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Old 27th May 2019, 2:33 pm   #41
Biggles
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Default Re: Tone Controls

Thanks for that Synchrodyne. I need a simple tone control circuit using an ECC83 for an amplifier I am building so that will do nicely as a starting point for some ideas.
Alan.
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Old 27th May 2019, 8:11 pm   #42
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Default Re: Tone Controls

Have a look at this website, you may already be familiar with it.

He has a very good section on tone controls and some example circuits, based as it happens on a 12AX7.

I hope this takes you to the relevant page

http://www.angelfire.com/electronic/...mp-Tone-A.html

Andy.

Last edited by bikerhifinut; 27th May 2019 at 8:15 pm. Reason: eventually managed to copy correct URL
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Old 27th May 2019, 8:18 pm   #43
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Default Re: Tone Controls

Cheers Andy, I will take a look at that. I must admit I am a bit lazy when it comes to designing my own stuff. It always seems easier to use sections of already proven circuits, and patch the bits together with a few minor modifications, particularly when it involves complicated mathematical equations.
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Old 27th May 2019, 10:08 pm   #44
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Default Re: Tone Controls

That's how we all start, Alan. Nicking a bit of circuitry here, a bit there.

Then we find incompatibilities and have to wonder why and get into the maths of a feedback loop... or worse still a phase-locked loop. You really can't do a good PLL without a feel for the maths. Once you've jumped in, the water temperature is OK and you acquire the tools to do a lot more than you thought AND you start to see things which were closed to you before.

Equations are just tools. Simple concepts, just written in a sort of alien language. Once you learn to read them and see what they're saying, life gets easier.

There's another magic moment, when you lose your temper with some piece of crappy electronics and declare "I could do better than that!" and then you do. A very large number of firms were started in this way.

Stuff that looks difficult IS difficult. But it's often because you haven't found the viewpoint which works for you.

Keep playing with nobbled circuits, but watch out for any easy openings into the layer above. It's worth it.

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Old 28th May 2019, 10:23 pm   #45
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Default Re: Tone Controls

Mentioned in post #36 was that the Radford SC24 control unit included emitter follower buffers in its two tone control feedback loops. The practice of including an emitter follower in the feedback loop may have started with the earlier Radford SCA30 integrated amplifier of mid-1968. (It was announced in Wireless World 1968 June.)

Here is the SCA30 tone control circuit:

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As I understand it, the SCA30 was based upon the combination of the Bailey 1968 fully complementary power amplifier (WW 1968 p.94ff) and to some extent at least, the Bailey 1966 control unit (WW 1966 December p.598ff.) For example, it used the Bailey 3-transistor RIAA stage. Here is the Bailey 1966 Baxandall tone control circuit:

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Quilter, in the letters pages of WW 1970 April pp.172, 173, observed that the Bailey circuit did not deliver its published curves, in part because of the loading effect of the tone control network on the single transistor. Bailey concurred with Quilter’s observation. Quilter went on to publish his own circuit (WW 1971 April, p.199ff) , which added an emitter follower at the output both to provide a buffer and to facilitate bootstrapping of the gain stage transistor output load, thus increasing open loop gain.

It may be supposed that Radford had seen this difficulty, and it addressed the tone control network loading issue by placing an emitter follower, Tr6, in the feedback loop ahead of the network. Given that the SCA30 dated from 1968, and that Bailey did not mention this approach in his 1970 response to Quilter, I think we may assume that the idea did not come from Bailey.

One may ask why did not Radford simply include an emitter follower after the gain stage, feeding both the output and the feedback loop. In fact there was an emitter follower in the output arm, namely Tr8. But this was used for the rumble (high-pass) filter, so would not have been able to feed an unmolested signal to the tome control network. Hence the need for a second emitter follower. Radford was also taking a bit more gain from the tone control stage than did Bailey. It does look though as if the feedback network part of the rumble filter goes back to the base of the tone control transistor Tr7, making it a case of overlapping loops.

Something of a paradox is that whilst Bailey evidently overlooked the loading effect of the tone control network when doing his 1966 design, a key feature of the latter was its three-transistor RIAA stage, developed from the two-transistor 1965 Dinsdale (WW 1965 January p.03ff) circuit by the addition of an emitter follower at the output, primarily to buffer the second transistor against the loading effect of the RIAA network.


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Old 28th May 2019, 10:28 pm   #46
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Default Re: Tone Controls

Continuing from the previous post, it is worth looking at what Baxandall said about the use of solid-state devices in his tone control.

In Amos (1977), after noting that the impedances for solid-state circuits were reduced by a factor of 10 as compared with the valved original, he said: “The cheapest versions employ just a single transistor, but lower distortion and lower output impedance may be obtained by using a transistor pair, or an integrated circuit operational amplifier…”

Similarly, in Talbot-Smith (1998), he said: “The cheapest versions employ just a single transistor, but lower distortion, more-symmetrical curves and lower output impedance may be obtained by using a transistor pair or an op-amp.”

The common emitter single transistor circuit was nonetheless used in some commercial hi-fi circuits, as well as by Dinsdale (1965) and Bailey (1966).

Next up from that was two transistors, common emitter followed by common collector (emitter follower), the latter feeding both the feedback and output arms. The Leak Stereo 70 and Stereo 30 Plus followed this pattern. (The Radford SCA30 circuit could be seen as a variation of this, with separate emitter followers for the feedback and output arms.)

Another variation on this was the use of a common source jfet and an emitter follower, used by Linsley Hood in his 1969 modular preamplifier. (An interesting sidebar here is that he used a three-transistor high-gain virtual earth amplifier for the shunt feedback RIAA stage, but elected to do differently for the tone control virtual earth amplifier.)

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The next level was modification of the basic two transistor circuit to include bootstrapping of the common emitter stage collector load, to obtain much higher open loop gain. This was done for example in the Quad 33 (1967; one imagines with the approval of Baxandall), and in 1971 by both Quilter (for whom the circuit was named) and by H.P. Walker.

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Ellis took it a stage further in 1973 by using a cascode pair in place of the common emitter stage, retaining bootstrapping of the load for the upper unit of the pair.

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Returning to 1970, Ambler saw fit to use Baxandall’s elegant virtual earth circuit as the basis for his tone balance control. He intended that this be used in addition to, not in place of the Baxandall control, and sketched out a control unit accordingly. He showed an op amp used for each virtual earth stage. These op amps were shown as common emitter-plus-emitter follower pairs with DC feedback.

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The Ambler control unit was evidently modelled on the Bailey 1966 design. The latter was used for the later Hart Electronics Bailey/Burrows/Quilter kitset preamplifier (which I’d guess might have been referred to informally as the BBQ pre-amp). Had Hart seen fit to add the Ambler tone balance control, then it would have been the Bailey/Burrows/Quilter/Ambler preamplifier. That said, the Burrows modification to the Bailey input stage did include a “tone balance” adjustment, although this was for the purpose of fine trimming of ceramic cartridge equalization.

In many, but not all cases, the tone control circuit was preceded by an emitter follower in order to provide a low impedance drive.


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Old 28th May 2019, 11:43 pm   #47
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Default Re: Tone Controls

The Baxandall arrangement can produce a fair amount of boost at either end of the spectrum and in a good design you want there to still be sufficient loop gain at these extremes to control distortion and to control impedances

In the circuits quoted above, the two transistor versions have one device as a voltage amplifier and the second as an emitter follower - but the output of the emitter follower is used to bootstrap the collector load of the first transistor as a way to squeeze the last drop of gain out of it. This works, but it's inelegant and has limitations. Fortunately, not too long after these circuits came along some good audio opamps like NE5534.

But some people have a solidly held belief that discretes are always better than ICs. Logic, mathematics. measurements and simulations are not allowed to interfere with such beliefs!

Anyway, there are enough circuits to go around, and enough varieties to not disturb people's beliefs.

I'd rather go for a decent opamp than the one or two transistor designs, but then I built some networks to replicate the Quad tilt curves for my preamp.

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Old 29th May 2019, 5:48 am   #48
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Default Re: Tone Controls

That prompted a look at an outline chronology for the progression from discrete devices to IC op-amps in high-quality tone control circuits. I am not sure when the NE5534 came into use in domestic audio equipment, but I think that Self’s “Precision Preamplifier” of 1983 was an early example of its use. (See WW 1983 October p.31ff.) The NE5534 was used for all stages, including the Baxandall tone control (and the Baxandall active gain control).

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The TL072 had preceded the NE5534, and that was what Quad had used in its 44 control unit of 1979, including for the tilt control circuit.

But before ICs suitable for high quality audio work became available, there were advocates for doing better using discrete circuitry. The Ellis tone control circuit of 1973, using three transistors, was mentioned in a preceding post.

Before Ellis, Meyer had advocated the use of discrete operational amplifiers for audio work, in WW 1972 July, p.309ff. He had noted that early audio ICs such as the MC1303 provided the desired higher open-loop gain, but fell short of the required performance level in other directions. Meyer’s answer was to construct an op-amp using discrete transistors. This was advocated for use in both the RIAA and Baxandall tone control stages:

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Linsley Hood appears to have been heading along the same vector with his Liniac of 1971 (WW 1971 September p.437ff). The Liniac was inverting, so quite suitable for use in a Baxandall tone control. (Unsurprisingly, JLH also advocated its use in the RIAA stage, with shunt feedback, then his favoured form.)

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Self used a complex discrete amplifying stage (with 9 transistors) for the modified Baxandall tone control in his 1976 Advanced Preamplifier (WW 1976 November p.41ff).

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In his simpler 1979 design (WW 1979 February p.40ff) he reverted to a transistor pair, common emitter gain stage with bootstrapped load resistor and emitter follower.

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That brings us back to the 1983 design, where Self said that it was then difficult or impossible to design a discrete stage that matched the 5534 low-noise op-amp without unacceptable complexity, the exception being low-noise, low-impedance stages such as moving coil head amplifiers. He mentioned that the earlier TL072 family was in many ways excellent, but noisier than discrete circuitry. Actually I think that whilst it was too noisy for say an RIAA stage, it was fine for line level circuitry such as tone controls. Anyway, in respect of the development of op-amps for high quality audio applications, one could say that the TL072 (1978-79?) was an early marker, with the NE5534 (1982?) being a nodal point.

For general-purpose audio applications, the MC1303 (c.1969) was quite early. There was also the LM381 (c.1972), which I think was op-amp like without actually being an op-amp. It seemed to have been quiet enough for use in hi-fi RIAA stages , and I have a vague recollection that Hi Fi News magazine published a suitable circuit in the early 1970s. But it might not have been cost-competitive with say the typical three-transistor circuits of the time, and if so, also not competitive for use in Baxandall tone controls. Curiously, the application note shows it used as a high-gain RIAA stage with a following passive tone control network, perhaps reflecting the fact that there would likely be customer resistance to using a second IC for the tone control.

An interesting audio IC from 1979 was the Philips/Mullard TDA1074, intended to be the basis for an electronically controlled treble and bass stereo tone control. It was typically used with the TDA1028 and TDA1029 audio switches, which also served as high-impedance, unity gain buffers.

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But some people have a solidly held belief that discretes are always better than ICs. Logic, mathematics. measurements and simulations are not allowed to interfere with such beliefs!
Yes, and some folk believe that the earth is flat…not much to do about that except just walk on by (but don’t wait on the corner!)


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Old 29th May 2019, 6:46 am   #49
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Quote:
Originally Posted by Radio Wrangler View Post
But some people have a solidly held belief that discretes are always better than ICs. Logic, mathematics. measurements and simulations are not allowed to interfere with such beliefs!
Yes, and some folk believe that the earth is flat…not much to do about that except just walk on by (but don’t wait on the corner!)


Cheers,[/QUOTE]

But don't walk too far or you'll fall off the edge

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Old 29th May 2019, 7:09 am   #50
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Default Re: Tone Controls

I did my preamp in 1980/81 and used an RIAA stage published in WW by Peter Baxendall, which he'd designed around an NE5534. It had a passive pole on the tail end to implement the missing time constant that was usually ignored.

I guess the NE5534 must have burst on the scene in 1978/79?. It takes a while for an interesting new part to get noticed and for word to spread. WW's lead time for a trusted author would be a couple of months at most.

Philips had a cover version of the NE5534, TDAxxxx whose number I don't remember, then Philips took over Signetics.

Except for moving coil cartridge inputs and some RFery, there isn't much need to go discrete in low power stages. You might do so just for the hell of it or as a learning exercise, but mostly it seems to come down to religion.

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Old 29th May 2019, 7:11 am   #51
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But don't walk too far or you'll fall off the edge
If you're lucky, you might land on one of the elephants, but how do you get back?

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Old 29th May 2019, 7:31 am   #52
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Default Re: Tone Controls

For the current state of the art in (FET) opamps, the very recent dual-amp OPA1656 is an astonishing design. 0.29ppm harmonic distortion, GBP of 55MHz, noise of 2.9nV/rootHz etc etc.

Surface mount only, I've just bought 10 pre-production samples

http://www.ti.com/lit/ds/symlink/opa1656.pdf

You would be seriously pushed to design a discrete op-amp to do that. Or compete with the ultra-low noise of the AD797 with 0.9nV/rootHz taking into account the other characteristics.

https://www.analog.com/media/en/tech...eets/AD797.pdf

The only disadvantage is that device is very sensitive to layout and decoupling, and can oscillate at VHF. And it is only a single op-amp per package.

I have no hesitation whatever in using opamps for audio. From the 5532 and LM4562 discrete, the OPA2134 (and now OPA1656) FET amplifiers to the low noise AD797. It is a superb audio amplifier toolkit.

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Old 29th May 2019, 8:51 am   #53
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Default Re: Tone Controls

I think you meant bipolar when you wrote 'discrete'?

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Old 29th May 2019, 9:12 am   #54
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No - if you are looking at the specs for an FET input op-amp, you have to use FET's as the input in a discrete component design, not bipolars.

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Old 29th May 2019, 9:30 am   #55
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Old 29th May 2019, 11:16 am   #56
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Ah, the opamp withe external FETs trick! sorry, misunderstood

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Old 29th May 2019, 3:20 pm   #57
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Well, I've done the layout for the Quad tilt control using their values - but scaled down in value.

Craig
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Old 2nd Jun 2019, 4:48 am   #58
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I guess the NE5534 must have burst on the scene in 1978/79?. It takes a while for an interesting new part to get noticed and for word to spread. WW's lead time for a trusted author would be a couple of months at most.

Philips had a cover version of the NE5534, TDAxxxx whose number I don't remember, then Philips took over Signetics.
The answer could be found “at home”, so as to speak. The development history of the NE5534/TDA1034 was kindly provided by Martin (Hartley118) in this earlier post:

https://vintage-radio.net/forum/show...0&postcount=15

And a comparative study of the TLA072 and NE5534 may be found in this thread: https://www.vintage-radio.net/forum/...d.php?t=144112.

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In the circuits quoted above, the two transistor versions have one device as a voltage amplifier and the second as an emitter follower - but the output of the emitter follower is used to bootstrap the collector load of the first transistor as a way to squeeze the last drop of gain out of it. This works, but it's inelegant and has limitations. Fortunately, not too long after these circuits came along some good audio opamps like NE5534.
I’d guess that in the pre-NE5534 days, the two-transistors-with-bootstrapping approach, however inelegant, was seen as a relatively economical way to obtain the desired level of open loop gain for an inverting amplifier for a Baxandall tone control circuit and other purposes. Of his 1971 circuit, H.P. Walker said “Turning to the amplifier itself, the bootstrapping circuit is used which gives a gain without feedback of 2000 and a distortion level of less than 1%. This enables very low distortion to be obtained even at maximum boost.” Walker also used bootstrapping in his microphone preamplifiers (both the low- and medium-impedance variants). These were effectively two-stage non-inverting amplifiers with emitter follower outputs. It may be noted that back in 1967, Revox had used bootstrapping for its A77 tape recorder signal input amplifier, which dealt with microphone as well as DIN and line inputs. In Walker’s 1971 mixer, the driver stage of the push-pull line amplifier had a bootstrapped load resistor. There was some adverse comment about bootstrapping in the letters pages of WW, but Walker evidently did not see what was said as a persuasive negative.

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As previously mentioned, Ellis in 1973 recommended that a complementary cascode pair be used for the gain stage in tone controls. He mentioned that the two-transistor bootstrap circuit suffered from latch-up when overdriven. Even so, his cascode circuit included bootstrapping of the collector load of the upper transistor of the cascode pair. Whether the Ellis tone control circuit was much used commercially, I do not know.

Walker did not use bootstrapping for his RIAA stage, which essentially followed the Bailey precedent. However, Self did add bootstrapping to the Bailey RIAA stage in his 1976 Advanced preamplifier, which he reckoned decreased close loop distortion by a factor of three. In his 1979 high-performance preamplifier, he replaced the output emitter follower with a push-pull class A amplifier, but retained the bootstrapping of the second transistor collector load. The 1979 design was intended in general to be a simpler, lower cost derivative of the 1976 design, using fewer transistors, but with close to the same performance. As already noted, the 1979 design reverted to the two-transistor bootstrapped tone control stage, in place of the complex 9-transistor circuit used in 1976, so presumably Self thought it was good enough in context.

Perhaps what one can take from the preceding empirical evidence was that the two-transistor bootstrapped tone control stage could provide relatively good performance overall if care was taken in its detailed design, and that it was at the knee of the benefit vs. complexity curve, beyond which small gains in measurable performance (and perhaps greater freedom from strange behaviours under very abnormal and low probability conditions) required very large increases in circuit complexities and device counts. This trade-off went away when opamps such as the NE5534 arrived. Perhaps for designers of commercial audio control units/preamplifiers that were competing in the mainstream and so could not be of the “cost no object” type, the discrete solid era was in some ways the most difficult period, requiring the use of odd circuit artifices such as bootstrapping, whereas the valve era before it and the IC era following were both a bit easier.


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Old 2nd Jun 2019, 4:51 am   #59
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An interesting audio IC from 1979 was the Philips/Mullard TDA1074, intended to be the basis for an electronically controlled treble and bass stereo tone control. It was typically used with the TDA1028 and TDA1029 audio switches, which also served as high-impedance, unity gain buffers.
Here is a schematic showing how the TDA1074 was used as a tone control.

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It looks like an electronically controlled version of the Baxandall. The TDA1074/TDA1028/TDA1029 combination was intended to be the basis of an all-IC control unit/preamplifier, in which the NE542 was used for the RIAA stage. The NE542 looks to have been very similar to the LM381. I can find no suggestion that the LM381 or NE542 were offered for tone control use. Rather they seem to have been oriented towards pickup, tape head and microphone input stage applications. Perhaps they were not very happy when used in the nominally unity gain inverting mode?

Another IC-based tone control was proposed by RCA, using its CA3140 op-amp, which I think just preceded the TI TL072 family.

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This used the op-amp in the non-inverting mode. In turn this required that the nominal gain be high enough to allow for the required maximum treble and bass cuts without going below unity gain. Overall unity gain was achieved by attenuating the input by the same amount as the nominal gain. Effectively the treble and bass potentiometers were balance controls, altering the input attenuation in a frequency selective way and at the same time altering the feedback level in the inverse way.

This appears to have been “going the long way around” as compared with the Baxandall. One advantage claimed was that it had a high input impedance. Presumably the implication was that it would not require a buffer stage ahead of it. Discrete Baxandall controls often had a preceding emitter follower in order to provide a low enough driving impedance. In the op-amp era, the buffer could have been a voltage follower. But if the preceding stage itself used an op-amp, then an intermediate buffer might not have been needed.


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Old 2nd Jun 2019, 10:52 am   #60
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Bootstrapping is a perfectly satisfactory technique in he right circumstances and offers some real advantages. The wrong circumstances are anywhere it might be hit with unconstrained transients into clipping. What should have been a short clip gets turned into a long hang-up before eventual recovery. You need bootstrap capacitors sized to do their job at the LF end of the signal frequency range, and this sets the time-constant of the recovery process. It's a bit like triggering a monostable multivibrator.

In general, if I don't know that the signal has been tamed, I don't use bootstrapping. If I do, then I can.

There's one in an AM transmitter I designed fairly recently.... Wozzat? Designing an AM transmitter in the 21st century? Yup. It's in current manufacture and sales. Certified for use in aeroplanes of all sizes and ages.

David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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