Preamplifier with only 6v

hantto · Aug 11, 2005

Hi!

I have designed this little preamp (or to be more exact, been looking at other designs and then put together one that would suit my needs). I designed it to operate from only 6v so that I could use 5v digital potentiometers instead of analog ones.

My question is simple, will this work, any errors? Thanks!

Roff · Aug 11, 2005

hantto said:
Hi!

I have designed this little preamp (or to be more exact, been looking at other designs and then put together one that would suit my needs). I designed it to operate from only 6v so that I could use 5v digital potentiometers instead of analog ones.

My question is simple, will this work, any errors? Thanks!

It looks OK to me, DC-wise. I can't comment on the tone control range.
Be aware that your input resistance is only 1k.

hantto · Aug 11, 2005

Thanks for pointing that out!

Nigel Goodwin · Aug 11, 2005

Such small power rails are VERY restricting, particularly when you're using tone controls, it will be very easy to clip the signal through these stages!.

There's no need to be concerned about the opamp power supply and that for digital potentiometers, you should run the digital side off 5V, and the opamps off their normal far higher supplies, preferably using a split supply. There's no problem doing so, most digital poteniometers used in audio circuits won't restrict the audio to tiny supply rails.

hantto · Aug 11, 2005

Nigel Goodwin said:
Such small power rails are VERY restricting, particularly when you're using tone controls, it will be very easy to clip the signal through these stages!.

There's no need to be concerned about the opamp power supply and that for digital potentiometers, you should run the digital side off 5V, and the opamps off their normal far higher supplies, preferably using a split supply. There's no problem doing so, most digital poteniometers used in audio circuits won't restrict the audio to tiny supply rails.

Ok, hm but wouldn't that destroy the pots? At least this is the response I got on another forum.

Then you will have to redesign the preamp so that it works from a nonsymmetrical supply. If you try and use the 5V digital pot chip on an opamp circuit thats passing signal swing of +/-15v, you will toast it.

Are 5v pots able to withstand higher potentials even if they're only running at 5v, and especially the negative side?

Nigel Goodwin · Aug 11, 2005

You really need to consult the manufacturers datasheets to see what they say about it?, and you could always simply arrange that the signals on the pots are kept within their limits.

Incidently, why are you wanting digital pots anyway?.

Are you aware that you can buy specific preamp control IC's, these include bass, treble and volume controls inside a single IC - they are usually stereo, and include a balance control as well.

hantto · Aug 11, 2005

Nigel Goodwin said:
You really need to consult the manufacturers datasheets to see what they say about it?, and you could always simply arrange that the signals on the pots are kept within their limits.

Incidently, why are you wanting digital pots anyway?.

Are you aware that you can buy specific preamp control IC's, these include bass, treble and volume controls inside a single IC - they are usually stereo, and include a balance control as well.

I'm not sure, that's why I tought it to be a good idea to keep the voltages down, anyway this is the pot i'm going to use: **broken link removed**

I need it to be remote controllable, that's why I need digital pots. And also for the innovation

Yes i'm aware of special IC's. The ones I have tried didn't satisfy me, the sound quality wasn't acceptable. Maybe this because I guess they weren't high-end ones. Another catch is that they are not readily available and I guess they're expencive. My application doesn't require balance control.

Nigel Goodwin · Aug 11, 2005

The Maxim datasheet doesn't really seem to say?.

I'd be inclined to try it and see what happens :lol:

hantto · Aug 11, 2005

Nigel Goodwin said:
The Maxim datasheet doesn't really seem to say?.

I'd be inclined to try it and see what happens :lol:

Yes, it would be intresting

But I don't dare to try

Guess i'll stick with 6v I have to be exact when adjusting the gain.

Nigel Goodwin · Aug 11, 2005

I don't see as anything 'nasty' would happen, I would suggest gradually increasing the signal level whilst monitoring the output with a scope, to see what happens as the signal goes above 5V.

What is the preamp actually for?, a 6V supply rail is pretty low for most purposes.

hantto · Aug 11, 2005

Unfortunately I do not own a scope. :<

It's for a car amp, the singe supply will make the design easier. But I think this will work like a charm I did some inital test, the sound was ok down to 2,5V! But the power amp I used wasn't the most quality full well, we'll se.

Roff · Aug 11, 2005

hantto said:
Nigel Goodwin said:

The Maxim datasheet doesn't really seem to say?.

I'd be inclined to try it and see what happens :lol:

Click to expand...

Yes, it would be intresting

But I don't dare to try

Guess i'll stick with 6v I have to be exact when adjusting the gain.

It does say (below). You basically have to keep the analog signal between the rails.

hantto · Aug 11, 2005

Didn't see that, thank you Ron H!

audioguru · Aug 11, 2005

I suggest using a logarithmic stepped digital pot for a volume control to match your hearing. A linear pot will be loud over most of its range.
Your linear stepped digital pots are necessary for the tone controls.

Your circuit has DC on its output pot. It is bad for a mechanical pot but necessary for a digital pot with your single supply.

hantto · Aug 12, 2005

audioguru said:
I suggest using a logarithmic stepped digital pot for a volume control to match your hearing. A linear pot will be loud over most of its range.
Your linear stepped digital pots are necessary for the tone controls.

Yes, I have a log pot for the volume control, and linear for tone controls.

audioguru said:
Your circuit has DC on its output pot. It is bad for a mechanical pot but necessary for a digital pot with your single supply.

I do not understand this? Must I alter the circuit in some way?

Thanks for helping!

audioguru · Aug 12, 2005

Hi Hantto,
With a mechanical pot a coupling cap is used to keep the DC from the output of the opamp away from the pot to reduce noise when it is turned.

If you direct-couple your digital pot to keep its signal swing within its power supply rails, then when you turn it far down or far up quickly it will cause a huge "pop" sound in the system's audio output.

If you direct couple the top of the pot and connect its bottom to the bias voltage of the opamp then it will work fine.
Make certain that the bias voltage is a low resistance and is well filtered to avoid positive feedback at the opamp or bass leakage when the digital pot is turned down. :lol:

hantto · Aug 12, 2005

Ah! I understand!

That was mostly welcome! Thank you!

Just a tought, wouldn't the digital pot work fine if I wired it in the mecanical pot's way?

audioguru · Aug 12, 2005

hantto said:
Ah! I understand!

I don't think so.
The signal for a digital pot must never exceed its power supply voltages.

Just a thought, wouldn't the digital pot work fine if I wired it in the mecanical pot's way?

No, it might be destroyed when the signal swings negative which is far beyond the 0V of its negative power supply voltage. If the current from the opamp isn't enough to destroy it then it surely will cause severe distortion of the signal's negative swings.
It would need a -6V negative supply if it is capacitor-coupled. :lol:

hantto · Aug 12, 2005

audioguru said:
hantto said:

Ah! I understand!

Click to expand...

I don't think so.
The signal for a digital pot must never exceed its power supply voltages.

Yes, that I already understood.

audioguru said:
hantto said:

Just a thought, wouldn't the digital pot work fine if I wired it in the mecanical pot's way?

Click to expand...

No, it might be destroyed when the signal swings negative which is far beyond the 0V of its negative power supply voltage. If the current from the opamp isn't enough to destroy it then it surely will cause severe distortion of the signal's negative swings.
It would need a -6V negative supply if it is capacitor-coupled. :lol:

Thanks for this info. Now I understand the capacitor coupling things better

I built this circuit, and what can I say, i'm very staisfied with the tests so far. It works very good. I built the tone controll part on a small board with smd componenets since the MAX5475 I received were SOT23. And the rest on breadbord.

audioguru · Aug 12, 2005

:lol: :lol: Great! :lol: :lol:

Preamplifier with only 6v

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