Electronics > Beginners

Building a sound mixer within my guitar amp with line IN & mic input

<< < (11/20) > >>

dazz:

--- Quote from: Zero999 on June 05, 2019, 10:37:55 am ---I think you'll struggle to get that circuit to run down to 9V, because you won't be able to get the voltage across the collector resistors to fall within the TL72's common mode range, at low voltages.

You could replace the emitter resistors with constant current sources: try 1mA. See link below. Note that the diodes and resistor provide a reference voltage, which can be used for both current sources.
http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT%20Current_Source/BJT_Current_Source.htm

I hope you can see why I suggested an IC, for simplicity's sake, but you won't learn so much and it'll still work out more expensive.

--- End quote ---

Yes, learning is priority #1. I might use the IC for the actual amp eventually, but I'm definitely building this preamp.

Probably a silly question, but can't I take the opamp inputs from the PNP emitters instead of the collectors where the voltage is close to Vref? I'm guessing the pnp stage needs to be emitter followers to act as buffers for impedance reasons maybe? I'm going nowhere until I understand bjt's reasonably well, so I'm going to address that before I continue messing with this circuit

Zero999:

--- Quote from: dazz on June 05, 2019, 11:22:40 am ---
--- Quote from: Zero999 on June 05, 2019, 10:37:55 am ---I think you'll struggle to get that circuit to run down to 9V, because you won't be able to get the voltage across the collector resistors to fall within the TL72's common mode range, at low voltages.

You could replace the emitter resistors with constant current sources: try 1mA. See link below. Note that the diodes and resistor provide a reference voltage, which can be used for both current sources.
http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT%20Current_Source/BJT_Current_Source.htm

I hope you can see why I suggested an IC, for simplicity's sake, but you won't learn so much and it'll still work out more expensive.

--- End quote ---

Yes, learning is priority #1. I might use the IC for the actual amp eventually, but I'm definitely building this preamp.

Probably a silly question, but can't I take the opamp inputs from the PNP emitters instead of the collectors where the voltage is close to Vref? I'm guessing the pnp stage needs to be emitter followers to act as buffers for impedance reasons maybe? I'm going nowhere until I understand bjt's reasonably well, so I'm going to address that before I continue messing with this circuit

--- End quote ---
The propose of the two PNP transistors is to boost the signal before the op-amp. The transistors are less noisy, than the op-amp so using them to amplify the signal before, improves the noise figure.

Taking the signal from the emitters would be pointless, because there is no voltage gain at that point: look up emitter follower.

dazz:

--- Quote from: Zero999 on June 05, 2019, 12:42:09 pm ---The propose of the two PNP transistors is to boost the signal before the op-amp. The transistors are less noisy, than the op-amp so using them to amplify the signal before, improves the noise figure.

Taking the signal from the emitters would be pointless, because there is no voltage gain at that point: look up emitter follower.

--- End quote ---

Yes, I know what an emitter follower is, but I'm only familiar with npn transistors and thought this was a common emitter job.  :palm:

I noticed something in the simulation: the dc bias at the opamp inputs is imbalanced, and that is what seems to be causing the distortion (see pic 1)

So I tweaked the value of one of the emitter resistors (from 15k to 20k) until I got pretty much the same dc voltage at the opamp inputs. The distortion is gone completely and the gain skyrocketed too! (see pic 2) Why would the gain increase so much?

I have no idea what I'm doing. LOL. Would a trimpot there be a good idea? Changing the Vref voltage didn't seem to help

dazz:
OK, I think the solution above was flawed. How about adding a cap like the one shown below? That seems to work fine, I get lots of gain with a few resistor tweaks so no need for another amplification stage.

Well, I got the transistors today, so time to start prototyping this thing and do some actual tests.

magic:

--- Quote from: Zero999 on June 04, 2019, 01:36:25 pm ---I'm currently designing an intercom system, using the NE5534 as a dynamic microphone pre-amplifier, simply because there's no point in using anything better.

--- End quote ---
Consider NJM2068, it's a dual, dirt cheap and very low noise - around 3nV/√Hz or so IIRC.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod