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Boris Lau Boris Lau is offline
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Default 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
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Boris Lau Boris Lau is offline
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Default 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
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Scott Dorsey Scott Dorsey is offline
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Default 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."
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Boris Lau Boris Lau is offline
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Default 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
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Scott Dorsey Scott Dorsey is offline
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Default 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."


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Boris Lau Boris Lau is offline
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Default 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
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Scott Dorsey Scott Dorsey is offline
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Default 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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