the circuit and speaker ground connector (this will make sure that short circuit impedance is always at least 10 ohms, though it can introduce sometime hummign problems because of increase grounding impendace).

John

Unless you are also powering this external amplifier from the very DC rectified sources on the Lionchief board- there is a chance of the audio ground or common being mismatched potentials creating a ground loop situation.

Yes, one could work around this with a speaker to line level converter that provided electrical isolation (often using a transformer- but there are other methods) to again prevent ground loop on a single ended signal like line level input.

Again it would be shear luck, that the amplifier you are using externally, in theory with a bridge rectifier and regulator, that the DC common would be noise free and at the same potential as the Lionchief amp speaker negative output.

Vernon, thank you for your insights on these ground issues. I believe John is using a buck converter to power the secondary amp.

Regardless, it seems that a check for DC potential between the onboard LC amp Out (-) and the secondary amp In(-) would be in order.

Isn't the speaker in the 0-8-0 rated at 3 watts? I think Lionel's separate amp board would provide plenty of sound. But I'm not an electronics engineer.

@Chuck Sartor posted:

Isn't the speaker in the 0-8-0 rated at 3 watts? I think Lionel's separate amp board would provide plenty of sound. But I'm not an electronics engineer.

You would think, but alas - No.  Completely drowned out by track noise even at low speeds unlike Santa Fe Super Chief starter set diesel loco.  Dramatic difference.

John

Update:

Here's where I am with this experiment.

1.  Definitely louder sounds with either the HiLetgo or Adafruit mini amps with a 4Ω speaker (tried two different ones) powered by an AC/DC Buck converter.   Seems to work the best without any attenuation on the Lionel amplifier speaker outputs.  Input level can be controlled with the Lionel remote.  Both mini amps also have trim pots.

The problem with it is that there is an interruption in the sound when it goes across the insulated sections of the FasTrack switches (see below)

Those are necessary to prevent shorts with certain wheels (Example:  Lionel Santa Fe Super Chief RTR set).

The only way to combat the interruptions would be for me to install a second pickup on the other truck which I really don't want to do.  Maybe someone else someday may want to try that.  Frankly I have had it with messing with mini amps inside this thing.

I removed all of it.

2.  So then, despite the warnings about overloading the Lionel amp I tried two different 4Ω 3W speakers in place of the stock 8Ω speaker and guess what?  It made no difference in the loudness at all.  Neither 4Ω speaker was any louder than the original 8Ω speaker.

So I back went the original setup with the pill bottle baffle for placebo effect.

I may try an 8Ω 0.5W speaker for the heck of it.  The aforementioned SF RTR set has one of those and sounds great.

Thank you everyone for all the help with this project. 

John

John, have you considered adding a bigger filter capacitor in parallel with the buck converter's output to prevent short power drops to the secondary amp?

Choosing capacitor sizes, isn't my forte, but I'm thinking maybe a 4000µF may do it, something like this.

https://www.mouser.com/Product...i7jDbygbqAvd2c3pw%3D

Maybe some else could suggest a better value than my rough guess...

@SteveH posted:

John, have you considered adding a bigger filter capacitor in parallel with the buck converter's output to prevent short power drops to the secondary amp?

Choosing capacitor sizes, isn't my forte, but I'm thinking maybe a 4000µF may do it, something like this.

https://www.mouser.com/Product...i7jDbygbqAvd2c3pw%3D

Maybe some else could suggest a better value than my rough guess...

I thought of adding a current keeper Steve.  I may try it after I try the lower wattage speaker, but the problem would probably be solved if I added a second pickup roller.  One would always be in contact with live current despite the taped over sections.   Still the cap would probably be easier.

John

John, out of curiosity, what are the microfarad and voltage values of the two bigger electrolytic caps on the buck converter?

I had the sound drop-out issue with all of the newest Legacy steam with the modified RS-Lite board.  I had to create a fix for it, and of course it involved large capacitors.

New Legacy RSL4 Sound Dropout Issue and Fix

@SteveH posted:

John, out of curiosity, what are the microfarad and voltage values of the two bigger electrolytic caps on the buck converter?

1000µF / 50v

470µF / 35v

John

@gunrunnerjohn What do you think about installing this 4000μF 35V Electrolytic in parallel with the Buck converter's output?

Should an inrush current limiting resistor be added in series with the DC+ out and the Cap's positive lead, maybe something like a 1/4 watt 100Ω?

But then connect the 2nd audio amp's input directly to the Cap positive lead?

A 100 ohm resistor would defeat the purpose of the large capacitor!  Also, the capacitor would be far more effective if it were on the input side of the buck converter after the rectification.

For the board pictured below, I'd install any bulk capacitance across the large filter cap prior to the switching section, that's the best place for the extra filtering.

Ok thanks!  That does make sense.  So then the extra cap should also be rated for 50V, like the existing 1000µF / 50V, right?

Well, it could probably be a 35V one since the most voltage you're going to apply is track power.  22V * 1.4 = 30.8 peak volts out of the bridge rectifier.

You're probably right.  I was thinking of matching the Voltage to the other 50V Cap as well as those pesky Transient Voltage Spikes I keep hearing so much about

The rating on the module is AC 5-30V DC, and DC 5V-48V, so they needed the 50V input cap rating for full range input voltage.  We're not using it that way, so no need to add the size or expense of a higher voltage rating.  Transient spikes will have no effect on the bulk capacitance.