Cathode Follower as DAC Output Stage?

cogitech · 22nd May 2015, 09:15 PM

I have seen that some people gain big perceived improvements in sound quality by omitting the opamps and surrounding circuitry from their DACs and replacing it with (for example) Jensen OPTs.

I am curious; can a similar thing be done using a cathode follower instead?

In other words, would the use of a cathode follower eliminate the need for the opamp (and related circuity) of a DAC?

Keit · 23rd May 2015, 10:15 AM

Any perceptable deterioration in sound arises from a poor choice of op-amp and/or poor circuit board layout.

Cathode followers need to be used with care. While a cathode follower has a low dynamic impedance output (but nowhere as good as an op-amp) it has very poor driving ability on negative peaks. This is because on postive peaks the tube can turn on as hard as required, but on negative peaks the only thing supplying the load current is the cathode resistor. It the load current needs to be bigger than what the resistor can let though, the tube will cut off completely, causing gross distortion.

Using a constant current source as the cathode load instead of a resistor can produce a dramatic improvement, as it can be arranged that the CCS ensures that the tube never cuts off regardless of signal.

Cathode followers were hardly ever used as outputs in the tube days in professional equipment for the above reason. If a low impedance was required, a high quality transformer fed from a common cathode stage was used.

You sometimes saw cathode followers in circuits published in hobby electronics magazines.

DF96 · 23rd May 2015, 10:24 AM

Many DAC chips produce a current output. This is designed to go into a virtual ground at an opamp input, and so produce almost no voltage. You can't use a cathode follower with these, unless you are happy to wildly exceed the datasheet spec for output signal voltage (as some NOS DACs with 'passive' I/V do).

DAC chips with a voltage output could use a CF.

Unless the original circuit was badly designed (e.g. by pushing fast edges straight into a slow opamp without a filter) the modification is likely to degrade sound reproduction, although this might make it sound more pleasant to some people.

cogitech · 23rd May 2015, 02:39 PM

Thank you both for the info. I do like the sound of the DAC as it is. I just have an insessant need to tweak and modify things. The DAC chip is V out, so I guess I could try it some time, but for now I will leave well enough alone. At least until I have built another DAC that I like more than the one I have. I don't like going without music for very long.

22nd May 2015, 09:15 PM	#1
cogitech diyAudio Member Join Date: Jan 2011 Location: Kamloops, BC	Cathode Follower as DAC Output Stage? I have seen that some people gain big perceived improvements in sound quality by omitting the opamps and surrounding circuitry from their DACs and replacing it with (for example) Jensen OPTs. I am curious; can a similar thing be done using a cathode follower instead? In other words, would the use of a cathode follower eliminate the need for the opamp (and related circuity) of a DAC?

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23rd May 2015, 10:15 AM	#2
Keit diyAudio Member Join Date: Feb 2014 Location: Australia	Any perceptable deterioration in sound arises from a poor choice of op-amp and/or poor circuit board layout. Cathode followers need to be used with care. While a cathode follower has a low dynamic impedance output (but nowhere as good as an op-amp) it has very poor driving ability on negative peaks. This is because on postive peaks the tube can turn on as hard as required, but on negative peaks the only thing supplying the load current is the cathode resistor. It the load current needs to be bigger than what the resistor can let though, the tube will cut off completely, causing gross distortion. Using a constant current source as the cathode load instead of a resistor can produce a dramatic improvement, as it can be arranged that the CCS ensures that the tube never cuts off regardless of signal. Cathode followers were hardly ever used as outputs in the tube days in professional equipment for the above reason. If a low impedance was required, a high quality transformer fed from a common cathode stage was used. You sometimes saw cathode followers in circuits published in hobby electronics magazines.

23rd May 2015, 10:24 AM	#3
DF96 R.I.P. Join Date: May 2007	Many DAC chips produce a current output. This is designed to go into a virtual ground at an opamp input, and so produce almost no voltage. You can't use a cathode follower with these, unless you are happy to wildly exceed the datasheet spec for output signal voltage (as some NOS DACs with 'passive' I/V do). DAC chips with a voltage output could use a CF. Unless the original circuit was badly designed (e.g. by pushing fast edges straight into a slow opamp without a filter) the modification is likely to degrade sound reproduction, although this might make it sound more pleasant to some people.

23rd May 2015, 02:39 PM	#4
cogitech diyAudio Member Join Date: Jan 2011 Location: Kamloops, BC	Thank you both for the info. I do like the sound of the DAC as it is. I just have an insessant need to tweak and modify things. The DAC chip is V out, so I guess I could try it some time, but for now I will leave well enough alone. At least until I have built another DAC that I like more than the one I have. I don't like going without music for very long.