QUAD 405 clone

[Mooly]

So the whole idea of large caps is wrong do you think

It always depends on the application. Low power opamp circuitry needs a stable noise free rail. There are no high current load transients that need consideration, you have essentially a constant current draw on the opamp rails in the Quad. There is no need for lots of capacitance and in some cases it can do more harm than good.

What exactly is the problem that you think 'needs to be addressed'?

This is exactly the point

You have to have a goal of what aspect are you trying to improve? If the rail is stable and has low measured noise (you need a wideband scope to look and check the noise) then adding caps will not change anything. In some cases, it might cause other unwanted effects. Every case is unique.

 

[goodguys]

Thanks ! Can you point out the 2.7k resistor you mentioned earlier.

 

[richb]

To me it looks like the ones highlighted in red, they are 3k3 in the original circuit. There is an additional 3k3 shown in green, but this looks like it is the one in parallel with the 330pF capacitor, so not the opamp supplies.

 

[ejp]

Not sure, thought a larger cap may give lower esr

Why do you think you need lower ESR at the opamp?

higher ripple current rating,

Why do you think you need higher ripple current rating at the opamp?

it may store more energy if needed in case of a big bass hit,

Why do you think the opamp will run out of current on a big bass hit? and how will a larger capacitor across the rails accomplish that?

or may act like a local power supply for the boards to pull on and provide b

The capacitor we are dsicussing isn't for the whole PCB. Only for the opamp. Its purpose is to couple the DC rails to each other to eliminate opamp oscillations. It has nothing to do with any of the purposes you mention.

 

[goodguys]

Why do you think you need lower ESR at the opamp?


Why do you think you need higher ripple current rating at the opamp?

Why do you think the opamp will run out of current on a big bass hit? and how will a larger capacitor across the rails accomplish that?

The capacitor we are dsicussing isn't for the whole PCB. Only for the opamp. Its purpose is to couple the DC rails to each other to eliminate opamp oscillations. It has nothing to do with any of the purposes you mention.

Thanks. No, i am talking about the amp boards in that instance, not the op-amp

 

[goodguys]

To me it looks like the ones highlighted in red, they are 3k3 in the original circuit. There is an additional 3k3 shown in green, but this looks like it is the one in parallel with the 330pF capacitor, so not the opamp supplies.

View attachment 1107051

Thanks! Is the zener diode 12 volt?
Mooly said to calculate the current : " If you have a 50 volt rail, a 2k7 resistor and a 12 volt Zener you have 14 milliamps available calculated as (50-12)/2700 "

In this case, if the zener diode is 12 volt, the resistor is 3300 ohms, the supply voltage in this case is +/- 40 volts, then that makes ( 40-12 ) / 3300, so the current should be 8ma

 

[Mooly]

If you want to use an opamp that pulls 14ma with 40 volt supplies then you need to drop the resistors to 1.2k 1 metal films. Those will run quite hot dissipating around 650 milliwatts each. 1.2k gives you 23 milliamps which allows 9 milliamps for the Zener.

 

[goodguys]

If you want to use an opamp that pulls 14ma with 40 volt supplies then you need to drop the resistors to 1.2k 1 metal films. Those will run quite hot dissipating around 650 milliwatts each. 1.2k gives you 23 milliamps which allows 9 milliamps for the Zener.

Thanks! Where are the resistors? I can't see 2.7k ohms on the circuit diagram. Can you please mark on the photo below.

 

[richb]

Thanks! Where are the resistors? I can't see 2.7k ohms on the circuit diagram. Can you please mark on the photo below.

View attachment 1107162

My best guess is in post #303 above! The original Quad values are 3k3.

 

[Mooly]

Where are the resistors?

If it follows the original 405 circuit they will connect directly to pins 4 and 7 of the opamp. Just check continuity to confirm.

 

[goodguys]

If it follows the original 405 circuit they will connect directly to pins 4 and 7 of the opamp. Just check continuity to confirm.

Thanks! You use the word " they " so is there more than one resistor

 

[Mooly]

One for each rail. Just check the continuity.

 

[goodguys]

Placing the probes on the 3.3kohm resistor that richb mentioned and on the op-amp V+ gives 6k ohms, from the 3.3kohm resistor to V- gives 30kohms

 

[Mooly]

If there is no continuity from pin 7 or pin 4 to those resistors then they are not the ones. Just do a visual check and see where the print goes.

 

[goodguys]

So should there

If there is no continuity from pin 7 or pin 4 to those resistors then they are not the ones. Just do a visual check and see where the print goes.

So should the ohmmeter read zero ohms when placed on the resistors , and pins 7 or 4 on the op-amp

 

[Mooly]

Pin 4 and pin 7 should have direct continuity to one end of the resistor and also the Zener. That means it should read zero ohms. For example in this diagram pin 7 should have direct continuity to one end of the Zener D1 and one end of the resistor R7.

 

[goodguys]

Thanks! I covered both sides of the amp boards with conformal coating, thinking it may help to improve the life of these boards, now i can't get a decent ohmmeter reading. My bad.

 

[atupi]

So op-amp rolling with the opa1641 definetly improves sound quality?

I don't have a OPA1641 on hand but last weekend played again and in my setup I can say that:
LME49710HA- plays very vivid and lively like a real concert
OPA604 - more laid back and pleasant, works nice with jazz and voices

My Quad405 clone is a heavily modified ebay module. (I posted here some time a go)

 

[goodguys]

Yep, the burr brown op-amps do have a reputation for being laid back, i find them boring, i much prefer analog devices, have much more attack - Each to his own.

 

[atupi]

I would say it is more about FET input opamp vs bipolar.