Racking RCA BA-71 pres

[what_noise?]

Just bought as pair of RCA BA-71 pres that im going to rack up.
http://www.ebay.com.au/itm/250908416094 ... 500wt_1413

This will be my first rack job so it'll be a bit of a learning experience
The plan is to get a power supply kit, and a couple of go between and variable pad kits.
Wheres the best place to order the rest of the parts, like jacks and a power transformer?
Also looking to get some large vintage style VU meters and knobs, does anyone do plastic replicas for a reasonable price instead of $30 for an old bakelite knob?

Any tips and advice before I dive into this will be much appreciated, been on prodigy pro but unfortunately most of their threads are down atm.

Cheers
Oli

[Hookemeister]

Hi Oli

They should be pretty sweet pre's when you get them up and running. I'm not sure about that particular version but if I remember correctly the early BA-71's used germanium transistors.

Probably be worth replacing the electrolytic caps as these units are from the 70's or earlier and if the caps haven't been changed, then nows the time to do it. There's probably less than a dozen in each one so it's worth the expense.
Get good quality 105 degree axial caps, not radials, you should be able to source them from RS Components or Farnell.
If you can't get the exact value, then go to the next value higher... if you can't get a 300uf then replace it with a 330uf. The same applies to voltage ratings. Always go higher.
Note: The only time you don't do this is when you're replacing caps in a filter circuit, but that doesn't apply here.

Before you start, take some high res photos of both sides of the PCB so you can use it as a reference in case you forget which caps go where.

Take that hi res photo and stick it in your computer then using Paint program or similar, text in the value and orientation (which end is +ve) of each electrolytic. This will make things a lot easier when you start to replace the caps.

Be careful when your desoldering the electrolytics, you don't want to lift the tracks off the pcb.
Best way to do this is to cut the capacitors out first, leaving the wire tails sticking up from the board, then with one hand, hold the remaining wire with a pair of pliers then heat the solder joint and when the solder melts, pull out the wire.

You can now clean the holes by heating the pad then inserting and twisting a stainless steel sewing needle thru the whole. The other method is to use a 0.8mm drill to open the whole. This is probably the safer method.

See if you can track down a schematic for them if not PM me your email address and I'll send you the BA-31 which is essentially the same unit but with a self contained power supply.

You will need a 30 volt single rail supply at around 1 amp.

JLM is a good place to get power supply components... they're in Brisbane but will post anywhere.
They have power supply kits, pad/phantom/phase switch kits, VU's and probably most of the things you'll need. They'll may also have some vintage style knobs.

Re: Power supply. If this is your first project then it's probably not a good idea to be wiring up 240volt transformers so get and external powerpack to supply a dc voltage to the rack. Talk to Joe at JLM and he'll sort you out with something suitable... a bit safer that way.

Rack hardware can be sourced from Altronics, Jaycar, etc.

Of course the hardest part is drilling and hole punching the rack case for XLR's, pots, meters, etc.

OK that should get you going.

Lastly... take your time and enjoy the process... it's very rewarding.

Remember this is vintage equipment and should be treated with the respect it deserves.

Hope this info helps you build a nice pre... everyone deserves one

Have fun.

Greg

[Ben M]

Hi Oli
+1 on everything Greg posted.

I used these knobs on my gpultecs.
http://www.don-audio.com/skirted-knob-b ... bs-collins

Should work well for your RCA's. 1/4" shaft.

[what_noise?]

Thanks for that, exactly what I needed and helpfull
I shall get hold of Joe, and organise everything I need.
I've read a little about people slowing the phantom power ramp up time on older pres to prevent damage, is this recommended or not really required?

cheers

[Hookemeister]

I've read a little about people slowing the phantom power ramp up time on older pres to prevent damage, is this recommended or not really required?

Hi Oli

That won't be a problem for you as the input to the pre is transformer isolated, no DC getting thru there.

Most important thing with phantom supply to the pre is to make sure the 2 distribution resistors are perfectly matched. It's not all that important that they are 6.8k but if one is say 6.785k you want the other one to be 6.785k.

What I used to do was buy a 100 x 6.8k 1% tolerance metal film resistors, open a bottle of red, get out the multimeter and spend an hour or two matching them into pairs. Note: Bottle of red is not mandatory!

1% metal film resistors are pretty cheap these days, I'd imagine you'd buy a 100 for less than $5... you'll spend more money on a good bottle of red!

When you're wiring up the 48v phantom to each module, connect it via a 100 ohm isolation resistor in series with the two 6.8k resistors. This protects the 48v supply and if you happen to short out the input on one module it won't affect the operation of the second module.

Hey Ben those knobs are pricey... I hope they add a bit warmth to that Pultec.

You can get the "chicken head" knobs locally, I think they're around $2 to $3 a piece.

Ok Oli hope the above makes sense.

Greg

[what_noise?]

I have one of these power supplies
http://docs-asia.electrocomponents.com/ ... ad2ea5.pdf
It has a 5, 12 and 24v outputs.
I also have two scully pres that need about 1A combined at 24v. It is 3A and I want to see if I can run both scully and rca pres off the same power supply. Is there any way of boosting the 24v to 30 for the RCA pres and getting a 48v phantom power output from this or am I being a bit optimistic? Or would it be ok to run the rca pres off 24V?

[rob]

toss that switch mode away for another job / another day

The RCA BA-71 wants -30V ... that's minus thirty volts. So you won't be sharing that with the 24V required for the Scully. Do the Scullys really require 500mA each?
The easist one shot solution is a JLM 3 rail PSU. Set up with a dual 25V transformer. Out of this you could get +24V, -30V and +48V.
I'd just be checking on the current draw of the Scullys as while the JLM PSU will do one amp, with a 25V transformer and set to +24V the regulators will get very hot. Thus good heatsinking will be required.

Rob

[rob]

oh .. .and hold the fort I've just found out that there are two versions of the BA71, one wants plus 30V, on wants minus 30V.

of course all this plus minus voltage stuff is all relative and you can make a -30V unit run on +30V but you need to have your wits about you to get all the grounding and relationships between the supply and the rest of the world the right way around.

[what_noise?]

cheers Rob, I think for now ill just stick with the original plan and run the 2 rca pres off there own jlm power supply and sort the scully's out another time.

[Hookemeister]

oh .. .and hold the fort I've just found out that there are two versions of the BA71, one wants plus 30V, on wants minus 30V.

I think you'll find they both require the same supply.

It's all to to do with your point of reference.

It's a little confusing but essentially it's a 30v supply with the minus side via an isolation resistor.
It also makes it simpler to show voltage references from the common rail (+30V).

Neve consoles nominate a B- and a B+ 24V supply when in reality it's a way of separating the chassis ground from the -ve rail of the supply.

I think for now ill just stick with the original plan and run the 2 rca pres off there own jlm power supply

Most definitely the way to go... not a good idea to be sharing a power supply when it comes to pro audio gear.

Greg