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Combining an Amplifier with a Voice Changer?

Jesuit24

Sep 3, 2014
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I'm trying to come up with a voice changer that also acts as an amplifer. I have two products that do the job separately, an Aker 1505 amp, and a cheap Halloween voice changer. Here they are:
http://www.amazon.com/Voice-Transformer/dp/B000CIS3GS
http://www.voicebooster.com/aker-mr1505-12watt-voice-amplifier-black.html

The problem is the voice changer is quiet with a crappy mic and speaker set and the amplifier doesn't change my voice. Is it possible for me to combine the two parts to take advantage of the superior headset mic, speaker and volume control of the Aker amp with the voice changing capability of the voice changer?

The Aker runs off a rechargeable 7.5 volt battery whereas the voice changer takes a 9 volt battery (I think; I don't have it yet).
 

KrisBlueNZ

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Yes, it should be possible. But you'll need to provide good clear photos of the circuit boards in both units (see https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/ for hints on how to take good photos) and you'll probably need to do some experimenting and testing as well. Then there are the mechanical aspects - do you intend to put the combined unit into a new enclosure?

Do you have a multimeter? An oscilloscope?

How do you know the voice changer is quiet and sounds crappy if you don't have it yet? Just wondering...

Edit: Oh, and welcome to Electronics Point :)
 

Jesuit24

Sep 3, 2014
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Okay will do once I get the voice changer.

The project is for a Boba Fett costume. I'm pretty much a beginner, so no multimeter or oscilloscope for me, just a soldering iron. I've been wiring up LEDs so far for the costume, but this is the one bit of the costume that's a bit above me.

Since the unit is going to be hidden away in a pouch, I don't intend to house it in a new box, I just really need the volume control to be accessible.

Basically, I've seen the reviews on Amazon complaining about the horrible feedback and crappy quality of the speaker and mic, which is pretty much across the board, and I'm not expecting a great product for the price. But the reason I chose it was because it was recommended for the costume, with video showing the speaker issues:
 

KrisBlueNZ

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OK. You may need to get a multimeter. You can pick up a "cheap and Chinese" one on eBay for under USD 10.
 

Jesuit24

Sep 3, 2014
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The voice changer arrived, and I bought a cheap multimeter off eBay so I'm ready. the voice changer is really shoddily made and the soldering is terrible. The solder on the button panel had come away so I had to resolder it to get it working, and just in taking pictures the negative battery wire came loose. Anyway, here are the pictures. You'll have to excuse the dirty window sill, only place I can get enough light to photo.

Amplifier:
(I removed the speaker to make it easier to photograph)
2ihuv0k.jpg

5arler.jpg

9hteo0.jpg


Voice Changer:
2a83sxe.jpg

witvtx.jpg
 

KrisBlueNZ

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OK, that's a good start.

I see what you mean about the voice changer. It's pretty nasty. It looks like one of the end wires on the grey ribbon may not be properly soldered on the switch board.

1. On the voice changer, can you test for continuity (direct electrical connection) between the black wire from the battery (marked A in the photo below) and the white wire from the microphone (marked B).

If your multimeter has a continuity test range (often indicated by the symbol for sound, several concentric arcs) where it beeps when you touch the probes together, use that, with a probe on each point in the circuit.

If it doesn't, set it to the lowest resistance (Ω) range - usually 200Ω - and measure the resistance between the points. If it reads less than a few Ω then there's continuity.

2. On the voice changer, with it switched ON, can you measure the DC voltage across the diode marked C in the photo below. It will be less than 20V so set your meter to the DCV section, and the 20V range. Connect the probes to the ends of the diode, either way round (the meter will read negative if they're the wrong way round but it doesn't matter - I just want to know the voltage).

3. On the voice changer can you post all of the markings you can see on the 8-pin IC marked D in the photo below. These are usually laser marked, so the lines are very fine and you often need to look at exactly the right angle to read them clearly. A bright light, and a magnifier, are also very helpful. If you can get a good photo of it, that's great, but otherwise, just the identifiable letters and numbers are fine. There will probably be a manufacturer's logo. It may help if you describe that.

2a83sxe.jpg

4. On the amplifier, can you post all the markings on the power amp IC. This is the black device at the left of the top side photo, with eight leads along each side, sitting among the five vertical black cylindrical electrolytic capacitors.
 

Jesuit24

Sep 3, 2014
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1) I don't have a continuity setting so I set it to 200 Ohms. When I touched the parts, it shot up to 250, then gradually settled down to 1.0 ish.

2) The diode is 4.39 volts. The battery I'm using isn't brand new though. Through the multimeter, I'm getting 7.7 volts directly off the battery.

3) The IC reads:
(Logo that looks like a squiggly CV connected together) MUAF
LM386
M-1

4) It reads:
TEA20258
G4320124
(another logo which looks like 691, with E3 in a circle next to it) G VW G4 CHN
 
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KrisBlueNZ

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OK. This should be pretty straightforward. The only tricky part will be matching the signal levels. We need to make sure that the signal level from the output of the microphone preamplifier on the amplifier board is properly matched to the input of the voice changer, and that the output of the voice changer is properly matched to the input of the power amplifier on the amplifier board.

The input to the power amplifier is on pin 10 of the TEA2025B (that's a B at the end, not an 8). The pins are numbered anticlockwise when viewed from above, starting with pin 1 just anticlockwise from the semicircular indentation. So referring to the second photo in post #5 (the one with the annotations for the connectors), pin 1 is at the bottom right of the IC. There are 8 pins per side, so the top right corner is pin 8, then the top left corner is pin 9, then the pin south of that is pin 10.

Can you take a closeup of that area of the board so I can see what pin 10 connects to. There are a couple of small SMT components in that area which I think will be involved, and there will be a track that runs over to the wiper (middle pin) of the volume control.

It will take me a little while to annotate up some drawings to show you what to do.
 

Jesuit24

Sep 3, 2014
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I think pin hits the C8 capacitor, and then travels up to R7 (which has 102 written on it), and then that seems to go into C5. That then goes into the left pin of the volume control (Not sure if it hits R3, the white line is in the way).
11r992a.jpg

29lcjlz.jpg
 
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KrisBlueNZ

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OK. I need to try to figure out how the volume control on the amplifier board is connected. Can you take a photo, as close as possible without being blurry, looking at the same angle as your top side photo with the "VOLUME", "MIC" etc annotations, but also with a bright direct light from behind the camera, to clearly illuminate the tracks around R3, R8 etc all the way up to the south edge of the volume control.

Also re the tracks in the R7/C5 area, there's one track running from near R3 to the south end of C5, one track from the north end of C5 to the north end of R7, and a track from the south end of R7 to the TEA2025B. Is that right? And no other tracks or connections in that area? You can use your multimeter on the 200Ω range to check the continuity of your suspected tracks.
 

Jesuit24

Sep 3, 2014
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R3 > south C5 = 3.8 Ohms
north C5 > north R7 = 1.0 Ohms
south R7 > IC = 1.2 Ohms

Also, pin 9 of TEA2025B picks up 3.4 Ohms on south C5.

Here's a picture. I don't have many lights to use, but I had a superbright LED I wired up for another project. Click the picture and it should turn massive. There's a couple of minuscule gaps in the tracks within the white squares.
2yvkv47.jpg
 

KrisBlueNZ

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Hi again. Sorry for the delay!

Can you make one more continuity measurement please. Can you check that there's continuity between the two circled points.

epoint 270132 check continuity between.jpg


You can also remove the battery from the voice changer and power it from the amplifier. Don't make any other changes yet to the voice changer. Here are the V+ and 0V connection points on the amplifier:

epoint 270132 amp power connection points.jpg

And here are all the connection points on the voice changer:

epoint 270132 put voice changer into amplifier - changer marked up.jpg

So just run a wire from amp V+ to voice changer V+, and a wire from amp 0V to voice changer 0V, and check that the voice changer still works as normal when it's powered from the amp.

Also check the voltage across the diode marked "C" in the picture in post #6 and make sure it's still at least 4.2V.

Do you have a microphone that you want to use with your amplifier to test with?
 

Jesuit24

Sep 3, 2014
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1) Yup, continuity of 1.0 Ohms.
I hooked up the wires and it runs. Question, how will I solder the wires on the amp board? I currently just threaded the wire through the holes, but without any metal contacts to solder to, I'm worried I'll burn the board when I want to permanently attach them.
The volume control on the amp also acts as the on/off switch, so it only works when that is turned on (the voice changer on/off switch now doesn't work)
2) The diode's voltage is now 4.44V.
I plan on using the mic supplied with the amp, which is for the 3.5mm plug.
 

KrisBlueNZ

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Exxxxcellent, as Mr. Burns would say :)

FYI that 1Ω multimeter reading is just the resistance of the multimeter probe leads and the meter's internal circuitry. If you touch the probes together you'll still measure 1Ω. So you can ignore it; if the meter shows 1Ω then there is full continuity between the points you're probing.

I think you'll be able to solder the wires to the insides of the "vias" (the holes that connect the copper on the top side to the bottom side). If not, you can scrape off the green solder mask that covers the copper, using a small sharp blade held near the end (for best control, to avoid having it slip off and leaving a big messy scratch on the board).

Now we have to connect the signals between the boards, and match the levels.

The signal flow in the amplifier unit is like this:

Microphone --> Mic socket --> mic preamp --> volume control --> TEA2025B power amp --> speaker.

We will be inserting the voice processor board into the signal path of the amplifier board, in between the output of the mic preamp, and the input of the volume control.

In the first picture in post #12 I marked the two points we need to connect to. The one on the top side, on the west end of R8, is the output of the mic preamp, and the one on the bottom, on the actual pin of the volume control, is obviously the input to the volume control.

Currently these points are connected together by the track that's marked with 'X' on the photo below. You need to cut that track somewhere, and the only accessible place is at the 'X'.

epoint 270132 amp connection points.jpg

Cut it as close as possible to R8 so that there's plenty of track sticking out from under the volume control, so that in future, if you want to return the amp unit to normal operation, you can scrape some solder mask off the track and connect it back to R8 with a blob of solder.

Then we need to make two connections from this board to the voice processor board:

1. Feed the signal from the output of the amp unit's mic preamp (on R8) into the input of the voice processor, and
2. Feed the signal from the voice processor's output back to the amp board, at the volume control.

The second connection is pretty straightforward. The last photo in post #12 shows the input and output points on the voice processor. You need to run a wire from the OUT point on the voice processor to the volume control pin. You MAY be able to connect it to the cut track, if there's enough copper, if you scrape off some solder mask. Otherwise you'll have to connect it to the pin of the volume control where it comes through on the underside.

The first connection is a bit trickier.

The output from the mic preamp is indicated on the photo. This point has the audio from the amp unit's mic preamp, which we will need to feed into the voice processor.

The signal path on the voice processor board is like this:

Microphone --> mic preamp and voice processor chip --> crappy power amp --> speaker.

So we will have to remove the microphone from the voice processor, and feed the signal from the amp unit's microphone preamp circuit (at R8) into the input of the voice processor (marked "IN" on the last photo in post #12).

However, that input is designed to take a signal straight from the little "electret condenser" microphone in the voice processor, which doesn't generate a very strong signal, so the voice processor's input will be quite sensitive.

The signal from the amp unit's microphone preamp will probably overload it and cause distortion. We will need to attenuate that signal. We can't change the input sensitivity because the "mic preamp and voice processor chip" section of the voice processor is all implemented in a single chip that is buried underneath the big blob of tarry stuff, and we can't get into it at all.

That input is also fed with a positive voltage, which is needed by the electret microphone. This requires a capacitor to be connected in series with the signal, otherwise that voltage could affect the microphone preamp in the amp unit.

My first suggestion is to insert a capacitor and a resistor in series, into the signal, like this:
Code:
Amp board                                                  R         -C+     Voice changer
Mic pre out (R8 west) ----------------------------------\/\/\/\/-----| |------ IN pad

The capacitor can be any electrolytic between 1 µF and 100 µF (1 µF is good) and the resistor value... well, you could start with 100 kΩ.

Then, test the circuit by talking into the microphone. As you adjust the volume, and talk louder and softer into the microphone, it should become clear when the voice changer is reaching its maximum input level - the signal will get badly distorted, and talking louder won't cause a louder signal in the speakers. If this is happening when you speak normally into the microphone, you'll need to attenuate the signal more, so increase the 100 kΩ resistor. If the distortion only occurs when you're very close to the microphone, you should attenuate the signal less, so reduce the 100 kΩ resistor.

You could also try a potentiometer instead of the resistor, and adjust it until you get distortion only when the sound at the microphone is louder than it would normally be in regular use.

You may discover that the voice processor's sound quality is not good. Some distortion may be audible at all times. Experiment with it to get the best performance you can.

If you have to set the amp unit's volume control near the bottom of its range, that means that the signal from the voice processor's output is too strong. You can attenuate it by inserting a resistor into the wire that feeds the top of the volume control (no capacitor should be needed here).
 

Jesuit24

Sep 3, 2014
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Okay, I removed the voice changer mic, battery pack and on/off switch from the board and soldered in the voice changer output to the volume control, and the preamp output to the voice changer input via a 1uF capacitor and a 820resistor (I don't have many resistors lying about, that was the biggest). As soon as I turn the amp board on, I'm getting a steady tone sound with or without the amp mic plugged in, so something has gone wrong at some stage.
I also cut the track, hopefully I didn't cut too much.
 

KrisBlueNZ

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Oh dear.

This tone comes from the speaker in the amp unit, right?
What happens when you adjust the volume control?
What happens when you plug in the microphone and talk into it?
 

Jesuit24

Sep 3, 2014
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The tone is from the voice changer speaker, the amp speaker doesn't work. The tone is consistent, when I raise the volume, there's no change and my voice doesn't come through the changer speaker at all when I talk into the amp mic.
 

KrisBlueNZ

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The voice changer's signal is supposed to come through the power amplifier in the amp unit. You should disconnect the voice changer's own internal speaker. See what happens then.
 

Jesuit24

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I'm now getting a tone(which changes depending on the voice changer setting) from the amp speaker when I turn it on. This time the volume raises and lowers with the volume switch. A couple times it turned on without the tone and I could pick up background noise coming through the speaker, although there was bad static coming through when I twiddled the volume (and any other wire) and I couldn't pick up my voice until it was set high. I'm not sure whether it's loose wiring, I cut more of the track than I should have (or not enough) or my resistor isn't right for the job.
 
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KrisBlueNZ

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I think it's probably the resistor value being far too low. Any signal at all from your microphone is being boosted by the microphone preamp in the amp unit, and feeds almost directly into the input of the voice changer, which is very sensitive. See if you can find a resistor that's at least 100 kΩ. And you do have a capacitor in series with it, right?
 
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