Hi, everyone.
Probably a basic question here. I've seen a couple of op-amp-based preamp and pedal schematics where the feedback loop leg going to ground is arranged as in the image attached, namely as a parallel combination of two RC networks.
What, if any, advantage does this setup have over the more simple approach, with just one resistor and one capacitor?
Thanks in advance for any input.
M.

Hello, everyone.
I've been reading teemuk's book and I came across some info that I'm trying to wrap my head around. On pages 210 and 211, he writes about operational amplifiers, their gain, configurations etc. He states that the resistor tethering the non-inverting input to reference voltage should, ideally, be equal to the parallel combination of the gain-setting resistors.
Now, I've seen a number of diagrams where that Vref resistor is simply a high-value one, probably to set the input impedance high, if I understand that correctly.

So, is it wrong to tie the non-inverting input to Vref with, say, 1M or 470k? What's the tradeoff?
EDIT: Teemuk writes that the value is important "To prevent the bias currents of the input from interfering
with the offset voltage", but what does it mean in layman's terms?
Thanks in advance.
M.

Good day, everyone.
I bought this class-D power amplifier module (pic attached). It came in the mail yesterday and I'm pretty excited to try it out. I was going to use it as a pedalboard amp due to its compact size. Before that happens, however, I'm going to test it to see how it performs.
The spec sheets I found online say it's capable of delivering 60W into 4 ohms at 10% THD. Personally, I take that with a pinch of salt, or even a handful perhaps, because, first of all, it doesn't employ a genuine TI TPA3118. The original chip has 32 pins, whereas this one has only 28, so some people online suggest it might be a relabeled (mislabeled?) TPA3110. No problem for me, though, as long as it behaves well.
I'm going to fan cool it because putting a heatsink on it looks tricky here. Correct me if I'm wrong, anyone.

I have questions, obviously.
I'm going to power it with a laptop charger / PSU capable of providing 20V, 3.25A. Good idea? Bad idea? Acceptable? I'm going to test it with an 8-ohm load.

Thanks for any help.
M.

Hello, everyone.
I've got this old solid-state 2x12 guitar combo from East Germany called Vermona Regent 1040 (schematic attached should anyone need it). Generally, the amp is operational. It does need some care and attention, like a cap job among others, but it works. I'm not too big on the preamp, though, so I figured I'd leave the output section intact and work on the A NEW preamp.
So, my idea was to first build some kind of input buffer with voltage limiting. Having researched the net, I figured I'd try something like in the attached image. So, if anyone more skilled than me could take a look and tell me whether there are any design flaws there, I'd greatly appreciate that.

Thanks in advance,
M.

-EDITED for clarification

Not sure if I'm posting this in the right place.

Anyway, I came across a thread by joecool85 where he wanted to use a relay for switching channels on his amp. To cut a long story short, it was necessary to lower the voltage needed for the relay.
J M Fahey suggested the following:

Quote from: J M Fahey on November 23, 2010, 10:12:58 AMThe series resistor is needed to bring those 19V down to needed 12V.
Value?: measure the DC resistance of the coil, and multiply it by 0.58 .
Example, if your relay coil has 1200 ohms, you'll need a (1200 * 0.58)=696 ohms.

My question is, why 0.58? At first I thought it was because it's the ratio of needed voltage to available voltage, but
12 / 19 = 0.62
Pretty close, and most probably good enough, but not exactly that.
So, what am I getting wrong?

Hello, Forum.
This topic has been inspired by J M Fahey, so, hopefully, he will chime in.
Because I sometimes modify small combos, I started wondering if anyone has ever tried any off-the-shelf midrange or woofer drivers in those. Such speakers can often be bought for peanuts (at least compared to speakers meant specifically for guitar) so they look pretty tempting. Do you have any experience with that?
Thanks in advance,
M.

Hi, everyone.
Firstly, I'd like to state that I wasn't sure where to start this topic because it's not exactly about an effects unit of any kind, but it is meant to switch effects, so here it is. If it's in the wrong place, I'd like to apologize and request that the Moderators move it somewhere proper. Thank you.

I've been thinking about building an effects switcher for my pedalboard to be able to turn on multiple pedals at the click of one footswitch. So, in search of ideas for selecting a given patch, I've found R. G. Keen's article at GeoFex and one by Rod Elliott, among others. Links here:
http://www.geofex.com/article_folders/fxswitchr/fxswitchr.htm
https://sound-au.com/project163.htm
Now, I'm wondering how to overcome the problem of accidentally engaging two (or possibly more) patches simultaneously if I wanted to build a CMOS-based controller. The way I understand R. G. Keen's design (schematic in question), if two footswitches are stomped on, two outputs of the 74HC373 go high, thus turning on two patches. The same is true for most designs provided by Rod Elliott (except the one built around the CD4017 Johnson counter and perhaps the one using SR latches).
I do understand that it'd be easier to use a microcontroller of sorts and make the code something along the lines of:

if input A is HIGH, output A is HIGH else LOW;

and duplicate it for other inputs.
However, I'd like to learn how to do that without turning to digital technology. I am also aware that it will add to the complexity of the circuit which will probably send me down digital path anyway. I'd like to learn nonetheless. Thanks for any suggestions, directions, and explanations.
Have a good one, everyone.
M.

Greetings, oh, collective wisdom.
I've got this solid-state guitar amp from the 1980s. It's operational, or at least it was last time I checked, but it's not quite the kind of tone I like. So, a thought of modifying it popped into my mind.

I was thinking of leaving the power section alone and changing the preamp to the one inspired by Joe Davisson's designs switchable to something else. It's not decided yet.

The amp as it is now uses a bipolar power supply and the preamp I was considering runs on a single PSU.
I'm wondering if it's not going to upset the power amp's PSU rail balance due to the extra current draw, which compared to the PA section is minuscule, I know, but it's still there.
Am I being paranoid here?

Hello, everyone.
First of all, I'm sorry if this question has been asked and dealt with before. I have done some searching, but haven't been able to figure out a straightforward answer. I'm new to solid state amplification. Now, to the point.
I've been looking at Rod Elliott's 40W power amp design and I don't know which components should be matched. I started reading teemuk's book, but for now it's quite a bit to digest, so I'm taking my time.
So, should I look for matched pairs of any devices found here?
The whole article:
https://sound-au.com/project215-p27-revisit.htm

Circuit diagram:
https://sound-au.com/p215-p27-revisit-f6.gif

Thanks for any input.
M.

Hi, everyone, first build-related post of mine   
Soon, I'm going to build Joe Davisson's 4-stage Vulcan preamp using the schematic posted on his website and I have a few questions before I embark on this project.
1. Has anyone tried it?
2. If so, how does it sound? Judging by the number of gain stages, it's a hard rock / heavy metal machine.
3. I'm going to use BC337 transistors because I have a lot of them (not much of anything else tbh). Apart from pinout and biasing, is there anything I should specifically pay attention to?
I'm going to breadboard it first and, of course, I'll keep the forum updated on the progress. That, however, may take a while because adult life is the way it is
Stay tuned.