replacement resistor?

[muizac]

Hi. im building my first pedal, a ts808 style OD. straight off the bat i'ive encounted a couple of issues:
1) i dont have a 510k restistor for the shaping (pre op amp) stage, i've stuck a 470k instead - am i likley to encouter any problems? (i dont suppose i can wire a  470k + 33k together?)
2) The plans are from JD's site - i have a schem called gringo 2.2 that looks pretty similar to RG's and the others, the problem im having is looking at the corralation between the schem and the board layout. (FYI http://www.generalguitargadgets.com/v2/diagrams/ts_lo_808.gif ) My main problem is that after the 510k resistor id expect this is vb - which im guessing could be a point between ground and possitive flows, so i was planning on conecting just after the jumper from IC socket 4 to ground perhaps? The flow however, indicates that after the 510k resitor it passes a 4k7 (r6) then 3x 10k's and bck into the IC etc.  - is this right or am i totally not yet able to read schems proberly?

thanks for your time
muizac

[brrt]

1) 470kOhm in series with 33kOhm makes 503 Ohm. So if you've got space on your board, you could series these two resistors. But i'd use a larger resistor erlier than a lower resistor at this point. Because using lower resistors wil give a lower input impedance = more "tone sucking".

2) Vb = 1.5*Vs, or in words: Vb is half the voltage of the supply voltage. This could be done by putting 2 resistors of the same value in series between supply voltage and ground.

Brrt

[Eric H]

1) 470kOhm in series with 33kOhm makes 503 Ohm. So if you've got space on your board, you could series these two resistors. But i'd use a larger resistor erlier than a lower resistor at this point. Because using lower resistors wil give a lower input impedance = more "tone sucking".

You can put two resistors in series like that --but you won't hear the difference --the 470k is fine. Also in a series circuit the order of components makes no difference to the signal --the signal "sees" 503k either way.

2)

Vb = 1.5*Vs, or in words: Vb is half the voltage of the supply voltage. This could be done by putting 2 resistors of the same value in series between supply voltage and ground.

Brrt

I assume 1.5 is a typo --should be .5*.  The Vb is taken from the mid-point of the 2 10k resistors (R9, R10) which are bypassed to ground by C7. You don't connect Vb to the board --it's taken care of for you by the divider.

-Eric

[muizac]

thanks guys.

im scratching my head a little. How would i know from looking at the schem, that after the 510k resistor where it says vb, that im to conenct it between 2 x 10k resitors, which are grounded from a 47uf ?

Is there a formula to work our what the diviser is / should be?

If i had a higher input resistance of, say 550k, would this give me the opposite of tone sucking ? (tone boosting would be great )

cheers fellas

muizac

[petemoore]

For voltage dividers, used to get 1/2 of supply voltage, I use LM386's.
 Pin 4 of course goes to ground, pin 5 [output] is tied to all the Vb, or 1/2v points, and pin 6 [Vin] goes to battery + lead.
 Very easy and gets very close to 1/2v everytime I use it, which is all the time...now. Thanks RG!

[Eric H]

For voltage dividers, used to get 1/2 of supply voltage, I use LM386's.
 Pin 4 of course goes to ground, pin 5 [output] is tied to all the Vb, or 1/2v points, and pin 6 [Vin] goes to battery + lead.
 Very easy and gets very close to 1/2v everytime I use it, which is all the time...now. Thanks RG!

Pete, The guy's trying to build his first pedal, one thing at a time ok?

-Eric

[Torchy]

TS-808 as a first pedal ??

The 510k resistor will have a Vr next to it (which is shortened Vref or Reference Voltage). All the points marked Vr are connected together, which you can see if you follow the traces on the pcb layout. Sometimes it is called Vb (Bias Voltage).

The 510k isnt an "input" resistance, its to hold the + input of the opamp at a fixed point, and the input signal is superimposed on that reference voltage. If its held to 1/2 supply voltage then the input signal can swing to almost supply voltage and almost ground. Sometimes called "headroom". The input signal travels via the input dc block capacitor (.02uf) and the 1k resistor to the transistor base.

[Eric H]

thanks guys.

im scratching my head a little. How would i know from looking at the schem, that after the 510k resistor where it says vb, that im to conenct it between 2 x 10k resitors, which are grounded from a 47uf ?

You would know if you read the FAQs at the top of this page.
Are you trying to build this on perfboard?

Is there a formula to work our what the diviser is / should be?

There is --and you don't want to bother, at this point The 10k/10k divider is usually used in this type of circuit.

If i had a higher input resistance of, say 550k, would this give me the opposite of tone sucking ? (tone boosting would be great )

No. Once the input impedance is high enough (470k is high enough) there is little to be gained by raising it futher. You would need to double it (1meg) to  have any chance of hearing a difference.There is an explanation of impedance at GEOFEX (link above, again) that will help with this  --read everything at that site, as a matter of fact

-Eric

[muizac]

yeah the ts808 will be my first pedal - its was either that or a EH16 as these are the only pedals that i need to play live.  (although thinking about it, my boss flanger could do with replacing  - where does it stop? )

im using perf board.

I read the FAQ's on aron's site about the vb terms - but i read it as though all connections would be put together - which i assumed would be like the ground wire.

OK - no formula for the time being. would it be safe to conenct all instances of vb to the point just after the 47uf cap? and go between the 10/10

i was sort of joking about the input resitance being higher, but it seems like this is a good thing after all

im going to read gofex tommorow - its 2.30am . cheers

[Eric H]

TS-808 as a first pedal ??

The 510k isnt an "input" resistance, its to hold the + input of the opamp at a fixed point, and the input signal is superimposed on that reference voltage. If its held to 1/2 supply voltage then the input signal can swing to almost supply voltage and almost ground. Sometimes called "headroom". The input signal travels via the input dc block capacitor (.02uf) and the 1k resistor to the transistor base.

Yes, the 510k sets the bias (and I just realized it's biasing the Transistor, not opamp) but the value also affects the input impedance.

-Eric