General Audio Research - REDD47 build thread

[Slenderchap]

Also the nominal turns ratio is 4:1 and I thought the REDD47 design needs a 7:1 but I may be wrong on that score.

We do also stock the 7:1 output transformer..... 9800R:200R..... it is the VTB2460

I will put them back in the on-line store, but we only have 6 in stock.

Colin

 

[BoBoSan]

You have a transformer at the input so you might as well go for balanced inputs. They will work just as well if the source is unbalanced. To keep noise to a minimum I would recommend 5K1 bus resistor on both the hot and the cold legs. With only 4 inputs per transformer you will need to slug the bus to match the transformer input. You can optimise this for a given transformer but if you use 220 ohms thenm you will not go far wrong. This will give you almost exactly 34dB of insertion loss which just so happens to be one of the preset gains of the REDD47.

Cheers

Ian

Greetings Ian

Silly question !!!
Is that possible to replace in the REDD47 PSU the OA2 regulator tubes with zener diode and keeping the rest of the schematic as it is? Or do you have any recommendations for a simple psu for a pair of REDD47 preamps

Thank you

 

[ruffrecords]

Greetings Ian

Silly question !!!
Is that possible to replace in the REDD47 PSU the OA2 regulator tubes with zener diode and keeping the rest of the schematic as it is? Or do you have any recommendations for a simple psu for a pair of REDD47 preamps

Thank you

I am not an expert on the REDD47 but I see no reason why the OA2 regulators could not be replaced by a pair of 90V zener diodes. If memory serves the RED47 draws about 8mA on the HT. Let's say it is 10mA and add 50% to ensure the zeners are always conduction. So worst case each 90V zener needs t carry 15mA which means its dissipation will be 90 x 15 milliwatts = 1.35 watts so I would suggest trying 5W rated zeners.

The REDD47 is a class A amplifier and these are remarkably tolerant of HT voltage so a regulated supply is not essential. When the REDD47 was designed semiconductor rectifiers and high value high voltage electrolytics did not exist so smoothing HT was difficult. These days silicon diodes can easily withstand the high peak currents that would be drawn by a high value reservoir capacitor and subsequent RC filtering can reduce ripple and noise to very low levels. My regular HT250 design with 220uF capacitors will easily reduce ripple by more than 120dB. Look in the Power folder on the DIY page of my website for details:

Custom Tube Consoles - DIY

Cheers

Ian

 

[moamps]

I am not an expert on the REDD47 but I see no reason why the OA2 regulators could not be replaced by a pair of 90V zener diodes. If memory serves the RED47 draws about 8mA on the HT. Let's say it is 10mA and add 50% to ensure the zeners are always conduction. So worst case each 90V zener needs t carry 15mA which means its dissipation will be 90 x 15 milliwatts = 1.35 watts so I would suggest trying 5W rated zeners.

OA2 -150V
IHT (REDD47)=20mA min

 

[My3gger]

You have a transformer at the input so you might as well go for balanced inputs. They will work just as well if the source is unbalanced. To keep noise to a minimum I would recommend 5K1 bus resistor on both the hot and the cold legs. With only 4 inputs per transformer you will need to slug the bus to match the transformer input. You can optimise this for a given transformer but if you use 220 ohms thenm you will not go far wrong. This will give you almost exactly 34dB of insertion loss which just so happens to be one of the preset gains of the REDD47.

Cheers

Ian

Hello. I noticed 34dB of gain is common in preamps/line amps like Redd47 or V72, Langeving AM5116b, UA1016 and MB-1 have 35dB positions, i need some sleep before figuring out DISA 91B21 or RCA BC-6B mixer schematics.
Would you say 34-35dB is common because some of them were used for passive summer gain make-up amps?
As you wrote original Redd47 had 7:1 output tx, they chose it for lighter load on the output stage, lower distortion, certain ratio between even and odd harmonics, etc.