Impedance matching pre -> power.

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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.
 
OK, here is a cut out of the signal input section.
 

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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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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:
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! :D
 
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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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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.
 
borg7x9 said:
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 ??
The two inputs are essentially the same, except XLR is balanced and the other is not. The switch merely shifts the ground connection from the primary centre-tap to one end. Apart from the balancing, anything which would work with one would work with the other.
 
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Do you have, or can you borrow, a voltmeter that has a low volt AC scale? 2V AC or less?
With that, you can measure the overall gain of the amp, then know if you really need the 17dB voltage gain of the input transformer. If not needed, it could be bypassed.

For that matter if you have some way to apply the signal at the 100K resistor and 1st grid, you'll find out very fast.
 
As Godfrey mentioned, you need a headphone amp to drive this power amp. You could design one that was essentially your preamp also. It's no big deal to me anyway. While many opamps claim to drive 600 ohms by themselves (LME49710 comes to mind) at very low distortion (but I assume this is at unity gain), I'd feel better if it was muscled up with a power buffer like the BUF634 or LME49600. Have you tried using a headphone amp to drive this power amp?
 
Dirkwright, I have a schiit Lyr with a pre out that is a fairly good amp. Are you suggesting that I run a headphone jack to rca and see what that is like. This amp also has a pre amp output at 75 ohms. I tried this it was ok but it was noisy.

I could try the head-out if you think this will yield different results.
 
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