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component reduction in signal chain

Hi, the first line of the picture is a block diagram of my system. The coupling capacitors are shown

2.2uf=capacitor before grid
10uf=output capacitor of tube stage
3.3uf=signal capacitor after volume pot , both inbuilt on tripath board.

Can I alter my chain to the second line arrangement?

Secondly , its a general question. If we look at the first line. As the capacitor after the volume pot in the tripath is 3.3uf, then surely the output capactor of the tube stage need only be 3.3uf too?
Thanks Alan
 

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There is nothing magical about a particular capacitor value; the issue is the product of capacitor value and the resistance it sees at that point in the circuit. Do some reading on first order high pass CR filters.

Moving the volume pot before the line stage may be OK; it will increase noise and may have a small effect on distortion (up or down). You will need to check that your DAC has low DC offset.

The best option may be to drop the line stage. In most setups it simply adds unwanted noise, distortion and gain. The gain then has to be lost in extra attenuation so even more noise is the result.
 

chokesrule

Member
2010-06-25 4:24 pm
Thanks for the replies.
Yes I have used smaller values in the past. I have messed around with them empiracally and have just ended up copying what I have seen in expensive commercial stuff.

The dac does have a very low dc off set because I have run it straight into the line stage without coupling caps. They are there because I just thought it was the thing to do

On the subject of not having the line stage, I have run dacs straight into the tripath but have found although the sound is crystal clear, its "weedy" So I hear what your saying about unwanted noise but to my ear I like this noise and the gain. You have just made me wonder tho perhaps the impedance of the tripath was too low for the dac and by adding a suitable resistor between the dac and tripath that would improve the sound and negate the line stage? Something I will have to try there. Thanks Alan
 
If the tripath has a low (but linear) input impedance which is too low (unlikely) for the DAC to drive and it has sufficient input sensitivity to cope with a signal drop, then a series resistor at the tripath end of the cable might help. A proper buffer might be better.

Note that most DACs have a very low output impedance so will drive anything. Exceptions may be those not using the standard opamp output circuit; these others may have poor driving ability.

Note that it is a standard human hearing characteristic to associate volume with quality. This means that almost all audio systems have too much gain, therefore need attenuating, therefore suffer from unnecessary noise.

It is not possible to arrive at the correct value of coupling capacitor "empiracally", or by copying other designs (unless you copy the entire circuit so have the same resistances too).