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Old 26th October 2009, 03:12 PM   #21
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Hi,

the wrong way to run an ESL is to run it by current.

Thats why any tube concept fails as long there is a stepup transformer involved. In order to magnetize the coils you need current and tubes are not capable to deliver current.

So if you use a stepup, a transistor amp is the better choice. But most people use amps with low supply voltage ranges (e.g. +/- 30 Volts). If you want to transform those values up to kilovolts, you need high stepup ratio. The performance of a stepup transformer suffers by a square factor with increasing stepup ratio. That means, that a ratio of 1:120 instead of 1:60, will increase the lower bandwith, decrease the upper bandwith and more important increase the phase shift by factor 4 !!!

I build myself a transistor amp using +/- 120 Volts powersupply. The only expensive parts are the capacitors in such voltage range.

Having such an amp you can reduce stepup ratio down to 1:30, which reduces the negative impact of the stepup drastically.

Capaciti
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Old 27th October 2009, 02:15 AM   #22
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Hi Capaciti,

What if I use a pair of 845 vacuum tubes using bias voltage of around 1kV and a step up transformer of 1:3~4. Do you think it will be good?

Wachara C.
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Old 27th October 2009, 07:19 AM   #23
Calvin is offline Calvin  Germany
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Hi,

or take a KR 845 with simple resistive load and drive it up to 2kV anode voltage
We had a thread here about such low-U trannies driven by rather conventional high-voltage amps. The problem remained that such a tranny must be wound on specification, because thereīs no commercial offer and the driving amp should be at minimum 100V capable. One should also keep in mind that a amp with supply voltages of op to 400V-500V (like lots of tube amps) need to be designed for higher impedances. This means it must be able to put out voltages of a couple of hundreds of V. It doesnīt need to put out so large currents though and the output impedance may be higher, so tubes would be fine for this job or HV-MOSFETs. The advantages of a HV-amp plus Tranny would be galvanic isolation (output signal would be free of DC), cheaper standard parts and īconventionalī circuitry.
Still though thereīs a (costly) coupling element between amp and ESL with all itīs restrictions and limits.

jauu
Calvin
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Old 27th October 2009, 07:00 PM   #24
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I looked into the SRPP output stage which has the advantage that you can optimize it for the capacitive load, or it says on John Broskie's site. This is a rather appealing approach because of the extend current capability of the circuit. I prefer tubes over semiconductors. The other logical approach is a conventional push-pull output stage but have it floating as in the circuits posted in this thread. Instead of choke load this task could be performed by the low ratio step-up transformer for the bass.

We have a pretty good transformer winder in the Netherlands who is willing to do single pieces and experiment and has plenty experience with high voltage and audio etc. The only downside is long waiting times (couple of months usually). Automatic Electric
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Old 28th October 2009, 02:23 AM   #25
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We are getting into something interesting now.

The output transformers can be easily made here in Thailand for any specifications you want. Normally I can get mine made within 1-2 weeks. How about we start looking for a suitable circuit? What circuit do you have in mind, Arend-jan?

Wachara C.
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Old 28th October 2009, 06:54 AM   #26
Calvin is offline Calvin  Germany
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Hi,

SRPPīs features sound seductive at first glance. High gain, high output current capability, low output impedance, very low parts count and low distortion without global feedback. But as it is usually implemented -one resisitor spanning between anode of the lower and cathode of the upper tube- it works correctly only into one specific resistive load. Off of the optimu, value it ceases beeing a symmetrical pushpull stage. Now practise isnīt as unforgiving as theory, so within a certain range of load resistor values the SRPP will function somehow sufficiently. But this doesnīt hold true for such an impedance variation a capacitor represents over the complete audio range. So all commercial amps that use such a simple stage rely on massive global feedback to linearize the amp (the beauty of a properly implemented SRPP is that You could omit with global feedback). You know that inherently nonlinear stages corrected by massive global feedback are not what our ears like to listen to.
The resistor between the tubes could be replaced by more complex circuitry to allow for wide bandwidth drive into a complex load like a cap/ESL as J.Brosky suggested. I havenīt seen this implemented yet.

As amp circuitry Iīd suggest a circlotron (sometimes implemented as OTL-amp). The low output impedance would help solve some problems with the tranny. The elevated impedance level of a low-U tranny design works in favour with output tubes. īSafeī for the load even when an output tube fails. It works stable into complex loads. The cons are :Youīd need two floating powersupplies and a driver stage that can supply the full output voltage range to the output tubes.

jauu
Calvin
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Old 30th October 2009, 02:16 AM   #27
ktuuri is offline ktuuri  Canada
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Just thought I would mention, I run Acoustat's with the Direct Drive Acoustat tube amps.
The amps have had slight upgrades. I have been running them for over a year with no problems at all, and on average they are run 6-8 hrs a day.
I have had 1 tube failure, which was strange because it happened when I was shutting down my system. I am using 6GE5 tubes as a sub for the 6HB5 tube. From what I have found from other owners, is that the brand of the tube ( Sylvania 6HB5) tubes are the ones that are a problem.
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Old 30th October 2009, 05:41 AM   #28
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Hi Ktuuri,

Could you tell us more about the direct drive amp? Is it a single ended or pushpull amp? Does it have output transformers or is it OTL? Do you happen to have it's schematic diagram?

Wachara C.
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Old 30th October 2009, 06:19 AM   #29
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There is a chapter dedicated to this subject in one of Menno van der Veen's books. I would have to look it up, but after some thorough investigation he concluded it is not the way to go..
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Old 30th October 2009, 06:52 AM   #30
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Quote:
Originally Posted by Miniwatt View Post
There is a chapter dedicated to this subject in one of Menno van der Veen's books. I would have to look it up, but after some thorough investigation he concluded it is not the way to go..
Really? What seems to be the problem?

Wachara C.
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