|28th April 2012, 02:09 PM||#21|
Join Date: Feb 2008
stratus46, Yes I have that very same issue that it was printed in.
That schematic has alot of errors, I have tried to work them out in circuitmaker but never got it to simulate.
Perhaps I will give it another try some time now that I know a little more about how amplifiers work.
I know of another cool cascaded circuit as well that uses bjt's driven from an opamp from the free circuits forum.
I will try to find that link in my archives as I had just switched from winXP to linux ,I don't have everything setup yet but it is going good.
I did do a cascaded version of your typical NPN output transistor type amplifier that runs on -/+ 500v bipolar supply when bridged will produce 2kv P-P.
I haven't built it yet but it simulates well and probably could be refined a little more now that I fully understand how that type of circuit works now.
I will dig that one up if anyone wishes to check it out as well.
I had considered a similar super simple HV Differental BJT amp using some 2SCxxxx Horizontal output transistors (kind of like the national circuit), but they are costly and getting hard to get compared to HV Fet's.
2kv P-P is plenty for headphones but is marginal for an ESL system depending on the size of the panels.
The typical Impedance of a large panel is in the order of around 8k ohms at 20Khz so you can imagine that the current required to fully drive one would be very great.
But for little panels of around 50pf the currents will be at a much safer level like below 30ma or 20ma.
Sorry I haven't been doing any circuit building lately maybe later after I get some new panels made as direct drive has always been my main goal.
From what I have read about driving capacitance loads is that you need a lot of current and think in some cases as much as 5 times more than the normal capability in order for the amplifier to stay stable.
Last edited by geraldfryjr; 28th April 2012 at 02:15 PM.
|28th April 2012, 05:35 PM||#22|
Join Date: Jan 2012
Location: Staines, UK
I think that a large panel will be too directional at 20 kHz so that it isn't necessary to drive it all. In my opinion the Quad ESL57s were too directional in the vertical plane and were quite limited in the horizontal plane using a panel width of 1.5 inches. The height was much greater.
It's difficult to get more than about +/- 2kV peak from semiconductors without using many output devices. I think that it could be sufficient for the mid range and upwards. It's probably best to use a conventional cone speaker for the bass. The Quad ESL57s were rather bass light, as good as a book shelf speaker but poor in comparison with a floor standing conventional louspeaker. Also the maximum level was rather limited.
|28th April 2012, 10:07 PM||#23|
Join Date: Feb 2008
Very good points.
For my little panel was getting something like 105db at 1 meter for 2.5kv P-P on test tones and about double the voltage for music at the same level.
But the panel failed and burned up before I had a chance to get any solid results for requirements at different levels.
But panel size and distance from the panel mostly rules those requirements.
Not really an issue for headphones.
My little panel were 3.25" x 9.75",roughly 4 times the size needed for a headphone driver and it easily produced 110db at the diagphram at that level.
Even 105db at 1 meter was much to loud for comfort after a while.
I will resume those experiments soon.
I wanted to chart voltage requirements and spl levels to use for building a direct drive amp.
But those original panels had decided that they have seen enough abuse for the last 9 years.He,he,he,he !
It was sad that all of the times that I pushed them really hard for long periods of time I didn't have a working SPL meter.
Even though I have never heard a 57 it sounds about right as far as bass goes.
The surface area with displacement is about that of an 8" woofer.
I have made a pair of 7.5" X 22" panels so I think I have a pretty good idea any how of what they might sound like.
I have given up the idea for a very large panel bass system for now but I will give it another look sometime at a later time.
Bass systems power requirements are much less depending on of course the panel size.
Woofers are still a better way to go when it comes to bass.
The amp I had in mind was going to run off of a MOT at about 3.5kv this would give me 7kv P-P and would be plenty considering it wouldn't be running at full volume all of the time.
My speaker system is about 14' from my listening point so I get about a 10db drop instantly.
I can measure 119db at the speaker but only 109.5 at my listening point.
I don't listen to it like that all of the time but every once in awhile I do.
to me 103db to 105 is a comfortable loud level when I get in the mood.
Last edited by geraldfryjr; 28th April 2012 at 10:18 PM.
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