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Old 3rd July 2012, 12:35 PM   #81
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Here is the literature on current mirror peaking. The extract from a book was written by Barry Gilbert.

The devices used to get the results were 3Ghz parts and i wonder what the results would be with 100 or 200 Mhz parts ?? Would there still be peaking and at what frequency?? As can be seen the 3 Ghz parts show peaking at or just below their ft.
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Old 3rd July 2012, 12:42 PM   #82
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Thanks for posting that. I will need to play around in spice and see if I can get a similar result. I don't normally use mirrors because my designs are all symmetrical, but this is one to keep in the back pocket if I ever go down that route.
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Old 3rd July 2012, 01:28 PM   #83
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This weekend I listened to both the e-Amp and the Ovation 250 with a guy that has a speaker manufacturing company here in Taiwan (I bought a pair of compact bookshelf speakers and a 24/192 DAC from him). Like me he is also a Jazz and acoustic music lover.

He wasted no time in re-positioning my speakers about 2m away from the wall (my wife looked on in horror :-o) and maybe a metre closer together than I normally have them. He asked me if I had a low bench to sit on, so I found a stool so that we were positioned correctly wrt to midrange and tweeters. I normally like to lounge in an easy chair (with a red wine), but I know that’s not good enough for critical listening.

His comments on the e-Amp: A very modern sounding power amp with lots of punch and snap and a great top end. He loved the sound staging and liked the bass as well.

His comments on the Ovation 250: That's the one he likes. It sounds ‘darker’ and more rounded than the e-Amp. He said its more laid back and sounds more ‘tubey’. The sound staging on the Ovation 250 is also very good.

The e-Amp is definitely more forward than the Ovation 250 - to my ears a little bit brighter.

Both amps do the front to back imaging very well – so the sound stage is very deep and there good imaging beyond the speakers to the left and right.

Interesting that these two amplifiers do sound different. The Ovation 250 uses slow output devices and has moderate amounts of feedback and was modified for TMC comp last July, while the e-Amp has a fast output stage and slightly higher feedback. The e-Amp was set for high loop gain and TMC for this listening test, so the amplifiers were like for like in terms of compensation.

The e-Amp uses a pretty conventional VAS with a beta enhancer, while the older Ovation 250 uses a beta enhanced VAS feeding a cascode. Both designs use an 5 x EF3 output stage. The Ovation 250 uses a very expensive high performance transformer and about half the rail capacitance of the e-Amp.

Both amps sound wonderful (ok, that's the view of only two people!) in my view, but they are different. No question about that.

I used the X-Altra Mini Preamp for the tests.

In a few weeks I will go to his place and we will trial the amps driving the top of the range Avalon’s. He’s got the big Dali’s and Ushers as well - I am really looking forward to that. He told me my 703’s let my system down, but I think he’s trying to line me up for a sale on something really high end – but my heart is set on a pair of big B&W’s – hell, I hope my stock appreciates in the next year or so! Of course, this has to be steered past the domestic boss . . .
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Old 12th July 2012, 02:48 PM   #84
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Originally Posted by Bonsai View Post
I don't normally use mirrors because my designs are all symmetrical, but this is one to keep in the back pocket if I ever go down that route.
Yes, thanks for that. Also on the subject of current mirrors - Did you ever examine the Baxandall+Swallow mirror?

Best wishes
David
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Old 13th July 2012, 12:51 AM   #85
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No, not yet - another one on the list of things to get around to!
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Old 20th January 2013, 09:05 AM   #86
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Hi Andrew. Just reminded of a detail on the e-amp that I never asked about at the time.
You connect the normally unused collector of the input LTP to the VAS emitter resistor and say that this reduces distortion. Seems attractive at first look. Do you have any references for this? I don't notice either Bob Cordell or D Self mention it, probably because they both like current mirrors and that would make it superfluous I think.
It also looks plausible to connect to the emitter resistor of the VAS beta enhancer (AKA emitter follower) instead. Any comments? or do I have to think for myself

Best wishes
David

Last edited by Dave Zan; 20th January 2013 at 09:10 AM.
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Old 20th January 2013, 02:20 PM   #87
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Hello Dave.

I first saw this in a James Bionjorno (spelling) amp years ago. You get a few ppm improvement on sims doing this, so you will not see any improvements if your distortion is high (improvement is swamped).

Re you point about returning the beta enhancement transistor - I have not tried that idea. Looks interesting though.
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Old 20th January 2013, 10:07 PM   #88
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I first saw this in a James Bionjorno (spelling) amp years ago.
...
Re you point about returning the beta enhancement transistor - I have not tried that idea.
Thanks. The beta enhancement transistor return is worth a simulation if it hasn't been positively rejected. It seems better balanced.
Did Bongiorno have a beta transistor and not use it, or did you put the beta transistor in front and not alter the connection?

I want to improve Bob Cordell's resistor fix for indeterminate VAS current in a double LTP + current mirrors IPS, and this looks to have promise.
I haven't seen this in the forum, anyone have any prior art?

Best wishes
David

Last edited by Dave Zan; 20th January 2013 at 10:09 PM.
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