stereo preamp advice

[snk]

Thank you so much both, that's super useful ! 
But after thinking about it, I think I like the pot-less design even more (so I can have the boost enclosure somewhat hidden, at least not like something I would be tweaking. It's less clutter in the studio, "set and forget" and voilà


(but the circuit you posted may be useful for another project I have - a stereo Red Llama, which I haven't build yet-, and for which I don't have dual gang 1M lin and 10k log pots -while I have many dual gang 100k pots)

[fryingpan]

Anyway, do get rid of those high value resistors. 10M, 22M, those are far too high. More noise, more susceptibility to noise on the supply, more offset.

Make R1 1M or smaller. R2 10k and C1 10u¹. R7-R8 22k, and C4 100u or 220u. You could make R10 larger, like 100kohm, since you will be plugging into a 10kohm input. The 5532 has plenty of drive, but the bigger the load, the better. I would also add overvoltage diodes on the input, just before R3. You can do this several ways. The easiest is an antiparallel pair to ground of a 1N1418 series with a (reversed) zener diode around 7V breakdown. The Blofeld won't get anywhere near frying the opamp's input, but should you use it with something else, better to be safe than sorry.

¹ : you don't need to make it that big for bandwidth reasons. Even 3.3u would probably suffice. That said, electrolytics are best chosen, like, at least three times larger (some say ten times).

[PRR]

powering a NE5532 (and alike) with 36V (+/- 18V) supply is like you're asking for trouble..

Where are youse guys buying your '5532??

The '5532 family is rated 44V, +/-22V. Yes, even in the 21st century. That is one of its best features.

https://www.ti.com/lit/ds/symlink/ne5532.pdf  table 7.1

[antonis]

We DO know 5532 specs but we've also experienced of those op-amps getting pretty warm at about 35V..
(after all, we don't sacrifice peace of mind for a couple of dBs more headroom..)

Also, under Table 7.1, TI declares:
<Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied.>

At Table 7.3 here, TI lists +/- 15V as Recommendend Operating Conditions

[snk]

Anyway, do get rid of those high value resistors. 10M, 22M, those are far too high. More noise, more susceptibility to noise on the supply, more offset.

Make R1 1M or smaller. R2 10k and C1 10u¹. R7-R8 22k, and C4 100u or 220u. You could make R10 larger, like 100kohm, since you will be plugging into a 10kohm input. The 5532 has plenty of drive, but the bigger the load, the better.[...]
¹ : you don't need to make it that big for bandwidth reasons. Even 3.3u would probably suffice. That said, electrolytics are best chosen, like, at least three times larger (some say ten times).

Thank you very much for all those advices.

I understand the need for lower R1 and R2 values (less noise).
I understand the need to increase capacitors values (more power filtering).

But could you elaborate about the advantage to make R7-R8 22k and R10 100k?
I see R7-R8 are needed for biasing : woudn't changing their values also change the biasing?
And wouldn't increasing R10 to 100k, despite changing the impedance (which I guess it the goal of mod), also change the output volume?
Are these mods safe in every way volume-wise and bandwidth-wise? (I mean : is there any other side effects)?

[antonis]

But could you elaborate about the advantage to make R7-R8 22k and R10 100k?
I see R7-R8 are needed for biasing : woudn't changing their values also change the biasing?

No, as long as their ratio is retained..
(by lowering their values you result into a more "stiff" resistive voltage divider..)

And wouldn't increasing R10 to 100k, despite changing the impedance (which I guess it the goal of mod), also change the output volume?

It will raise output volume about 4.5%..
( compare 10k/(10k+470R) to 100k/(100k+470R), considering op-amp output impedance almost zero..)

You can use a 100k pot in place of R10 as an output Volume control..

[fryingpan]

We DO know 5532 specs but we've also experienced of those op-amps getting pretty warm at about 35V..
(after all, we don't sacrifice peace of mind for a couple of dBs more headroom..)

Also, under Table 7.1, TI declares:
<Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied.>

At Table 7.3 here, TI lists +/- 15V as Recommendend Operating Conditions

That's what I remembered too. Anyway, 30V is plenty (about 18dB headroom relative to +4dBu). I suppose that +/-18V (36V) is doable too with a heatsink.

[mzy12]

There are a few criteria here you need to look at. The NE5532 isn't exactly the most modern device and there are better options out there when it comes to noise and power consumption. Also, it's got a regular BJT inut, which means input impedance is low even in a non-inverting configuration.

The OPA1642 is a great low noise, high input impedance JFET dual opamp. Great for things like buffers in guitar signals or clean gain boosts as a utility. With the high input impedance, it doesn't matter what the output impedance of your synth is (could be really low with modern designs or unusually high for arbitrary reasons, can't see anything in the waldorf manual).

Are you planning on designing a PCB for this or are you perfboarding it (OPA1642 is smd only)? And have you decided on how you are powering the circuit?

[fryingpan]

It will raise output volume about 4.5%..
( compare 10k/(10k+470R) to 100k/(100k+470R), considering op-amp output impedance almost zero..)

You can use a 100k pot in place of R10 as an output Volume control..

Also, the intended load is 10kohm (a line input). With an already existing 10kohm load, that becomes 5kohm (10kohm//10kohm), which the 5532 can certainly manage fine, but the higher the load, "the best it feels", so to speak. The noise should mostly be "absorbed" by the low output impedance of the opamp, similarly to how a 1M input resistor rarely causes noise when you plug in a guitar.

[fryingpan]

There are a few criteria here you need to look at. The NE5532 isn't exactly the most modern device and there are better options out there when it comes to noise and power consumption. Also, it's got a regular BJT inut, which means input impedance is low even in a non-inverting configuration.

The OPA1642 is a great low noise, high input impedance JFET dual opamp. Great for things like buffers in guitar signals or clean gain boosts as a utility. With the high input impedance, it doesn't matter what the output impedance of your synth is (could be really low with modern designs or unusually high for arbitrary reasons, can't see anything in the waldorf manual).

Are you planning on designing a PCB for this or are you perfboarding it (OPA1642 is smd only)? And have you decided on how you are powering the circuit?

If I were to swap opamps (but the 5532 is mighty fine) I'd go with the 4562 honestly, considering that it's intended to be used with a low source impedance. That opamp is basically silent and very low input offset too (not that it matters in this application). And it can be found through-hole too. With SMD devices there are loads (well, relatively) of opamps to choose from.

[mzy12]

considering that it's intended to be used with a low source impedance

The 'problem' is we don't know the output impedance of the blofeld. At least I couldn't find anything on the internet about it.

I'd also recommend moving R10 of the MXR microamp 'behind' R9, to avoid creating a resistor divider on the output, unnecessarily dumping the output volume. And of course increasing the value of the power supply filtering caps and adding a 100nF ceramic cap as close to the opamp power rails as possible between power and ground (that's just good practice to do all the time).

[fryingpan]

considering that it's intended to be used with a low source impedance

The 'problem' is we don't know the output impedance of the blofeld. At least I couldn't find anything on the internet about it.

I'd also recommend moving R10 of the MXR microamp 'behind' R9, to avoid creating a resistor divider on the output, unnecessarily dumping the output volume. And of course increasing the value of the power supply filtering caps and adding a 100nF ceramic cap as close to the opamp power rails as possible between power and ground (that's just good practice to do all the time).

Waldorf says nothing, but a Greek website says it is 550 ohms (1100 ohms stereo, which makes no sense). Regardless, it would be weird if the output impedance were much higher. The DAC's output is connected to an opamp and then to the line out.

[mzy12]

1100 ohms stereo, which makes no sense

Stereo, twice the impedance, twice the goodness!   

[snk]

So I built a version on veroboard today and it is not working... Obviously I made a mistake somewhere, so I will try to debug the build... I will let you know how it goes!

[mzy12]

Here's a quick schematic for a bipolar power supply with a 'clean' gain of 10. It's optimised for low noise as best my knowledge allows me. I still haven't decided on what ferrite bead would be the best choice for this application. Also ignore the suggested opamp and use what you want, that's just what I was building it around 

Schematic

EDIT: Images aren't working lol

EDIT AGAIN: Just inserted a link.

[antonis]

I built a version on veroboard today and it is not working...

Distorting, dead silent, oscillating, smoking..

[snk]

I built a version on veroboard today and it is not working...

Distorting, dead silent, oscillating, smoking..

Sorry, I didn't give any detail (it was late, and I didn't take much time to investigate).
By "not working", I meant "no sound at all". It is not buzzing, oscillating, distorting or smoking.
But since I had built it on veroboard, from a layout I did myself (and I am by no means an expert), with a dual gang pot at the output, there are plenty reasons why it could not work   
I need to take some time to check the wiring first.
I can also post my layout, but I don't know if it would be useful?

[duck_arse]

I can also post my layout, but I don't know if it would be useful?

it is always useful. but with a circuit diagram. and photos of your build. easy peasy. please.

[PRR]

it is not working...

Longshoremen went out on strike this week.  EDIT strike on hold for now.

[fryingpan]

Here's a quick schematic for a bipolar power supply with a 'clean' gain of 10. It's optimised for low noise as best my knowledge allows me. I still haven't decided on what ferrite bead would be the best choice for this application. Also ignore the suggested opamp and use what you want, that's just what I was building it around 

Schematic

EDIT: Images aren't working lol

EDIT AGAIN: Just inserted a link.

I never understood how this overvoltage protection approach works. I mean, it will limit input voltages by V± ± 0.7V but the absolute maximum limits for many opamps dictate input voltages no higher than the supply.