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Old 2nd December 2011, 11:32 PM   #11
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Originally Posted by kevinkr View Post
Interesting, any reason you would have an axe to grind with me? In what past life have I transgressed against you?
Huh? You may care to note that I did not respond to your comments.
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Old 2nd December 2011, 11:47 PM   #12
kevinkr is offline kevinkr  United States
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I made the original suggestion he endorsed..

I thought it was reasonable given the original OP's interest in driving this amplifier with a tube pre-amp.. The version I suggested can drive 64 ohm headphones, but there are still a lot of issues to be considered including things like turn on/turn off transients which could be dealt with using a shunt relay and appropriate timer circuitry, along with appropriate power sequencing of the pre-amp and power amp..

I'd be the first to admit that the idea is not without risk, I have done it with with other solid state amps and tube pre-amps, but nothing with an unbalanced input with a 1K input impedance. He'll have to decide whether or not it is worth the risk - the designer with whom he has contact should be able to provide guidance as to whether or not this is a risky proposition.

Personally I would choose transformer coupling and avoid any risk of dc following the amplifier's input circuitry. One could also do a parafeed output transformer arrangement with the white CF in the Aikido, ratios could be 1:1 to 3:1 for 0 to -10dB of attenuation. The 3:1 would reflect a 9K load to the Aikido which is not a difficult load for the version I suggested, would allow the use of a much smaller and presumably higher quality film coupling cap at the output, and still leave you with about 12dB of gain which is enough for most applications.
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Old 2nd December 2011, 11:56 PM   #13
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Watch out kevinkr, remember that Mr. Beeblebrox has two heads - and three arms!
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Old 2nd December 2011, 11:57 PM   #14
kevinkr is offline kevinkr  United States
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Watch out kevinkr, remember that Mr. Beeblebrox has two heads - and three arms!
I'll have to google the reference, thanks for the heads up..
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Old 3rd December 2011, 12:04 AM   #15
kevinkr is offline kevinkr  United States
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Ah, The Hitchhiker's Guide To The Galaxy.. Not a fan of the book or series..
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Old 3rd December 2011, 12:14 AM   #16
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Originally Posted by kevinkr View Post
I made the original suggestion he endorsed..
I understand that. I confined my comments to those made by cjcc67.
Quote:
Originally Posted by kevinkr View Post

I thought it was reasonable given the original OP's interest in driving this amplifier with a tube pre-amp..
Indeed, given your lack of knowledge about the specific product, your comments were reasonable. Which is why I did not address them.

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Originally Posted by kevinkr View Post
The version I suggested can drive 64 ohm headphones, but there are still a lot of issues to be considered including things like turn on/turn off transients which could be dealt with using a shunt relay and appropriate timer circuitry, along with appropriate power sequencing of the pre-amp and power amp..
Precisely. The ME550 is fundamentally unsuited to a large number of preamps that are commonly available. That includes even quite heroic valve preamps, such as suggested by you. The ME550 SHOULD be driven by a source impedance of less than 10 Ohms. 100 Ohms would provide average to poor results. The ME550 has:

* DC coupling throughout.
* No DC protection system, outside full power shutdown, if DC is detected.

It is perfectly suited to any preamp which:

* Exhibits a source impedance of less than 10 Ohms.
* Is completely stable, with better than 1 or 2mV of offset.
* Does not deliver any switch on thumps, DC shifts or other nasties.

Quote:
Originally Posted by kevinkr View Post
I'd be the first to admit that the idea is not without risk, I have done it with with other solid state amps and tube pre-amps, but nothing with an unbalanced input with a 1K input impedance. He'll have to decide whether or not it is worth the risk - the designer with whom he has contact should be able to provide guidance as to whether or not this is a risky proposition.
Tigerscent has already been provided with appropriate guidance by those who have very long experience with the ME550, used with a very wide range of suitable and unsuitable sources. That includes the person who designed and manufactured the ME550. He was clearly and unequivocally told that use of a valve preamp would provide sub-optimal (at the very least) results.

Quote:
Originally Posted by kevinkr View Post
Personally I would choose transformer coupling and avoid any risk of dc following the amplifier's input circuitry.
And excellent suggestion.

Quote:
Originally Posted by kevinkr View Post
One could also do a parafeed output transformer arrangement with the white CF in the Aikido, ratios could be 1:1 to 3:1 for 0 to -10dB of attenuation. The 3:1 would reflect a 9K load to the Aikido which is not a difficult load for the version I suggested, would allow the use of a much smaller and presumably higher quality film coupling cap at the output, and still leave you with about 12dB of gain which is enough for most applications.
Or Tigerscent could use the preamp that was expressly designed for the ME550. That preamp exhibits a 2 Ohm (open loop) output impedance, unconditional stability, a frequency response from DC ~ 150kHz, low distortion, no DC shifts at switch on and zero global NFB. It is the safest, most appropriate option. And one that Tigerscent has yet to try.

I am not against the use of a valve preamp, per se. In fact, I would encourage Tigerscent to try an appropriate, low output impedance one, AFTER he has tried the preamp that the manufacturer suggests.

If Porsche suggsts the use of (say) Pirelli P-zero tyres, then would you fit (say) $100.00 tyres to one and expect the vehicle to perform?

Same deal with the ME550. A fine amplifier deserves the most appropriate match in preamp. A suitable preamp from ME, Krell, Mark Levinson or Gryphon are likely candidates. Most valve preamps are not suitable. Unless quite heroic alterations are made.
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Old 3rd December 2011, 12:19 AM   #17
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Old 3rd December 2011, 12:24 AM   #18
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Quote:
Originally Posted by Zaphod Beeblebrox View Post
I understand that. I confined my comments to those made by cjcc67.


Indeed, given your lack of knowledge about the specific product, your comments were reasonable. Which is why I did not address them.



<snip>

If Porsche suggsts the use of (say) Pirelli P-zero tyres, then would you fit (say) $100.00 tyres to one and expect the vehicle to perform?

Same deal with the ME550. A fine amplifier deserves the most appropriate match in preamp. A suitable preamp from ME, Krell, Mark Levinson or Gryphon are likely candidates. Most valve preamps are not suitable. Unless quite heroic alterations are made.
Your comments do make sense, and nothing I would inherently disagree with.. There are purpose built designs that would be suitable but that presupposes the OP has the ability to build such a design. It does seem to me that the matching pre-amp ought to be evaluated as a minimum.. FWIW there are not too many SS pre-amps that have such low output Z either if for no other reason than the series resistors generally found at their outputs.
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Old 3rd December 2011, 12:45 AM   #19
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Originally Posted by kevinkr View Post
Your comments do make sense, and nothing I would inherently disagree with.. There are purpose built designs that would be suitable but that presupposes the OP has the ability to build such a design. It does seem to me that the matching pre-amp ought to be evaluated as a minimum..
My point exactly. It always makes sense to start with a known yardstick and move on from there. Once Tigerscent has evaluated the 'standard' he can move onto other ideas and see how they compare. My point to Tigerscent has always been that the matching preamps are plentiful and modestly priced in Australia. He should have little difficulty in borrowing or buying one to try.

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FWIW there are not too many SS pre-amps that have such low output Z either if for no other reason than the series resistors generally found at their outputs.
Indeed. I've long wondered at the laziness/incompetence of SS designers who build such devices, given the ease that a low source impedance can be accomplished with SS devices. At least valve designers have an excuse, though it would seem 100 Ohms is pretty easy to acheive using cathode followers. And, as you say, transformers could reduce this still further, albeit at the a monetary and sonic cost.
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Old 3rd December 2011, 12:54 AM   #20
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Quote:
Originally Posted by Zaphod Beeblebrox View Post
<snip>

Indeed. I've long wondered at the laziness/incompetence of SS designers who build such devices, given the ease that a low source impedance can be accomplished with SS devices. At least valve designers have an excuse, though it would seem 100 Ohms is pretty easy to acheive using cathode followers. And, as you say, transformers could reduce this still further, albeit at the a monetary and sonic cost.
I suspect it has more than a little to do with the massive amounts of feedback employed to get good looking numbers and insufficient phase margin to safely drive typical audiophile capacitors, err.. cables..

Long ago I did a tube phono stage and specifically told the dealer who was evaluating it that it needed to used with low capacitance cables into a 100K load minimum - they ignored my comments and the cables they used had about 500pF of capacitance and needless to say this design did not acquit itself very well under the circumstances - this was supposed to be a very minimalist phono stage that would retail for under $500. I learned a valuable lesson, that most dealers and audiophiles ain't that bright, and that I should not design things with a 10K output impedance..
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