TC SPARK BOOSTER MINI  [schematic]

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TC SPARK BOOSTER MINI  [schematic]

Postby rcustoms » 07 Oct 2016, 04:14

Hi folks ,last week i trace ,a spark booster mini from a friend of mine.just only for learn how is work.
i triple check de values and the schematics.minus the switching system ,is very complicated, and we can put instead , a 3p3t switch.

spark mini.jpg


the layout , parts and pcb

SPARK MINI PCB.jpg



enjoy.
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 07 Oct 2016, 08:52

Yay! :) Discrete op-amps for the win!!! I love seeing this kind of stuff :horsey:
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Re: TC SPARK BOOSTER MINI

Postby rtp_burnsville » 08 Oct 2016, 12:10

Thanks for posting, nice job!

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Re: TC SPARK BOOSTER MINI

Postby bulldozer1984 » 27 Apr 2017, 13:17

Hey guys,

I have experience modding tube amplifiers but not pedals. I would like to turn this into a tube screamer type boost that rolls off some highs and lows.
Where would be the best places in the circuit to do this ?

I would imagine a 22nF (starting point) capacitor would cut some lows and then a 1nF (starting point) capacitor to ground would be a good place to start with the rolling off of the highs but where should I implement them ?

Thanks fellas..
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Re: TC SPARK BOOSTER MINI

Postby newly » 11 Nov 2017, 16:14

What is the sv1 and sv2? And how it is connected between?
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Re: TC SPARK BOOSTER MINI

Postby rcustoms » 11 Nov 2017, 17:55

newly wrote:What is the sv1 and sv2? And how it is connected between?

gain pot conector ,pin 1 to pin 1,pin 2 to pin 2 pin 3 to pin 3,easy
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Re: TC SPARK BOOSTER MINI

Postby bmxguitarsbmx » 12 Nov 2017, 19:16

bulldozer1984 wrote:Hey guys,

I have experience modding tube amplifiers but not pedals. I would like to turn this into a tube screamer type boost that rolls off some highs and lows.
Where would be the best places in the circuit to do this ?

I would imagine a 22nF (starting point) capacitor would cut some lows and then a 1nF (starting point) capacitor to ground would be a good place to start with the rolling off of the highs but where should I implement them ?

Thanks fellas..


Decrease C5 for more low cut, Increase c8 for more high cut. You could also mess with c1 for some bass trimming.
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 12 Nov 2017, 21:58

bmxguitarsbmx wrote:
bulldozer1984 wrote:Hey guys,

I have experience modding tube amplifiers but not pedals. I would like to turn this into a tube screamer type boost that rolls off some highs and lows.
Where would be the best places in the circuit to do this ?

I would imagine a 22nF (starting point) capacitor would cut some lows and then a 1nF (starting point) capacitor to ground would be a good place to start with the rolling off of the highs but where should I implement them ?

Thanks fellas..


Decrease C5 for more low cut, Increase c8 for more high cut. You could also mess with c1 for some bass trimming.


Also: C6 is in charge of the op amps dominant pole compensation. Increasing that capacitance would decrease the gain/bandwidth product of the op amp - thus rolling off high frequencies earlier.
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Re: TC SPARK BOOSTER MINI

Postby bmxguitarsbmx » 13 Nov 2017, 23:45

Increasing C6 is a creative idea and should be experimented with. Think of C6 as being part of the open loop gain calculation. If C6 is increased, the feedback mechanism will try to compensate for this. At larger and larger values of C6, you will get greater and greater slew-rate limiting. The amplifier cannot provide the current to charge C6 fast enough. So, creating a frequency response with C6 will be unpredictable. Not that it shouldn't be tried! :)
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 14 Nov 2017, 00:04

bmxguitarsbmx wrote:Increasing C6 is a creative idea and should be experimented with. Think of C6 as being part of the open loop gain calculation. If C6 is increased, the feedback mechanism will try to compensate for this. At larger and larger values of C6, you will get greater and greater slew-rate limiting. The amplifier cannot provide the current to charge C6 fast enough. So, creating a frequency response with C6 will be unpredictable. Not that it shouldn't be tried! :)

Au contraire; shaping the frequency response of an operational amplifier with dominant pole compensation is extremely predictable, which is why it is used practically everywhere.
The slew rate however relates directly to the transient response and then in turn also to the highest possible reproduced frequency given a certain closed loop gain and desired output amplitude :)
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Re: TC SPARK BOOSTER MINI

Postby bmxguitarsbmx » 14 Nov 2017, 04:19

It's predictable? What is the formula to predict it? Let's put -3dB @ 1kHz. What is the value of C6?

It is not conventional to shape the frequency response with the same circuitry that is used to compensate the dominant pole. But, as I said, give it a try.
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Re: TC SPARK BOOSTER MINI

Postby bmxguitarsbmx » 14 Nov 2017, 04:24

You do technically have a point about the dominant pole. Adjusting C8 changes the dominant pole. C6 however, like I said- Try it! Slew rate distortion might be the next big thing.
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 14 Nov 2017, 06:41

bmxguitarsbmx wrote:It's predictable? What is the formula to predict it? Let's put -3dB @ 1kHz. What is the value of C6?

Once you calculate your small signal equivalents its trivial to employ the Miller theorem in order to calculate the open loop frequency response. I'll try to give an example, when i get off work :)
Looking back on the last few posts, it looks like we both agree, the dominant pole in the open loop dictates both the GBWP and slew rate in this amplifier. The only thing i disagree on is your statement about dominant pole compensation being unpredictable ;)
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 14 Nov 2017, 06:43

bmxguitarsbmx wrote:You do technically have a point about the dominant pole. Adjusting C8 changes the dominant pole. C6 however, like I said- Try it! Slew rate distortion might be the next big thing.

Also; C8 is in the negative feedback loop, and as such it is affecting the closed loop frequency response, not the open loop frequency response ;)
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Re: TC SPARK BOOSTER MINI

Postby bmxguitarsbmx » 14 Nov 2017, 20:06

You are correct that miller capacitance is involved. You also need to know the current gain of the pnp, the re of the pnp, and the output impedance of the j-fet stage. Once that is all done, it doesn't really matter because the feedback loop will correct the response (to the limits that it can). That is what I call unpredictable.

My basic point is that changing C6 will do nothing but increase slew limiting until the limits of the feedback mechanism are reached.

Like I said, TRY IT! Definitely try it before you recommend it anymore.
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 30 Dec 2017, 12:36

For a fairly accurate estimate, assume the usual textbook conditions for DC analysis; Vbe=0.7V, VT=25mV and Hfe=infinite:
The bias current for T1 and T4 is half of the current generated by T2 and T3:
IC2=IR8=Vbe/R8=7mA
IE1=IE4=IC2/2=3.5mA

To find the gain of the differential stage, we find re and then use the formula for gain in a differential stage with single ended output.
re1=re4=VT/IE1=7.1ohm
AVdiff=RC1/(re1+re4)=7042V/V

Next is the gain for Q1:
VR11=VB4=VB1=VCC*R14/(R3+R14)=4.5V
IEQ1=ICQ1=IR11=4.5mA
reQ1=VT/IEQ1=5.6ohm
AVQ1=R11/reQ1=179V/V

Assuming the parasitic capacitances of Q1 to be negligible, the break frequency is dominated by C6 and T1's output impedance:
RoT1=R5+R12=101Kohm

C6 looks much larger according to Miller's theorem:
C6'=C6*(1+AVQ1)=18nF

Thus the break frequency becomes:
fb=1/(2*pi*RoT1*C6')=88Hz

The open- and closed loop gains below fb assuming the 100Kohm gain pot is all the way up:
Ao=AVdiff*AVQ1=1.26MV/V
Acl=1+P1/R10=11V/V

As the gain bandwidth product is constant, we get
GBWP=fb*Ao=111e6

Closed loop linear bandwidth then becomes:
fb_cl=GBWP/Acl=10.1MHz

If we wanted linear bandwidth to be 1KHz at Acl, we would get:
GBWP=1KHz*Acl=11e3

Then the open loop break frequency becomes:
fb=GBWP/Ao=9mHz

And the compensation capacitor is found:
C'=1/(2*pi*RoT1*fb)=175µF
C=C'/(1+AVQ1)=972nF ~ 1µF
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Re: TC SPARK BOOSTER MINI

Postby grrrunge » 30 Dec 2017, 13:02

bmxguitarsbmx wrote:You are correct that miller capacitance is involved. You also need to know the current gain of the pnp, the re of the pnp, and the output impedance of the j-fet stage. Once that is all done, it doesn't really matter because the feedback loop will correct the response (to the limits that it can). That is what I call unpredictable.

My basic point is that changing C6 will do nothing but increase slew limiting until the limits of the feedback mechanism are reached.

Like I said, TRY IT! Definitely try it before you recommend it anymore.

I don't see any JFET's in this circuit - only a two stage purely BJT based op-amp. Are we talking about the same thing here? ;)

With modern, general purpose, small signal BJT's typical Hfe ranges are so high that the effect of Hfe in terms of voltage gain is less than 0.01%. I find that rather insignificant.

Also, I am well aware that slew rate limiting in the audible spectrum might not be desirable, but to each his own. Setting the effective break frequency at 1KHz seems rather silly, but defining the linear frequency spectrum to somewhere around 12-18KHz would seem reasonable in my opinion, as most guitar related speakers doesn't reproduce signals above 5-10KHz.
On the other hand the 10MHz linear frequency range in the original circuit seems like overkill in that context, as a lower bandwidth will help you achieve a lower noise floor as a result of less amplified high frequency noise. Hence my suggestion to alter the dominant pole - not for frequency response shaping in the audible spectrum, but as a mean to lower the noise floor by another 20dB/decade above the unity gain frequency set by the feedback network :)
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