General Audio Research - REDD47 build thread

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What do you think about this board from China that i bought. It uses double Gz4 tube as rectifier bridge and zener diodes to regulate power supply value. Can somebody give me advice about power transformer value that will fit best for this build. To get 280-290vdc. Plan is to power dual redd 47 boards. Thanks
 

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What's wrong with unregulated B+ and dc unregulated heaters ?

I would start with the simplest approach first
 
Cinemag and Jensen equivalents with full mu metal screening cans are the same price as this.

Cheers

Ian
I found both those brands a bit difficult to purchase in the EU. I would be happy for someone to point me to a good source in the EU.
Don-Audio has some CineMag, but not for this project as far as i can see.
 
You lost me. Why is that a problem?. Both before and after the UK was in the EU I bought transformers direct from Cinemag and Jensen without a problem. Does the EU have problems now with buying from the USA?

Cheers

Ian
This is pretty OT but yes I prefer buying from inside the EU with some exceptions. Shipping gets expensive since I am a hobbyist looking to buy one or two transformers at a time, shipping usually takes a long time and is unpredictable, Swedish custom usually takes a long time and splits my cost over two payments, returns are functionally impossible.
More specific to those manufacturers is that neither have a proper webstore or even prices online.
I buy from Digikey in the US all the time but the offer a low threshold for free shipping and charges me Swedish VAT at purchase and ship quickly.
 
Currently i am looking at the layout in the case i think i have settled on this option. Any objections? :)
be careful the power pcb is not made for 2 preamp red47 the filament regulator will not resist
I recommend a power supply pcb per preamp
and I recommend putting a big heatsink for the 7806
and if it must be fixed directly on the rack chassis
you can also remove a capacitor to save space and increase the value of the first one
 
Hi,

I've planned to add a simple passive HPF in my build using a DPDT switch so, where do you guys think I should insert it ?
 
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Here's my version of the REDD.47
I had trouble with 2 NOS TESLA EF806s tubes. First one blew up at first start, second one is not far to do the same, anyway it's damaged also. Both channels works great with an old NATIONAL EF86 tube I had around. The seller of the NOS tubes told me that the EF806s 3.2mA emission was too strong for my circuit, I'm sure Ian @ruffrecords could tell me if he's right or not and give me an explanation. Anyway, I'll have to order a matched pair of EF86 to get both channels working.

IMG_3296.jpgIMG_3297.jpgIMG_3299.jpgIMG_3298.jpg
 
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The seller of the NOS tubes told me that the EF806s 3.2mA emission was too strong for my circuit, I'm sure Ian could tell me if he's right or not and give me an explanation.
Ok, I'm not Ian and I'm sure he can explain this better than I can but one thing is for sure, this statement is bullsh*t.

The stated 3.2mA current is a max capability that the tube can emit under certain circumstances.

What current actually flows determines the biasing of the tube in the circuit.

If a tube can basically supply a little more current, it won't break because of that. Nonsense.

The tubes were just old and at the end of their life time.

Beautiful build by the way!👍

Edit: EF806 and EF86 are basically the same - the EF806 can deliver 0.2mA(!) more, if needed. 🤩
 
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Vigortronix was set up by ex employees of OEP and they make many very similar products. Neither the Vigortonix recommended or OEP equivalkent output transformers are really man enough for this job. They are both input transformers not output ones. Edcor XSM series would be much better.

Cheers

Ian
Greetings Ian
I have a question
It will be ok if i will use a gapped CA-18-VTB2290 - Transformer for te output of REDD47 ?

Thank you
 
Maybe, maybe not. The VTB2290 as you say is a gapped transformer. This means it has much lower inductance that its ungapped brother the VTB2291 so its bass response will not be as good. 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.

Cheers

Ian
 
Maybe, maybe not. The VTB2290 as you say is a gapped transformer. This means it has much lower inductance that its ungapped brother the VTB2291 so its bass response will not be as good. 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.

Cheers

Ian
Thank you Ian
I guess I will have to choose the VTB2380, this might work well I guess
Also can I ask your opinion ?
Planing to build with this ( two REDD47) a summing mixer with
4 stereo channels for line inputs
Something simple without any faders or any EQ.
What resistor values should I choose for the summing buss ? And should I go for a balanced or unbalanced ?

Thank you
 
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
 
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