Ross playing up voltages included.

[Papa_lazerous]

Ok I built my ross clone using the tonepad schematic I have used 1% resistors and 2N3904 trannys

The effect works to a fashion and I thought it was ok as I'd never played through a compressor before.  I've heard others now and its clear this one isnt working properly.  I found the posted voltages on Fuzz central I deviate in only a few places and for the life of me I cannot find out why.

I am only listing the voltages that are wrong...

Q1

B = 1.35v

pin2 + 3  = 3.9v

everything else is on the money give or take a tiny amount.  I dont understand how the voltage on Q1 can be so low at the base but seem correct at C & E.  Looks like a Vb problem but I checked everything.  I cant get 2.1V at the base. I get about 2.9v between the 56k and 27k which I would say is perfect.  but from there on in I dont know.

I ripped it all off the bread board and started again 3 times I get the same voltages.

I have tested all the trannys they are all ok.  I got an analyser all showed similar gian all showed 0.8v forward voaltage drop and all showed zero leakage.  I am stumped.

If I havent included enough here please let me know I will add to it.  I cant see how I am not doing this properly but at the same time something is wrong for sure

[Papa_lazerous]

wow 24 reads and no replys.  Someone must have a suggestion?

[gez]

Hallo Daaave.  (sorry)

Link to schematic wouldn't go amis. 

[Papa_lazerous]

ok here's the schematic over at tonepad. 
http://www.tonepad.com/getFile.asp?id=9

I dont get why the voltage is low though at those points as everything on the schematic is what I have done.  Most confusing

[gez]

I'll have a look later tonight if nobody else posts a reply (will need to print it off and scrible in values to understand things better).

[Papa_lazerous]

Thanks Gez.

Just for reference I tested all my trannys on this toy http://www.maplin.co.uk/Module.aspx?ModuleNo=31538&criteria=analyser&doy=18m12 so I know that they are fine.

Across the voltage divider set by the 56k & 27k I get perfect voltage.  from there there are 2 470k resistors in series going to the base of Q1.  Thats where the voltage comes out too low.   I dont know how to apply Ohms law to actually calculate what I should be getting etc...

I cant understand it as the tranny is good its the one specified its correctly orrientated and the components leading up to it are spot on.  When I re built the circuit I started off by just putting in the voltage divider for vbias and the Q1 tranny and the 2 10k resistors one to c and one to e.  The voltage should be correct there without anything else connected but it wasnt.  The same goes for the voltage at pin 2 and 3 of the IC I tried building the circuit little by little and checking as I went and there is no rason I can think of that the voltage is low there I am doing things correct.  Its made worse by building it on breadboard as there wont be a breadboard layout to check against

[gez]

Presumably the correct voltages are posted somewhere, though I haven't searched for them and still haven't gone thru the schematic in any detail yet.  Wanted to ask this first:

If the emitter of Q1 is at the right voltage (yes?), then the dodgy base reading might just be your meter's impedance pulling it down.

If that is the case, then the same might be true of the OTA's inputs (if its buffer's emitter - Q2 - is at the right voltage).

Are you getting any compression at all?

[Papa_lazerous]

Yeah I do get some compression I'm not sure I am getting full function out of it though. 

You may have a point about the meter pulling it down I know between Q3 & Q4 where the 150k resistor feeds into the 10uF cap and also the collector of the the two trannys that when I measure the voltage it is spot on where it should be for like a split second but as I hold the probes on the voltage drops.  I figured that the meter was drawing all the juice out of the 10uF.  This could be a similar situation.  its there another way of checking it? as other people are getting correct voltae readings.

oh the voltages are herehttp://fuzzcentral.ssguitar.com/ross.php

[gez]

is there another way of checking it? as other people are getting correct voltae readings.

Buffer the point you wish to measure.  Take a single op-amp, preferably FET input, wire up the power connections then connect the output to the - input.  Now connect the + input to the point you wish to measure and take the measurement with you meter from the amp's output.  If you don't have a single amp then wire up both sides of a dual as above and take the measurement from either amp's output.  Wire everything up before you apply power.

Yeah I do get some compression I'm not sure I am getting full function out of it though. 

I still haven't worked thru the schematic yet, but just glancing at some of the values it does suggest your meter is the culprit and that the circuit is working.  Does you guitar have single coil pickups?  This could just be a case of a weak signal not providing enough drive for the envelope follower to do its thing, in which case that section will need tweaking a little.

[MartyMart]

Put a fresh battery in the meter first  !!
Mine was doing all kinds of "wierd" stufff, til I replaced the two "AAA"s in it 

.... just a thought ....

[gez]

Put a fresh battery in the meter first  !!
Mine was doing all kinds of "wierd" stufff, til I replaced the two "AAA"s in it 

Possibly, but more likely the impedance of the multimeter is the problem.  Ra is 1M.  When you connect a meter to the base of Q1, the meter's input impedance acts as a resistor to ground and forms a divider with Ra.  Some cheap meters only have 1M input impedance which is, therefore, going to pull the base of Q1 down (you've basically divided down Vb by two).

Likewise, the input impedance will be in parallel with those two 1M resistors to ground (plus whatever resistance is on the trim) which bias the OTA inputs, so will pull them down.

Q1 is only acting as a buffer so I wouldn't worry too much about it.  If the source is at the correct voltage and it's passing signal, forget about it. 

[Papa_lazerous]

gez you are spot on the money with what you say,  I blame the meter.  I did allot of messing around and can say conclusively the readings are wrong. 

I have come to the conlcusion that it is working ok but needs some tweaks to suit my style of playing.

[gez]

Never built this, but I'm guessing that (slightly) reducing the value of the resistor connected to the collectors of Q3 & Q4 - the 150k -  should make things a little more friskier.  Either that or reduce the 27k to the Iabc pin?  If you do the latter, don't reduce it too much or you'll fry the chip (try 24k/22k)*

*  Hold on, I'll check the data sheet (can't remember what these chips can take)

[gez]

Data sheet says absolute max 2mA, though it would be unwise to take it anywhere close to that amount. 

[TELEFUNKON]

10k is O.K.

for 9V pedals.

[Papa_lazerous]

10k is O.K.

for 9V pedals.

Did I miss something?

[gez]

10k is often a recomended safe minimum value for the resistor to the Iabc pin.  Any less and there's a danger of damaging the chip (though in theory you can use a lower value resistor with 9V supply).

[Jay Doyle]

That bias point is feeding a BJT and the 1Meg of resistance doing so is going to cause the BJT to really drop the current through that 470k/470k string so I don't think you have a problem.

What is the issue soundwise? How does it differ from what you expect or have heard in other examples of how it should be?

Please post all of the voltages so that we can take a look at the whole circuit.

[Papa_lazerous]

I think I got i going all good now,  I used an audio probe on the ciruit to see what was going on that helped allot.  I guess Q1 is used to buffer the signal into the circuit.  how do I calculate the gain on that Transistor?  I am just curious now on how to work it out

[gez]

I think I got i going all good now,  I used an audio probe on the circuit to see what was going on that helped allot.  I guess Q1 is used to buffer the signal into the circuit.  how do I calculate the gain on that Transistor?  I am just curious now on how to work it out

It's a buffer.  Gain is just under unity due to minuscule loss within the transistor (internal resistance).  Google emitter follower or common collector amplifier for explanation.