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...

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!

Hey,

I think it's a simple basic question, but I need to know it because it confuses me more and more  :loco
Sometimes you see two parallel resistors with a coupling cap in between, or two resistors that form a voltage divider but with a coupling cap in between.

Like the inputstage of this schematic for example:


Or the 1k/470n/22k after te 1e gainstage in this schematic:


I was learnt that a coupling cap decouples such resistors and that there is no interaction between them (for DC). So the parallel resistors aren't parallel anymore and the resistors don't form a voltage divider. But sometimes you read differently.

So what is true?

Hi,

My first guitaramp was a Vox Pathfinder that I really liked, but sold it to buy something else (as usual). I'm still thinking of it once and a while, and wonder if I could build the preamp and stick it into my diy combo? I allready found the schematic in this thread (you need to be logged-in to download it).

The thing is, I would only like to build the 'normal' channel of it. So I don't need the boost or the tremolo, nor the poweramp (for I allready have one). I can do an educated guess but I'm not that good at reading schematics to say what parts exactly can be left out and what needs to stay in. Can you help me out please?

Hi,

I'm looking for simple but effective discrete poweramp designs that can be build easily and perform wel in a guitaramp. I did a view diy-builds but I've never build a poweramp myself, most schematics of poweramps seem way too complicated to start with.

Googling around I find schematics like this:



It comes from an 18W audio amp. But since I haven't much theoretical knowledge, I can't tell wether a design like this perfoms well or is stable enough?
Can you give me some advice?

Hi,

You often hear this said/written by many (if not all) guitarists who use or have been using a solid state amplifier in a (live) band situation. They hire a studio/barn with their band and use the PA and gear that's allready in there, or just use their own ss-amp that they bought once for home use. But then they have a really hard time to try to get through the mix and be heard. And when they get through, their gear have never sounded worse than with that amp. And they need at least 3 times more power than with a tubeamp!

Actually this is a wide spread and accepted opinion and has allmost become a definition of ss-amps. But I'm wondering... is it really because of the amp being solid state, or are there other factors involved? I mean, the technology is whidely accepted and preffered in audio equipment and bass-amplifiers, so why is it so bad in combination with electrical guitars?

This topic is meant to bust the myths about solid state guitar amplifiers. So please shoot and share your opinions, experiences, stories and knowledge!

Hi,

I'm planning on building my first small ss-guitaramp. I've bought a LM1875 kit on ebay and want to build a Marshall Lead 12 preamp myself. There is however one thing that confuses me in the Lead 12 schematic, probably due to my limited knowledge.

Here is the schematic: http://music-electronics-forum.com/attachments/2552d1212538675-3005.gif

There is a connection from pin 5 of IC1B back to the ring of the low-input. What does it do, is it a feedbackloop? What confuses me is that actually it's just connected to ground (via the ring). Does this mean that I can just make a connection to ground? Are does it work differently?

Second question is: according to the schematic the MC1458 opamp runs at +/-16V, will it be safe to run it at +/- 18V? That is it's maximum volt rating so I don't know if that's okay.

Hi,

I ordered an LM1875 kit to use in a guitaramp. I have a speaker which I want to use that rates 15 Watt max. To prevent the speaker from blowing out, I don't want the amp to put out more than 15W.

According to the datasheet the LM1875 poors out 20W with a 18V trafo. Is that in theory or is that the real output power? In that case, what trafo do I need to use to keep the power at max 15Watt? 15V?

Hi,

I'm new here and quite new to the diy world. I built myself a ROG Ruby amp with some personal adjustments, now I want to build a "bigger" amplifier based on the LM1875 with a nice and simple pre-amp. Actually all I want the LM1875 poweramp to do is increase (clean) volume, I don't want it to clip or alter my tone. All the toneshaping and clipping will take place in the pre-amp. I prefer this because then I have a basic poweramp, and I only need to mod or swap the preamp to shape my tone.

Now I wonder, how much inputsignal can the LM1875 handle before it starts to clip? Can I find this out with the datasheet?
I like to get the maximum clean power out of the circuit, so I wonder if I need to stick with fet-based preamps (like those ROG ampsims) or if I can mess around with MOSFET's and mu-followers?

Next question is: in some schematics of the LM1875 you'll find an inductor in parallel with a resistor right after the LM1875.
http://www.eleccircuit.com/wp-content/uploads/2007/11/amp20w_lm1875.gif
What does it do?

Thanks!