Hi,
I've been tasked with designing a 'robot voice generator'. I've gone for a design involving a preamp, transformer-diode ring modulator and power amplifier. I had it running satisfactorily on a breadboard and then on stripboard, powered from a bench power supply. However, when I run it from a pair of 9V batteries, I get loud carrier bleed and it doesn't work properly at all. I'm totally at a loss to explain why this is. Any ideas would be greatly appreciated.
Thanks,
James
I've been tasked with designing a 'robot voice generator'. I've gone for a design involving a preamp, transformer-diode ring modulator and power amplifier. I had it running satisfactorily on a breadboard and then on stripboard, powered from a bench power supply. However, when I run it from a pair of 9V batteries, I get loud carrier bleed and it doesn't work properly at all. I'm totally at a loss to explain why this is. Any ideas would be greatly appreciated.
Thanks,
James
I've got the negative of one battery connected to the positive of the other, connected to the ground. When I'm running it from the bench power supply, I've got the +9V, -9V and 0V lines connected to the battery snaps.
Use your DVM to verify all the circuit DC voltages with respect to ground (the common battery terminals).
Could you have one battery reversed?
Could you have one battery reversed?
I've just checked the voltages. I'm getting the correct DC supply voltages at the correct points. The circuit does 'work' on the batteries but the carrier bleed is unacceptably loud whereas it's virtually inaudible with the power supply.
Logically, the presences of a carrier tone, means there is a pin-out mistake in the transformers or the bridge diodes in-between them. The carrier tone should cancel.
I would avoid transformers but, of course, they make it simple. A truly modern design would be digital.
I would avoid transformers but, of course, they make it simple. A truly modern design would be digital.
9 volt batteries are weak compared to a power supply. Internal resistance of 9 volters is huge. Bypass caps will only get you so far. You may just have to use beefier batteries, like two packs of some lithium type, or two small gel cells.
Looks like 555 timer for square wave oscillator.
Depending on chip type they draw a lot of current
and will make a lot of noise on the power rail.
The big current spikes it produces will basically be
the same frequency as the oscillator. Since its basically
turning on and off to make the square wave.
So anything else on the same power rail will thump thump thump
along with it.
That noise will leak into the op amps and output amplifier.
Basically all the decoupling capacitors on the power rail
should be much larger.
The schematic doesn't seem to show any power rail decoupling cap
for the 555 timer at all...which is a big no no.
Add a decoupling capacitor as close as possibly to the 555 timer
power pin 8
Likely large as possible 100u, 220u 470u etc etc.
As with the opamps it is standard practice to place the 100u and 100n
decoupling capacitors absolutely as close as possible to the opamp power pins.
Also U3 opamp is driving external power transistors.
Im assuming to directly drive a speaker.
It is essentially a rudimentary power amplifier.
Which also has absolutely no power rail decoupling either.
It also needs coupling capcitors as close a possibly to Q3 / Q4
220 to 470u likely needed.
Once you decouple the 555 timer and the power amplifier.
The thump thump thump of the timer wont be on the rails
with battery power.
Depending on chip type they draw a lot of current
and will make a lot of noise on the power rail.
The big current spikes it produces will basically be
the same frequency as the oscillator. Since its basically
turning on and off to make the square wave.
So anything else on the same power rail will thump thump thump
along with it.
That noise will leak into the op amps and output amplifier.
Basically all the decoupling capacitors on the power rail
should be much larger.
The schematic doesn't seem to show any power rail decoupling cap
for the 555 timer at all...which is a big no no.
Add a decoupling capacitor as close as possibly to the 555 timer
power pin 8
Likely large as possible 100u, 220u 470u etc etc.
As with the opamps it is standard practice to place the 100u and 100n
decoupling capacitors absolutely as close as possible to the opamp power pins.
Also U3 opamp is driving external power transistors.
Im assuming to directly drive a speaker.
It is essentially a rudimentary power amplifier.
Which also has absolutely no power rail decoupling either.
It also needs coupling capcitors as close a possibly to Q3 / Q4
220 to 470u likely needed.
Once you decouple the 555 timer and the power amplifier.
The thump thump thump of the timer wont be on the rails
with battery power.
Thanks for the replies. I do have decoupling caps, they're just drawn at the top on the schematic rather than directly next to each component.
I have a pair of 100uF next to the output transistors, which is where the batteries are connected.
There is a pair of 100nF on each of the Op Amps.
I don't have any on the 555 at the moment.
So, if I've read correctly, I should increase the value of the electrolytics by the output transistors to perhaps 1000uF, and add a cap next to the 555? Will I need an electrolytic and a ceramic in that position, do you think?
Thanks
I have a pair of 100uF next to the output transistors, which is where the batteries are connected.
There is a pair of 100nF on each of the Op Amps.
I don't have any on the 555 at the moment.
So, if I've read correctly, I should increase the value of the electrolytics by the output transistors to perhaps 1000uF, and add a cap next to the 555? Will I need an electrolytic and a ceramic in that position, do you think?
Thanks
Click image to enlarge of course.
Excuse my rudimentary schematic additions quickly done with paint.
Dont show polarity of caps, polarity should be obvious and done
as needed.
likely if you have only 100u available in your stash.
Adding decoupling to the 555 power pin might fix the problem.
The power amp driving a speaker likely very much is needing decoupling
for final design when on battery.
Far as I can tell 555 timer usually draws 10 to 20ma
but can source up to 200ma
Without calculating assume whatever it is driving
its probably pulling 40 to 100ma.
And with no decoupling on battery power.
Everything on that rail is going to thump away.
At the same frequency of the oscillator.
Why it sounds like it is bleeding.
I completely forget the suffix for 55 timers.
There is original standard timers which can draw high current
then their is low current versions.
Not sure if you have high current or low current version.
Depending on the load it is driving. It might need to be high
current. Or could be switched over to low current.
Either way it needs power rail decoupling as shown
1000uF would be excessive, was not aware you decoupled the power amplifier already with 100u
Could try double the value to 220u
they need to be as close as possible to the transistors.
Same with opamps, caps should be close as possible.
Most the issue is the 555 putting noise on the rail.
Decouple it with 100 to 470u and see what happens.
Basically every time it goes high/low
it causes current surges.
Basic known behavior of Cmos style oscillators
of this type.
With transformer type ring modulators for special effects
usually the oscillator needs a buffer to direct drive
a transformer for current. Why its common to use 555
because they can source up to 200ma.
Otherwise it is basically a analog multiplier.
So to do away with diodes and transformers.
You can use Phase Lock loop or PLL chips
Since most have built in VCO for the oscillator
And they have a chip based Multiplier or Mixer section
all on one chip. So no diodes or transformers needed.
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Im sure the net has plenty examples now shown
for free, But back in the day you had to buy the book
Craig Anderton designed a Ring Modulator using a Phase Lock Loop ( PLL)
chip. As described.
You get the oscillator and mixer/multiplier all on one chip.
Ironically just discussing Signetics earlier for the 5532 opamp.
Far as I remember they made the 565 PLL
never seen one LOL from them.
Built my first ring modulator with a LM565.
I put it inside a Wah Wah pedal case.
So I could change the carrier frequency with my foot.
Either way all fun. Decouple the 555 see what happens.
Having built numerous modulation effects for battery power.
Oscillator bleed is pretty normal problem. Specially with a current
sucker like a 555
Use a 40106 schmitt-trigger oscillator and power it from both batteries so the diodes get driven symmetrically. This also solves the problem that the positive 9 V battery does phut before the negative. Viewed from a power drain perspective a 555 is suboptimal.
I've give this all a try soon.
I was originally going to use a 40106, due to the lower component count, but thought it probably wouldn't be able to drive the transformers. I'll try hooking one up.
Do you think it will be okay powered between the two batteries? That's right at the limit of the voltage rating according to the datasheet. If I've understood correctly, you're suggesting that I connect the power pins to -9V and +9V, and connect the capacitor to -9V, thus producing a square wave that oscillates between +9V and -9V. Keeping the centre tap of the second transformer connected to ground, I'll have a carrier wave oscillating symmetrically about ground. Is that correct?
I wish I'd been aware of the PLL option before. It would be cheaper than the transformers, too. I don't think I have time now to develop a new version.
Thanks
I was originally going to use a 40106, due to the lower component count, but thought it probably wouldn't be able to drive the transformers. I'll try hooking one up.
Do you think it will be okay powered between the two batteries? That's right at the limit of the voltage rating according to the datasheet. If I've understood correctly, you're suggesting that I connect the power pins to -9V and +9V, and connect the capacitor to -9V, thus producing a square wave that oscillates between +9V and -9V. Keeping the centre tap of the second transformer connected to ground, I'll have a carrier wave oscillating symmetrically about ground. Is that correct?
I wish I'd been aware of the PLL option before. It would be cheaper than the transformers, too. I don't think I have time now to develop a new version.
Thanks
For the most part.
They also have full chips for voice changers etc etc.
Often labeled Ghost . Alien Monster etc etc
when used in toys or Halloween changers in stores
In reality it is a chip with 3 effects
Ring modulation
Phaser
Pitch Shifter
Around now is best time to buy them in stores being close to Halloween.
At least back in the day they still used Dip packages.
I use to hunt down the oldest voice changer toys I could find
hoping they had Dip packages
Now its usually on a untouchable black blob on the PC board.
Before I would mod them and add a pot. Which depending on the Mode
In phaser mode it changes the speed, in Ring Modulation changes carrier
frequency. In pitch shifting changes the pitch.
Common chip if I remember was HT8950
dont remember if it has three modes or not.
Same company makes 3 mode chip whatever the suffix is.
Velleman also makes a kit MK171
believe it is using HT8950 as well.
If really lucky, I have found them. Some toys have hidden
board mounted pot on the board for same functions.
I would just add a external pot.
Main interest was the horrible 8 bit pitch shifter LOL
lots of LoFi fun.
They also have full chips for voice changers etc etc.
Often labeled Ghost . Alien Monster etc etc
when used in toys or Halloween changers in stores
In reality it is a chip with 3 effects
Ring modulation
Phaser
Pitch Shifter
Around now is best time to buy them in stores being close to Halloween.
At least back in the day they still used Dip packages.
I use to hunt down the oldest voice changer toys I could find
hoping they had Dip packages
Now its usually on a untouchable black blob on the PC board.
Before I would mod them and add a pot. Which depending on the Mode
In phaser mode it changes the speed, in Ring Modulation changes carrier
frequency. In pitch shifting changes the pitch.
Common chip if I remember was HT8950
dont remember if it has three modes or not.
Same company makes 3 mode chip whatever the suffix is.
Velleman also makes a kit MK171
believe it is using HT8950 as well.
If really lucky, I have found them. Some toys have hidden
board mounted pot on the board for same functions.
I would just add a external pot.
Main interest was the horrible 8 bit pitch shifter LOL
lots of LoFi fun.
No need to worry I would be proud of it.
I've have built ring modulators before in many cases.
From musician pedal types to rack mount modular synthesizer cases.
Have always wanted to build transformer type just for nostalgia
or being rather unique. But never built one.
So it is pretty cool you got one.
Chips come and go, so depends on goals LM565 and HT8950
can be hard to find.
Doing a quick search seems MK171 kit is gone
it is now WSAH171
And seems maybe only pitch shifter and vibrato
Would have to look at data sheets.
I haven't played with these circuits in maybe 15 years.
Last time I found a LM565 when National Semi was still around
and a engineer had a few in his desk LOL. My dad worked for them.
Asked me what I was doing with old Signetics junk.
Maybe LM1496 could work or Analog devices had the 833
Basically multipliers or balanced modulators
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I've done some work on it. I've replaced the 555 with a Schmitt astable, using a potential divider to output 1V p-p to the transformer.
I first ran it from my function generator to decide on the required amplitude and with that, there was essentially no carrier bleed.
The problem I have now, aside from still having some carrier bleed, is that it goes crazy when the volume is turned up full. I had this with both the function generator and the Schmitt astable.
I've made a video to illustrate the problem. Excuse my very messy workbench!
Thanks
I first ran it from my function generator to decide on the required amplitude and with that, there was essentially no carrier bleed.
The problem I have now, aside from still having some carrier bleed, is that it goes crazy when the volume is turned up full. I had this with both the function generator and the Schmitt astable.
I've made a video to illustrate the problem. Excuse my very messy workbench!
Thanks