Sound Quality Vs. Measurements

[nigel pearson]

[IMG][/IMG]

This is a very posh version of what many studios still use . BC 560 and 741 in a very simpler arrangement typical .

If going to a studio and see this we add the 10 K resistor to try to rescue it .

Is that 10 K any use now ?

Thanks to ESP for circuit .

[scott wurcer]

Originally Posted by nigel pearson

[IMG][/IMG]

This is a very posh version of what many studios still use . BC 560 and 741 in a very simpler arrangement typical .

If going to a studio and see this we add the 10 K resistor to try to rescue it .

Is that 10 K any use now ?

Thanks to ESP for circuit .

What's R14 for? Nothing like having 1000uF in series with the input signal, I gather the pros don't care.

[simon7000]

R14 is there so that when the pot wiper lifts you don't blow stuff up, like the entire speaker stack! ESP is a repository of pretty much so average circuit design. There is not enough gain in front of the TL0 series for serious use either. (IMO)

[DF96]

R14 does not protect against wiper lift, but track open circuit (presumably at the end contacts). Wiper lift is protected by the track itself, provided the PCB designer does not omit the 'redundant' connection to the LH end of the track.

[bcarso]

Yes, R14 would be trying to set some minimum gain, and I suppose might be selected to trim end-to-end resistance in the pot, which of course can have a huge spread. Also I don't see where open circuits in the potentiometer would blow much of anything up. Although they could make a bit of a racket as they became intermittent.

[simon7000]

Originally Posted by DF96

R14 does not protect against wiper lift, but track open circuit (presumably at the end contacts). Wiper lift is protected by the track itself, provided the PCB designer does not omit the 'redundant' connection to the LH end of the track.

The schematic shows both ends connected. I have seen it implemented the wrong way. That is how the resistor probably made it's way into the design. Also due to the careful handling low end band gear often gets R14 will help when the pot gets torn off the board!

I have a hand held wireless microphone transmitter on my bench. The surface mount power switch has been pushed apart and off the PC card. Apparently if you push the switch to turn it on and it doesn't come on, rather than check the battery, just push the switch harder!

[simon7000]

Of course it also could be C1 was originally in series with the pot to keep DC noise out while trimming and R14 was the DC path.

[bcarso]

I'm doing a battery test on a low-power product and periodically turn on the microammeter to see how the current looks, and turn it off again after settling and observation to conserve the meter battery. I'm already worried that I'm going to wear the power switch out on the Fluke meter! At least it is not surface-mount, rather a nice old-fashioned slide switch.

Of course I could contrive an a.c. adapter and leave the meter on.

[dvv]

Originally Posted by nigel pearson

[IMG][/IMG]

This is a very posh version of what many studios still use . BC 560 and 741 in a very simpler arrangement typical .

If going to a studio and see this we add the 10 K resistor to try to rescue it .

Is that 10 K any use now ?

Thanks to ESP for circuit .

Nige, I would strongly suggest yout try one of these.

That's what I mean when I say "current boosting" the op amps. Pictures 01 and 02 are in fact the same thing, your choice between standard diode or a trannie. FYI, Revox for example uses the circuit in 02 as a matter of routine.

Obviously, 03 is when you need a BIG current boost. Add another pair of power trannies, recalculate the values, and you have a straight power amplifier.

Since +/- 15V rails won't get you very far (out to 7 or 8 Watts into 8 Ohms), you may want to consider using something like BB 2604 op ap, which can be run off +/- 24V rails no problemo, good for about 22 or 23 Wpc into 8 Ohms. And the 2604 has a FETtie input - one of the rare few op amps I honestly think can sound really good.

[DF96]

Originally Posted by simon7000

I have a hand held wireless microphone transmitter on my bench. The surface mount power switch has been pushed apart and off the PC card.

Yes, I have seen that on a Sennheiser radio mike. The switch was a tiny SMD one only held in place by solder. Solder is not designed to provide much mechanical strength. I was surprised, as I expected better mechanical design from Germany.