Re: Monte McGuire's SX-202 mods

[Monte P McGuire]

In article >,
A. & G. Reiswig > wrote:
>Okay, here's a question for those of you who've taken apart an SX-202.
>
>I managed to figure out the key to getting the XLR jacks separated from the
>case, but now I can't seem to get a slothead screwdriver (what I remember
>using before) to engage the little key that locks the jack to the case.
>Sheesh!
>
>Is it an allen screw? Phillips? Do I have to sacrifice an animal before I
>try it?

Flat blade screwdriver, about 2mm or so will do it. A tiny
screwdriver by regular standards... The little cam turns from about
10:00 to 2:00 (or is it the other way around...) You'll feel it fit
into the slot though, so it's not hard to figure out - if it lines up
at 2:00, go to 10:00, and vice versa.

A goofy connector, to say the least, but with the right screwdriver,
it works well.


Regards,

Monte McGuire

[Monte P McGuire]

In article >,
Justin Ulysses Morse > wrote:
>The biggest trouble I've had has been finding a unit and a schematic
>whose circuit and/or part numbers correspond to one another, or with
>Monte's recipe. It's not a terribly complex circuit, but I found this
>to be a source of confusion because Symetrix made some circuit changes
>and totally re-arranged the part numbers, so a few things took a while
>to find.

I found this out while helping George with his project. Although I
have a pretty sizeable pile of 202s around here, I apparently don't
have all varieties of PC boards. They did a lot of tinkering with the
design, and the only schematic on line isn't even the latest one.

>I did replace op-amps in one of my units but never got around
>to doin a lot of the mods. I still plan to, but I don't really hear
>anything wrong with the stock units so I feel no urgent need. When I
>don't want that sound, I use some of my other preamps. Still, I'd love
>to get one of my units totally souped up and someday actually hear this
>glorious clarity I'm supposed to be missing.

The one mod you'll get the most bang-per-buck out of, after replacing
the output chips, is recompensating the SSM2015 chip. It's only three
small caps per 2015 chip, easy to do.


SSM 2015 Compensation:
----------------------------------------------------------------------
The next area for improvement is to change the way the SSM 2015 mike
amp stage is compensated. The SSM 2015 is the heart of the mike amp
and it provides all but 6dB of the gain of the completed circuit. The
SSM 2015 requires three small valued capacitors to set the internal
feedback structure at high frequencies. By changing these values, not
only is the stability of the circuit affected, but the high frequency
distortion and slewing behavior are also greatly affected.

The rev. C boards have less than optimal values, but the rev. G boards
have values that drastically increase HF distortion with no other
benefits. The capacitor betwen pin 1 and pin 13 of the 2015 is the
most critical capacitor, and the rev. G value of 47pF is simply too
large. Changing this to 7.5pF will move the frequency at which
distortion sharply rises up an entire decade from 3-4KHz to over
40KHz. The common mode amplifier capacitor should be increased to
56pF. I do not know why this improves HF distortion, but it does.

These caps are located at both ends of the 14 pin SSM 2015 chip and
look like little orange-ish tan discs with two leads. For rev. C
boards, change C9 and C18 from 47pF to 7.5pF, change C8 and C17 from
10pF to 7.5pF and change C7 and C16 from 47pF to 56pF. For rev. G
boards, change C12 and C29 to from 47pF to 7.5pF, change C13 and C30
from 47pF to 7.5pF and change C16 and C32 from 47pF to 56pF.

In case I got any component legends wrong, make sure these are the
capacitors connected to the SSM 2015 chip after you do the mod: 7.5 pF
between pins 1 and 13, 7.5 pF from pin 1 and ground and 56pF between
pin 6 and pin 7.

Select a quality monolithic ceramic capacitor with .2" radial lead
spacings that uses the NP0 / COG dielectric. You could use a
polystyrene capacitor instead, but it must be noninductively wound and
be connected with short leads to the PC board or it will not function
properly. Note that the NP0 dielectric is a very linear dielectric,
far different than conventional ceramics used in bypass caps. Since
the capacitance is so small, degradation is very unlikely even if the
dielectric weren't so clean, so, in my opinion, there is little
benefit from using anything but a high quality NP0 ceramic capacitor.

In my units, I made the 7.5pF capacitor up out of two series connected
15pF NP0 surface mount (0805 size) chip caps. It takes a little
dexterity and a tiny bit of hookup wire to arrange this, but it can
work out well. Alternatively, through hole parts can easily be used
here too.

To illustrate the benefit of this mod, a stock rev. G with 5532 output
amplifiers but the stock compensation caps will produce almost 1-2%
distortion at 100KHz, whereas the same amp with the recommended
compensation caps will produce less than .01% distortion at 100KHz.
This is measured with the circuit set for 40dB of midband gain, 150
ohm source, 600 ohm load and a -20dBV input. That's +20 into 600 ohms
at 100KHz... at .01% distortion!
----------------------------------------------------------------------


Have fun,

Monte McGuire

[Tom Loredo]

Monte, would you happen to know where to get pots to drop in as
replacements for the stock pots? In my SX202, it seems I was
unlucky and got a pair of pots at opposite ends of the tolerance
range, so when I set them for the same gain (measured with an
analyzer), they have incredibly different front panel settings.
I don't expect them to match exactly, but they aren't even in
the right ballpark. Both channels measure fine in other respects
(freq response, THD), so I'm pretty sure it's just bad tolerance
on a pot.

-Tom

--

To respond by email, replace "somewhere" with "astro" in the
return address.

[Mike Rivers]

In article > writes:

> Monte, would you happen to know where to get pots to drop in as
> replacements for the stock pots? In my SX202, it seems I was
> unlucky and got a pair of pots at opposite ends of the tolerance
> range, so when I set them for the same gain (measured with an
> analyzer), they have incredibly different front panel settings.

I would suspect that there's something else wrong, not the pots,
unless your idea of "incredibly different" is incredibly different
from mine. But before I'd replace pots (unless they're noisy, which
may not be the pots either) I'd check the rest of the preamp. Is the
gain of both channels pretty close at the full gain setting? If they're
straightforward attenuators, perhaps the gain of one stage is different
than the other. If the gain control is in the feedback loop (as I suspect
it might be from another discussion here) then the absolute value of
the pot could indeed affect the overall gain.

If the indicators are only a few degrees apart in the normal operating
range, can you pull one of the knobs off, rotate it to match the other
one, and put it back on the shaft? If there's a set screw on a flat,
you probably can't, but most other knob attachments can be
"calibrated" in this manner.


--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

[R. Foote]

(Mike Rivers) wrote in message news:<znr1076966797k@trad>...

>If you look inside a Great River MP2, you'l see a
> whole bunch of capacitors wired in parallel. Dan told me that he could
> have used one big capacitor instead, but this just sounded better.
> Possibly for the same reasons you're asking about.

Mike

What sort of caps? Polypropylene? I have wondered about this practice,
but usually opt for a large polypropylene...

Any opinions about paralleling, say, 4- 8uf caps for a nominal 32uf
value?

[Mike Rivers]

In article > writes:

> >If you look inside a Great River MP2, you'l see a
> > whole bunch of capacitors wired in parallel.

> What sort of caps? Polypropylene? I have wondered about this practice,
> but usually opt for a large polypropylene...

It's been a few years since I looked, but I think these were
electrolytics, probably making up a large bypass capacitor.

> Any opinions about paralleling, say, 4- 8uf caps for a nominal 32uf
> value?

Only if you have to do that for the sake of availability or making
things fit.


--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

[Monte P McGuire]

In article >,
R. Foote > wrote:
(Mike Rivers) wrote in message news:<znr1076966797k@trad>...
>
>>If you look inside a Great River MP2, you'l see a
>> whole bunch of capacitors wired in parallel. Dan told me that he could
>> have used one big capacitor instead, but this just sounded better.
>> Possibly for the same reasons you're asking about.
>
>What sort of caps? Polypropylene? I have wondered about this practice,
>but usually opt for a large polypropylene...

These were Panasonic HFQ electrolytic caps in a traditionally annoying
part of an amplifier circuit, the part where the bottom of the gain
resistor is coupled to ground. The problem is that when you run a
preamp at high gains, the gain resistor is usually a very low value,
perhaps 20-50 ohms. The problem is trying to couple that to ground
and still get coupling at low frequencies. You need a lot of uF to do
that with 20 ohms or so.

If this were polypropylene, one channel would definitely not fit into
a 1U box. But, then again, since there's no signal voltage across
this cap, electrolytics won't sound bad at all. So, while
polypropylene will certainly last longer, it's not clear that it will
sound better in this particular circuit application.

>Any opinions about paralleling, say, 4- 8uf caps for a nominal 32uf
>value?

No opinions needed - 4 8uF caps in parallel is definitely 32 uF. The
ESR will be 1/4 of each cap alone, but whether that's greater or less
than the ESR of a 32uF is up to you to determine... as is whether ESR
matters to you or not.

As I said above, polypropylene has some advantages, but depending on
the circuit, these may not matter. Electrolytics can be used for
audio quite successfully as long as there's no audio signal voltage
across them. In other words, they have to not be used as a filter
component, only as a coupling component, and they must be made large
enough to assure that no signal will ever develop across them.
Leakage and useful life will still be very different than using a film
cap, but again, this is something for you to work out in the context
of a particular application. Generalities there are useless.


Regards,

Monte McGuire

[Monte P McGuire]

In article >,
Tom Loredo > wrote:
>Monte, would you happen to know where to get pots to drop in as
>replacements for the stock pots? In my SX202, it seems I was
>unlucky and got a pair of pots at opposite ends of the tolerance
>range, so when I set them for the same gain (measured with an
>analyzer), they have incredibly different front panel settings.
>I don't expect them to match exactly, but they aren't even in
>the right ballpark. Both channels measure fine in other respects
>(freq response, THD), so I'm pretty sure it's just bad tolerance
>on a pot.

Incredibly different as in 9 o'clock and 12 o'clock? This pot is
wired as a variable resistor, and there are other resistors involved
in determining the actual gain of the circuit. So, the pot may be
only part of the problem.

Also, the switches used in the 202 are not bulletproof either, so
perhaps the pad is not switching out of the circuit properly? Some
ProGold may help that out...

I replaced my pots with a switch and 12 resistors scaled to give the
gains I wanted. Pots can work, but finding a quality reverse log
taper 2K5 pot is nearly impossible, and even if you find one, a
quality switch and film resistors will certainly work better
electrically. While measuring the 202, I found that sometimes, with
some pots positions, the wiper contact was not good enough to prevent
some odd distortions and noise from happening. A quality switch will
never do this, so I decided to go with that instead.

I used some small Alcoswitch 12 position 1/2" switches (wish I had the
part number handy) and they were only a minor pain to install in the
202. Try to get a switch with gold contacts if you can...


Best of luck,

Monte McGuire

[A. & G. Reiswig]

All,
I'm having some other problems after performing the modification Monte
recommends to couple the preamp chip output to the output amplifiers. This
preamp has three outputs: Channel 1, channel 2, and a summed output. Here's
the text from Monte's description:

"Coupling between the 2015 and output stage:
----------------------------------------------------------------------
The original rev. C boards simply connected the 2015 stage's output to
the output op amp circuit, and this was found to cause stability
problems. Most SX202s will have a 511 ohm resistor retrofitted in
series between the 2015 and the output amplifier to tame some
stability issues caused presumably by output to input coupling. In
the rev G board, this part was designed in and called R78 and R79.

If your unit does not have this retrofit, it should, or else it will
probably suffer from marginal stability and the resulting increase in
distortion. In all units I have seen, this resistor is installed, but
there may be a few early rev. C units that do not have the resistor.
If that's the case, you will have to cut some traces and install the
resistor yourself. I decided to use 432 ohms because it was handy.

As part of the coupling cap modifications, place a small film coupling
cap in series with the resistor to block any DC developed in the SSM
2015 gain stage. If DC is removed here, the coupling caps after the
buffer stage, which have to be much larger to handle low impedance
loads, can be removed. This coupling cap is loaded by the 10K input
resistors of the summed ch 1+2 output stage, (R37, R38 on rev G
boards, R51, R52 on rev C boards), so it must be scaled accordingly.
I decided to use a .22uF film-foil polypropylene cap, so the stock 10K
load would not be appropriate. On some units, I have removed the
summing stage entirely (by removing R37 and R38 or R51 or R52) and
placed a 150K resistor across pins 3 and 5 of the two remaining output
amplifier chips. This yields a single order 4.8Hz highpass filter,
which is only .24dB down at 20Hz."

As I have mentioned before, I learned early on that some of the component
numbers on my Rev. G board do NOT match up with the Rev G numbers Monte has
listed. In this case, I could not see an R37, 38, or 39. Furthermore,
tracing back from the output jacks themselves led me to the conclusion that
the chip numbers (U4, U5, etc.) aren't even the same on my board.

However, I was able to find R78 and R79 (the 510 ohm resistors that follow
the output of the 2015) on my board. Those numbers match. So I lifted one
end of those resistors and put a film cap there.

Then I used the schematic at
http://www.symetrixaudio.com/tech_support/schematics/202_1A0.pdf , and
ASSumed that the resistors that Monte is talking about (R37, 38, 39) are
those same components on the schematic. Again, tracing back from the
outputs, I concluded that the summed output chip in my board is actually U4,
and that the 10k resistors corresponding to R37. 38, and 39 are R42, 52, and
53 on my board. I changed those out to 150k resistors as per Monte's
instructions.

I then jumpered all 6 of the output caps, which on the schematic are C19,
20, 21, 22, 18, and 17.

This is where the trouble begins. Measuring offset relative to ground on
tip and ring of the outputs revealed serious problems: #1 out had -+2.3V on
tip and ring. #2 had +-.23V. The summed out had -+.16V. Something is very
wrong.

I've emailed Monte a few times, and the man is patient beyond description,
but also busy as a beaver. The question I have is this: I *thought* R78 and
R79 on my board coincided best with R46 and R48 on the schematic, leading to
Pin 3 of the non-sum output amps. Is this the right place to put the film
coupling cap, or do you want it *before* the split (i.e. immediately after
the output of the 2015)? If the latter, I'm going to have to figure out a
way to break into that trace, I guess. But I don't know what else to do. I
can always put it back to stock, though! ;-)

George Reiswig
Song of the River Music

[maxime]

Hello Folks
This is my first post over here....
I am replying to your posts you about some detail upon a "symetrix sx202" mod. I saw this great article about the upgrade and just finished the wiring of the upgrade today.
I was testing it before feeding it with a real microphone. I was a bit surprised by the value of the phantom power being only 43.4 volts, perfectly symetrical though . I started to check around the schematics and found that some 43volts (1n5206) Zener was connnected at the base of the TIP29a transistor, explaining the low value maybe...
My question is the following: there some compromise in the design, in my preamp or is the 43 volts value perfectly acceptable ? is there something wrong in my particular unit?


Best wishes

Thank you

Maxime Bodson (from Brussels)