General Audio Research - REDD47 build thread

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Urskov

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Great! I'm building this too, and was wondering about transformers and stuff. Interesting with those UTM transformers! Gonna be 👀 here- and await whether the input transformer is shielded. If not - is it possible to shield it ourselves? Donaudio has these in a lot of different sizes: Small Flanged Aluminum Enclosure, 13,30 €

If all else fails, tho - Edcor output and Sowter input. If the Sowter waiting time is long, I found another GDIY thread recommending the Lundahl 1576: Lundahl LL1576 Audio transformer, 92,28 €

For PT for European builders, I found this with exact specs, but it's sadly out of stock: Audiograde toroidal 15VA 230//200V-30mA/6.3V-1.1A power transformer for tube amp
 
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rock soderstrom

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Great! I'm building this too, and was wondering about transformers and stuff. Interesting with those UTM transformers! Gonna be 👀 here- and await whether the input transformer is shielded. If not - is it possible to shield it ourselves? Donaudio has these in a lot of different sizes: Small Flanged Aluminum Enclosure, 13,30 €
Basically a good idea, but Aluminium doesn't work that good for that job.

Your enemy are magnetic fields, think guitar pickup. You need something like Mu-metal. ("Mu-metal is a nickeliron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields.")
 

HyprDrivr

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The UTM2555 looks interesting, but no information if this guy is shielded or not?

Got a quick response from UTM.

"Yes, it's double shielded. The one shield is a copper layer tape between windings connected to pin #6.
The other one is a mu-metal shield as a housing of the transformers connected to the mounting screws."
 

rock soderstrom

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Got a quick response from UTM.

"Yes, it's double shielded. The one shield is a copper layer tape between windings connected to pin #6.
The other one is a mu-metal shield as a housing of the transformers connected to the mounting screws."
Thats good news! 1:5.6 stepup is a little on the low side but I guess it will work.
 

rock soderstrom

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I would very much like to be educated on the effects of the different stepup ratios, i have tried to figure it out but i cant find solid info on this.

I am not a transformer expert at all, but the turns ratio of a transformer indicates the voltage gain. The original REDD47 has a 1:7 step-up transformer. This results in 16.90dB voltage gain.

A 1:5.6 step-up transformer on the other hand has a voltage gain of 14.96dB.

This means the overall gain of the amplifier drops a little.

So does the impedance matching. The impedance is matched quadratically, so a 1:7 matches the input impedance by a factor of x49. With 1:5.6 it is correspondingly slightly less.

Microphone tube amplifiers usually use larger step-up ratios to amplify the weak signal with less noise. 1:10 is more or less standard, there are also many transformers that go far beyond that (1:20, 1:40!). Such transformers are difficult to produce and expensive for various reasons.

Low step-up ratio therefore will result in somewhat worse signal to noise (SNR) ratio.
 
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HyprDrivr

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I am not a transformer expert at all, but the turns ratio of a transformer indicates the voltage gain. The original REDD47 has a 1:7 step-up transformer. This results in 16.90dB voltage gain.

A 1:5.6 stepup transformer on the other hand has a voltage gain of 14.96dB.

This means the overall gain of the amplifier drops a little.

So does the impedance matching. The impedance is matched quadratically, so a 1:7 matches the input impedance by a factor of x49. With 1:5.6 it is correspondingly slightly less.

Microphone tube amplifiers usually use larger step-up ratios to amplify the weak signal with less noise. 1:10 is more or less standard, there are also many transformers that go far beyond that (1:20, 1:40!). Such transformers are difficult to produce and expensive for various reasons.

Low step-up ratios therefore will result in somewhat worse signal to noise (SNR) ratios.
Thank you very much for the explanation! Its sort of what i had guessed
 

innercityman

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I have just finished stuffing the boards.

I finally decided to use Vishay/Dale MetalFilm resistors going against the BOM recomendation of Carbon. It was just very difficult to find the correct values. I know Sammy from GAR uses Amtrans AMRG but those are unironically 100 times more expensive than standard resistors, and i have trouble finding them in the EU. 2W resistors is a mix of what i could find.

MKP caps is Vishay with the exception of 0,022uf where i could only find a fitting Nichicon. 0,47uf cap is oversized for the board due to faulty information when I ordered the parts. Cornell Dubilier MICA caps and Würth electrolythics because they are pretty :)

Currently i am looking at transfomer options as the recomended ones dont work for me for different reasons. Sowter is too expensive and have a long waiting period, The Jensen seems out of stock everywhere and difficult to get in the EU and the Vigotronics just seem a bit cheap to me and i dont like the mounting options. Leaning towards the UTM at the moment.
I also found seven vintage EF86 tubes (Telefunken, Philip and Osram) in the hoard so i am looking forward to giving those a go.

View attachment 85923
I've just started this build too. I've chosen UTM Xfmrs... 15K:600, 600:15K, 5.1 and 1.5 ratios, recommended by Sammy himself. These are exact copies of Carnhill VTB2380 and 2381
 

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HyprDrivr

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The FedEx shipment for 2 x UTM2555 in Europe is 17.50 Eur. Expensive?
Its not expensive for FedEx, but i would like to have a standard postal option around 10€

But i retract the "insanely" part ;)
 

HyprDrivr

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I've just started this build too. I've chosen UTM Xfmrs... 15K:600, 600:15K, 5.1 and 1.5 ratios, recommended by Sammy himself. These are exact copies of Carnhill VTB2380 and 2381
Cool. I just ordered from them as well.
I think that the UTM2581 in your image is 1:4 ratio rather than 1:5
 

ruffrecords

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I would very much like to be educated on the effects of the different stepup ratios, i have tried to figure it out but i cant find solid info on this.
Basically tubes are relatively noisy high impedance input devices. To obtain decent noise performance from a tube mic you need to raise the input signal as much as is feasible before feeding it to the grid of the first stage (which is where most of the noise is added). Fortunately transformers provide almost noiseless gain which indicates you should use a high ratio input transformer. There are however practical limitations in frequency response and noise (from the resistance of the windings of the transformer itself) . A 1:5 input transformer is relatively straightforward to make whilst retaining an excellent frequency response and contributing very little noise. With care a 1:10 transformer can be make with little compromise in frequency response and noise. With extreme care, and a lot of money, a 1:20 transformer can be made.

In practice I have found a 1:10 transformer is sufficient to ensure the first stage tube contributes very little to the overall noise at the preamp output. A 1:5 transformer would generally be about 5dB noisier. A 1:20 transformer would be a little quieter but not much. Sowter, Cinemag and Jensen all make excellent fully screened 1:10 mic input transformers. A lower cost alternative would be the OEP A187A15C:

https://carnhill.co.uk/wp-content/uploads/2019/07/A187A15C.pdf

available here: A187A15C | Through Hole Audio Transformer 11kΩ | RS Components

Cheers

Ian
 

HyprDrivr

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Basically tubes are relatively noisy high impedance input devices. To obtain decent noise performance from a tube mic you need to raise the input signal as much as is feasible before feeding it to the grid of the first stage (which is where most of the noise is added). Fortunately transformers provide almost noiseless gain which indicates you should use a high ratio input transformer. There are however practical limitations in frequency response and noise (from the resistance of the windings of the transformer itself) . A 1:5 input transformer is relatively straightforward to make whilst retaining an excellent frequency response and contributing very little noise. With care a 1:10 transformer can be make with little compromise in frequency response and noise. With extreme care, and a lot of money, a 1:20 transformer can be made.

In practice I have found a 1:10 transformer is sufficient to ensure the first stage tube contributes very little to the overall noise at the preamp output. A 1:5 transformer would generally be about 5dB noisier. A 1:20 transformer would be a little quieter but not much. Sowter, Cinemag and Jensen all make excellent fully screened 1:10 mic input transformers. A lower cost alternative would be the OEP A187A15C:

https://carnhill.co.uk/wp-content/uploads/2019/07/A187A15C.pdf

available here: A187A15C | Through Hole Audio Transformer 11kΩ | RS Components

Cheers

Ian
Super! Thanks for the good info.
 

koki

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Is possible to use OEP A262A3E for input ? I think i saw some bilds with that transformer as input.
 

Ilya

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I've built REDD with 1:7 Cinemag input tx, and the unit has rather low gain overall. I wouldn't advise going any lower than 1:7 with an input transformer - you're risking to get a preamp that doesn't have enough gain for low-output mics.
 

ruffrecords

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Is possible to use OEP A262A3E for input ? I think i saw some bilds with that transformer as input.
Yes and no. It has a 1:6.25 ratio which is reasonable but it is unscreened which is no good for a mic pre input transformer. A screened version is available I believe.

Cheers

Ian
 

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