Doug Self Preamp from Linear Audio #5

[jan.didden]

Originally Posted by bk856er

I replaced R5/R6 with 100ohm and R9/R10 with 1kohm.

Still have stable +/-17V with 170mA load, but now with 0.144V across R5/R6 and 1.41V across R9/R10. Temps are the same.

I had no issues, but made the change on Jan's suggestion as good practice. Thanks everyone for the good discussion!

BK

For those new to this stuff: in the circuit you see that the opamp drives the pass devices through R5. That drive shoulf flow into the base of the pass driver, which in turn drives the final pass device. Very little current is required, for instance for a 100mA output and both transistors have a gain of 70 (total 4900), the required current is 100mA/4900=20uA. The voltage wrt the output is 2 x Vbe, say 1.4V.

But someone decided to place R9 of 200 ohms in parallel with that 1.4V drive which in itself required 1.4/200=7mA, much much more than what the drive requires. In itself no problem for the opamp, but it has to send that current through R5 at 560ohms, which means you lose 560x7mA=3.9V. So the opamp output must be much higher than the output voltage just to overcome R5.

Probably R5 was included in an attempt to promote stability but anything above 50 ohms will do that. Similar story for R6, to promote a quick removal of the base charge thus helping fast reaction to a drop in load current. Here one or two k should be enough.

Jan

[Hicoco]

Originally Posted by jan.didden

For those new to this stuff: in the circuit you see that the opamp drives the pass devices through R5. That drive shoulf flow into the base of the pass driver, which in turn drives the final pass device. Very little current is required, for instance for a 100mA output and both transistors have a gain of 70 (total 4900), the required current is 100mA/4900=20uA. The voltage wrt the output is 2 x Vbe, say 1.4V.

But someone decided to place R9 of 200 ohms in parallel with that 1.4V drive which in itself required 1.4/200=7mA, much much more than what the drive requires. In itself no problem for the opamp, but it has to send that current through R5 at 560ohms, which means you lose 560x7mA=3.9V. So the opamp output must be much higher than the output voltage just to overcome R5.

Probably R5 was included in an attempt to promote stability but anything above 50 ohms will do that. Similar story for R6, to promote a quick removal of the base charge thus helping fast reaction to a drop in load current. Here one or two k should be enough.

Jan

Thanks Jan, that help a lot the community to understand why this circuit did not work perfectly and particularly when we have to adapt to a situation like mine. (240V +-10%). We always should try to understand the situation and find a solution adapted...and do not let people with no answer, Thanks again Jan

[bk856er]

Can anyone share a rear panel fpd file that's been cut/proved?

Hicoco, I looked at yours and there seems to be some offset on the XLR mounting holes.

Received my chassis today, so one step closer.

BK

[still4given]

This is the one I made.

[tcpip]

Are board sets still available for this build?

I also wanted to buy boards from SignalTransfer but no one answered my mail; it's been 4-5 days now. Is that service still active?

[bk856er]

Originally Posted by still4given

This is the one I made.

Looks good - I'll print it out and compare with mine. I spent a couple hours today and took careful measurements + consulted the datasheets. Ended up with the attached. Need to carefully review one more time. Millimeters matter when it comes to CNC and board-mounted connectors!

BK

[Hicoco]

Originally Posted by bk856er

Looks good - I'll print it out and compare with mine. I spent a couple hours today and took careful measurements + consulted the datasheets. Ended up with the attached. Need to carefully review one more time. Millimeters matter when it comes to CNC and board-mounted connectors!

BK

Yes this is the file posted by RickG post #876, i used the file posted by Jwilhelm post #1305 for my rear panel.
For the hole center heights i used the horizontal midline of the rear panel.
Did not received my panel yet with the cuts and engravings..Change txt with fdp

[Hicoco]

This is the Front panel..

[bk856er]

Originally Posted by Hicoco

Yes this is the file posted by RickG post #876, i used the file posted by Jwilhelm post #1305 for my rear panel.
For the hole center heights i used the horizontal midline of the rear panel.
Did not received my panel yet with the cuts and engravings..Change txt with fdp

That looks good and thanks for clarifying the origins. I've seen a few designs now with the upper RCAs labelled as L. Are the upper RED RCAs not the R channel??

BK

[Hicoco]

Originally Posted by bk856er

I've seen a few designs now with the upper RCAs labelled as L. Are the upper RED RCAs not the R channel??

BK

I did not make attention of that, but look likes yes in my I/O board.. May be Carl could answer better than me.