Feedback lead/lag compensation

[Alan0354]

For a guitar amp you should think more about how it sounds when overdriven than about stability and fidelity.

Speaking of home stereo, for me coloration is the one obstacle that I am getting rid of. Learning to design guitar effects and analog synthesizers I better realized how to make distortions disappear, i.e. what to fight with, and what to leave alone because some errors are audible, other errors are not.

You design guitar effect and/or guitar amp? Those are very different from the topics here. Now you are talking about clipping, not just distortion like harmonics.

 

[famousmockingbird]

Hi Wavebourn,
I just got to looking around your website and can say I dig it. I see you have a proprietary optical compressor? I am thinking it is a transparent studio compressor....correct? The reason I ask is because I recently have been obsessed with compression (for guitar use). I don't know what it is but I have a Ross compressor clone, which is more or less a dynacomp Anyway I am obsessed with the "clicky" "poppy" sound of this thing. I guess I am curious to different types of compressors and have never tried an optical compressor. Can they sound like the Ross?

I think I threadjacked my own thread

 

[Fast Eddie D]

All I am saying is "fidelity" is only what people define in their mind. The real raw music sounds very different from any of the recording. If people really want fidelity, then none of the recorded music even come close. So, it's better to talk about what pleases you rather than talk about no distortion because what you hear in any system are way distorted from the real music.

Don't believe me, go listen to live music particular performance that doesn't even go through the PA. Even simpler, if you have someone that sing good enough. Have the person sing straight to you without mic, then sing through a karaoke machine, you'll hear the voice sound so different. To me, fidelity is if you can reproduce the raw voice of the person or the instrument.

As for me, I just want a system that pleases me, distortion or not.

You're so correct. Most people don't realize this.

Back in the 60s, some of those consoles did give very pleasant, nice room filling sound. People bragged about their systems and how the cabinets "resonated" (they really did) like a finely crafted musical instrument. Of course this is anathema to what many of us do here.

I have been guilty of intentionally "dirtying" up some of my designs, so they sound more "musical." A little bass boost, a little lift to the treble, makes things sound more "listenable" sometimes. My recently built headphone amp has a deliberate slight "tilt" towards the bass hard wired into the circuit. Is that a bad thing?

My computer sound system has speakers that are decidedly bass heavy. The bass is kind of boomy and poorly defined. Everything else sounds OK. Now I really don't like that sound (but I'm not complaining because the speakers came out of the dumpster), but everybody else that hears it thinks it's the bomb shizzle. They're amazed that so much bass can come from such small cabinets, but in reality it's more of an illusion than anything else. But if people like it, is it really a bad thing?

 

[DF96]

I don't know your term of conditional or unconditional stable.

Unconditional stability is when a reduction in loop gain is guaranteed not to lead to instability. Conditional stability is when a reduction in loop gain may lead to instability, because of phase response causes a loop in the locus to cross and re-cross the axis beyond the critical point.

Sadly, there is another quite different use of these two terms: an amp is conditionally/unconditionally stable if adding any load to the output may/will not cause instability.

The ONLY way to have it unconditionally stable is IF you have a single pole role off in the whole open loop gain. That is a dominant pole compensation.

No. You could have an unconditionally stable amp with two similar poles if the loop gain is small enough.

 

[Alan0354]

Unconditional stability is when a reduction in loop gain is guaranteed not to lead to instability. Conditional stability is when a reduction in loop gain may lead to instability, because of phase response causes a loop in the locus to cross and re-cross the axis beyond the critical point.

Sadly, there is another quite different use of these two terms: an amp is conditionally/unconditionally stable if adding any load to the output may/will not cause instability.


No. You could have an unconditionally stable amp with two similar poles if the loop gain is small enough.

Yes, I misused the term, it's been a long time since I studied this.

Yes you "could" have conditional stability with two pole as long as the second pole is close to the 0dB crossover as it's only 45 deg right at the pole only, you still have 45 deg phase margin. The ultimate criteria is you don't have gain of -1 at 0dB crossover. But relying on just phase margin calculation is not as robust, you don't have 90 deg phase margin gurarantied. I tend not to do that and design to guaranty one pole cross over. I always worry that one pole that I missed might sneak up on me.

Yes, the load is part of the open loop gain calculation as it introduce pole and/or zeros. That is more troublesome for tube amp as the output impedance is relatively higher than SS output.

 

[Alan0354]

You're so correct. Most people don't realize this.

Back in the 60s, some of those consoles did give very pleasant, nice room filling sound. People bragged about their systems and how the cabinets "resonated" (they really did) like a finely crafted musical instrument. Of course this is anathema to what many of us do here.

I have been guilty of intentionally "dirtying" up some of my designs, so they sound more "musical." A little bass boost, a little lift to the treble, makes things sound more "listenable" sometimes. My recently built headphone amp has a deliberate slight "tilt" towards the bass hard wired into the circuit. Is that a bad thing?

My computer sound system has speakers that are decidedly bass heavy. The bass is kind of boomy and poorly defined. Everything else sounds OK. Now I really don't like that sound (but I'm not complaining because the speakers came out of the dumpster), but everybody else that hears it thinks it's the bomb shizzle. They're amazed that so much bass can come from such small cabinets, but in reality it's more of an illusion than anything else. But if people like it, is it really a bad thing?

I personally rather hear the crashing of the cymbal in drums through recording than live, also I like Sax, but mainly through mic and recording as both are so harsh and raw in real life, it's not pretty.

One thing amaze me about high quality hifi system, it really gives you the 3D effect when you close your eyes. It can create a sound stage that sounds big. I don't understand how that can happen and explain in electronics.

 

[Wavebourn]

You design guitar effect and/or guitar amp? Those are very different from the topics here. Now you are talking about clipping, not just distortion like harmonics.

I used to design guitar effects and analog synthesizers when I was young. And no, it was not about clipping only.

It was about how to express emotions using electronics.

 

[Alan0354]

I used to design guitar effects and analog synthesizers when I was young. And no, it was not about clipping only.

It was about how to express emotions using electronics.

Are you in business or been in business selling your effect electronics? What kind of effect you design? You are the second one from forums I know in the Bay Area that involves in guitar electronics. The only other person I know is a pickup maker. I live in Sunnyvale, about an hour and half from where you live.

I have been designing OD pedals and high gain amps lately just for the fun of it. I just have a US patent granted in a noise cancellation design for single coil pickups. My interest is mostly in OD and distortion sound, not synthesizers.

 

[richwalters]

Making it stable on a dummy load, is just the first step. It does not mean it is stable connected to real speakers, you should use the scope to check that too.

Deceptive game, this stability stuff....When many AB amps go into near "blocking" with the cusp of a waveform forcing the issue; that partial cutoff scenario by output stage overdrive, a careful ear may be able to hear "squegging" and other strange noises occurring at that instant.
This is another sure sign that open loop stability is not as stable as might think it is by going into oscillatory mode on peaks. A negative impedance load, ie piezo tweeter or higher order crossover network can aggravate the problem even further.

richy

 

[famousmockingbird]

I am happy with the sound and performance for the moment, or until I get my hands on better testing gear. It isn't that I am not listening to anyone in particular I just wanted to play with the circuit a little, after all this is a hobby. So I did lower open loop gain by going EF86 triode and I did end up with 20db of feedback which is what I wanted and I got to keep the same input sensitivity. For lack of test equipment I empirically adjusted the lag network until the square waves looked their best, I ended up with a value larger than I expected (330pF with a 4.7k resistor) but it seemed to give the best results.

Listening tests revealed good transient response and it sounds nicely balanced. I did notice that the highs were less "shrilly" which is probably a result of moving the first -1 pole down with the 330pF value.

 

[famousmockingbird]

*Note: The strange "squeal" on power up is gone too

 

[costis_n]

The 5-20 did use a lot of nfb, but was still stable, with the intended OPT. (Nobody has mentioned that so far.) My own way of checking stability in practice is simply checking with a 4 - 5 kHz square wave (I am a little lazy to draw Bode plots!). One can observe quite nicely what effect the compensation has. And I agree: An amp should be stable up to open load - or at least with a permanent say 470 - 820 load resistor fitted just to keep some check on a no-load situation.

Since it was raised, I have a problem with using a triode as input stage; Miller-effect viz-a-viz an unspecified input impedance. As Andreas was the only person to mention so far: What about the (to my judgment) poor choice of a 12AX7 as a phase-inverter/driver? High impedances all over the place, Miller capacitance to input stage, etc.? With due respect to Mr Byrith for his alternative, I would much rather use a more suitable phase inverter like perhaps a 12AT7 or even lower µ like an ECC88 instead of going to a triode input stage. One has a better situation regarding output tube Miller-C, coping with the odd grid overdrive peak, etc.

I have always fancied a pentode input stage as more desirable (the extra noise because of the extra grid present is academic in power amplifiers). The screen grid bypass can further serve as a measure to regulate l.f. phase response for l.f. stability where needed with its phase angle returning to zero - not to further expound here (OT?). I would agree that satisfactory low distortion should be possible with some 20 - 23 dB global nfb. (The Williamson achieved 0,06% distortion with 20 dB of nfb.) My own experience with several designs using input pentode - ECC88 phase inverter - 6L6GC output pair in UL has been highly satisfactory and docile, with open-loop frequency response flat over the audio range. (Again, of course, the OPT must be up to its task regarding its own specs.)

Exactly. Using an ECC82 at the input gives a lower Miller effect (because of the low mu). And an ECC88 is a good choice for the phase inverter/driver because of the high(ish) mu and more importantly the high transconductance. Who knows, maybe if ECC88 was a Philips/Mullard product (and available at the time), they would have made it into the design.

 

[DF96]

And an ECC88 is a good choice for the phase inverter/driver because of the high(ish) mu and more importantly the high transconductance. Who knows, maybe if ECC88 was a Philips/Mullard product (and available at the time), they would have made it into the design.

Unlikely that Mullard would have used an ECC88 LTP phase splitter, even if it were available at the time.

Max Va too low (130V)
Mu (33) too low for good balance, so would need a CCS tail or unequal anode resistors
Low anode impedance not much of an advantage when driving EL34
Low cathode-grid spacing could lead to flashover before the first stage warms up

They were right to choose the ECC83. Its only weak point in this circuit is the limited current driving ability into the output stage Miller capacitance. They could have used an ECC81 instead if they thought this would be a problem.

 

[costis_n]

Unlikely that Mullard would have used an ECC88 LTP phase splitter, even if it were available at the time.

Max Va too low (130V)
Mu (33) too low for good balance, so would need a CCS tail or unequal anode resistors
Low anode impedance not much of an advantage when driving EL34
Low cathode-grid spacing could lead to flashover before the first stage warms up

They were right to choose the ECC83. Its only weak point in this circuit is the limited current driving ability into the output stage Miller capacitance. They could have used an ECC81 instead if they thought this would be a problem.

Interesting, you didn’t seem to notice that when I presented my build....

 

[DF96]

Interesting, you didn’t seem to notice that when I presented my build....

I don't always notice and comment on everything put in front of me.

I don't always comment on popular but, in my view, mistaken valve choices.

Note that my remarks relate to EL34 (and EL84) output. Beam tetrode outputs often need smaller grid leak resistors (presumably because of worse grid current - but I don't know why) so then an ECC83 will suffer from anode loading. An ECC81 makes an acceptable substitute, with adjustments to resistor values to give more current, as an LTP will largely cancel its second-order distortion.