ESP Death of Zen preamp vs BOZ

[gpsmithii]

I have completed the BOZ, and have made the same observation with regard to its clipping the negative portion of the signal as it approaches 2 volts at the input. I have followed the various threads and am a bit confused as to why one would want to change the bias point verses using a voltage divider at the input or some other means of attenuation. It is my understanding that the hot bias is what makes the BOZ sound good and measure well. My logic says that we want the device to be biased up to a point of most linear operation. How would it make any difference when driving a ZEN or some variation of a more conventional, but less musical current gain device? If it clips as result of being over driven it matters not what it is driving. Or am I missing something? Thanks in advance for your replies.

[Mike L.]

Hello,

I too am planning to build a preamp. I've already decided on the DOZ but would like to use +/- 15V rather than the +30V for which it is designed. At the ESP site, Mr. Elliot says that this is possible but not trivial and alludes to circuit changes required but does not define them. Could someone knowledgeable in this area tell me what modifications are needed?

Thank you.

Mike L.

[Nelson Pass]

Originally posted by gpsmithii
I have completed the BOZ, and have made the same observation with regard to its clipping the negative portion of the signal as it approaches 2 volts at the input. I have followed the various threads and am a bit confused as to why one would want to change the bias point verses using a voltage divider at the input or some other means of attenuation. It is my understanding that the hot bias is what makes the BOZ sound good and measure well. My logic says that we want the device to be biased up to a point of most linear operation. How would it make any difference when driving a ZEN or some variation of a more conventional, but less musical current gain device? If it clips as result of being over driven it matters not what it is driving. Or am I missing something? Thanks in advance for your replies.

The attenuator is before the input coupling cap, and does not
alter the bias.

[PMA]

Originally posted by Mike L.
Hello,

I too am planning to build a preamp. I've already decided on the DOZ but would like to use +/- 15V rather than the +30V for which it is designed. At the ESP site, Mr. Elliot says that this is possible but not trivial and alludes to circuit changes required but does not define them. Could someone knowledgeable in this area tell me what modifications are needed?

Thank you.

Mike L.

Rod's DoZ preamp is a typical single supply circuit. It's common point (ground, reference) is at emitter of Q3, which is also a point of single-polarity supply 0V voltage. Output from the DoZ preamp is between collectors of Q3 (second stage transistor) and Q2 (constant current source) is at half of power supply voltage, that's why it needs output cap. It is really not good idea to try to convert this circuit into non-capacitor output one with 2 power supplies. Despite that this could be done by connecting +Vs to R7/D1 point, -Vs to emitter of Q3, input signal should be connected between C1 and 0V of +/-Vs supplies (center point), output would be against 0V (center point). VR1 trim would set 0V at the output (collectors of Q2/Q3). I suppose that output dc offset would be unstable, as it is a very simple circuit. Probably better to connect 2 of them as differential DoZ, but the best to keep it as it is with output capacitor. You do not need great value and electrolytic type, this depends on input impedance of the following amp.