SM Pro Audio PR8E Schematic (completed) + Mod

[moley1390]

Hi all, this is my first post on GroupDIY - a draft schematic!

I couldn't find any schematics for this preamp (the newer version of the PR8, for which a schematic exists), so I've just finished tracing the connections and drafting a schematic (attached). There are lots of things which could obviously be improved, but this is what I've got for now (note: I excluded the +48, phase and Clip circuitry from this post for simplicity, but I have these if people want them. Also note that P1A and P1B are the same pot, but as is shown, the connections are on opposite ends). I'll post component values later, if people are interested, but for now, I'd really appreciate if people could have a look over and see if things make sense. EDIT: Values Below

The op-amps are JRC 4558 Ds, and i'm planning to swap these out and try a few different op-amps and see how they sound.

I'm also planning to change the signal path electrolytics (if anybody had advice on this, it'd be greatly appreciated). So far I've identified these to be C23, 22, 20, 19, 8, 7, 2, 1 - Is this correct? Would there be any benefit in changing any others? I was just planning to replace them with caps of the same value by a better manufacturer (panasonic?) or maybe have tantalums on a few channels.

In this thread http://www.groupdiy.com/index.php?topic=344.0 there is a suggestion to cut three caps - in my schematic these come out as C6, 9 and 11. As mentioned in this thread http://www.groupdiy.com/index.php?topic=19187.60 the values have changed from 470pf to 100pf, so clipping them may not be necessary. Some people have also suggested using small ceramic capacitors from pin 4/8 to ground on the op-amps, but this appears to be taken care of in the new model by C3 & C4? Or does the fact that they're polarised (electrolytic) change things?

Could anybody explain the function of R25 and C16 between the inputs? I assume it's a filter of some kind, but I don't really understand why it's BETWEEN the inputs (something to do with Common Mode?)

Thanks in advance for any help/info!

--Scott

EDIT: Here are the component values:
Resistors:
R1 47R
R2 47k
R3 100k
R4 10k
R5 47R
R6 10k
R7 47k
R10 280R
R11 68k
R12 68k
R13 47k
R14 150R
R15 30k
R16 100k
R17 8k2
R18 4k7
R19 2k4
R20 2k4
R21 8k2
R22 4k7
R23 33R
R25 22k
R26 10k
R28 10k
R29 47k
R30 240R
R31 1k2
R32 1k2
R33 47k

Capacitors:
C1 22uF/25V
C2 22uF/25V
C3 100uF/25V
C4 100uF/25V
C5 104 (film)
C6 22pf (cer)
C7 47uF/25V
C8 10uF/50V
C9 100pf (cer)
C10 470pf (cer)
C11 100pf (cer)
C12 470pf (cer)
C15 470pf (cer)
C16 680pf (cer)
C17 680pf (cer)
C18 680pf (cer)
C19 47uF/63V
C20 47uF/63V
C22 10uF/50V
C23 10uF/50V

Diodes:
D1 1N4148
D2 1N4149
D3 1N4150
D4 1N4151
D5 1N4007
D6 1N4730
D7 1N4730
D8 1N4007

ICs:
IC1 JRC4558D
IC2 JRC4558D

Transistors:
Q2 9015
Q3 9015

Pot: 5k

 

[moley1390]

Hi everybody, a few days and I've collected 67 views but no responses Having read a lot of the threads on this forum I was hoping there'd be more enthusiasm, but I understand that my post is a little lengthy. Here's a more condensed, direct question list:

1) Should I bother posting the values (since I got no replies I thought it might be a waste of time)?

2) Could somebody confirm my identification of the signal path capacitors (i.e., the ones to upgrade)? These are:
C: 23 + 22 + 8 (10uF/50V); 20 + 19 (47uF/63V); 7 (47uF/25V); 1 + 2 (22uF/25V)

3) Can anybody explain the function of R25 (22k) and C 16 (680pf ceramic) between the inputs?

4) Can anybody comment on C6 (22pF) C9 + C11 (100pF) and whether these should be changed/removed?

Thanks,

Scott

 

[wolfgang]

1) Should I bother posting the values (since I got no replies I thought it might be a waste of time)?

It always hepls to have values and IC-Parts!

1)
2) Could somebody confirm my identification of the signal path capacitors (i.e., the ones to upgrade)? These are:
C: 23 + 22 + 8 (10uF/50V); 20 + 19 (47uF/63V); 7 (47uF/25V); 1 + 2 (22uF/25V)

Yes this is right

1)
3) Can anybody explain the function of R25 (22k) and C 16 (680pf ceramic) between the inputs?

Dont know really but i guess it has somenthing to do with input impedance  and i think the capacitor is shorting out higher frequencies before entering the signal path. But maybe you´ll get an answer from a member with more in the brain!

1)
4) Can anybody comment on C6 (22pF) C9 + C11 (100pF) and whether these should be changed/removed

These are there to prevent the opamps from oscillating at higher frequencies. They are building a low-pass with their parallel resistors.

I think these should be left in!

regards,
Wolfgang

 

[moley1390]

Thanks Wolfgang!

I'll post the values shortly.

2) I've replaced these on one channel for Panasonic caps (p.s., when FM series aren't available is FC/FR the next best bet?) and it has seemed to make quite an improvement

3) Curious...

4) There are substantial threads about the previous model of this preamp, and they nearly all suggest removing these caps (BLA do this in their mod as well). Seeing as I'll be playing with lots of different op amps, I guess i'll leave them in just in case (oscillation, etc) but people seemed to think that this was unnecessary. Any thoughts?

 

[moley1390]

Values posted in my original post (to make them easier to find)

 

[ricardo]

1) Should I bother posting the values (since I got no replies I thought it might be a waste of time)?

It's a waste of time for anyone to comment on schematics that don't have the values ON them.  Even a separate list is a PITA.

 

[clintrubber]

2) Could somebody confirm my identification of the signal path capacitors (i.e., the ones to upgrade)? These are:
C: 23 + 22 + 8 (10uF/50V); 20 + 19 (47uF/63V); 7 (47uF/25V); 1 + 2 (22uF/25V)

Thanks Scott for the traced schematic.

Strictly speaking, C15 is in the signal path as well.

The value-list states 470pF (cer), but it'll be more something like 470 uF or 1000 uF, right ?

Regards

 

[clintrubber]

It's actually a remarkable situation:

- people having trouble with the quick overloading of this preamp

- internal jumpers (after the input-connector) already present to enable a pad, as also seen on the traced schematic

- despite the manufacturer being so nice to implement this pad (it'll be around -20 dB), there's no mentioning of this in the manual


The PR8E-box is a bit elaborate to open (more than a few turns of a screwdriver), but once inside one could think of enabling the pad on a few channels - by simply reconfiguring those jumpers.

Or if you need to be able to change it on the fly and don't want external padding, you could consider re-purposing the rear-mounted polarity-switch to drive a relay that switches the pads (I wouldn't route the input-signal itself back & forth)

Bye

 

[Indie Air]

Hello there,

i just purchased this preamp and i would like to do the pad modification to allow it to be controlled by the phase switch. Is it Possible?
Any clue how to do it?

 

[clintrubber]

Like mentioned two posts up, a relatively easy schematic could realize a relay that's located near the pad-resistors and the polarity switch controls the relay (those things called phase switch are in fact usually polarity-flipping switches, but that's another topic).


Might be best though not to do this for all channels (you may not need it on all eight either, but it depends of course on what you're recording), and if you do, perhaps better take a more sturdy external adapter (same Volts, higher Amperage).


Don't know your level of DIY-experience (your first post, welcome here!  ), so if a switch driving a relay is relatively new to you I'd suggest to keep the box closed and simply realize all your padding needs externally (have a look at the nice Uneeda-article on padding: http://www.uneeda-audio.com/pads/).

Regards