Got Bored Today

[DoomPixie]

Hey guy's!
I got bored earlier and decided to try something a little "strange"... I built an amplifier that is based around a single Tip110 darlington transistor.. Just because i had a couple in my parts pox and i was bored..
I have NOT tested this amplifier yet as i'm at my g/f's in harpenden and didnt bring a speaker with me **DOH!!** Also she uses a mac and i havent been able to find a visual spice simulator that will run on it yet (mac os x if anyone knows one that will work?)
I have powered it up and nothing caught fire and no magic smoke escaped which is a good sign i think..
Am i missing anything obvious? is there an obvious reason why this will not work?

 

[anatech]

Hi DoomPixie,
I'd bias the base at a fixed voltage and couple the input capacitively at the base instead. Your speaker will pump as you adjust the volume otherwise.

-Chris

 

[AKN]

Hi DoomPixie,
I'd bias the base at a fixed voltage and couple the input capacitively at the base instead. Your speaker will pump as you adjust the volume otherwise.

-Chris

hi!

I think that is what he has just done. It's not a volume pot, instead it's DC working point pot.

/ Anders

 

[bigwill]

Am I right in thinking that when the wiper on that pot is close to ground it will also shunt a lot of the signal to ground?

 

[DoomPixie]

the pot isnt a volume pot...
when it is set towards ground it will feed signal to ground aswell.. hopefully it will never be anywhere near ground though.. as i said, i have no way to simulate it and no speaker to test it so it is very likley that i will have to change the values and change the layout a bit to get it working right.. if it will work atall...
Owen

 

[AKN]

Am I right in thinking that when the wiper on that pot is close to ground it will also shunt a lot of the signal to ground?

Yes and not just close to ground, the same thing will happen close to Vcc. You could add resistor from base to center of pot to get a min resistanse approx to value of resisor (not involving base current).

But on the otherhand the pot should be close to center to get working point close to half of supply voltage. Pot could be replaced by two resistors.

/ Anders

 

[DoomPixie]

true, i could just replace it with two resistors.. Or i mabey i could use ±15V supply and referance input and output to ground and do away with the pot altogether? the power supply i am runing this prototype from has outputs for ±15V...
Owen

 

[anatech]

Another vote for me using my glasses?

I'd go for fixed resistors. The nework will load the input a bit.

-Chris

 

[DoomPixie]

true.. I will see how it works soon, hopefully i can get some kind of cheap little speaker to test it with tommorow...

 

[sawreyrw]

DoomPixie,

This amp will amplify the input signal if the 10K pot is adjusted so that the TIP110 is in it's active region. The ideal gain of the amp will be 8, but if the input swings more than a few tenths of a volt the output will be badly distorted. This is the reason almost all solid state amps use negative feedback. In addition, the 500 ohm collector load can't drive much current to whatever is on the output side of the 2200uF cap. Good idea, but it won't do what you want.

Rick

 

[Tim__x]

the pot isnt a volume pot...
when it is set towards ground it will feed signal to ground aswell.. hopefully it will never be anywhere near ground though.. as i said, i have no way to simulate it and no speaker to test it so it is very likley that i will have to change the values and change the layout a bit to get it working right.. if it will work atall...
Owen

With such a simple circuit there's no need for a sim, at least not for DC bias calculations. Please excuse me as I make up component labels as I go along

Q1+2 should saturate to around 0.6v, small enough to ignore with such a rough circuit and a 15v supply.
Rc (500ohm) determines output impedance and should be adjusted to taste.
Rc/Re(40ohm) determines gain, the higher the higher the max swing but the lower the bias pot needs to be.

Neglecting base current, with Re=0 the pot will need to be at the 8% point giving you a 740ohm input impedance. All other cases are better.

P.S. I agree with Sawreyrw, this amp, though it has the excuse of simplicity, won't sound very good.

 

[DoomPixie]

I never for a minute thought it would sound good, i was just bored and decided i'd have a look and see if i could make an amp that would "work" useing a single transistor... and yes.. i know a darlington is technically two.. but i couldnt think of anything else to do with a TIP110..lol.. I sill fancy testing it out and seeing what happens
Owen

 

[anatech]

Hi Owen,

i know a darlington is technically two.. but i couldnt think of anything else to do with a TIP110

Regulated power supply?

-Chris

 

[DoomPixie]

good thinking.. i have about 15 TIP110's and about 20 TIP126's that came in a big bundle of transistors i bought..

 

[DoomPixie]

i just tested the amp out.. it works.. kind off... i was useing passive computer speakers as the speakers.. and the headphone out on a cd player to test it..
it was actually louder to just plug the passive speakers into the headphone socket..lol.. but thats no fun is it?
if i turn the volume up on the cd player it started to distort and i had to adjust the pot on the amp so it was biased more towards the V+ and then if i turned the volume down on the cd player i had to tur the pot back down aswell otherwise it sounded like it was latching .. suddenly it would just buzz.. other than that it was working ok..
i may well play around with it some more later.. change the 500Ohm resistor for something smaller mabey.. see what happens.. it is currently made up of 2 x 1K 1 Watt in parralell so mabey i will try adding another 1K in paralell with them and see if it makes a differance to the performance..
Owen