Hi there,
I'm a relative newbie with a wierd question. In looking at the available schematics for a 50-75 watt amplifier, using 12v, I have come across the plans using a TDA1562 chip by Philips. Looks interesting, but there is no volume control. Is it reasonable to expect to control the volume with the gain pot on a preamp? According to the layout drawing of the TDA1562 chip, it has a preamp already built in. So, can I run another preamp into that, and if so, what signal input voltages should I aim for? I have a preamp already designed, with treble/bass controls, and I can vary the input voltage by proper placement of resistors to control the gain. Will a variable output of 0 to 200 uV work? (0 -.2V)
Thanx
The Happy Hippy
I'm a relative newbie with a wierd question. In looking at the available schematics for a 50-75 watt amplifier, using 12v, I have come across the plans using a TDA1562 chip by Philips. Looks interesting, but there is no volume control. Is it reasonable to expect to control the volume with the gain pot on a preamp? According to the layout drawing of the TDA1562 chip, it has a preamp already built in. So, can I run another preamp into that, and if so, what signal input voltages should I aim for? I have a preamp already designed, with treble/bass controls, and I can vary the input voltage by proper placement of resistors to control the gain. Will a variable output of 0 to 200 uV work? (0 -.2V)
Thanx
The Happy Hippy
Volume control is the thorn in the DIYers side. It seems simple but when you get into it there are so many signal-to-noise issues that need to be compromised.
For a generic rule, most power amplifiers will approach their rated output power at about 1Vrms signal input level. Note that this means peak signal can be 10V!
Assuming voltage transfer, most pre-amps have low output impedance and most amps have high input impedance. Thus with a pot in the 10-100Kohm (log) range, you can easily do a passive vol control with zero noise/dynamic range impact.
When you go balanced, stereo or bi-amp/tri-amp things get more complicated.
Please search on these key words and you will be rewarded with a treasure trove of knowledge. Post here with follow on questions.
good luck!
For a generic rule, most power amplifiers will approach their rated output power at about 1Vrms signal input level. Note that this means peak signal can be 10V!
Assuming voltage transfer, most pre-amps have low output impedance and most amps have high input impedance. Thus with a pot in the 10-100Kohm (log) range, you can easily do a passive vol control with zero noise/dynamic range impact.
When you go balanced, stereo or bi-amp/tri-amp things get more complicated.
Please search on these key words and you will be rewarded with a treasure trove of knowledge. Post here with follow on questions.
good luck!
Preamp for TDA1532
OK The preamp is based on an OPA2132 from TI. (Dual version of the OPA132) It's basically a circuit with an inverting opamp, a baxandall tone control, and another inverting opamp. I'll be happy to send a copy of the schematic, if I can figure out how to do it and what format to send it in.
I'm using the TINA-TI Spice program to check this circuit; according to it, I can vary the output voltage by varying the voltage of one of the resistors in the second opamp. I've checked it with input voltages running from .00001 volt to .5 volt, and the output voltage does not vary. I can vary the voltage at the output pot from 0 to .200v. A centered pot puts out approximately .100v. The biggest question is, will this "feed" the proper signal into the TDA1532?
Thanx
The Happy Hippy
PS I give up. How do I send a .JPG file as an attachment?
OK The preamp is based on an OPA2132 from TI. (Dual version of the OPA132) It's basically a circuit with an inverting opamp, a baxandall tone control, and another inverting opamp. I'll be happy to send a copy of the schematic, if I can figure out how to do it and what format to send it in.
I'm using the TINA-TI Spice program to check this circuit; according to it, I can vary the output voltage by varying the voltage of one of the resistors in the second opamp. I've checked it with input voltages running from .00001 volt to .5 volt, and the output voltage does not vary. I can vary the voltage at the output pot from 0 to .200v. A centered pot puts out approximately .100v. The biggest question is, will this "feed" the proper signal into the TDA1532?
Thanx
The Happy Hippy
PS I give up. How do I send a .JPG file as an attachment?
See you managed it lol
Your preamp doesn't invert from input to output...
Using virtual earth configuration means the passive tone stage is loaded by R3 (That sets the impedance it works into) which may be to low.
Here's an "active" tone control,
http://sound.westhost.com/project97.htm
Where did you get the circuit from ?
The volume pot is OK... but we need to see what it works into. 100K is high and will form a LPF (low pass filter) with any capacitance on the wiper. 10k would be better... or feed the volume control into an opamp and use the opamp output to feed the power amp.
Your preamp doesn't invert from input to output...
Using virtual earth configuration means the passive tone stage is loaded by R3 (That sets the impedance it works into) which may be to low.
Here's an "active" tone control,
http://sound.westhost.com/project97.htm
Where did you get the circuit from ?
The volume pot is OK... but we need to see what it works into. 100K is high and will form a LPF (low pass filter) with any capacitance on the wiper. 10k would be better... or feed the volume control into an opamp and use the opamp output to feed the power amp.
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