L-Pad Question

[owennewton]

the Star Wars cell that I turned into a clock has had a lot of play (cheater switch) and has been on 24/7 as of the middle of November last year. last month I attempted to hook up speakers to it (never did as I never figured out where to mount them) and the amp still works fine and sounds as clear and loud as ever.

[FTKServices]

Thanks you very much for clear that up Daverob. I have the small pots on all of my machine & have never had a problem with distortion, heat or one burning up.

[Sederien]

The amps will not be damaged by disconnecting the speakers.

Spiffy.

That means I can test my yet-to-have-volume-controls pachinko without worrying about the neighbors.

I'll try this on New Road Construction later today. Thanks, daverob.

[MrGoodBurn]

I can't tell anyone here that I have ever had an amp burn up on me from running cheap pots, or by disconnecting the speakers. I have never tried running a Pachi with disconnected speakers since it has always been ill advised. I have a few machines that still have the cheap pots both "chip" style and the standard ones that Hiro installs. I have had several of the pots burn up on me in the last few months. One of the "chip" style just quit working on my DRM machine, but would make an attempt to pass audio while moving the adjustment around. Six of the standard pots installed by Hiro have failed; two failed within the first ten minutes of of machine use. The last one went up in smoke (literally) yesterday during a fever round on Galaxy Train. The Sankyo machines appear to be the ones that are eating pots due to their high power level. I have removed volume controls from two of my machines (Maharaja and Cutie Honey) since the volume levels are not that bad.

So here is my point of view on the Pot vs. L-Pad debate. If you are happy with your pots, and they are not burning up on you; go for it... I will personally continue to convert my machines over to L-Pads because the pots are burning up on me, and I would rather not take any chances with my precious Pachi's. It's only $9.00 plus shipping for a stereo 15w/8ohm L-Pad, and the standard pots are $5.00 plus shipping for two. I would rather spend the $4.00 more for a reliable, long life solution that I don't have to replace all the time.

[Sederien]

MrGoodBurn, did you have any of those machins set on high volume mode before adjusting with pots?

I would assume that could cause such issues easily. I have all of mine set on chisai (low).

[MrGoodBurn]

MrGoodBurn, did you have any of those machins set on high volume mode before adjusting with pots?

I would assume that could cause such issues easily. I have all of mine set on chisai (low).

I have each one set to the lowest of the three volume levels. The four machines that kill volume control pots are Julie Pinball, Star Wars, Indiana Jones, and Galaxy Train. All are Sankyo Lumina frame machines. Star Trek was about to kill a pot the other day as well. I turned them all up when I started to smell some burning from one of the lower controls. The volume switch on Star Trek was also at the minimum level.

[JohnD0406]

Ok I'm fed up with the untruths that the L-Pad brigade use to justify their obsession.

I'll assume you're not referring to me, as everything I've posted is absolutely correct. Call me out on anything you may disagree with and I'll prove my case.

The amps will not be damaged by disconnecting the speakers, or by connecting them to a load that is not 8 ohms.

This is a myth based on fact. High power audio amps are damaged when run at high volume without a load. How do I know it's not a myth? I burned up my home audio amp by doing just that, before my audio engineering training. It was a 200W amp, and it let the smoke out. I agree with you though - low power IC's are far less of a risk.

As for the "load that is not 8 ohms", you do go on to qualify that statement with the minimum 2 ohm load. Anything less can easily burn up the amp. Certainly 0 ohms (a short circuit) would. The speakers in these machines are rated for 4 ohms (nominal), so I would guess they can dip to 2 ohms at certain frequencies. I personally wouldn't want to present my machines with a 2 ohm nominal load, just in case...

Another undeniable fact of running a low impedance is higher heat. Over time (a long time), that heat burns the circuit boards. This can cause the traces to lift and crack. I've fixed many boards with that problem. Again, that's long term, but I plan to keep my machines long term.

The ONLY benefits of using an L-Pad is reliability (as they're usually rated to carry the full output power of the amplifier), and audio quality at full load (as small pots that are not quite up to the power will cause distortion due to thermal effects).

Those are two very good reasons! My primary reason, by far, is audio quality. Being an audio engineer, hearing distortion drives me crazy. Those little 1K pots cause severe distortion, and I can't listen to that.

On a pachinko machine a small preset pot is absolutely fine providing that it is adequately rated for it's use (ie not rated at 1/8W or 1/4W or made from carbon film

That's the primary problem with these "volume controls" - they're NOT rated for the power these machines deliver, in most cases (qualification, as I have not personally tested every dealers' pots).

The bottom line is in an audio circuit, resistors at the output stage are the wrong way to go about things. It's ok in some cases, but we don't want to train everyone that it's ok in every audio application. L-pads are the correct way to control audio ouput, in every case.

[daverob]

I'll assume you're not referring to me, as everything I've posted is absolutely correct.

I wasn't referring to you directly. What you have posted is indeed true, whether it's entirely relevant to this situation or not.

This is a myth based on fact. High power audio amps are damaged when run at high volume without a load.

This can be true with some designs of audio amp that are intended for the upper end of the market (ie audiophiles). In these designs they sometimes use current feedback in the output stages to improve the quality of the audio. They also tend not to have any protection circuits as the additional components are thought to negatively affect the audio quality, and so can be vulnerable if the output impedance specifications are not adhered to.

Of course this isn't the case for the integrated circuit amplifiers that are designed for consumer equipment. If your iPod exploded because you powered it up without the headphones connected, or your surround amp started smoking because you only connected the front speakers, then you'd have a lot of unhappy customers.

The speakers in these machines are rated for 4 ohms (nominal), so I would guess they can dip to 2 ohms at certain frequencies. I personally wouldn't want to present my machines with a 2 ohm nominal load, just in case...

All the pachinkos I've seen use 8 ohms speakers running off BTL amplifiers, though the amplifiers will be usually be specced at 4 ohms. Besides the pots are wired in series with the speakers, so they will only ever increase the impedance seen by the amplifier, and thus decrease the output current driven by the amplifier and the overall power dissipated by the amplifier. So I would say that your argument of low impedance causing excess heat doesn't apply here either.

Being an audio engineer, hearing distortion drives me crazy.

Yet you recommended people using L-Pads to set their volume control switches to maximum. On Lumina framed machines this puts the audio amplifier chip into the 5-10% THD range (soft clipping), whereas set to the Middle and Low settings it's around 0.2% (check the datasheet if you're not sure).

That's the primary problem with these "volume controls" - they're NOT rated for the power these machines deliver, in most cases (qualification, as I have not personally tested every dealers' pots).

I completely agree with you. An underspecced pot is bad and many dealers supply machines with pots that are not adequate for the power output of these machines. But it doesn't necessarily mean that a correctly specified pot is also bad and an L-Pad is the only 'good' way, which is the way the argument comes across whenever L-Pads are mentioned around here.

The bottom line is in an audio circuit, resistors at the output stage are the wrong way to go about things.

And right on cue you completely validate my argument by stating 'Resistors wrong, L-Pads correct.'

Resistors are not appropriate in every case, but there's nothing inherently wrong about them. You'd be laughed at if you suggested that they are OK for audiophile circuits (unless maybe they are non-spiral wound oxygen-free resistors with gold plated terminals ), but in these cases apart from the possibility of a little more distortion they're perfectly fine.

[teqkng]

well now if we want to go all that far why not just put a line level converter in the system and pipe them all to a central amp.

[BigBearSteve]

:offtopic: C'mon Guys, I didn't start this thread to get into the battle of L-pad vs resistor vs no speakers blowin' up amps.

My concern was the amount of heat that my L-pads were putting off. Nothing more, nothing less.

[teqkng]

No steve we are just putting out all of the possible solutions to the heat issue. Every one is actually valid. I think we just got carried away with some of the tech spec. Some of these things are beyond the capability of the average. It does however make a good reference thread for the future because there are MANY different options represented and their ups and downs. And besides the fact the answer will vary somewhat for every different machine and amplifier spec in them. I think I will get an agreement from everyone posting that while an Lpad is likely the best "Generic" or "Overall" solution there are plenty of others that are options. I just set up a little system of jacks on a bunch of my rental machines that play a low volume MP3 track we made of a bunch of machines being played. I use a little linked amp and a cheap mp3 player to pipe it through all of the machines to attract attention to them I have had no issues with any of them as yet but I did forget to check to make sure all of the machines with the mod were working at the same impedence which could lead to issues so I need to double check my work and make the proper adjustments I am using directional High pass filters to aleviate the Feedback issue and they are working well. but this post helped.

[JohnD0406]

I wasn't referring to you directly. What you have posted is indeed true, whether it's entirely relevant to this situation or not.

I can live with that..

This can be true with some designs of audio amp that are intended for the upper end of the market (ie audiophiles).

Guilty as charged. I've been into high-end audio since I was a kid. My first experience with electronics repair was due to my 'experimenting' with amps and letting the smoke out - everything from running the amp with no load to test it's thermal properties (I later learned to use an 8-ohm ceramic power resistor the size of a small lead pipe, as well as for burn-in), to testing the amp to see if it would pass 100KHz :oops:. I still have that amp I've abused over the years... Can't count how many times I've repaired it. Still works great...

Of course this isn't the case for the integrated circuit amplifiers that are designed for consumer equipment. If your iPod exploded because you powered it up without the headphones connected, or your surround amp started smoking because you only connected the front speakers, then you'd have a lot of unhappy customers.

Good point - the consumer stuff must be very well idiot-proofed (thus 47Kohm input impedance vs 600ohm on the high-end/pro gear), though this is professional gear not intended to be modified, and I haven't reverse engineered them enough to make informed decisions as to what's safe and what's not.

All the pachinkos I've seen use 8 ohms speakers running off BTL amplifiers, though the amplifiers will be usually be specced at 4 ohms.

You made me take another look at my machines just now... 2 of the frames (Lumina and Victory) have 8ohm Onkyo speakers; 1 other frame I can't get to the speakers without disassembly; 1 other frame doesn't list the impedance. I'd swear in one of my L-pad jobs I saw 4ohm speakers, but maybe not...

Besides the pots are wired in series with the speakers, so they will only ever increase the impedance seen by the amplifier, and thus decrease the output current driven by the amplifier and the overall power dissipated by the amplifier.

Then why does the signal distort when using a variable resistor? Isn't the amp current clipping?

Yet you recommended people using L-Pads to set their volume control switches to maximum.

I said it depends on where the volume switch is integrated into the amp circuit. If it's pre-amp, then setting it on low will drive the amp easier, giving it more life, whereas setting it on high would work the amp harder and increase THD as you noted. However, if the volume switch is post-amp, then setting the volume to low is causing the same distortion you're trying to avoid by using an L-pad in the first place, whereas setting the volume on high would prevent that distortion. I haven't looked into each model to know how they're designed, so at this time I don't know what the best setting would be.

Do you happen to know if the switch is pre or post amp, and if it's a universal design? A friend has a machine with a volume control built-in (I think it's Jurassic Park). Adjusting the volume on it doesn't cause any distortion, so it's safe to say it's implemented pre-amp.

But it doesn't necessarily mean that a correctly specified pot is also bad and an L-Pad is the only 'good' way, which is the way the argument comes across whenever L-Pads are mentioned around here.

Pots are bad, if for no other reason than the distortion they create. Taking that one step further into the psychological realm, being a solid-state amp, it's distoring in odd order harmonics, thus causing subconscious listening fatigue and possibly ill feelings toward the machine! Ok, that's far fetched, but it is solid science in the high-end and professional audio world.

And right on cue you completely validate my argument by stating 'Resistors wrong, L-Pads correct.'

You can't deny that in the realm of electronics design, variable resistors at the output stage of an amp are incorrect design, whether it works or not, right? I wouldn't want people to start developing overall bad habits when it comes to electronics, possibly leading to them letting the smoke out in a similar circumstance in the future.

little more distortion they're perfectly fine.

A little? It's subjective I suppose... Perhaps someday I'll measure it, but I'm hearing about 40% THD with pots, and none with L-pads. For me, there's no choice to be made here...
______________________________________________________

well now if we want to go all that far why not just put a line level converter in the system

I just re-read what you wrote... I missed the "converter" bit on the first read. A line-level volume pot (pre-amp) would be most ideal, but requires more in-depth modification by opening the sealed boardsets, and directly accessing the electronics. One good static shock and you're buying a new machine. Unless someone is familiar with working with electronics, I would never recommend people open up their machines. I haven't done it to mine because installing an L-pad is so much easier (and gives me front access to the volume while playing the machine).

[daverob]

Apologies to Steve for hijacking his thread, but this is PachiTalk...

Steve, at full volume an average pachinko machine will be putting around 5W into each of it's speakers and your L-Pad will most likely be taking the majority of that power. If you think that small soldering irons start at 12W and they heat up pretty good, then your L-Pad getting warm to the touch should be considered to be perfectly normal.


Anyway, back to the scheduled rant. (BTW I do feel a lot better after this, so there is a positive side to it after all )

I'd swear in one of my L-pad jobs I saw 4ohm speakers, but maybe not...

I didn't say that there weren't 4 ohm speakers in pachinko machines. I just haven't come across any yet. As the new generations of machines are getting louder all the time, I wouldn't be surprised to find them turning up though. Now all I need to do is to go away and work out what an 8 ohm L-Pad would do if it's connected to a 4 ohm speaker and whether this is likely to trigger the end of the universe as we know it.

Then why does the signal distort when using a variable resistor? Isn't the amp current clipping?

There will be some extra distortion from the amp when driving into a higher impedance, but it won't be current clipping, as the current will be proportionally lower into a higher impedance. I would suspect that there will be higher crossover distortion (were the signal crosses over the zero volt point), as there could be some offsets at the zero-crossing point as there is less current to bias the output stages.

But from my experience, there is less distortion using a fixed resistor than using a variable resistor set to the same value. So I suspect that the problem could be a physical one, where the wiper contact of the pot could be a factor. Whether this is a thermal issue (the heat causing expanding and contracting and therefore physical movement at the contact point), or just a plain poor contact due to vibration or other issues.

I said it depends on where the volume switch is integrated into the amp circuit.

Sorry, I missed that when I read your post earlier. The volume controls on the machines I've examined in detail are all pre-amplifier, and to my ears they all sound more distorted when they are switched to maximum, no matter what method of external volume control is used. If I was using an L-Pad I'd probably recommend setting the volume to the middle setting, and (of course!) always to the low setting when using a variable resistor.

Pots are bad, if for no other reason than the distortion they create.

I have no problem with that statement, but if the owner of the machine is happy with the quality of the audio, then they are perfectly adequate. I'm fine with the argument that an L-Pad is a better quality solution, I just disagree with those who say that they are unsafe or may cause damage to your machine.

You can't deny that in the realm of electronics design, variable resistors at the output stage of an amp are incorrect design,

Oh yes I can! But it is only a matter of semantics. They are not best design practice and we both agree that they have fundamental flaws, but I would never state that they were 'wrong' or an 'incorrect design'. Going by this argument I could say that using an L-Pad is an incorrect design as a better solution would be to break open the sealed cases and install volume controls at the pre-amp stage.

There will always be better ways of doing something. If all designs had to be perfect (regardless of budget) or deemed to be 'wrong' or 'incorrectly designed' then we'd all be driving Ferraris, Aston Martins and Bentleys, and all our computers would be running Windows Vista* / Mac OS-X* / Linux* (*delete evil empire of choice! ). However many of us are happy settling for a less perfect experience, providing that we don't have to stretch the budget so far. On this side of the pond I'd need to pay $20 each for stereo L-pads, and when I can get a pair of decent quality preset pots and still have change left from a dollar it's honestly no contest.

All I'm trying to do is provide a balance point to the L-Pad argument. I'm happy to admit that they are better than a cheap variable resistor. But I don't want newbies coming in here, and reading what are almost scare stories and feeling that they have to replace their regular volume controls because someone has claimed that they are unsafe (or dare I say it 'incorrect' !) and that L-Pads are the only true way.

[JohnD0406]

I had to break out the scope and have a quick look at what the audio switch is doing. I didn't calibrate the scope, and the blurry cameraphone picture actually helps reveal what's happening.

The audio is clipping a lot more when the switch is set to HIGH, and not clipping when set to LOW. This tells me the switch is pre-amp in the circuit, and not only safe to set to low, but better. Had the switch been post-amp, the results would have been the opposite.

The machine violated for this test was Julie the Pinball, which has distortion built into the recorded audio. The lack of clipping when the volume is set to low indicates it's intentionally distorted (it is rock music after all).

[dattia]

I just love a good provocative technical thread... don't understand them, but love them just the same! Nice work guys!

[teqkng]

the funny thing is that everyone is almost agreeing, Different machines DO sometimes require different solutions. Perhaps instead of dickering we should just look at the situation by manufacturer and then by machine and make reasonable recomendations as to the method of secondary volume reduction.

[MrGoodBurn]

the funny thing is that everyone is almost agreeing, Different machines DO sometimes require different solutions. Perhaps instead of dickering we should just look at the situation by manufacturer and then by machine and make reasonable recomendations as to the method of secondary volume reduction.

Agreed... Take Taiyo Elec Top Gun in the Antler frame for instance. I have notice a set of wires on this machine that appear to be the preamp leads from one board to another. A pot should connect in this location to adjust the volume safely before the main amp. This will eliminate the need to use an L-Pad, and should cut down on the possibility of distortion. Not all machines have this, and will need to be handled on a per machine basis.

I have not tried to add a pot in this location, but it should work if I can figure out the best value pot to use.

[hanabi]

I just love a good provocative technical thread... don't understand them, but love them just the same! Nice work guys!

[Sid]

weird really when i tried installing pots on my Thunderzone machine...

the only thing i got was Cactus...
S5002210 (Medium).JPG

[luckydog]

speaking of pot

E! News - Gilligan's Mary Ann Goes to Pot