Thanks Enzo, I placed new resistors and have a decent voltage on the IC's now.  :dbtu:

No problem Phatt, I got my answer, so the thread can go about anything else we can learn from!  :cheesy:

Well, it's a bit too late for regulators now. It's an existing powersupply that I only need to mod slightly, so I just need to replace the dropresistors for another value.

I found out I did the math totally wrong (and too complicated), so here is a new calculation.

The datasheet tells me that one 4558 needs 3,5 mA with a Vcc of +/- 18V, times 2 makes it 7 mA. The voltage needs to drop 9,5V, so the resistor will need to have a value of 9,5 / 0,007 =1357 Ohm. The heat dissipated will be 0,007 x 9,5 = 67 mW. So I'll pick a 1k5 resistor of 0,25W for each rail.

Am I right?

please...?

Ah, so I just got lucky... it sounds great though  :cheesy:

I somehow calculated the resistor in the powersupply the wrong way. The powersupply is a 'traditional' one, with elco's and resistors. Can you help me out calculate the right value of the resistor that will make it (just below) 18V?

I have a 30VAC/18V trafo, after the rectifier I get +/-27,5V DC and I need to get it down to 18V. It powers a preamp with two 4558 chips in it (4 opamps).

So I've installed the buffer at the end of the preamp and changed the powersupply of the preamp so the IC's would all run on +/- 18V. I did the last thing wrong, and measurements told me the there's +/-25V on the IC's. I thought the IC's should be smoked out but turned the amp on anyway.... and it works! 

But how? Datasheet tells me a 4558 can run at +/-18V max.

Thanks Fahey! So only when the opamp is 'isolated' by series capacitors, a new bias is needed.

Hi,

I have a (simple) question about biasresistors. I want to add a simple opampbuffer after a preamp (for impedance matching). The preamp runs on a dual power supply and so does the buffer. Is it necessary to add a biasresistor to ground at the + side of the opamp? And why, or why not?

It seems to me that once the bias is set in a circuit, you're done. But I might be wrong...

@Fahey: yes you're right, building stuff is fun! I've done a few builds over the last year, without too much knowledge, and it worked out great But I'm at a stage now that I want to know what I'm doing and why, for me that's the other part of fun

Actually those questions I ask come from my attempt to design a simple fx-loop myself
Yes voltage swing is an issue as well, I see a lot of fx-loops that have a simple voltagedivider to cut down the voltageswing to a value that is suteable for effects. Diodes are another way to limit the output, maybe nice to combine that by using it as an overdrivestage as well. 

Well, I don't mind a long explanation, as long as it makes things clear. And it does, thanks

I've read the so called 'contradiction' a couple of times and see where I went wrong.

Somehow I really need to understand what's going on in these circuits before I start to build something. I just don't want to simply copy/paste things and hope for the best.

Thanks for the replies, it helps a lot!

@Fahey: I'm a little confused about this:

QuoteRs in parallel with the much lower internal impedance

Quoteit (Re) still has 26 ohms in series with the emitter

It looks like a contradiction to me, or are 'Rs' and 'emitter' different things in your example?

About the source follower: I understand that in real life it's quite impossible to calculate the right Zout. So am I right by saying that in the above example Zout will just be 'very low', in the ballpark of say <100 Ohm? And when there is (for example) some other resistor between source and Vout, this will define the output impedance of the whole gain stage (in parallel with Rs)?

@phatt: well, it's not that you cannot care, does it? When you design for example an fx-loop I think you need to think of the right Zout of your send-fx to get it to work properly.

Besides that, I like bonkaz  :cheesy:

@Roly: thanks for the theory and the equations

"If you want to follow any of these with a pot you can treat it simply as a combination of resistors (and apply ohms Law a lot)."
Do you mean that I need to treat it as a combination with, in your example, R4 (Rload)? So any following resistors I need to calculate as being in series/parallel with R4?


And when the gainstage is a bufferstage (sourcefollower), does the calculation of the Zout work about the same?



So will Zout in this case be: Zout=Rs/2 ?

Hi,

I have a hard time to understand what defines the value of the output impedance of a gainstage? Whatever it is build around an opamp or a FET, etc.

Mostly the inputimpedance of a gainstage is set by a resistor from input to ground. But I read different things about what sets the output impedance. I understand the outputimpedance of a FET/opamp is normally very low. But what happens when you put for example a pot right after the FET/opamp, or a simple voltagedivider or high-/lowpassfilter? What impedance will the next stage see then? And are there simple ways to calculate it?

Thanks!

Roly, thank you very much again
I never thought there would be so much to ask about couplingcaps!

One more thing. I was wondering about how designs with different supplies and reference voltages can match? Like for example a preamp with a dual power supply and a reference of 0V and stompboxes with a single supply and a reference of say 4,5V, or stompboxes with different reference voltages. It would seem to me that they would mess with each others reference voltages and could cause a decrease of headroom and unwanted rail clipping (asymmetrical).

Do couplingcaps also take care of this problem?

Thanks again Roly for extensive reply, very educational indeed

As you may understand, as a starter I find it pleasant to stick to "consistive laws", so I won't do anything stupid  But I understand there are many things to consider and compromises to make.

Reading your reply I tend to follow the same principle as many stompbox-manufactorers (so there is any consistency) and at least have an output cap in my builds. Further more I tend to isolate pots with coupling caps whenever possible. Would that be a good starting point?

Speaking of this, it reminds me that I've build a marshall lead 12 preamp where there's no couplingcap existing at all, none in the whole design, and it works fine. Probably due to the fet-opamps in it.