3 transistor buffer for lm3886 help

Thread Starter

carpetride

Joined Sep 10, 2018
23
hi could some one clarify some questions on this 3 transistor buffer for lm3886 !as ime reletively new to building amps ect and my knoledge is limited and prob will be using 18volts
link = http://www.decdun.me.uk/gainclone2.html


1. can this buffer be powered by a plugin wallmart power supply or do i need to make a dual power supply as it shows (15v-20v +) (15v-20v - ) (0v)


2. it mentions input capacitor on input and output !!! what value capacitors ? input -capacitor- ground ? output-capacitor - ground ?
any help appreciated
 

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Thread Starter

carpetride

Joined Sep 10, 2018
23
Discrete buffers.
Now, just to make matters even more complicated, I received a design for a discrete transistor buffer from Andrew Rothwell of Rothwell Audio Products. It is actually based on a circuit by the late Les Sage who wrote in the Audio Conversions DIY hi-fi magazine. The design as you can see below, is very simple and operates in class A. Transistors recommended by Mr Sage were BFQ232's (NPN) but I have used BC547's and ZTX653's and the sound is very good. In fact, I think that this may resolve my opamp dilema!









Simple class A discrete buffer circuit. (Note you must use a DC blocking cap on the input as well as the output of this circuit!)

Moderators note : removed commercial link
 
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wayneh

Joined Sep 9, 2010
17,153
Simple class A discrete buffer circuit. (Note you must use a DC blocking cap on the input as well as the output of this circuit!)
Ah, OK, got it. Well the drawing recommends C1>2µF so I'd probably start with a 4.7µF and experiment with a 10 or 20µF to see if it changes the volume at all.

More or less the same approach on the input, in series in between "in" and the junction of R6 with base of Q1. Start with a 0.1µF ceramic and increase up to 1µF is you can detect a volume increase. You probably won't.

Both of these capacitors will pass the AC signal but block DC - caused by an offset in bias - from flowing in or out of the circuit.

I believe it's possible you might be able to get that to work on a single supply but you'd have to bias things differently. Someone with more chops in this area than me will have to school us.
 
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Audioguru

Joined Dec 20, 2007
11,249
Why do you need the 3-transistors input buffer? It adds a little even harmonics distortion like a vacuum tube, adds a little hiss and maybe the capacitors are too small and cut low frequencies.
The text about the opamp buffer was written by an audiophool who is hearing improvements that are not possible. The input buffers are not needed.

NEVER use a ceramic capacitor to pass or filter audio frequencies. Use film capacitors instead.
 
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