B1 Buffer Preamp

[dirkwright]

Originally Posted by Salas

MOSFETS are much noisier than JFETs in general. In a line buffer, that can be of minimum relative consequence, given the signal level and lack of gain while the MOSFET's higher transconductance benefits Zo, and THD as the signal level rises in peaks. TO-220 and TO-247 devices have way too high input capacitance to be useful here driven from a practical value pot, I have stashed away some BS170 MOSFETS though to experiment with some day. Only problem is to find nice solid TO-92 sinks widely available, since to realize the gm benefit, bias should be appreciably high, at least 100mA. Now imagine the ridicule if I will ever come up with a DC-BS buffer.

I would think that a depletion mode MOSFET would be easier to use in the B1.

[Salas]

Got BSS135 Siemens TO-92 DMOS too but it has 110-150pF Ciss for 10 to 40mS only. Better a JFET. When BS170 has 30pF and 300mS. If there is a dynamic gm and low Ciss DMOS all to the better.

[jacco vermeulen]

Originally Posted by Salas

Only problem is to find nice solid TO-92 sinks widely available

Ha !

(even prettier is shaving the body to the bare minimum thickness, and/or encase it into a larger high thermal epoxy body)

[dirkwright]

Well, all I'm saying is that this circuit treats the upper transistor as a depletion mode MOSFET instead of a jfet. It seems to be made for those kinds of devices.

[jameshillj]

Patrick (EUVL) developed a rather good higher current buffer running about 250mA called the"DAO Buffer (Taylored) with the Lu1014 jFets plus cascode - driver for the AKG k-701 headphones - extremely good, simple device especially with the Shunt Regs, but don't know if it's actually a B1 variant ....

[RKH]

All,
I apologize if these measurements exist in this long thread. I am (finally) getting around to completing my B-1 with Pass Boards. I didn't burn anything up on initial power up (at least no smoke)... but I'm not getting anything from the output... save for some very faint audio. (yes, it is connected to my power amp)

Can anyone tell me where I might find voltage measurements at some points in the circuit so that I may trouble shoot and identify where I might have made a mistake? It seems silly that it's not working given the simplicity of the circuit and the great board. Alas... obviously I've done something incorrectly.

I have a 24 volt supply. I know the current draw of the circuit is approximately 20 mA. The voltage drop across R1 doesn't tell me anything, i.e. no stable measurement with my DVM. The power indicator LED lights up just fine.

Thanks for the help.

Ryan

[6L6]

RKH - well lit, detailed photos please!

[intrator]

Check the voltage on the drains of the JFET Q100, Q200. They should be around +10V each, and each source should be at or near ground. Q201 and Q101 should each be sourcing about 10mA. Also are both channels the same? That could be a clue.

[intrator]

Just finished a fully differential version of the B1 Buffer. Details about design, execution, and how it sounds are posted at

Audio Equipment

It sounds wonderful. Much improved over a passive attenuator we were using before this.

[Skywave-Rider]

Originally Posted by intrator

Just finished a fully differential version of the B1 Buffer. Details about design, execution, and how it sounds are posted at

Audio Equipment

It sounds wonderful. Much improved over a passive attenuator we were using before this.

Great! I am thinking about going transformer i/o, but unbalanced. Did you consider any other transformer manufacturers like Lundahl or Cinemag?