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#1
Posted to rec.audio.pro,rec.audio.tech
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Building an amp for my talkbox
Hi there,
I've got a custom made talkbox with an amp inside. I don't like the amp anymore. It's 18W at 4 Ohms, but my speaker has 8 Ohms and it's just not loud enough before the amp distorts. Since it's not easy to find an amp module that fits inside the box and runs of 12-24 Volts, I decided to build my own using an amp IC. My plan is to use the Philipps tda 1562, which seems to do the job pretty well according to the data sheets. To protect the speaker and to save power, I want to put a high-pass before the amp. To achieve this I need to decouple the input, otherwise the output impedance of the instrument hooked up to the talkbox would mess up the high-pass cut-off frequency. How can I do that? I thought of using a TL 084 OP-amp with 1:1 feedback, but I heard that this amp is pretty noisy. How bad is it in my case, since I don't amplify in that stage? Any other thoughts or recommendations for doing that job? Boris -- http://www.borislau.de - computer science, music, photos |
#2
Posted to rec.audio.pro,rec.audio.tech
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Building an amp for my talkbox
ok, back to r.a.p. with this thread. Seems to be more efficient ;-)
Boris Lau wrote: Hi Scott, thanks for your reply. Are you sure it's the amp and not the speaker? It shouldn't actually be a speaker but a compression driver. It is a compression driver with 50 W, the amp has 18 W at 4 Ohms. The signal at the output of the amp looks highly distorted on an oscilloscope, when I drive it at the level that I need. What instrument is it? If it's a high-Z guitar pickup or something like that, you will need more gain and a higher input Z than the TDA 1262 will give you, so you'll need an input stage anyway. You can put a passive filter network or even an active filter between your FET input stage and the output amp. I mainly use a keyboard to drive that thing, but I would like to be able to use a guitar as well. I have a smooth distortion circuit that I can switch into the signal path, that provides additional gain when using the guitar. Can't I just put a resistor with 100k Ohms or 1M Ohms between my unbalanced signal and GND to get a high Z that works for both keyboard and guitar? Boris -- http://www.borislau.de - computer science, music, photos |
#3
Posted to rec.audio.pro
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Building an amp for my talkbox
Boris Lau wrote:
Are you sure it's the amp and not the speaker? It shouldn't actually be a speaker but a compression driver. It is a compression driver with 50 W, the amp has 18 W at 4 Ohms. The signal at the output of the amp looks highly distorted on an oscilloscope, when I drive it at the level that I need. The power rating is meaningless. But if the amp is clipping on a scope, you need an amp with higher voltage rails. What instrument is it? If it's a high-Z guitar pickup or something like that, you will need more gain and a higher input Z than the TDA 1262 will give you, so you'll need an input stage anyway. You can put a passive filter network or even an active filter between your FET input stage and the output amp. I mainly use a keyboard to drive that thing, but I would like to be able to use a guitar as well. I have a smooth distortion circuit that I can switch into the signal path, that provides additional gain when using the guitar. You need more than just gain, you need a very high-Z input. Can't I just put a resistor with 100k Ohms or 1M Ohms between my unbalanced signal and GND to get a high Z that works for both keyboard and guitar? Sure, but if you have a 1K input impedance, and you put a 1M series resistor in place... you just lost, what, 40 dB of signal in the process? Better to use a FET input stage. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#4
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Building an amp for my talkbox
Scott Dorsey wrote:
You need more than just gain, you need a very high-Z input. Can't I just put a resistor with 100k Ohms or 1M Ohms between my unbalanced signal and GND to get a high Z that works for both keyboard and guitar? Sure, but if you have a 1K input impedance, and you put a 1M series resistor in place... you just lost, what, 40 dB of signal in the process? Better to use a FET input stage. Well, I mean 1M not in series with the FET amp input, but in parallel. That would work for both guitar and keys, wouldn't it? Boris -- http://www.borislau.de - computer science, music, photos |
#5
Posted to rec.audio.pro
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Building an amp for my talkbox
Boris Lau wrote:
Sure, but if you have a 1K input impedance, and you put a 1M series resistor in place... you just lost, what, 40 dB of signal in the process? Better to use a FET input stage. Well, I mean 1M not in series with the FET amp input, but in parallel. That LOWERS the input impedance, not raises it. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#6
Posted to rec.audio.pro
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Building an amp for my talkbox
Scott Dorsey wrote:
That LOWERS the input impedance, not raises it. of course, you're right. So just the FET? I'll draw a schematic and get back to you (the thread). Boris -- http://www.borislau.de - computer science, music, photos |
#7
Posted to rec.audio.pro
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Building an amp for my talkbox
Boris Lau wrote:
Scott Dorsey wrote: That LOWERS the input impedance, not raises it. of course, you're right. So just the FET? I'll draw a schematic and get back to you (the thread). Yes, just standard FET input stage. Or use a FET-input op-amp. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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