My simple preamps for GCs (or whatever)

[Russ White]

Here are a couple simple preamps I drew for protoyping. It is built on the protoboard you can get from futurlec.

These two designed work great for me.

The power supply is not shown, but any decent power supply from +/-10V to +/-18V should work fine.

I build mine with a regulated +/- 12V supply.

Only one channel is shown. You can build two channels per side of the board, which if you are doing a stereo preamp allows you to use the entire other side for the power supply which is what I did.

I can send a drawing of the PS and photos if you wish.

Drawing #1 is the opa627/637 + buf634 version.

Drawing #2 is a LT1015 + LT1010 version with 2 channels shown. This version may benefit from a 5-100pf cap soldered direct between the negative input and the output of the LT1015 if you see any instability. The biasing for the LT1010 will make it run pretty hot. You can safely omit the biasing resistor, or use a high value like 1K.

Both also make excellent headphone amps.

Humbly submitted in the hope you have some fun.
[IMGDEAD]http://www.sequenet.com/images/preamp1_a.jpg[/IMGDEAD]

[IMGDEAD]http://www.sequenet.com/images/preamp2_a.jpg[/IMGDEAD]

 

[Russ White]

I forgot to mention that I used a simple 10K stereo pot for volume control.

Also note that I did not use input capacitors since I know that I have no DC coming from my source. If you think you might you should add at least a .2uf capacitor either before or after the pot (I have seen people do both).

 

[Russ White]

pics of finished preamp

Here is the LT1115 version finished.

[IMGDEAD]http://www.sequenet.com/images/pre1b1.jpg[/IMGDEAD]
[IMGDEAD]http://www.sequenet.com/images/pre1b2.jpg[/IMGDEAD]
[IMGDEAD]http://www.sequenet.com/images/pre1b3.jpg[/IMGDEAD]

 

[mod_evil]

Hi! Its look good. Can you post the schematic?

Thanks.

 

[Russ White]

Thanks man, I will try to post a schematic. But really it is quite similar to the schematics shown on the buf634 and LT1010 data sheets. Its a pretty vanilla circuit. The LT1010 datasheet is probably the closest.

 

[mod_evil]

Ohh thanks. How much ampers this trafo can supply? How much the circuit consumes? which one is better?

Thanks for attention.

 

[Russ White]

The trafo is 2amp 25VCT it actually puts out closer to 30V idle.

After full bridge rectifier I have ~ +/-16V which is then regulated to +/- 12.

Of these two designs I really like the 1115 and 1010 combo best so far. But really I think I like AD8610/8620 best (different pre).

The circuit draws very little idle (a few ma), but it could drive about 150ma per channel.


One thing I am changing tonight is I will make the input impedance 100K instead of 1M. I am getting too much line noise.

 

[mod_evil]

If the circuit consumes 150mA, why you're using a 2A trafo? Are you feeling an increase in quality using a most powerful trafo? How much uF you're using in regulator input?

Thanks.

 

[Russ White]

If the circuit consumes 150mA, why you're using a 2A trafo? Are you feeling an increase in quality using a most powerful trafo? How much uF you're using in regulator input?

Well actually it is not 100% efficient, and that is 150ma per channel. So i would say it could actually draw 400-500ma at its full potential. But still the trafo I have is certainly overkill. I got it cheap a while back at an estate sale. Plus I know it to be a very quiet well built trafo with next to no EMI.

You could probably use a 400-500ma trafo fine with either of these circuits.

I have 1,500uf on each rail prior to the regulators, then there is 1000uf per rail after the regulators.

 

[mod_evil]

Hmmm its looks good! I will use 4700uf on each rail, and 800ma trafo. What you using to regulator? 7812/7912? Do you using ceramic capacitors in the regulator?

tnks.

 

[Russ White]

You are cortrect 7812 and 7912 for regulators. They get warm, but not hot. A heat sink on each would not hurt.

 

[mod_evil]

Hmmm... The datasheet of the 78xx and 79xx specify one ceramic capacitor ( 100nf to 220nf ) in the output and the input.

Tnks.

 

[Russ White]

The output capactitor can be as large as you like or even omitted (mine is 1000uf) you only need the small input cap if your regulators are not close to the power supply(filter) caps. Mine are about 3/4" from the 1500uf filter caps. So I do not need the additional filter cap on the input to the regulators.

 

[Russ White]

On further review based on input from the group I will likely remove the large caps at the regulator output.

 

[mod_evil]

It improved the quality of the sound?

Tnks.

Please Russ, post the schematic.

Thanks.

 

[Russ White]

It did improve the sound. Funny thing though. The regs are running cooler, but the trafo warmer now... wild.

In any case I got great input from Jan, Mauro, macboy, and peranders in no particular order.

The pre-amp sounds great.

For all those who have not thought about it the LT1115 and the LT1110 are quite a combo.

I will next try the opa637/buf634 in the same circuit.

I will also try the opa637/LT1010 as well as the LT1115/buf634.

Thanks to all who helped me. I am sure I forgot a few.

 

[Russ White]

About the schematic, I am working on it. Can anyone recommend good(cheap) software?