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.  

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THEY DON'T STATE IT

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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.

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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.

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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