Bellari RP520 Tube Mic Pre

I just bought one of these used to perhaps mod.
Two channels, each with one 5751 tube.
Does anybody have a schematic?

I have one of these, and have only been able to find the schematic for the RP110, which seems to be related.
I'm assuming you found the web info about replacing ICs, and input xformers.
The interesting thing about these is the low input impedance (600 ohms), which seems to make them ideal for dynamic mics, but leaves a bit to be desired for anything that wants to see a higher impedance, though it can tame some of the "Chinese Mic Syndrome".
It's a solid performer in my humble rig, and one day I'll try some Cinemags on the input and see what I think. Already replaced ICs and tubes and there is a some improvement there.

Thanks! I started tracing out the circuit and it seems similar.  If I get a better schematic I'll post it.
I found info about replacing the tubes, chips, and switches (http://www.voodoo-labs.com/index/gearslutz_data/bellarimods.htm)
What is the info on replacing the input transformers?

I ran an RMAA test on it and it has some rolloff of the low end and at moderate gains a high end loss.

And pretty good noise... left and right were the same.

Don't remember the source or what forum, but I copied the info and will post it below. He's talking about the RP220, which is essentially the same pre, and I think he mistakenly refers to the 583, when he means the 520 (though I might be mistaken myself), so take it all with a healthy dose of salt.
Thanks for the plots. Wonder if different tubes would affect the freq plot.
Seems like the previous owner must've put the 5751s in there. I think they come stock with some Chinese cheapo 12AX7s. Have some Mullards in mine. Smooth.
I personally am not so fond of BB2604, and took the Voodoo Labs advice using the ADs. Just recorded some bass with it yesterday (mic and direct out from a SWR), and it's pretty nice. Not Summit Audio quality, but at least within my budget.
So far, the switches have been OK.
Would love to have a proper schemo if you come across one.
Thanks.


"I've redone an Rp220 with the following changes. The same genearl things could be done to the 583 I imagine - not familier with the insides a that one tho.
I replaced the Ba4560s with Burr-Brown 2604s, all the electrolytics with Nichicon, Elna,or BC or Black Gate, any other sketchy mylar caps with polypro, the input transformer with a Jensn je115 or equivalent OEP, Cinemag, UTC, Thordarson, Stancor,Triad etc. And the cheeseball 12ax7s I'd replace with NOS RCAs or the Tesla/JJ Telefunken copies that Uncle Ned sells for $7 or so.
Bypass R15 & R41 with 120uF caps to stop possible oscillation an I think you will be able to leave out C8,C9,C18 and C37. Double the coupling cap size too.
This removed the wolliness from the preamp and made it pretty aceptable."

Why would the high end would rolloff at moderate gain? The circuit is very similar to other one tube preamps (other than the weird input pad???).
I'm guessing the low end rolloff is from the input transformers - I have a pair of 10:1 Beyer that I was saving for a preamp that I could try in it. I should have run an RMAA test into the DI jack before taking it apart.
The Voodoo labs site says they came stock with Phillips JAN 5751s, which is what mine had in it.

it's feedback based gain, so there's less correction at higher gain. 

I would expect the Beyer to have worse bottom roll-off, but might be surprised.  Probably worse level handling. 

the 1st half of that circuit looks similar to a nyd 1-bottle...One could ditch the chip buffer and run unbalanced out with a few alterations

Most probable cause for HF rolloff is Miller effect combined with poor performance of input transformer.
The way the input pad works is an absolute design fault. I just don't understand by what mental aberration they ended up doing that... Low input impedance is not good for any decent microphone, dynamics included.
IMO, rewiring (or getting rid of) the pad is the most significant improvement that can be done, more than anything else; and, in second position, replacing the input xfmr - and installing a proper Zobel.
Then some attention may be applied to reducing Miller effect on the first stage.
The "balanced" output stage relies on proper balancing of the equipment it is connected to for correct operation (I presume they recommend disconnecting pin 3 when connecting to unbalanced equipment).
For me, it is an anthology of wrong answers to the usual compromises a designer has to make when having to design at a cost.
Booh!

Thanks. I'm going to put in a balanced input pad and experiment with the input transformer. I was also curious if installing C4, the bypass cap on the second tube stage, would help low end response (not installed currently). The NYD one-bottle has a 470uf there, and if I recall correctly, this cap compensates for the tube bias point changing for LF grid signals, which lowers LF gain of the stage.
If I were to put in a output transformer, would 1:1 be ok? Or does the tube stage like to see a certain impedance the way tube power amps do?

Most probable cause for HF rolloff is Miller effect combined with poor performance of input transformer.

Click to expand...

The input transformer was seeing the same level for both the low gain and moderate gain tests, as was the output stage. The rolloff was in the tube stages.

The "balanced" output stage relies on proper balancing of the equipment it is connected to for correct operation

Click to expand...

I don't understand, but I'm guessing it wants to see a 10k input impedance and might have problems with 600? Is this because of the IC selection or component values? The balancing stage designed by Audix is similar layout - would it be a benefit to change to that? (http://www.groupdiy.com/index.php?topic=29333.0) or... should I put in an output transformer?

Reading about the NYD 1Bottle & MILA, I think this type of circuit should use a 10k:600 output transformer... I'm not sure about the ins & outs of this, but I thought I'd try a JT-10KB-D that I have lying around (4:1, 40k:2.4K), and see... ratio is right, but not designed for a 600 ohm line output.

posted on gmail

mikeyB said:

posted on gmail

Click to expand...

Thanks Mikey, You made my day. Been wanting to see that for a while.
A Guiness to you...
JW

I know this thread is long dead (now undead :-0) but I ended up getting hold of an unpopulated PCB from a Bellari RP220.  It looks like a schematic eventually surfaced on this thread. Does anyone have it? It's a long gone product from the '94-95 timeframe.

Thanks in advance.

BT

I don't have any more than what is posted in this thread.
Maybe someone else does?

dmp said:

Thanks! I started tracing out the circuit and it seems similar.  If I get a better schematic I'll post it.
I found info about replacing the tubes, chips, and switches (http://www.voodoo-labs.com/index/gearslutz_data/bellarimods.htm)
What is the info on replacing the input transformers?

Click to expand...

Since this old thread was resurrected, I clicked the link, and I stopped reading after this:

Voodoo Labs said:

Now there is something that must be told to you, there is a certain amount of 'Burn In' time required to
get the full Sonic Impact from these mods. It takes about 50 hours (at least!) to burn in a Tube and the Op Amps.

Click to expand...

Morons.

Andy Peters said:

Since this old thread was resurrected, I clicked the link, and I stopped reading after this:

Morons.

Click to expand...

Everywhere........quite unsettling nowadays.

I ended up converting this to the NYD 1 bottle circuit which was very easy and then uses the cheap and available 12av7 tube.
It's a great sounding tube DI right away (you don't have to wait 50 hrs  ;D)
it's been our go to for the last 5 yrs.

Andy Peters said:

Since this old thread was resurrected, I clicked the link, and I stopped reading after this:

Morons.

Click to expand...

Back in the '80s I wrote a magazine column called audio mythology and this was the kind of stuff I would write about.

Yes tubes change over time but I wouldn't expect audible change over that short of a time scale. If the circuit is that sensitive to small parameter shifts it is not a very good design.

Solid state devices change very very slowly.  More like 50 years than 50 hours, if that. One solid state mechanism and that is a stretch is self-annealing. Low noise device b-e junctions can be degraded by allowing them to zener (reverse voltage breakdown at roughly 7V). That's why we always see reverse clamp diodes across low noise transistors in mic preamps. These damaged junctions still work but suffer from higher noise and lower Hfe. These junctions can be healed somewhat by annealing (running a high forward current through them). So a very hypothetical stretch to come up with a solid state mechanism is a junction that gets reversed at turn on/off and self anneals (or heals ) over time on.  Nah very unlikely.

The more likely basis for the wait 50 hours and it will sound better is some merchant trying to get customers who don't hear the magic right away to leave him alone. Over time our ears will adapt somewhat to response errors if we are only listening to one system.

Back in the 70's my diy speakers were a 16 driver variant on the Bose 901 ("no highs no lows, must be a bose") ;D .  I applied the typical EQ, but no amount of HF boost was going to get those 4 1/2" paper cone drivers to rock the top octave.  One day I walked into a friend's recording studio and his control room monitor speakers nearly took my head off, I was so accustomed to the soft top end, that a flat system was uncomfortable. (I ended up adding some cheap piezo tweeters to my decade old DIY creations for some top end. The piezos didn't sound that bad with their plastic horns removed.). I traded some of my kit product for real speakers shortly thereafter.

So yes.. burning in for improved sound quality is 99.99% BS... in production burn in all about catching assembly errors and perhaps some infant failures (product failures follow the bathtub curve... higher at the very beginning and very end, low and steady for the flat middle time period. ).

JR

PS: The morons might be the ones buying the sales pitch... but I don't like calling people names, their bank account depletion should be suitable punishment.