I recently came across this gem in the X Chapters supplement to the Art of Electronics 3rd edition. (Not reproducing here because of copyright. You need to buy the book anyway, it’s brilliant 😉 )
Of course my first idea was to convert it to dominant pole compensation (C2) and bodge a standard audio power amp output stage onto it.
This worked very nicely in simulation, so the next step was to slap on an opamp front end ala Quad 405 or Cordell Super Gain Clone.
The resulting circuit should give a lot of bang for the buck when used with a good quality FET input opamp. I expect it to outperform the Alexander CFB design in areas that matter, like HF THD, PSRR, playing well with the standard Bailey current limiter circuit, and not depending on an obsolete opamp or catching fire when overdriven.
This is a voltage feedback design, not current feedback, and won’t beat the Alexander’s slew rate without running the transistors at crazy idle currents. Besides being CFB, the Alexander driver circuit operated in Class AB and could call on arbitrary amounts of current when slewing was required. The price it paid for this was catching fire when overdriven 🙂 and also crossover distortion generated in the opamp’s output stage, which is very audible if you use any other opamp than the original SSM2131. (And kind of audible even with the SSM2131 imo…)
So overall I am happy with this set of compromises and looking forward to trying it out in real life.
Regular readers will know that I’m a fan of Douglas Self’s Blameless philosophy of audio power amp design. (An uncontroversial choice, as the Blameless topology is not really that different to designs you would have seen in Wireless World in the 1970s, or indeed the innards of Bob Widlar’s opamps.)
Probably the biggest problem I have with the Blameless is the huge bias and offset currents at the inputs, which require a low resistance feedback network to avoid a huge DC offset at the output. This sets off a chain of design compromises and ultimately the carbuncle of C1 and D9 (Fig.1) pops up.
In my previous experiments I tried a matched pair for the input stage, but it made absolutely no difference. The collector currents aren’t necessarily matched, so neither will be the base currents. The biggest improvement was had by using high beta input transistors such as the now-obsolete BC213C. (MMBT5087 would be a suitable 21st century replacement.)
Revisiting this, I discovered that my first attempt at building it was badly unbalanced because of the clip detector (Q14 in fig 1) which robs a lot of base current from Q9. The other side of the mirror has Q12 robbing from it, but Q14 takes much more as it’s a high voltage part with low beta.
By adding a beta helper transistor to the input mirror (Q14 in fig 2) the imbalance due to base currents can be eliminated. The beta helper’s base current is drawn from one side of the mirror, and the base current of the Darlington VAS (Q2, Q17) comes from the other side. By setting R15=R7, Q2 and Q14 will run at the same collector current, so if they are matched for beta, their base currents will also be the same, and the whole input stage will be perfectly balanced to a first order.
(It’s not balanced to a second order because the current drawn from Q14 emitter by Q8-Q11 bases isn’t necessarily the same as that drawn from Q2 by Q17 base, but the imbalance due to this should be tiny. The clip detector can now be hung off Q8, Q9 bases with impunity. 2 transistors are now used to get a 2:1 ratio as required for balance with the current sourced by Q3. I used the same type of transistors as Q8, Q9 for improved balance, so a cascode Q23 is now needed because of their limited voltage rating.)
The other side of the problem was limited beta in the input stage. One of the pillars of the Blameless philosophy is to run the input stage at a high current to give it lots of gm, which you then throw away with emitter resistors. The goal is good slew rate, and the price paid is the high input bias currents.
We attacked the input offset current issue by improving balance of the collector currents (and hence base currents) but if we could reduce the input bias current too, the offset current would decrease still further in proportion.
A promising approach seemed to be replacing the input transistors for Sziklai pairs. (Q6, Q15, and Q7, Q16 in Fig 2.) Now the actual input transistor Q6 can run at a low current (300uA in this case) while Q15 conducts the remainder of the high current needed for decent slew rate. We have a double win because the Sziklai pair is more linear than a plain transistor.
Adding more and more transistors inside a feedback loop is always dodgy, and running one of them at low current where its Ft will be reduced especially so. In this case though, I think the bandwidth will still be adequate. The whole mess is cascoded by Q12, Q13, so there will be no Miller effect to slow Q6, Q7 down. The degeneration from R5, R6 should also help to stabilise things.
The LTSpice simulation of this showed an overall DC offset of 20uV, a huge improvement on my previous Blameless driver board. Of course this is with perfectly matched virtual transistors, but I think the real thing will do pretty well when built with BCM846/856 matched pairs. They specify Vbe to be matched within 1mV, and (what really matters for this circuit) beta to be matched within 10%, which works out at a 100nA worst case input offset current.
The overall result is that the DC offset and clip indicator trims from my original design can be done away with, and the impedances in the input and feedback networks can be increased by a factor of 10, for the same worst case DC offset of about 10mV at the speaker terminals. This means that C1 the obnoxious 1000uF electrolytic can be replaced by something more audiophile grade: the same 10uF plastic film capacitor that used to be the input DC block.
This is what the simulation says, so now I have to build one and see if it works in real life… 🙂
For more aluminium welding practice I decided to weld up the knob holes in the original Corvette front panel. The front looks OK but you don’t want to see the other side. I then made a new layout using MAD- permanent Marker Aided Design.
Next step was to finish wiring up the Marshall 2204 preamp.
This winter I discovered that blocking up the fireplace makes the living room much warmer and cuts down the gas bill substantially. All good for the Conner Labs carbon footprint. 🙂 However I guess an open fire has some sort of primeval appeal. Before I knew it I had bought this “vintage retro” piece of junk on Ebay.
I immediately regretted it in case it turned out to be full of asbestos.
It was :/
As it was also caked full of dust, and to be honest smelt a bit suspect, I decided to take it outdoors and wash it down with soap and water. Everything including the wiring, to get the asbestos wet for safe removal.
It was hardly the most complicated assembly so I stripped it down to the last nut and bolt and cleaned everything. The reflector was polished using T-Cut.
Rewiring with bare copper wire in modern high temperature fibreglass sleeving. The switch marked X had somewhat melted contacts. I couldn’t find a replacement so I retensioned them as best I could and used that switch on the lowest powered heating element.
Modern toggle switch doesn’t have 1/10 of the vintage mojo. Doesn’t fit the panel hole anyway.
Red fireglow lamp is arguably the most important part 🙂 25W filament ones are still available.
About now I realised the error of using WD40 to free off the nuts holding the heating elements in place, as great clouds of WD40 flavoured smoke belched forth. 🙂
Once the fumes had dissipated it turned out to heat the living room better than the old gas fire and cost less to run.
Soon the phase inverter and power amp were finished and working.
One big difference is that the 2204 uses negative feedback around the output stage while the original Corvette didn’t. So I decided to go with the NFB, and include the transistor output stage in the feedback loop too for an extra challenge.
It was perfectly stable first time! LOL just kidding… It suffered from high frequency parasitics-
And these comically chaotic LF oscillations could be provoked by overdriving it at low frequencies.
After much trial and error I ended up with something like this. The 0.68uF/5 ohm RC snubber killed the HF oscillations, and removing C17 and C18 (this schematic) stopped the motorboating. With these values it was just barely stable with the load disconnected and a 220k NFB resistor (vs 100k in the original 2203 circuit)
Note that when the transistor output stage is in play, the OPT secondary becomes bootstrapped and flies around with the speaker output, so the NFB takeoff point I used sees the output voltage of the transistor stage plus the output voltage of the valve OPT.
Removing C17 and C18 demanded quite a lot of extra current from the bias generator, but it seemed to deliver it no problem, so no changes were required there.
Resistor values were also changed to reduce the current gain of the transistor output stage, due to the increased output of the valve part of the circuit.
I got bored of the Ninja Corvette Hybrid and decided to transform it into something with a little more “FU”.
The plan I came up with was to strip out the valve part of the circuit and replace it with a clone of a Marshall 2204. This is a classic rock amp that I hadn’t had much experience with.
I decided to use 6AQ5 power tubes running off 250V, for a modest apartment-friendly power output. The 3 position power switching would be retained, giving power levels of 1W, 10W and 40W.
The Marshall 2204 circuit has 5 valves, but there are only 2 holes in the chassis…
A new output transformer was also required, as the original one was single-ended. I used the cheapest PP one I could find at TAD. I also TIG welded a bracket for it, as I’ve been watching way too much Project Binky.
I was looking for an aluminium welding project that wouldn’t kill anyone if it failed. And also some speaker stands that wouldn’t take up any desk space.
I started with a 1m length of 3″ x 3″ x 1/8″ box section.
After a whole afternoon of hacksawing, jigsawing and filing it was reduced to 2 columns and some wedge shaped pieces.
The pieces were then welded together into a giant C clamp shape and a 3mm plate was welded to the top to support the speaker. I decided to only tack weld the plate because I was worried the heat of a full weld would warp it.
I find the fillet weld the most difficult. This doesn’t exactly look great but it’s my best yet.
The stand base screws to the underside of the desk with some hefty wood screws.
The desk is completely clear and there is plenty of room underneath for oscilloscopes, soldering irons and so on.
I got fed up with the thumb controls on the torch that came with my welder and decided to try a foot pedal. They are extremely expensive to buy so I decided to make one out of an unwanted wah pedal.
The main difference between a wah pedal and a TIG foot pedal is that the switch engages as you first apply pressure to the pedal, telling the welder to open the gas valve and go through its preflow and ignition cycle. Depressing the pedal further then controls the welding current.
To allow this operating mode I threw away the existing stomp switch and replaced it with a microswitch operated by a cam.
The other difference is that it needs a rather strong return spring to avoid igniting the welder by accident. I found this out by trial and error. D: Keeping with the musical instrument theme, I used some Strat tremolo springs from eBay.
I connected it to the socket for the hand controller as a spare plug for this was supplied with the welder. It thinks it is a torch with 1 button and thumbwheel. This seems to work fine, but I will try the foot pedal socket if I ever get round to figuring out the mating plug for it.
It was time to put it all together! (This actually happened in March- these are post hoc posts 🙂 )
First Odin’s control electronics had to be converted to run off 24V DC instead of the original 240V AC. (And mounted in a Eurorack while I was at it…) This wasn’t too difficult as they already used 20-something volts DC internally, derived from the mains with a traditional iron cored transformer and rectifier, and regulated to 15V.
I added DC input sockets to the driver and gate drive amplifier modules, and changed the fan for a 24V one too. The original 240V AC inputs are retained in case the PFC breaks down and I need to change back to the old power supply.
The PFC will be situated at the operator’s position with long cables for 750V and 24V running to the coil. This made everything simpler, as there was no need for remote control and the circuit breaker on the PFC could be the emergency shutoff for the whole system. But it did leave the 24V cable vulnerable to strikes and general pickup of the extreme levels of EMI around a Tesla coil. My solution was to make a DC input module using a surplus Traco 40W DC-DC converter to give galvanic isolation, and lots of EMI filtering on both input and output.
The red module is the receiver for my Teslink system that sends multiplexed control signals over a Toslink optical fibre. I finally got round to completing it (and making a Eurorack mounting transmitter too)
The idea is that the PFC accepts single or 3 phase power at anywhere between 208 and 415V, and supplies 24V DC to the Tesla coil electronics from its own control power supply. I didn’t want the hassle of having to change taps on control power transformers, or rather the carnage of connecting it to 415V with the taps set to 240. (I have done this before- it was messy)
The Tesla coil primary was set up using a water-filled steel pan as a dummy load.
It didn’t explode! 😀
The next step would have been to take the PFC and immersion heater bucket to a lab with 3 phase 415V power. Unfortunately this was made impossible by the COVID-19 lockdown. The debut was to have been the Nottingham Gaussfest, but this was also cancelled. Insert corona joke here 🙁