Transformerless discrete balanced micamp

thanks PPR,

Is the output OK to take unbal?

Can I try this out at bi-polar 24 volts? 28 volts? What are it's limits with these same transistors?


This preamp is really very good and would be an excellent candidate for
those on a budget or just wanting a first all discrete mic amp. I'm using it after just using TELE V72, V78, Neve 1290,QE CA127 etc and it sure holds it own.

I'm too behind on my projects to learn how to do PCB layouts, so I hope
someone else trys this design out.

Lance

> Is the output OK to take unbal?

Absolutely. (You could omit one output emitter follower if you are not using it.)

> Can I try this out at bi-polar 24 volts? 28 volts?

Up above +/-30V, this may be able to deliver much more signal than any modern input can swallow. But distortion at some "sane" level will drop as you feed it more supply.

Somewhere around +/-35V you will stress low-price transistors.

Over +/-24V, the output emitter followers' 2K2 resistors need to be bigger than common 1/4 Watt. Over +/-35V, they will dissipate over 1 Watt, and the 4K7 resistors will need to be bigger than common 1/4 Watt. Rather than wear out my slide-rule, crank it up slow, check for hot-spots, and upgrade resistors as needed.

thanks again PRR,

This is all good news and I'll be trying this fella out in some other applications. I'll keep ya all posted.

cheers,
Lance

can someone check my head on this one before I finish the layout.

The only thing not shown here is the pot, which I'm still working out the part for, that, and I need to fix polarity on caps.

UPDATED Schematic:

Regards

ju

R24 should be 10 ohms, not 100R.

If the first stage has a gain of 45 dB so if that much gain is enough could the second stage just be bypassed or removed altogether? With the first stage and the EF's it would be a nice little circuit.

Thanks, fixed that, and other things. I updated the schematic posted above as well. Here's a rough of the board layout. Still have to check the transistor pinouts.



Comments?

ju

Hi Fum

Should the emitter of Q1 not go to the collector of Q2 on the schematic?

Any chance of a single sided PCB?

Regards
Peter

reworking, we'll see what I end up with =)

ju

Hi Fum
Are you The Shiny Box guy.

I have one of your mics down here in thailand, a 23.

I like it a lot, we did some dual backing vocals with it and it really was killer.
Using an Aphex Thermionix preamp.

The tener sax got an old Ben Webster sound the sax player loved.
He's retro in tastes.

And we did some lead guitars with it and it had a nice room feel and pleasing tone.

I am wondering how you like this PRR preamp still,
and if you like it with your ribbons.
I am looking to build a dedicated pre amp for the 23.

And I have great respect for PRR's throw away ideas,
more than many peoples finished ones.

Hi Fum
Are you The Shiny Box guy.

I have one of your mics down here in thailand, a 23.

I like it a lot, we did some dual backing vocals with it and it really was killer.
Using an Aphex Thermionix preamp.

The tener sax got an old Ben Webster sound the sax player loved.
He's retro in tastes.

And we did some lead guitars with it and it had a nice room feel and pleasing tone.

I am wondering how you like this PRR preamp still,
and if you like it with your ribbons.
I am looking to build a dedicated pre amp for the 23.

And I have great respect for PRR's throw away ideas,
more than many peoples finished ones.

What about removing the second stage of this one?

PRR said:

Click picture for readable copy.

Click to expand...

Will this work? What about C2/C3, can they be omitted? I know that the maximum gain will be reduced to about 45 dB, that is okey for me.




Roger

For reference, the original: http://sound.westhost.com/project66.htm

Yes, I know!

It is also strange to see that Rod Elliot uses no RFI protection at the input. I want the output to be differential.


Roger

Roger Gustavsson said:

What about removing the second stage of this one?

Will this work? What about C2/C3, can they be omitted? I know that the maximum gain will be reduced to about 45 dB, that is okey for me.

Roger

Click to expand...

Funny I just draw the same pic yesterday but without the caps (didn't post though). If you really need 45 dB gain, or more than say 25 dB, it's better to share it between multiple gain stages to get better transient response and thicker sound. Opamps aren't necessary that bad anyway. Anyway, I was about to ask a question on discrete pre design (not necessary trafoless) using 48V single supply. Found this one and the other PRR's design idea that didn't work for some reason.