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Messages - SemiConductive

#16
Trying to understand what you said about R5/6 & C6 not being in the signal path...

So, they are doing some filtering, but the "signal" passes through pin 5(6) of IC1-B? And R5/6 & C6 are not "signal path"?

What I'm a little confused about is how the filtering is operating. I understand how an R/C network filters off treble e.g. a "tone control" (capacitor is grounded, bleeds off high frequencies, resistor limits bleed to cap). But in that scenario, the cap is always bleeding treble to ground. In this case it looks like it's bleeding it around IC1-B (?).

I'm also familiar with using a small cap to filter bass... and that looks a lot like the arrangement of R6 & C6, but again, turning on IC1-B filters more treble, not more bass. So if I carry that logic through, is R6 & C6 filtering (more) bass around IC1-B? So it's not reducing treble, it's adding bass?

Or have I missed this entirely - and R5 is involved in the filtering too... and somehow the grounding through Q1 is pulling off treble? I thought the Q1 circuit was strictly handling the foot (overdrive) switch. Does it do something else?

Thanks,

#17
Update on the hands-on side: Successful test!

I lifted the diodes from ground and put a toggle in there so I could do back-to-back testing.

The diodes were definitely clipping at fairly low signal levels... as soon as the gain pot was set to above minimal. By disconnecting them with the switch, I can get a lot more clean headroom, which is what my goal is for this amp... and as expected.

Turning on the "overdrive" switch in the panel and bringing IC1-B into play bumps the volume fairly cleanly. But with that on and the gain turned up high you still get into distortion territory, again as expected.

There is a little bit of a treble cut when you switch on IC1-B, even at low levels where it stays very clean. Very slight cut, just taking off the upper edge a little. Not unpleasant but a minor change. Perhaps the resistors and caps hanging above IC1-B are doing some filtering when it's on? Or maybe it's the opposite... there's filtering when the signal runs fully through the resistors/caps above with IC1-B shut off and turning it on eliminates that? I'm a little confused by the fact that the caps appear to be "in-line" (not going to ground) and I would think they'd filter bass, if anything, in that configuration.



#18
Thanks. I'll ask you one question, as I'll need to study a bit to ask more intelligent ones. (I'm really light when we get beyond basics and 'following directions').

Why do we invert at IC1B? It it because "that's the way a 4558 rolls"? Or, is it to accommodate what the power amp wants to see and/or another inversion happening there?
#19
Quote from: phatt on April 06, 2022, 09:42:26 PM...

Just a note on the schematic;
Unless I'm missing something??? IC1B is labeled wrong.
Pin 5 is the Positive input and is normally grounded in this configuration,, It looks like it's + & - input labels are flipped. I can't read the label Numbers,, they both read 5 to me.

Phil.

Yeah, this is actually the schematic from the manufacturer too. LOL.

From what I see, you are correct. Pins 3 & 5 are positive inputs. Pins 2 & 6 head to ground.

You're saying they'd normally flip the positive the other way when using both halves of the 4558? I'll look when I'm in there in a few days to see how it's really wired. 
#20
Thanks all.

Makes sense. I had a home-built distortion pedal I made that had a pot just like that... controlling the amount of clipping. I'll probably try that at least as a test to find the balance between clipping and power amp distortion.

Thoughts on the proper value for the pot to reduce LED throughput to zero? 10K? 100K? 250K? I know I can just head to the big side, but if ya'll have a suggestion, I'll start there.

Thanks for the insight on C9. I thought it might be a filter with that resistor, but I'm weak in that regard... and I definitely don't understand that tone stack. Different than anything I've seen or in Duncan's Tone Stack Calculator. Also, I am familiar with the "harsh treble" issue for SS amps! All to common. This one works though - the amp has very nice tone / range.

#21
Thanks for the insight.

I'm OK with lifting them to test the sound... I'm just looking for confirmation that that's an OK thing to do. If they are limiters to subdue the signal of the dual 4558 amps and I'm going to fry the 1875 power amp by letting too large a current in, then I clearly wouldn't want to do it.

Also, I'm not sure what to do with C9 - leave it or lift it with the LED's. I don't know enough to understand, but I know enough to ask first :-).
#22
Circuit link below. Sorry for the poor quality. This amp was not popular and this is the best I've been able to find.

Background: I have some basic understanding of SS components and circuits. But I'm really a newb. Trying to increase my knowledge and perhaps mod this amp.

Signal comes in and through IC A / 4558 and moderated by the gain control POT 1. It then flows down into the other half of of the 4558, IC B (IC B is switchable by panel or footswitch through Q1). From there, the signal runs past a pair of Red LED clipping diodes LD1 & LD2. But, they are unswitched/directly grounded.

Are they always clipping? Or are the Red LEDs chosen because they only conduct over a higher gain signal coming through?

Possible Mod: What I'm thinking is that if the diodes themselves had a switch to ground that I might be able to get additional clean headroom with the primary gain control. But, I'm also thinking that by running the signal hotter (no clipping at all) that perhaps it is going to overdrive the poweramp... and maybe there's not much clean headroom to be added because of that (?). Perhaps the clipping is a "required limiter" or sorts?  Any danger to the other circuit components by switch-controlling the LED's?

Also, there's a capacitor C9 in parallel with the LEDs. Is it shown there simply for circuit drawing convenience in the schematic? Or would it need to be switched along with the LED's if I switch them?



EDIT: Schematic cleaned and waxed: Zoomable circuit diagram: https://i.imgur.com/WjfO8n1.jpg