[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

[haima]

dmp - re: rotary switch, if you can find one that is small enough in diameter it might be able to fit without it touching the pcb... but it would have to be seriously small, you'd probably need to drill out the mounting hole in the panel and bracket too.

if i was you i'd just build one or two with the standard pots from jeff - at least to begin with... and go from there.

good luck!

 

[dmp]

Thanks haima,
I ordered the standard pot to do just what you suggest...
I'll try the rotary swith on a later project...

 

[dmp]

I have my preamps assembled and just waiting on the psu to try them out...
I was wondering about the output attenuator in this application?
If the preamp sounds better with a 600 ohm load on the output, shouldn't one put a 600 ohm resistor on the secondary, if omitting the attenuator? My recording interface has a much larger input impedance ~10k.
Any links to the theory on how a 600 ohm load affects the 2520 output versus a higher (~10k) would be appreciated. Frequency filtering from the inductance of the transformer? Give me something to do while waiting on my psu board...
Thanks!

 

[haima]

dmp - the output attenuator presents a 600 ohm load to the secondary of the output transformer - performing the same task as a 600 ohm termination resistor... so you are all good.

the advantage of the output attenuator is that you can run your preamp a little hotter to get more colouration - and then attenuate the output to suit your recording format . this is similar to the technique of turning the preamp up and the fader down that many people do on consoles.

i'm not an expert on the sonic effect of 600 ohm loads on API preamps - i do know some output transformers require proper termination to be flat in the top end (old neve's etc). i don't think this is the case with API as these output transformers are very flat out to 100s of khz. however, 600 ohms is a heavier load than 10k - which DOES loads down the opamp and makes it work a little bit harder, which will effect the sound - more harmonics, etc.

try it with and without a 600R load and see what you like...

i'm guessing the effects of this load change would probably be fairly subtle, the 2520 is quite a burly opamp - it can handle some tough conditions - but like everything it does have a sound. it's not 100% clean, but it's not a fuzz box either by any means. remember that many great sounding albums have been made entirely using these console preamps. "mixing and matching" preamps is a relatively new thing. william wittman (cyndi lauper) is one engineer i can think of who prefers not to buy into this recent trend - instead he prefers to use one preamp design for all sources - coincidentally he often uses API consoles...

i guess what i'm saying is that these preamps sound good enough to use for pretty much everything

have fun!

 

[gar381]

If you are omitting the attenuator
and drop a 604R or 620R (what ever you got)
accoss the trafo output
your Pre won't mind it a bit and your 10k input
of your next device won't mind it either.  
Listen to it both ways and pick what you like.

GARY

 

[Sleeper]

Just about to start mine up.
I'm going to rack mount them but I have a question

I already have a -2, -5, -10, -20db bridged-H   600 ohm stepped pad
I'm thinking about this on the output...  
It's all 1/4 watt though
I looked up the clarostat 70 series and it looks like the output attenuator is 1/2 watt .
I'll admit to being a tad lazy here--(I've found the info and calculated something similar before but I was hoping to avoid doing math today)
I'll get back to searching in a few hours, but maybe one of you guys already knows if 1/4 watt will be OK.
Couldn't hurt to ask.
Thanks
Kelly

 

[dmp]

Well, Power = V^2/R (from ohm's law, V=IR and P=VI)
so for a 1/4 watt, a 600 ohm load can handle Voltage = ~sqrt(.25*600) = ~12V
This would be a pretty high RMS output from a preamp wouldn't it? 24 dBv I think?
I'm sure others know a lot more, but I think it would be fine.

 

[Sleeper]

Hi DMP and thanks for looking at that.
I've been running some of the calculators on the net, but my confusion stems from the API specs which (with a plus minus 20v psu) claim 30 dbM output.  24.4949 vRMS  according to this calculator.
http://www.sengpielaudio.com/calculator-volt.htm
That's a full 1 watt of power (unless I'm plugging these figures in the wrong way)
Not that I'll be running sine waves at full level and padding down the output...

Wouldn't the power dissapation resistor requirements for a balanced signal (post-transformer and all) be halved with a bridged-H because I'm attenuating on both legs?  gotta check this.
But OTOH it's close to the edge and I'm going to try what I have on hand anyways...
I'm pretty sure in real practice this will not be an issue at all.
We'll see.
Thanks again
Kelly

 

[jsteiger]

The VP26 has a headroom of about +25.8dBu when driving a 600 ohm load. +27.4 without being terminated.

+26dBm is about .4 watts. +24dBm being about .25 watts.

Those levels are more like absolute peaks, not an average signal levels.

With an average level of say +18dBu, which is very high IMHO, you have about .063 watts. Well below the capacities of the Clarostat t-pad or even 1/4 watt resistors. I would say a real world average level may be more like +6dBu or so. That would give you a wattage of .004.

BTW, the Clarostat series 70 pots capacity drops from 1/2 watt to 1/4 watt when going to a 3 deck configuration. I'm not exactly sure why.

In a real world application, there should be no issues at all. If one plans on running sine waves at +25dBu for long term...there will be some issues!  I know this will never happen.

Someone correct me if my math is wrong.

Cheers, Jeff

 

[Sleeper]

Hi Jeff,
Thanks. and that's also awesome because my attenuator should work
Your math is in line with what I got.
I'm just so lousy at the maths- I'm always concerned that I'm not plugging things into the formula correctly.
I'm pretty sure the balanced H should be great for this application, and I have room so away we go.
Thanks again
K

 

[jsteiger]

...I'm pretty sure the balanced H should be great for this application, and I have room so away we go...

Yes, I agree. Let us know how things work out.

Best, Jeff

 

[TheGuitarist]

Could anyone describe the differences between the vp25 and 26 please?

 

[funkymonksf]

Does anyone have a FPD file for the face plate that I can use to design my own faceplate with. I want to send it to frontpanelexpress.com and make mine look oh so sexy. Anyone? I would highly appreciate it.

[email protected]

 

[haima]

TheGuitarist - the only difference is the output transformer.

the VP25 uses the 2503 output transformer (as used in the 312 and most of the api made 500 series)

the VP26 uses  2623-1 output transformer. these are what was used in the api consoles i believe. this transformer is $10 cheaper... slightly smaller, but it is wound differently so that doesn't necessarily mean it's lower headroom.

i have made VP25s and i'm very happy with them - i chose that model as i already had 2503s in stock. i'm interested to compare the two - i have no doubt they are both awesome. i imagine the difference would be subtle, they are both pretty high headroom transformers.

jeff has more info on his site:

http://classicapi.com/catalog/product_info.php?products_id=33

http://classicapi.com/catalog/product_info.php?products_id=79

 

[Sleeper]

Does anyone have a FPD file for the face plate that I can use to design my own faceplate with. I want to send it to frontpanelexpress.com and make mine look oh so sexy. Anyone? I would highly appreciate it.

OK this post reminds me, I said I was going to start next week and that was a few weeks ago, but I've got a nice break coming up so I'm really getting started.

I DO have to design a front panel and since I'm out of town at the moment, it would be a good time to knock this design together. I'm making mine as a side by side pair in 1 rack space.  I was planning on taking my measurements off the boards, but I didn't bring them with me on this trip. (much as I'm excited to get started)

If there's an FPD or illustrator file maybe someone can post it or PM me.
or Jeff- if you know the spacing between the front panel items,  that's all I really need to know.
It would be a great help.
Thanks
Sleeper

 

[funkymonksf]

That exactly what I need to is the spacing information between the holes. If anyone has these, can you please send them to me at [email protected].

-Thanks

 

[surfkat]

Just got a pair of these beautiful kits today with the GAR2520 DOA kits. Can't wait to build these and slide them into my Lunchbox for a test drive. Thanks to Jeff and Gary for making these kits and to all others who participated in the development.

On another note, what became of Silent:arts' re-measurements of the various DOAs? Did I miss it?

 

[Whoops]

Hi,
I finish building my VP26 and unfortunately it seems I'm the first one to have any problems.

Every time I connect this Pre there's smoke coming out of the GAR2520 Opamp.

I rechecked Caps polarity, diodes resistors and solder joints and everything seems fine.

I swapped the burnt OPamp for another GAR2520 that I had and it also burned imediatly after I applied power to the rack, I couldn't figure out from what component was the smoke coming from maybe the BD139, Im not sure, but it was definitely coming from the opamp.

Any ideas of what might be the problem?

thanks a lot

here are some images:

 

[haima]

maybe some hi res shots of the solder side? maybe you've bridged something...

my experience smoke usually happens when you've got the power supply reversed somewhere or the power supply voltage is too high for a component, or you're drawing WAY too much current somewhere. a short could cause things like that...

did you make the opamps yourself? is there a chance both of them have a reversed component or bridged track?

 

[haima]

what rack are you using, out of interest?