Balanced L'espressivo

[FallenAngel]

Hey guys,

I've always loved to follow Doug's work and his L'espressivo amp has caught my eye years ago. After a few incredibly informative conversations, I've decided that it's an amp I'd like to build... with a few twists.

I've read about the theoretical balanced version here and am planning this build.

This is going to be a build log and a place for planning as well as discussion.

Budget is not specifically set for this build but as I see this design as a potential MONSTER, I will certainly not slouch where not necessary.

More to come soon. Comments are welcome at any time.

Thanks

[FallenAngel]

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[FallenAngel]

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[Uncle Erik]

It is a nice design! I've looked it over a number of times and it's been on my "someday" list for awhile. What output transformers are you planning to use?

[dsavitsk]

Hi Pavel,

To save people the time, here's the basic schematic



This is basically either a Raven, or the K&K linestage and can be built either way. The difference between those two is the type of output iron used, and there are costs and benefits to each. The Raven uses standard push-pull iron with DC through the primary while the K&K is parafeed with a blocking cap. In theory, the K&K can use higher quality iron with better coupling between the primary and the secondary. But, it does need the cap which will impact on fidelity.

That said, the basic issues to building one of these are output iron, input iron, and what tube to use. And, to a lesser extent, power supply -- to a lesser extent because half the point of the design is that the PS is less of a factor than for a single ended amp.

For input iron, you basically get what you pay for. The Edcor WSM's in the schematic are about $10 each, and sound and work fine. They lack some dynamics and may have a little distortion. Good for breadboarding, though. For a little more money (between ~$50 and $80 each), you can get Cinemag's or Jensens or Sowters. They will all be a significant step up from the Edcor. Or, you can splurge and get some amorphous core Lundahls. These are the best input transformers I've used, hands down. But, they aren't free.

For tubes, anything with decent gain and Gm -- 6H30, ECC99, 6N6p, 5687. Or, use a quad of something like 5842, 6688, D3a, C3g, etc. I use this circuit as the output stage to a DAC with 6688's and it works great. Lots of good options.

That leaves the output iron which is the trickiest. For Low Z phones, you are basically looking for something with a big enough ratio and with decent primary inductance. The power toroid in the schematic is actually great for breadboarding. I don't think you'd want it long term, but it is a good and cheap way to get the circuit working. The won't deal well with DC though, so they are parafeed only. Note, too, that the dual windings make the schematic look nicer, but are not necessary. Single winding parafeed transformers will work fine, too. A decent option is the Lundahl LL1930. These are 5.8+5.8:1+1. This means that you can get a Z ratio of ~4500:32. This isn't ideal, but the inductance of these is such that it is a little better than that number suggests. Unfortunatly, that's about the best I am aware of unless you get something custom made. To that end, Electra-Print, or perhaps Intact Audio, are good sources.

For standard push-pull iron, well, there still aren't a lot of choices. Something like a Lundahl LL1630 or LL1689 might be a good choice. Otherwise, again, custom might be the order of the day.

Finally, for the PS, the K&K version has CCSes to isolate the PS from the circuit (in addition to the fact that the circuit is isolated anyway.) So, PS hardly matters. Use decent diodes to keep out the switching noise, and a small filter -- CLCRC would do just fine. For the Raven-esque version, sticking with a shunt regulator of some sort, especially one fed by a CCS would be a good choice. The shunt element can be a glow tube, or a zener, or some other clever concoction. It probably won't make a huge difference.

Oh, all of those tubes will run happily with about ~160V on the plate, less for some of them. With the Raven-esque topology, you would thus need a ~160V PS. For the K&K, you'll need to drop some voltage across the CCS, so ~220V is probably a decent spot. Also, if you use the K&K topology, note that while the blocking cap can see full B+ at startup, if the tubes are perfectly matched, there will be 0V across it at idle. The whole point of it being there is that perfectly matched tubes is unlikely, but it is basically plate to plate, so the voltage across it is actually minimal. You still need to account for startup voltages.

One last thing, due to the high voltages involved, you really need to ground one side of the output transformer secondary. I know this is no fun as it is no longer "balanced," but this is the reality of high voltages. You might find a transformer with a center tap that can be grounded and run full balanced, but that will require something to be custom made.

[Koyaan I. Sqatsi]

Neat little circuit.

But at the end of the day, ya gotta ask yourself, do you REALLY want to build an amp designed by THIS guy?





Hiya, Doug!

k

[luvdunhill]

Originally Posted by dsavitsk

One last thing, due to the high voltages involved, you really need to ground one side of the output transformer secondary. I know this is no fun as it is no longer "balanced," but this is the reality of high voltages.

ah yes, the "balanced" versus balanced debate. I guess pabbi1 is still at the beach, but we all know how much he loves that debate.

[FallenAngel]

Hmm... OK, I guess I'll start with the unbalanced version first.

I'm thinking I'll be starting with this version ecp.cc

As a "basic" start:
Tubes: 6688 ($9 each from my favorite tube shop - Vacuum Tubes Inc) - 6.3V @ 300mA heaters
Output Transformers: Hammond 119DA (Mouser)
Power transformer: I'm thinking R-Core R0026-009 from VT4C. It has 2x 0-220V 50mA and 2x 0-6.3V .8A secondaries.
Choke: Again from VT4C, 010-10H100 10H 100mA.
Coupling caps: Still deciding on this one. What voltage does it really need to be?
CSS: I'll go with 2x 10M45's per channel, but I don't get the whole Hi-Z vs Low-Z output, can you please elaborate.

[dsavitsk]

Originally Posted by FallenAngel

Hmm... OK, I guess I'll start with the unbalanced version first.

How come?

Originally Posted by FallenAngel

As a "basic" start:
Tubes: 6688 ($9 each from my favorite tube shop - Vacuum Tubes Inc) - 6.3V @ 300mA heaters
Output Transformers: Hammond 119DA (Mouser)

These transformers are cheaper at both AES and Radiodaze ... plus, they both have tubes (and sockets and power transformers) which will save you on shipping.

Anyway, 119DA into grados, off the 8R tap are 2400:32. Off the 4R tap they're 4800:32. Rp of the 6688 is ~3500. It is generally a good idea for primary Z to be 3x to 5x rp. Bending this is probably OK here, but this is probably not a great match. A tube with an rp in the 1200 to 1600 range is a better choice.

I would say the best option for the 119DA is a single 6H30 or maybe a 6N6p. Nice tubes, easy to find, good specs all around. Other options are things like the 6C45, 7788, etc. But, these can be difficult to work with as they will oscillate easily.

Otherwise, if you want to stick with the 6688, look into the power toroids to get a better Z ratio.

Originally Posted by FallenAngel

Coupling caps: Still deciding on this one. What voltage does it really need to be?

With a CCS load, at least equal to B+ when unloaded. Which, by the way, can be pretty low with the 6H30. A 120:120 isolation transformer, available for about $12 from everywhere, with solid state diodes will easily get you ~150V which is more than plenty here.

Originally Posted by FallenAngel

CSS: I'll go with 2x 10M45's per channel, but I don't get the whole Hi-Z vs Low-Z output, can you please elaborate.

Just use the high Z out -- the low Z, also referred to as the mu output, has a lower output impedance (though, frankly, not lower enough that it matters here) but it also has less isolation from the power supply. This might help PimmLabs

-d

[FallenAngel]

Originally Posted by dsavitsk

How come?

The whole "balanced" thing. If I ground the one of the taps on the output transformer, it's no longer balanced or do I not understand how you want to actually "ground" it.

Originally Posted by dsavitsk

These transformers are cheaper at both AES and Radiodaze ... plus, they both have tubes (and sockets and power transformers) which will save you on shipping.

Which power transformer should I be looking at from here?

Originally Posted by dsavitsk

Anyway, 119DA into grados, off the 8R tap are 2400:32. Off the 4R tap they're 4800:32. Rp of the 6688 is ~3500. It is generally a good idea for primary Z to be 3x to 5x rp. Bending this is probably OK here, but this is probably not a great match. A tube with an rp in the 1200 to 1600 range is a better choice.

I would say the best option for the 119DA is a single 6H30 or maybe a 6N6p. Nice tubes, easy to find, good specs all around. Other options are things like the 6C45, 7788, etc. But, these can be difficult to work with as they will oscillate easily.

Otherwise, if you want to stick with the 6688, look into the power toroids to get a better Z ratio.

Originally Posted by dsavitsk

With a CCS load, at least equal to B+ when unloaded. Which, by the way, can be pretty low with the 6H30. A 120:120 isolation transformer, available for about $12 from everywhere, with solid state diodes will easily get you ~150V which is more than plenty here.

Cool, this opens up an nice little option - I've got a pair of 4.7uF V-Cap OIMP that I don't have much "use for", that I wouldn't mind stuffing in this. This does make it sound like I should be considering 2 builds - a high-voltage one with other caps and a lower voltage one so I can use these. DIY gets crazy quickly. I also have a 2-channel DACT stepper I don't have a project for either.

Originally Posted by dsavitsk

Just use the high Z out -- the low Z, also referred to as the mu output, has a lower output impedance (though, frankly, not lower enough that it matters here) but it also has less isolation from the power supply. This might help PimmLabs

-d

Thanks, will use High-Z.

----

Alright, back to planning the balanced build.

Lets go with something reasonably "low-voltage", but I really wouldn't mind a tube rectifier. My V-Caps are 250V, so I'll aim for about 200-220V on the plates. With some tube rectifiers I've seen, (recommendations very welcome), I've seen about 20-50V drop, so what power transformer should I be looking at?

With the schematic I just realized that it looks like there is no volume control in that schematic - just a 50k pot but I'm guessing that for balancing input, not volume control.

Input transformer recommendations?

Another thought - though I know the CSS makes the PSU considerably less important, what do you think about throwing in the Bijou regulated PSU?

I'm thinking I want to start with a "decent" budget build (no VCaps or DACT), with 119DA's, 6H30's and decent specs.

[luvdunhill]

Doug, are you sure you don't need a true RC filter after the input transformer?

I'd probably use a proper 10K volume control in this case myself, as the fewer things in the signal path, the better, right

[luvdunhill]

Originally Posted by FallenAngel

With the schematic I just realized that it looks like there is no volume control in that schematic - just a 50k pot but I'm guessing that for balancing input, not volume control.

Input transformer recommendations?

nope, it's there. I'd go with the Cinemag's personally.

[Koyaan I. Sqatsi]

Originally Posted by luvdunhill

nope, it's there. I'd go with the Cinemag's personally.

Ditto.

k

[pabbi1]

Originally Posted by luvdunhill

ah yes, the "balanced" versus balanced debate. I guess pabbi1 is still at the beach, but we all know how much he loves that debate.

Just got back - fishing until 1am, then getting up @5am to drive 10 hours back makes for a kinda long day... and no chance to add anything coherent, 'cept...

The BH is 'balanced', so other dual stereo boards wired for +/- aren't? That's where I lose the fox.

Anyhoo, great to see more folks wandering this way... oh, and FA, using that balanced DACT for this build?

This is the version I followed, with the pricey Lundahl iron... pity I can't quite finish it off.

[dsavitsk]

Originally Posted by FallenAngel

The whole "balanced" thing. If I ground the one of the taps on the output transformer, it's no longer balanced

well, the circuit is still balanced which seems more important to me.

Originally Posted by FallenAngel

Alright, back to planning the balanced build.

With the schematic I just realized that it looks like there is no volume control in that schematic - just a 50k pot but I'm guessing that for balancing input, not volume control.

No, it's a volume control. The advantage of doing it this way is that there is only one wiper so both sides are the same. With a 4 gang pot, there is imbalance which means that it is not quite in balance.

Originally Posted by FallenAngel

Another thought - though I know the CSS makes the PSU considerably less important, what do you think about throwing in the Bijou regulated PSU?

It should work fine.

Originally Posted by FallenAngel

I'm thinking I want to start with a "decent" budget build (no VCaps or DACT), with 119DA's, 6H30's and decent specs.

For balanced? I think I'd use something with a higher ratio -- this is good for SE, though.