Sorry, I don't do a lot of transistor cascades.
I'm just looking at your amplifier circuit.
What is the operating point of the amplifier as a whole?
I'm guessing that when the DC offset at the output goes over 3.4V the voltage on the base of the input transistor starts to climb over 0.6V (through the NFB divider R4, R5) and since the emitter is grounded the transistor starts to conduct, pulling down the voltage at the collector, and hence, through the darlington, the voltage at the output, which settles at some unknown point over 3.4 V.
Taking 3.4V the voltage at the first transistor collector must be about +1.2V i.e. 5.6V so the voltage drop across R2, R3 is ~6.4V and the quiescent current is 6.4/440 = ~14mA. DC current gain of the BC550 is 110 min 800 max so current into the base is <125uA and can be as low as 15. However at 3,4v the current down thru R4, R5 is 3.4/68000 = 50uA so this is not a stiff source as far as the base of Q1 is concerned
Given the wide variation of DC gain of the BC550 this topology may result in wide variations of the operating point from unit to unit and may contribute to a poor PSRR.
Anyway how did you arrive at this design?
w
I'm just looking at your amplifier circuit.
What is the operating point of the amplifier as a whole?
I'm guessing that when the DC offset at the output goes over 3.4V the voltage on the base of the input transistor starts to climb over 0.6V (through the NFB divider R4, R5) and since the emitter is grounded the transistor starts to conduct, pulling down the voltage at the collector, and hence, through the darlington, the voltage at the output, which settles at some unknown point over 3.4 V.
Taking 3.4V the voltage at the first transistor collector must be about +1.2V i.e. 5.6V so the voltage drop across R2, R3 is ~6.4V and the quiescent current is 6.4/440 = ~14mA. DC current gain of the BC550 is 110 min 800 max so current into the base is <125uA and can be as low as 15. However at 3,4v the current down thru R4, R5 is 3.4/68000 = 50uA so this is not a stiff source as far as the base of Q1 is concerned
Given the wide variation of DC gain of the BC550 this topology may result in wide variations of the operating point from unit to unit and may contribute to a poor PSRR.
Anyway how did you arrive at this design?
w
wakibaki said:
Whee-hooo... all greek to me!!
So you think that the hum occures due to over volting?
This design is based on some basic transistor circuits and gentle nudges from friends in general (good) directions. The circuit works beautifully exept for the hum using mains... battery is fine.
D
GlidingDutchman said:
Whee-hooo... all greek to me!!
Your amp circuit has poor Power Supply Rejection Ratio (PSRR) - that means any voltage noise on the 12V seeps on to the output. That's why you hear hum with the mains psu but not with the battery. The noise is dominated by this, not shielding problems or anything.
Either you need to change the amp circuit to increase its PSRR or you need a regulated psu. Hey, that's what you asked for in the first place!
I missed this earlier comment; I now see why AndrewT was compelled to repeat himself. I also missd the posts about regulators whose message I redundantly repeated. Sorry.
You appear to have two problems here. One is psu ripple noise getting to the output, the other is either a grounding problem or a stability problem which is excited when you attach the "phono stage".
Is it now the case that without the phono stage attached the amp has little or no hum. When you attach the stage then it hums loudly?
You seem very sure. How can you tell the two apart?
traderbam said:
Let me try to clear some confusion here.
The amplifier circuit functions beautifully - I coudnt ask for a better sonic response.
I get ZERO hum/buzz (50/100Hz) from a fully charged lead acid battery
I get ZERO hum/buzz from a switch-mode, regulated desktop PSU.
I DO GET hum/buzz from a normal transformer with a standard rectifier bridge and single smoothing cap.
My phonostage is an EAR 834P Signature with level control and this works A-OK with the h-amp.
Also my PC soundcard works A-OK with the h-amp.
I know, from basic knowledge that the hum/buzz is not a ground fault as I am all too familiar with ground faults in phono systems - I do design and build turntables for a "living".
What I suspect, from previous experience, is that a simple POSITIVE regulator with solve the hum problem as that is how the clever folks at Pro-Ject gets away with a single rail AC supply in their phonostages - spitting the single rail with two diodes and using one POS and one NEG regulator for dual power rails 9-0-9v.
I live in a rural village, 320kms from Johannesburg and thus cant quickly hop in a veheacle and wizz off to a parts supplier - I have to order online.
Any other questions?
D
yet i havent
understood what exactly your setup is ....all the things mentioned above have absolutely nothing to do with your problem ....
no regulator will ever elliminate the noise .....
yet again and of course andrew t is right .....
the only reason that you battery doesnt produce nois is that the baterys ground is diferential to the rest of the system
also the only reason that the pc power supply doesnt produce noise is also because the ground is isolated and dieferential to the rest of the system also let us not forget that pc psu is working in a few hundred kilohertz i think so even if produced noise it wouldnt be audible .....
my only complain from diy foroum is that people often miss the obvious and create circuits to go to mars instead of just a simple power supply used everywhere .....
there you go
understood what exactly your setup is ....all the things mentioned above have absolutely nothing to do with your problem ....
no regulator will ever elliminate the noise .....
yet again and of course andrew t is right .....
the only reason that you battery doesnt produce nois is that the baterys ground is diferential to the rest of the system
also the only reason that the pc power supply doesnt produce noise is also because the ground is isolated and dieferential to the rest of the system also let us not forget that pc psu is working in a few hundred kilohertz i think so even if produced noise it wouldnt be audible .....
my only complain from diy foroum is that people often miss the obvious and create circuits to go to mars instead of just a simple power supply used everywhere .....
there you go
Attachments
What you say in the earlier post seems to be inconsistent with what you just said.
Previously, you said that when using Mooly's psu the hum only arises when you attach the phono stage.
Now, you seem to be saying that the hum has nothing to do with whether the phono stage is attached, the hum arises when you use a poorly regulated psu.
Would you clarify this?
Previously, you said that when using Mooly's psu the hum only arises when you attach the phono stage.
Now, you seem to be saying that the hum has nothing to do with whether the phono stage is attached, the hum arises when you use a poorly regulated psu.
Would you clarify this?
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