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Old 14th May 2013, 08:58 AM   #21
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Yes Mooly,
I still think the manual is wrong.
It is more likely to be stating the required source impedance rather than the actual input impedance.

The actual input impedance needs to be checked before any decisions are made about purchases or modifications.
I'm saying nothing

Where did I get dragged into all this ?
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Old 14th May 2013, 11:01 AM   #22
borg7x9 is offline borg7x9  New Zealand
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Looking at the circuit diagram there are two 300ohm resisters in series across the front of the input transformer. giving a 300ohm resistance for ballanced and 600ohm for SE. the imput transformer is PM600:30k and it has a 100k resister before feeding the first half of the CV181 input tube. I've got no experience reading schematics though. It does lok like its designed to present a 600 ohm load !!! Curious.
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Old 14th May 2013, 11:31 AM   #23
Pano is offline Pano  United States
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Quote:
Pano, would you like to see the circuit diagram of the power amp ??
Yes, but if it's copyrighted then we don't allow it to be posted here. If it's online somewhere, post the link. Or send me a PM and I'll look at it and post here the relevant parts. Thanks.
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Old 14th May 2013, 09:03 PM   #24
borg7x9 is offline borg7x9  New Zealand
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Pano, I'm not able to attach a file to a PM. I can attach it as a thread reply. But I am not keen to do that as I'm not clear on the copyright status. If I'm being dense can someone tell me how to attach a file to a PM.

Thanks.
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Old 14th May 2013, 09:12 PM   #25
Pano is offline Pano  United States
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I sent you my email. Send me the schematic and I'll have a look at it.
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Old 14th May 2013, 09:30 PM   #26
borg7x9 is offline borg7x9  New Zealand
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OK, here is a cut out of the signal input section.
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Old 14th May 2013, 10:04 PM   #27
Pano is offline Pano  United States
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You beat me to it.

It looks to me like the primary will see about 2K, what with the 100K and the input tube grid behind it. Add that resistors into that and you really are and 600 ohms. They are using the input transformer as a voltage gain stage (gain of 7x). Not sure you really need it.

You can see the input section in my edit of the schematic.
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Old 14th May 2013, 10:52 PM   #28
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by Pano View Post
It looks to me like the primary will see about 2K, what with the 100K...
And that's in parallel with the 600 Ohms, so the actual input impedance is about 461 Ohms. A headphone amp would be able to drive this easily, but that just adds more complication and weirdness.

It's almost tempting to suggest removing the 300R resistors, or replacing them with higher value ones but that could end badly if the transformer's primary inductance is too low (and it would invalidate the warranty, closing the door on the "return for refund" option).

Something that might be worth trying is simply adding a resistor in series with the input. e.g. adding a 1.5K resistor changes the sensitivity from 200mV to 850mV and lifts the input impedance to 2K. That might be an easier task for most "normal" preamps. At any rate, it costs almost nothing to try and doesn't hurt the warranty.

edit:
Quote:
Originally Posted by Pano View Post
You can see the input section in my edit of the schematic.
That's a little too revealing, Pano. What do we have bottom right hand corner? Could it be evil global feedback, applied via a big ugly electrolytic cap, no less? That's gonna step on on some audiofool toes, that is!

Last edited by godfrey; 14th May 2013 at 11:07 PM.
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Old 14th May 2013, 11:00 PM   #29
borg7x9 is offline borg7x9  New Zealand
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Thanks for looking at that Pano. It is very much appreciated.....So in this case do you think that a pre-amp which is impedance matched and transformer coupled could work well ?? I've been chatting with the Audionote guys (their pre-amps are transformer output). and they still recommend driving the single ended input with -10x the input impedance <60ohms, but say that the XLR input should work well at 600ohms matched. Do you agree with this opinion ?? Thanks again.
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Old 15th May 2013, 03:28 AM   #30
Pano is offline Pano  United States
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Quote:
Originally Posted by godfrey View Post
That's a little too revealing, Pano. What do we have bottom right hand corner? Could it be evil global feedback, applied via a big ugly electrolytic cap, no less?
Close! It's a local feedback loop taken from the secondary of the interstage transo that drives the output tubes. Cap is probably film as it's marked 0.47uF 600V

I guess we really need to know if the amp needs the 7X voltage gain at the input. That's mostly what the input transfo does.
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