RCA BC-2B

I'm thinking about building this, do you think its worth my time? It's so simple.

I've built one with UTCs on input and output. If I recall it was A11 on in and A25 on out. Sounds good, but gain is low. Not for ribbons.

Bah!  Plenty for ribbons.  If you need more gain, get it after.  I had a singer last week , 2 feet back from a ribbon, RCA BA-2A preamp, and getting +24 dBm peaks.  You just need an actual talented unfettered artist, or proper caveman hitting the drums.  Etc.  I frequently have ribbons into 40 dB preamps, with plenty of output level, sometimes even with a 20 dB pad. 

Anyone know the sonic difference between the 2B and 2C?

None.  I've said it before, it's a mechanical change: the way the electrolytics mount.  A BA-2 is a BA-2. 

buildafriend said:

It's so simple.

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Simple is neither a quality nor a flaw. Actually, there is no reason for a mic pre to be complicated. You need to know the consequences of its simplicity.
There is no NFB, so the actual performance depends on the tubes' linearity, transformers quality, and PSU's integrity.

The BC-2B console preamp has about 10 dB NFB....now that we've started talking about 2 different preamps.  Both original amps sound good. Quality transformers are extremely important with either.

I've still never figured out where that BC-2B schematic came from. Both it and the other floating around are not the official drawing from the manual. 

. . . . and how many BA2s, C, B, etc will sound exactly the same ever?

Resistor & cap tolerances are always referenced but no one ever mentions that same model iron doesn't exist as <<<1%, perfect clones of each other either.

Okay, so im a retard and I don't know how to select transformers. I don't see any UTC's available with a quick google search besides some on ebay. No A11's are available currently on ebay and I found an A24 which I would guess is close to the A25.

http://www.ebay.com/itm/VINTAGE-UTC-A-24-OUTPUT-TRANSFORMER-FOR-TELETRONICS-LA2A-/221024387420?pt=Vintage_Electronics_R2&hash=item337614815c#ht_500wt_1048

I also noticed that the PSU supply rail on the schemo does not have a voltage or ampere rating. I have been told that standard is something around 250-300V. What are the amps?

Where is that 2k7 resistor going?

abbey road d enfer said:

buildafriend said:

It's so simple.

Click to expand...

Simple is neither a quality nor a flaw. Actually, there is no reason for a mic pre to be complicated. You need to know the consequences of its simplicity.
There is no NFB, so the actual performance depends on the tubes' linearity, transformers quality, and PSU's integrity.

Click to expand...

I remember in school that I was shown a few different biases on transistors.. base.. emitter.. collector bias.. Then we were shown how to stabilize things but that was 2 years ago these are tubes. I understand that I am at a compromise, but I still dont really understand why or how to make things more stable and less reliant on other parts tolerances.

buildafriend said:

Where is that 2k7 resistor going?

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That's a dropping resistor coming from the power supply "B+" rail.

and I found an A24 which I would guess is close to the A25.

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Impedance ratio wise yes, but they are different animals.  A-25 is designed to take unbalaced DC on the primary.  A-24 is not.  You want A-25 for either RCA circuit.  They work good as general substitutes.  I'm happy with the A-25s I have on my BA2s. With the current price of A-25s and all the LS stuff (grapevine has echoed that the LS-27 kind of sucks  , FWIW) a Lundahl or the Cinemag CM2092-CF seems the best way to go unless you can track down some of the originals.


UTC A-11 is fine for the input on either amp.  Cinemag CMMI-10C probably better.

If you build the stock power supply it should already be stable, so no worries.  For resistor ratings you just need to know how much current draw.  Assume about 3.5 to 4.5ma for either amp per channel if doing a pair.

Okay ill scoop that stuff up.

Im using angela capacitors.. copper foil in oil.. http://angela.com/angelacopperfoilpaperinoil22uf630vdccapacitor.aspx
I can't find the 20mfd capacitor.. like I said I'm a moron.

These resistors http://www.mouser.com:80/ProjectManager/ProjectDetail.aspx?AccessID=4861715fd1

Ill get cinemag for the input.. I want the UTC A25 for the OP xfrmr, since its not available ill source something else. 

emrr said:

I've still never figured out where that BC-2B schematic came from. Both it and the other floating around are not the official drawing from the manual.

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Years ago 2003 ish I think when I had my first attempt and DIY audio , I came across this schematic & a hand drawn schematic from a guy called Larry Sayer .....not sure of the spelling

buildafriend said:

I can't find the 20mfd capacitor.. like I said I'm a moron.

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Use 22 uF, that won't make a difference; most capacitors used at the time had a 10 or 20% tolerance.

These resistors http://www.mouser.com:80/ProjectManager/ProjectDetail.aspx?AccessID=4861715fd1

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Don't get overexcited about components, they don't make that much difference.

buildafriend said:

Okay ill scoop that stuff up.

Im using angela capacitors.. copper foil in oil.. http://angela.com/angelacopperfoilpaperinoil22uf630vdccapacitor.aspx
I can't find the 20mfd capacitor.. like I said I'm a moron.

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Why, they are incredibly expensive.

A regular 22uF 350V or higher electrolytic part will do fine for this component.

These resistors http://www.mouser.com:80/ProjectManager/ProjectDetail.aspx?AccessID=4861715fd1 else.

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Again why? They are expensive and of poor specification. Use regular metal film resistors.

Cheers

Ian

The carbon comp resistors will sound different.  Softer around the edges.  With the angela caps, you don't buy the multi-mfd values, just the sub-mfd values. Use standard electrolytic for the polar types. 

If going all carbon comp, buy at Newark for better prices.

http://www.newark.com/jsp/search/browse.jsp?N=422+200625+810090234+810092253&No=0&getResults=true&appliedparametrics=true&locale=en_US&divisionLocale=en_US&catalogId=&skipManufacturer=false&skipParametricAttributeId=&prevNValues=422+200625&mm=1000194||,1000359||,&filtersHidden=false&appliedHidden=false&autoApply=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D422%2B200625%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_US%26divisionLocale%3Den_US%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D422%2B200625

Is there a power transformer available with the appropriate specifications?
Otherwise I'm guessing you would just require a 230V supply and a 6.3V heater supply for the tubes?

emrr said:

The carbon comp resistors will sound different.  Softer around the edges. 

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Urban myth. They may be noisier, though.