Adjusting Discrete Class A buffer for gain?

[TheSeekerr]

Hi all,
I've currently got this http://myweb.tiscali.co.uk/nuukspot/decdun/gc/ls.buffer.gif circuit as a buffer in a basic preamp/source selector box. However, I've found that some of my sources (notably the Very Simple Phono Stage and my portable player) can't supply sufficient voltage and/or current to make the most of the amplifier.

Thus, I'm wondering if it's possible to modify this circuit so that rather than providing unity gain, it provided a modest +6dB or so, as a stopgap solution until I get around to building Rod Elliots discrete preamp (the DoZ Preamp).

Thanks everyone,
Chris

 

[MikeB]

In a simple word: No
The circuit is a simple emitter follower biased with a ccs, it can't provide voltage gain, only current gain.
For voltage amplifying you need a completely different circuit.

Mike

 

[TheSeekerr]

Thankyou for your honest and simple answer!

OK, in another question - are there any circuits of a similar complexity that I could perhaps reuse some of the parts in? I'm not after perfection, as I said, it is merely a stopgap until I can afford to build something of higher quality.

 

[Allen Wright]

Chris,
You can quite easily change that buffer circuit to provide some gain by adding one PNP transistor and a couple of resistors. It's called a complimentary feedback gain pair, or words of similar meaning.

The problem is that I don't have a schematic in electronic format to post - or a working scanner - but if you can give me a fax number - I can fax it to you.

Regards, Allen

 

[TheSeekerr]

Hi,

I've contacted Allen off board, but haven't yet heard back - is there anyone else who can demonstrate the arrangement he describers? More importantly, will it be stable? Will it sound bad, or merely ordinary?

Chris

 

[Allen Wright]

Chris,
You should have the fax OK by now - it went through from this end at least.

The circuit has been used by all sorts of good audio companies (like Revox) since the birth of time, or at least the birth of transistors anyway - and sounds OK.

I've been a tube guy for 25 years now, but was used in my last commercial solid state preamp.

But if you want to do the job properly, try this circuit for your new phono capable preamp:

http://www.vacuumstate.com/phono_secrets.htm

Regards, Allen

 

[TheSeekerr]

Hi Allen, trying to catch you while you're online:

I can't read the component labels/values on your fax, can you please attempt to explain (in terms of top left, bottom-most, etc) what they should read?

Chris

 

[homemodder]

Have a look here, this has all the basics from a audio designer. Look at the other pages too, very informative.

www.dself.dsl.pipex.com/ampins/discrete/singleq.htm

Cheers

 

[Allen Wright]

Chris,
The resistor at the collector of the original emitter follower NPN is 680 ohms.

The resistor (Ra) from the emitter of the original NPN to the negative rail is the same number in kohms as your negative supply is in volts. So if the neg rail is 15V, then this should be 15k. If 20V, then it needs to be 20k.

The resistor (Rb) from the collector of the new PNP to the emitter of the original NPN is the one that sets the gain of the overall gain block, by the formula:

Gain = (Ra + Rb)/Ra

So for your desired 6dB gain, Rb will be something like 5-6 times bigger than Ra.

OK?

You may have to tweak the original transistor's 680 ohm collector resistor to get the collector of the PNP to sit somewhere around zero, which allows you the most linear output swing.

Regards, Allen