Does anyone have the schematic for this amp? I can only find one for the LH1000 which is similar but I believe the issue I am having is with the limiter which to me looks like the limiter switch shunts R17 to ground. I think the LH500 is different in this section due it not having two channels to bridge and it would be nice to look at the correct schematic.
Awesome thanks guys!!
For some reason it's easier for me to actually see the real schematic without U9 and U5 and everything else in there, it does indeed go straight to the power amp.
So bare with me here, I am not well versed in compressor tech so I may have a few questions here. Is this feedback compression? It looks to me that the voltage control is taken after the recovery/gain device (U2). I guess I could model the whole thing in LTSpice but if someone doesn't mind filling me in it could help me understand a lot. PRR did a great job labeling all the components and that helps significantly. I just have never had to trouble shoot a compressor before. What's going on is when the limiter is off the amp works as it should, but when the limiter is engaged I get cutting out where it drops and then swells back up. I can post some scope images if it helps.
For some reason it's easier for me to actually see the real schematic without U9 and U5 and everything else in there, it does indeed go straight to the power amp.
So bare with me here, I am not well versed in compressor tech so I may have a few questions here. Is this feedback compression? It looks to me that the voltage control is taken after the recovery/gain device (U2). I guess I could model the whole thing in LTSpice but if someone doesn't mind filling me in it could help me understand a lot. PRR did a great job labeling all the components and that helps significantly. I just have never had to trouble shoot a compressor before. What's going on is when the limiter is off the amp works as it should, but when the limiter is engaged I get cutting out where it drops and then swells back up. I can post some scope images if it helps.
Yes, and yes. 🙂 Closed-loop compression, rather than open-loop.
This usually happens when the attack time constant is very short, and the decay time constant is rather long, and the initial transient (or signal strength) is way too high. This causes the limiter to over-react to the initial signal, and then take a long time to recover.
You don't seem to have adjustable time constants here, though, so my next thought concerns the factory-provided adjustments (
R74/ R153). Do you have access to factory information on the proper way to set these trimpots?
-Gnobuddy
Before you go poking at resistor values which probably worked, replace the electrolytic caps (hint: I added "+" signs). They leak. If the DC is screwed-up, the level detection is bungled. The amp is old enough to be having old-cap troubles, and I don't think there's a buck's worth in this area.
That's the strange thing is the attack and recovery are fixed😕
I do not have any factory information on setting the limit. The only way I could maybe see this being an issue is from the Bass vibrations possibly slowly moving the pot. Another thing I am confused about which I think is a typo is that the schematic says adjust R153 but that is a fixed resistor (47k). R152 is a 200k linear trimmer pot. Is the limit adjustment made to R152 and not R153?
I hate poking around with anything unless I have certainly identified the problem. With that said the caps are cheap enough to try and replace to see if things get better. Also I suspect I could measure the current value of R152 and always return it there if things don't improve after adjusting it.
When I apply a signal the DC at the junction of D15 and D16 modulates the gate of Q5. I am getting -2.6v (at the gate) and when I apply signal it goes positive, the larger the amplitude the more positive it goes and then recovers.
Age, and corrosion or dirt between wiper and carbon track, are possibilities. I've seen trimpots change their resistance setting over time, though the physical position of the wiper remains unchanged.
Looking at this circuit, if your R152 trimpot ages and the wiper completely loses contact with the track, U4-A goes to maximum gain. This would make the limiter hyper-sensitive, and likely to over-react, squashing the life out of a normal-sized signal.
This scenario seems to match quite well with your description of the problem. So my hunch is that this is in fact the problem: R152 wiper has gone open-circuit, or high-resistance, resulting in premature and excessive limiting.
I'm not ruling out PRR's input about old capacitors, though. It looks as though there is a double time-constant acting on the JFET; a fast one (attack, R143/C121), and a much slower one (release, R144/C121; strictly speaking, not just R144, but a combination of R144, R142,R137.)
Now, C121 appears to be a metalized foil cap, not an electrolytic. These sorts of caps usually are quite reliable and don't usually suffer from the aging problems of old electrolytic caps.
So my first suspect is still the sensitivity trimpot, R152. Measure it, just to get an initial value; take a quick photo so you have a visual reference of the original position; give it a clean with an appropriate contact-cleaner, wipe it back and forth a couple of times, re-set to the same physical position as before, see if your problem goes away.
I think you're right. R152 changes the gain of U4-A, which in turn changes the amount of signal needed to get the JFET (Q5) to start conducting.
That sounds entirely normal and correct.
The JFET is normally held off by the -2.6V gate bias, so it does nothing to the audio signal. If the signal gets big enough, the gate voltage goes sufficiently positive for the JFET to start conducting. When it conducts, it acts as a volume control in conjunction with R134/R135, lowering the signal voltage that passes through to the power amp. Limiting it, in other words.
-Gnobuddy
I didn't have time today to look at it again but tomorrow I have a game plan. I will start with the trimpot and do as you say. I forgot to mention that it's a sealed trimpot so I can't spray any deoxit in it and that's why I didn't think it was corrosion but I suppose anything is possible. If that doesn't work I will replace some caps.
Alright that's what I figured.
Got it thanks, that is how I thought it worked and looks normal. I just don't know how much gain U4 is supposed to have. Like I said I will measure R152 first and see what I get and go from there, If it's open I will make note of the position of the trimpot adjustment screw and count how many turns etc....
Thanks guys for all the assistance!!!
Attack is 1K+ against 224 which seems to be 0.22uFd. Works out to 0.2 milliSeconds, which is rather short. Diode impedance adds to this.
Release is that 0.22u against the 4.7Meg, about 1 Second. (Yes, R142 and friends enter into it, but not for release.) That's not wrong. Opinion may vary.
If we were really sure Hartke screwed this up, you could raise the 1K to say 5K and a 1mS attack. Dropping 4.7Meg to 1Meg makes 0.2Sec release, which may be short for this purpose, 2meg may be inneresting.
> I am getting -2.6v (at the gate)
Gate is many Megs out from the bias voltage. About 9Megs out. Meter-loading will be severe. Assuming 10Meg meter, you should have ~~half of the -3.2V at C120. Something is not as documented, but not sure what.
Agree that bad R152 will screw things up. "B200K". Tack a 100K across it. If it was bad, this will give some sanity, though not to factory spec. Yes, wiper observation will get you ballparked. Generally you would run test-tone and trim so the limiter holds the output just below power amp clipping. However there's more variable stuff after this, so not sure just the test setup should be.
Release is that 0.22u against the 4.7Meg, about 1 Second. (Yes, R142 and friends enter into it, but not for release.) That's not wrong. Opinion may vary.
If we were really sure Hartke screwed this up, you could raise the 1K to say 5K and a 1mS attack. Dropping 4.7Meg to 1Meg makes 0.2Sec release, which may be short for this purpose, 2meg may be inneresting.
> I am getting -2.6v (at the gate)
Gate is many Megs out from the bias voltage. About 9Megs out. Meter-loading will be severe. Assuming 10Meg meter, you should have ~~half of the -3.2V at C120. Something is not as documented, but not sure what.
Agree that bad R152 will screw things up. "B200K". Tack a 100K across it. If it was bad, this will give some sanity, though not to factory spec. Yes, wiper observation will get you ballparked. Generally you would run test-tone and trim so the limiter holds the output just below power amp clipping. However there's more variable stuff after this, so not sure just the test setup should be.
R152 measured 108k
That's the thing I am not sure what the test setup should be either. Compared to my compressor for my guitar which the attack is 5ms and the release is 1s. Maybe I could adjust things but I would rather figure out the limit first. On load the amp with limiter engaged goes up to about 70v peak (which is clipping) and then drops down to 50v peak and holds. I can adjust R152 to where it settles to just under clipping. Not really sure if that is what I am supposed to do.
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I think the designer of your amp opted for a very quick attack in the limiter, so that amp overload would only occur for a very short time. Given the amount of power, and the high likelihood of damaging speakers and hearing, it was probably a good choice.
As you say, guitar compressors tend to have a slightly slower attack, often in the 5mS -> 10 mS range. This is still very fast compared to our human senses, less than a hundredth of a second. Guitar compressors have a different job than this limiter, though.
That makes sense to me - I think that's exactly what the limiter is supposed to be doing. Staying out of the way unless the power amp is overdriven, in which case, it limits the amp to maximum unclipped output.
Once adjusted this way, is the limiter behaving properly under normal program material? Staying out of the way, causing no audible effects?
-Gnobuddy
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