Fredxx wrote:
On 02/04/2021 11:06, Max Demian wrote:
On 02/04/2021 01:07, Fredxx wrote:
On 01/04/2021 23:28, JNugent wrote:
On 01/04/2021 09:51 pm, Tim+ wrote:
We already have to have a small pre-amp as theres no phono input
on my
amp.
The ceramic cartridge (eg, as found on the classic Garrard, Collaro
or BSR autochanger and fitted with a flip-over mechanism for
swapping between vinyl and shellac styluses) doesn't need a pre-amp
stage. You can just run the output direct to your soundcard input.
You might loose quite a bit of bass depending on the input impedance.
I guess an sound equaliser can make up some of the loss.
Does that means that a piezo cartridge has a very high impedance,
suited to valve amps? I used to have a transistorised table radio with
the obligatory "gram" input that worked all right with a piezo
cartridge. Maybe quality wasn't an issue.
They are effectively a voltage source in series with a capacitor, so
input resistance of the sounds-card is critical. They are normally made
a value to minimise pickup, high in terms of normal needs but low in
comparison with preferred input resistance for a ceramic/crystal cartridge.
This explains better than I can.
https://www.petervis.com/record_play...pacitance.html
Transistor amplifiers can be designed with very high input
impedance/resistances, certainly on par with valves.
The same site (arrived at by a Google search), has an amplifier
based on a 2N7000.
https://www.petervis.com/record_play...p-circuit.html
The analog R and C component values are near the bottom of the
page. R1 and R2 are 20 million ohms each (general purpose resistors
come in values up to 22 million ohms). Removal of C1 and C2
would change the frequency response of the circuit ("flat").
The value of R1 and R2 is meant to indicate what kind of high impedance
circuit this is (very high, suited to piezo cartridge). In a pinch,
the NP 4.7uF could be made from two polarized 10uF in series
(with opposing polarization). I just buy the NP ones instead.
I've only ever owned two or three of those.
The location of the amplifier is important too. The amplifier
and its nine volt battery, should be positioned near the
phono arm, where the wire comes off. The circuit doubles as
a "cable buffer", and then you can run a suitable cable from
there over to the Line In on the destination device. Without
that, if you put a hundred feet of cable right
off the phono cartridge, the frequency response would be
missing all treble. By using the amplifier up near the
arm, it functions as a low impedance cable driver. I still
would not try a hundred foot of cable though, six feet
would be plenty.
The phono cartridge is electrically floating. The amplifier
and nine volt battery are floating. A desktop computer sound card
has a grounded input, and it is the ground on the computer
sound card that is the only ground in the circuit. (We need
this knowledge, to avoid ground loops.) Any tube amplifiers
are not involved, as long as the phono cartridge is no longer
connected to any tube amp inside the player device.
In cases where a media device is ground referenced at the
source, and you seek to input to the computer Line In, a
"hum breaker" can be purchased, to isolate the source from
the computer. This is used to break the ground loop (stop hum),
but at the expense of mutilating the frequency response (passes up
to around 10KHz). The transformer must be carefully wound
for best results in those. When you use things like
battery powered cassette players, then those are floating,
and the computer is the only ground in the circuit.
There are softwares for hobbyist usage. The first one
is mentioned in web reviews regarding computer audio.
And Audacity is a general purpose tool.
https://audio.rightmark.org/index_new.shtml
https://www.audacityteam.org/download
I find when using Audacity for this sort of application
(frequency response), you need two tools. A separate
player application to generate a LineOut stimulus, and
Audacity to record on LineIn. That's if you needed to
check the amplifier response for some reason (sweep frequency
waveform). I don't think Audacity can do output and
input at the same time. You can also use two computers
and two soundcards, for characterization work, and then
two copies of Audacity are all that is needed (one on
each computer).
Paul