B4 Bootstrap Buffer Preamp schematic

[a.palfreyman]

Well, I noticed during twiddling of the offsets that for each mV change at the input, you get about 10mV change on the output, same direction pos. or neg. Having got the inputs very good with about 2mV on the outputs, I therefore tweaked the front ends by just a couple of tenths and now have <1mV offsets all around and these are very stable over a long period as I ran it with test tones for a few hours yesterday. Will therefore remove the input and output caps.
What is the consensus over volume pots? I have 47k loading on the input (R5) so was going to go for 20k pot which means that at full volume, the 47k parallel to this will reduce the load to the source to 14k. In actual fact, with an input pot and no coupling cap, can the 47k be removed?

 

[Arkless Electronics]

Varying the resistance to ground at the input may well vary the offset. You may also get some noise when operating the pot from the DC on it.

 

[a.palfreyman]

Jez,
Sound of penny dropping....
That's why you said remove the O/P caps at such a low offset level (ergo leave the I/P caps in-situ).
Will try 100k at R5, that way I can get away with 0.33uF for a corner frequency of 5Hz. Might find a better input cap for that sort of value.
Cheers,
AP

 

[toprepairman]

That's a really weird circuit, doesn't look correct. Sadly don't have the time just now to knock it up to disprove it.

 

[a.palfreyman]

It works.... as many will testify

 

[Arkless Electronics]

That's a really weird circuit, doesn't look correct. Sadly don't have the time just now to knock it up to disprove it.

It looks straightforward enough to me.... The buffer circuitry is entirely bootstrapped via the two emitter followers which take their input from the buffer output... there's a bit of "the snake eating its own tail" to it but as there is no gain it works great. Note the constant current source diodes in use. If in a cursory glance at the circuit they were taken as just diodes then no it wouldn't work if they were

 

[337alant]

[QUOTE]That's a really weird circuit, doesn't look correct. Sadly don't have the time just now to knock it up to disprove it.[/QUOTE]

The B4 bootstap buffer circuit was designed by Pigletsdad
https://pinkfishmedia.net/forum/threads/b4-pre-amp.107753/

alan

 

[toprepairman]

Thanks Jez, that makes sense, they are current sources, not diodes as drawn.

 

[a.palfreyman]

Made some very minor tweaks last night. Changed R5 to 100k and re-adjusted the dc offsets slightly as a consequence. Also, removed the output caps. At the very worst, when touching one of the BC550 / BC560 transistors the output drifts to about + or - 5mV, depending which one is warmed. This temperature difference is very unlikely once it is mounted in a box so I am well happy. Reason for changing R5 to 100k is that I have some 0.45uF polystyrene caps: http://cgi.ebay.com/ws/eBayISAPI.dl...mchn=em&s=ci&mail=sys&xt=104038&unp_adv=false
coming from alexer1 (Ukrainian seller on ebay I'm sure some of you are familiar with) and wanted to keep the input corner frequency low. Will try and get it into a makeshift box with pot at the weekend and run on the polyester input caps that are fitted. Hopefully these new caps will be here late Feb / early March.