[Ian Bell[_2_]]

Patrick Turner wrote:

Ian Bell wrote:
Patrick Turner wrote:
John Byrns wrote:
In article ,
Ian Bell wrote:

Eeyore wrote:
Ian Bell wrote:

On several preamp PSUs I have built in that last couple of years I have
noticed the same thing. A very low frequency low level variation in the
output voltage. My latest PSU uses a series of five 100uF caps with 1K
resistors in between and it still exhibits this problem. The 'noise'
seems to be below 1Hz in frequency, fairly random and peaks typically
between +- 10mV with occasional excursions to +20 to 30mV.
Well what do you expect without regulation ?
Unhelpful replies like yours I guess.
His reply may have been unhelpful, but it was certainly to the point. I don't
get it, if you don't want to use regulation how can you expect to avoid low
frequency voltage fluctuations? You should probably count yourself as lucky
that you are seeing only "occasional excursions to +20 to 30mV", I would expect
several volts, or more, without regulation.

--
Regards,

John Byrns
Considerable self regulation occurs in mains supplies in populos cities
and towns because where you have thousands of people all using something
and averaging say 1kW mains input per household 24/7, then if the mains
voltage is 240V, the current average is 4 amps and the load must be 60
ohms. Say there are 3,000 housholds sharing your circuit locally and
that the wiring between them is low resistance
then the load connected to the circuit you are on is 60 / 3,000, ie, a
very low load indeed on average, but one that changes its value
enough to cause Ian's observed LF voltages on a B+ rail of +/- 30mV.
That's about what I see here.

After several R&C stages of filtering the rail noise movements are much
less than +/-30mV.

If you have a CCS type of load supplying dc to a triode anode, then the
low Ra of ther triode makes a divider with the CCS, or with a resistance
load for dc feed, and the anode LF noise voltage will be much lower than
at the rail.

Yes, that's my other problem. The simple CC cascade pair with NFB I am
using has a very poor PSRR.

Smooth thy B+ rails lest ye suffer noise...

'Tis sunday here, and I've been to church.

My church has walls extending from horizon to horizon, and the ceiling
is the sky above. The God Of Triodes resides in His Regality of the
Firmament, and for me to attend this Church I ride 100km on bicycle in
the presence of a few like minded souls and under watchful eye of the
God Of Bicycles, or GOB to ye who are not aware of His Presence.


But where you have a pentode or cascode pair of triodes then Ra is high,
and any noise on the rail appears at the anode. The other source of LF
noise in phono and line level stages is where you have a large grid bias
R to bias a typical gain triode.
Its grid may be cap coupled to a preceding triode gain stage with low
Ra. At say 1kHz, the low Ra and low XC shunt the high resistance of the
grid biasing R, say 470k or 1M. But as F becomes lower, the XC becomes a
high impedance, and effectively the grid is then coupled toa good noise
source, ie, the high resistance. Its most important in phono or
microphone first stages to have a relatively low grid biasing R,
and 47k is typical for MM inputs. The carts are usually direct coupled
to a grid, and the standard 47k is so the grid is biased when no cart in
connected and the 47 happens to be the most common recommended loading
for most MM carts.
That's not a problem in this case. The mic transformer is connected
direct to the first stage grid.

OK. So its a high gain mic amp we have here.
I assume the response is level between at least 20Hz to 20kHz.

What is the maximum voltage gain at say 1kHz?


24dB

What is the noise at the amp output with maximum gain and with input
grid directly shunted to 0V close to the input?


Not possible to measure accurately at the moment as the LF blips whack
the meter needle all over the place one you try to see noise below 1mV.
That said, looking at it on a scope you can see the broadband noise
underneath the LF blips and I would estimate the noise at the output
with the input shorted as about 50uV rms.

What happens if you temporally connect a spare 1,000uF or more to be in
parallel to the last 100uF cap in the filter line up, ie, the filter cap
giving the B+ supply to stage 1 of the mic amp?


Not tried that yet, I have a spare 470uF or two so I'll try that.

Noise should fall a lot with the extra C added where it'll do the most
good.

Think big, use enormous C values if you cannot bring yourself to make
what might be a very simple shunt regulator in your preamp.


Yes and no. I was using just a couple of RC stages using 470uF but then
I realised the five RC stage of 100Uf each would perform better. I
suppose I could go bananas and replace all the 100uF caps with 470uF ones.

Beware using simple zener diode based shunt regs close to mic input
stages though. The LF noise of the zener will find its way into signal
paths.

Agreed. I have been looking at the Maida regulator as a means of
eliminating the LF noise *prior* to the normal RC string.

Cheers

Ian

Patrick Turner.