Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › A DIY electrostatic amp for intermediate DIYers?
New Posts  All Forums:Forum Nav:

A DIY electrostatic amp for intermediate DIYers? - Page 7

post #91 of 593
2/3/09 at 6:48pm
Quote:
Originally Posted by runeight View Post
Voltage stresses are relieved everywhere, particularly on the input devices. We can use 6S4s in the GG configuration. As you can see biasing them properly needs a static grid voltage divider.
That is NOT grounded grid. The 100k ohm resistor has a BIG effect here.

You *may* be able to get away with replacing the 100K ohm resistor with a zener diode or voltage reference tube, but they may add noise and will waste HV current.
Quote:
There is still the issue of frying the source, but that's easy to fix. And there is still O/P protection. How do other people do that?
The easiest thing is coupling caps at the output.
Next up is a coupling cap somewhere in the middle. Between the input stage and the 2sc2705 with an adjustable voltage reference for each 2sc2705 would allow grounded grid, and output voltage offset adjustment.
Quote:
How are we doing?
Please remove the +15V reference for the tail CCS in the input stage. The input stage of the amp is the the only thing that uses it, and although it improves performance and saves a cap it adds complexity & cost. Realistically: an amp like this is either going to "mission creep" to the point of looking like a BH with currently available transistors or have caps.

since the rail voltage has dropped to 350, 400V caps can be used, bonus round for the frugal-fi'ers
Reply
post #92 of 593
2/3/09 at 7:49pm
Why yes, there is some effect. But is it really that big? The grid current changes are small even during full plate voltage swings. This is really just a grid leak resistor (and fairly small as they go). So, it is effectively a GG. However, I might be wrong about this so would you be kind enough to follow up? We need to get this right.

Coupling caps at the input. Done.

I know we want to avoid caps at the output. I don't see how an internal coupling cap helps. The O/P stage will still need an adjustment and can still go to the positive rail if the tube dies.

If this is true, then we can't protect with an internal cap. Or, perhaps, I am just missing something?

If we don't use an internal coupling cap then the absolute O/P stage offset can be adjusted by changing the current source on the tail of the diff amp. And the relative offset between the two sides can be adjusted using a trimpot between the emitters of the diff pair.

However, in anticipation of a fair critique of this concept, these controls will be very sensitive and possibly impractical.

So, the other reason I added the grid leak resistors is because they offer the perfect place to adjust the O/P offset individually for each side. Putting a 75k resistor there in series with a 50k pot (or something like that) would make each side independently adjustable and reasonably, but not perfectly, stable.

Thoughts????
Reply
post #93 of 593
2/3/09 at 11:24pm
A totally newb question. I have no idea how any of these voltage swings and biases apply to stats. If my target headphone, which I hear is a good choice with an uprated amp, is the Koss ESP-950, would this amp be considered "uprated" and be able to bias the ESP-950 properly. Note, I have no idea what it means to bias a stat nor why it matters, and will have to research that.
Reply
post #94 of 593
2/3/09 at 11:37pm
Thread Starter 
Quote:
Originally Posted by holland View Post
A totally newb question. I have no idea how any of these voltage swings and biases apply to stats. If my target headphone, which I hear is a good choice with an uprated amp, is the Koss ESP-950, would this amp be considered "uprated" and be able to bias the ESP-950 properly. Note, I have no idea what it means to bias a stat nor why it matters, and will have to research that.
Most 'stats can be driven reasonably well by the same amps if they are supplied with the correct bias voltage.

Old stax normal bias was 230v. Pro bias is 580v. The esp950 is 620v iirc.

The esp950 can be driven quite well by pro-bias stax amps even at the 580v bias. It is of course better to provide the designed voltage.

If my plans come to fruition, I hope that this amp will have a bias supply flexible enough for any halfway normal electrostatic headphone. Delivering 1kv bias for jecklin float electrostatics is probably outside of the scope of this amp. They also have panels so big that you'd need much more power than even an omega needs.

The short answer is: Yes. And it should sound much better than the E90 amp.
Reply
post #95 of 593
2/4/09 at 12:08am
the Koss spec for 950 bias is 600V. I have tried 580/600/620V in my JRM transformer box driving the 950, and didn't hear obvious difference. The 620v might be beneficial in a blue hawaii, but I doubt it will do much in a more moderate amp.

20V difference is less than 5% (of the 600V), after all.
Reply
post #96 of 593
2/4/09 at 1:46am
Quote:
Originally Posted by AudioCats View Post
the Koss spec for 950 bias is 600V. I have tried 580/600/620V in my JRM transformer box driving the 950, and didn't hear obvious difference. The 620v might be beneficial in a blue hawaii, but I doubt it will do much in a more moderate amp.

20V difference is less than 5% (of the 600V), after all.
In a KGSS, it _is_ really quite noticeable, so, perhaps it would make a difference here as well. If is done cheaply through resistors, certainly worth a try. It will simply have to be tried to know for sure.
Reply
post #97 of 593
2/4/09 at 4:54am
I did most of my bias tests with a T1 doing the driving and the effect was very noticeable.

As for the bias a +350v supply gives us ample headroom for all of the bias voltage we really need. Like Eric said we shouldn't be looking at this amp to drive floats as they not only need more bias but also a whole lot more voltage swing. So instead of the normal load resistor we could put spots for two or even three resistor pairs on the board so the amp can have 230, 500 and 580v bias or what ever the builder wants.
Reply
post #98 of 593
2/4/09 at 6:26am
Quote:
Originally Posted by spritzer View Post
Like Eric said we shouldn't be looking at this amp to drive floats as they not only need more bias but also a whole lot more voltage swing.
my dreams are now officially shattered
Reply
post #99 of 593
2/4/09 at 6:41am
Quote:
Originally Posted by bjarnetv View Post
my dreams are now officially shattered
Just get Justin to build a 1.2kV bias supply into a BHSE.
Reply
post #100 of 593
2/4/09 at 7:06am
Quote:
Originally Posted by spritzer View Post
Just get Justin to build a 1.2kV bias supply into a BHSE.
if i sell my car, and speakers... and all my headphones except the floats, then i might just afford it
Reply
post #101 of 593
2/4/09 at 1:20pm
Quote:
Originally Posted by nikongod View Post
That is NOT grounded grid. The 100k ohm resistor has a BIG effect here.
Aha, I see what you mean. We'll have to fix this . . .
Reply
post #102 of 593
2/4/09 at 2:43pm
Quote:
Originally Posted by FrankCooter View Post
Nobody commented on my suggestion to use a small push-pull tube amp (a la Broskie) as the basis of a Stax amp. I'll throw it out again if for no other purpose than to have it formally dismissed. How about a long tailed pair of triode strapped 12gn7a (Pete Milletts fovorite cheapo pentode driver)connected to push-pull triode strapped el34s driving a hammond transformer? Use the primary of the transformer as a center tapped plate choke. Pull the output off the el34s via cheap but exellent 630v Russian p.i.o. coupling caps. Build a 560vdc power supply using an Antec toroid and some motor run oil capacitors. The bias takes care of itself. Parts available in about 30 minutes and a total cost of under $400.00. Simple, cheap, and fairly foolproof. Tell me what I've got wrong here.
Frank:

Would would be the reason to do the above over either the Egmont or SR-Xh (I think that's what it's called... aka ES-1) circuits? Don't these two circuits offer great bang-for-the-buck in some sense?
Reply
post #103 of 593
2/4/09 at 5:21pm
Thread Starter 
Quote:
Originally Posted by FrankCooter View Post
Nobody commented on my suggestion to use a small push-pull tube amp (a la Broskie) as the basis of a Stax amp. I'll throw it out again if for no other purpose than to have it formally dismissed. How about a long tailed pair of triode strapped 12gn7a (Pete Milletts fovorite cheapo pentode driver)connected to push-pull triode strapped el34s driving a hammond transformer? Use the primary of the transformer as a center tapped plate choke. Pull the output off the el34s via cheap but exellent 630v Russian p.i.o. coupling caps. Build a 560vdc power supply using an Antec toroid and some motor run oil capacitors. The bias takes care of itself. Parts available in about 30 minutes and a total cost of under $400.00. Simple, cheap, and fairly foolproof. Tell me what I've got wrong here.
"under $400" fails to meet the spec of "under $300".

I don't want to build an amp with a $55 chunk of iron on each channel.

It's a good idea to be sure, if you already have the push-pull amp. Every time i see an old console stereo at a flea market i get my hopes up.
Reply
post #104 of 593
2/4/09 at 6:53pm
Im not sure that the design runeight is drawing up will be buildable for less than $400 either.

Increasing the number of HV caps required for the amp and added complexities in controlling the bias of the output stage precisely may even come out more expensive.

Unless I misunderstood, the signal does not PASS through the transformers in franks description: they are just there to act as a plate choke, so quality becomes somewhat secondary to what you can get to fit in the chassis and budget. He mentioned Hammond iron, but considering that signal dosnt really go through the transformer a "wallet friendly" Asian part could probably be used with adequate results.

Anyways:
On the front of runeight's design, here is what I think:
the output stage, with 6s4(a) tube, here is the operating point, just so I don't REALLY screw this up.
330V from plate to cathode, 20V cathode to grid, and about 5mA idle current.

So the crux move for DC coupling after the input is getting and keeping 20V on the emitters of the last transistor while allowing adjustments to fine-tune the bias.
Reply
post #105 of 593
2/4/09 at 7:10pm
Quote:
Originally Posted by nikongod View Post
Im not sure that the design runeight is drawing up will be buildable for less than $400 either.

Increasing the number of HV caps required for the amp and added complexities in controlling the bias of the output stage precisely may even come out more expensive.
The amp would be around 50$ per channel but that leaves the PSU...

Quote:
Originally Posted by nikongod View Post
Unless I misunderstood, the signal does not PASS through the transformers in franks description: they are just there to act as a plate choke, so quality becomes somewhat secondary to what you can get to fit in the chassis and budget. He mentioned Hammond iron, but considering that signal dosnt really go through the transformer a "wallet friendly" Asian part could probably be used with adequate results.
That's true, just about anything could be used between the tubes but then we have output caps to deal with. They have to be chosen carefully as they can "sing" along. The cheap Russian teflon's should work just fine but I'd rather direct couple the transducers...
Reply
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › A DIY electrostatic amp for intermediate DIYers?