EL84 as Preamp and as Crossover

[navin]

I am thinking of building a tube preamp, crossover and power amp.

the pre amp will be about 15 meters from the crossover and power amp.

Since the distance between the preamp and crossover is so long would it make sense to use a EL84 as a preamp?

Also could the EL84 be used as a Crossover (12db/oct)?

If so does anyone know of a schematic using the EL84 as a preamp and/or crossover?

I assume that 1 EL84 per channel would suffice for a preamp and 1 EL84 (triode) would suffice for the crossover.

 

[hacknet]

i don`t know if you can get 12dB/oct with 1 stage. from what i know. 1 RC filter will get you only 6dB/oct. you`ll have to stack if you want more.

 

[navin]

i don`t know if you can get 12dB/oct with 1 stage. from what i know. 1 RC filter will get you only 6dB/oct. you`ll have to stack if you want more.

ok so i will need 2 EL84 to get a 12db/oct slope. thanks.

any schematic that you know of?

 

[hacknet]

why would you want to use a el84 as a crossover? its rather distortion ridden and microphonic. why not go for something more efficient like an opamp?

 

[ilimzn]

i don`t know if you can get 12dB/oct with 1 stage. from what i know. 1 RC filter will get you only 6dB/oct. you`ll have to stack if you want more.

If you don't know it, it does not mean it is not possible 🙂
One of the implementations of a 12dB/oct active filter requires an amplifier with Av=1, i.e. a follower - which should be fairly easy to do with an EL84. Triode mode will have a gain somewat less than 1, which will produce an error in the filter's transfer characteristic, but this could be reasonably compensated.

 

[rdf]

I've played around with triode connected EL84s as drivers for an EL84SE. Of the brands tried (EH, JJ, Sovtek and Russian EL84M) my experience was hacknet is right when he says they're too microphonic at that signal level.

 

[Wodgy]

15 meters? If you're planning a line-level cable run that long, your priority should be building a preamp with balanced outputs.

 

[ErikdeBest]

15 meters? If you're planning a line-level cable run that long, your priority should be building a preamp with balanced outputs.

Agree on that - and while you're also thinking about building power amps, I would suggest them to be along the lines of this one: as far as my knowledge goes you could easily turn them into balanced amplifiers.
http://www.wdehaan.demon.nl/tubeamps/el84dc/

For higher power with EL34.
http://www.vacuumstate.com/schematics/pp-1c_s.gif

For the low output impedance needed to drive the 15m of cable a white cathode follower with a regulated PS would be good, even using some 'small' tube (a double triode).

And in case you haven't seen it yet...
http://www.tubecad.com/articles_2001/Active_Crossovers_and_Filters/
http://www.tubecad.com/articles_2001/Tube-Based_Crossovers/

Erik

 

[anatech]

Hi navin,
Unless there is a good reason to run the signal that far, move the amp(s) back to the rest of the equipment. The signal between the pre and power amps is the most delicate, unless we talk about a turntable. The difference between 2 feet of wire and 20 feet of wire is tiny and overshadowed by other negative things.

-Chris

 

[navin]

Hi navin,
Unless there is a good reason to run the signal that far, move the amp(s) back to the rest of the equipment. The signal between the pre and power amps is the most delicate, unless we talk about a turntable. The difference between 2 feet of wire and 20 feet of wire is tiny and overshadowed by other negative things.

-Chris

thanks everyone. wow what a response! I feel special.

the object of locating the power amp that far away is to located them behind the speaker and make them more discrete. this way WAF only sees the preamp and that too is only because of the volume control.

Presently I have a home made SS preamp and 200W SS power amp. The preamp uses relay switching a pair of OP275 and an ALPS pot. The power amp uses Hitachi MOSFETS (3 pair per channel) and is similar to B&K ST140 in topology. Both were built by a friend.

Anyway my wife wonders why I need such large equipment when most of our friends are happy with their Bose Acoustimass and B&W VM1.

Tube power amps will be bigger considering that I am thinking of using an active XO and biamping. OPTs and such are not really WAF compatible.

I got interested in tubes when the same friend got a Prima Luna Prologue 2 KT88 40W amp to replace his 200W solid state. Yes the max SPL is lower but the sound quality is nice. I cannot afford the Prima Luna he has.

Any other options?
SS preamp feeding tube power amps perhaps?
Tube preamp feeding small gain clones?

 

[navin]

For higher power with EL34.
http://www.vacuumstate.com/schematics/pp-1c_s.gif

For the low output impedance needed to drive the 15m of cable a white cathode follower with a regulated PS would be good, even using some 'small' tube (a double triode).

And in case you haven't seen it yet...
http://www.tubecad.com/articles_2001/Active_Crossovers_and_Filters/
http://www.tubecad.com/articles_2001/Tube-Based_Crossovers/

Erik

hey thanks i have seen the PP-1 by Allen Heath. I hear there is a PP-2C ckt out there as well.

in fact i find most amp ckts listed with amp models and not wattage or tubes used like this link below

http://www.one-electron.com/FC_Consumer.html

i have rummaged through some of these to look at the various EL34 ckts.

Now the reason i have shortlisted the EL34 is that the KT88 requires higher voltages and I felt that a Class AB1 Pentode EL84 PP (15W?) might be a bit less for the woofer (88db in a 12' x21' x9' room). Since I wanted to use the same valve for tweeter and woofer I decided to use EL34 AB1 PP for both and just use triode mode (which is only 8W but I'm told has a less grainy high end) for the tweeter.

Given that I got 4 EL34 per channel, plus driver tubes, then at least 1 tube per crossover (4 more tubes) I decided to break the project into 3 chassis (left power, right power and pre). Even with using a SS current sink; 8 El34 plus atleast 8 smaller tubes (4 driver, 4 for crossovers), plus 4 OPTs would be a bit much for one chassis right? I would need only one big power transformer though.

Visually the chassis would have to be 22-24" wide with 5 transfomers (4OPTs and 1 Power) laid out behind a row 8 EL 84 tubes and the smaller driver and crossover tubes.

now some crazy questions refering the to the PP-1C schematic.

1. Can the driver tubes (ECC88 in the PP-1C for example) be also used as Low Pass and High Pass filters?

2. Can all 8 EL34 (if I was to build the huge stereo amp described above) have a common current sink? If yes, Allen uses and IRF 820 but this wont be adequate for 8 EL34 so what are the alternates? In fact I would have to upgrade the entire power supply for the PP-1C but where would I get the 10H choke that Allen uses in this supply. It assume it would be big and expensive so is there an alternate?

3. In the PP-1C photo (http://www.vacuumstate.com/schematics/vt1ab_s.jpg) there are 2 OPTs shown for each PP-1C. in the schematic there is only 1. what am i missing? I assume the black box is the power transformer and the 2 silver boxes are OPTs.

 

[navin]

maybe i can reconfigure this into one big integrated amp. to do this i would need schematics with low part count for the premp, crossover and EL34 power amp.

say an EF86 or alternate input mated to 1 or 2 ECC81 or 83 or alternate. configured to be a LPF / HPF which drives the EL 34 PP Triode. that's just 4 tubes per channel. possible? remember no need for RIAA or phono preamplification. 🙂 my inputs are CD player, Ipod (via dock), Cable TV box, and CD Recorder (analog out).

some EL34 ckts I have considered....

http://www.arrakis.es/~igapop/avantic.htm

http://www.drtube.com/schematics/jolida/sj-502a.gif

http://home.mira.net/~kiewavly/Leak/Circuits/Stereo50.gif

http://werple.net.au/~kiewavly/Leak/Circuits/tl25Plus.gif

however i like the last one best because it looks simplest with the fewest part count.

any other low part count options?

 

[navin]

Visually the chassis would have to be 22-24" wide with 5 transfomers (4OPTs and 1 Power) laid out behind a row 8 EL 84 tubes and the smaller driver and crossover tubes..

the "EL84" should read as "EL34" sorry.

 

[ErikdeBest]

Hi Navin

I am not an expert at all... Most of what I am saying are just thoughts...

I know I said you could just select right RC combination between stages to create a crossover, but for a first project I wouldn't go this way. To me it seems there are too much new things happening at the same time, and when the final results are dissapointing you won't know it it's the loudspeaker, the amp, the preamp, the crossover or your fear all that investment being lost. Some people advice to use full range amps and a Behringer crossover/equaliser for adjustments of crossover frequencies and level's, and when it sounds fine you can keep the Behringer, substitute it with a tubed crossover or adjust RC combinations between stages to adjust crossover frequencies.

2. Can all 8 EL34 (if I was to build the huge stereo amp described above) have a common current sink? If yes, Allen uses and IRF 820 but this wont be adequate for 8 EL34 so what are the alternates? In fact I would have to upgrade the entire power supply for the PP-1C but where would I get the 10H choke that Allen uses in this supply. It assume it would be big and expensive so is there an alternate?

In this amp the EL34's are cathode biased (the big 470R/10W resistor in the schematic). The IRF820 is used as a sink for the differential amp consisting of cascoded ECC88's. This sink is mandatory for good balance in a differential amp. Maybe you can use a LM317 to bias the EL34's...maybe you need the HV version.http://www.glass-ware.com/tubecircuits/Tube_Auto_Biasing.html and again more information available at tubecad.

About the photo. Probably there is one PS trafo and one output trafo. The black cap is probably used to hide capacitors, chokes, etc.

If you decide to go for this design, check this forum, there has been some discussion about this amp. I remember something about exceeding maximum ratings for the ECC88.

Here is another design with the EL34, using just one tube for driver and phase splitter
http://www.plitron.com/PDF/PB/Article/Atcl_9.pdf

And finally - how to put the whole thing together!? I don't know - there are some many options, which sounds best??? As I lack time, interest and money to build every single available circuit, I am 'developing' some prototyping circuits - you can see some pictures of it at this thread - there are also other possibilites shown in the thread.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=68315

good luck, i am gonna work now

Erik

 

[navin]

... Most of what I am saying are just thoughts...

I know I said you could just select right RC combination between stages to create a crossover, ...when the final results are dissapointing you won't know it it's the loudspeaker, the amp, the preamp, the crossover or your fear all that investment being lost.

In this amp the EL34's are cathode biased (the big 470R/10W resistor in the schematic). .... Maybe you can use a LM317 to bias the EL34's...maybe you need the HV version.http://www.glass-ware.com/tubecircuits/Tube_Auto_Biasing.html and again more information available at tubecad.

About the photo. Probably there is one PS trafo and one output trafo. The black cap is probably used to hide capacitors, chokes, etc.

Here is another design with the EL34, using just one tube for driver and phase splitter
http://www.plitron.com/PDF/PB/Article/Atcl_9.pdf

And finally - how to put the whole thing together!? I don't know - there are some many options, which sounds best???

thanks for the all the help. even if you dont profess to be an expert your knowledge is far more than mine.

my idea was to build 2 fullrange stereo power amp's one in pentode mode and one in triode mode (EL34 Class AB1 PP). see if i hear a difference between them.

then add the volume control and input switch,

then add the crossover network (RC) making the triode amps HP and the pentode amps LP.

this what i really would be doing is limiting the bandwidth of an otherwise fullrange amp

I like the idea of using an LM317 to bias the cathode. BTW what is a HV version?

i dont like the big 10H choke in Allen's amp. It is bulky and expensive. so I hope there is a way around it.

the piltron amp is EL34 PPP that would double the El84 I need or maybe I dont biamp at all! just use the bigger 46W triode version. The reason I like biamping is that the amp is connected directly to the driver. It makes the amp really work well. also in from what i have learnt the sweetness of the triode is more apparent in the HF so why waste triode on the woofer? i understand the damping factor with triode is higher so it does help the bass too but not to the same degree right? (again this is dependant on the amp - speaker combination right?).

thanks again.

 

[Wodgy]

then add the crossover network (RC) making the triode amps HP and the pentode amps LP.

Do you currently use a first order crossover in your speakers? I'm asking because there are relatively few speaker drivers that can handle a first order crossover (potentially 2nd order acoustic) like that. In car audio, it's fine, of course.

If you're looking for better sound, it probably would make sense to try a 4th order (acoustic) passive crossover before going active.

If your goal is just to biamp, I'd leave the passive crossover networks in the speakers the way they are, particularly if they're higher order networks. You'd need additional tubes to implement a 4th order active Linkwitz-Riley filter. In any case, I'd prototype my active crossover with op-amps before trying to implement a tube version.

 

[navin]

Do you currently use a first order crossover in your speakers?

my existing crossover is a quasi 2nd order (electrical) network built using LSP CAD and tinkered with over the years.

Quasi because the slopes are not 12db/oct exactly but 12db/oct would be a very close approximation.

i was under the impression that such a XO could be implemented using 1 or 2 tubes.

 

[anatech]

Hi Navin,
I have found that an ultra linear works well in your room size. You can use EL34 / 6CA7 or 6L6 for that. My favorite is 35 WPC running full range. It's extremely clean and has adequate bass (which is very good by tube standards). My hint to you is oversize the output transformers for the expected power. This gives you much better bass and damping.

I would build a stereo amp, or stereo pair. This gets the amp up running before you worry about active crossovers. Once that stuff is running well, build your control amplifier / crossover. You can run your cables under the floor if you are lucky. Maybe later the amps can come live near the pre after the wife gets use to them.

I'm lucky. After 19 yrs, my wife has gone from "I don't want to see it" to "gee I like the sound". Now there are three sets of large speakers in three rooms, in walls and small speakers everywhere. Including the master bathroom. Not to mention all the gear. Now I have to build a cabinet to house the main systems. Given a choice, she will accept reasonable amounts of gear and speakers. She doesn't like the sound of the alternatives. She even lets me change gear around without getting too upset!

-Chris

 

[ErikdeBest]

Hi Navin

I like the order in which you're planning to put things together.

The LM317HV is a Vigh Voltage version of the LM317. Because the difference between input and output are limited on a LM317, you can use it up to 40V of bias, no more (maybe enough for the EL34, I don't know), with the HV version you can go a bit higher!

You are talking about EL34's in pentode mode. Off course there is nothing wrong with it, but I believe most amplifiers with pentodes will use a bit of negative feedback, and I don't know how you would have to apply this feedback to Allen's amp (the original design is with trioded EL34's, no feedback).

Chokes in the PS are not mandatory, you can use a CRC (capacitor, resistor, capacitor) filter... probably chokes would be better, but you have to make compromises.

The plitron design is using PPP, but you can just skeep a pair of tubes and use 2 per channel. I actually posted it to show you the driver and phase splitter stage's, plain simple, using an ECC82.

Erik

 

[navin]

I like the order in which you're planning to put things together.

The LM317HV is a Vigh Voltage version of the LM317. ... (maybe enough for the EL34, I don't know), with the HV version you can go a bit higher!

You are talking about EL34's in pentode mode. Off course there is nothing wrong with it, but I believe most amplifiers with pentodes will use a bit of negative feedback, and I don't know how you would have to apply this feedback to Allen's amp (the original design is with trioded EL34's, no feedback).

Chokes in the PS are not mandatory, you can use a CRC (capacitor, resistor, capacitor) filter... probably chokes would be better, but you have to make compromises.

sorry for the delay. i was a bit swamped. thanks for the complemetn though. i am feeling so lost (as can be seen from my posts on another thread - see link below) that any complment is welcome 🙂

http://www.diyaudio.com/forums/showthread.php?s=&threadid=69042&pagenumber=4

I have not distilled my thoughts completely but so far the plan is to mate Steve Bench's ECC88C based filter to the HIF 87.

build a ECC88C based filter/input
let the ECC88C output drive the 6SN7 (no need for teh ECC83 input)
then the 6SN7 will feed the EL34

what do you think?

all else (power ckts) will be Solid State.