preamp/mixer combo

[zoidbergslo]

Hello DIYers

First of all I know these things have been on here quite a few times and I have read most of the threads about it but I still have few questions.
Ok so here it goes

First of all I am trying to do something like this:


Will there be a big problem if I put all this in same case?

Ok so there will be ODAC or something similar + two external stereo inputs one RCA and one 1/4" jack.

Everything will be fed into inverting input on opamp (don't know whitch one yet) and after the first one it will go to second one to correct phase. Is second one even needed?

After this stage I don't know exactly how to continue. How to connect both speaker and headphone amp? can both be driven from opamp output or should there be something in between?
I want to use one potentiometer for volume for both amps where should I put it on input or output of opamp?

Here is schematic I have so far. Only one channel shown


Thanks

 

[udok]

You can use a potentiometer instead of R5/R6. With a little playing around you can get an approximation to logarithmic scale (loudness follows logarithmic law).
The second OPV can then drive the headphone amplifier and the power amplifier.
Udo

 

[zoidbergslo]

Hello

So I played around a bit and I got something like this.

New schematic


And this would correspond to gain changes like this


But now I could play a bit more with resistors to get better curve. Should I use lower value pot and lower value resistors or is this good enough.

 

[udok]

Hello,
if you want a gain > 1, you should reduce R6 to 1k. The resistor across the pot (R5) can be omited.
Greatings,
Udo

 

[zoidbergslo]

Ok after some simulation that is probably wrong I got some results

Schematic in spice (resistor numbering not the same)


and result is


I'll try without R5 across pot.

Is this changing volume with gain same thing as attenuating at input? What is better?

Now about PS for this would it be better to have regulators on same board or on separate?

 

[udok]

Hello,
The circuit looks already good. Use 10u for C1,C2,C3 (better availability and similar price). C5 is to low. Your need at least 10u if the input impedance of the next step is taken into account.

It is also possible to use an attenuator at the input. Small advantage of your current circuit is, that no current flows through the middle pin of the potentiometer.

It is not essential that the power supply regulator (LM317 or similar) is on the same board. If it is on the same board, the output impedance of the regulator is somewhat smaller (no cable).
It is important to have decoupling capacitors near the opv.

Good luck,
Udo

 

[zoidbergslo]

Ok started working on board layout. Ground is not finished.

 

[zoidbergslo]

Ok bit of an update where I am so far. Layout is probably pretty bad.

Potentiometer will be mounted on front plate of the case so I just left sockets for it there.

I also tried to implement some sort of relays for turning outputs on and off.

Any comments.

 

[udok]

Can't read the circuit values...

 

[zoidbergslo]

I still have to clean around second opamp gain settings. ceramic capacitor is in wrong position I guess and board layout could be a lot better around there.

 

[zoidbergslo]

Ok I fixed gain settings for second stage a bit. I left 10k there for now. After I test it on breadboard I'll remove it of leave it permanently.

 

[udok]

The power suply looks wrong. Why not use a bridge rectifier? Also the diode at the output of the regulator is suspicious.

 

[Zero D]

Originally Posted by udok

The power suply looks wrong. Why not use a bridge rectifier?

It looks as if he is using a transformer with only one secondary winding ? If so it's ok, if not i would use both windings & use a bridge

I don't know the 1N5818's are there either ?

 

[zoidbergslo]

Well power supply is copy of O2 power supply section.

I was planing on using one transformer with two secondaries one for headphone amp and one for preamp. But since I am not good at this stuff there could probably be better way.

 

[mondogenerator]

Nice layout Zoidberg

 

[zoidbergslo]

So I can skip 1n5818's there?

Should I go for some other regulators too? Maybe two stage since there is some space around there

 

[zoidbergslo]

Small board for potentiometer RK27/RK097 for now.

Or I could go with one of those stepped attenuators from ebay such as

DACT Type 21 Stepped Attenuator Potentiometer 10K
Valab 23 Step Attenuator Potentiometer 10K Stereo Log
Valab 23 Step Ladder Type Attenuator Potentiometer 10K Log Stereo

 

[udok]

The Diodes after the regulator are unnecessary.

You can find some basics here: HTTP 301 This page has been moved

A online calculator is here (in German): Spannungsversorgung und Netzteile

 

[zoidbergslo]

Hey

I kinda put this project aside for a while now. But no I hope I'll actually continue.

First of all thanks udok for those links I helped to read a bit more about regulators.

Ok so this past month I was still thinking about this circuit and decided on few changes to it.
First of all two inputs will be good enough One for DAC that will sit inside same enclosure and one for external gear. After talking with one of my co-workers he nudged me in SMD direction . So with this change I have enough space on same PCB for preamp and headphone section. Now I can make powersupply from two secondaries not with voltage doubler like I had before.

Found this link and maybe I should implement something similar in my design. Like add buffers on input and buffer inside gain control.

I'll draw new schematic today or tomorrow.

 

[zoidbergslo]

I have put together schematic now. I'll have to spice it to get better component values.

Since it is pretty big I have cut it in sections