Build a better Volume Control

[websherpa]

In my undieing pursuit of better volume mods... I did a little research and came up with this simple tweak on adding linear potentiometers (pots) that sweetened the sensitivity on my external volume controls for King Camel. (I just wasn't getting good results with L-Pads and I wanted to have external volume controls on all my Pachis.)

http://sound.westhost.com/pots-f8.gif

Reference: http://sound.westhost.com/pots.htm

VOL is the potentiometer, and R is a simple resistor. I used a ratio of 5:1 for the resistance of VOL(linear pot) : R, so for the single woofer, I used a 500 Ohm Panel Mount Linear 1/2 watt potentiometer, with a 1/2 watt 100 Ohm resistor for R, and for the stereo piezo tweeters I used a 5k Ohm Dual Gang Panel Mount Linear 1/2 watt potentiometer, with a 1/2 watt 1k Ohm resistor for R.

It does mean tapping into the negative gnd for the speaker so it'a an extra wire and a little more soldering.

Works like a charm and so much more variability between loud and soft. So far sounds so good (you can tune them just until the speaker hum (squeal) disappears, and still have a decent volume). You still get hum at the peak volumes though - though nothing beats those overdriven bass cues that King Camel puts out when you spin!

[Eddie]

Which Potentiometers and resister did you use ? Did you get them from Radioshack?

[websherpa]

Hi,

The info you requested is in the post "so for the single woofer, I used a 500 Ohm Panel Mount Linear 1/2 watt potentiometer, with a 1/2 watt 100 Ohm resistor for R, and for the stereo piezo tweeters I used a 5k Ohm Dual Gang Panel Mount Linear 1/2 watt potentiometer, with a 1/2 watt 1k Ohm resistor for R."

Parts came from various sources, surplus electronics and mouser.com I think.

[Oldpapa]

I hope this helps..
You should move the 100 ohm resistor to the input side of the control. Reason is to keep a constant lower resistance to the amp output. The squeal you here is oscillation which will destroy the IC's.

In fact, a lower value resistor and a bit higher wattage and not a carbon type is good. Then the control could be any value. The lower the value of the control means better volume adjustments. So a 10 ohm or 100 ohm can work.

I have just started playing with these machines as a side adventure. Got 12 really poor machines and out of the 12, got 3 in really good condition, and 2 that need locks and keys. Another with no sound, missing a board I think. However, I worked on the one with no sound and the IC rate high wattage. They run off the 24-28 volt supply and demand some current. They are like 20 watt amps and you do not want them to run without the correct load.

BTW, this site really is great.. Kudo's to the owner. I will try my best to help on the electronic side.. Been doing it since I was a kid.. like 10 or 12.. 57 now.. so a few years. Remember Tubes?

Old..Out

[websherpa]

I hope this helps..
You should move the 100 ohm resistor to the input side of the control. Reason is to keep a constant lower resistance to the amp output. The squeal you here is oscillation which will destroy the IC's.

In fact, a lower value resistor and a bit higher wattage and not a carbon type is good. Then the control could be any value. The lower the value of the control means better volume adjustments. So a 10 ohm or 100 ohm can work.

I have just started playing with these machines as a side adventure. Got 12 really poor machines and out of the 12, got 3 in really good condition, and 2 that need locks and keys. Another with no sound, missing a board I think. However, I worked on the one with no sound and the IC rate high wattage. They run off the 24-28 volt supply and demand some current. They are like 20 watt amps and you do not want them to run without the correct load.

BTW, this site really is great.. Kudo's to the owner. I will try my best to help on the electronic side.. Been doing it since I was a kid.. like 10 or 12.. 57 now.. so a few years. Remember Tubes?

Old..Out

For some reason I missed this when first posted. Thank you for the info! Can you post a diagram so that I and others can understand the connection better.

I will have to check where I put the resistance R (The SAmmy's squeal without the external pots and limiting resistors in too.)

I have a 14 watt (7 watts per channel) 300B Stereo Tube amp that I built from a kit and it warms that Jazz just so nicely that everyone who hears them on my Mini-MKIII Paradigm's (not the best but no slouches either) with their eyse closed believes the singer is in the same room (well on some recordings). And because of the efficiency of the speakers and the clean (albiet warmly distorted in the mid range), 7 watts is most of what you need for any room and it will rock!

Tubes rock, ask any experienced guitarist.

One thing that confused me about this schematic as just what "Out" and "In" referred to. I assumed because of the direction of the arrow tabs that "Out meant the lead coming Out of the Amp, and "In" meant the lead going into the speaker.

Did I get that right or is it the opposite ("In" meaning into the circuit, and "Out" meaning out of the circuit). (So you see I may have wired it up correctly as yo suggested because I confused the direction of the circuit.

Thanks for any light you can shed!

[Harleybum]

Without getting TOO technical I will try to explain exactly why a "L-Pad" is the way to go.
Some of the audio power amps are single ended, a single device driving the load thru a electrolytic cap for DC isolation. Others use a totem-pole configuration.Still others use a DC coupled circuit where the speaker is connected between two transistors and one is fed 180 degrees out of phase.
#1 rule of thumb, NEVER and mean NEVER connect either speaker wire to chassis ground.It must ALWAYS referance the return wire.Connecting them to ground or power (more than likely) will destroy the audio output stage.
Single ended/Cap coupled amps do not care about the load impedance.They usually referance ground as the return.Check with a meter from either side of the speaker BEFORE you modify.If you find a static DC voltage there BE CAREFUL..
Constant impedance is very important in DC coupled circuits.This is where the "L-Pad" comes in handy. The radio shack units are in reality tapped transformers. They work great.The primary is constant at 16 Ohms, the secondary is tapped and does not affect the load on the primary side.

Simply inserting resistors in series is OK on the Single ended amps, NOT OK on the DC coupled/Totem-pole type outputs or the PNP-NPN pair driven out of phase.
The use of a "L-Pad" is the safe way to go. Yes..they are bigger, more expensive than a 500 1/2 pot..but overall it will last forever, sound better and the chance of damage is the same as the factory speaker.
The engineering of the Radio Shack tapped transformer unit is good, exactly what the doctor ordered.
Another way to do this is to find the signal INPUT to the final amp, cut the trace on the board with an exacto knife and install a 10K ohm pot there and reduce the work of the audio amp.This is NOT for the first timer
Hope this helps explain the "L-Pad"

H.B.

[websherpa]

Thank you!

The general consensus on the board also has been that L-Pads are best, but that an inline pot is much cheaper and if you don't lower the volume to nil, then there is still a load on the amp. So far I haven't read of anyone losing their sound board (although a few have blow low wattage trim pots).

When I saw the circuit below I assumed that the fixed resistor would keep a load on the amp (and additional heat dissipated through the pot). The values I used were from researching the article referenced, not practice (which I veered away from some 25 years ago (still dabble - but I gave up on learning the theory and switched to software).

I use L-Pads on my Hanabi Hyakkei pachislo (with a very narrow range of volume control). But on the King Camel, it made no difference (pot with trim resistor vs. L-Pad) and the configuration in the schematic below not only sounded a little better, it gave a much better "range" to the volume control (plus I think because of their lower resistance, the L-Pads didn't work on the piezo speakers).

The ground referred to in the schematic is the speaker's negative terminal.

I agree the RadioShack version with its isolating transformer is a great option, engineered to control volume for specific room zone speakers coming from a single amp (and still keeps a load on the amp at zero volume). It's just that is's 1) Big! 2) Expensive - squeezing one into a Pachinko would be a challenge.

Any more theory / practicum you can add to this discussion (volume mods for pachinkos and pachislos) would be greatly appreciated by all here I am sure since we all toy with it eventually (unless you're already hard of hearing I guess) .

[Harleybum]

OK..no problem, glad to help anyone I can. We can go ahead with Audio 101.B if you like
DC circuits are easy to deal with, they make sense and follow simple Ohm's law.
AC circuits on the other hand are differant animals entirely. AC reactance, Free air resonance and capacitor coupling all have effects that dont make sense in a DC world.
Here is the deal, you will find that the smaller speakers and "Tweeters" are highly efficient, in fact the added volume controls hardly affect them...here is why.
"Tweeters" will produce more sound pressure for much less wattage due to the size."Piezos" even more.Since they are wired in parallel, they produce a lot of sound at lower power output.The larger the speaker, the lower the free air resonance frequency.The larger it is the more power it takes to move the mass and the farther away you go from the resonance the more power it needs to do its job.This is why they run over 10 watts of audio in these machines.
Smoking resistors and controls......
You will notice at lower levels all is OK. Even with high pitched sounds they still perform.When the speaker needs to reproduce a LOW pitched sound,, the amp works hard, the speaker pulls the power needed to do the job and the 1/2 watt device suddenly is handling 10 watts of power.Imagine holding 2 4 watt nightlights in your hand for a few minutes.Same idea...power dissapation is power dissapation.For the modification to be successful, the control needs to be able to handle 10 watts of power.The smaller 1/2 watt 300-1000 ohm controls WILL work on the smaller speakers.They will not work right on 3-5 inch speakers at all.
A common design cheat is to wire the smaller speakers directly in parallel with the big speakers, BUT to include a 1MFD cap in series. It stops the Bass and midrange cold, protects the small speaker from burnout and passes all the high freq audio to there it belongs.In the old days they called this a "Crossover" network.
Bottom line, you CAN run regular "pots" if you make them at least 10 watt devices.About 25 ohms will do the trick.
Why 25 Ohms you ask? I will explain (I am not a good teacher).
Here is the deal, lets say you have the control adjusted to half. The amp "sees" the full 25 ohms, but now there is a 16 ohm speaker in parallel with HALF the pot.the Amp really "sees" 12 ohms with the other 12 ohms and a 16 ohm in parallel.The 12 ohms and the 16 in parallel is about 7 ohms. 7 ohms and 12 added together in SERIES equals 19 ohms...close to the original 16 ohms circuit. This keeps the amp happy.
Going below the rated output resistance can cause the output device to sink WAY more current than it was designed for.(NOT all designs).
Some audio output stages will oscillate if you upset the output impedance.THis CAN BE DEADLY for the output device.They can and will oscillate at a freq that you can not hear! The speakers will sink the current and the device will commit hairy-cairy.It will pull current until it melts down. There is no warning.
Best thing to do is this, try to keep the impedance close to the designed amount. If the factory speaker is 8 ohms, 16 ohms even 40 ohms...try to match it.

If your machine has a 1 watt amp..a little LM386 chip or a TAD2002 or similar no sweat. They are single ended, you cant kill them. But if you have a modern Pachislo with hefty 10/15/20 watt amp...be very careful.
Hope this helps, dont mean to be long winded...

H.B.

[websherpa]

Long winded is good, pictures even better (at a thousand words each, much more efficient).

What I'm wondering then is why all of us with 1k trimmers on our more modern Pachislos aren't experiencing more problems? My circuits (with pot instead of trimmer, plus the extra resistor in parallel) certainly have held out no problem on my exceedingly loud and low bass King Camel. Teach me, I'm a student of all things needed to keep my pachis "alive"! Man if we could only develop a way to eliminate Sammy Squeal (at full volume) while adding volume controls, you'd be famous. In my rough guestimation, I think the squeal is related to some signal interference or RF leaks. When one moves their body around inside the machine (it's not easy, but generally it's just an arm), the squeal can sometimes be modulated.

[Harleybum]

Hi Wayne,
I have not been able to reproduce the "Sammy Squeal" on my popeye. Squeal at full volume is usually due to power supply foldback. A lot of times if you "Stiffen up" the supply with a larger filter cap that will stop.Sometimes its due to board layout, the traces are too small either on the Power side, or it can be the grounds around the power output devices.The squeal you mention where it changes as you move your hand about the case is an interesting one.It is possible that the audio section is oscillating.I have a handheld scope I use on Solid state pins I can use on the Sammy and see if theres anything there. There is SO much reserve gain in these machines, getting good audio should be an easy task.I will look into this sometime this week, next weekend is a long one for me.It may be an easy "fix" such as tacking a 100Mfd cap across the power pins on the audio preamp or power amp. The "Popeye" is all SMC so it's not as easy as a regular PCB.

[websherpa]

I have not been able to reproduce the "Sammy Squeal" on my popeye.

I wonder if that's because it's a newer model and they've worked out the kinks. Does Rupan or anyone whose played a Sammy like King Camel in a real parlour know if they hear squeal on the original house supply and ground (I ground all my pachis, by the way, but this made no effect on the Sammy Squeal), that is, if you could actually hear anything inside a loud parlour.

[DI9FFTR731]

Much cheaper, and more stable than using a variable L-pad is to just make a permanent L-pad with two resistors.

If you don't know how to do the math, here is an on-line calculator to determine the ohms of the two resistors you will need, depending on how many dB you want to lower the audio.

30 dB seems like a good choice for the loud bottom speaker, the top two can be controlled with the 3 position dip switch on the audio board.

L-Pad (Driver Attenuation Circuit) Designer / Calculator

Select 8 ohms for the driver impedance (Z) if you have an 8 ohm speaker. (speaker will show how many ohms and watts on the magnet)

If you don't have the right value resistor, you can combine resistors in parallel to get what you need.

Here is a parallel resistance calculator...

PARALLEL RESISTOR CALCULATOR

Most know if you put two 20 ohm resistors in series, you get 40 ohms, but if you put two 20 ohm resistors in parallel, it cuts down to 10 ohms.

You can handle more power if you use resistors in parallel too.

[PALADIN]

Another way to do this is to find the signal INPUT to the final amp, cut the trace on the board with an exacto knife and install a 10K ohm pot there and reduce the work of the audio amp.This is NOT for the first timer

Wouldn't it be easier, and ultimately cheaper and safer to control volume with smaller cheaper pots at the amp input rather than output? It seems the better way to go. There don't seem to be any posts about how to do that--yours being the only mention of it I've found. I would be grateful for any info on this approach for a Sammy Aladdin II evolution.