R*E*D Forty Seven

[bradb]

Hey guys,

I'm doing the Drip build at the moment and homing in on the transformers.  Issues I'm confronting are: mounting type, shipping costs and best sound.

The board is set up for threaded bushing mount input and output transformers, so I have a predilection for this format for ease of assembly and noise rejection (only an issue for the output transformer as you shall see).  Cinemag makes an input transformer for the Redd 47 with this mounting type and after reading the raves from guitarmaker, I'm sold...

For input
Cinemag's is:
CMMI-7C with threaded bushing

The sowter input transformer is the 9970 Microphone input transformer which also has a threaded bushing option.


The output iron is a different story... If i could avoid Sowter (and the INSANE shipping cost) I'd prefer that, but again, the Cinemag doesn't have the threaded bushing mount.  I mean, we are at line level so maybe its not a big deal, but its also easier.

For output
Cinemag's selections are:
CM-9661A-H (high-nickel core):    $135.03
CM-9661A-L (50% high-nickel + 50% steel):  $100.41

Sowter's is:  9980 Output transformer

The 50/50 core is supposed to be more vintagey, and the high nickel core is supposed to be more hi-fi.  I'd prefer something with some character.... Cinemag recommended the high-nickel core version and according to Cinemag, many people like that better



So, all in all, whats the best output transformer for the REDD47??

 

[CJ]

most of the character will come from the input in my experience, unless you really overdrive a Ni core output.

 

[syn]

PM sent
thanks

 

[bernbrue]

Hi,
I used CMMI-7C for input and CM-9661A for output. For me it´s a dreamteam. Going to try sowters for the next one, just to have another colour. I have Sowter 3195 and 9050 PP in mind.
regards
Bernd

 

[gary o]

Hmm I have a 9050... push pull output tran that one might try that ... I cant get this pre to set me alight yet not sure why, transformers, maybe just my taste, seems to have tiny bit more top than some of my others diys but not the guts or thickness of some of the others.... I just read so many ravs about this pre but havent heard it yet....il keep listening

 

[gary o]

Tried the Sowter 9050 ....sounded thinner less bass didnt like it.....dont no if thats cos its for push pull usually...I dont no much about transformers scares me a little I think I understand the ratio stuff I have been using a Sowter 8940 3:1 ratio it works well the 9050 was 6:1 so less level thats fine but didnt like the sound....I read the REDDs usual is a 7:1 sowters version is 40% Mu metal 50% M6....Im not sure what the different metals M6 MU would do to the sound ? both the transformers I tried were 50% Mu....I have a Sowter 9129 meant to be for a V74 100% M6 I wonder how this will sound, its cheaper to buy than other trans.....I dont understand the ratio on this its 8+1+1=7 ?....anyway I will try it & listen

 

[gary o]

Tried the V74 transformer ...I prefer its sound....gonna do some more listening.

Ive been running my red on a PSU I made for a V72 its just a full wave bridge rect 3x 8UF caps & 2x 2K5 resistors as per V72 schematic it works fine I notice the Red power supply is bit more complicated 6 caps 6 resistors and 2 reculators......why is this ? will it made the pre sound beter ? quieter ? or is PSU designed to rum more pres at once or something ? .....thanks for reading

 

[guavatone]

The R£DD PS is just has larger total capacitance and 2 Valves that in a very simple way are kind of like zeners. 

 

[gary o]

Thanks yep the oa2 s keep the voltage clamped at 300V ......I was wondering why ...but I think its cos there was more than one pre running in the consule and depending on what each amp was doing in would effect the supply to the others...well thats my guess.

So to answer a question in my head....I wont need the 300 reg for my one RED type pre.....I have mine running on a simple V72 type PSU 3X 8uf  2X res powered by old Grundig tape recorder transformer....amp sounds good noise to my ears is low,Im think making the PSU as the orig red in my case wont make it any different, I havent learnt a good way to measure noise yet, still learning but as a simple test for my DIY stuff I plug pre under test into sound force via my M audio Delta input socket and just take note of the sensitive input meters in sound forge, noise seems to be around -69( Im not sure about db scales yet again still learning) near the bottom of the meter, a little lower than some of my other DIY pres.

 

[Winston OBoogie]

Why not?

A regulated H.T. has an impact on the sound in my book.  You may like it, you may not.  Some folks might say "Well, regulating the H.T. is cheating coz then the bass just gets all impressive and shit!".  Maybe it does?
A regulator can help reduce ripple on your H.T. supply but so can a simple cap multiplier.  So can lots of L or R and C for that matter.  But too much of that sort of stuff can also slow down your supply and lots of R can raise the impedance of the supply to the point of causing other issues.  A reg can help keep your circuit isolated from some nasty crap that happened on your A.C. line that would otherwise have ruined a good take.
What type of regulator are we talking about anyway? series? shunt? squalid-state?, hollow-state? 
Generally, I find that the cap that closes the current loop of a circuit is the one that you can hear the sound of more than the others.  If you use a simple cap multiplier (with a voltge ref or not) then you can use a higher quality but smaller value cap such as a poly-prop instead of an elecky on the output.  And remember! keep your current loops as small as possible. 
Simple shunt regs with neons are nice too I.M.O. but are even nicer if you feed them with an active current source rather than a resistive one.  A small value cap (.047uF ish) across the output of the neons is sometimes a good thing but put too big a cap there and you'll know about it!!!!

I built about a 100 of these REDD thingy pre amps.  I built 'em a few different ways with different types of H.T. regs. A couple without too.  I generally mixed it up a bit when I'd got bored with doing it one particular way and felt like I could do it better.
I spent lots of time fiddling with other bits and pieces related to the supply too.  I learned a bit by looking at things such as the duty cycles of the power transformers I was using and things like measuring the crud that diodes inject into the H.T. and back into the AC line.  Some of the fixes for this stuff turned out to be really simple and cost me pennies. 
I think a regulator for the H.T. can be a good thing.  But, if I had to choose one today, I think it'd be one that I didn't hear working and it'd be a shunt type.  Better still might be a really fast passive filter network with ripple that's low enough not to offend, although I haven't yet come up with one that quite does everything I want.   It's worth downloading the Duncan Amps power supply simulator and playing around with some values.

Ramble over.

 

[gyraf]

Now that's clear talk. Welcome back, Mr. O'Boogie....

Jakob E.

 

[gary o]

Thanks for great ramble mr boogie man.....I wish I had the understanding that most you guys have of electronics, so much goes over my head sadly.....Im all for cheating...if neon regs or what ever makes pre sound good/ different then in they go, il run LT on a car battery if sounds better dont care how it looks....

 

[bradb]

Interesting to see you say "slow down the power supply"

I always thought a whole lot of capacitance in the supply would provide a reserve of power as the pre transiently works hard.... but all that capacitance is also going to slow down that power LEAVING the supply.


does that make sense?

 

[jensenmann]

no, for serious filtering you need RC filters. These filters have a time delay t=RxC. If you have several stages to get the ripple low enough the delay sums up. Even worse the output resistance increases because all filtering resistors are basically connected in series. If you lower R then you have increase C to still get rid of ripple but this is a problem with high voltage caps. You don´t get them - or at least don´t want to pay them - in high capacity values. And high cap values - see formula - will give you a high delay time.

 

[Winston OBoogie]

Thanks for great ramble mr boogie man.....I wish I had the understanding that most you guys have of electronics, so much goes over my head sadly.....Im all for cheating...if neon regs or what ever makes pre sound good/ different then in they go, il run LT on a car battery if sounds better dont care how it looks....

I think it will sound different yes.

As an example: The REDD47 has a standing current of approx 20mA.  When using neon bulbs or simple zeners as shunt regs, we supply the shunts with enough current for the circuit load on them, plus some current to turn the shunts on. 30ma total would be a good value in the REDD. 
So, if our pre reg'ed but filtered H.T. were 350 volts (under load) and our final reg'ed voltage will be 290 volts, using ohms law and plugging in the voltage difference between 290 and 350 along with the 30mA current for the shunts, we would use a 2K feed resistor.  Try this and see if you like or dislike it.  Of course, plug in your own values for the voltages to get the appropriate value resistor. 
After doing that, and if you feel like going further, substitute an active current source (set for the same 30mA) for what was, in my example, the 2K resistor.  See if you like that. 


At the end of the day, it's all about the power supply.  That's what we're listening to.  Anything you can do to help is good in my book.  As further experimentation: On a scope, you can look at the noise from the rectifying diodes.  Some diodes are better than others in regard to the hash they produce, but they all produce it.  Think about some ways to lower this hash at the source rather than leaving it all to the H.T. filter bank.  There is also LOTS of crap coming in from your A.C. wall outlet.  Think about some ways to get rid of some of that stuff too.  I generally do it on the secondary of my power transformer.

Naturally, none of this is exclusive to the REDD47, it applies to all power supplies.   

One thing I said before was:  "Generally, I find that the cap that closes the current loop of a circuit is the one that you can hear the sound of more than the others...  keep your current loops as small as possible."
I see a lot of PCB layouts and p2p layouts where a loop takes a circuitous route.  It's easy to avoid this.

W.

 

[bradb]

Question to everyone who used the Cinemag CMMI-7C input...

seems like the secondaries of that tranny do not have a center tap, yet the schematic calls for a center tapped secondary.  That center tap going between that cap and the 100k resistor (by memory)

how is the cinemag hooked up then?

 

[Winston OBoogie]

Question to everyone who used the Cinemag CMMI-7C input...

seems like the secondaries of that tranny do not have a center tap, yet the schematic calls for a center tapped secondary.  That center tap going between that cap and the 100k resistor (by memory)

how is the cinemag hooked up then?

I can answer that question.  Easy.  The original transformer (an A92 of which a friend of mine just sold a pair I owned to a forum member) does NOT have a centre tap on the secondary.
What you see on the original REDD47 schematic is a cap and a resistor (100pF and 100K damping network) soldered to an un-used pin on the top of the can.  The pin that is used as this tie point is pin number 6 and it does not go to the winding.  Of course, if you use the CMMI-7C, which I did myself, then you would use a different and appropriate damping network as the resonance is higher on the CMMI-7C. 
The CMMI-7C was a transformer I had a small hand in designing many years ago. I had had Marco who used to work for Reichenbach Engineering wind it for me.  It was based on an older Reichenbach design but I asked for larger input level handling and for the peak to be higher so that I could decide if I wanted to let it ring at some ungodly high place or damp it with a network.  The transformer was initially stamped as an R47-IT.  When Tom Reichenbach bought the assets of RE and turned the company around (along with David) into the great company that is today Cinemag, I had him make the same transformer for use in a few pre amps for UA and another company who shall remain nameless.  At that time, it got assigned the Cinemag number CMMI-7C.

W.

 

[pH]

Nice to have you back in the mix, John. I'd like to thank you for your work on the Cinemag transformer (which I used in my build) and your schematic found here on the forum. Good to have someone who's built this circuit seven ways 'til sunday giving his two cents. It's the most used mic pre in my studio, and I wouldn't have built it without your contributions.

I built mine PTP into a recycled Akai tape preamp with what is probably an undersized power transformer (250-0-250). I used OA2 tubes to regulate and sometimes they strobe! I have 290v before the neons so I don't think they have enough to work with, so I basically run it without. Since this one is a recycle job I might just leave it as it is unregulated and eventually build another one with different regulation. You wouldn't want to donate a nice PSU, eh?  ;D

Cheers,
Paul

 

[Bryson]

Here's a thread at PSW about a guy building a REDD 51 desk from scratch.
http://recforums.prosoundweb.com/index.php/t/27583/833/

 

[Winston OBoogie]

I have 290v before the neons so I don't think they have enough to work with, so I basically run it without.

Do you mean you have 290V feeding the current sourcing resistor?  That isn't enough no. With 250V AC from your transformer you have about 350v rectified. If you're careful with the amount of resistance in your C-R-C filter banks (maybe use bigger value C's to compensate for smaller R's or use chokes with low DCR), you should be able to filter enough ripple and have 30 volts or so available across the current resistor.  If you like your pre without the regs, then you could just leave it as is too.

You wouldn't want to donate a nice PSU, eh?  ;D
Cheers,
Paul

How about a pcb with a solid state regulated H.T. supply with L.T. supply and phantom supply + a suitable power transformer to go with it?  It'd be a supply that's useful for various tube projects, not just a REDD47.   It won't be my latest and greatest but it'll be a good supply that has been used in LOTS of units.
If so, as it happens, I was recently speaking on the phone with a regular long time forum member here about putting something together and handing it over to him to make it available.

W.