Simple Pre-Amp with Variable High-Pass - This right?

domjohnson · 8th January 2012, 02:00 AM

Hi there,

I've just finished frankensteining this circuit from a couple of others around the t'interwebs, and wanted to check that what I've done will, you know, work...namely, are the XLRs connected up properly, or does there need to be a third connection on the third pin? (I assumed that pin was left disconnected unless I'd added something to the circuit that would be giving out a balanced connection, which I haven't).

Also, will the bypass switch on the highpass actually bypass the high-pass, or have I royally messed that up?

And of course, if there's anything else thats blindingly obvious, please tell me!
Finally, thanks for reading

Speedskater · 8th January 2012, 04:34 PM

Well the connector is labeled "XLR" but the symbol is not that of an XLR connector. Also XLR connectors are mostly used in balanced interconnect systems. This circuit is not balanced.

domjohnson · 8th January 2012, 05:01 PM

Quote:

Originally Posted by Speedskater

Well the connector is labeled "XLR" but the symbol is not that of an XLR connector. Also XLR connectors are mostly used in balanced interconnect systems. This circuit is not balanced.

Hi,

Thanks

Yeah, I couldn't see an XLR symbol, and didn't intend it (at first) to be looked at by anyone other than myself. Thinking of going with just 1/4" Jack connectors instead, considering the lack of a balanced circuit.

Also - should there be a connection from above R3 to before (to the left of) C1, to give the voltage that it needs to operate, or is that supplied from the conection that goes via R1?

Apologies - ultra-noob here

domjohnson · 8th January 2012, 06:15 PM

*to give the voltage that the transistor needs to operate.

Apologies for the post instead of editing...can't seem to find an edit button.

DF96 · 8th January 2012, 08:37 PM

'Edit' is only available for a limited time after a post.

This circuit will 'work' as what? High distortion is likely for signals over a few mV.

domjohnson · 8th January 2012, 09:52 PM

Quote:

Originally Posted by DF96

'Edit' is only available for a limited time after a post.

This circuit will 'work' as what? High distortion is likely for signals over a few mV.

Its meant to be a pre-amp for dynamic microphones on drums, so I would imagine that would exceed the "few mV you mention"...How would I stop it distorting? Is it the transistor I'm using, or resistor values?

DF96 · 9th January 2012, 11:00 AM

No, the circuit. You can't just feed a signal straight into a common emitter stage taken straight from an elementary textbook, unless the signal is very small. Rule of thumb to remember: for a BJT fed a signal of X mV peak at the base the second harmonic distortion will be X %.

domjohnson · 9th January 2012, 05:00 PM

Right...After a little bit of research on designing Bipolar Transistor preamps, it looks like I should have a resistor on the base and on the emitter...is that the main problem here? Or are there other things I'm missing - should I have a darlington pair in there to make it more usable (i.e, will there be a decent amount of gain coming from just one transistor, or will I need/will it help a lot if I have the amount of gain given from a darlington)? Can you point me in the direction of an article that discusses how to design a pre that is simple, but works (reasonably) well-ish, or a fairly simple schematic?

domjohnson · 10th January 2012, 07:17 PM

Would this be a better design to base mine upon? Or this?

DF96 · 10th January 2012, 09:15 PM

You need to read "The Art of Electronics" by Horowitz and Hill.

8th January 2012, 02:00 AM	#1
domjohnson diyAudio Member Join Date: Jan 2012	Simple Pre-Amp with Variable High-Pass - This right? Hi there, I've just finished frankensteining this circuit from a couple of others around the t'interwebs, and wanted to check that what I've done will, you know, work...namely, are the XLRs connected up properly, or does there need to be a third connection on the third pin? (I assumed that pin was left disconnected unless I'd added something to the circuit that would be giving out a balanced connection, which I haven't). Also, will the bypass switch on the highpass actually bypass the high-pass, or have I royally messed that up? And of course, if there's anything else thats blindingly obvious, please tell me! Finally, thanks for reading

8th January 2012, 04:34 PM	#2
Speedskater diyAudio Member Join Date: Sep 2002 Location: Lakewood, Ohio	Well the connector is labeled "XLR" but the symbol is not that of an XLR connector. Also XLR connectors are mostly used in balanced interconnect systems. This circuit is not balanced. __________________ Kevin

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8th January 2012, 06:15 PM	#4
domjohnson diyAudio Member Join Date: Jan 2012	*to give the voltage that the transistor needs to operate. Apologies for the post instead of editing...can't seem to find an edit button.

8th January 2012, 08:37 PM	#5
DF96 diyAudio Member Join Date: May 2007	'Edit' is only available for a limited time after a post. This circuit will 'work' as what? High distortion is likely for signals over a few mV.

9th January 2012, 11:00 AM	#7
DF96 diyAudio Member Join Date: May 2007	No, the circuit. You can't just feed a signal straight into a common emitter stage taken straight from an elementary textbook, unless the signal is very small. Rule of thumb to remember: for a BJT fed a signal of X mV peak at the base the second harmonic distortion will be X %.

9th January 2012, 05:00 PM	#8
domjohnson diyAudio Member Join Date: Jan 2012	Right...After a little bit of research on designing Bipolar Transistor preamps, it looks like I should have a resistor on the base and on the emitter...is that the main problem here? Or are there other things I'm missing - should I have a darlington pair in there to make it more usable (i.e, will there be a decent amount of gain coming from just one transistor, or will I need/will it help a lot if I have the amount of gain given from a darlington)? Can you point me in the direction of an article that discusses how to design a pre that is simple, but works (reasonably) well-ish, or a fairly simple schematic?

10th January 2012, 07:17 PM	#9
domjohnson diyAudio Member Join Date: Jan 2012	Would this be a better design to base mine upon? Or this?

10th January 2012, 09:15 PM	#10
DF96 diyAudio Member Join Date: May 2007	You need to read "The Art of Electronics" by Horowitz and Hill.

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