Bypassing caps? How about with wire?

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This is a follow-on thread to the recent "Bypassing Caps" thread.

I too have a Soundcraft 200 series console waiting to be gone through. (A
reasonable beast to work on; lots of room on the boards and Harmon was
good about emailing the schematics.)

My plan has been to replace the TLO7x chips with better opamps that also
have zero (or darn near zero, <1mV) offset regardless of rail asymmetries,
and then hopefully getting rid of many coupling caps. (The OP275 is one
such device that usually holds zero offset under most power conditions.)

I've gone back and looked at several old threads related to this topic;
some have been quite interesting, but here are some additional questions:

Several threads had someone who was "about to" rechip a console, but I
didn't see any follow up as to which chip they finally liked best, or how
things turned out.

My current choices, based in part on personal experience (OP275) and also
several excellent posts by Monte McGuire (among others; sorry for not
mentioning all the great posters by name), are the OP275, 2604, and
OPA2132. There are other possibilities, but the prices start getting
rather high.

Any more recent thoughts on the best amps to use? My application is
primarily acoustic and classical recording/mixing; I'm not after a
particular "sound".

I will be adding power decoupling at each chip and will be watching
for oscillation, and am buying new linear power supplies. I will be upping
the wattage of the current limit resistors in each module to accommodate
the hungrier chips.

Any cautions about replacing coupling caps with a piece of wire? (PS
failure is one concern, but supposedly chips such as the 275s will
continue with zero offset even with a rail gone. I do plan to run
redundant supplies, coupled through diodes.)

Obviously I'll leave in the mic input couplers (pres are transformerless),
and perhaps one or two where the signal interfaces to the outside world,
but this console seems to have a large number of coupling caps in the
signal path.

Thanks for any new advice. I will make a report when this project is
concluded (the plan also includes adding EQ bypass switches in several
places and insert pre-post switching -- lots to do).

Frank
--
.
 
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While I haven't rechipped & "de-capped" entire consoles, I do agree entirely with eleminating
coupling caps entirely where possible, replace with polypropylene and/or styrene film caps
where needed & as physically possible, and bypassing fresh Nichicon Muse lytics with styrene
films where there's not enough room for whole film.
I would strongly advocate using the OPA2134PA, with the OP275GP being my next choice.

--
Stephen Sank, Owner & Ribbon Mic Restorer
Talking Dog Transducer Company
http://stephensank.com
5517 Carmelita Drive N.E.
Albuquerque, New Mexico [87111]
505-332-0336
Auth. Nakamichi & McIntosh servicer
Payments preferred through Paypal.com
"Frank Stearns" <franks.pacifier.com@pacifier.net> wrote in message
news:10g3ss83vot7d34@corp.supernews.com...
> This is a follow-on thread to the recent "Bypassing Caps" thread.
>
> I too have a Soundcraft 200 series console waiting to be gone through. (A
> reasonable beast to work on; lots of room on the boards and Harmon was
> good about emailing the schematics.)
>
> My plan has been to replace the TLO7x chips with better opamps that also
> have zero (or darn near zero, <1mV) offset regardless of rail asymmetries,
> and then hopefully getting rid of many coupling caps. (The OP275 is one
> such device that usually holds zero offset under most power conditions.)
>
> I've gone back and looked at several old threads related to this topic;
> some have been quite interesting, but here are some additional questions:
>
> Several threads had someone who was "about to" rechip a console, but I
> didn't see any follow up as to which chip they finally liked best, or how
> things turned out.
>
> My current choices, based in part on personal experience (OP275) and also
> several excellent posts by Monte McGuire (among others; sorry for not
> mentioning all the great posters by name), are the OP275, 2604, and
> OPA2132. There are other possibilities, but the prices start getting
> rather high.
>
> Any more recent thoughts on the best amps to use? My application is
> primarily acoustic and classical recording/mixing; I'm not after a
> particular "sound".
>
> I will be adding power decoupling at each chip and will be watching
> for oscillation, and am buying new linear power supplies. I will be upping
> the wattage of the current limit resistors in each module to accommodate
> the hungrier chips.
>
> Any cautions about replacing coupling caps with a piece of wire? (PS
> failure is one concern, but supposedly chips such as the 275s will
> continue with zero offset even with a rail gone. I do plan to run
> redundant supplies, coupled through diodes.)
>
> Obviously I'll leave in the mic input couplers (pres are transformerless),
> and perhaps one or two where the signal interfaces to the outside world,
> but this console seems to have a large number of coupling caps in the
> signal path.
>
> Thanks for any new advice. I will make a report when this project is
> concluded (the plan also includes adding EQ bypass switches in several
> places and insert pre-post switching -- lots to do).
>
> Frank
> --
> .
 
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"Frank Stearns"

> I too have a Soundcraft 200 series console waiting to be gone through. (A
> reasonable beast to work on; lots of room on the boards and Harmon was
> good about emailing the schematics.)
>
> My plan has been to replace the TLO7x chips with better opamps that also
> have zero (or darn near zero, <1mV) offset regardless of rail asymmetries,
> and then hopefully getting rid of many coupling caps. (The OP275 is one
> such device that usually holds zero offset under most power conditions.)
>

** Be aware of the input bias current for the (bi-polar) OP275 is up to
400 nA. If the input resistance is say 100kohms - this means 40 mV input
offset is possible. The input bias current of the humble (bi-fet) TL072 is
about 1000 times less.


>
> Obviously I'll leave in the mic input couplers (pres are transformerless),
> and perhaps one or two where the signal interfaces to the outside world,
> but this console seems to have a large number of coupling caps in the
> signal path.
>


** It actually has remarkably few ( per channel) - they are mostly only
fitted prior to switches and pots / faders to eliminate clicks and noise
when rotated / moved. Bi-polar electros do this job very well without the
THD issues at low frequencies of polarised electros.





............. Phil
 
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Phil Allison <philallison@tpg.com.au> wrote:
>"Frank Stearns"
>
>> I too have a Soundcraft 200 series console waiting to be gone through. (A
>> reasonable beast to work on; lots of room on the boards and Harmon was
>> good about emailing the schematics.)
>>
>> My plan has been to replace the TLO7x chips with better opamps that also
>> have zero (or darn near zero, <1mV) offset regardless of rail asymmetries,
>> and then hopefully getting rid of many coupling caps. (The OP275 is one
>> such device that usually holds zero offset under most power conditions.)
>>
>
> ** Be aware of the input bias current for the (bi-polar) OP275 is up to
>400 nA. If the input resistance is say 100kohms - this means 40 mV input
>offset is possible. The input bias current of the humble (bi-fet) TL072 is
>about 1000 times less.

Much as I dislike Phil's quoting style, he has a point here. I'll also
put in a good word for the sound quality of the TL072, which is really
not bad at all in a well-designed circuit.

I suggest you put sockets on one channel strip, then try a couple different
types of ICs in different spots and get a sense of how they sound and what
they do. BUT, if your intention is to later eliminate coupling capacitors,
you may want to be careful about offset of op-amps.

You may find that the TL072 is the best-sounding chip in at least some of
the locations. It's certainly among the best bifet chips that you can get
these days now that the Motorola stuff is all discontinued.

Do you care if the controls pop? If you can live with clicking and popping,
say when you enable and disable the EQ, you can live with substantially more
DC offset than the console was originally designed with. Do keep DC off of
the pots, of course. Of course, if you're using it as a PA console, the
popping is a major problem....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
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"Scott Dorsey" <kludge@panix.com> wrote in message
news:cdto5r$mu2$1@panix3.panix.com...

> You may find that the TL072 is the best-sounding chip in at least some of
> the locations. It's certainly among the best bifet chips that you can get
> these days now that the Motorola stuff is all discontinued.

Be careful about loading, though; the TL07x has an insipid output stage, and
it hates driving anything under 10k (don't forget to include the feedback
network when you calculate total load). It's really happiest with a 50k load
or higher.

Peace,
Paul
 
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"Paul Stamler" <pstamlerhell@pobox.com> wrote in message
news:7gvMc.316443$Gx4.46615@bgtnsc04-news.ops.worldnet.att.net
> "Scott Dorsey" <kludge@panix.com> wrote in message
> news:cdto5r$mu2$1@panix3.panix.com...
>
>> You may find that the TL072 is the best-sounding chip in at least
>> some of the locations. It's certainly among the best bifet chips
>> that you can get these days now that the Motorola stuff is all
>> discontinued.
>
> Be careful about loading, though; the TL07x has an insipid output
> stage, and it hates driving anything under 10k (don't forget to
> include the feedback network when you calculate total load). It's
> really happiest with a 50k load or higher.

Not to mention the opportunities for latch-up.

http://focus.ti.com/docs/prod/folders/print/tl072.html seems to claim that
the current parts are "latchup-free". That's a tacit admission that there
was once a problem. I wonder when they fixed it?
 

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"Stephen Sank" <bk11@thuntek.net> wrote in message news:<cdub0d$uco$1@reader2.nmix.net>...
> While I haven't rechipped & "de-capped" entire consoles, I do agree entirely with eleminating
> coupling caps entirely where possible, replace with polypropylene and/or styrene film caps
> where needed & as physically possible, and bypassing fresh Nichicon Muse lytics with styrene
> films where there's not enough room for whole film.
> I would strongly advocate using the OPA2134PA, with the OP275GP being my next choice.
>

How about somebody do this to say one channel only and then post a
before and after pic of a waveform or spectrum analyzer or other
measurable difference you think this makes...

Lets see a spectun alayzer pic of low frequency distortion caused by
electrolytic caps and see it go away when the cap is replaced. (And
I'm not talking about replacing a defective cap that is leaky causing
a shift of the bias point. If you want to tell me that electolytic
caps have a poor reliability record, OK, but I would ask that you show
me evidence of a non leaky electrolytic cap causing distortion.)

Lets get some science going here.

Mark
 
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Paul Stamler <pstamlerhell@pobox.com> wrote:
>"Scott Dorsey" <kludge@panix.com> wrote in message
>news:cdto5r$mu2$1@panix3.panix.com...
>
>> You may find that the TL072 is the best-sounding chip in at least some of
>> the locations. It's certainly among the best bifet chips that you can get
>> these days now that the Motorola stuff is all discontinued.
>
>Be careful about loading, though; the TL07x has an insipid output stage, and
>it hates driving anything under 10k (don't forget to include the feedback
>network when you calculate total load). It's really happiest with a 50k load
>or higher.

Hmm... how is the output stage biased? Is the current-drive poor because
the output transistors are too small, or because they are biased way linear?

The Motorolas had no current drive ability to speak of, since the output
stage was running into class A.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
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Thank you, gentlemen, for many excellent ideas and observations regarding
rechipping and decapping my old Soundcraft 200.

This project is still 30-60 days out; will try to post some "here's what I
finally did" followups then.

Frank
--
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One of the most enormous failings of the audio electronics industry through the years has been
a direct result of people like you, Mark. "If it looks good on paper(and on test equipment),
it has to sound good." One of the WORST sounding power amps I have ever heard was the Yamaha
M70. THD & TIM were rated 0.002% from 250mw to 250w at 8, 4 or 2 ohms. Slew rate 200V/us.
S/N 105db below rated output. Drew an absolutely perfect squarewave at any audio frequency &
any power level. But the amp, with music, sounded absolutely HORRIBLE. Regardless of speaker,
preamp, cables or any other peripherals, the upper treble was downright screechy, and despite
the apparently lively treble, detail in the high treble was obsured badly. When I switched
from that amp(which I regrettably owned for a while) to a much older McIntosh MC2105, which is
also a transistor amp, and which has far "worse" measured performance in every way, it was such
a VAST improvement in sound in every describable way, I couldn't believe I'd ever been able to
stand listening to the Yamaha.
In short, if you are actually trying to assert that any non-defective electrolytic sounds the
same as any other non-defective electrolytic, you are either completely lacking in actual
listening experience, have quite a lot of hearing loss, or are self deluded. Even
*manufacturer's* published data will plainly spell out that different grades of lytics can have
very different ESR curves, with most published ESR curves showing very significant upswing of
resistance well within the audio band, often even starting below 10kHz. If you think such
differences ALONE aren't enough to audibly effect the sound of lytics used as coupling caps,
then you defy even your own "If it looks good on paper, it must sound good" apparent mentality.
BTW, where did I mention "low frequency distortion" in my previous post? Not that there ain't
any with lytic coupling, but I did not state anything to that effect.

--
Stephen Sank, Owner & Ribbon Mic Restorer
Talking Dog Transducer Company
http://stephensank.com
5517 Carmelita Drive N.E.
Albuquerque, New Mexico [87111]
505-332-0336
Auth. Nakamichi & McIntosh servicer
Payments preferred through Paypal.com
"Mark" <makolber@yahoo.com> wrote in message
news:3367f36e.0407241657.6bd385fb@posting.google.com...
> "Stephen Sank" <bk11@thuntek.net> wrote in message news:<cdub0d$uco$1@reader2.nmix.net>...
> > While I haven't rechipped & "de-capped" entire consoles, I do agree entirely with
eleminating
> > coupling caps entirely where possible, replace with polypropylene and/or styrene film caps
> > where needed & as physically possible, and bypassing fresh Nichicon Muse lytics with
styrene
> > films where there's not enough room for whole film.
> > I would strongly advocate using the OPA2134PA, with the OP275GP being my next choice.
> >
>
> How about somebody do this to say one channel only and then post a
> before and after pic of a waveform or spectrum analyzer or other
> measurable difference you think this makes...
>
> Lets see a spectun alayzer pic of low frequency distortion caused by
> electrolytic caps and see it go away when the cap is replaced. (And
> I'm not talking about replacing a defective cap that is leaky causing
> a shift of the bias point. If you want to tell me that electolytic
> caps have a poor reliability record, OK, but I would ask that you show
> me evidence of a non leaky electrolytic cap causing distortion.)
>
> Lets get some science going here.
>
> Mark
 
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"Scott Dorsey" <kludge@panix.com> wrote in message
news:cduoa6$s68$1@panix3.panix.com...
> Paul Stamler <pstamlerhell@pobox.com> wrote:
> >Be careful about loading, though; the TL07x has an insipid output stage,
and
> >it hates driving anything under 10k (don't forget to include the feedback
> >network when you calculate total load). It's really happiest with a 50k
load
> >or higher.
>
> Hmm... how is the output stage biased? Is the current-drive poor because
> the output transistors are too small, or because they are biased way
linear?

IIRC (it's been years since the last set of lab tests I did), the output
transistors are biased quite low. The harmonic distortion was ugly and
spiky-looking, much nastier than, say, an OPA604 or OPA134 (these are the
one-amp-to-a-package versions of the 2604 and 2134). As Monte noted, they're
also quite susceptible to high distortion when driven by high impedances at
high levels, in voltage-follower situations.

At lower impedances, driving higher impedance loads, they can be decent.
Unlike some other users, I've never had any problem with latchup. But you do
have to be careful about how you apply them.

Peace,
Paul
 
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"Arny Krueger" <
> "Paul Stamler"
> >
> > Be careful about loading, though; the TL07x has an insipid output
> > stage, and it hates driving anything under 10k (don't forget to
> > include the feedback network when you calculate total load). It's
> > really happiest with a 50k load or higher.


** The TL072 in fact drives loads down to around 2 kohms with 0.001 % THD.

See: http://www.dself.dsl.pipex.com/ampins/webbop/072.htm

Several other op-amps are detailed on similar pages at the same site -
well worth a read.


> Not to mention the opportunities for latch-up.
>
> http://focus.ti.com/docs/prod/folders/print/tl072.html seems to claim that
> the current parts are "latchup-free". That's a tacit admission that there
> was once a problem. I wonder when they fixed it?


** The TL0xx series op-amps were described by TI as "latch up free" from
the day they were released - I have a 1977 brochure which states that.

However, they will phase invert the output if the inputs are driven close
to a supply rail making for a *very nasty* overdrive sound.




............... Phil
 
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In article <2mgj2qFmm4egU1@uni-berlin.de>,
"Phil Allison" <philallison@tpg.com.au> wrote:

> "Arny Krueger" <
> > "Paul Stamler"
> > >
> > > Be careful about loading, though; the TL07x has an insipid output
> > > stage, and it hates driving anything under 10k (don't forget to
> > > include the feedback network when you calculate total load). It's
> > > really happiest with a 50k load or higher.
>
>
> ** The TL072 in fact drives loads down to around 2 kohms with 0.001 % THD.

If you think that's good enough, then have at it. Chips like the
OPA2132 can do a lot better and the end result sounds that way too.
Also check the THD at something other than 1KHz. It's not so pretty as
you go further up.

One of the more annoying aspects of a JFET input amp is that it ends up
producing a lot of second harmonic when it is asked to run with lots of
input signal presented as a high impedance common mode signal, basically
when it is run as a noninverting stage with a high source Z, something
you'd think a JFET would be perfect for.

IIRC, Walt Jung built a circuit that bootstrapped the op amp's substrate
(V-) with the common mode input voltage to nullify the effects of the
nonlinear drain to substrate capacitance (that causes the common mode
problems) and a bunch of distortion went away. The other solution is to
use a low source Z, in which case you don't need a JFET input in the
first place.


Regards,

Monte McGuire
monte.mcguire@verizon.net
 
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On Sun, 25 Jul 2004 05:09:02 GMT, Monte McGuire
<monte.mcguire@verizon.net> wrote:

>IIRC, Walt Jung built a circuit that bootstrapped the op amp's substrate
>(V-) with the common mode input voltage to nullify the effects of the
>nonlinear drain to substrate capacitance (that causes the common mode
>problems) and a bunch of distortion went away. The other solution is to
>use a low source Z, in which case you don't need a JFET input in the
>first place.

Yet another is to cascode the input diff pair(s). Not too easy
for a given monolythic op-amp, natch.

Chris Hornbeck
"Vote or Die" - P. Diddy
 
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"Monte McGuire" <monte.mcguire@verizon.net> wrote in message
news:monte.mcguire-6A1C1D.01090525072004@news.verizon.net


> IIRC, Walt Jung built a circuit that bootstrapped the op amp's
> substrate (V-) with the common mode input voltage to nullify the
> effects of the nonlinear drain to substrate capacitance (that causes
> the common mode problems) and a bunch of distortion went away. The
> other solution is to use a low source Z, in which case you don't need
> a JFET input in the first place.

This seems to be described in

http://www.analog.com/UploadedFiles/Application_Notes/742022599AN232.pdf

The primary means of dealing with this problem that was recommended was to
drive both inputs of the opamp with similar source impedances.
 
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makolber@yahoo.com (Mark) writes:

> "Stephen Sank" <bk11@thuntek.net> wrote in message news:<cdub0d$uco$1@reader2.nmix.net>...
>> While I haven't rechipped & "de-capped" entire consoles, I do agree entirely with eleminating
>> coupling caps entirely where possible, replace with polypropylene and/or styrene film caps
>> where needed & as physically possible, and bypassing fresh Nichicon Muse lytics with styrene
>> films where there's not enough room for whole film.
>> I would strongly advocate using the OPA2134PA, with the OP275GP being my next choice.
>>

> How about somebody do this to say one channel only and then post a
> before and after pic of a waveform or spectrum analyzer or other
> measurable difference you think this makes...

Get theee to a library! Wireless World did this, and more some years ago.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
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"Monte McGuire"
> "Phil Allison" :
> > "Paul Stamler"
> > > >
> > > > Be careful about loading, though; the TL07x has an insipid output
> > > > stage, and it hates driving anything under 10k (don't forget to
> > > > include the feedback network when you calculate total load). It's
> > > > really happiest with a 50k load or higher.
> >
> >
> > ** The TL072 in fact drives loads down to around 2 kohms with 0.001 %
THD.
>
> If you think that's good enough, then have at it. Chips like the
> OPA2132 can do a lot better and the end result sounds that way too.


** Got DBT results to prove that ??

Thought not.


> Also check the THD at something other than 1KHz. It's not so pretty as
> you go further up.


** 0.003% at 10 kHz with a 3.3k ohms load sure aint ugly.


>
> IIRC, Walt Jung built a circuit that bootstrapped the op amp's substrate
> (V-) with the common mode input voltage to nullify the effects of the
> nonlinear drain to substrate capacitance (that causes the common mode
> problems) and a bunch of distortion went away. The other solution is to
> use a low source Z, in which case you don't need a JFET input in the
> first place.


** As I already pointed out, the very low input bias current is major plus
with the TL072 almost 10,000 times less than good bi-polar op-amps like the
5532. If you are after low *output* offset voltages then fet op-amps are
the way to go.

The DC source resistance seen by the inputs of an op-amp is a seperate
issue from the signal's AC source impedance.



............ Phil
 

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"Stephen Sank" <bk11@thuntek.net> wrote in message news:<cdv52t$j08$1@reader2.nmix.net>...
> One of the most enormous failings of the audio electronics industry through the years has been
> a direct result of people like you, Mark. "If it looks good on paper(and on test equipment),
> it has to sound good." One of the WORST sounding power amps I have ever heard was the Yamaha
> M70. THD & TIM were rated 0.002% from 250mw to 250w at 8, 4 or 2 ohms. Slew rate 200V/us.
> S/N 105db below rated output. Drew an absolutely perfect squarewave at any audio frequency &
> any power level. But the amp, with music, sounded absolutely HORRIBLE. Regardless of speaker,
> preamp, cables or any other peripherals, the upper treble was downright screechy, and despite
> the apparently lively treble, detail in the high treble was obsured badly. When I switched
> from that amp(which I regrettably owned for a while) to a much older McIntosh MC2105, which is
> also a transistor amp, and which has far "worse" measured performance in every way, it was such
> a VAST improvement in sound in every describable way, I couldn't believe I'd ever been able to
> stand listening to the Yamaha.


Why did the Yamaha sound horrible Stephen. Were you able to make any
measurment that would account for what you (thought you) heard? Was
it crossover distortion, high frequency intermod, or what, you can't
just say, I hear it but I don't know what it is.

I also understand that some people think their speakers sound better
when they lift the speakers cable off the nylon carpet. Something
about lossy dielectrics.

Science please. If you think you hear something, fine, maybe you do,
but the job isn't done until you identify what it is that you hear.

Mark
 
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On 25 Jul 2004 18:58:33 -0700, makolber@yahoo.com (Mark) wrote:

> you can't
>just say, I hear it but I don't know what it is.

Really? Why not?

Chris Hornbeck
"Vote or Die" - P. Diddy
 
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On Mon, 26 Jul 2004 05:40:10 -0400, "Arny Krueger" <arnyk@hotpop.com>
wrote:

>In each of these situations, the case that the UUT measures good and sounds
>bad is in essence, based on ignorance.

I believe that's what Stephen said, too.

Chris Hornbeck
"Vote or Die" - P. Diddy
 

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