SS Driver and splitter stages used for PP tube output??? - diyAudio
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Old 9th September 2009, 03:01 AM   #1
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Default SS Driver and splitter stages used for PP tube output???

Hi,
I am still a bit of a newbee but I have built several tube amps in the past 10 months (started this addiction 10 months ago). I have built a few PTP tube preamps from schematics as well as the Oddblocks and a couple of others. Now that I am starting to understand this stuff a little it occurs to me that it is probably possible to use a SS chip in place of the driver and splitter stages of a PP arrangement. I am not yet saavy enought to step out on my own and figure something out like this by myself but I am very interested to learn more about this concept. Has anybody out there done or seen this done? If so could you refer me to some resources or schematics on this typpe of arrangement?
Thanks for your help!
Jeff
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Old 9th September 2009, 03:22 AM   #2
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Jack Eliano at Electra-Print has always made some of the best transformers for tube amps. I had a long conversation with him several years ago discussing our different beliefs in vacuum tube amp design. He was pretty adamantly against using silicon in the signal path and was pretty vocal about using mosfets to drive a triode into A2 (PowerDrive).

Well time has a way of changing things, never mind the mosfets, how about chip amps driving triodes:

http://www.electra-print.com/pushpull_a2.php
http://www.electra-print.com/singleended_a2.php
http://www.electra-print.com/211a2.php
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Old 9th September 2009, 03:52 AM   #3
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FWIW, I'd steer clear of SS voltage gain blocks, as there are significant linearity issues requiring a good deal of corrective NFB.

OTOH, it's easy enough to use an enhancement mode MOSFET as a DC coupled "concertina" phase splitter. Unlike a triode, the pentode like FET can swing reasonably close to the PSU rail.

What sort of O/P tubes do you have in mind? EL34s and, perhaps, 6L6s will work with a SS "concertina" splitter.
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Old 9th September 2009, 05:32 AM   #4
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I had in mind KT88, KT77, 6L6, or even EL84's.

I have built several gainclone type chip amps, in fact that is how I got into tube gear. I have played with inverting vs. non-inverting chips and thought recently how it should be possible to use both the invet and non-invet outs at the same time to drive the PP power stage of some tubes like above. I really don't know enough at this point in my learning to understand the interplay between these two circuits. What I mean is I don't know enough to know what the inout and output impeadances of the 2 circuits would need to look like and how to achieve proper matching as well as the voltage swings needed in these circuits and even what the voltage swing looks like coming out of, lets say, an LM3875 or LM1875 chip. I know the chips put out a lot more power than the driver stage of a PP amp. Like I said, I am still learning alot about these types of circuits. It is one thing to have the technical skills to copy another's work and build a great tube amp (like I have done) and even understand pretty much what is going on in the circuits I have copied and built. It is quite another thing to understand this stuff enough to design a circuit from scratch or even know how to pick the proper "silicon" as you say, to give the desired results.

I just have read in many of the tube PP circuits output power can be limited by the THD of the previous stage. I know that there are too many to count chips out there that could provide extremely low distorion and could possibly be used as the driver/splitter in place of a valve.

I take it what we are looking for in a driver stage is voltage gain and there should be plenty of chips out there that could do this. I looked at the circuits at electraprint and the TDA2030 appears to be doing just that. Also the circuit makes an impressive wattage. The TDA must be providing something a tube didn't in that circuit to be able to get that kind of power. There is an interstage transformer after the TDA, Is that going to always be needed or could there be a chip out there that has just the right amount of voltage and output impeadence to be able to couple the two togeter with a simple capacitor; use the invert of one leg and the non-invert of the other and couple them with caps directly to the top and bottom of the PP circuit? My questions aren't meant to be answered directly like "yes Jeff, get a XYZ123 chip and hook it up to a couple of EL84's". I guess what I am asking is would it be possible and what would I need to know about the circuit and chip to do it. What questions will I even need to ask or answer to do this?

Take for instance Bruce Heran's Oddblocks which I have built. I know this is not a typical PP circuit b/c of it's self inverting nature but let me say I wanted to use the non-inverting output of an opamp to drive the following power stage. I would need to find an opamp that basically provided the same output impeadence and voltage of the 12SL7 right. My point is, the opamp should have similar characteristics as the tube. My question is, what are those characteristics I need to match to make it work. I figure if I can answer that question than it is just a matter of finding the opamp that mimics the tube and "plug it in" right? Or am I way to simple minded about this? Heck, maybe there is no such thing as an opamp that is similar in characteristics to a driver tube. It appears I have to do some homework on the TDA2030, it sounds like an opamp.

Let me know if I am at least way off base or not.

Thanks for listening to my ideas, I will probably look back in a year or two and think "what was I thinking" as I have already done many times in the past year as my knowledge has grown in this stuff.

Jeff
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Old 9th September 2009, 07:48 AM   #5
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by tubelab.com View Post
Jack Eliano at Electra-Print has always made some of the best transformers for tube amps.

...

http://www.electra-print.com/pushpull_a2.php
http://www.electra-print.com/singleended_a2.php
http://www.electra-print.com/211a2.php
However remember that those circuits are designed specifically to sell transformers and chokes. (For example, if you build the Puss-pull A2 6A3 as drawn you end up buying 7 lumps of iron from them!).

Surely if you are going to mix tube and SS you would want to optimise each bit according to the strengths of the technology? High feedback SS voltage gain stages rarely match the subjective performance of an open-loop tube stage. And tube outputs never achieve the low output impedance of a SS output stage. So Jeff, I think your approach may be the reverse of the ideal. Perhaps you could consider a hybrid of an open-loop triode based gain stage feeding a single-ended solid state emitter-follower / source-follower output stage.

Off the top of my head I would say a 6H30 using a constant-current anode load (at roughly 25 mA current, -3V bias, 90V anode voltage) capacitor coupled to a FET output follower, such as from forum member PMA... http://web.telecom.cz/macura/follower_e.html .

Personally when I use solid state I use it for voltage regulation, constant current generation and impedance buffering (either at the output, or interstage like George does in his tubelab designs). I prefer to use tubes for voltage amplification.
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Old 9th September 2009, 04:38 PM   #6
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Hi again, I have spent some time at Electraprint's website studying the stuff there. So it appears they have done in part what I was thinking about. He basically is using a chip amp to drive a tube powerstage. Looks like they have designed a special transformer/phase spitter to match the inpeadences and drop the power from one stage to the next. I studied the TDA 2030 spec sheet, it is basicaly alot like the LM1875. I was thinking of using 2 chips, one inverting and the other leg non-inverting to drive either side of the PP cicuit. They have solved both issues by using one chip and the special transformer. Pretty cool stuff. I tried to follow the tech blog on the website about driving into A2 using the chip but it is still a bit over my head. Appearently they use the chip to solve the problem of distortion when driving into A2, how the chip can drive into A2 and not distort is what I am having trouble understanding.

The phase splitter they used appears to have the quality of a line output transformer but kinda backward with the primary and secondaries in reverse to step down the power from the chip amp.

Well it is becomeing more and more appearent that I have much more to learn. Thanks for entertaining my thoughts!

I would love to try to build one of those designs at Electraprint but wow, those could get very expensive!

Jeff
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Old 9th September 2009, 06:12 PM   #7
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"Apparently they use the chip to solve the problem of distortion when driving into A2, how the chip can drive into A2 and not distort is what I am having trouble understanding."

The very low output Z of the chip allows them to drive in A2 mode without much distortion. It's able to handle the sudden increase in tube grid current when the grid voltage passes above 0V (with respect to cathode). A2 mode makes the SS driver somewhat sensible here, but just a SS follower, like Tubelab uses, will work fine for an A2 tube driver.

Most of the SS IC power amps I've seen only have one polarity of output, although some are dual channel units where one could be operated as an inverse amp versus the other. There are some IC driver chips that have two outputs from one driver, but they are in phase, just voltage shifted for operating the P and N devices in the output totem pole.

Don
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Old 9th September 2009, 06:50 PM   #8
ChrisA is offline ChrisA  United States
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I've been biuybf "junk" amps for parts. Some of these are from a Hammond organ. Some Ive had some motivation to study the Hammond desings. I really like what I see there. Cost must not have been a factor there because Hammond seems to have "throught tunes and iron at the problem" in a big way. But if I were to build a hybred SS/Vacuum system I'd used the Hammon power amp design

The typical organ amp is feed a "balanced" signal. Today with modern electronics "balanced is very common in proaudio system but not used in consumer electronics. Consumer electronics uses unbalance (single ended RCA cables) where the signal is referenced to ground. The balanced signal is sent over two wires that are 180 degrees out of phase. The cable typically has the third grounded wire as well. Typically either 1/4 inch TRS, or XLR connectors are used.

The Hammond amps lokk like two parallel single ended gain and driver stages that finally are connected by the push-pull output transformer. So for example each side of the input jack runs to int's own triode (1/2 of a 12AU7) and then to an EL34 or 6L6 and then the 6L6 contes to one side of the OPT.

Build a solid state preamp section that outputs an "industry standard" balanced line level signal to an XLR jack. Or you can simply buy such a device.
Designing around an industry standard lets you literally "plug into" an entire ecosystem of equipment so you are not required to re-invent every wheel.

What this does different then your idea is to push the phase splitting back into the preamp where it is done at low signal levels, maybe even inside a small audio transformer or with opamps. and not with the "gain clode" chip amps.

Last edited by ChrisA; 9th September 2009 at 07:14 PM.
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Old 9th September 2009, 09:01 PM   #9
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Quote:
Originally Posted by Eli Duttman View Post
FWIW, I'd steer clear of SS voltage gain blocks, as there are significant linearity issues requiring a good deal of corrective NFB.

OTOH, it's easy enough to use an enhancement mode MOSFET as a DC coupled "concertina" phase splitter. Unlike a triode, the pentode like FET can swing reasonably close to the PSU rail.

What sort of O/P tubes do you have in mind? EL34s and, perhaps, 6L6s will work with a SS "concertina" splitter.
As with any MOSFET implementation, I'd suggest using it in cascode. That leaves open a LTP circuit as well. Cascode CCS tail load, and cascode diff amp. With a negative rail of ~20V makes things very much simpler.

Matter of fact, eliminate the heaters entirely... see Gary Pimm's SS Tabor amp( google his site and have a good read ).
cheers,
Douglas
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Old 6th October 2009, 08:40 PM   #10
Empee is offline Empee  Netherlands
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Hi !

This subject makes me think of Tom Evans' work;

The "Linear A" amplifier:
Discrete opamps driving 4 parallel EL84's in a UL-single ended topology (!)

And his "Linear B";
again discrete opamps, one phase inverted, driving 8 EL84's in a 4-by-4 UL
push pull circuit..........


Too bad I cannot find any more details about these designs,
I bet Mr. Evans uses some sort of CCS in the cathodes as well.

I don't know about you, but I find this marriage between sand & vacuum
HIGHLY interesting

Anybody know more about these amps ?


regards,
Empee
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