REDD 47 In/Out Transformers

[Fablab]

I like Edcor transformers but I wouldn't usean unshielded transformer as mic input in a tube preamp
Consider that with a little more you can buy the EA 2622 from classicapi.com that is a 1:7 step up transformer

 

[ungifted]

Fablab, thank you for linking to EA2622, I think I'll order one!

Anyway Edcor MXL10 is 1:10 and price is $23 (EA2622 - $42).
We can diy copper shield and perhaps it's possible to find some other shielding... need to look at dimentions, maybe OPE mu metal shield is suitable.
How do you think? No chance? 

 

[Fablab]

I don't think you can do it and anyway it is always better to use a factory shielded transformer

 

[imo]

One thing to keep in mind is that this preamp doesn't have a ton of gain to start with, so i would make sure you aren't losing any unnecessarily. I have the cinemag's in mine and have been quite pleased for a while

 

[iomegaman]

I used the edcors on my Orange 86 which is essentially the same preamp...hum was a beast to tame, copper will not get rid of magnetic hum and your PSU tranx is probably huge...

I ended up moving stuff around in the case (big 4u enclosure) and actually building a metal shield for the edcor out of steel which helped immensely...

By far my favorite preamp now...

 

[lassoharp]

P.S.
I know that Sowter and Cinemag produces some specific transformers for this preamp but I want to spend less

Cinemag is not less $$ than Lundahl no??    Cinemag probably best bang for buck if ordering in USA.



My two cents is to spend whatever is needed to get the sound you like most for a preamp like this.  When you consider the time and work involved . . . .

 

[iomegaman]

Sorta an update, I dropped a Jensen jt-13k7 in mine to see how it would sound, quite a bit "cleaner" and less noise, but also less gain/difference which I knew going into it, the REDD calls for something like 1:7 or 8 I think and the Jensen here is only 1:5 but is extremely well shielded...I have an older Stancor that beefs up the output level that I might try just to see how it affects moving the rations around...

If you go in 1:5 instead of 1:7 and go out 13:1 instead of 7:1...I have no idea what it will do...but I am not afraid to try...

 

[All Ears]

To any of the iron and wire heads out there...

I'm considering trying to wind my own output transformer for the Redd 47 for a 30k:600 ratio. Not for cost reasons, just because.

Assuming I have a similar core, could anyone take a guess at the rough number of turns I would most probably be looking at (either on primary or secondary) to achieve something pretty close to this kind of thing - http://cinemag.biz/output/PDF/CM-27101.pdf  ?

 

[CJ]

pri 4,000 to 5,000

 

[DaveP]

It's worth pointing out that the original REDD47 TX ratios were designed for Abbey Road's 200 ohm standard connection regime.  If you want 600 ohm connections then the ratio will be different.  I have successfully used OEP's mumetal pot X187B on this pre (600:50k) http://uk.rs-online.com/web/p/audio-transformers/2106352/

This is better than the other OEP transformers as they are totally enclosed, not just a mumetal top to cover a PCB.

To make a decent mic pre you must have good shielding on the IPT otherwise it's all a waste of time and money.  Maybe you could compromise by using an open frame Edcor for the OPT as the levels are so much higher.

If it's possible, use a remote power supply as well, it will repay you with better S/N ratio.

DaveP

 

[shabtek]

oh what i'd give to have an RS stateside...

 

[All Ears]

CJ - thanks again, pro - I still have to calculate gauge...I guess I'm looking at some kind of spiderweb to achieve 4000-5000 turns of my handcranked 'scatter-tangle'.

DaveP - I assume you were actually talking about the input TX,  rather than attempting to use this i/p type as an o/p. That right ?Funnily enough, I actually have two vintage nickel 600:50k tx's from an old hifi tube amp which are in bulky heavily shielded cylindrical cans and have a sort of nickel soft-squishy sound. Since they have a beefy kind of look I was actually considering testing them as O/P's as well as I read of someone doing this with a 600:50k. Not sure if that is a silly idea - mic pre  input TX's usually have very high inductance vs output TX's as well as a number of other key differences plus concern about handling o/p power.

 

[DaveP]

Yes just the IPT.
It would be a waste to use one of yours for an OPT, an Edcor would probably be fine.
best
DaveP

 

[All Ears]

Hi DaveP,

With your 600:50k as mic input transformer, did you strap an RLoad across the secondary ? If so, what value did you go for ?

Also, did you use a zobel on the secondary of the 600:50k IPT ?

Cheers

All Ears

 

[DaveP]

I checked up my R47 circuit and realised I used two Sowter transformers.

These were mu-metal cased but 1:4 & 4:1 ratios, this was a long while ago and I had forgotten, sorry about the mix-up.



As you can see, I optimised the square wave with a 470pF and 10k.  Unless you get excessive ringing or other effects that need taming, it is better to leave the IPT unterminated.  If you do need a resistor then I would recommend using the stated impedance, i.e. 50k. for the OEP or in this case it was 10k to give 600 ohms input Z (9600/ 4x4 = 600)

best
DaveP

 

[All Ears]

Wow Dave,

Firstly - your schematics are very pro - better than most  book published stuff...Nice !

Secondly - jeez ! - what are those huge tube beasts that you have pics of in your photobucket account ?? That is some really beautiful DIY stuff...I'm floored by the care and attention to detail that you've put into that stuff. Some of the best diy tube stuff i've seen for a while. Are they two all tube compressors you've built ?

Thirdly - how did your Redd47 turn  out compared to that interesting EL84 mic preamp that you had a schematic of ? Which one do you like better ?

 

[DaveP]

The first comp I made was a modified LA-2A and I used the art work from that schematic as a template for everything else I made.  I quite like re-drawing them neatly in Paint, it helps me understand what's going on and is part of the research I do for each project.

Most of my projects should be searchable on the forum, usually starting "x...........x from scratch".

I was a little disappointed by the R47, it seemed kind of dried up and lifeless, if that makes sense?  After I finished it, I started reading up about non-feedback amps and the way the harmonics get treated under feedback.  The R47 has a hell of a lot of feedback and it gives it a great spec, but I am not convinced about the sound, so  I converted it to the EL84 amp.

Most of the classic pre-war amps did not use  feedback, or very little, say 6dB,  but the records made in the late forties and fifties using those amps sound great.  I prefer my triode wired EL84 amp, it probably has a few percent of 2nd harmonics but that does no harm, a 12 string guitar is a six string with built in 2nd harmonics and no-one complains about that.

best
DaveP

 

[kambo]

a 12 string guitar is a six string with built in 2nd harmonics and no-one complains about that.

haha, i like this, totally agree

 

[All Ears]

Very helpful and timely advice, Dave

Your builds are inspiring too - neat, robust, mil-spec, good wiring.

Actually, what you say about the R47 and neg feedback, I have heard said by 3 or 4 others, so there is clearly something in it.

I am not particularly sold on any supposed 'Beatles magic aura' for these preamps - so I might in fact have a crack at your EL84 build if that's ok with you - I also would prefer less NFB rather than more. Any issues or quirks I should know about your EL84 pre  ?

Cheers for your input on this one - you may have saved me a fair bit of time and trouble in the long run.

 

[My3gger]

The first comp I made was a modified LA-2A and I used the art work from that schematic as a template for everything else I made.  I quite like re-drawing them neatly in Paint, it helps me understand what's going on and is part of the research I do for each project.

Most of my projects should be searchable on the forum, usually starting "x...........x from scratch".

I was a little disappointed by the R47, it seemed kind of dried up and lifeless, if that makes sense?  After I finished it, I started reading up about non-feedback amps and the way the harmonics get treated under feedback.  The R47 has a hell of a lot of feedback and it gives it a great spec, but I am not convinced about the sound, so  I converted it to the EL84 amp.

Most of the classic pre-war amps did not use  feedback, or very little, say 6dB,  but the records made in the late forties and fifties using those amps sound great.  I prefer my triode wired EL84 amp, it probably has a few percent of 2nd harmonics but that does no harm, a 12 string guitar is a six string with built in 2nd harmonics and no-one complains about that.

best
DaveP

I find your work very inspiring too, thanks for what you are giving to us by describing builds in great detail. Since PRR pointed to EL84 as possible mic amp output tube it always interested me, would you please comment why you chose it, how much gain whole preamp has, would higher ratio input transformer work better there, etc.?

Built a few 47s and some preamps without NFB too, i think they all have their place. If i started with non NFB preamps i would probably have hard time liking NFB ones, at least for some time.
I agree with Lassoharp that preamp like this needs certain transformers to sound good, seems like 47 isn't like some that work very well with lesser transformers, whatever that is. I've just done some tests with OEP A262A3E on input and CT Cinemag at 3,5:1 on the output. It doesn't sound that good at all, judging by my previous tests it seems 47 really needs higher OT ratio original like 7:1. Also, E88CC impedances seem a little high for very low ratios and i see this preamp like a kind of English V72; makes no sense building it without very good correct transformers. I really like their CMMI-7C here, if it was my first one i would choose CM-9661-A (50Ni-50Fe) OT.
Btw, i never understood why people like Beatles sound so much and want to recreate it without the talent, same instruments, tape...maybe because we come from different generations.