Floating ground dual class amplification A + B/C/D/etc...

[Roberto]

Hi,

another idea that came to my mind while reading this:
http://www.thatraymond.com/downloads...ttala_v1.0.pdf
(see page 112)

Is the possibility to have a floating ground tube power amp working in class A amp plus a class B/C/whatever with high efficiency and high power. The secondary of the tube amp (and the iron as well I would say) has to be able to supply the needed current for the full swing of voltage of the D class amp + the A class tube amp.

The C/D/whatever amp, together with the preamp, will be grounded, while the PA & OT will not, but will use the floating ground controllet by the C/D/blabla.

Do you see any reason why this could not work as planned?

[nickb]

Can you draw a block diagram of what you have in mind?

[Roberto]

Sure, tomorrow I'll sketch that, but it's really just the same thing of the linked book, just with the A class switched to vacuum.

[Enzo]

But "same thing" requires me to scan through teemu's entire book looking for what you might be referring to, while a block diagram answers most questions in seconds. And we would see what YOU intend, rather than what may or may not really be the same in his fine book.

I think when posing anything, it helps to state what problems it solves or what enhancements it provides.

[Roberto]

Enzo,

No need to scan the entyre book, because I've clearly written the page where there's already the schematic of what I was referring to.

Furthermore, I already wrote I will post the block diagram. I just had no possibilities to do it today in a clear way.

Thanks anyway.

[Enzo]

Sorry, perhaps I am dense today. I know you already said you'd make a drawing, I was responding to the page in the book being the same thing. it isn't clear to me where a class D anything would be put and how it would interact with the analog stuff.

[Gingertube]

These Class A + amps have been around for a while.
The "traditional" HiFi arrangement (e.g. the NAD Current Dumping Amps) ran a low power Class A Amp with a high power Class B Amp, with the speaker connected in a bridge arrangement between the 2 Amp outputs (no ground connection to speaker). The small Class A Amp hadles the critical (for HiFi) first watt and error signal.

The schematic on page 112 shows a variation of this. The small Class A Amp handles the "first watt" as before but the whole Small Class A Amp is "pumped" up and down by the output of the High Power Class B (or other) power Amp (The +/- 5V rails to the small Class A amp ride on the High power Amp output). The advantage over the NAD Current Dumping scheme is that the speaker output is still ground referenced.

I would GUESS that it would be possible, but difficult to do the small amp as a tube Class A amp. It would need a floating B+ supply riding on the high power amp output.

When I apply the KISS engineering principal then I would probably do it differently, Use a small Class A push pull amp with current sense resistors in the cathodes of the output tubes and then use the voltage developed across the cathode resistors to drive MOSFETS pulling additional current through the FULL power rated output transformer. That way you get the "sound" of the small amp but heaps more power. Kevin O'Connor discusses this in one of the TUT series books (TUT4 maybe) and he calls this scheme "gm multiplication".

Cheers,
Ian

[Roberto]

When I apply the KISS engineering principal then I would probably do it differently, Use a small Class A push pull amp with current sense resistors in the cathodes of the output tubes and then use the voltage developed across the cathode resistors to drive MOSFETS pulling additional current through the FULL power rated output transformer. That way you get the "sound" of the small amp but heaps more power. Kevin O'Connor discusses this in one of the TUT series books (TUT4 maybe) and he calls this scheme "gm multiplication".

Thanks Ian,

the reason of my questions is looking at the power attenuators for guitar amps on the opposite side:
Instead of carrying a 30 kg amp and dissipate all the power, use a godd sounding small amp (like the attached Anderton '64 Marshall 145 is: http://m.youtube.com/watch?v=w71tBeuLn2Y) and beef-up the power to any needed level.
A lightweight, cheap, almost free of maintenance amp.

...and a good option for the project we were talking together via PM about the shred guitar + bass amp head.
Is it still going?

[Gingertube]

Ah! A travel amp perhaps, to go with a travel guitar to suit a busy international traveller - and then a booster for home and gigs.

That posted schematic is a nice little amp, cathodyne phase splitter with a 12AU7 triode class A push pull output stage (which means no nasties from the cathodyne splitter in overdrive) for around 1 Watt max. output.
Also for guys wanting to build a clone , then, the Class A triode mode output with low rp triodes (12AU7) means that the "standard" ,readily available, Fender Reverb transformer (for Push pull 12At7,, 22K Raa) would almost certainly work well.

When "forming" your sound with an amp like this you don't want to "mess" with that sound in your "booster" amp, direct injection to the PA is the go, but if you want to be "stand-alone" then any HiFi'ish power block would suit. Lots of commercial offerings at cheap prices. I would not contemplate designing and building my own "booster" amp for this purpose, BUT if I had to (and use tubes), I would "borrow" another KOC idea, run a small output tranny reverse connected at the input (that is: drive the secondary) and use its push pull primaries to drive the grids of a big pair or quad of output tubes directly. So your "booster" amp is just input tranny, output tubes and tranny and power supply, output tube bias applied to the input tranny "primary" centre tap..

ASIDE: My least favourite HiFi Amp (in my system) of all time was a 210W per Channel 0.0001% distortion top off the line ROTEL power amp. I disliked it so much that I thought there must be something wrong with it. I bought a Service manual (A$50) and did a full set up making no difference at all. I eventually sold it to a local shreader guitar player, who used it with the 2 Channels bridged for 400W and fed from his Line6 DSP Guitar Preamp, it was seriously great at reproducing distortion but didn't have enough of the "right" distortion of its own for a HiFi Amp.

OFF THREAD, CONVERSATION, END OF THE DAY OFF
Roberto etc.
The other "All things to All Men" you alluded to (clean channel able to handle bass and a lead channel (doesn't have to handle bass) capable of shread, with professional audio equipment compatible effects loop and balanced line out etc.etc.project, I've put on hold till I have finished the current build. Also the friend I'm to build it for has hit hard times and may well not have the cash any longer, so pressure is off. Regardless it is a worthy project which will find someone who wants it. AND for those of us who play casually with friends on Sunday afternoons with a cheese platter and a bottle or 3 of Red (or Bourbon in the Winter) does all you might want from an amp. I will eventually start a dedicated thread for it in "proper" Engineers fashion by posting a Requirements Specification (so we know when the job is finished) and follow that up with preliminary design discussion and schematics for review/critic/alternatives.
I want to attempt to do this "properly" and progress that design here, just like I would in the day job when the Technical Manager hands me a Requirement Specification and says go away and design something to do this job, be prepared to present your preliminary design to a review of your peers by date X. Make changes as per the design review, and present the CRITICAL design review (again by your peers) on date Y, document final design and pass to production, write test specification etc. etc.
NOT SURE how that might pan out But if we don't try we will never know.!!

My design options maybe influenced by the fact that his initial requirement was he wanted an amp which sounded just like mine, which is pretty much a KOC London Power Standard, 2 Channel preamp with 6SL7 substituted for the 12Ax7s and a Fender 5E3 power amp with 6sl7 gain and concertina splitter and push pull vintage 6V6G (the old 'ST' coke bottle shape). Then he added all the "bells and whistles" that he would like. He uses my amp a lot (for both guitar and bass) into his own sealed cab which has a single 15" JBL K140 or K130 (the bass one). I use it into old Sanyo HiFi box, the 10" bass speaker has been replaced with a vintage Oz Etone Guitar speaker and the old 5" midrange and 3" tweeter holes are left open, for some "loose" port tuning (if you believe in that) or just me being slack.

Current build to complete first:
Is a GA40T Les Paul inspired Tremolo amp with 6BR7 pentode (BRIMARs version of an EF86) front end, Tremolo via varying the anode and screen voltage to the input pentode, and an Aussie Goldentone/PlayMaster inspired power section using push pull 6DQ6 Horizontal Output TV tubes with B+ of only 300V (150V screens) for 30 Watts out. A pair of 6DQ6 pushed to the limit can give you 85 Watts, so in this design the tubes should last forever. The power Amp design is by "default" since this is a conversion job on an old AWA PA Amp which ran 6CM5 or 6DQ6 output tubes. While I mostly gutted it for the conversion, and replaced the 70V line output tranny with a 50W Randall Output Tranny (Raa = 3k4) I had lying around, the power tranny in particular, suited the low B+ and the 1/2 B+ screen supply required by the 6CM5/6DQ6, so I stuck with them. PLUS my most recent restoration was a Oz Vintage Goldentone running push pull 6DQ6 for 60W out and a cathodyne splitter. It was stunning. A ex-pat Aussie who now tours the world as a Vintage Rock and Blues artist tried it at my place and promptly asked to use it at all his remaining Aussie gigs, at the end of which he just said "How Much?" I said A$1000 which he paid on the spot. A search of EBay the next day showed that it was very rare and the last one which had come up, 2 years earlier went for A$2300. Oh well, I get my satisfaction from the fact he loves it and has the skill to get the best from it, plus that it is actually being used rather than being a museum piece in someone's collection..

The new ATTAM GIT Amp Project after that and after that FINALLY new HiFi Monoblocks for myself.

Now back on thread.

Cheers,
Ian

[Zozobra]

I've never noticed that the NFB of those 1W marshalls goes to the grid of the cathodyne! Some interesting stuff in them for sure.

I've been playing around with an ecc99 poweramp at the moment and I must say I sounds surprisingly large. I was using a toroidal PT but now I'm experimenting with a ZVS driver and a laptop brick for the PSU. One of the things I want to try is to build a reactive dummy load for taking a line out to record. I'm hoping I can get away with using regular inductors, rather than aircore ones since the power is low at maybe 3-4W as they'll be cheap and compact. I do want to then stick a cheap class D module on to it so I have more power if I need it. Not quite as elegant as what you're proposing here I know, but I'll be sure to report on it nevertheless

[Roberto]

Thanks Ian,

Basically you suggest an "ultimate attenuator".
There could be two outputs, if you do not plug into the pure tube one, a load will be connected to the OT and the amp can be reamped by the SS amp.

As for the other project, I'm fully available to give you my contribution.
Not that you need to, but may just be another point of view.

[Roberto]

Zozobra, the v-twin pedal preamp had one of the best (line level) cabinet emulators I've ever used.
Could be a good starting point for your project.

Then when no miking was possible, I used to use a H&K passive Red Box.
Sometimes a bit harsh, but nothing impossible to adjust with the mixer's eq.