#26 pre amp

[pred]

That is great post, I am in travels from Saturday and will be reading this index with great pleasure

Thanks so much 12B4A

[merlin el mago]

cool

[H.Michi]

I just completed modification to my 26 pre-amp "from battery to the fillament biasing".
As a result I feel the sound surely improved.
However, I feel a residual hum? noise (0.31mV) which was not included in battery bias. I measured by ac voltmeter(Kenwood VT-171) at the output terminal (600 ohm).
Under such circumstances, I wonder are there any suggestions or idea in getting rid of the said residual noise?

[Rod Coleman]

Hiro, such a high level of noise is not expected - especially with the excellent filtering of raw dc shown in your circuit.

my suggestions for checks:

1. Please measure ac voltage (common-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - connected at cathode of 26 (GND).

2. Please measure ac voltage (differential-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - at Raw dc -. This should be <100mV

3. Please measure ac voltage (common-mode) at output of filament Regulator: Meter + at filament +, Meter - connected at cathode of 26 (GND). This should be < 1mV

differential mode ac voltage (Filament + to Filament -) should be << 1mV

4. filament transformer has low capacitance from primary to secondary? 50 to 90pF is a good value.
Also, transformer construction is E-I ?

4. possibly, a capacitor of 1000uF to 3300uF connected at Regulator PCB (input terminals), may be useful. Low impedance type: Panasonic FC, Nichicon PM etc.

5. Snubber network R-C (R=47R 1/2W; C=47nF stacked film) placed across filament transformer secondary is helpful to eliminate leakage-inductance waveform problems (large spikes).

[merlin el mago]

Rod YHPM

[H.Michi]

Originally Posted by Rod Coleman

Hiro, such a high level of noise is not expected - especially with the excellent filtering of raw dc shown in your circuit.

my suggestions for checks:

1. Please measure ac voltage (common-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - connected at cathode of 26 (GND).

2. Please measure ac voltage (differential-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - at Raw dc -. This should be <100mV

3. Please measure ac voltage (common-mode) at output of filament Regulator: Meter + at filament +, Meter - connected at cathode of 26 (GND). This should be < 1mV

differential mode ac voltage (Filament + to Filament -) should be << 1mV

4. filament transformer has low capacitance from primary to secondary? 50 to 90pF is a good value.
Also, transformer construction is E-I ?

4. possibly, a capacitor of 1000uF to 3300uF connected at Regulator PCB (input terminals), may be useful. Low impedance type: Panasonic FC, Nichicon PM etc.

5. Snubber network R-C (R=47R 1/2W; C=47nF stacked film) placed across filament transformer secondary is helpful to eliminate leakage-inductance waveform problems (large spikes).

Hi Rod, Thank you very much for your very quick responce!!
I will check following your advices.
BTW, my previous schematic was wrong one.

[tyo94]

Originally Posted by H.Michi

I just completed modification to my 26 pre-amp "from battery to the fillament biasing".
As a result I feel the sound surely improved.
However, I feel a residual hum? noise (0.31mV) which was not included in battery bias. I measured by ac voltmeter(Kenwood VT-171) at the output terminal (600 ohm).
Under such circumstances, I wonder are there any suggestions or idea in getting rid of the said residual noise?

Hiro,

What kind of Line Output Transformer do you use within you preamp? Seems interesting that it has finemet core. Is that feastrex? How is the sound?

[H.Michi]

Originally Posted by tyo94

Hiro,

What kind of Line Output Transformer do you use within you preamp? Seems interesting that it has finemet core. Is that feastrex? How is the sound?

Ty094,
I bought a custom-designed OPT (FM15K(200H)-150x2) via my friend and honestly I do not know the maker`s name.
As far as I know, they are small company in Saitama and they may be supply the finemet core OPT`s to Andix Audio or may be Noguchi.

Specification of FM15K(200H)-150x2:
winding ratio: 1:0.22
Inductance: 212H(5Hz) 15K
216H(100Hz)
256H(1KHz)
Maximum current: 15mA/15K
DCR primary 783 ohm x2
Secondary 43ohm x2
Frequency response 8Hz ~ 30KHz (-3dB)

The sound impression is warm, brilliant and less distortion.

[tcqanh]

Originally Posted by H.Michi

I just completed modification to my 26 pre-amp "from battery to the fillament biasing".

Hi Michi,

Do you compare the sound between your pre 71A and 26 ? Which makes you satisfied ?

Regards,

QA.

[H.Michi]

tcqanh,

I`d like 26pre rather than 71a.
Although 71a was a capacitor cuppling with normal biasing.
I will try 71a with fillament bias later.

Regards
Hiro