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G8MNY > TECH 19.01.26 16:51l 77 Lines 4013 Bytes #8 (0) @ WW
BID : 49864_GB7CIP
Subj: HiFi Dynamic Noise Limiter
Path: JH4XSY<N3HYM<GB7YEW<GB7CIP
Sent: 260119/0740Z @:GB7CIP.#32.GBR.EURO #:49864 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Clarification Mar 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is an old circuit I have used for years, only 4 transistors, originally
designed by Philips, as a hiss remover for tape recorders. I have used it for
the HiFi sound channel on 23cms ATV repeater GB3HV etc. Although treble gating
can sound bad if set up badly, with the right level of noise floor, it works
very well removing just the hiss & no useful treble.
Phase Inverting Treble Clipping Treble Treble
buffer Amp Amp Gate
ﾚﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄR19ﾄ+12-18V
ｳ R3 ｳ R10 R12 R14 C7 +ｳ 100R
ｳ 680R C2 ｳ 1K8 2K 4K7 n33 D3 & 4 === C12
R1 ｳ 20n R8 ｳ R11 ｳ ﾃﾄｴﾃﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄｿ _ｳ_ 100u
68K ﾃﾄﾄｴﾃﾄｿ 330K ｳ ﾚﾄ330Kﾄﾄｴ ｳ/ R16 _ｳ_ _ｳ_
In +ｳ ｳ/ ｳ ｳ ﾃﾄｴﾃﾄｴ ﾃﾄﾄｴT4 330k /_\ \_/
>ﾄｴﾃﾄﾅﾄｴT1 ｳ C3 C4 ｳ ｳ C5 ｳ ｳ/ ｳ\e ｳ ﾚﾄﾄｴ ﾃﾄﾄｿ
C1 ｳ ｳ\e ｳ 3n3 n47ｳ ｳ/ 1n8 ﾃﾄﾄﾄﾄﾄｴT3 ﾃﾄﾄﾄﾄﾙ ｳ _ｳ_ _ｳ_ ｳ C10 C11
1u ｳ ﾃﾄﾄﾂﾄﾄ)ﾄｴﾃﾄﾂﾄｴﾃﾄﾅﾄｴT2 ｳD1 ｳ\e C6 ｳ ｳ /_\ \_/ ｳ R18 10n +1u
ｳ ｳ R5 ｳ R6 ｳ ｳ\e ﾃﾄｴ>ﾃｿ ｳ 20nﾃﾄR17ﾄﾄ)ﾄﾄﾁﾄﾄﾄﾁﾄﾄ)ﾄ56Kﾄｴﾃﾂﾄｴﾃﾄ>
R2 ｳ 1K ｳ 8K2 ｳ ｳ ｳD2 ﾃﾄﾄ)ﾄｴﾃﾄｴ 22K ｳ D5 & 6 ｳ ｳ Out
56K R4 ｳ ｳ ﾃﾄﾄﾄﾄ)ﾄﾄﾄﾙ ﾀﾄｴ<ﾃﾙ ｳ ｳ ｳ C8 C9 ｳ ｳ
ｳ 680R ﾀﾄﾄｴ R7 R9 R13 R16 ===4n7 4n7=== ｳ
ｳ ｳ ｳ 1K 100K 680R 680R ｳ ｳ ｳ
ﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄ)ﾄﾄﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄ)ﾄﾄﾄﾄ
T1-4 ｳ VR1 ｳ
BC107/8 Type ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ4K7ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
Null
HOW IT WORKS
This is an unusual circuit. It uses treble phase cancellation to achieve the
high level of treble reduction (20dB) at low treble levels (-50dBm).
T1 produces 2 phases of signal, one via a fine level null control goes to the
output. The other feeds a treble Butterworth filter T2 then on to T3 that forms
a clipped treble amplifier. T4 buffers this & produces an amplified version
that drives 2 diodes D3 & 4 to quickly charge up C8 & 9 to control the treble
gate D5 & 6. This gate gently attenuates the out of phase treble so it cancels
it out if it is weaker than -40dBm.
SETUP
Set VR1 to provide the minimum hiss on a weak hiss source >-52dBm (2mV RMS).
Then you should have a noise reduction that is like this...
ATTENUATION LEVEL dBm mV
0dBｴ----------...____________________.........-------- -40 7.7
ｳ ~~""":---------------ﾄﾄﾄﾄ~~~~~~~~~ -42
ｳ ~"":-----------ﾄﾄﾄﾄﾄ~~~~~~~ -44
-10ｴ ~"":------______--ﾄ~~~ -46 4
ｳ ~":-..._______... -48
ｳ "-...____... -50
-20ｴ "-.__.-- -52 2
ｳ "-._
ﾀﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾂﾄ
2k 3k 4k 5k 6k 7k 10k 15kHz FREQUENCY
So set up the input level to this circuit to give the noise floor @-52dBm (2mV)
so that you get maximum hiss attenuation. Then as programme material treble
content gets louder the noise gate gently reduces the top attenuation.
CONCLUSION
In mono it is very impressive for such a simple circuit. But for stereo some
combination of the left & right channel DC at C8 & C9 (e.g. a pair of 1k
bridges between the 2 channel's C8s & C9s) improves the stereo, by blurring the
the spatial image (stereo location) of the soft switching points.
Also see my Tech Bul "BHI Noise Eliminating DSP LS"
Why don't U sent an interesting Bul?
73 De John, G8MNY @ GB7CIP
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