Monoblock
Phono Stage Pre-Amp
Click
here for picture
Click
here for schematic
I made this amp because
my friend Mr. A. Bing S. Wijaya from Wijaya Audio who is vinyl lover
requested me. I don't use turntable actually.
I built this Phono Stage in the form of
Monoblock to separate both channel and preventing crosstalk. I also put
power transformer in the diferent casing.
One of my friend who is also a vinyl lover
told me that it was the first time he saw
phono stage pre amp in the form of monoblock
during 25 years experience as high end seller.
Circuit Description
As you can see from the
schematic the circuit consist of three gain stage, first stage is paraleled
anode follower using 12AY7 tube, second stage is also anoda follower using
12AY7, and finally the third stage is anoda follower using 6J5 tube. I
use paraleled 12AY7 in the first stage to reduce pick-up noise and also
internal noise from the tube. It is very important to pay serious
attention to this first stage because all the noise occurs in this stage
will become greater and greater on the next stage because of cascade configuration.
The first stage provides
30 dB gain, second stage 29 dB, and finally the third stage provides 22
db gain. I designed the circuit to work with MC catridge, but it
is also posible to use MM catridge without concerning about aver amplification,
because I provide 20 dB attenuation switch on the input of third stage
that can be use when the circuits work with MM catridge.
I used 12AY7 and 6J5 because they have sweet
sound character, especially 12AY7 which is recommended tube for small signal
application such as phono pre amp.
On this design I'll show you two of my audio
phylosophy, Zero Feedback Design
and
Pure
Class A design. Pure Class A Design indicated by the high bias
current on every stage especially the third stage which use 7.5 mA bias
current, and Zero Feedbak Design indicated by the absence of NFB loop in
the whole of circuit.
RIAA
Equalization is something have to be done
in order to achieve flat frequency response during playback.
As seen on the equalization curve there
are three cut of point, 50, 500 and 2120 Hz. I try to achive 80 dB gain
for MC catridge application, but if this pre amp is going to be used with
MM catridge which only require 60 dB gain Attenuation switch will helps.
To apply 2120 Hz cutt
off point I use C5 and R5, to apply 50 and 500Hz cut-off point I use the
combination between C8, C9, R8, R9, R11.
Power Supply
The ciruit itself requires
250V supply voltage and total current consumsion is around 11mA. I apply
zener-source follower regulator to get 250 VDC. The series combination
of 47V zener and 33V zener diode are use to reach 250V zener voltage. Noise
voltage from zener diode reduced by 100nF Siemen Oil Cap
As the source follower
drived by zener diode I use IRF530 enchancement type MOSFET.
As I already described in My Audio Phylosophy
about the application of Ultra Regulated
Power Supply, here you can see that I use totally 4 RC filter stage
plus Zener-Mosfet Regulator. Those are
a) First RC stage consist of CB, RC13,
dan C13, C14
b) Second RC stage consist of R16, C15 and C16
c) Zener regulator and MOSFET source follower
d) Third RC stage consist of R18, C17 and C18
e) Fourth RC Stage consist of R3 and C1
Heater regulator stage apply the combination
between LM317 floating type regulator and 2N3055 bipolar transistor as
current gain amplifier.
The heater regulator circuit also completed
by soft start function capability to maintain the filament lifetime.
