Aleph-x input-coupling-capacitors

[oliverniekamp]

Hello,
I implemented the BG 10uF capacitors in the two ways shown on the image. The main difference is, if they are before, or after the 10K input resistor. What suprises me is the big difference in sound. When the capacitor is in front of the resistor, voices or instruments are quite small and very precise in focus. When the capacitors are behind the 10k resistor the sound is very laid back and the timing is relaxed. But voices are much bigger and out of focus. I have no real explanation of this phänomenon, as the 68k groundresistor is responsible for the resistance the capacitor faces. Could it be the feedback is different?
This shouldn´t be a discussion about BG´s or the reasons for input caps, just a question about the influence of their position in the circuit.
Oliver

 

[oliverniekamp]

Hello,
Does someone know how the iputcapacitor is implemented in the origina XA? Or do you have ideas, why the difference in the sound is so big? I know that the inputcap comes behind the input resistor normaly, but the other way voices sound much better.
Oliver

 

[EUVL]

If you connected the feedback capacitor to the junction of 10k/100k and before the cap in the lower figure, then you just added 10uF to the input capacitance of the input diff pair, I think.

I would connect it as per the upper figure, and I do not think you need the 100k before the cap. The 100k feedback resistor goes to the junction of 10k/68k, as in the original schematic.

Patrick

 

[oliverniekamp]

Not the feedbackcapacitor, but the inputcapacitor. The difference to the inputcap of preamps is that i have the zener on the other side of the capacitor. Could this be wrong?
Oliver

 

[EUVL]

Sorry. Typing error. I meant the 100k feedback resistor.
The Zener is in the right place.

May I ask why you want an input cap in the first place ?

Patrick

 

[oliverniekamp]

It´s just my wish to be carefull, because in a friends preamp, the outputcapacitor had a breakthrough, and the aleph-x has dc gain, if there are no inputcaps. That´s the reason for inputcaps in the XA.
The Coral chassis of my speakers are not replaceable.
Oliver

 

[oliverniekamp]

In the aleph L and Aleph P the diodes are on the oposite side of the inputcaps, that´s why i´m asking.

 

[EUVL]

In my test circuit, I put an input cap before the 10k resistor. It works fine. The rest of the circuit stays the same if you want to remove the cap later.

Patrick

 

[Netlist]

Oliver, what's the use of the 100k resistor.
I can't see it in the original AlephX.

/Hugo

 

[oliverniekamp]

It´s common to have a groundresistor in front of inputcaps, in case there is some dc, i guess. Actually it doesn´t make a difference in the sound if i leave it out. I tried.
Oliver

 

[oliverniekamp]

Patrick,
the 100k feedbackresistor is not in my drawing, but it´s in the correct place on the board. The cap position in the upper drawing is the one that sounds better, but i don´t know why. And i want to know.
Oliver

 

[oliverniekamp]

Everything is fine now, the 10k input resistor has to be in front of the input cap, if you have the speakers in phase, it sounds great.
Just don´t let someone else connect your speakers, it could be irritating. But back to the question of the position of the inputcapacitors. If they are in front of the inputresistors, it sounds more uptight and dull, if the resistor comes first the sound is more relaxed or laid back. It´s similar to high voltage preamp powersupplys, when you use to large capacitors, the sound gets dull and uptight too.
Oliver

 

[Netlist]

Do I understand it corectly that:
Input>cap>resistor sounds better than Input>resistor>cap ?
I'd like an explanation for that as well.
Interesting find.

/Hugo

 

[oliverniekamp]

The reason is clear, the Mosfet has an infinite dc resistance, but it´s an capacitive input and has a low ac resistance, on ac signals the 221Ohm resistor is what the input capacitor sees, and than 10k is definitive to big, thats why the 10k must be in front of the input cap.
Oliver

 

[oliverniekamp]

Sorry Hugo, you were faster,input resistor capacitor is good, input capacitor resistor sounds dull.
oliver