Pink noise generator circuit mod

[culomax]

Hi All,

First post on here. Please bare with me as I have very limited knowledge in electronics. Hopefully some can help/advise.

Basically Im looking at the VELLEMAN Pink Noise Generator kit (PNG) which I would like put into the cabinet of an active loudspeaker (L/S) with an external on/off control switch.

I have two issues that I can see. Firstly the output voltage from the PNG is only 150mVrms where as I need something similar to the output from an iphone or laptop so I can get the speaker up to approx 130 - 140 dB at 1m. From searching around I believe the iphone has an output of approx 1.1Vrms. So first question is can the circuit be modified to increase the output.

Second issue is I need to add some sort of protection for the speaker cones i.e If the amp it turned up max volume and some switches on the PNG it may pop the cones therefore I would ideally like that output of the PNG to ramp up slowly. I assume a capacitor would take care of this but not sure.

I cant add an image to the post but the circuit diagram can be found here: http://www.esr.co.uk/manuals/k4301.pdf

So does anybody think this is possible or should I look elsewhere for a better circuit?

Any comments/suggestions welcome, thanks.

 

[CocaCola]

similar to the output from an iphone or laptop so I can get the speaker up to approx 130 - 140 dB at 1m.

Are you sure about that? 130-140 is the threshold for extreme human pain and permanent ear damage, that is equivalent to front row at a loud rock concert directly in front of the speakers, or on the runway with jets taking off and you believe you are getting that volume level out of an iPhone or laptop at 1m unassisted?

 

[KrisBlueNZ]

In addition to the issue raised by CocaCola, here are two other issues you need to think about.

First, yes you will need to increase the output level of the pink noise generator, to suit the input requirements of the amplifier inside your powered speaker. You can do this with a fixed-gain amplifier stage; the simplest way is to use an op-amp.

Second, it is not simple to vary the volume the way you want, to avoid pops and prevent speaker damage. You can't use a capacitor; a capacitor will give you a gradually changing DC voltage, but you actually need to vary the amplitude of the AC signal. If you really want this amplitude ramp-up to be automatic, you'll need to add a circuit called a variable-gain amplifier (VGA) or voltage-controlled amplifier (VCA). You can then use a capacitor to control the gain, starting at zero (silent) and ramping up to a predetermined amplitude.

The maximum output amplitude you can get at the output of the circuit is limited by the +V rail voltage, which is 8V (determined by voltage regulator VR1), which will give you at least 1.5V RMS clean output output before the op-amp or VCA starts to clip. If you need more than that to fully drive the amplifier in your powered speaker, you can increase the circuit's +V rail voltage up as far as 15V by using a regulator with a different output voltage, but the voltage going INTO the regulator needs to be at least 3V higher than the +V rail voltage, due to the "dropout" voltage requirement of the regulator. If you already have a REGULATED DC supply of up to 15V, you can ditch the regulator altogether and feed +V from that, but it must be a clean, regulated supply. In this case you can remove the five components in the top left corner of the PCB to make space for new components.

In either case (fixed gain or variable gain) you will need to add at least five components to the circuit. You can make your add-on circuit on a separate piece of stripboard, or tack it onto an unused area on the existing PCB, perhaps using the "dead bug" construction technique - glue the IC(s) upside down on the board, and solder the small components between the pins, supported by the IC. This method is also called "skeleton wiring" because of the resemblance to the bones of a skeleton, with larger parts supporting smaller lighter parts.

You can increase the output amplitude with a fixed-gain amplifier stage using an op-amp. This is the simplest way, and will add only five components to the circuit. It will give you a higher output amplitude, and you can then feed that signal through a potentiometer to adjust the volume (if there isn't already one on the powered speaker). To avoid pops and overloading, manually set the volume control to zero, switch on the powered speaker and the pink noise generator, then turn the volume control to get the desired volume from the speaker. For more info see http://en.wikipedia.org/wiki/Operational_amplifier#Non-inverting_amplifier. For audio use I like the MC34071 series and would use that in preference to old standard parts like the 741, 308 and 358.

To provide an automatic ramp-up of the amplitude you need a VGA or VCA (see http://en.wikipedia.org/wiki/Variable-gain_amplifier) and a capacitor to provide a ramp voltage to control the gain. A VCA can be made with an IC known as an operational transconductance amplifier (OTA), but this would require a significant number of components, and in this case it would be simpler and equally appropriate to use the variable gate-source resistance of a junction field effect transistor (JFET) as the method of varying the gain. The simplest topology for the JFET solution actually acts as a variable attenuator, not an amplifier, so you would need an op-amp to amplify and buffer the signal after the JFET. The gain of the op-amp amplifier stage would be adjusted to set the maximum output signal amplitude (i.e. the final amplitude, when the ramp-up has finished). A JFET-based attenuator will be easier to design if the +V rail voltage is higher. 15V or 12V would be good. 8V is a little hard to work with.

If you want to pursue these ideas, respond to all the issues I've raised in this post, and include any questions you have, and include ANY details that might be relevant, and I'll be happy to draw you a schematic for the method you want to use. More details save time and increase the chance that I will design something suitable for your application.

 

[culomax]

Are you sure about that? 130-140 is the threshold for extreme human pain and permanent ear damage, that is equivalent to front row at a loud rock concert directly in front of the speakers, or on the runway with jets taking off and you believe you are getting that volume level out of an iPhone or laptop at 1m unassisted?

Ok so maybe I stretched it a bit but certainly 120 - 125 dBA at 1m is possible. I measured an average of 108dBA today in a small room using my phone and 2 speakers. Obviously in larger rooms that will drop. I just want the option of having max output from the speakers without blowing them.

I know if I hook up the laptop it will give me more than a phone can.

 

[culomax]

Kris,

Thanks for reply. Its Fri nite where I am and had a few after work so will have to re-read your post when fully coherent. I have a busy w'end ahead with looking for new place to live and such but will get back to this next week. I just wanted to put some feelers out to see if it was possible and by the looks of it..it is!

Please check back next week when I can provide some more info>

Many thanks.

PS: Just had a re-read and I hope to power the circuit off the power supply for the amp. I guess it all hinges on this. FYI the speakers are http://www.dbtechnologies.com/index.php?id=22&L=0&tx_amdbt_pi1[category]=16&tx_amdbt_pi1[product]=149 but have not found any circuit diagram for the amps yet.

 

[CocaCola]

Ok so maybe I stretched it a bit but certainly 120 - 125 dBA at 1m is possible. I measured an average of 108dBA today in a small room using my phone and 2 speakers. Obviously in larger rooms that will drop. I just want the option of having max output from the speakers without blowing them.

I know if I hook up the laptop it will give me more than a phone can.

108dB to 120dB is a HUGE jump, to achieve that jump requires about 12 times the power... So if you are getting 108dB with say a 100 watt amplifier it would take a 1200 watt amplifier to get to 120dB...

What kind of speaker are you using that you are getting that volume? They have to have a built in amplifier and a very sizable one at that to achieve 108dB @ 1m or they have to have very expensive and efficient speakers with a large amplifier... What is the sensitivity rating of the speakers?

To achieve 108dB you are generally looking at about 100-200 watts...

 

[culomax]

Cola,

Now I'm sober or hungover I can answer your question. The speakers I'm using are Opera 410D which can be found here:

http://www.dbtechnologies.com/index...t]=149&cHash=a7ca66018998a0b9f7eaf9a3d1cf5230

The manufacturer's specify a SPL of 127 dB which I assume is at 1m. What I was measuring was an average of 108 dBA in a small room with no absorption using 2 speakers. This was the reverberant sound field so not directly in front of the speakers.

The spec for the amp can be found on the website but manufacturer's specify 400W. They are new speakers and I was well impressed with the performance.

Usually use a 900W Peavey amp with a pair of JBL's that the amps have been blown in and they would give a similar output but are bigger cabinets.

 

[KrisBlueNZ]

PS: Just had a re-read and I hope to power the circuit off the power supply for the amp. I guess it all hinges on this.

That would be good. The circuit draws hardly any current so the amp won't mind the extra load. It will have a positive rail that you can drop down to 12V or 15V with a 78L12 or 78L15 regulator, which accepts up to 35V input voltage. If the amp's positve more than 35V you can drop it down with a zener or something so don't worry about that. The amp quite likely has several voltage rails to choose from anyway. You just need access to the amp board, and preferably, a schematic.

FYI the speakers are http://www.dbtechnologies.com/index.php?id=22&L=0&tx_amdbt_pi1[category]=16&tx_amdbt_pi1[product]=149 but have not found any circuit diagram for the amps yet.

That link didn't work for me - it just took me to the dbtechnologies home page, not a specific product.
The maximum sound output you will be able to get before clipping (or compression, if the amp is smart enough to have it) is limited not only by the signal level you can provide into the amp's input, but by the amp's power output and the speaker efficiency. You can't just keep increasing the signal level and expect the sound pressure level to follow. I guess you knew that but I'm not sure.

 

[(*steve*)]

Your questioning a drunk man

Hahaha. No worries. We can wait out your hangover too.

 

[culomax]

Thanks All for your posts.

So just to summarise what you have said Kris.

I will need to add an op-amp to the circuit to increase the output voltage from the PNG. Firstly I need to set the maximum output voltage. The speaker manual ( Opera 410 D) does not specify a maximum input voltage therefore I assume it has some sort of circuit protection as you said, but I dont really want to put that to the test. So what is a sensible output without killing the speakers? Max 1.5Vrms with a variable resistor to control PNG output. I can then adjust the output of the PNG until the speaker reach 127 dB at 1m as specified by the manufacturer. Does this make sense? The speakers amp does have its own gain control which I would set to max before adjusting the Op-amp.

For speaker protection I will need to add a JFET with a charging capacitor to control the gate which will allow the voltage across the circuit to ramp up. I assume the size of the capacitor determines the speed at which the JFET opens or am I completely wrong!?

So at this stage I need to start ordering parts (if I knew what parts to order). Kris I would be very grateful for your input/guidance on what I need to do next?

Many thanks.

PS: just to add that there does not appear to be a circuit diagram available for the Speakers but I will contact the manufacturer to see if they can supply one.

 

[CocaCola]

Consumer line level input is 0.316VRMS, if it's designed to attenuate a speaker level fed into it, then anything goes and you need to contact the manufacture to see what the max input level is...

I assume it has some sort of circuit protection

IMO, never assume that, and never assume even with protection that you are not exceeding the protection levels since they are unknown...

until the speaker reach 127 dB at 1m as specified by the manufacturer.

That is an absolute rating and usually with a measurable level of distortion and almost certainly taken at a single frequency that suited the amplifier and speaker pair... I doubt you will see that level with a varying tone, and regardless you should never run continuous at maximum levels anyway...

 

[KrisBlueNZ]

Those speakers have an internal DSP-based processor for the active crossover (it is bi-amped - separate power ampifiers for the two speaker drivers), which also does compression, so it should be very difficult to overload it with too much input signal. It would be preferable to operate it below the compression threshold, since compression would affect the reproduction accuracy at the very-low-frequency end.

Right, you should set the speaker's gain control to maximum, so you can get repeatable behaviour during your testing. Yes, you can put a potentiometer to adjust the output signal level, and you could replace it with a preset (screwdriver-adjustable) potentiometer later.
You would still want the JFET circuit to give you the ramp-up of signal level after power-on. Yes, the resistor and capacitor connected to the gate will determine the output signal level's ramp-up time at power-on. I would not call it "speaker protection". That protection is built into the powered speaker. The circuitry in that speaker is higher-tech than the pink noise generator!

I doubt the manufacturer will give you a schematic, but it might be worth a try.

I'll draw up a schematic for some changes to your pink noise generator board and post it soon.

 

[culomax]

Thanks for your input guys and look forward to the schematic.

I will probably just use a preset (screwdriver-adjustable) potentiometer in the first instance for gain control as once it goes into the cabinet I hope not to have to open them again. My only concern is that the pot will stay at its fixed preset with vibration and other knocks and bangs the speakers will take over their lifetime.

 

[KrisBlueNZ]

Here's a first draft of a schematic. It's turned out a bit more complicated than I expected. Have a look and see if you want to have a go at building it. I'm sure you could, but you may want to start with something simpler to get the hang of things if you haven't done electronic construction before.

I will post a circuit description later.

I wouldn't worry about a preset potentiometer moving under vibration. If you're really concerned, you could use a ten-turn trimpot.

 

[culomax]

Nice Kris,

Many thanks. I dont think I will have a problem building it as I've put together PCB's in a previous job.

I will put together a components list and get ordering tomorrow. How would you add the additional components. Would you uses an additional board or just skeleton mount as you previously mentioned?

 

[KrisBlueNZ]

Here's an updated schematic and a circuit description. Let me know what you think.

Circuit description:

The low-volume signal from T1 in the pink noise generator enters through CA and RA and is amplified by op-amp UA and fed to the speaker amplifier. RA and the drain-source resistance of QA form a voltage divider that attenuates the signal at point "A" according to the resistance of QA, which is determined by its gate voltage, which comes from point "C".

The +V rail from the pink noise generator is fed through RD and decoupled by CD. This network ensures that noise on +V (due to switching of the ICs in the pink noise generator) doesn't get into the VS power supply rail used by the following circuitry.

US is a TLE2426CLP supply rail splitter IC in a TO-92 package (it looks like a transistor). It takes VS and splits it in half at its output. So its output will be 6V or 7.5V for a +V rail voltage of 12V or 15V. (Actually a bit less, as some voltage is dropped across RD.) This half-rail voltage, marked VHH, is used as a derived ground reference for the attenuator and op-amp.

When the whole circuit is initially powered up, CT is discharged (has no voltage across it), so the voltage at point "C" is initially near to VS. This voltage is coupled via RT into the gate of QA. A positive voltage on QA gate (relative to its source, which is held at VHH) causes QA's drain-to-source resistance to become very low, typically a few hundred ohms, very low compared to RA, so the AC voltage at point "A" is almost completely eliminated (attenuated). (The DC voltage at "A" is equal to VHH.)

Current will flow through RT and RR, slowly charging up CT and causing the voltage at "C" to drop. When "C" reaches the VHH voltage and starts to drop below it, QA's gate voltage drops below its source voltage and QA's drain-to-source resistance starts to increase, allowing more of the audio signal to pass through RA and increasing the signal level at "A". (The DC voltage at "A" does not change.)

As the voltage at "C" continues to drop, QA's drain-to-source resistance increases in a corresponding way, so QA attenuates the signal at "A" less and less. This gives the ramping up in output signal level.

When the voltage at "C" drops below VHH minus QA's "gate-source pinch-off voltage", which is typically a few volts, QA's source-drain resistance is practically infinite, and it doesn't attenuate the signal at all. RB will have a slight attenuating effect. (Its purpose is to keep the DC voltage at "A" equal to VHH.) C7 will continue to charge through RR until "C" almost reaches the ground rail.

The signal at "A" is amplified by UA, which is configured as a non-inverting amplifier, with an AC gain calculated as (1 + (RG / RY)). The values given allow the gain to be adjusted from 1 to about 21. With an input voltage round 150 mV RMS the output voltage will be adjustable up to about 3V RMS. UA's output has a DC voltage equal to VHH so CC couples the AC signal to the output of the circuit, while blocking DC. RZ ensures that the circuit output has a DC voltage equal to GND.

When the circuit is switched off and the +V rail goes to zero, D7 will conduct and discharge CT, ensuring that CT is discharged ready for the next power-up sequence.

During the initial silent delay at startup before the ramp-up begins, "C" is above VHH and QA's gate draws current via RT. Current also flows in RR. Therefore this silent time is determined by CT in conjunction with RT and RR.

Once "C" drops below VHH, as the output signal starts to ramp up, current flow through RT stops; therefore the ramp-up duration is determined only by RR. If you want to play around with the silence and ramp-up durations, you can replace RR and/or RT with 200K trimpots. You can also vary CT if you need to change these times by a large amount. With those values, the silent time should be around 5 seconds and the ramp-up time should be around 2 seconds.

CA is needed because the voltage at T1 emitter is slightly less than VHH so C12 ends up having the wrong polarity. You can remove C12 altogether (typass that position with a short circuit).

QA is specified as a BFR30 or BFR31. These are both available from Digikey in unit quantities. It appears there are not many depletion-mode JFETs available nowadays. These devices are both surface-mount. You can mount them on stripboard (aka veroboard) fairly easily with one track cut. QA is an 8-pin through-hole part (its pins go through the holes in the stripboard and solder underneath, like the components on the pink noise generator circuit).

Re your recent post. There are really too many components to skeleton wire. I would use a bit of stripboard.

 

[culomax]

Kris,

Many thanks again for taking the time to explain the circuit. I will need to re-read it to fully understand whats going on.

My only comment would be that I would like to reduce the quiet time at start-up to about 2 seconds and maybe stretch the ramp up time to 3-4 sec. Would this be to much trouble to change?

Thanks again.

After re-reading I guess I just add trimpots for RR and RT.

 

[KrisBlueNZ]

No problem.
The time durations are controlled by RT and RR but are also affected by QA's gate characteristics, which vary significantly from one device to the next, so the best way to get them how you want them is by experimentation using trimpots, at least initially. To reduce the quiet time and increase the ramp-up time, decrease RT and increase RR.
Your speaker will probably have a delay of a second or so before it enables its output after power-up. You'll need to make sure your circuit waits longer than that before the start of the ramp, otherwise the noise will come on suddenly.
Also when you're testing, make sure that +V drops to zero (or close - less than 0.5V) in between tests, to ensure that CT is completely (or nearly completely) discharged at the start.

 

[culomax]

Very good point about the speaker startup time. I can check that out and adjust the PNG accordingly. I guess I will power the circuit of a battery for testing purposes so will watch that CT is fully discharged.

Ok time for a components list.

Just out of curiosity have you build the circuit in a simulator and tested it?

One more thing. I guess Im adding the switch at the input to the PNG. I know its a dumb question but just checking!

 

[KrisBlueNZ]

No I haven't simulated or prototyped the circuit! I have reasonable confidence in my ability to design a circuit like this, and I have designed similar but not identical circuits.

I actually have very little experience with simulators. I have traditionally tested low-frequency circuits like this on breadboard and/or stripboard. If you are comfortable with a simulator, and you can generate the necessary signals, please go ahead and simulate it. I would be interested to see the results.

Otherwise, you might want to prototype it on breadboard, but I personally would go straight to stripboard because it's not hard to make any small changes needed.

I anticipate that it may not work perfectly first time - either due to a design problem or a construction error. I assumed that we could communicate through Skype to diagnose any problems that come up. Would you be prepared to do that if/when necessary?